Combating Pathogens Using Carbon-Fiber Ionizers (CFIs) for Air Purification: A Narrative Review

Abstract

Airborne pathogens, though a minor fraction of airborne particles, can cause infections, intoxications, or allergic reactions through respiration, dermal contact, or ingestion. The SARS-CoV-2 pandemic has underscored the significance of mitigating airborne microbial threats. Traditional air ionization methods, such as dielectric barrier discharge and metal tip corona discharge, produce ozone, a reactive and potentially harmful byproduct. However, carbon-fiber ionizers (CFIs) generate high concentrations of ions with minimal ozone production, making them a safer alternative. Operating at voltages below 5 kV, CFIs are more efficient than their metallic counterparts. This review focuses on the antimicrobial efficacy of CFIs, which produce unipolar ions that can disrupt microbial membranes, leading to cell death. Compared to ultraviolet light sterilization, CFIs are cost-effective and suitable for small spaces. The literature review highlights the need for comprehensive studies to evaluate the real-world application and effectiveness of CFIs. Many existing studies are limited by small-scale testing and insufficient data reporting, complicating comparative analyses. Our work aims to provide a detailed perspective on CFIs, examining their impact on various microorganisms, ion efficacy, ionization outcomes, and ozone generation levels. By addressing these aspects, the review seeks to offer an updated understanding of CFIs’ antimicrobial capabilities and to identify limitations in current research, paving the way for more informed and effective air purification strategies.

1. Introduction

Microorganisms play a key role in planetary well-being, not just in humans but in plants and animals as well. Pathogenic microbes can significantly impact human health, potentially leading to global pandemics with severe and, to a certain extent, unpredictable consequences. While airborne pathogens constitute a small fraction of airborne particles, they can readily infiltrate the human body via respiration, dermal contact, and the gastrointestinal tract, which can culminate in infection, intoxication, or an allergic reaction [1]. The threat posed by airborne microbes was most recently highlighted by the SARS-CoV-2 pandemic.

Among the physical means of disease prevention, there are several technologies aimed at reducing the infectious burden carried by pathogen-laden aerosols. The main goal of air purification technologies is to either influence the viability of microorganisms or simply eliminate microbes from the circulating air. Air ionization is known to enhance the natural deposition of particulate matter through electrostatic effects [2,3]. It is assumed to be able, to some extent, to also inactivate or weaken microorganisms [4]. Small air ions for the purposes of air purification have to be generated artificially. Two types of artificially produced air ionization, which are mostly used in practice, are the dielectric barrier discharge (DBD) and the corona discharge at the metallic electrode tip. Both types of small air ion generation produce ozone as a byproduct, a sterilizing gas that is highly reactive and, in higher concentrations, harmful to living organisms. Recently, the use of carbon fiber corona discharge has become highly popular. Unlike the thin metal tip electrode, carbon fibers consist of entire bundles of extremely thin spikes, the fiber tips, allowing them to produce remarkably high concentrations of ions with negligible ozone production [5,6]. As a result, the minimal voltage to produce carbon fiber-based ionization is below 5 kV, whereas about 7 kV is required when working with the metallic electrodes. Apart from the practical merit of later use of carbon fiber corona discharge ionizers, it is also highly important from an experimental point of view to use ozone-free ionization. Namely, in studies investigating the biological effects of microbe exposure to small air ions, it is important to distinguish the effects caused by the small air ions from those caused by the efficient but unsafe administration of ozone. It is not possible to claim with certainty what exactly caused the observed effect unless the conducted experiments were ozone-free. Therefore, this review will mainly focus on the studies using a CFI. Some of the important results where there was ozone generation to some extent will be briefly mentioned in order to perform objective comparisons and draw reliable conclusions.

Considering the risk posed by airborne microbes, notably underscored by the recent SARS-CoV-2 pandemic, there exists an understandable trend towards investigating technologies designed to mitigate pathogen-laden aerosols. Among these approaches are those utilizing ions to influence the viability of microorganisms or outright eliminate them from the air circulation.

The bipolar ionizer generates both positive and negative ions, initiating the formation of hydroxyl radicals. These radicals can attach to hydrogen groups on the microorganism’s membrane, causing its structural integrity to break down and resulting in microbial death [7]. This technology competes favorably with ultraviolet light sterilization in terms of microbial eradication capacity, albeit with lower production and upkeep expenses [8]. However, due to its effectiveness, its application is limited to small spaces [9]. As regards unipolar ions, there have been reports of carbon-fiber ionizers (CFIs) producing unipolar ions (with remarkably low generated concentrations of O₃) in sufficiently high air ion concentrations [5,6,10,11,12].

However, the effectiveness of numerous emerging technologies has not been sufficiently investigated yet, with testing often conducted on a small scale and not reflective of real-world applications. Partial or not sufficiently detailed data reporting, when assessing and comparing air treatment technologies, has made the comparison of results across different studies or types of technology quite challenging, as noted in the work by Ratliff and colleagues [13]. Therein, a consistent assessment of several air purification technologies has been conducted.

The literature appears to abound in research concerning the impact of ions on microorganisms. However, this review gives a more specific perspective, delving into the role of CFIs in microbe elimination and excluding the studies presenting mixed effects due to air ionization and the simultaneous action of other anti-microbial means. We examine the existing publications, with a focus on the type of microorganism studied, the efficacy of specific ions, the outcomes of ionization, and the generated ozone levels. The aim of this review is to provide a current viewpoint on the antimicrobial effectiveness of CFIs and comprehensively address any limitations encountered.

2. Material and Methods

A literature search was performed using the electronic databases Google Scholar, PubMed^®, and the Cochrane organization.

On 16 April 2024, a search was conducted on the Google Scholar search engine using the terms “carbon-fiber ionizer AND pathogen”, which yielded 111 results. The search utilizing the same search syntax on the PubMed database on 24 May 2024 retrieved 0 results. The search on the Cochrane collaboration was performed on 24 May 2024 using the following search word: “carbon-fiber ionizer AND pathogen”, “carbon-fiber ionizer”, and “carbon-fiber”, again yielding no results. Apart from the pertinent papers from the aforementioned search, additional relevant literature, found in the reference lists of several publications, was also utilized.

