Radiation in Contemporary Dentistry: Health Hazards and Oral Microbiome Implications

Abstract

Modern dentistry increasingly relies on light-curing units (LCUs) and lasers in essential clinical procedures such as composite resin polymerization, caries treatment, and periodontal therapy. This review aims to outline the evolution of light-emitting technologies and to assess their potential biological risks, with particular emphasis on effects on the visual system, oral tissues, and microbiome. The development of curing devices is presented chronologically, from the first-generation ultraviolet (UV-A) lamps introduced in the 1970s to current light-emitting diode (LED-LCU) systems and dental lasers (e.g., Er:YAG, Nd:YAG). The progressive increase in light intensity—now exceeding 3000 mW/cm²—has shortened curing times but simultaneously raised safety concerns. Major hazards include the so-called blue-light hazard, where exposure to high-energy visible (HEV) blue light may accelerate macular degeneration, and temperature elevations in the pulp chamber, which may damage the dentin–pulp complex. Laser radiation also exerts significant microbiological effects: Er:YAG and diode lasers demonstrate bactericidal activity against biofilms and oral pathogens (e.g., P. gingivalis), although therapeutic outcomes depend on wavelength, dose, and exposure time. Suboptimal parameters may lead to microbiome disturbances, whereas low-level laser therapy (LLLT; 600–1200 nm) supports tissue regeneration and helps restore microbial balance. The individualization of irradiation parameters, combined with thorough theoretical knowledge, operator expertise, and technical understanding of LCUs and lasers, is essential for maximizing clinical benefits while minimizing health risks and preserving oral microbiome homeostasis.

1. Introduction

A broad spectrum of electromagnetic radiation is applied in dentistry—ranging from UV to infrared—including blue light, UV-C radiation and laser radiation (VIS and IR). The wavelength determines the type of application—from the activation of photoinitiators in composite materials to the stimulation of tissue regeneration and precision cutting. Modern dentistry involves the intensive use of radiation curing (e.g., LED light, halogen lamps) and lasers in key clinical procedures, such as the curing of composite materials, the adhesion of veneers and caries, and periodontal treatment [1,2,3,4]. Light curing units (LCU) provide the basis of many modern dental procedures, including direct composite restorations, the adhesion of veneers and orthodontic brackets, and the sealing of fissures. This mechanism is based on light-initiated polymerization of monomers, in which photoinitiators present in adhesive materials and systems (including camphorquinone, TPO) absorb radiation within the UV-VIS range—mainly in the blue band—which triggers polymer cross-linking. Although ultraviolet light (UV-A) systems were originally used for photopolymerization, due to health concerns, these devices have evolved towards sources dominant in the blue light band. Currently, dental offices mainly use light-emitting diodes (LEDs) [3,4,5]. Continuous technological improvements have led to a significant increase in radiation intensity, reaching up to 3000 mW/cm² or more. This technological advance has brought clinical benefits, such as reduced work time and less patient discomfort [2,4]. However, increased power and higher radiation intensity also entail potential biological hazards for patients, operators, and support staff. The primary safety concerns are related to the direct and cumulative effects of radiation. The most important hazard here is the risk posed to eyesight (i.e., “blue-light hazard”), because high-energy blue light (HEV), especially with a wavelength of about 440 nm, can penetrate through to the retina, causing oxidative stress and damage to the retina. Due to long hours of often cumulative exposure resulting from light reflection, operators are particularly at risk here. Moreover, the use of LCU involves a degree of thermal risk, which may result in higher temperatures in the pulp chamber of the tooth, thereby causing damage to the pulp–dentin complex and possible burns in the mucosa [1,2,6,7,8]. Possible phototoxic and photoallergic reactions should also be taken into account, triggered by the absorption of radiation by endogenous substances (e.g., porphyrins), exogenous substances (e.g., residues of oral hygiene products, photosensitizing drugs taken) or substances present in the oral cavity, such as unreacted monomers, i.e., “leachables”. Exposure may also have microbiological implications, affect the complex ecosystem of the oral microbiota, and potentially lead to alterations or imbalance (dysbiosis) [1,9,10].

In light of these multidimensional risks, matching the technical parameters of the LCUs used (including emission spectra) to the absorption profile of the photoinitiators and strictly controlling the dose and exposure time are vital to ensure complete material curing while limiting biological risk. One important safety concern is the protection provided for the eyesight of both the operator and patient. Scientific reports and reviews of clinical practice indicate the need to verify radiation intensity, the spectral profile of the lamp, exposure geometry and the absolute use of personal protective equipment (glasses with an appropriate cut-off wavelength ensuring protection against, among other things, potential “blue light hazard”), as well as minimizing the exposure time outside the treatment field [1,3,10]. The objective of this review is to provide a detailed analysis of the health risks associated with the radiation applied in dental practice, with a particular emphasis on the consequences of such treatment on the eye, oral soft tissue and perioral skin, as well as its potential impact on the oral microbiome. The aim of the work is to provide a synthetic approach to emission parameters, light–material interaction mechanisms, and the tissue and microbiome of the oral cavity, as well as to formulate practical recommendations for safe exposure. This topic is important for raising clinical awareness and encouraging the implementation of necessary preventive actions given the continuously increasing power and technological diversity of modern curing lamps.

2. Literature Selection Methodology

The publications were curated through a targeted, non-systematic literature search tailored to the scope of this review, focusing on dental light-curing units (LCUs) and lasers, their applications in dentistry, and associated health hazards (including ocular, thermal, and phototoxic effects), as well as their impacts on the oral microbiome and recommendations for dental staff. Irrelevant topics (general ultraviolet radiation without an oral health context) were excluded. The inclusion criteria comprised peer-reviewed original research articles, narrative and systematic reviews, clinical studies, and official statements (e.g., from the FDI World Dental Federation and the International Commission on Non-Ionizing Radiation Protection). Non-peer-reviewed sources were excluded (e.g., blogs, unless authoritative). The search was limited to publications from 2000 to 2025, capturing the technological evolution from quartz–tungsten–halogen (QTH) lamps to modern polywave light-emitting diode (LED) systems and antimicrobial blue/UV light applications. Only articles published in English and Polish were considered. Studies were excluded if the full text or DOI was unavailable. The following databases were searched PubMed/MEDLINE, Scopus and Web of Science.

