Advancing Greenhouse Air Filtration: Biodegradable Nanofiber Filters with Sustained Antimicrobial Performance

Abstract

Air quality management in greenhouses is critical to safeguarding plant health and occupational safety, yet conventional filtration methods often fall short in performance and sustainability. These enclosed environments are prone to the accumulation of bioaerosols, including fungi, bacteria, pollen, and dust particles, which can compromise crop productivity and pose health risks to workers. This review explores recent advancements in air filtration technologies for controlled environments such as greenhouses, where airborne particulate matter, bioaerosols, and volatile organic compounds (VOCs) present ongoing challenges. Special focus is given to the development of filtration media based on electrospun nanofibers, which offer high surface area, tunable porosity, and low airflow resistance. The use of biodegradable polymers in these systems to support environmental sustainability is examined, along with electrospinning techniques that enable precise control over fiber morphology and functionalization. Antimicrobial enhancements are discussed, including inorganic agents such as metal nanoparticles and bio-based options like essential oils. Essential oils, known for their broad-spectrum antimicrobial properties, are assessed for their potential in long-term, controlled-release applications through nanofiber encapsulation. Overall, this paper highlights the potential of integrating sustainable materials, innovative fiber fabrication techniques, and nature-derived antimicrobials to advance air filtration performance while meeting ecological and health-related standards.

1. Introduction

Greenhouse cultivation involves growing plants in controlled environments with glass or plastic structures equipped with irrigation, heating, and ventilation systems [1]. This method protects plants from adverse weather conditions and fosters the growth of fruits and vegetables year-round, contributing significantly to sustainable agricultural practices, including organic farming [2,3]. Despite the benefits, controlling the environment and air quality within greenhouses poses significant challenges. Ensuring air quality is crucial for a productive and healthy growing environment [1]. Particulate matter (PM) also poses specific risks in greenhouse environments, affecting air quality, worker health, and crop productivity [4]. Poor air circulation not only contributes to the accumulation of PM but also leads to elevated carbon dioxide levels and high humidity, conditions that promote mold and microbial growth [5]. Airborne pathogens, dust, and volatile organic compounds (VOCs) can also adversely affect crop quality and worker health [6]. Dust and particulate matter on plant surfaces hinder photosynthesis, highlighting the need for regular cleaning and dust control [7]. Harmful volatile organic compounds and unpleasant odors in greenhouses, such as carbonyl compounds like formaldehyde and acetaldehyde, often originate from sources including combustion processes, fertilizers, and decomposition of organic matter [8]. These VOCs negatively affect crop quality by causing physiological stress and reduced growth, and they pose risks to human health, leading to respiratory issues and irritation of the eyes and mucous membranes [5,7]. Implementing sustainable practices such as advanced filtration techniques to remove pollen, particulate matter, and other airborne contaminants is essential for maintaining optimal air quality in vegetable greenhouses and indoor environments [9]. These methods include the use of mechanical filters, electrostatic filters, and advanced materials such as electrospun nanofibers, which provide superior filtration efficiency and reduced pressure drop [10]. Unlike traditional filters, which often face high air resistance and limited dust-holding capacity, electrospun nanofibers can achieve high filtration efficiency while maintaining low pressure drop [11,12]. Among these electrospun nanofibers, biodegradable ones such as those derived from poly(lactic acid) or cellulose acetate have gained particular interest for their ability to effectively capture fine particulates and bioaerosols while degrading into non-toxic byproducts, thus offering both high filtration performance and environmental sustainability [13].

In this article, a comprehensive overview is presented on the sources of airborne contaminants in indoor environments, with a particular focus on greenhouse environments. Key air filtration principles and the primary parameters influencing filtration performance are discussed. Special attention is given to various methods for fabricating air filters, particularly electrospinning techniques. Furthermore, the potential of electrospun nanofibers integrated with antimicrobial agents is explored for their effectiveness in capturing dust and bioaerosols while simultaneously inactivating microorganisms. In particular, essential oils are examined as promising antimicrobial additives that offer a safe and sustainable approach for enhancing air filtration in agricultural environments. In addition to antimicrobial functionality, the use of biodegradable polymers is discussed as a complementary strategy that addresses the significant waste generated by frequent filter replacement in greenhouses, making these advanced nanofiber systems both functionally effective and environmentally responsible. This combined approach is especially relevant for greenhouses, where high humidity promotes microbial growth and frequent filter replacement makes biodegradable and antimicrobial functions equally important. The overall concept and key filtration pathways of a biodegradable antibacterial electrospun nanofiber filter are schematically illustrated in Figure 1.

2. Filtration and Evaluation of Air Quality in Greenhouses

Ensuring proper ventilation and humidity control is crucial for a healthy greenhouse environment. Bioaerosols, recognized as potential pollutants in greenhouses, consist of various components, including fungi, bacteria, endotoxins, β-glucans, mycotoxins, and allergens [14]. Bioaerosols are small airborne particles (0.001 to 100 μm) that originate from biological sources, including plants, animals, and microorganisms, and due to their small size, bioaerosols can easily migrate across environments [15,16]. They may contain living organisms or fragments such as viruses, bacteria, and fungi and can significantly impact human health, causing diseases ranging from respiratory infections to cancer [17]. It is essential to understand the identification, quantification, distribution, and health impacts of these bioaerosols, some of which are shown in

Table 1. Overview of Bioaerosol Components, Sources, and Health Impacts [18,19,20,21,22].

Component	Description	Common Sources	Associated Health Effects
Fungi	Common fungi like Aspergillus and Penicillium thrive in moist environments and are prevalent in indoor and outdoor air. Seasonal variations affect their concentrations.	Air conditioning units, water-damaged materials	Respiratory symptoms, allergic reactions
Bacteria	Bacteria such as Staphylococcus are found in indoor and outdoor air, especially in moist environments.	Air conditioning units, water-damaged materials	Infections, respiratory issues
Endotoxins	Toxic components of Gram-negative bacteria that cause inflammation and respiratory issues. They are often attached to dust and easily inhalable.	Agricultural settings, dust	Inflammation, respiratory issues
β-Glucans	Polysaccharides present in the cell walls of fungi, bacteria, and plants, capable of inducing immune responses and respiratory symptoms when inhaled.	Fungi, bacteria, plants	Immune responses, respiratory symptoms
Mycotoxins	Toxic secondary metabolites produced by fungi, leading to various health issues, including immune suppression and cancer, depending on exposure duration.	Fungi	Immune suppression, cancer, various other health issues
Allergens	Substances from fungi, plants, and animals that trigger allergic reactions and asthma.	Mold spores, pollen, dust mites	Allergic reactions, asthma

[18].

While numerous reviews have examined air filtration technologies in industrial and indoor settings, limited attention has been given to greenhouses, which present distinct challenges such as high humidity, confined air circulation, and a complex mix of biological and chemical pollutants. This review addresses this gap by exploring recent advances in nanofiber-based filtration systems with potential applicability to greenhouse environments.

2.1. Potential Health Impacts of Bioaerosols

Bioaerosols can spread diseases through direct or indirect contact, airborne transmission, and vectors, and exposure to them can lead to respiratory issues such as asthma and allergies [23]. Common bioaerosol contributors such as pollen, mold, and endotoxins are known to cause airway inflammation, reduced lung function, and respiratory infections like tuberculosis, influenza, and legionellosis, as well as allergic reactions triggered by allergens and mycotoxins, resulting in symptoms such as sneezing, coughing, and asthma [24]. This is largely due to inhaling harmful biological materials, such as viruses and endotoxins, that may have carcinogenic effects [18]. Figure 2 illustrates the origins, components and health hazards associated with bioaerosol exposure in the greenhouse environment.

Bioaerosols in greenhouses originate from natural sources such as plants, soil, and water, as well as human activities like watering and cleaning, which release particulate matter [25]. The humid and enclosed nature of greenhouse structures fosters microbial growth, leading to elevated levels of fungi, bacteria, and endotoxins [26]. Ventilation systems and misting can disperse bioaerosols, exacerbating contamination. Therefore, effective air filtration and proper humidity control are essential to mitigate bioaerosol exposure and ensure healthier environments for both plants and workers [27]. Key terms related to bioaerosol exposure include CFU/m³ (Colony-Forming Units per cubic meter), which measures the concentration of viable bacterial or fungal cells in the air. Another important term is EU/m³ (Endotoxin Units per cubic meter), indicating the level of endotoxins and reflecting Gram-negative bacterial contamination [28,29].

Table 2. Airborne Biological Contaminants: Size, Associated Diseases, and Transmission Pathways [18,22,23,25].

Category	Causative Organism	Approximate Size	Resulting Disease	Infection/Transmission
Bacteria	Mycobacterium tuberculosis	Length: 2–4 μm, Width: 0.2–0.5 μm	Tuberculosis (T.B.)	Person to person through the air
	Legionella pneumophila	Length: 2 μm, Width: 0.3–0.9 μm	Legionnaires’ disease	Inhalation of a water aerosol containing the bacteria—hot water sources
	Y. pestis	length: 1.0–3.0 µm, Width: 0.5–0.8 µm	Plague Whooping cough	Direct contact or inhalation of airborne droplets—infected fleas
	Bacillus anthracis spore-B. anthracis	Length: 3–5 μm, Width: 1.0–1.2 μm	Anthrax	Contact with infected animals and flies and breathing air containing anthrax spores
	Bordetella pertussis	Length: 40–100 nm, Diameter: 2 nm	Whooping cough	Direct contact or inhalation of airborne droplets
	Vibrio cholerae	Length: 1.4–2.6 μm, Width: 0.5–0.8 μm	Cholera	Ingestion of contaminated food or water
	Salmonella Typhi	Length: 0.7–1.5 μm, Thickness: 28 μm	Typhoid	Through contaminated food or water and occasionally through direct contact with someone who is infected
Fungi	Alternaria spp.	Length: 18–83 µm Width: 7–18 µm	Asthma, rhinitis	Outdoor air, damp surfaces
	Histoplasma spp.	2–4 µm for yeast form Width: 1 to 3 µm	Histoplasmosis	Bird droppings
	Aspergillus (aflatoxin)	Length: 2.5 –3.5 µm Width: 2.5–8 µm	Cancer	Damp surfaces
	Penicillium spp.	Length: 2.5–5.0 µm Width: 1.5–5.0 µm	Penicilliosis	Mold-contaminated building
	Microsporum Trichophyton	Length: 5–100 µm, Width: 3–8 µm	Ringworm	Direct or indirect contact with skin or scalp lesions of infected people, animals, or fomites
Virus	Variola vera	Length: 220–450 nm, Width: 140–260 nm	Smallpox	Inhalation of variola virus, close contact with infected individuals or contaminated materials
	Herpesviridae, HHV-3	Diameter: 150–200 nm	Chickenpox and shingles	Direct contact with fluid from the rash blisters caused by shingles
	Morbillivirus (measles)	Length: 125–250 nm, Diameter: 21 nm	Measles, mumps, rubella	Bodily fluids: drops of saliva, mucus from the nose, coughing or sneezing, tears from the eyes, etc.
	Coronavirus (SARS-CoV-2)	60–140 nm	Coronavirus disease	Human: respiratory droplets, close contact

presents key airborne biological contaminants, their sizes, related diseases, and transmission routes. A large proportion of these airborne biological contaminants could potentially be present in greenhouse environments, highlighting the need for awareness and mitigation strategies.