The articles included in this review were published peer-reviewed articles in English concerning the application of the examined technology, which pertains to indoor environments as well. The major aim of the study was to evaluate carbon fiber ionization as a viable technique for mitigating pathogens in indoor settings. Unpublished manuscripts, books, doctoral dissertations, and publications in languages other than English were excluded from the literature analysis. The following data were taken into consideration: type of ions generated, ion microbicidal performance, pathogen characteristics (viruses, bacteria, or fungi), and ozone generation. The search methodology and types of data are further illustrated in Figure 1. The database search results are represented in

Table 1. The microbe-eliminating performance of CFI-generated ions, highlighting the effects of specific ions in relation to ozone levels and various microorganisms.

Microorganism	Outcome of Ionization *	Ion Type and Performance	Ozone Levels	References
Surrogates (particle sizes equivalent to pathogen dimensions)	Particle deposition	− ions better than bipolar ions	<5 ppb	[3]
Bacteria
Staphylococcus epidermidis, Escherichia coli	Microbial destruction b (membrane disruption)	+ ions better than − ions	<10 ppb	[10]
Staphylococcus epidermidis, Escherichia coli	Microbial destruction (membrane disruption)	+ ions	21.8–26.0 ppb	[14]
Staphylococcus epidermidis, Escherichia coli (see also viruses)	Microorganism inactivation	− ions	3.0–3.5 ppb (emission rate 0.026 mg/h)	[15]
Staphylococcus epidermidis	Antibacterial effect Cell contraction	Bipolar ions eliminate bacteria Unipolar (+) activity not antibacterial	<25 ppb	[16]
Staphylococcus epidermidis, Serratia marcescens	Microbial inactivation	− ions	68 ppb c	[17]
Staphylococcus aureus, Escherichia coli, Bacillus subtilis, Enterococcus faecalis	Potent bactericidal effect	− and + air ions	~35 ppb	[18]
Staphylococcus aureus	Microbial deposition Microbial destruction	− ions better than + ions	Not measured	[19]
Staphylococcus aureus, Escherichia coli	Decreased viability -On Petri dishes (104 CFU/mL): S. aureus up to 86% after 3 h exposure and 95% after 8–12 h; E. coli up to 51% after 3 h exposure and 70% after 8–12 h; -On filters soaked with 104 CFU/mL: S. aureus up to 78% on PP filters and 82% on PET filters after 3 h; E. coli up to 52% on PP filters and PET filters after 3 h.	− and + air ions	N/A	[20]
Staphylococcus aureus, Escherichia coli	Agglutination of microbial products, removed from air, microbicidal effects. More noticeable effect on gram + bacteria Exceptional antibacterial activity via oxidative damage	− and + air ions	Only passively mentioned	[21]
Staphylococcus aureus, Escherichia coli	Bactericidal, more noticeable effect on gram-bacteria	− air ionization with oxidation effect	N/A	[22]
Escherichia coli	Enhanced pathogenic removal efficiency a	+ ions	N/A	[11]
Escherichia coli (see also viruses)	Complete inactivation with more than a 5-log reduction (99.999%) in 90 min	bipolar	<24 ppb	[23]
Escherichia coli (see also fungi)	Pre-charging enhances collection efficiency	− ions	N/A	[24]
Pseudomonas fluorescens, Bacillus atropheus (see also fungi)	Particle deposition (microorganism viability not measured)	− ions	39 ppb	[25]
Pseudomonas fluorescens, Bacillus anthracis (see also fungi)	Easier and more efficient collection (from 70% without charge to 80–90%)	+ ions	N/A	[26]
Pseudomonas fluorescens	Synergistic bactericidal action (decreased microbial load), morphologically deformed	Combined − air ions and C. citratus essential oil vapor	N/A	[27]
Pseudomonas fluorescens, Erwinia carotovora, Escherichia coli	Bactericidal effect, P. fluorescens most vulnerable	− air ions	Yes (noted synergy between ozone and NAIs)	[28]
Escherichia coli	Antibacterial	Released + ions from copper/silver (metals proved to be synergistic)	N/A	[29]
Escherichia coli	Disinfection	− ions generated by a cold plasma tube	No ozone, but oxygen species and oxygen-containing radicals, UV-C, and short-term heating of microorganisms	[30,31]
Escherichia coli	Inactivate and decontaminate E. coli via oxidation	− and + ions, free radicals, all fall into the category of the fourth state of matter (cold plasma)	Harnessed ozone as part of the study to maximize disinfection	[32]
Serratia marcescens	Significant bactericidal effects	− and + air ions (NAIs showing slightly stronger repercussions)	N/A	[33]
Pseudomonas veronii	Destroyed cells in a starved, and thus highly impenetrable, state via the predicted ionic porous formation of the cell wall (due to ion accumulation on surface)	Both − and + ions of electric corona	N/A	[34]
Bacillus subtilis	Antimicrobial effects, reduced number of bioaerosols	− air ions created by ionizer, tested with concurrent ozone	Yes	[35]
Campylobacter jejuni, E. coli, Salmonella enteritidis, Listeria monocytogenes, Staphylococcus aureus, Bacillus stearothermophilu	Significantly decreased microbial load (levels in biofilm)	Supercharged − air ions	N/A	[36]
Mycobacterium parafortuitum	Cell inactivation and biocidal qualities via electroporous mechanisms	− air ions	Yes, but not the principal cause of destruction	[37]
Mycobacterium tuberculosis	Prevented most airborne TB	− air ions	N/A	[38]
Legionella	Ions find the − charged cell walls of pathogens and destroy them	+ ions in water systems	N/A	[39,40,41,42]
Salmonella enteritidis	Statistically significant decrease in infection via airborne transmission, attracted to ground surfaces, direct organism killing	− air ions	N/A	[43,44,45]
Staphylococcus albus	Bactericidal effects	− air ions in synergy with superoxide radical anion	Inadvertently, as superoxide radicals may be chain carriers for ozonation (O3 decomposition) and since it has been remarked that ozone and superoxide combine in corona discharge	[46]
Clostridioides difficile, drug-resistant strains of Staphylococcus aureus, Pseudomonas aeruginosa, and Klebsiella pneumoniae (see also fungi and viruses)	Reduction Bacteria 94.4–99.9%; Virus 94%;	bipolar	22–66 ppb	[23]
Bacterial/viral agents	Same polarity of ions on respiratory protective masks (N95 and surgical) leads to electrostatic protection	− ions	N/A	[47]
Bacteria (review)	Bactericidal effects	− ions	N/A	[48]
Fungi
Penicillium notatum	Lowered penicillin production (mostly by − ions), reduced germination of spores (mostly by + ions), lowered CO2 production	− and + air ions	N/A	[49]
Penicillium chrysogenum (see also bacteria)	Particle deposition (microorganism viability not measured)	− ions	39 ppb	[25]
Penicillium brevicompactum (see also bacteria)	Easier and more efficient collection (from 70% without charge to 80–90%)	+ ions	N/A	[26]
Aspergillus fumigatus, Candida albicans (see also bacteria and viruses)	Reduction Fungi 32.4–87.3%	bipolar	22–66 ppb	[23]
Candida albicans (see also bacteria)	Pre-charging enhances collection efficiency	− ions	N/A	[24]
Candida albicans (10 strains)	Inhibited growth	− air ions	Yes, additionally hypothesizes potential microbicidal role of ozone	[50]
Viruses
H5N1 avian influenza virus	Neutralizes up to 26% of airborne pathogens	− ions	<50 ppb	[51,52]
RNA/DNA Viruses	Free ionic Ag+ inactivated ssRNA MS2 and ssDNA PhiX 174 (specifically in neutral and alkaline environments)	+ charged copper/silver ions in water, synergistic effect	N/A	[53]
SARS-CoV-2	Reduced aerosolized pathogens	− ions created by plant-based ionizer	No ozone detected	[54]
SARS-CoV-2	Pathogens agglutinate and ‘fall’ down	− ions	Varies between generations/low concentration	[55,56]
SARS-CoV-2 and Influenza A virus	Inactivation—fixed on surfaces: >99.98% after 1 h of exposure; Disinfection—aerosolized: after 10 min of exposure at a 30 cm height—89.96% for SARS-CoV-2 and 91.27% for influenza A virus. At a 50 cm height, 87.77% for SARS-CoV-2 and 89.50% for the influenza A virus.	− ions	<50 ppb	[57]
Human coronavirus 229E	Reduction Virus 94%;	bipolar	22–66 ppb	[23]
Newcastle disease virus	Facilitate pathogenic aerosol decay, wire-gauze completely prevented transmission	− ions	N/A	[58,59]
MS2 phage (see also bacteria and fungi)	Complete inactivation with more than a 5-log reduction (99.999%) in 30 min	bipolar	<24 ppb	[23]
MS2 bacteriophage, H1N1 influenza virus	Particle deposition	− ions	~10 ppb (varying)	[60]
MS2 bacteriophage	Microbial inactivation	− ions	1.6 ppb	[61]
MS2 bacteriophage	Reduction unipolar ions: 46.1%, 78.8%, and 83.7% after 15, 30, and 45 min of exposure, respectively, and up to 97.4% for bipolar ions	bipolar better than unipolar	unipolar ions: 2–10 ppb; bipolar ions: ~30 ppb	[4]
virus (P22 and Φ6 bacteriophages) (see also bacteria)	Microorganism inactivation	− ions	3.0–3.5 ppb (emission rate 0.026 mg/h)	[15]
Φ6 bacteriophage (SARS-CoV-2 surrogate)	Particle removal Antiviral performance	− ions	Did not measure d	[62]
Canine calicivirus (CaCV), rhesus rotavirus (RRV), influenza A virus (H3N2)	Reduced infectivity of aerosolized CaCV and RRV (>97%); Active ionizer prevented 100% of guinea pigs from infection by H3N2.	− ions	<2 ppb	[63]