3. Different Generations of Devices for Curing Composite Fillings

The ultraviolet range encompasses UV-A (315–400 nm), UV-B (280–315 nm) and UV-C (200–280 nm); in the context of dentistry, it has been UV-A that has been primarily used for photopolymerization purposes. Nowadays, however, sources in the visible (blue) band dominate, and UV-C is the main tool for ensuring environmental disinfection (surfaces, air) due to its strong germicidal properties and compliance with safety standards [11]. Blue light includes the highest energy levels in the visible spectrum (approx. 380–500 nm) and is divided into blue violet/HEV (high-energy visible; approx. 380–450 nm) and blue turquoise (approx. 450–500 nm). In practice, it is of paramount importance that the emission peak of the lamp is precisely matched to the absorption spectrum of the photoinitiator and that the irradiance and exposure time are controlled.

In dentistry, devices initiating the polymerization reaction of light-cured composite materials (LCUs) were first used in the 1970s [12]. Technological development also concerns devices used for light-curing dental materials. Currently, the most widely used are light-curing units based on light-emitting diode technology (LED-LCUs), which have gained the greatest popularity [13,14,15].

We propose dividing LCUs into four generations based on the emitted light source. In the literature, including the work of Mangat et al. (2014) [13], LCUs can be divided into five generations. The argon-ion laser is listed as the fifth generation [13]. This device was used for polymerizing composite fillings with a wavelength of 450 to 500 nm. The laser lamps had narrow optical fibres, which are not ideal for filling cavities, and the depth of polymerization of the composite resin was greater than with other LCDs [13]. Furthermore, argon-ion lasers were large and they did not gain popularity among clinicians [14]. They differ from the other four generations, and therefore we propose dividing LCUs into four generations.

3.1. First Generation: UV Lamps

The first lamp was designed for curing composite materials, Nuva Light (developed by Dentsply/Caulk), utilized ultraviolet light (UV) to emit ultraviolet light (approximately 365 nm) through a quartz rod from a high-pressure mercury source. The exposure time was 20 s, with better results achieved after 60 s [13]. However, these devices were eventually withdrawn from use as they exposed the human body to the potentially harmful effects of short-wave UV radiation, including possible damage to eyesight (corneal burns, cataract formation) and the risk of changes to oral microflora [13]. In addition, with such devices there were problems regarding light penetration into the filling, and thus with material curing depth [12].

3.2. Second Generation: Visible Light Curing Units (QTH-LCUs)

In the following years, UV-based lamps for curing composite materials were succeeded by quartz–tungsten–halogen (QTH) lamps (halogen curing light replaced UV curing light) with quartz–halogen bulbs emitting visible light. The light source is a bulb connected to a filter [4,13]. In a quartz bulb, a tungsten filament is heated in halogen gas, and the light from the bulb is reflected from a mirror and collected so that blue light is delivered via optical fibres. QTH lamps are capable of emitting a broad spectrum of violet and blue light [13]. The longer wavelengths of visible light emitted by the device ensure greater penetration of the curing light and light energy [13]. Additionally, their power and radiation intensity were low, as a consequence of which a considerable exposure time of 30–60 s was required when polymerizing 2 mm of resin [4], which is longer than in the case of the light-emitting diode (LED) polymerization lamps described below [13]. Because the device was powered by the mains, it could not be wireless, and the bulb required changing every 30–50 h. The filament produced high temperatures, and the excess heat and visible light thus generated were not needed for polymerization and required filtration. Thanks to the emission of a broad spectrum of radiation within the range of blue and violet light, QTH lamps can activate all photoinitiators used in dental composites. The polymerization process occurs under the influence of visible light with a wavelength of 410–500 nm [4,13].

3.3. Third Generation: Plasma Arc Curing Lamps, Plasma-Arc Lamps (PALs)

In 1998, a plasma arc curing lamp (PAL) was introduced, emitting light from a glowing plasma, i.e., a mixture of ionized xenon particles and electrons [13]. PAL is a device in which tungsten rods, surrounded by xenon gas, release radiation, much of which escapes, as a consequence of which PAL requires high radiation filtration [4]. The device was characterized by considerable intensity levels within a narrow wavelength range, i.e., around 470 nm. The manufacturer’s claim of a rapid, 3 s hardening process has been completely refuted [13]. Moreover, the high cost and inferior quality of PAL compared to the QTH lamp undermined its popularity [4,14].

3.4. 4th Generation: Light-Emitting Diode (LED) Units, (LED-LCUs)

The most popular polymerization lamps with light-emitting diodes (LED-LCUs) were first used in the late 1990s [4]. Light-emitting diodes (LEDs) are special semiconductors that produce electroluminescence [16]. The spectral flux of such a diode is concentrated in a narrower band than the spectral flux of QTH diodes [4] and plasma arc hardening [13]. LED-LCUs are lightweight, can be wireless [17,18], battery-powered [18,19] and have an operating life of more than 10,000 h [4,13]. Moreover, they are characterized by high radiation intensity at the lamp tip [19] and do not require the use of band-pass filters to isolate blue light [13,18]. Thanks to all these features and their greater efficiency compared to QTH lamps, these devices are commonplace nowadays, replacing the previously used QTH lamps [14]. The development of LED technology encompasses three generations of diodes. LCU-LEDs of the first and second generations are single-peak emission devices (monowave, single-peak, single-wave) [4]. They have the ability to emit one colour of light (blue) with a wavelength above 420 nm. The first-generation LEDs were developed in 1999–2002 [14]. The included several low-power LEDs with a longer operating time [4]. They had a narrow spectrum [20], with a maximum wavelength of 465–470 nm [14], and were suitable for dental materials containing the photoinitiator camphorquinone (CQ) [18,20]. CQ is a popular photoinitiator, a component of numerous resin-based materials [17,21]. It is characterized by a wavelength absorption range from 360 to 510 nm [22], with the maximum absorption of camphorquinone light occurring at a wavelength of 468 nm [18]. CQ has low biocompatibility, is yellow in colour and must be used in combination with tertiary amines that act as reducing agents [22]. Photoinitiators, which are an integral component of the composite material, absorb light and are necessary for the polymerization of the resin and monomers in the composite. Therefore, the polymerization of the composite material is influenced by the wavelength needed to activate the photoinitiator contained within [4,17,18]. In addition to CQ, which can be activated by almost all LCUs [4], photoinitiators include, among other things, ivocerin [17], bisacylphosphine oxide (BAPO), monoacylphosphine oxide (MAPO), and 1-phenyl-1,2-propanedione (PPD), and less yellow than CQ [22] trimethylbenzoyldiphenylphosphine oxide (TPO), which is used to achieve whiter-coloured material composites [18]. In summary, mono-peak LEDs are unable to activate initiators requiring wavelengths below 420 nm due to the narrow range of the length of the light they emit, unlike multi-peak LEDs, which can also emit light with wavelengths below 420 nm and are capable of activating all photoinitiators. This is similar to QTH-LCUs, which, by emitting a wide spectrum of blue and violet light, possess the ability to activate all photoinitiators used in dental composites [4]. Therefore, first-generation LED-LCUs with a low power intensity range (low irradiance range of 100–400 mW/cm² [14] were unable to activate all types of photoinitiators, such as those used in bleach shades [13]. They were less effective in hardening composite materials compared to QTH-LCUs [14]. The narrow spectra of devices with first- and second-generation LEDs correlated with the absorption spectrum of the CQ photoinitiator [17].