One of the few detailed investigations into bioaerosol exposure in greenhouse settings was a comprehensive 2.5-year-long study conducted in a Danish flower greenhouse growing Campanula, Lavandula, Rhipsalideae, and Helleborus, which assessed occupational exposure to airborne dust, endotoxins, fungi, and bacteria using personal full-shift measurements and electronic task logs [30]. Exposure levels varied significantly across plant types, seasons, and specific tasks. Inhalable dust concentrations ranged from 0.04 to 2.41 mg/m³, with the highest levels during packing of Lavandula. Endotoxin levels spanned 0.84 to 1097 EU/m³, exceeding the health-based limit of 90 EU/m³ in 30% of samples, particularly during sticking of Campanula. Fungal exposure reached up to 3.4 × 10⁶ CFU/m³, highest during the packing and dumping of Campanula, while bacterial levels peaked at 4.2 × 10⁵ CFU/m³, largely driven by tasks such as sticking Campanula and working with biopesticides. The study highlighted how exposure is strongly task- and season-dependent, supporting task-specific interventions to reduce greenhouse worker exposure [30]. In another study, greenhouse workplaces reported average endotoxin concentrations of 3.87 EU/m³, with total bacterial levels at 2.06 × 10³ CFU/m³ and Gram-negative bacteria at 0.05 × 10³ CFU/m³. Although these values are lower than those typically found in livestock facilities, they still pose health risks, including respiratory problems and potential long-term effects. Warm (27.2 °C) and humid (45.9%) greenhouse conditions further promote bioaerosol proliferation, underscoring the importance of effective air filtration and protective measures [26].

Filters must be selected based on specific pollutants, crop types, and operational conditions, alongside routine monitoring and maintenance. However, many conventional filters clog quickly and require frequent replacement with non-biodegradable materials, making this practice both environmentally and economically unsustainable. These challenges emphasize the need for more efficient, sustainable filtration solutions [4].

2.2. Principles of Filtration

Air-filtration processes can operate in either a steady-state regime, where filtration efficiency remains constant over time, or an unsteady-state regime, where efficiency changes due to particle accumulation. Early models by Freundlich and Kaufmann integrated Brownian motion and inertial deposition and later evolved into single-fiber filtration theory [31]. Figure 3a illustrates the four primary mechanisms involved in particle capture during fiber filtration: interception, impaction, diffusion, and electrostatic effects [10,19,23]. While aerogels and particles are subject to gravitational forces, this effect is negligible for smaller particles due to their low mass. However, as particle size and mass increase, the efficiency of the gravitational deposition becomes more significant [4].

Interception Effect: This occurs when the center of a particle, moving along with the airstream, is captured by a fiber because the distance between the fiber and the particle is less than or equal to the particle’s radius [32]. Larger particles are more prone to this type of interception, as their size increases the likelihood of coming into contact with the fiber [33].

Impaction Effect: Due to their inertia, particles are unable to stay aligned with rapidly changing air streamlines and are subsequently deposited onto the fiber [31]. This inertial impaction is more pronounced in particles larger than 0.3–1 µm or those moving at higher velocities, as their momentum prevents them from following the airflow [34].

Diffusion Effect: Tiny particles near the fiber undergo irregular Brownian motion, which increases the likelihood of their contact with and deposition on the fiber surface. This mechanism is dominant for particles smaller than 1 μm and becomes more effective at lower gas velocities, as random motion causes particles to deviate from streamlines and migrate toward the fibers over time [35].

Electrostatic Effect: The electrostatic effect significantly enhances filtration efficiency, especially when either the fibers or the particles carry an electric charge [31]. Charged particles can be attracted to oppositely charged fibers via Coulombic forces, while even neutral particles can be polarized and drawn toward charged fibers through dielectrophoretic attraction. These interactions result in strong adhesion between particles and fibers, improving overall filtration performance [36].

To conclude the discussion on general filtration mechanisms, it is important to note that various filter designs, such as HEPA filters, electrostatic precipitators, membranes, and activated carbon filters, incorporate these mechanisms in distinct ways [37]. While these designs are effective in specific applications, fabric-based filters stand out for their versatility in design and potential for optimization, and are therefore a focus of this review.

Figure 3. (a) Filtration mechanisms illustrating impaction, interception, diffusion, and electrostatic attraction. Reprinted with permission under CC BY 4.0 from Han et al. [31]. Surface filtration mechanisms: (b) Complete blocking filtration; (c) Bridging filtration. Depth filtration. (d) Depth straining mechanism; (e) Depth retaining mechanism. Arrows show the flow direction, and circles represent the particles. Adapted and redrawn from Mukhopadhyay [38].

Figure 3. (a) Filtration mechanisms illustrating impaction, interception, diffusion, and electrostatic attraction. Reprinted with permission under CC BY 4.0 from Han et al. [31]. Surface filtration mechanisms: (b) Complete blocking filtration; (c) Bridging filtration. Depth filtration. (d) Depth straining mechanism; (e) Depth retaining mechanism. Arrows show the flow direction, and circles represent the particles. Adapted and redrawn from Mukhopadhyay [38].

2.3. Filtration Mechanisms in Fabric-Based Filters

Filtration processes in filter fabrics can be broadly classified into surface filtration (Figure 3b,c) and depth filtration (Figure 3d,e). Surface filtration, which includes cake filtration, occurs on the surface of the filter, whereas depth filtration involves the capture of particles within the filter media.

2.3.1. Surface Filtration

Surface filtration retains particles through mechanical mechanisms such as sieving and bridging. In this process, larger particles are captured directly by sieving, while smaller ones form bridges across the filter pores, gradually creating a stable cake layer on the surface. Blocking and bridging, caused by particle accumulation, obstruct airflow and play a key role in dust-cake formation [38].

Cake filtration involves the accumulation of dust particles on the surface of the filter fabric. As more particles are trapped, they form a layer, or “cake,” which acts as an additional filtration medium. This cake layer comprises both large and small particles, enhancing the overall filtration efficiency by capturing smaller particles that penetrate the initial filter layer. The structure and efficiency of the cake are influenced by factors such as the cake’s permeability, pore size, particle size, and compressibility [39]. As the filtration process continues, the thickness of the cake layer increases, leading to higher filtration efficiency but also greater flow resistance and pressure drop. The growth and compaction of the cake are accelerated by higher flow velocities, which cause the cake to become more compact and dense. The increased resistance due to cake formation necessitates periodic cleaning to maintain the filter’s efficiency and functionality. In some applications, the filtration cake is the main filtration component and may need to be recovered, particularly when the captured material is of economic or process value [38].

2.3.2. Depth Filtration

Depth filtration captures particles within the filter media as they pass through funnel-shaped pores. As particles travel deeper, they encounter narrower pore spaces that eventually become too small, effectively trapping them. This process leads to increased flow resistance and pressure drop as particles accumulate within the filter structure [39]. For in-depth filters, particularly those made from nonwoven materials, fabric strength is critical for maintaining performance under operating conditions. One important parameter is bursting strength, which indicates the material’s resistance to pressure applied from all directions. In needle-punched filters, this strength can be enhanced by optimizing the needling process and incorporating reinforcement layers. These improvements increase mechanical stability and extend the filter’s durability during prolonged use [40].

2.4. Evaluation of the Filtration Performance

Several parameters are used to evaluate air filter performance, including filtration efficiency, pressure drop, quality factor, dust-holding capacity, and air permeability [41]. Among these, filtration efficiency, pressure drop, and quality factor are considered the most critical due to their direct impact on both particle capture and energy consumption [31,32,33].

2.4.1. Pressure Drop

The pressure drop across a filter arises from resistance in the dense fibrous structure [42]. Under equivalent filtration efficiency, filters composed of smaller-diameter fibers offer a larger specific surface area and a more interconnected pore structure. This design improves particle capture while facilitating airflow, resulting in a relatively lower pressure drop compared to filters made with thicker fibers [43]. Additionally, thin fibers enhance van der Waals interactions with fine particles, further improving capture efficiency without substantially increasing resistance [44]. The pressure drop is closely related to the drag coefficient, as greater aerodynamic resistance around the fibers leads to a higher pressure drop across the filter [45].

Drag Coefficient (C_D)

The Drag Coefficient (C_D) is a dimensionless number that describes the resistance (or drag) experienced by an object moving through a fluid (such as air). In the context of air filtration, the drag coefficient relates to the resistance experienced by air as it flows through the filter material [45]. The drag coefficient is directly related to the pressure drop across the filter, which is expressed as:

$${\mathrm{C}}_{\mathrm{D}} ={\displaystyle \frac{2 \Delta \mathrm{P} }{{\mathsf{\rho }\mathrm{v}}^2}}$$

(1)

where $\Delta P$ is the Pressure difference across the object (Pa), $\rho$ is the Fluid density (kg/m³) and $v$ is the velocity (m/s) of the fluid flowing past the object. A higher drag coefficient indicates greater resistance, which can lead to a higher pressure drop [35]. The drag coefficient in the context of air filtration can be expressed using the following equation:

$${\mathrm{C}}_{\mathrm{D}} = {\displaystyle \frac{2{\mathrm{k}}_1{\mathsf{\mu }}^2{\mathrm{d}}_{\mathrm{f}}}{\mathsf{\rho }\mathrm{vL} \ln \left({\mathrm{k}}_2{\mathrm{d}}_{\mathrm{b}}\right)}}$$

(2)

K₁ and K₂ are empirical constants that account for the shape and orientation of fibers within the filter structure, d_f is the diameter of the fibers in the filter material, $v$ represents the speed of the air flowing through the filter, μ is the dynamic viscosity of the fluid (usually air) flowing through the filter, and fiber Spacing d_b, the distance between adjacent fibers in the filter, are the main parameters influencing the drag coefficient [46,47].

2.4.2. Filtration Efficiency

Filtration efficiency is a general measure of how effectively a filter captures PM from the air and reduces particle concentration [48]. In the case of nanofiber membranes, high filtration efficiency is often attributed to their small fiber diameters and high surface area, which enhance particle capture mechanisms [49].

$$\mathsf{\eta }=\left[1-{\displaystyle \frac{{\mathrm{C}}_{\mathrm{down} \mathrm{stream}}}{{\mathrm{C}}_{\mathrm{upstream}}}}\right]\times 100\%$$

(3)

where η is the capture efficiency, C_upstream represents the particle concentration before filtration, while C_downstream is the particle concentration after filtration [48].

2.4.3. Quality Factor

The criterion for filtering performance, defined by Chen in 1955, incorporates both the pressure drop and filtration efficiency to provide a comprehensive measure of filter performance [2,39].

$$Q_f={\displaystyle \frac{-\ln \left(1-\eta \right)}{\Delta p}}$$

(4)

where $Q_{f }$ is the quality factor, η is the filtration efficiency defined earlier, and ΔP is the pressure drop across the filter [49].

The Kuwabara model is another key parameter used to estimate single-fiber efficiency, which influences total filtration efficiency and, in turn, the quality factor (Qf), as it depends on both efficiency and pressure drop [50].

Kuwabara Model

The Kuwabara model is a mathematical approach used to predict the filtration efficiency of fibrous filters, particularly in air filtration applications. Although originally developed for steady-state filtration processes, it can, with appropriate assumptions, be extended to model filtration behavior more broadly, offering valuable insight into fiber-level performance [51]. This model is particularly suited for predicting particle capture under low Reynolds number conditions (Re < 1), where inertial effects are negligible and particles tend to follow airflow streamlines. It is commonly used to estimate the single-fiber efficiency for the collection of such inertialess particles [52]. The model is described as [51]:

$$\mathsf{\eta } = 1-\exp [{\displaystyle \raisebox{1ex}{$-\left(4{\mathsf{\eta }}_s\mathsf{\alpha }\mathrm{L}\right) $}\!\left/ \!\raisebox{-1ex}{$ ({\mathsf{\pi }\mathrm{d}}_f\left(1 - \mathsf{\alpha }\right) $}\right.}]$$

(5)

Key parameters involved in the model include the fiber volume fraction (α), which represents the proportion of the filter volume occupied by fibers, and the fiber diameter (d_f), referring to the size of individual fibers in the filter. Additionally, the model considers the filter thickness (L), which defines the depth of the filter medium, and the filtration efficiency (η), representing the overall effectiveness of the fiber membrane in filtering particles [51].