CFI—carbon-fiber ionizer; ppb—parts per billion; N/A—did not mention. * Outcomes of ionization were labeled as per the wording found within a specific study. ^a The results have met the ISO14698 safety standard [64]. ^b The efficacy of inactivation fluctuated based on factors such as voltage, material selection, and flow velocity, resulting in variable success rates ranging from 42.9 ± 29.1% up to 99.9 ± 0.2%. ^c The emission rate would lead to indoor concentrations lower than 50 ppb, as stated by the authors. ^d Although the authors did not directly evaluate the ozone-generation rate, the experimental method itself was used to account for possible ozone influence.

(Section 3), along with relevant information and the respective studies referenced.

3. Database Search Results

The search results are presented in Table 1, listing the microorganism type, ion type, and measured ozone levels where applicable. A short summary of outcomes is presented for each of the studied references.

4. Discussion

This paper explores the efficacy of carbon fiber ionization technology in controlling pathogens in indoor environments. It assesses this method’s effectiveness against a spectrum of pathogens, considers operational limitations, and evaluates potential impacts on indoor air quality and human safety. By focusing on carbon fiber ionization, this research aims to contribute to developing safer and more efficient pathogen control solutions for indoor public and private settings. Interestingly, Ouyang et al. [65] highlighted carbon fiber ionization as an emerging technology in the control of bioaerosols within animal agriculture settings, stressing the importance of managing ozone levels as excessive ozone can be harmful to both animals and humans. It was particularly noted for its potential to be integrated into mechanically driven ventilation systems, helping to prevent disease spread from facility to facility. Both bipolar and unipolar ionization have the ability to charge airborne particles, enhancing their removal from the air. However, bipolar ionization offers advantages in terms of more effective particle collision and agglomeration, potentially leading to more efficient air purification. Moreover, the effectiveness of unipolar ionization can be highly dependent on ambient conditions such as temperature and humidity, which can affect ion mobility and lifespan. Available studies increasingly support the potential benefits of air ionization based on the use of carbon fibers in reducing bacterial accumulation on surfaces and deactivating airborne bacteria, viruses, and fungi. However, since there are significantly varied approaches to proving this, potentially hindering a valid interpretation of the results, caution should be exercised in drawing conclusions from the studies.