The second generation of LEDs (2002–2004) had a power intensity range of 500–1400 mW/cm², which is higher than that of the first generation of LEDs [14]. The devices for light-curing second-generation light-emitting diodes (e.g., Bluephase, Elipar Freelight 2, LE Demetron 1, Radii) have a large emission surface and a high energy output, in the form of a single high-power diode [13]. They are able to harden composites containing CQ effectively. However, in the presence of other photoinitiators, such as TPO or PPD, polymerization fails to proceed properly due to their absorption range [14].

Third-generation LEDs, the production of which commenced in 2004, are multi-wave diodes (multi-peak, multi-peak, polywave, multipeak) [4]. They are capable of emitting light of more than one wavelength [20], i.e., both blue and violet light [17]. They are effective not only on CQ but also on other photoinitiators, e.g., TPO. In practice, this translates into the possible application of many different materials using one type of lamp. Furthermore, the lamp has a higher radiation intensity than previous generations, ranging from 1000 mW/cm² to 3000, and according to others, values reaching even up to 5000 mW/cm² [20]. The device is additionally equipped with an emission peak, thanks to which it is now possible to activate photoinitiators other than CQ [17].

Recently, the latest generation of LED-LCUs has also appeared on the market. The technology in this case is based on “wavelength scanning”. The operator can choose the operating mode of the device depending on the type of material being hardened [17]. It is possible to choose various settings, not only regarding time, but also light emission. Recommendations have been made to shorten the polymerization time by means of higher device power using high-energy LED lamps [23]. Reducing the procedure time in some dental procedures, e.g., when installing orthodontic brackets, improves patient comfort [17].

The physicians must know the absorption spectrum of the photoinitiators used in the composite material to properly match the appropriate LCU to it [14]. For example, UltraLume 5, a third-generation LED, emits light in various ranges, thanks to which photoinitiators such as CQ and other alternatives can be activated. From a clinical point of view, the risk of heat generation by these devices is significant [13]. Devices using Quadwave technology have also appeared on the market. Quadwave produces four different wavelength bands: violet, blue, infrared, and near-infrared (NIR) [19].

4. Types of Lasers and Their Application

Lasers have become an integral part of modern dentistry. They are applied in many fields of dentistry, including soft tissue surgery, periodontology, endodontics, prosthetics, pediatric dentistry, and even in whitening treatments [24]. Despite the many benefits resulting from their use, there is some risk associated with their incorrect application.

Lasers in dentistry can be classified according to various criteria: the type of laser used, the chromophore employed to activate the laser, the tissue on which they can be applied and the wavelength range. Lasers operate at different frequencies, intensities and specific wavelengths (all in the infrared range), which in turn determines their absorption and penetration rates. These factors vary depending on the type of tissue on which the laser acts. Malcangi et al. [25] propose dividing lasers into three groups: solid-state lasers (Nd:YAG, Er:YAG, ErCr:YSGG), gaseous-body (CO₂) lasers and semiconductor lasers (diode lasers). Among these, we can distinguish those used in soft and hard tissue procedures, such as Nd:YAG, Er:YAG, ErCr:YSGG, as well as lasers designed exclusively for soft tissue procedures (CO₂ and diode laser). The same authors describe the application of a low-energy laser for photobiomodulation purposes. A focal lens was used to achieve diode effects in low-power laser therapy (LLLT), which, thanks to photobiostimulation, enables good tissue penetration. Photobiomodulation is a photochemical and non-thermal biological reaction that utilizes red light with a wavelength of 600–700 nm and infrared light with a wavelength of 770–1200 nm to accelerate healing and reduce tissue inflammation. LLLT serves as an alternative to the diode laser. An additional advantage of this device is that there is no risk of tissue overheating despite its deep penetration [25]. The low-energy LLLT laser helps alleviate the symptoms of trigeminal neuralgia, reduces pain and accelerates wound healing in recurrent aphthoses and herpes simplex. It stimulates bone growth around the implant and increases its osseointegration [26]. The CO₂ laser emits infrared light with a wavelength ranging from 9400 nm to 10,600 nm, either pulsed or in the form of a continuous wave.

It is used in the treatment of oral ulcers as well as in the abrasion of water-rich soft tissues. Moreover, thanks to its hydrophilic properties, it ensures proper healing of post-operative wounds. To prevent overheating of soft tissue and subsequent necrosis, it should be operated in pulsating mode. It then works to a depth of 0.1 mm and does not make contact with the tissue. Unlike the CO₂ laser, the wavelength of the Nd:YAG laser is 1064 nm. The generated energy is extinguished using long pulses, which are converted into heat in the tissue. These lasers have many applications, but their use in endodontics should be particularly highlighted—they help remove pathogenic microorganisms and blockages in the root canal. Compared to other lasers applied in dentistry, the Nd:YAG laser has greater tissue penetration, low affinity for water and high-affinity binding to blood vessels, which explains its good hemostatic properties [25]. The Er:YAG laser is a device that can operate at two different wavelengths: Er and Cr lasers, yttrium scandium gallium garnet (YSGG) at a wavelength of 2780 nm and Er:YAG lasers at a wavelength of 2940 nm [27]. Both types of laser have good affinity for hydroxyapatite and hydrophilicity, thanks to which it is suitable for treating hard tooth tissues. It helps maintain healthy tooth structure with a reduced need for anesthesia in the treatment of caries. The Er:YAG laser has excellent cutting properties and causes little tissue damage, which makes it an appropriate tool for the surgical removal of lesions [28]. The argon laser at a wavelength of 572 nm can operate in both pulsed- and continuous-wave modes. It is used in tooth whitening and remineralization. The diode laser emits light with a wavelength of up to 810 to 980 nm using a solid-state semiconductor made of aluminum and gallium. It enables both continuous and impulse operation. Diode laser therapy optimizes the elimination of infectious processes in the tooth apex. It also stimulates osseointegration, which makes it a device that can be used in dental implantation. In addition, laser technology allows for the treatment of oral ulcers, herpes and dentin hypersensitivity [25].