2.5. Advanced Methods for Predicting Particle Capture

Advanced modeling techniques are increasingly used to analyze airflow behavior and pressure drops in nanofiber filters. In a detailed study [53], a comprehensive computational fluid dynamics (CFD) framework was applied to model flow through nanofiber networks across 56 combinations of fiber diameter (50–800 nm), packing density (0.02–0.08), face velocity, and thickness. This modeling approach incorporated aerodynamic slip to realistically capture nanoscale gas–fiber interactions, allowing the simulations to probe flow fields and drag behavior that cannot be easily measured experimentally. The results showed that ultra-thin nanofiber layers do not follow the simple linear relationship between thickness and pressure drop suggested by classical filtration theory. Instead, the drag term depends on packing density, Knudsen number, and the ratio of thickness to fiber diameter, highlighting the importance of nanoscale flow modeling in predicting airflow resistance [53]. Data-driven modeling has also been applied to predict filtration efficiency in nanofiber membranes. Sohrabi et al. [54] used an artificial neural network–genetic algorithm (ANN–GA) approach to link electrospinning parameters with nanofiber diameter, thickness, and basis weight in a polyurethane nanofiber system, all of which directly influence particle capture performance. Through morphological optimization (fiber diameter reduced from 316 to 277 nm, thickness from 10 to 6.2 µm, and basis weight from 0.64 to 0.52 gsm), the PM₃₀₀ (particulate matter less than 300 µm) filtration efficiency increased from 0.73 to 0.96, while the pressure drop rose from 79.23 Pa to 110.23 Pa. Despite this increase in airflow resistance, the overall quality factor improved markedly from 0.0167 to 0.0297 Pa⁻¹, demonstrating that AI-guided control of fiber morphology can significantly enhance separation performance without excessive material loading.

2.6. Air Filter Types and Their Fabrication Methods

Absolute filters are commonly applied in environments requiring a high level of air cleanliness. According to the European standard CSN EN 1822 [55], these filters are categorized into three main types based on their particle removal efficiency: Efficient Particulate Air (EPA), High-Efficiency Particulate Air (HEPA), and Ultra-Low Penetration Air (ULPA) filters [56]. There are several key methods for fabricating air filters, including electrospinning [42,57], melt-blown [58], needle-punched [59], and, more recently, 3D printing [60,61]. While electrospinning remains the most established technique for fabricating high-efficiency air filters, recent studies have demonstrated the potential of hybrid approaches, combining 3D printing with electrospinning to enhance design flexibility, mechanical stability, and air permeability through tailored filter architectures [62]. Electrospinning utilizes a high-voltage electric field to stretch a polymer solution or, in some cases, a polymer melt into fine fibers [63]. This process produces nanofibrous membranes with a high surface area, making them ideal for applications requiring high filtration efficiency, such as air and water filtration [57,64]. The melt-blown technique involves extruding molten polymer through small nozzles and blowing it with high-speed air, creating microfibers that are collected into a nonwoven fabric, commonly used in applications like face masks and HEPA filters due to their fine structure [65]. Needle-punched filters are produced by mechanically entangling fibers through repeated piercing with barbed needles, creating a durable and porous structure that is often employed in industrial applications such as wastewater treatment and dust collection [59]. 3D printing has recently emerged as a promising technique for air filter fabrication, allowing the use of recycled polymers and functional coatings while offering precise control over porosity and surface properties to enable customizable, multifunctional filters with features such as gas adsorption and antibacterial activity [63]. These methods offer a range of filtration capabilities suitable for various applications. Another key parameter influencing fabric-based filters is area density, or grammage, which represents the weight of the filter material per unit area (g/m²) and plays a crucial role in determining filtration efficiency, pressure drop, and overall performance. It is influenced by material composition, fiber diameter and distribution, layer construction, and manufacturing processes such as electrospinning or melt blowing. Higher area density often corresponds to improved filtration but can increase the pressure drop [66].

2.7. Guidelines for Greenhouse Filtration

Greenhouse filtration standards often address acceptable levels of CO₂, fungi, dust, and pollen. These standards help ensure optimal plant growth conditions and the safety of greenhouse workers. Studies have shown that the indoor air of glasshouses can contain higher concentrations of fine particles compared to outdoor environments [14]. This highlights the need for effective filtration strategies to control airborne pollutants and ensure worker and crop safety.

2.7.1. Optimal CO₂ Concentration for Plant Growth

Maintaining optimal CO₂ levels is important for plant growth in greenhouses, with most crops performing best between 800 and 1200 ppm [67]. While filtration systems do not regulate CO₂ concentration directly, they are part of broader air management strategies that help maintain controlled environments and limit the introduction of unwanted pollutants during ventilation.

2.7.2. Worker Safety Standards

To assess potential health effects of CO₂ exposure, Zhang et al. [67] investigated the effects of short-term (2.5 h) exposure to elevated CO₂ levels (up to 5000 ppm) in a controlled low-emission chamber. Ten healthy adults were exposed to reference (500 ppm) and high-CO₂ (4900 ppm) conditions. Elevated CO₂ did not significantly affect perceived air quality, health symptoms, or cognitive performance. The only physiological change was a slight increase in end-tidal CO₂ (5.3 kPa vs. 5.1 kPa), a marker of exhaled CO₂ levels, which was not harmful. The study concludes that short-term exposure to CO₂ at 5000 ppm poses no significant risks but highlights the need for research on longer exposures and varied populations [67]. In addition to CO₂, biological contaminants are a key concern for worker safety. Background levels of microorganisms in indoor environments are generally around 500 CFU/m³. In composting plants, levels can reach 10⁴–10⁶ CFU/m³ for Aspergillus and 10³–10⁵ CFU/m³ for Gram-negative bacteria. The Dutch guideline suggests that concentrations above 10,000 CFU/m³ for total bacteria and fungi, or above 500 CFU/m³ for specific groups or potentially pathogenic species, pose a health risk [68]. For Gram-negative bacteria, the threshold is 1000 CFU/m³ [69]. Chemical pollutants such as hydrogen sulfide also pose occupational risks in some greenhouse settings, with the Dutch threshold limit value set at 10 ppm. Typical indoor concentrations of VOCs are between 1 and 50 µg/m³, with threshold limit values ranging from 100 to 800 mg/m³ [68]. Research indicates that toxicity starts to manifest when airborne endotoxin levels surpass 30 EU/m³. Endotoxin exposure at the level of 100 EU/m³ can cause respiratory tract inflammation, and exposure to 2000 EU/m³ could lead to severe pneumonia symptoms. The Netherlands, one of the few countries with a formal guideline for airborne endotoxin exposure, recommends a general population limit of 30 EU/m³, derived by applying a safety factor of three to the occupational limit of 90 EU/m³ [37,38]. In South Korea, for example, in multi-use facilities, such as medical institutions, postpartum care centers, nursing homes, day care centers and all offices, the total bacteria in the indoor air should be less than 800 CFU/m³ [26]. On the other hand, high levels of dust not only pose health risks but also reduce light penetration, negatively impacting plant productivity [7]. Maintaining relative humidity above 85% significantly enhances the efficacy of microbial agents like Beauveria bassiana in greenhouses, while levels between 60 and 70% balance fungal control and prevent excessive mold growth [70].

3. Key Technologies and Factors in Greenhouse Air Filtration

Although empirical studies on air filtration implementation in vegetable greenhouses are limited, research in analogous indoor environments has demonstrated that advanced filtration systems can effectively reduce airborne particulate matter, such as PM_2.5 (particulate matter less than 2.5 µm); one controlled study reported a 48% reduction in PM_2.5 levels, underscoring the potential applicability of such technologies to greenhouse air quality management [69]. Particulate filters, such as HEPA filters, are effective in capturing airborne particles, maintaining a clean and healthy plant environment [71]. Activated carbon filters are valuable for adsorbing gases, odors, and volatile organic compounds (VOCs), thereby enhancing overall air quality by reducing unwanted smells and harmful airborne chemicals [72]. Electrostatic filters leverage an electrostatic charge to attract and capture particles, providing superior filtration efficiency compared to non-electrostatic filters. One example involves antimicrobial air filters fabricated from polyurethane fibers, as shown in Figure 4a,b; the electrostatic version in Figure 4b captures a substantially higher number of nanosized potassium chloride (KCl) particles than the non-electrostatic filter in Figure 4a, owing to the enhanced particle attraction provided by the electrostatic charge. KCl particles, commonly used as test aerosols in filtration studies due to their stable size and chemical properties, enable accurate assessment of filter performance [73]. UV-C light filters leverage ultraviolet light to disinfect and eliminate microorganisms, contributing significantly to pathogen reduction [74]. Biofilters utilize biological processes to remove contaminants, fitting well into organic and sustainable greenhouse practices. Additionally, pleated panel filters, commonly used in heating, ventilation, and air conditioning (HVAC) systems, are cost-effective and easily replaceable, effectively trapping dust and larger particles [75]. However, the recurring clogging of these filters, along with limited pollutant removal efficiency, necessitates frequent replacements that are often non-biodegradable and environmentally burdensome. This leads to increased waste generation and higher costs associated with additional air quality control measures [34]. In response, recent research has focused on developing biodegradable filters such as those used in face masks that balance environmental sustainability with high filtration performance, using natural polymers like cellulose derivatives, chitosan, and polylactic acid (PLA) as promising alternatives to conventional synthetic materials [37,76,77]. Advanced techniques such as electrospinning and 3D printing are being employed to enhance breathability and fine particle capture while preserving biodegradability [59,66,67].

3.1. Air Quality Issues

Greenhouses, being enclosed systems, often struggle with maintaining optimal air quality. Such environments can accumulate gaseous pollutants and biological contaminants if not properly ventilated [5]. In addition, exposure to bioaerosols poses ongoing health concerns, reinforcing the importance of targeted filtration strategies [9,38]. Occupational exposure continues to result in elevated reports of respiratory symptoms among greenhouse workers, even with current mitigation practices [18,46]. Despite existing air quality practices, recent studies reveal that certain fungal spore taxa, like the allergenic Aspergillus/Penicillium, predominantly originate inside greenhouses or can colonize the indoor environment under favorable conditions [78]. For most tasks, preventive measures to impede the growth or aerosolization of microorganisms present a viable alternative, thereby reducing exposure.

Common air filters, such as commercial HEPA filters, are widely used for air filtration in greenhouses. However, these filters, particularly those made from melt-blown glass fiber, can face limitations such as high resistance to airflow (pressure drop), inefficiency in capturing very fine particles, and challenges in recycling due to the non-biodegradable nature of their waste [79]. This reduces their efficiency and increases energy consumption in maintaining adequate airflow. Electrospun nanofibers are suggested as a promising alternative due to their smaller-diameter fibers, which offer high filtration efficiency with lower resistance to airflow [33,42,57]. Figure 4c shows the scanning electron microscopy (SEM) image of traditional fiberglass media, while Figure 4d displays electrospun polyacrylonitrile nanofibers for comparison. The finer and more uniform nanofiber morphology observed in the electrospun fibers highlights their potential for enhanced filtration efficiency and lower flow resistance, positioning them as a promising alternative to conventional HEPA filter materials [80]. Targeted strategies are needed to manage airborne pollen and fungal spores. Handling large amounts of plant material in greenhouses releases bioaerosols, including fungi, bacteria, and endotoxins, which are a significant source of occupational exposure. These task-related emissions emphasize the need for targeted mitigation strategies in areas where plant materials are actively processed [30].