4.1. Defining the Testing Protocols and Parameter Reporting for a Consistent Research Approach

Among the factors hindering a comparison of results across studies, inconsistent and/or insufficient reporting on parameter space (defining the conducted study) and on the testing protocols that were adopted might be the most significant obstacle. Naturally, when the field of study is relatively broad, a subset of all experimental conditions is often examined. Even so, the metric space comprising all relevant parameters should be defined, and the spans of parameters covered in each study should be carefully and accurately reported. Firstly, such an approach facilitates comparisons of experiments, their repeatability by other groups, and reaching valid conclusions. Secondly, detailed data reporting allows for the construction of complex conceptual models, such as the ones developed in Refs. [66,67], to study the intricate interconnections of groups of parameters. Moreover, subsets of parameters can be used to infer the influence of particular parameters on system behavior.

Regarding the application of small air ions to remove and inactivate pathogens from indoor air, the most important parameters are ion polarity (bipolar, positive, and negative air ions—NAIs), polarity ratio (bipolar ionization typically produces different amounts of positive and negative ions), ion concentration, concentration of ions compared with the concentration of bioaerosols (ratio), type of ionizing devices used, particularly in connection with the production of ozone as a byproduct, and exposure time (due to the short lifetime of the air ions, exposure time can be taken to coincide with the time the source is turned on). For example, the dielectric barrier discharge (DBD)-based ionizer type uses voltage to tear apart the air molecules, thereby creating ions of unpredictable chemical composition (possibly also harmful to humans). In contrast to that, the corona discharge is considered a clean technology because it produces electrons that bond with the existing air molecules (not harmful in any way). All of the aforementioned parameters have to be measured, controlled, and reported. A certain set of parameters may be of interest for a particular experiment, but the reporting must be as accurate as possible. The ozone concentration is routinely monitored in all of the experiments in our group, although we use the CFI with “zero ozone production” declared by the manufacturer.

Testing protocols (the physics part) depend on the type of experiment conducted. In an open or partially open interaction volume, ions need to be steered by the airflow into the experimental volume. To work with the microorganisms, we design customized chambers (often like the well-known Goldberg rotating chamber) and setups that provide highly controlled experimental conditions, thus allowing reliable and accurate measurements even when a relatively small quantity of pathogens is used. All of these details are thoroughly described, which we recommend as a good practice.

Regarding the biological parameters, the ones necessary to report are microorganism type, size of the microorganism, as well as the average sizes of aerosol droplets and pathogen surface potential. The size of microorganisms and their surface charge significantly influence their interaction with the CFI. Smaller microorganisms, such as viruses (20–300 nm), have a higher surface area-to-volume ratio, allowing more extensive contact with the CFI, which can disrupt their surface coats. Bacteria (0.5–5 µm) and larger fungal spores (2–100 µm) also interact with the CFI, but their larger size means that gravitational settling plays a more prominent role in their deposition. Additionally, the surface charge of microorganisms affects their attraction to the CFI; negatively charged microbes are more likely to be attracted to a positively charged CFI, enhancing their deposition and potential inactivation.

Regarding the testing protocols, a combination of polymerase chain reaction (PCR) and cultivation methods can be utilized to accurately evaluate the microbicidal or microbiostatic effects of the CFI and its impact on microbial deposition on various surfaces. Initially, bioaerosolized microorganisms, dispersed in a chamber and exposed to the CFI, can be collected from surfaces after deposition. For PCR analysis, microbial DNA is extracted from these samples, and quantitative PCR (qPCR) is employed to measure changes in DNA abundance, providing quantitative data on the reduction in microbial populations. This method offers a precise assessment of the microbial load, reflecting the effectiveness of the CFI in microbial inactivation or growth inhibition. Additionally, standardized cultivation methods involve inoculating the samples on agar media or cell culture, followed by incubation. The enumeration of viable microorganisms post-exposure is performed based on the number of colony-forming units (CFUs) or by determining the viral titer by the TCID50 method (50% tissue culture infectious dose). By comparing CFUs or TCID50 counts from treated surfaces with those from control surfaces, the effects of the CFI on microbial deposition and viability can be assessed. Collectively, these methodologies provide a comprehensive analysis of both antimicrobial efficacy and the impact on microbial surface deposition.

4.2. Air Ionization and Microorganisms—A Familiar Interplay?

The control of pathogenic organisms in indoor environments is a pivotal aspect of public health, particularly because individuals spend a substantial amount of their time indoors, where air quality directly impacts their health and well-being [68]. Indoor environments often facilitate airborne transmission, serving as a significant route for the spread of various pathogens, including viruses [69]. Ensuring these environments are free from harmful pathogens is crucial, especially in sensitive areas like hospitals, schools, and offices. Traditional approaches for mitigating pathogen spread indoors include mechanical filtration, chemical disinfection, and ultraviolet germicidal irradiation (UVGI). Each method, however, has its limitations. Mechanical filters often fail to capture all microbial contaminants, especially viruses, due to their small size [70,71]. Chemical methods can leave harmful residues and have variable efficacy depending on the pathogen and the environment [72]. Meanwhile, UVGI, while effective at inactivating pathogens, can pose risks to human health and is limited by the need for direct exposure of the pathogens to UV light [71,73].

Given these challenges, there is significant interest in exploring more effective and less invasive technologies. Ionization, particularly using carbon fibers, emerges as a promising solution [71,74]. Ionization technologies work by releasing ions that attach to and deactivate harmful particles in the air, including pathogens, and also increase the rates of natural particle deposition [75,76]. This method is distinct from HEPA filters, which trap particles through a dense fibrous network and can remove 99.97% of particles down to 0.3 μm; however, regular filter maintenance is challenging and involves ongoing costs [73].