5. Health Risks Associated with the Use of LED-LCUs

5.1. Threats to the Visual System Resulting from Exposure to Blue Light and Possible Means of Their Prevention

The modern human eye is exposed to a higher degree of radiation stimulation than in the past. This is due to the use of devices such as computers and electronic displays, among other things [29]. Most LED lamps currently available in dentistry for hardening dental materials emit light with a wavelength of 430 to 480 nm [15]. Blue light with a wavelength of approximately 440 nm is most harmful to the retina [4,15,30,31,32], which is within the range of radiation emitted by dental LCU devices. Additionally, the band of blue radiation emitted by LED-LCUs is narrow and is not always able to induce a protective aversion reaction in the eye [32]. Not only are high levels of blue light hazardous for the eye, as they can cause irreversible retinal burns, but so are chronically low levels of blue light [4,32], because they may contribute to retinal degeneration, can cause chronic photochemical damage to the retina, and can also accelerate the process of premature age-related macular degeneration [4,31,32]. Phototoxicity affects the development of retinitis pigmentosa [29]. Healthcare personnel may also be exposed to possible visual damage from accumulated radiation levels from LCU devices. The aforementioned risk applies not only to dental staff, but also to patients. People who have undergone cataract surgery and take certain medications, such as photosensitizing drugs (antimalarials, chlorpromazine, St. John’s wort, dimethylchlortetracycline and 8-methoxypsoralen) are at greater risk of retinal damage than others [31]. Preventive measures are required [4,31] to reduce the amount of blue light reaching the eye [29]. With this goal in mind, blue light filters (filtering glasses) have been used, as these devices are capable of reducing the transmission of light with a wavelength of less than 500 nm down to less than 1% [4,31,32]. Filters may be in the form of orange (amber) glasses, blades, or polymerization covers. The use of orange (amber) glasses as shields against blue light greatly facilitates the doctor’s work, making it easier for him or her to observe light during dental procedures [4,32]. Orange covers have been shown to block more blue light than brown equivalents [29]. Although most manufacturers of LED-LCU devices used in dentistry supply them with protective glasses, ensuring staff wear them is another matter altogether. Ordinary prescription glasses do not prevent the penetration of blue light [31,32].

Of key importance for both polymerization and eye protection is ensuring correct positioning of the lamp tip during its operation. It should be placed close to the resin to ensure the proper polymerization of the entire material [4]. Many organizations, including ACGIH and ICNIRP, have published their own standards in the case of human exposure not only to lasers, but also to incoherent optical sources, including LEDs. Unlike lasers they have limits based on radiance (brightness). According to the recommendations of the American Conference of Governmental Industrial Hygienists (ACGIH), the greatest risk of retinal damage occurs when the optical fibre is located on the palatal side of the tooth, with partial exposure beyond its incisal edge [31]. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has developed its own guidelines regarding optical radiation [32]. According to ICNIRP, although LEDs in dental lamps are safer than lasers, they still carry a risk of retinal damage with long exposure due to the use of blue light. The dangers of using infrared are also highlighted because it is not visible to the eye and no protective reflex occurs in less than 0.25 s, which helps shield the retina when looking at a very bright source of visible light (e.g., the sun, welding, a light bulb). Attempts to cause vision damage using high-power LEDs in laboratory conditions proved successful, which was not the case with diode lasers. As a consequence, current safety limits classify LEDs as inconsistent sources. Further research into the safety and effectiveness of LCUs is recommended [33,34]. A study on chronic dental lighting effects analyzed 14,523 individuals, revealing that dentists face a 3.6 times higher risk of vision disorders compared to other occupations, linked to blue and white LED exposure significantly disrupting blood–retinal barrier homeostasis. Experimental rat models in the same research exposed to 200–1000 Lux for 8 h daily, 5 days a week, over 6 months showed substantial retinal damage from these sources, advocating reduced blue light intensity and safer halogen alternatives. Dentists should minimize light intensity under good-visibility conditions, opt for low-intensity halogen lights as safer alternatives, and avoid direct exposure to harmful blue light from curing lamps by using protective measures. White LED dental lights, which contain blue components that damage retinal barriers, should be redesigned during manufacturing to provide adequate brightness without short-wave light [35]. Chronic risk cannot be adequately described by a single numerical threshold, as it is influenced by multiple parameters, including total exposure duration, frequency of repeated exposure, irradiance, and the individual susceptibility of retinal tissues. An example of an important factor in risk assessment is the distance from the light source, as irradiance decreases markedly with increasing distance, thereby reducing retinal exposure. This is clearly demonstrated in the study by Lehmann et al., which shows that maintaining a greater distance from the curing lamp can significantly lower ocular risk while still ensuring sufficient light intensity for effective dental procedures [36]. In the context of dental professionals, both cumulative occupational exposure and individual exposure events should be considered. Therefore, the long-term effects on the human body of exposure to new lighting technologies have to be investigated.