Figure 4. The SEM micrograph of the (a) non-electrostatic and (b) electrostatic polyurethane filters after filtration, reprinted with permission from Sim et al., Elsevier [73]. SEM images comparing (c) melt-blown borosilicate glass HEPA filter media and (d) electrospun/stabilized polyacrylonitrile (PAN) filter media at similar resolution. From Beckman et al., MDPI, [79] CC BY 4.0. Image of fungal growth on HEPA filter (e) where air first contacts the filter, indicating initial contamination; (f) confirms fungal growth on the supply side. Redrawn based on Price et al. [81].

Figure 4. The SEM micrograph of the (a) non-electrostatic and (b) electrostatic polyurethane filters after filtration, reprinted with permission from Sim et al., Elsevier [73]. SEM images comparing (c) melt-blown borosilicate glass HEPA filter media and (d) electrospun/stabilized polyacrylonitrile (PAN) filter media at similar resolution. From Beckman et al., MDPI, [79] CC BY 4.0. Image of fungal growth on HEPA filter (e) where air first contacts the filter, indicating initial contamination; (f) confirms fungal growth on the supply side. Redrawn based on Price et al. [81].

3.2. Biological Contamination

A study conducted over an eight-year period in hospitals and commercial buildings across the southeastern United States demonstrated that HEPA filters, while effective at trapping airborne particles, can also serve as breeding grounds for fungi under high humidity conditions [35,81]. Figure 4e,f illustrate this issue, showing fungal growth on both the load (intake) side (Figure 4e) and the supply (clean air) side (Figure 4f) of untreated HEPA filters—evidence of microbial colonization and penetration through the filter medium [81]. Without proper maintenance, such filters may become secondary sources of bioaerosol emissions, emphasizing the need for sterilizing capabilities or supplementary filtration technologies to prevent microbial contamination [35].

3.3. Considerations in Sustainable Greenhouse Filtration Systems

High humidity levels can lead to the growth of mold and microbial contamination in the filters themselves, thereby degrading filter performance and lifespan [82]. Once saturated, even high-performing filters may support bacterial or fungal proliferation. If not properly managed or sterilized, these filters become sources of secondary contamination. Moreover, improper disposal of synthetic filters adds environmental burden through microplastic release and biohazard risks [35]. Biodegradable filters present a sustainable alternative, as they break down naturally and reduce the risk of environmental pollution [33,83]. However, their long-term mechanical durability and compatibility with high-efficiency filtration systems remain active areas of research.

3.4. Biocontainment Sampling Methods

Various methods are used to detect airborne biological contaminants in greenhouse environments, including active and passive sampling, filtration, and impingement. Active sampling draws air through a device over a defined period, while passive sampling relies on particle deposition onto agar plates or surfaces [84]. Filtration uses membranes to trap microorganisms, while impingement directs air into a liquid medium where biological particles are suspended for further analysis [28]. For instance, Benis et al. [85] investigated bioaerosol concentrations in a building-integrated rooftop greenhouse using volumetric suction traps to monitor daily pollen and fungal spore levels indoors and outdoors. The study revealed strong correlations between indoor and outdoor concentrations, indicating that ambient air is a major source of greenhouse bioaerosols. Although indoor fungal levels remained below the occupational threshold of 10⁵ spores/m³ [86], they exceeded the 10³ spores/m³ guideline recommended for sensitive individuals [87], highlighting potential health risks for vulnerable occupants. These findings underscore the importance of effective air monitoring and filtration strategies to reduce exposure to allergenic fungi such as Cladosporium and Aspergillus/Penicillium, thereby improving worker safety and air quality in greenhouse environments [24].

3.5. Analysis Techniques

Following sample collection, various techniques are employed to analyze the collected bio-contaminant samples. Culture-based methods involve growing microorganisms on suitable media to identify and count colonies, while microscopy is used for the morphological identification of bacteria and fungi [88]. More advanced methods include Polymerase Chain Reaction (PCR), which amplifies specific genetic markers for precise microbial identification, and flow cytometry, which rapidly detects and quantifies microbial cells based on their optical and fluorescence properties, providing a comprehensive analysis of microbial populations [83]. In essence, a combination of traditional and molecular approaches enhances the accuracy and comprehensiveness of bioaerosol monitoring in greenhouse environments [3]. The adoption of biodegradable filters, renewable energy for systems, and integrated pest management (IPM) can reduce the environmental impact while maintaining air quality. Innovations like advanced sensors can optimize system efficiency, creating a more sustainable and healthier greenhouse environment [89].

4. Polymeric Materials and Micro–Nanofiber Structures in Filtration Applications

Over the years, membranes based on various polymers have been extensively utilized for air and water filtration applications. Polypropylene-based melt-blown membranes, for instance, have been effectively used for capturing PM_2.5, demonstrating their utility in improving air quality [58]. In a study by Park et al. [60], recycled polypropylene (rPP) was 3D-printed into filter templates and then functionalized with activated carbon (AC) to improve their filtration and antibacterial performance. Two surface modification methods were applied, as illustrated in Figure 5a: dip coating, where the filters were immersed in an AC solution, and direct coating, which involved applying AC directly to the surface for more uniform coverage. As shown in Figure 5a, the direct-coated filters achieved better adhesion and more consistent AC distribution than dip-coated ones, resulting in higher SO₂ gas adsorption and approximately 49% reduction in E. coli. Figure 5b illustrates how both filter geometry and AC type influenced performance. Fabric-shaped filters outperformed plate-shaped ones due to increased porosity and surface area, while filters coated with recycled AC (rAC) from PET waste surpassed those with commercial AC (cAC), thanks to rAC’s higher surface area and more developed pore structure [60]. The commonly used microfiber air filters, such as melt-blown and glass fibers, often exhibit an inevitable trade-off between filtration efficiency and air permeability; this is primarily due to their micrometer-scale fiber diameters (5–50 μm), thick structure, and low porosity (<60%) [57]. Electrospun nanofiber membranes possess superior characteristics such as high porosity, small pore size, and excellent connectivity, making them outstanding candidates for high-efficiency particulate air (HEPA) filters [57]. These nanofibers possess properties that make them especially well-suited for various filtration applications, particularly in air filtration [26,75]. Figure 5c presents a simplified setup for the electrospinning process, showcasing the fundamental components and configuration required to produce nanofibers. Unlike traditional filters, which often grapple with high air resistance and limited dust-holding capacity, electrospun nanofibers can achieve high filtration efficiency while maintaining low pressure drop [11,12]. These nanofibrous membranes offer enhanced surface area and porosity, making them effective in trapping fine particles and contaminants while maintaining lower energy consumption, which is critical for sustainable and efficient filtration systems [57]. This performance is attributed to the nanometer-scale diameter of the fibers, which provides a large surface area and improves filtration capabilities. These advantages meet the increasing demand for more efficient air filters across various applications, including industrial air purification, HVAC systems, and personal protective equipment, addressing health concerns and reducing environmental impact by lowering energy use and extending filter lifespan [34]. According to a study by Kim et al. [71], a Nylon 6 nanofiber filter membrane, produced from fibers with diameters ranging between 80 and 200 nanometers and a basis weight of 10.75 g per square meter, demonstrated superior filtration efficiency compared to a commercial HEPA filter. The researchers measured performance at airspeeds of 3 and 10 cm/s for particles with a diameter of 300 nanometers. For reference, standard HEPA filters are required to capture at least 99.97% of particles 300 nanometers in size, a benchmark used for evaluating filtration performance.

Numerous polymers are utilized in the production of electrospun nanofiber filters, each offering distinct advantages. One of the most widely used is polyacrylonitrile (PAN), valued for its excellent chemical resistance, thermal stability, and mechanical strength. Its ease of functionalization and cost-effectiveness make PAN especially suitable for efficient and durable air filtration applications [90]. Thermoplastic Polyurethane (TPU) is also widely used for air filtration due to its flexibility [11,12,66,91]. In one study, electrospun polyvinyl chloride (PVC) filters exhibited higher filtration efficiency and lower pressure drop compared to traditional commercial filters [92]. Electrospun polyvinylidene fluoride (PVDF) nanofiber membranes, as developed in the study by Roche and Yalcinkaya [40], achieved over 99% filtration efficiency for both PM_2.5 and PM_0.1 (particulate matter less than 0.1 μm) particles. This specific nanofiber structure was optimized through lamination onto a polypropylene spunbond substrate, demonstrating the potential of PVDF membranes for high-performance air filtration applications. In parallel, polylactic acid (PLA) nanofibrous filters were highlighted for their biodegradability and adequate mechanical properties, positioning them as promising sustainable materials for air filtration applications [32].

These polymers can be modified or combined with functional particles such as silver nanoparticles (Ag NPs) [90], activated carbon [93], nanoclay [94], and metal–organic frameworks (MOFs) [95] to enhance characteristics such as antibacterial activity, mechanical strength, chemical resistance, and VOC adsorption capacity. As highlighted by Seraji et al. [94], the incorporation of nanoclay into polyamide 6 (PA6) fibers significantly improved their crystallinity and mechanical properties, such as tensile strength and elongation at break, which can be utilized to enhance the structural integrity and overall performance of fibrous materials. Pioneering applications of electrospun nanofibers as filtration materials have demonstrated their potential to significantly outperform conventional alternatives [92,96].

4.1. Overview of Electrospinning for Air Filtration

Electrospinning is a highly effective and versatile method for fabricating nanofibrous membranes. This process is favored due to its controllability, high production efficiency, and cost-effectiveness [57]. It involves applying a high-voltage DC power supply to a polymer solution or melt, which creates a jet that forms ultrathin fibers upon solvent evaporation or melt solidification. These fibers are then collected to form a nanofibrous membrane with desirable properties for various applications, including air filtration [34,97].

4.2. Primary Techniques for Electrospinning

Solution Electrospinning: In this approach, a polymer is dissolved in a solvent to create a homogeneous solution. This solution is then loaded into a syringe, and a specific voltage is applied, transforming the solution into nanofibers. Despite the drawbacks of low production rates and solvent residues, the method’s simplicity and versatility in handling various polymers make it highly popular [63].

Melt Electrospinning: Melt electrospinning applies a DC voltage to a polymer melt, eliminating the need for toxic solvents. This makes the process more environmentally sustainable, safer, and cost-effective. The resulting fibers offer excellent structural integrity, minimal defects, and enhanced mechanical properties, making the method suitable for high-quality, eco-friendly filtration applications [98].

4.3. Advanced Electrospinning Techniques

Needleless Electrospinning: Needleless electrospinning eliminates the use of needle-like spinnerets, thereby avoiding common problems such as clogging and low productivity associated with single-needle systems. Various spinneret designs, such as rotating disks and cylindrical electrodes, enhance the process by allowing multiple jets of polymer solution to form simultaneously, significantly increasing productivity. This technique is suitable for producing large quantities of nanofibers for industrial applications due to its high productivity and stable production process [99,100].

Multi-Needle Electrospinning: This method increases production rates by using multiple nozzles simultaneously, though it may encounter challenges such as nozzle clogging and inconsistent electric fields. To overcome these limitations, researchers have developed specialized spinneret designs that improve electric field uniformity and enhance fiber quality [97,100].

Coaxial Electrospinning: In this method, two different polymer solutions are fed through a central needle and a surrounding sheath, allowing for the creation of core–shell fibers. These fibers have unique properties, enabling encapsulation of drugs, sensitive materials, or functional agents [101].

Hybrid Electrospinning: This technique simultaneously uses two or more different solvents and immiscible polymers to fabricate composite nanofibers with tailored properties such as tunable pore size, improved tensile strength, and specialized surface chemistries for selective adsorption [102]. It is particularly useful in biomedical and filtration applications where specific mechanical performance, permeability, or functionality is required [100].