Various methods exist for generating air ions, which are used to charge aerosols, facilitating their deposition on different surfaces. Air ions, whether positive (PAI), negative (NAI), or bipolar, can enhance the rate at which particles are removed from the air; however, each type influences this rate differently [4]. Air ionization devices fall into two categories: those that emit only negative ions, known as unipolar ionizers, and those that emit both positive and negative ions, referred to as bipolar ionizers [65]. Common technologies like corona discharge (also sometimes denoted as needlepoint bipolar ionization—NBPI) typically produce similar ions that operate through the same mechanisms [77]. Studies have differentiated the effects of positive and negative ions, with each showing distinct benefits in pathogen control. While the specific processes behind the biocidal effects of positive and negative ions are not yet fully understood, the proposed mechanism suggests that these ions cluster around microorganisms. During exposure, both negative and positive ions trigger oxidative stress in bacteria. This stress leads to cell death due to oxidative damage that affects cellular components, including DNA [18].

Many studies have focused more on the antimicrobial efficacy of NAIs than PAIs, which have been shown to have a lesser bactericidal effect, as evidenced by the results of Fletcher et al. [37] concerning mycobacteria. Escombe et al. [38] demonstrated that NAIs effectively reduced tuberculosis transmission in hospital wards, suggesting potential against other airborne pathogens. Furthermore, Asadgol et al. [78] suggested that NAIs can effectively reduce the concentration of airborne microorganisms, thereby potentially minimizing the risk of respiratory infections. Jiang et al. [76] discussed the importance and functions of NAIs, highlighting their effectiveness in filtering particles, especially ones smaller than 10 μm (PM10), making them valuable for air purification purposes. Previous research, such as the study by Mitchell and King in 1994 [58], has demonstrated that NAI generation can effectively reduce Newcastle disease virus transmission in animal models. Similarly, in a controlled experiment, Hagbom et al. [63] successfully demonstrated the prevention of airborne transmission of influenza A virus between animals through the use of NAIs. A robust inhibitory effect on the viability of airborne bacteria, namely Staphylococcus aureus and Escherichia coli, exposed to positive and negative ions was demonstrated by Comini et al. [20]. Moreover, ion exposure reduced the viability of bacteria soaked in air filters, suggesting potential applications for air purification systems that incorporate ionization technology to prevent the spread of infections. Interestingly, Fletcher et al. [37] indicated that the principal cause of bacterial inactivation was exposure to ozone generated during ionization rather than the direct effect of both positive and negative ions, which should be taken into account in every study concerning the assessment of ionizing devices on microorganisms [18].

It was noted that positive, negative, and bipolar combinations of air ions can all effectively remove bioaerosols from the air, although each functions at its own specific role and rate [4]. Air ionization is especially helpful when dealing with a high viral load in enclosed spaces, where degradation is sped up and filtration, along with removal, is more effective. Negative air ions target bacteria, molds, and viruses in indoor air. The effectiveness of removing particles depends on both the rate at which ions are emitted in a confined area and the volume of the space itself [79]. Recently, it has been revealed that while positive ions enforce their bactericidal effects through physical means, negative ions operate equally through physical and chemical mechanisms [22].

Consider that the process of sterilizing the air may forgo the need for particle removal, rendering the pathogen benign [71]. Traditional ionizers produce ozone, an unwanted byproduct, while newer models do not carry this risk [55]. Electrostatic removal by simulating corona discharge, hence ionizing the field, disinfecting, and eliminating any biological aerosols, was recommended as a public measure in large-scale public institutions [22]. Electrostatic precipitator methods serve two purposes simultaneously: the physical removal of pathogens and biological disinfection [80]. In ionized electric fields, particles acquire a charge and are drawn towards collection plates due to the Coulomb force, a process known as physical removal. Additionally, ions produced by corona discharge have the ability to deactivate bacteria and other microorganisms [30,31].

Other researchers noted that treatment with negative air ions (NAIs) alone had no killing effect on any of the investigated bacterial species, exerting its effects through the strong interplay between ozone and NAIs [28]. Even bacterial aerosols like E. coli were analyzed and found to be removed by UVD, but not UVA or UVC, whose disinfectant properties were attributed to the generation of ozone [81]. In another study, seven bacterial species were introduced to both positive and negative air ions, and the primary mechanism of cell death among the bacteria studied was exposure to ozone, with electroporation due to NAIs playing only a secondary role. The authors even suggested that any bactericidal or therapeutic effects ascribed to negative air ions by previous researchers may perhaps be blown out of proportion [37,76]. All in all, it seems that the vulnerability of bacterial strains to NAIs varies based on their strain type, physiological condition, and their proximity in relation to the source [82].

4.3. The Other Side of the Coin: How to Avoid Harmful Byproducts of Air Ionization?

Despite its potential benefits, air ion generation, especially associated with corona discharge or electrostatic methods, often produces ozone as a byproduct, posing risks to human health [73,76,83]. Park et al. [18] highlighted the effectiveness of ionizers equipped to minimize ozone production, focusing on the generation of ions as the main method of achieving bactericidal effects. The health and safety issues stemming from ozone generation by air ionizers degrading indoor air quality are often emphasized [84]. Ozone, as a strong oxidant, can cause respiratory problems and worsen conditions such as asthma, potentially negating the benefits of removing airborne pollutants. Additionally, ozone can react with volatile organic compounds (VOCs) in indoor air to form harmful secondary pollutants, such as formaldehyde and ultrafine particles, which can cause more harm than the original pollutants [83,84,85]. This creates a complex situation in which devices intended to purify the air can paradoxically release harmful ozone, negating the benefits of removing airborne pollutants. Therefore, it is essential to evaluate air purifiers not only on their ability to remove pollutants but also on their ability to emit ozone by meeting stringent regulatory standards. The World Health Organization’s (WHO) ozone guidance emphasizes managing ozone concentrations to minimize respiratory health risks caused by high ozone concentrations. These guidelines serve as a benchmark for national air quality standards, calling on states to monitor and manage ozone, especially from industrial and automotive sources, to achieve cleaner air and minimize health impacts [86].