5.2. Threats to the Pulp and Soft Tissues Resulting from Exposure to Blue Light and Possible Means of Their Prevention

Not only are the plasma and halogen lamps of earlier years capable of increasing the temperature in the tooth pulp chamber, but so too are the more recent LED-LCU devices. In a 2010 study, Leprince et al. demonstrated that the temperature in the tooth pulp chamber under the influence of LED lamps increased by approximately 3.47–7.03 °C [37]. In an article published in 2023, Maucoski et al. [19] described various increases in temperature in the pulp chamber, depending on the exposure of a tooth, when using polymerization lamps with different light emission technologies and spectral characteristics: high-power-LED, narrowband, quadwave LED lamps, as well as a diode laser. Furthermore, measurements were taken at a distance of 6 mm and 0 mm from the surface of the tested tooth. All the devices caused an increase in the temperature in the pulp chamber. The researchers observed a correlation between the distance of the device from the tooth and the increase in temperature, such that a greater distance from the examined tooth correlated with a smaller increase in temperature, with the largest increase observed for the Monet and PinkWave diode laser [19]. An increase in temperature may result in damage to the pulp–dentin complex [17]. Extending the hardening time may also result in damage to the pulp and oral tissues due to a higher temperature in the tooth [18]. One possible way to protect the pulp might be to reduce the power or the distance from the device. However, the outcome of such an action would be a deterioration in the quality of the polymerization process. Blue light generates heat that may have a harmful effect not only on the pulp, but also on the soft tissues of the oral cavity [37], which may result in thermal injuries [29]. Therefore, it is recommended that the operator minimize exposure time and avoid directing light onto mucous membranes [37]. In 2012, three clinical cases were reported in which oral tissue burns were diagnosed in anesthetized patients following the use of LED-LCU lamps [32]. Research by Rueggeberg et al. and Alasiri et al. [2,12] also includes reports of patients who experienced burning sensations in soft tissues when receiving high doses of blue light [17,38].

More recent LED-LCU models with radiation intensity values higher than 2000 mW/cm², despite speeding up the procedure and increasing comfort, are another cause for concern, as they are associated with an elevated risk of damage to the pulp and soft tissues [19]. Hence, it is recommended that the operator wait a few seconds between material exposure cycles to minimize the temperature increase [32]. Hence, there is a danger of damage to eyesight due to the increased amount of radiation received by the pupil in such a situation. Further research in this area is required [29].

5.3. The Effects of Ultraviolet Radiation on the Oral Microflora in Dentistry

Ultraviolet (UV) radiation is very commonly applied in dental practice, mainly in the context of hardening composite materials, surface decontamination and supporting certain therapeutic procedures. In particular, UV-A light is used in curing lamps, while UV-C radiation is gaining popularity as a means of infection control. UV-C radiation directly affects the DNA of microorganisms, causing the formation of thymidine dimers, which result in the death of bacterial and fungal cells [39]. However, more and more scientific reports suggest that the impact of UV on the oral environment, although beneficial in terms of eliminating pathogenic microorganisms, may also disturb the balance of the microbiome, which has clinical consequences, the importance of which are still not fully understood [40,41].

5.4. Influence of UV on the Physiological Microflora of the Oral Cavity

The oral microbiota is a complex ecosystem comprising over 700 species of microorganisms, including bacteria, fungi, viruses and archaea, the balance of which determines the health of oral tissues and systemic homeostasis [42]. Disturbances in this balance, called dysbiosis, may trigger the onset of numerous pathologies, including caries, gingivitis, periodontopathy, and even systemic inflammation [43]. Exposure to UV radiation, especially within the UV-C range, although effective in eliminating pathogenic bacteria such as Porphyromonas gingivalis or Fusobacterium nucleatum, may also negatively affect the presence of commensal microorganisms such as Streptococcus salivarius, Rothia dentocariosa or Lactobacillus spp. [44,45]. Studies have shown that UV-C reduces the number of living microbial cells in the oral biofilm [46].

5.5. Effects of Long-Term UV Exposure in a Microbiological Context

Although the use of UV light in dentistry is usually short-term and local, there is evidence that frequent exposure, e.g., during repeated reconstructive procedures, can lead to chronic changes in the structure of the microbiome. Importantly, in vitro studies have shown that commensal microorganisms exposed to repeated UV procedures can undergo selection and develop resistance to environmental factors, such as the production of photoprotective pigments or the activation of DNA repair enzymes [39,47]. These changes may promote colonization by opportunistic pathogens, including yeasts of the genus Candida, which are more resistant to UV radiation and chemical antiseptics [48]. A single, universal exposure threshold at which the antibacterial benefit of UV-C radiation translates directly into a risk of dysbiosis in the patient, has not been definitely identified. However, research indicates clear differences between the doses necessary to eliminate pathogens and the doses that can damage mammalian cells or affect the balance of the microbiome. The sensitivity of the microbiome to the UV-C radiation depends on its composition and location. Gram-negative bacteria (e.g., E. coli) are typically more sensitive to UV-C than Gram-positive bacteria or fungi [49,50]. UV-C radiation (especially with a wavelength of 254 nm or 207–222 nm) has high selectivity, which allows the destruction of bacteria at doses that are safe for the patient’s tissues. A dose of approximately 2 mJ/cm²–5 mJ/cm² may be sufficient to inhibit 90–99% the growth of many pathogenic bacteria (e.g., S. aureus, P. aeruginosa) in vitro. Nevertheless, overexposure can selectively eliminate sensitive species, promoting the growth of radiation-resistant microorganisms such as Deinococcus and some Micrococcus species. Bacteria in mature biofilms require significantly higher doses for elimination (e.g., 946.7 mJ/cm² for a 3–4 log reduction on stainless steel), up to 10–100 times higher (e.g., >500 mJ/cm²) than planktonic bacterial cells. Applying such high doses near healthy tissue significantly increases the risk of damage to host cells and microorganisms forming the microbiome [50]. The risk of dysbiosis increases when the doses used exceed the minimum values necessary to control the infection, especially in the case of repeated exposure, which may lead to the selection of resistant strains and genetic mutations in microorganisms. The benefit outweighs the risk at doses below 20–25 mJ/cm² [51,52,53]. Above this limit, although bactericidal efficacy increases, the risk of nonspecific destruction of beneficial microflora and irritation of host tissues begins to increase.

5.6. Mechanisms of Host Response to UV Exposure

UV radiation not only affects microorganisms, but also has a direct impact on host cells. Keratinocytes of the oral mucosa and fibroblasts are sensitive to oxidative stress caused by UV-A, which may help activate inflammatory pathways, as well as DNA damage and apoptosis [29,54]. These effects have the potential to impact the local microbial environment by disrupting host–microbiota communication and influencing the expression of surface receptors associated with microbial recognition [55].