4.4. Electrospinning Parameters and Process Optimization

Electrospinning performance is governed by several key parameters, including solution properties, solvent type, applied voltage, flow rate, needle-to-collector distance, collector design, electric field configuration, and environmental conditions [63,100,103]. Among these, the polymer solution properties, solvent selection, and applied voltage are particularly critical for achieving optimal fiber morphology and filter performance [100]. The viscosity, surface tension, and conductivity of the polymer solution influence jet stability and fiber formation. Low-viscosity solutions often lead to bead formation, whereas higher viscosity supports uniform fiber development [104]. The solvent choice affects both the evaporation rate and dielectric properties, which are essential for forming a stable Taylor cone and achieving proper fiber solidification [63]. Applied voltage controls the initiation and elongation of the polymer jet. While increased voltage generally reduces fiber diameter, excessively high voltages may destabilize the jet, resulting in irregular or beaded structures [104]. Optimizing these parameters is essential for producing nanofibers with controlled morphology and high filtration efficiency [91].

4.5. Superior Characteristics of Electrospun Filters

4.5.1. High Surface Area

Electrospun nanofibers, with their extremely thin diameters ranging from nanometers to micrometers, possess a large specific surface area that enhances their particle capture ability by increasing contact points with airborne particulates and improving the efficiency of physical interception and diffusion mechanisms. Additionally, surface modifications such as nano-protrusions and hierarchical structures can further boost surface area, thereby improving filtration efficiency [105].

4.5.2. Low Pressure Drop

The thinner diameter of electrospun micro- and nanofibers results in lower pressure drops across the filter media, minimizing airflow resistance and energy consumption. This characteristic makes them more energy-efficient and cost-effective than traditional filters, which typically exhibit higher pressure losses [34]. Maintaining a low pressure drop is crucial for ensuring breathability and airflow in filtration systems. Strategies such as the creation of bead-on-string nanofibers [106], fluffy 3D structures [107], and wrinkled fiber structures allow for larger pore sizes without sacrificing filtration efficiency. Figure 6a,b show the SEM images of porous beads, taken at different magnifications, that are formed due to rapid solvent evaporation and phase separation in the polymer solution during electrospinning. Water vapor condenses on the surface of the polymer jet, creating small pores that remain after the droplets evaporate. Low solution viscosity helps stretch the fibers, leading to the formation of large, porous beads [106]. Figure 6c,d schematics illustrate the fluffy fibers in membranes composed of polyethylene oxide (PEO), polyacrylonitrile (PAN), and polysulfone (PSU), referred to as PEO@PAN/PSU [108]. In this composite, PEO serves as a bonding agent, stabilizing the structure and enhancing air permeability by reducing packing density while maintaining high filtration efficiency. This reduces packing density, allowing air to flow more easily, which lowers the pressure drop while maintaining high filtration efficiency [107].

4.5.3. High Filtration Efficiency

Electrospun fibers offer high filtration efficiency, especially for capturing submicron particles. Figure 6e, adapted from Zhang et al. [105], shows a schematic of a PLLA electrospun fiber layer combined with nonwoven fabric for filtration. During electrospinning, aligned polymer dipoles generate piezoelectric charges on the fiber surface, which enhance particle capture through electrostatic attraction, particularly for PM_2.5 (Figure 6f). These charges arise from airflow-induced mechanical stress during filtration, producing localized surface charges that improve submicron particle capture. Figure 6g compares the quality factors of PLLA, PDLLA, and a 3M respirator for PM₁₀ (particulate matter less than 10 μm), showing that PLLA outperforms the others, partly due to its piezoelectric properties. SEM images in Figure 6h,i show the PLLA membrane before and after filtration, respectively, with the latter revealing visible particle deposition that confirms both effective capture and structural stability during operation [105]. The fine structure of micro–nanofibers allows them to achieve filtration efficiencies comparable to or even exceeding those of HEPA filters, which are the gold standard in air filtration. Electrospun nanofibers, due to their delicate nature, require support from a rigid medium to provide stability and prevent deformation. Nanofiber layers can often be reinforced with nonwoven substrates to enhance strength and stiffness [109]. This support ensures that the nanofiber layer does not delaminate or break during operational use. Additionally, the structural stability of a nanofiber layer can be enhanced by stacking multiple layers of nanofibers, thereby increasing the overall mechanical strength and packing density [39].

In designing antimicrobial electrospun nanofiber filters, several inherent trade-offs must be considered. According to a detailed numerical study on PAN nanofiber membranes [53], reducing fiber diameter and increasing packing density can enhance particle capture efficiency, but these structural changes also result in substantially higher pressure drops. The study showed, for example, that at a thickness of 0.5 μm, the pressure drop increased from 19.5 Pa for 100 nm fibers to 82.4 Pa for 50 nm fibers. Increasing thickness had a similar effect, where for 100 nm fibers, raising thickness from 0.5 μm to 4 μm increased pressure drop from 19.5 Pa to 212.2 Pa. Given their high surface area, low pressure drop, and high filtration efficiency, micro–nanofibers have significant potential to replace or supplement traditional commercial filters in various applications. In greenhouses, these filters can provide superior performance, capturing fine particles and reducing energy consumption. The adaptability of micro–nanofibers allows for customization based on specific filtration needs, enhancing their suitability for various environments.

5. Filters from Biodegradable Polymer Materials

Biodegradable filters have become increasingly important, especially after the COVID-19 pandemic, which saw a surge in the use of disposable masks and filtration materials [33]. Traditional masks and filters contribute significantly to plastic waste, but biodegradable alternatives made from polymers like PLA and PCL offer effective filtration while degrading into harmless substances post-use. These biodegradable filters are particularly advantageous in healthcare and environmental protection, offering a sustainable solution to waste management issues associated with conventional filters [108]. Recent advancements in green electrospinning techniques, including solvent-free and aqueous solution methods, have further enhanced the development of biodegradable nanofiber filters. These methods not only reduce reliance on toxic solvents but also improve the environmental sustainability of the production process [110]. Biodegradable polymers are materials designed to break down after their intended use, returning to the environment without leaving harmful residues [111]. These polymers are composed of monomers linked by degradable bonds that break down under environmental conditions such as exposure to microorganisms, moisture, or sunlight, thereby helping to mitigate the long-term environmental impact associated with conventional plastics, which can persist in ecosystems for centuries [33]. There are various types of biodegradable polymers, each with distinct characteristics and applications.

Aliphatic polyesters are a prominent category, which includes polylactic acid (PLA), polycaprolactone (PCL), and polyglycolic acid (PGA) [112]. PLA, derived from renewable resources like cornstarch or sugarcane, is widely used in filtration systems, medical implants, and 3D printing due to its biocompatibility and biodegradability [32,113]. PCL, known for its low melting point and high flexibility, is used in sutures, wound dressings, and as a matrix for tissue engineering. PGA, which degrades quickly in the body, is employed in absorbable sutures and tissue engineering scaffolds [114].

Polyhydroxyalkanoates (PHAs), produced by bacterial fermentation, are used in packaging, agricultural films, and biomedical applications such as drug delivery and wound healing [115]. Additional biodegradable polymers include polyanhydrides, used in drug delivery systems for their surface-eroding properties, and poly(ortho esters), utilized in controlled drug release applications [116]. These polymers demonstrate diverse degradation profiles and functional properties, broadening their potential in environmentally sustainable material applications.

Peptides and proteins, derived from naturally occurring amino acids, include materials such as collagen, gelatin, and silk, which are widely studied for biodegradable filtration and biomedical applications [117]. Collagen, the major component of mammalian connective tissue [118], gelatin, derived from partially hydrolyzed collagen [119] and silk [64], known for its extraordinary strength, are studied for use in filter substrates, tissue-engineering scaffolds and biomedical materials.

Polysaccharides are polymers composed of various sugar units, with common examples including cellulose, starch, and chitosan [120]. Chitosan-based biopolymers, rich in ionizable amino and hydroxyl groups, are adept at binding with negatively charged particles, thus augmenting filtration performance through electrostatic adsorption [121]. Biodegradable polymers degrade primarily through hydrolysis and microbial action. Hydrolysis, involving the reaction of the polymer with water, breaks ester bonds (Figure 7) in polymers like PLA and PCL, forming oligomers and monomers (e.g., lactic acid, caproic acid) that microorganisms can further assimilate [117].

These polymers, such as polylactic acid (PLA), polycaprolactone (PCL), chitosan, and cellulose, offer the dual benefits of functionality and environmental sustainability. Regarding sustainable filters, Wang et al. [111] developed a biodegradable and high-performance nanofiber membrane as mask filter media based on PLA. The resultant mask filter exhibited a high filtration efficiency (PM_0.3–99.996%) and a low pressure drop (104 Pa), superior to the commercial N95 filter. The biodegradability test showed complete degradation after 150 days. Additionally, enzymatic degradation studies on the true-scale PLA nanofibers revealed progressive fragmentation over time, accompanied by a continuous increase in weight loss [111]. Microbial degradation involves the consumption of the polymer by bacteria and fungi, which use it as a carbon source and convert it into non-toxic end products like carbon dioxide, water, and biomass that easily integrate into natural biogeochemical cycles [122]. Figure 8a,b show the progressive degradation of chitosan nanowhiskers over a 28-day period and the enzymatic degradation of chitosan nanofibers within 7 h, respectively [123]. The shorter degradation time of chitosan fibers compared to PLA fibers is due to differences in polymer structure, molecular weight, and enzymatic susceptibility. Zhang et al. [105] also reported that increasing the PLLA solution concentration led to larger fiber diameters and pore sizes, which reduced filtration efficiency but lowered the pressure drop; notably, PLLA nanofibers produced from an optimized concentration exhibited a 15% improvement in quality factor for PM_2.5 compared to a commercial 3M respirator. In another study focused solely on biodegradation, PLA/chitosan/cellulose nanofiber (CNF) biocomposites were prepared by blending PLA and chitosan (85:15) with varying CNF contents (1–5 wt%). While neat PLA showed limited degradation (~1–10% mass loss in five months), the addition of chitosan and CNF significantly accelerated the process, resulting in up to ~40% mass loss due to enhanced hydrophilicity and microbial activity [124].

Liu et al. [111] developed poly(ε-caprolactone) (PCL)/zein/Ag nanoparticle (AgNP) fibrous membranes using ultrasonication-assisted electrospinning for efficient air filtration. The optimal membrane, with 1% AgNP and 30 min of ultrasonication, featured a bead-on-string structure, high porosity, and interconnected airflow channels. It achieved over 97% filtration efficiency for NaCl aerosol particles (0.3–1.0 µm), surpassing N95 masks, and demonstrated significant antibacterial activity against E. coli and S. aureus. Thus, the PCL/zein/1%Ag-30 membrane offers a sustainable and effective solution for air filtration, combining high efficiency, antimicrobial properties, and biodegradability. In another study by Xie et al. [108], an electrospun hydrophobic nanofibrous membrane was fabricated using biodegradable zein blended with curcumin, which significantly enhanced the filtration efficiency, achieving over 98% removal for particles larger than 0.5 µm. The tensile strength of the nanofibers increased from 0.21 MPa (without curcumin) to 0.72 MPa (with 1% curcumin), attributed to hydrogen bonding between curcumin and zein that enhanced the mechanical strength [108]. In a recent investigation, poly(3-hydroxybutyrate) (PHB) was electrospun into uniform nanofibers optimized for PM_2.5 filtration. The resulting membranes, with fiber diameters close to 600 nm, achieved about 95% removal of PM_2.5 while maintaining a low pressure drop below 5 mm H₂O per square centimeter. Their tensile strength ranged from 2.4 to 5.01 MPa, which is higher than that of many commercial mask filter materials, indicating that PHB-based nanofibers can serve as a practical and sustainable option for fine particulate filtration [116]. In many biodegradable polymer systems, rapid degradation or the presence of highly degradable components can compromise mechanical integrity over time. As a result, blending and reinforcement (e.g., biocomposites or polymer blends) are widely used to recover mechanical strength while maintaining biodegradability [125].