Ozone exposure can have significant effects on human health, depending on the dosage delivered to the respiratory tract and the duration of exposure. Short-term exposure to ozone can cause inflammation and damage to the airways, followed by respiratory symptoms such as coughing and throat irritation, thus exacerbating preexisting respiratory conditions. Having this in mind, individual responses to ozone exposure can vary widely. Short-term ozone exposure is linked to respiratory issues, leading to hospital admissions for conditions like asthma and chronic obstructive pulmonary disease (COPD). Ozone levels between 60 and 80 ppb over an 8-h period are used to assess potential health risks for both healthy individuals and vulnerable groups, such as children and the elderly [86,87]. Epidemiological studies on short-term exposure thresholds show inconsistent evidence, and health impact calculations assume linear concentration–response relationships with appropriate cut-off points: one at 10 ppb for daily maximum 8-h ozone and one at 35 ppb [87].

Long-term exposure to ozone is associated with chronic respiratory diseases and increased respiratory mortality, particularly in individuals with predisposing conditions, even at lower threshold levels around 40–60 ppb. It is also linked with deficits in lung function growth and increased asthma incidence, especially among children [86,87,88].

The number of electrons produced and the ion emission rate, as well as the ozone output, are correlated with the input power [89]. However, it is possible to effectively generate air ions while minimizing ozone generation, for example, through the use of carbon fiber brushes. Deployment of carbon fiber brushes has the potential to increase ion emission rates while reducing or even eliminating ozone production [11,90]. The use of carbon fiber brushes has been shown to optimize the performance of ionizing air filtration systems by effectively charging and capturing particles, potentially reducing particle aggregation and improving dispersion and persistence of ions without the high ozone emissions typical of traditional ionizers [71,73,91].

Ozone gas has proven to inactivate norovirus and may be utilized in decontaminating empty areas; however, any spaces with people should be avoided due to its well-known toxic attributes [92]. Interestingly enough, Park et al. postulated that ozone has little to no bactericidal effects [18]. Corona discharge may create ozone, a harmful byproduct made up of three oxygen atoms, which is especially problematic when attempting to use an ionizer in closed environments or near humans [91]. It is crucial to note that both the quantity of electrons produced, which inadvertently influences the rate of ion emission, and the rate of ozone generation are directly linked to the amount of energy imputed into the system [89]. Some studies have attempted to decrease the ozone emission rate by placing a graphene coat over the metal wire [93]. Additionally, carbon brush ionizers have been noted to only discharge negligible amounts of ozone, defining a new age of biomedical engineering [5]. New innovative designs also include bipolar ionizers emitting negative corona discharge to avoid submicron ammonium nitrate buildup, which is a prominent concern when discussing ionizer micro-contamination [94].

4.4. Carbon-Fiber Ionizers: The Pros and Cons

A carbon-fiber ionizer, which can be carefully positioned in front of an air filter, produces charged particles via corona discharge and traps the pathogens through its electrostatic interaction [4,95]. It is an efficient measure for removing microorganisms from the air. Carbon-fiber ionizers can emit stable unipolar ions that efficiently charge surrounding air particles, with extraordinarily low or comparably lower ozone levels than standard corona ionizers due to the large number of sharp-edged fibers comprising the ionizer tips [96].

The World Health Organization (WHO) and the National Ambient Air Quality Standards (NAAQS) guidelines set specific margins for ozone concentration: <50 ppb for 8 h (2006) [97] and average 70 ppb for 8 h (2018) [98], respectively. In most studies, the CFI ozone levels fall below the designated threshold values, indicating that CFIs are a reliably safe option as ion-generating sources. The presence of ozone in some experiments underlines the role of controlling the input power to the ionizers, the importance of well-planned experiments, as well as care in the proper utilization of CFIs as powerful technological remedies against air pollution.

Carbon-fiber ionizers were recognized to destroy the cell membranes of aerosolized bacteria such as S. epidermidis and E. coli [10]. Electrostatic disruption of the procaryotic organisms was described as the leading antibacterial outcome. Interestingly, positive ions exhibited greater antibacterial effectiveness compared to negative ions against both types of microbes, despite the fact that the concentration of negative air ions surpassed that of positive air ions in this study. Cellular membrane disruption was also evidenced after utilizing CFI-generated ions in the work by Park et al. [14]. It should be noted, however, that although ozone was generated during the experiment, the latter work differentiates between ozone- and ion-mediated cell destruction, with bacterial lysis mainly attributed by the authors to the air ions. Moreover, Han et al. reported that ozone was not generated with CFIs at voltages up to −4 kV [5]. Varying degrees of effectiveness of CFIs were reported in another study, where ions were used to inactivate the MS2 bacteriophage [61]. The success of inactivation showed considerable variation, ranging from 42.9 ± 29.1% to 99.9 ± 0.2%, and was contingent upon factors such as voltage, material choice, and flow velocity. Consequently, the selection of the most suitable material, voltage, and airflow rate appears pivotal in ensuring effective elimination of microorganisms.

The CFIs were employed in another setting to distribute ions onto a filter surface, followed by the assessment of their microbicidal efficacy. Negative ions demonstrated greater effectiveness compared to positive ions, a phenomenon ascribed by the authors to their differing concentrations [19]. Moreover, the inactivation rates showed an increase corresponding to the number of ionizers utilized. The great potential of negative ions to enhance indoor air quality was also described elsewhere against the bacterial pathogens Serratia marcescens and Staphylococcus epidermidis [17], with the effect of negative ions described in the case of a virus (Φ6 bacteriophage) as well [62]. Further confirmation of the ability of negative ions to impinge on microorganisms is reported in the study of Lu et al. [15], where their effect was investigated on Staphylococcus epidermidis, Escherichia coli, P22, and Φ6 bacteriophages. In another study conducted on Staphylococcus epidermidis, positive ions were found to be lacking in their antibacterial effect, while bipolar emissions demonstrated antimicrobial influence [16].

In a study by Kolarž and colleagues [3], the investigators did not use live microorganisms per se but utilized particles of approximately corresponding diameters as surrogates; subsequently, this particulate matter was subjected to different types of ions (bipolar or negative ionization). The authors reported on unipolar ions being much more efficient in increasing deposition rates for the size range up to 1 μm. Figure 2 represents the relationship between deposition increase and particle size, with a comparison of the effects of negative and positive ions. The increase in the deposition coefficient (i.e., the deposition of particles itself) under the influence of small ions in the air is most pronounced for particle sizes within the middle range of the size spectrum (100–1000 nm) and is particularly significant for SARS-CoV-2, Haemophilus influenzae, cytomegalovirus, Staphylococcus aureus, and Mycoplasma pneumoniae. It may be concluded that the action of small ions provides an efficient mechanism for reducing the concentration of infectious particles in the air, including some that would otherwise linger for quite a long time as bioaerosols.