5.7. Clinical Implications and Future Research Directions

The use of UV radiation in dentistry remains beneficial in many aspects. However, its potential consequences on the oral microbiome must also be considered. Possible clinical consequences include an increased risk of recurrent infections, chronic inflammation, and difficulties in rebuilding the microflora after antiseptic therapy. In the face of these challenges, exposure protocols should be developed so as to minimize damage to the microbiome, e.g., by using selective wavelengths or supporting probiotic therapy [56]. Further clinical trials in patients are recommended, including analysis of the microbiome composition before and after UV exposure, as well as observation of long-term therapeutic effects. Modern techniques such as metagenomic sequencing and metabolomics analysis may serve as valuable tools in assessing the impact of UV on oral microbial homeostasis.

In the clinical next-generation sequencing study of Uğur Aydin et al. (2026) [57], the effect of photodynamic antibacterial therapy on the root canal microbiome in patients with general health and type 2 diabetes were analyzed. They show that aPDT combined with chemical–mechanical preparation leads to a profound restructuring of the microbiota, reducing its stability in metabolically challenged patients [57]. Another study conducted by Wang et al. (2021) [58], who performed 16S rRNA sequencing, showed that LED irradiation (e.g., 405 nm) significantly alters the alpha and beta diversity of interdental biofilms. Among other things, a decrease in the abundance of pathogens such as Fusobacterium was observed, while commensal species such as Veillonella increased, suggesting the possibility of regulating microbiome balance without inducing resistance [58].

While UV radiation exhibits potent, direct genotoxic activity against oral pathogens, its clinical use is limited due to potential damage to host soft tissues. This provides a starting point for exploration within the visible spectrum, where antibacterial blue light (aBL) is emerging as a safer alternative. The shift from mechanisms based on DNA degradation (characteristic of UV) toward the induction of endogenous porphyrins and the production of reactive oxygen species (typical of wavelengths 400–470 nm) allows for the selective photoinactivation of microorganisms while maintaining the integrity of mammalian cells.

5.8. Desirable vs. Undesirable Effects in the Oral and Skin Microbiome

Antimicrobial blue light (aBL) 400–470 nm inactivates bacteria by inducing endogenous porphyrins/flavins and inducing ROS; 405 nm is often most effective against biofilms, while 450–470 nm is effective against species, especially P. aeruginosa. For pathogen targeting, 395–405 nm is most versatile against numerous periodontal and skin pathogens; 450–470 nm is highly effective against P. aeruginosa, and moderately effective against S. aureus (higher doses).

The effect on the microbiome can be selective: 405 nm can shift communities towards more favourable profiles, but results depend on the dose, oxygen conditions and biofilm structure. NIR 810–980 nm reduces pathogens mainly indirectly (photothermally) as an addition to SRP; in single applications, the effects on the microbial composition may be limited or transient [9,59].

The antimicrobial efficacy of radiation, typical dosages, and biophysical mechanisms against various bacterial species as well as biofilms are shown in Table 1 and Table 2.

Table 1. Antimicrobial efficacy, typical dosages, and biophysical mechanisms of visible and near-infrared (NIR) radiation against various bacterial species and biofilms [60,61,62,63].

Wavelength	Typical Dosage/Comments	Target Bacteria (Examples)	Effect on Bacteria	Mechanistic Commentary
395–405 nm (violet/blue)	5–30 J/cm2 (soft tissue therapy); higher dosage for biofilm (100 J/cm2)	S. aureus, P. aeruginosa, P. gingivalis, Prevotella spp., E. coli	Strong, often the most lethal; biofilms reduced especially at 395–405 nm; P. gingivalis sensitive at 400–410 nm	Induction of porphyrins/flavins → ROS; 405 nm often the “sweet spot”
420 nm	Close to 405 nm, but sometimes weaker	P. aeruginosa, mixed biofilms	Effective, but sometimes less than 405 nm, depending on species and dose	Similar ROS mechanism; lower absorption in some species
450–470 nm (blue)	1.5–30 J/cm2 in clinical healing studies; for a bactericidal effect, often 10–15 J/cm2+	P. aeruginosa, S. aureus (partly), bacteria of skin and oral cavity	470 nm: highly effective against P. aeruginosa; on S. aureus, effective at higher doses; 450–470 nm shows high level of antibacterial activity with low cytotoxicity in periodontal models	ROS through endogenous chromophores; species-dependent absorbency
400–454 nm (aBL range)	Dose–time relationship; pulses can increase effectiveness at low energies	A. baumannii, S. aureus, P. aeruginosa, E. coli (biofilm)	All showed a decrease in viability; 395/405/420 nm most effective on biofilms; longer wavelengths may even stimulate the growth of S. aureus in specific conditions	Selection of precise waveform and modulation (continuous/puls) is critical
780–810 nm (NIR diodes)	Fluences of 3–20 J/cm2 (PBM); clinical powers in periodontology (e.g., 1–1.5 W) with “sweeping” movements	S. mutans, Lactobacillus spp., Actinomyces spp. (indirectly), periodontal pathogens	May reduce pathogens and improve clinical metrics as an adjunct to SRP; antibacterial effects more often indirect/thermal; mixed results	Mainly photothermal/biomodulation; not aBL
904 nm (pulse)	Low fluences PBM (~3 J/cm2)	S. aureus (changes in cell topography)	Induces ultrastructural changes; reports of growth inhibition depending on settings	Photothermal effect/biomodulation
980 nm (NIR diodes)	Surgical/antiseptic parameters; effectiveness depends on power/time	P. gingivalis and other periodontopathogens	Clinical trials showed a strong bactericidal effect as an adjuvant in acute periodontal interventions	Hemoglobin/water absorption → controlled thermal effect

Table 2. Phototherapeutic modulation of the human microbiome: selective antimicrobial effects and risks of dysbiosis induced by visible and near-infrared (NIR) radiation [61,62,64,65,66].