In brief, biodegradable polymers present a promising alternative to conventional plastics, addressing environmental concerns through their ability to degrade into non-toxic byproducts [80]. Their applications span across medical devices, food packaging, and filtration systems, highlighting their versatility and importance in fostering sustainable practices [126,127,128]. As research and development in this field continue to advance, the adoption of biodegradable polymers is expected to grow, contributing significantly to environmental conservation and waste reduction [4,129].

6. Antibacterial Treatments in Micro- and Nanofiber-Based Filtration

The integration of antibacterial treatments into micro–nanofiber-based filtration media has received particular attention due to its potential to improve filtration efficiency and ensure hygiene in a variety of applications. These advanced filtration materials not only trap particles, but they also reduce the risk of microbial contamination, making them ideal for use in the healthcare, environmental, and industrial sectors [43,130]. Several key factors must be considered in this context.

6.1. Evaluation of the Antimicrobial Properties

To evaluate antimicrobial activity, standardized tests and assays are commonly employed, with some of the most widely used methods listed and discussed in this section. The Disk Diffusion Test, also known as the Kirby-Bauer method, measures antibiotic effectiveness by placing antibiotic-impregnated disks on a bacterial-covered agar plate [131]. After incubation, the clear zones around the disks indicate bacterial inhibition, helping classify bacteria as susceptible, intermediate, or resistant [132,133]. The Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) tests are used to assess antibacterial activity. MIC is defined as the lowest concentration of an agent that inhibits visible bacterial growth, while MBC is the lowest concentration that kills 99.9% of the bacteria, confirmed by subculturing onto agar plates [134]. In a study by Wen et al. [128], these tests were used to assess the antibacterial effectiveness of electrospun micro–nanofibers composed of Polyvinyl alcohol (PVA), cinnamon essential oil (CEO), and β-cyclodextrin (β-CD). The CEO was encapsulated in β-CD and incorporated into the fibers to enable controlled release. MIC and MBC results against E. coli and S. aureus demonstrated that the CEO-loaded fibers had strong antibacterial properties, supporting their potential for active antimicrobial filtration or packaging applications. Antibacterial treatments in micro- and nanofiber-based filtration media involve incorporating agents that inhibit bacterial growth, enhancing the filter’s ability to protect against microbial contamination. Common antibacterial agents used include silver nanoparticles [97,135], copper oxide [136], zinc oxide [135,137], and titanium dioxide [135]. These agents can be integrated into the fiber matrix during the electrospinning process or coated onto the surface of the fibers [80]. This dual functionality makes micro- and nanofiber-based filters highly effective in environments where both particulate and microbial contaminations are of concern [42,138].

6.2. Antimicrobial Agents

The development of antimicrobial agents for filtration media has become a critical focus in enhancing air and water purification systems, with recent advancements introducing long-lasting compounds that preserve the filtration media’s performance [80]. Antimicrobial agents eliminate bacteria through various mechanisms. Quaternary ammonium salts disrupt cell membranes via electrostatic interactions, while metal nanoparticles physically rupture cell walls, disrupt membranes, and generate reactive oxygen species (ROS) that oxidize and destroy cellular components [138,139]. Essential oils exert their antibacterial effects by targeting and disrupting bacterial cell membranes, altering their permeability, and causing the leakage of cytoplasmic contents such as nucleic acids and proteins [140]. Certain nanostructures, particularly metal- and metal oxide-based ones such as ZnO nanoparticles, exhibit antibacterial activity through the generation of reactive oxygen species (ROS), including superoxide anions (O₂⁻), hydrogen peroxide (H₂O₂), and hydroxyl radicals (•OH), which damage bacterial proteins, DNA, and other vital components [137,141]. Additionally, these nanostructures can release metal ions like Zn²⁺, disrupting membrane fluidity and integrity, ultimately leading to bacterial inactivation [142]. Recent studies also emphasize that these ROS-mediated and ion-release mechanisms act synergistically, enhancing membrane disruption and supporting the effectiveness of ZnO-based systems in filtration and antimicrobial surface applications [137]. These antibacterial mechanisms have made such nanomaterials valuable candidates for integration into filtration systems, as explored in the following section.

6.2.1. Carbon Dots (CDs)

Carbon Dots (CDs) are a relatively new class of carbon nanomaterials with unique optical properties and biocompatibility, ideal for antibacterial applications [143,144]. For instance, CDs were incorporated into a nanoporous polyvinylidene fluoride (PVDF) membrane developed for use in facemasks. The CDs absorb sunlight and convert it into heat, enabling self-sterilization by destroying bacterial and viral particles. This membrane effectively filters particles over 100 nm, such as those from COVID-19, while remaining breathable [142]. Khan et al. [145] fabricated a hybrid bio-organic cellulose nanofiber (CNF) membrane functionalized with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidation and coupled with nitrogen/sulfur-doped carbon dots (N/S-CDs). The air filter demonstrated high mechanical strength (9.3 ± 1.9 MPa) and excellent biocompatibility. It achieved effective removal of 0.1 μm and 0.3 μm aerosol particles, including bacteria and viruses, with filtration efficiencies ranging from 50 to 81%, making it a promising solution for sustainable air filtration using biodegradable materials.

6.2.2. Essential Oils

Essential oils (EOs) are natural oils extracted from plants, known for their antibacterial, antifungal, and antiviral properties. Thyme, lavender, lemongrass, oregano, cinnamon, and tea tree essential oils are widely used for their antibacterial properties [93,140]. Bartošová et al. developed polyvinylidene fluoride (PVDF) nanofibrous membranes incorporating five different essential oils—thymol, eugenol, linalool, carvacrol, and cinnamaldehyde—using a needleless electrospinning method for antibacterial and antifouling filtration applications [146]. Figure 9a displays the results of disk diffusion assays performed on EO-loaded PVDF films, where clear inhibition zones, especially for thymol and cinnamaldehyde, indicate strong antibacterial activity against Escherichia coli. In Figure 9b, SEM reveals both the nanofiber morphology and bacterial adhesion after exposure to S. aureus. The pure PVDF and carvacrol-loaded membranes showed dense bacterial colonization, while fibers loaded with thymol and eugenol exhibited smoother surfaces with significantly fewer attached bacterial cells. These findings confirm the role of essential oil incorporation in enhancing the antibacterial performance of electrospun membranes, reinforcing their suitability for use in filtration systems where microbial contamination is a concern [147]. Despite their proven antimicrobial efficacy in other applications, the potential of essential oil-infused nanofibers in air filtration remains underexplored, highlighting a need for further research in this area.

6.2.3. Nanoparticles

Nanoparticles are gaining attention as potential antimicrobial agents in air filtration due to their ability to inhibit microbial growth [148]. Huang et al. [149] developed a hierarchically porous poly(L-lactic acid) (PLLA)/copper composite fibrous membrane using electrospinning, acetone treatment, and eco-friendly copper nanoparticle deposition with L-ascorbic acid. The membrane showed excellent air filtration performance, reducing PM_2.5 levels from approximately 1700 μg/m³ to below 50 μg/m³ within 45 min. It also demonstrated strong antibacterial activity, achieving 99.9% and 99.999% reduction in Staphylococcus aureus and Escherichia coli, respectively, through reactive oxygen species (ROS) generation and membrane disruption. The study also noted measurable copper ion release, underscoring the need to consider potential safety concerns such as ion leaching and long-term exposure when applying such materials in air filtration systems [149]. In another study, Liu et al. [43] developed a nanofibrous membrane based on poly(ε-caprolactone) (PCL), zein, and silver nanoparticles (AgNPs) using ultrasonication-assisted electrospinning, as illustrated by the SEM images in Figure 9c, which showed a bead-on-string structure with uniform AgNP distribution. As shown in Figure 9d,e, the antibacterial properties of the membrane increased with the rising AgNP concentration, reaching over 97% efficiency against Staphylococcus aureus and Escherichia coli at higher Ag% levels. Gold nanoparticles, known for their biocompatibility and versatility, and zinc oxide nanoparticles, effective against both Gram-positive and Gram-negative bacteria, are commonly utilized for their strong antibacterial properties [146].

6.2.4. Peptide-Based Antibacterial Agents

Antimicrobial peptides (AMPs) are small, heat-stable polypeptides that serve as a front-line defense in living organisms, capable of eliminating a wide range of microorganisms, including bacteria, viruses, fungi, and parasites, with some even showing anti-cancer properties; they exhibit broad-spectrum antibacterial activity [42]. An advanced electrospun air filter combining LL-37, a human-derived antimicrobial peptide, with puncturable nanostructures was studied by Kim et al. [150]. LL-37 is known for its amphiphilic and cationic properties, enabling it to attract negatively charged bioaerosols and disrupt their membranes. This filter demonstrated synergistic chemo-mechano antimicrobial action, efficiently capturing bioaerosols and achieving a 97.7% bacterial reduction. Filtration tests showed a 28.2% improvement in bioaerosol removal efficiency over bare filters. This innovative dual-action filter ensures enhanced air purification and robust pathogen neutralization, making it ideal for health-sensitive applications [150].

6.2.5. Metal Ions and Complexes

Metal-based materials such as ZnO, CuO, and Ag₂O release antimicrobial ions (e.g., Zn²⁺, Cu²⁺, Ag⁺) that contribute to strong antibacterial and antiviral activity [138]. Electrospinning is an effective technique to embed these nanoparticles into nanofibers, enhancing their antimicrobial activity by disrupting microbial processes [42,103]. In a study by Sohrabi et al. [54], a nanofibrous filter based on PAN with silver nanoparticles was developed. Silver nanoparticles in electrospun nanofibers release silver ions (Ag⁺), disrupting bacterial cells and leading to their death. This, along with the high surface area, makes them effective against Gram-positive and Gram-negative bacteria. The membranes were applied for air filtration, achieving 99.27% efficiency for PM_0.1. Studies have also shown that zinc-containing layered materials are effective antimicrobial agents. For example, zinc basic salt (ZBS) incorporated into a polyvinyl alcohol–polyethylene imine (PVA–PEI) matrix produced clear inhibition zones against E. coli, attributed to Zn²⁺ release and electrostatic attraction, demonstrating the usefulness of Zn-based fillers in antimicrobial composites [151].

6.2.6. Quaternary Ammonium Compounds (QACs)

Quaternary ammonium compounds (QACs) are widely used for their antimicrobial properties, effectively disrupting microbial cell membranes. Jiang et al. [152] developed antimicrobial indoor air filters made from polypropylene fibers treated with QACs via a spray-coating method, achieving over 99.9% antibacterial efficiency against both Gram-positive and Gram-negative bacteria. These polypropylene filters offer a practical solution for preventing respiratory infections by significantly reducing microbial contamination in the air, making them particularly suitable for environments like healthcare settings, offices, and public transportation, where air quality is crucial [152].

6.2.7. Photocatalytic Agents

Photocatalysts like TiO₂ and ZnO are effective antimicrobial agents, generating reactive oxygen species (ROS) under UV light, causing oxidative damage to microbial membranes, proteins, and DNA [140]. The high-reactive (001) facet of TiO₂ enhances its photocatalytic and antimicrobial efficiency. Wanwong et al. [64] developed an advanced air filter based on silk nanofibers loaded with 1 wt% Ag-doped TiO₂ nanoparticles (Ag-TiO₂-silk), which demonstrated outstanding performance, with a PM_2.5 removal efficiency of 99.04 ± 1.70%, a low pressure drop of 34.3 Pa, and effective photocatalytic degradation of formaldehyde. Additionally, Ag-TiO₂-silk exhibited strong antibacterial activity, making it an ideal material for multifunctional air filtration.