Young et al. tested the effectiveness of a bipolar ionization system using carbon fiber brushes to control airborne particles, especially ultrafine particles [99]. The study evaluated the impact of ionization technology on particle sizes from ultrafine to fine, with a particular focus on particles from 49.6 nm to 201.7 nm, further demonstrating broader effectiveness against pathogens in the air. Jeong et al. explored carbon fiber ionization technology to control pathogens in indoor environments and demonstrated the effectiveness of an electrostatic precipitator (ESP) that uses a carbon fiber ionizer to charge and remove pathogens [62]. This technology demonstrates the high efficiency of an ESP equipped with copper (Cu) collector plates in removing airborne pathogens. Comini et al. [20] explained the rationale for applying carbon fiber ionization technology in healthcare and other environments to prevent or control infections. Their experiments showed strong antibacterial effects of PAIs and NAIs on Gram-positive and Gram-negative bacteria grown in Petri dishes and soaked on filters. Variation in bacterial viability is attributed to differences in their outer layers, with Gram-negative bacteria being less affected by ionization than Gram-positive bacteria. Kanesaka et al. demonstrated the potential of bipolar ionization using a carbon fiber ionizer as an effective complementary technology in healthcare facilities to reduce the burden of hospital-acquired infections (HAIs) [23]. The experiment was performed in an acrylic chamber, and significant reductions were demonstrated for both bacterial and viral pathogens, while ozone levels were monitored and maintained at low levels (mostly below the detection limit) to distinguish the effects of ionization from potential ozone-induced deactivation of pathogens. However, this study showed no significant inactivation of fungi.

Park et al. developed a carbon fiber-based ionizer that produces low amounts of ozone and effectively kills Gram-positive and Gram-negative bacteria [18]. The study was conducted in a controlled environment in a sealed plastic chamber, where air was circulated by a fan and bacteria were grown on a 0.45 µm filter. The study also found that the bactericidal effect was due to oxidative stress caused by negative and positive ions on the bacteria, rather than from the produced ozone. This finding contrasts with that of Fletcher et al. [37], who attributed the bactericidal effect mainly to ozone. The study by Hagbom et al. [63] used a carbon fiber-based ionizer with an ion-flow ionization technology, demonstrating that negative ions can effectively reduce airborne virus concentrations without generating significant levels of ozone, thus addressing the main health and safety concerns associated with the use of ionizers. Aerosol tests were performed in a 19 m³ room, where ionization reduced the infectivity of aerosolized rotavirus and calicivirus by more than 97%. Additionally, the study highlighted that the ionizer completely prevented airborne transmission of influenza A virus between animals in a guinea pig model.

The carbon fiber ionization process is very effective in generating significant amounts of negative ions while keeping ozone levels within safe limits. This achievement significantly increases its applicability to indoor environments [99]. The authors developed a portable cold-plasma detergent device that is effective against aerosolized pathogens and is capable of reducing total bacteria and viruses by more than 99% within a short operating time (<90 min) in indoor environments tested in sizes up to 3 × 2.4 × 2.4 m³.

4.5. Possible Challenges in the Development of Real-World CFI-Based Air Purification Systems

While the CFIs show great potential when tested in laboratory settings, their successful real-world application requires elaborate planning and careful system design. A single CFI with well-planned air streaming may be an ideal solution for relatively small closed spaces, such as elevators, hallways, and public transportation. In the case of very elongated spaces, two or three periodically placed CFIs with a well-designed air streaming system could provide excellent air purification. The scalability and applicability to larger or more varied environments could also depend on the available infrastructure. As a more elaborate air exchange system might be needed for large spaces to optimize the CFI performance, the implementation of the CFI might be easier in newer and/or higher build quality places. In hospitals and schools, skillful integration of CFIs with the existing ventilation systems might be the best practical solution. Finally, CFIs have the potential to be used in combination with other air purification technologies (please see below) to obtain the best overall results.

4.6. Effectiveness of Air Ionization in Comparison with Other Pathogen Mitigation Measures

To provide a wider perspective on air purification technologies, we give a short comparison of the different available measures. Firstly, the very efficient HEPA filters originated from military requirements for protection against warfare agents of different types [100]. Today, they are used in industrial systems and medical buildings, inside HVAC systems (heating, ventilation, and air conditioning), and beyond. The use of portable HEPA filtration systems is also on the rise for the removal of airborne contaminants in emergency situations [101]. Dense in construction, HEPA filters can collect up to 99.97% of airborne particles with a size of 300 nm. With a particle size smaller or larger, this filter is even more efficient [102]. The two main characteristics of these filters are the pressure drop and the particle collection efficiency, which vary based on the constitution and density of the used fibers as well as the fiber thickness and air velocity [100,103]. HEPA filters are safe for humans. Exposure to collected microorganisms can happen only during HEPA filter removal. Some microorganisms captured on HEPA filters could survive under normal operating conditions for up to 200 days [104]. Because of their high density, HEPA filters require powerful fans to maintain air circulation throughout the system, which leads to high energy consumption. If the air flow rate is constant, pressure drop increases due to filter clogging [104], so these filters need regular maintenance and replacement. In combination, the high maintenance, high energy consumption, and high cost of HEPA filters prevent their wide use in general. HEPA filter characteristics are summarized in

Table 2. Comparison of major types of air purification technologies.

Air contaminants Removal Technology	Benefits	Limitations	Representative Examples
HEPA (High Efficiency Particulate Air) filters	- Safe for humans - High efficacy - (99.97% or more)	- High cost - High energy consumption - Regular maintenance and replacement needed - Exposure to airborne contaminants during removal or assessment	[100,101,102,103,104]
UVGI (UltraViolet Germicidal Irradiation)	- High efficacy - Fast disinfection - Low maintenance	- Not safe for humans - Need air flow assistance	[105,106,107,108,109]
Ionizers also producing ozone	- High efficacy - Fast disinfection	- Not safe for humans	[18,28,37,61,99]
Ionizers with negligible ozone production	- Good efficacy - Safe for humans - Health benefits from exposure	- Better performance with air flow assistance	[10,62,63,110,111]

.