Wave Range	Typical Dosage/Comments	Desired Effects (Microbiome)	Undesired Effects (Microbiome)
395–405 nm (violet/blue)	5–30 J/cm2 for soft tissues; Biofilm eradication usually requires more energy/time	Significant inactivation of many pathogens and biofilms (e.g., periopathogens, P. aeruginosa, S. aureus); potential “balancing” of the community with the appropriate dose and schedule; modulation of dysbiosis.	Risk of too-broad eradication of sensitive commensals and disruption of eubiosis at excessive doses/time; species-dependent reactions, possible temporary composition shifts.
420 nm	Close to 405 nm, sometimes weaker	Reduction in pathogens in biofilm with adjusted dose; lower level of cytotoxicity in a shorter time.	Potentially lower selectivity and need for longer exposure → higher risk of impact on commensals.
450–470 nm (blue)	10–30 J/cm2 for antibacterial effect; effectiveness depends on species	Highly effective against P. aeruginosa; useful additional instrument against skin/oral pathogens; capable of controlling biofilm with lower cytotoxicity.	Less effective against some Gram-positives (e.g., S. aureus), at low doses may result in resistance selection/imbalance if the protocol is suboptimal.
400–454 nm (aBL overall; continuous vs. pulse)	Pulsing may improve the effect/side effects ratio	Effective reduction in many pathogens in aerobic conditions; can be adapted to specific communities.	In anaerobic conditions, some species (e.g., S. mutans) are less sensitive → risk of species disproportion following exposure.
780–810 nm (NIR diodes)	Adjuvant for SRP; power approx. 1–1.5 W, sweeping movement	Improving clinical metrics and pathogen reduction with multi-session protocols; healing support.	A single application may not improve the microbial composition and may even temporarily increase selected taxa; the effect on the microbiome tends to be inconsistent.
904–980 nm (NIR)	Main photothermal effect; critical parameters	Reduction in periopathogens and yeasts with selected parameters; potential clinical effect.	Excessive thermal effects can dysregulate local biofilms and the host; the lack of any standardized parameters promotes variable results.

5.9. Photosensitisation: Phototoxic and Photoallergic Reactions

Another threat to the oral mucosa of patients as well as to the eyes and skin of the hands of operators is posed by possible phototoxic and photoallergic reactions caused by the accumulation of endo- and exogenous substances that absorb radiation in these tissues [1,29]. Photosensitization is a collective term defining reactions that depend on the presence of specific photochemical substances (photosensitizers) in tissues and the action of radiation in the range of 320–800 nm. The risk of phototoxic and photoallergic reactions is particularly important in the case of LCU polymerization lamp usage, because the resulting radiation may be absorbed by substances accumulated in the oral mucosa of patients and in the eye tissue and skin (hands) of operators. Many commonly used drugs are potential photosensitizers, including anticancer drugs, antidepressants, antibiotics, neuroleptics, diuretics, and oral contraceptives. Patients taking photosensitizing substances such as antimalarials, chlorpromazine, hypericin, dimethylchlortetracycline and 8-methoxypsoralen, are particularly vulnerable to retinal damage [1,67,68,69,70,71]. Likewise, dental materials, such as photoinitiators, leachables and degraded products of dental materials, as well as residues from oral hygiene products, can absorb radiation and contribute to photosensitivity reactions. In several clinical cases reported in the literature, erythematous reactions have been attributed to a combination of long-term antibiotic therapy (trimethoprim) and exposure to radiation from laboratory curing lamps. To prevent risk, lamp use should be avoided in patients with a history of photobiological reactions or in patients taking photosensitizing medications [1].

6. Hazards Associated with Laser Use

The use of lasers in dentistry has brought many benefits, such as coagulation, precision cutting, reduced pain during tissue preparation, and modern biostimulation therapies. On the other hand, they are also associated with various hazards due to their biological effects, incorrect usage, and the failure to apply appropriate protective measures [25,72].

Thermal hazards

Radiation from high-energy lasers such as Er:YAG, Nd:YAG can significantly increase the temperature within the enamel, dentin and pulp. A rise in the temperature of 5–7 °C can result in pulp necrosis and pulp inflammation, and impede or worsen healing. Histopathological tests have shown that laser emissions that are too intense may cause irreversible damage to the pulp and lead to microburns within the dentin [73]. Incorrect positioning and exposure time may also result in burns to the gums, tongue, mucous membrane, and periodontal tissue [25].

Mechanical and physical hazards

The use of air-cooled lasers during surgical procedures may result in the dangerous injection of air into soft tissues [74]. If the operator is inexperienced or selects the wrong laser parameters (power, pulse length, operating mode), it can result in excessive tissue ablation, charring, or microcracks [25,75].

Damage to dental materials

Lasers used near prosthetic restorations or close to teeth with fillings during whitening procedures may weaken the bond with the tooth surface, cause microcracks or result in surface degradation. This is mainly noticeable when using CO₂ and Nd:YAG lasers. Composites, compomers and dental ceramics may be damaged [76,77].

Damage to eyesight

Laser radiation may damage the cornea, lens, or retina of the eye. This is mainly the case in the infrared and ultraviolet ranges. Not only can direct radiation lead to temporary flash blindness, permanent damage to the macula, and corneal burns, but so can wave reflection from the metal surface of tools or wet surfaces. Consequently, not is the operator required to wear protective glasses appropriate for a given wavelength, but so is the patient and the assistant [78,79].

Inhalation hazards—laser smoke (plume)

One phenomenon that occurs during the ablation of soft or hard tissues is ‘laser smoke’. This is an aerosol containing biological particles such as viruses and bacteria, toxic compounds such as formaldehyde or phenols, and microparticles of metals and dental materials. Inhalation of laser smoke may irritate the mucosa from the throat, nose, and eyes, as well as trigger allergic reactions. There is also an increased risk of infection. To protect against the inhalation hazard, local ventilation, high-power surgical suction devices, and FFP2/FFP3 protective masks should be used [25,72].

Risks associated with an operator’s lack of competence

An operator who has received insufficient training in using a laser device poses a risk factor. The most common mistakes made by such operators include applying too much power, failing to ensure cooling or suction of the laser smoke, selecting the wrong wavelength for the tissue, and improper work performed around anatomical structures, for example around vessels or sinuses [25]. Many countries require laser operators to complete additional specialized certification courses.