Table 3. Electrospun nanofibers integrated with various antimicrobial agents for filtration applications. The (–) and (+) signs denote the absence and presence of the indicated feature, respectively, throughout the table.

Material	Fiber Diameter (nm) Optimum	Active Material (%)	Application	Antimicrobial/Antiviral Efficiency	Filtration Efficiency (%)	Pressure Drop (Pa)	Refs.
Polylactic acid (PLA)	37	-	Air filtration (Mask)	-	99.996% for (PM0.3)	104	[111]
Poly(l-lactide) (PLLA)	500	-	Air filtration	-	98.92 for (PM2.5)	96	[105]
Polylactic acid (PLA)/ Polyhydroxybutyrate (PHB)	500	ammonium-based ionic liquid (IL)	Air filtration	+	95.7% for (PM0.3)	40 ± 10	[153]
Polylactic acid (PLA)	264	Zn-doped titanium dioxide (Zn-TIO)	Air filtration in healthcare settings	+ 98.7% reduction in Escherichia coli and Staphylococcus epidermidis	98.7% for (PM0.3)	–	[126]
Cellulose Acetate (CA)	–	cetylpyridinium bromide (CPB)	Surgical face masks, air filtration	+ 100% bacterial reduction for E. coli and S. aureus	63–77% (NaCl aerosol)	115.13–207.73	[154]
Cellulose Acetate (CA)	239 (ranging from 113 to 398)	cetylpyridinium bromide (CPB)	Air filtration, mask filters	Potential antiviral protection	~100% for NaCl aerosol (7–300 nm)	1800 (1.6 cm/s)	[155]
Cellulose acetate/quaternary chitosan (CA/Q-CS)	–	quaternary chitosan (CA/Q-CS)	Air filters for PM and antibacterial protection	98.27% for E. coli, 98.65% for S. aureus	96.4% for (PM0.3), 99.9% for (PM1.0), 100% for (PM2.5)	48	[156]
Zein/Polyvinyl alcohol (PVA)	750–1170	Curcumin (1%)	Air Filtration	+	98% for (PM0.5)	–	[108]
PVA (Polyvinyl Alcohol)	100–250	P(ADMH-NVF) (N-halamine-based antibacterial polymer)	Air filtration, antibacterial protection	Excellent for E. coli and S. aureus	99.3% (NaCl), 99.4% (DEHS)	183 (NaCl), 238 (DEHS)	[157]
Polycaprolactone (PCL)/Zein	922	AgNp (1%)	Air Filtration	+	97.1% for (Pm0.3)	290	[43]
Soy Protein Isolate (SPI)/Polyamide-6 (PA6)	450	silver nanoparticles (AgNPs)	Air filtration, antibacterial protection	99.99% for E. coli and B. subtilis	95% for (PM0.3)	233	[158]
Polyacrylonitrile (PAN)	–	Zn-CB nanoparticles	Air filtration, personal protective masks	99.99% bacterial interception rate	>99% for (PM0.3)	–	[159]
Silk	129–194	Ag-TiO2 (1%)	Air filter for PM2.5	99.999% antibacterial efficiency against S. aureus and E. coli	99.04 ± 1.70 (PM2.5)	34.3	[64]

presents the integration of various antimicrobial agents into electrospun nanofibers to enhance filtration efficiency.

These examples highlight the diversity of antibacterial agents, ranging from advanced nanomaterials to natural extracts. Each has unique properties and mechanisms of action, making them suitable for various applications in combating bacterial infections. The use of electrospinning to incorporate antimicrobial agents into air filters can enhance their efficiency. Moreover, combining two or more types of antimicrobial agents in air filters may offer enhanced protection against bacterial infections [64].

While antimicrobial agents are added to fibers to eliminate microorganisms, their use raises significant safety and environmental concerns. A recent study by Pollard et al. [160] reported that silver and copper ions leaching from functional textiles can accumulate in air or water, potentially entering the human body, especially in humid settings such as greenhouses, and negatively impacting ecosystems. Additionally, microbial resistance to such agents may develop over time, and the high cost of nano-biocidal particles like silver limits their feasibility for large-scale applications [91,161]. Concerns about resistance are particularly relevant when antimicrobial agents are released slowly from fiber surfaces. Leaching systems, especially those using quaternary ammonium compounds, may promote resistance when microbes experience prolonged low-level exposure [162]. The rising global problem of antimicrobial resistance, identified by international health authorities as a threat that could lead to a post-antibiotic era, further highlights the need for careful selection and design of antimicrobial materials [103]. Immobilizing active agents within the fiber structure, rather than allowing continuous release, has been suggested as a way to help reduce this risk [162]. Increasing the loading of antimicrobial nanofiller particles generally enhances antibacterial performance; however, higher nanoparticle contents promote agglomeration and restrict polymer chain mobility, which in turn reduces tensile strength and overall mechanical integrity of the nanofibers [163]. Essential oils, with their natural antibacterial properties, offer a safer and more eco-friendly alternative. Encapsulating these oils in polymeric nanofibers through electrospinning protects their volatile compounds and ensures controlled release [93].

7. Effectiveness of Essential Oils as Antimicrobial Agents in Filtration Media

Essential oils (EOs) are natural, plant-derived compounds with potent antimicrobial properties, offering an eco-friendly alternative to synthetic agents [164]. They have gained attention as natural alternatives for improving indoor air quality due to their antifungal and antimicrobial properties, effectively addressing issues like mold and fungal contamination while offering sustainable and health-conscious air management solutions [165]. Due to their affordability and safety, essential oils have the potential to serve as effective agents for air filtration systems [164]. These oils are commonly extracted from plant parts such as flowers, leaves, roots, and seeds through steam distillation, a process that releases volatile compounds with antimicrobial properties, which are then condensed and separated for use [166,167,168]. The antimicrobial effectiveness of essential oils is largely attributed to phenolic compounds that disrupt microbial membranes and induce cell death, making them effective against a broad spectrum of Gram-positive and Gram-negative bacteria, as well as fungi [140]. As previously discussed, incorporating essential oils into electrospun nanofibers protects their volatile components, allows controlled release, and enhances antimicrobial performance as a natural alternative to synthetic agents [140]. Essential oils encapsulated in nanofibers can be used in a variety of applications, including air filtration, food packaging, wound dressings, and other biomedical devices [127,128,169]. In air filtration, nanofibers containing essential oils like thyme or oregano significantly improve air quality by inhibiting airborne pathogens [170]. For food packaging, essential oil-infused nanofibers, such as those with lemongrass or cinnamon oil, help extend shelf life by preventing microbial growth and maintaining food safety [128,168].

In a review by Mele, properties of thyme, lavender, lemongrass, oregano, and cinnamon essential oils, when embedded in electrospun nanofibers, have been extensively explored for their antibacterial efficacy. Thyme essential oil, rich in thymol, has been successfully electrospun into nanofibers to create antibacterial surfaces capable of inhibiting Staphylococcus aureus and Escherichia coli by disrupting their cell membranes. This makes it suitable for wound dressings and air filtration systems [171]. Lavender essential oil, rich in linalool and linalyl acetate, offers both antibacterial and soothing properties, making it suitable for wound dressings and skin applications [172]. Lemon and lemongrass essential oils, which are rich in citral, effectively inhibit foodborne pathogens, making them valuable for enhancing indoor air quality and extending the shelf life of packaged foods [140,165]. Oregano essential oil, rich in carvacrol and thymol, exhibits potent antibacterial activity, particularly against resistant strains like Methicillin-resistant Staphylococcus aureus (MRSA), with its ability to disrupt protective bacterial biofilms further enhancing its effectiveness [173]. Cinnamon essential oil, rich in cinnamaldehyde, exhibits strong antibacterial activity against Gram-positive and Gram-negative bacteria by damaging their cell membranes. Incorporating the oil into electrospun fibers enhances its stability and provides long-lasting antibacterial protection for applications in improving air filtration and medical fields [93]. Celebioglu et al. [174] developed electrospun pullulan nanofibrous mats incorporating cinnamaldehyde, complexed with gamma-cyclodextrin. The use of cyclodextrin significantly enhanced the loading efficiency, achieving 62% compared to just 10% without complexation, by stabilizing the volatile cinnamaldehyde during the electrospinning process. These mats demonstrated strong antibacterial activity, with inhibition zones of 32.8 mm against E. coli and 35.9 mm against S. aureus. They also featured a pore size of 390 nm, making them highly suitable as antibacterial and air filtration layers [174]. In another study by Shen et al. [175], bio-based nanofibrous membranes composed of zein and cinnamaldehyde (CMA) were developed using a green electrospinning method. The addition of CMA significantly enhanced air filtration performance, achieving a filtration efficiency of 99.25% for PM_0.3 particles, a pressure drop of 58.2 Pa, and a quality factor of 0.084 Pa⁻¹, comparable to N95 standards. CMA also improved the hydrophobicity and antibacterial properties of the membranes, ensuring greater stability and protection in air filtration applications. Clove essential oil, with its active compound eugenol, exhibits strong antimicrobial properties by disrupting bacterial membranes and walls. This causes leakage of essential substances and ultimately leads to bacterial cell death, effectively dismantling the pathogen’s defenses rather than merely inhibiting growth [140]. The active terpene structures of these essential oils are illustrated in Figure 10a [176]. To better address the role of essential oils in filtration systems, this part is divided into two subsections: one focusing on their incorporation into nanofibrous materials and the other on their release behavior, which is essential for sustained antimicrobial effectiveness.

7.1. Essential Oil-Loaded Nanofibers

Encapsulating essential oils in nanofibers combines mechanical filtration with antimicrobial action, significantly improving filtration efficiency. The electrospinning process is ideal for creating nanofibers with high surface area-to-volume ratios, ensuring a controlled release of essential oils and maintaining their effectiveness over time [140]. Studies have shown that essential oil-infused nanofibers exhibit superior antimicrobial efficacy compared to untreated fibers. Wen et al. [128] evaluated the efficiency of cinnamon essential oil (EO) infused into PVA electrospun mats against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), comparing results to kanamycin sulfate. The mats created inhibition zones of about 28.9 mm for E. coli and 30.5 mm for S. aureus, slightly larger than those of kanamycin sulfate. The minimum inhibitory concentration (MIC) was 1.0 mg/mL for E. coli and 0.9 mg/mL for S. aureus, whereas kanamycin sulfate required 4–5 µg/mL. The minimum bactericidal concentration (MBC) to exterminate 99.99% of bacteria after 24 h was 8.0 mg/mL for E. coli and 7.0 mg/mL for S. aureus, equivalent to 70–80 µg/mL of cinnamon EO. Byun et al. [177] engineered an antimicrobial air filter using polyethylene terephthalate (PET) as the base material. Plant extracts, including tea tree oil, rosemary, and garlic, were applied to the PET filters through a silicate polymeric coating process. The coated filters demonstrated inactivation rates of 99.99% for Micrococcus luteus and significant reductions in Escherichia coli. Tea tree oil demonstrated the highest efficacy, with up to 99.97% inactivation at a concentration of 2.89 mg/cm², while rosemary and garlic extracts achieved 93.3–99.9% inactivation, depending on the extract concentration [177]. A filter medium incorporating thyme essential oil into polyacrylonitrile (PAN) fibers via centrifugal spinning has shown outstanding antimicrobial and filtration performance [178]. This material achieved a 99.999% reduction in Escherichia coli and Staphylococcus aureus while also demonstrating 99% collection efficiency for microparticles and 58% for nanoparticles. These results highlight its potential for effectively capturing and neutralizing airborne pathogens in air filtration applications [178].