UltraViolet Germicidal Irradiation (UVGI) kills or inactivates microorganisms by damaging their DNA or RNA. A large number of studies have shown that UVGI is very fast and effective in eliminating viruses [105,106]. This type of radiation has been used in hospital and industrial systems for the removal of microorganisms from surfaces, air, and liquids. The UVC spectrum of UV radiation shows the strongest decontamination properties [106]. As UV radiation is harmful for humans, indoor spaces can only be decontaminated in the absence of humans, which cannot ensure that harmful particles will not accumulate while people are in the previously treated indoor space. Another approach is the installation of UVGI in the upper room zone, but with proper installation to avoid interaction with human skin and eyes [107,108]. This installation should be combined with a vertical air flow mechanism in order to adequately clean the air in the entire volume of indoor space [109].

Small air ions have been shown to efficiently decontaminate air. While generating ions through the use of high voltage, both ions and ozone are formed. Exposure to ozone affects pathogens by causing damage, and in combination with ions, it can lead to their elimination [18,37]. Some research papers have shown that the combination of ozone and negative air ions (NAIs) has a strong effect on bacterial cell death [28]. Air ion disinfection using NAIs in ventilation ductwork has been studied in Ref. [61], showing that the disinfection efficiency is relatively good, but with a high ozone concentration of 68 ppb at 1.5 m/s velocity of air. Some studies aimed to assess the effect of bipolar ionization in a controlled environment, aiming to mitigate the effect of ozone [99]. A significant disadvantage of this approach is that ozone is harmful for humans, so it can only be used in empty spaces. This also raises questions about the accumulation of harmful particles during human presence while not being able to eliminate airborne pathogens from the breathing zone in a timely manner.

Recently, there has been increased interest in investigating the effects of small air ions, produced in a way that generates no ozone or negligible ozone concentrations, on airborne pathogens [110]. Carbon fiber ionizers use low enough voltage efficiently, thus producing ions without ozone formation. The effects of unipolar air ions on bacteria in Ref. [10] included increasing antibacterial efficiency with the increasing concentration of ions. Also, NAIs showed lower efficiency than positive ions. Ionizing air has been shown to affect influenza viruses and prevent airborne transmission [63]. In HVAC systems, research shows that they can be decontaminated using air ions against aerosolized bacteria in a ventilation duct flow [62]. Employing air ionization can inactivate pathogens in indoor spaces, either in combination with or separately from air filtration. It has been shown that it can even bring additional health benefits to humans [111].

4.7. Limitations

There has been a growing interest in technologies purported to decrease levels of airborne pathogens in indoor spaces. In this study, we thoroughly reviewed the existing literature to explore studies investigating ion emissions in CFI systems, aiming to derive insights into their effectiveness in eliminating microorganisms from the human environment.

We feel that it is necessary to tackle various issues pinpointed as constraints within specific studies or the broader realm of microbe inactivation induced by CFIs. The studies we examined differed on a number of relevant parameters, such as:

• Different particle sizes investigated;
• Varying degrees of ozone emissions (or simply a lack of O₃ measurement in a particular study);
• Range of microorganisms studied (some studies report on microbes that are not infectious to humans; many others, although they analyze the effect of ions on human pathogens, do so on only bacteria or viruses; the spectrum of microorganisms analyzed per study is restricted to only several species; ultimately, some microorganisms may be inherently more resistant to ions than others);
• Vegetative bacterial forms (in this instance, we are still uncertain about how ions impact bacterial spores);
• Different methods to measure microorganism viability (which can negatively impact interstudy comparisons; viability was also not measured in every study);
• CFI-related parameters (voltage, material used, and flow velocity);
• Definition of ion source (an example is a study that does not define whether a CFI unit is utilized);
• Type of most efficient ions (the optimal type of ions is also disputed, as in several instances a frank contradiction between study results was observed (Cf. Table 1).

An important issue to note is the lack of real-world data, i.e., conclusions derived from experiments conducted under real-life indoor and/or outdoor conditions (e.g., the CFI effect on removing airborne particulate matter from the breathing zone in offices, storage facilities, education institutions, parks, etc.) where temperature, air currents, and moisture have temporal variations. While creating an appropriate experimental setup to mimic these environments poses challenges, we believe that relying solely on extrapolated data from lab conditions should be approached cautiously. However, it is probable that these limitations are less pronounced for HVAC systems and similar structures.

5. Conclusions

Carbon fiber used in ionization processes demonstrates a superior capacity for ion emission due to the material’s electrical properties, which allow for a more efficient generation of ions when an electric current is applied. The increased density of ions effectively neutralizes airborne particles, thus aiding in their removal from the air. The unique properties of carbon fibers help minimize the electric field strength required for ion generation, thereby limiting ozone production. This aspect is crucial for indoor environments where ozone accumulation could pose health risks. This makes it a promising solution for managing indoor air quality in various settings, particularly where the minimization of ozone and other harmful byproducts is critical.

Regardless of the wide range of experimental conditions and pathogens involved, they collectively lead to a singular inference: ions evidently serve as an effective method for combating microorganisms in our surroundings. In conclusion, CFI systems apparently show significant promise for the destruction of microbes. Choosing the optimal material, voltage, and airflow rate seems crucial for ensuring the successful eradication of microorganisms. However, in order to reach more definite findings regarding the effectiveness of CFIs in containing microorganisms, it is necessary to conduct studies that encompass a broader range of microorganisms (relying on human pathogens) and employ more standardized methodologies/study approaches.

We require a standardized, large-scale testing method that includes replicated trials and time-matched control conditions. Such an approach is essential for contextualizing laboratory efficacy findings, translating them into real-world scenarios, and facilitating comparisons between technologies. A systematic review combined with meta-analysis could provide deeper insights into the findings of the research conducted thus far.