7. The Role of the Doctor in the Selection of Radiation

The lamp is a medical device. The doctor is obliged to verify the validity of the lamp as a medical instrument, examine the certification label, and read the operating instructions. It is important that the emission spectrum of the LCU lamp is compatible with the absorption spectrum of the photoinitiators used in light-cured materials [40]. The physician’s responsibilities also include checking the output power of the device. The tip of the lamp should be used appropriately, held perpendicular to the material and the appropriate disinfectants applied [4], because some of these may reduce light transmission through the optical fibres and degrade the plastic body of the device. However, repeated sterilization of the optical fibres in an autoclave may result in diminishing the light’s power. An important technical parameter is the diameter of the active optical tip [15,38], which should have a size in the range of 9–11 mm+. For example, when the diameter of the active tip is reduced, the light-emitting surface is also reduced, and more radiation is emitted (increased radiation intensity), even though there has been no change in the radiation power. Changes in the diameter of the active LCU tip affect the output radiation value. Exceeding the current output radiation standard (ISO 10650 standard) results in uneven hardening of the resin and possible damage to soft tissues [15,80]. Hence, the physician is required to have technical knowledge of LED-LCU devices in order to be able to effectively select the appropriate device for a given clinical task. There is a risk of adverse events resulting from these devices, including possible damage to pulp tissues [17].

LCUs are subject to the standards set by ISO 10650:2018 and ISO 4049:2019 [80,81]. Physicians should expect manufacturers to provide precise specifications of the devices they describe, including the emitted radiation power and radiant exitance. In accordance with the recommendations of the FDI (Fédération Dentaire Internationale, World Dental Federation), dentists are obliged to check the compliance of the wavelengths emitted by the LCU with the requirements of the material manufacturer, as well as, in accordance with the manufacturer’s recommendations, to apply the appropriate thickness of the material layer and to comply with the recommended exposure time. According to these guidelines, the average LCU radiation density should be 500–2000 mW/cm². If values are below this level, there is a risk of insufficient hardening of the material, while higher values may result in overheating and damage to soft tissues. It is essential to keep the LCU clean and to periodically check its performance [33]. Dental staff exposed to light from polymerization lamps should be monitored for the occurrence of any retinal pathologies [82]. Although the blue light used in LED-LCU is essential to the process, it poses a real hazard if appropriate procedures are not followed, including regarding proper work technique, device inspection and the use of special filters. Lasers are a valuable tool for dentistry. They ensure a high degree of accuracy, patient comfort, and faster wound healing. However, there is a risk of serious clinical consequences if the device is used incorrectly. To be able to work safely with lasers, operator requires theoretical knowledge and practical experience, must comply with occupational health and safety rules and should take the necessary protective measures, as should their assistants and patients. For this reason, lasers should only be used by trained operators after careful planning and risk analysis.

7.1. Recommendations for Dentists Regarding the Use of Devices for Material Polymerization

• The operation of an LCU device should be monitored, its properties should be well understood, the exposure mode should be adapted to a specific composite material, the device’s operating instructions should be read [32], and the technical specifications of the device should be familiar [17].
• Inspect the device, including the lamp tip, for any damage, remove remnants of any previously hardened materials, and check the exposure parameters [32].
• The exposure time should not be longer than necessary [29].
• The oral mucosa should be protected from light, the LCU tip should be positioned directly above the filling surface [32], and direct light onto the mucosa should be avoided [37].
• Infection control and tip hygiene control should be ensured [32,37].
• Lamp operation should be monitored [37].
• There should be increased awareness of the need for regular inspections of LED-LCUs [17].
• To minimize temperature increases, breaks of several seconds between material exposure cycles should be ensured [31].
• Eyes and skin should be protected by limiting exposure to blue light— breaks from intense LED light should be taken, and blue light filters should be used [34].
• Flickering should be prevented by using LED lamps with low or eliminated flicker and glare (glare) by avoiding direct light into the eyes, adjusting the direction of the light, and using appropriate fixtures [34].
• Staff should be supervised and instructed by a physician regarding the rules for the operation of polymerization lamps, as well as provided necessary eye protection and training in the proper use of an LCU [33].
• The eyes of all people in the dental office should be protected by protective orange safety glasses (including staff and patients) [32].

7.2. Recommendations for the Laser Operator

Minimize the risks associated with the use of lasers:

• Be aware of all people in the laser field [78,79].
• If possible, due to the lower risk of overheating, it is recommended that the operators work in pulse mode.
• Select parameters depending on the type of tissue involved and the purpose of the procedure [26,75].
• Avoid long-term irradiation of the same place [73].
• Use active cooling (water, air) [74].
• Use smoke extraction systems and appropriate masks [25,72].
• Undergo regular training in the use of a given type of laser.

8. Conclusions

This review synthesizes the evolution of light-emitting technologies in dentistry—from 1970s ultraviolet (UV) systems to contemporary fifth-generation LED light-curing units (LCUs) and specialized lasers. It integrates technical emission parameters with their multidimensional biological impacts, specifically emphasizing the often-overlooked effects on the oral microbiome, alongside risks to the visual system and dental pulp. By bridging the gap between physics (wavelengths, irradiance) and clinical biology, this work highlights key considerations for developing protocols in modern practice. While modern high-intensity devices exceeding 3000 mW/cm² enhance clinical efficiency, they may carry significant biological risks, including the blue-light hazard where exposure to high-energy visible light (approximately 440 nm) can potentially induce oxidative stress and accelerate macular degeneration. Furthermore, thermal dynamics are a critical concern, as radiation can increase the temperature in the pulp chamber, potentially leading to pulp-dentin complex damage or necrosis. Ensuring clinical safety requires operators to possess deep technical knowledge to match the lamp’s emission spectrum with the absorption profiles of specific photoinitiators, such as camphorquinone or TPO, while strictly adhering to ISO 10650 and ISO 4049 standards. Essential protective measures include the consistent use of orange-filtered eyewear for both staff and patients and thorough screening for photosensitizing medications, which significantly lower the threshold for tissue damage. In the microbiological context, while specific wavelengths like 405 nm effectively target pathogens, they may also induce dysbiosis by eliminating beneficial commensal microorganisms like Streptococcus salivarius. Future research should prioritize evaluating the long-term cumulative effects of radiation on dental staff and the safety of emerging “wavelength scanning” and “quadwave” systems. Finally, there is a clear need for closer collaboration between academia, clinicians, and industry to develop standardized, evidence-based, and universally applicable safety protocols for light-based technologies in dentistry. Establishing such guidelines will be critical to ensuring both clinical efficacy and long-term biological safety for patients and dental professionals.