Son et al. [93] developed a nanofibrous air filter by incorporating activated carbon (AC) and cinnamon essential oil (CO) into a 12 wt% polyurethane (PU) matrix via electrospinning. AC was stirred into CO for adsorption, then dispersed in the PU solution. SEM and TEM confirmed AC integration, with bead formation at higher CO levels due to saturation. The resulting AC–CO–PU filters demonstrated approximately 35% higher filtration efficiency and a low pressure drop of 8.8 Pa, outperforming conventional PP and pure PU filters. Under NaCl aerosol testing (300–500 nm, 32 L/min), filtration efficiency reached up to 68.2%. Antibacterial tests against S. aureus and E. coli showed strong inhibition zones, particularly in mats with 15 wt% CO. AC enhanced both CO stability and controlled release, contributing to improved filtration and antimicrobial performance [93]. According to Mishra et al. [179], encapsulating lemongrass essential oil (LGEO) into chitosan (CH)-nanocellulose fibers (CNF) for indoor air filtration produced a composite that effectively reduced culturable bacterial levels. Bacterial tests revealed that LGEO’s aroma compounds penetrated cell membranes, disrupted cellular processes, induced leakage, and inhibited growth, as evidenced by elevated reactive oxygen species (ROS) levels. The composite showed greater inhibitory activity against Gram-positive bacteria, likely due to differences in cell wall structure. Notably, the CH/CNF composite containing 500 μL of LGEO achieved the highest bacterial reduction, suggesting this concentration merits further study [179].

7.2. Release of Essential Oils

Embedding essential oils in biodegradable nanofibers ensures a controlled, sustained release crucial for antimicrobial efficacy [180]. Encapsulation in polymers like chitosan, PLA, PVA, and PCL protects the volatile oils from environmental degradation and premature evaporation [121]. The use of essential oils in these applications highlights a shift towards more sustainable and biocompatible antibacterial treatments, aligning with growing environmental and health concerns [140]. Electrospinning techniques, particularly coaxial and emulsion electrospinning, are commonly used to fabricate core/shell nanofibers, where the essential oil is encapsulated in the core and the polymer forms the protective shell [180]. This structure allows for the gradual release of the essential oils over time, ensuring prolonged antimicrobial activity while mitigating any burst release that could reduce their effectiveness or lead to rapid depletion [102]. Min et al. [181] fabricated electrospun poly(lactic acid) (PLA) and chitosan nanofibers encapsulating curcumin in core and shell layers. The release profile shows that curcumin in the core provides a more sustained release compared to the shell, minimizing the burst effect. Figure 10b demonstrates that the release is also influenced by relative humidity, with higher humidity (80% RH) accelerating the release, while lower humidity (20% RH) results in a slower, more controlled release, making it suitable for applications requiring prolonged activity [181]. Figure 10c shows the release of curcumin from electrospun PLA/chitosan core/shell nanofibers, with curcumin in both the core and shell layers. The release from the shell displays a rapid burst, reaching about 30% within the first few hours. In contrast, the core layer demonstrates a slower, sustained release, achieving the same percentage over a much longer period, indicating that encapsulating curcumin in the core effectively shifts the release from burst to sustained [102]. Similarly, peppermint essential oil encapsulated in chitosan and PVA nanofibers exhibited enhanced antimicrobial properties while maintaining the structural integrity of the fibers. This encapsulation technique demonstrated a significant reduction in bacterial growth, making it highly suitable for air filtration systems where long-term antimicrobial activity is essential [121]. In another study by Kamrudi et al. [182], thyme essential oil (TEO) was encapsulated within nylon-6 nanofibers via electrospinning, with polyamidoamine (PAMAM) dendritic polymers used as nano-carriers to enable controlled, stepwise release. The mats containing PAMAM (2% and 10% wt) demonstrated extended fragrance release up to 9 and 12 days, respectively, compared to just 4 days for fibers without PAMAM. This prolonged release was attributed to the branched structure and internal cavities of PAMAM, which effectively trapped the essential oil. Antibacterial tests against Escherichia coli and Staphylococcus aureus showed that PAMAM-containing mats maintained superior antibacterial activity over time, achieving up to 99.9% bacterial reduction even two weeks after fabrication. These results confirmed that PAMAM enhances both the longevity of essential oil release and the sustained antimicrobial performance of the electrospun nanofibrous filters [182]. Rahman et al. [183] explored the impact of infusing polycaprolactone (PCL) nanofibers with neem and lavender oil nanoemulsions on filtration performance. The integration of the nanoemulsions increased the nanofiber diameter from 50 ± 12 nm to 100 ± 32 nm, which is indicative of improved physical filtration capabilities. Additionally, the antimicrobial properties of neem and lavender oils contributed to biological filtration, effectively inhibiting airborne bacteria such as Escherichia coli, Bacillus subtilis, and Staphylococcus aureus. This dual enhancement highlights the potential of using essential oil-infused PCL nanofibers for advanced antimicrobial filtration applications [183].

Essential oils (EOs) have demonstrated significant potential for improving indoor air quality by leveraging their natural antimicrobial and antifungal properties, effectively reducing microbial contamination and addressing issues like asthma and allergic reactions linked to poor air quality [165,184]. Despite the remarkable potential of essential oils, their practical usage faces challenges due to several factors, including high production costs, rapid evaporation, significant volatility, short-lasting effects, strong odor, and occasional phytotoxicity. These challenges, combined with their sensitivity to environmental conditions, hinder widespread application, particularly in sensitive areas like plant-based environments [185]. Moreover, as Milhem et al. [186] highlighted, the volatile and reactive nature of essential oils may lead to the formation of secondary pollutants, such as formaldehyde and secondary organic aerosols, when interacting with indoor oxidants. These secondary products can degrade air quality and pose health risks, contradicting the intended benefits of essential oils in confined spaces [186].

Although encapsulating essential oils (EOs) in nanofibers enables sustained release and helps protect the oils from volatilization and rapid degradation, the stability and longevity of essential oil-based antimicrobial systems still depend strongly on environmental conditions such as humidity, temperature, and light exposure. In particular, higher humidity generally leads to a faster release of EOs due to polymer swelling and increased molecular diffusion within the nanofiber matrix [184]. Likewise, elevated temperature significantly accelerates the loss of essential oils from electrospun structures. Hu et al. [187] incorporated mint essential oil into PVA/PEG (polyethylene glycol)/porous graphene nanofibers and compared EO retention at different temperatures. At room temperature, more than 90% of the oil was retained after 120 h, whereas at 75 °C, retention dropped to below 20% within only 10 min, demonstrating that increasing temperature dramatically accelerates EO depletion from the nanofiber matrix [187]. However, the stability and antimicrobial longevity of such systems under realistic greenhouse conditions remain largely unexplored. Therefore, comprehensive, long-term performance evaluations under simulated and real greenhouse environments are still required to validate their practical applicability.

8. Conclusions

Greenhouse cultivation involves growing plants in controlled environments equipped with advanced systems for irrigation, heating, and ventilation. While these environments offer numerous advantages, maintaining optimal air quality remains a significant challenge. Poor air quality, driven by harmful volatile organic compounds (VOCs) like formaldehyde and acetaldehyde, and bioaerosols such as mold, bacteria, and pollen, can negatively impact both crop productivity and worker health. VOCs, often originating from fertilizers, combustion processes, and organic decomposition, can cause physiological stress in plants, reducing growth while also posing risks to human health, including respiratory irritation and allergies. The enclosed, humid nature of greenhouses further exacerbates microbial growth, creating an environment that demands effective air filtration solutions. Conventional air filtration systems, such as HEPA filters, are widely used in greenhouses but face notable limitations. These include high resistance to airflow, inefficiency in capturing very fine particles, frequent clogging, and environmental concerns stemming from their non-biodegradable nature. The frequent replacements of these filters generate waste that challenges sustainability goals. Addressing these issues, advanced nanofiber-based filtration technologies have emerged as a promising alternative. Electrospun nanofibrous filters feature a highly porous structure that enhances particle capture while minimizing air resistance. Moreover, the use of biodegradable polymers in these filters not only maintains high filtration performance but also addresses long-term sustainability concerns by minimizing plastic waste and environmental persistence. The addition of nanostructures further strengthens the filters’ durability and antimicrobial properties, tailoring them for specific greenhouse challenges.

Among antimicrobial agents used in filtration, metallic nanoparticles like silver and copper are prevalent for their effectiveness against a broad spectrum of pathogens. However, in moist greenhouse conditions, these nanoparticles may be released into the environment, posing potential risks to both human health and ecological systems. This highlights the need for safer, more sustainable antimicrobial solutions. Essential oils (EOs) have garnered attention as a natural alternative for improving indoor air quality due to their potent antifungal and antimicrobial properties. They are effective against Gram-positive and Gram-negative bacteria, as well as fungi, owing to their phenolic compounds that disrupt microbial cell membranes and cause cell death. Encapsulating essential oils in electrospun polymeric nanofibers enhances their stability, prevents degradation, and allows for sustained, controlled release. This approach ensures prolonged antimicrobial activity, mitigates burst release, and maintains structural integrity.

The practical application of essential oils in greenhouse environments faces several challenges, primarily due to their inherent volatility, susceptibility to oxidation, and limited stability under fluctuating temperature and humidity conditions. Although embedding essential oils within electrospun nanofibers can promote a more sustained and controlled release, the actual efficacy, release kinetics, and functional lifetime of these compounds in real greenhouse conditions remain largely unexplored and should be systematically evaluated in future studies. Long-term exposure to UV radiation, moisture, and continuous airflow may also significantly influence oil retention, antimicrobial durability, and overall filter performance. Therefore, further research is required to optimize encapsulation strategies, polymer–oil interactions, and loading concentrations to ensure reliable long-term functionality in practical greenhouse settings.

Despite the clear potential of biodegradable antimicrobial nanofiber filters, several practical and scientific challenges must still be resolved before they can be widely implemented in greenhouse environments. Recent electrospinning approaches, such as needleless and multi-jet systems, have increased production rates; however, current output levels remain insufficient for manufacturing the large-area filter panels required in commercial greenhouses. Improvements in continuous production methods and the development of cost-effective, industrial-scale equipment will therefore be critical for future deployment.

Another important gap is the lack of real-world testing in greenhouse environments. Although air contaminants in greenhouses are well documented, and a substantial number of studies have investigated air filtration performance in indoor and industrial settings, far fewer have evaluated how nanofiber filters perform under actual greenhouse conditions. Factors such as fluctuating humidity, dust accumulation, microbial load, and long-term structural stability remain poorly explored in these agricultural environments. Additionally, biodegradable polymers also present specific challenges. Materials such as PLA, PCL and PHAs offer clear environmental benefits, but their cost, mechanical stability and sensitivity to moisture may limit their performance in real greenhouse conditions. In many cases, these polymers degrade efficiently only under controlled composting conditions, which do not reflect typical disposal environments. Improving polymer blends and reinforcing them with suitable additives will help overcome these limitations.

Field-scale studies and standardized evaluation protocols will be essential to validate long-term performance and guide the practical implementation of biodegradable nanofiber filters in greenhouse environments. Successfully addressing scalability, durability, and real-world operational constraints will accelerate the transition from experimental systems to commercially viable solutions. As demand for sustainable and efficient air-filtration technologies continues to grow, nanofiber-based filters enhanced with functional materials represent a promising pathway toward cleaner, safer, and more environmentally responsible greenhouse cultivation.