Theoretical Basis And Potential Of Far UV Applications In Disinfection

The International Ultraviolet Association (IUVA) has published a white paper on far-ultraviolet disinfection, concluding that “far-ultraviolet is a promising technology“. It shows that far UV disinfection is positively viewed and expected technology in the field of UV, but there are many unresolved and as yet inconclusive issues.

This paper investigates, summarizes, and analyzes the theoretical development of far UV-C disinfection, human friendliness, and ozone generation, as well as various results and perspectives in research and application at this stage; clarifies the known, unknown and ambiguous points about far UV-C at this stage; and provides an outlook on the research and development of far UV technology and its future development direction from academic and technical perspectives.

Let’s dive right in now, and you can click on the question that interest you, 

Introduction

1 Far UV wavelength range

2 Far UV disinfection mechanism characteristics

3 About far UV photons friendly to the human body

4 Far UV light source

5 Ozone issues

6 Some points of contention about far UV disinfection

7 Summary & Outlook

Introduction

Far UV-C (far UV) is UV-C in the wavelength range of 200 to 230 nm. Based on the strong absorption of far UV-C by proteins, etc., a team of researchers from Columbia University, USA, has published a series of research papers on far UV disinfection and human friendliness starting from 2013. Many other scholars have also conducted research in this area, as shown in Tables 1 and 2. This UV disinfection technology has received a high level of attention with the outbreak and continuation of the COVID-19 outbreak. In the early stages of the outbreak, both pharmaceutical disinfection and UV disinfection were used in large doses for emergency purposes. Pharmaceutical disinfection is the act of chemicals entering the environment and must be carefully examined from the perspective of environmental protection if used in large quantities over a long period of time. With the normalization of anti-epidemic, and even if the COVID-19 is completely defeated, early prevention of other and next new viruses or disease-causing microorganisms such as superbugs is necessary. The environmentally friendly nature of UV disinfection is its strength, but the unfriendly nature of UV disinfection (harming human skin and eyes) is its weakness, limiting its use in public places where disinfection is most needed. As a result, far UV-C technology, which can disinfect without harming humans, has become a hot technology of interest. The International Ultraviolet Association (IUVA) discouraged the use of far UV-C light for direct human exposure during the initial phase of the COVID-19 epidemic, given the inadequate data on the safety of far UV-C light. The IUVA recently published a white paper on far UV-C, which describes and discusses in detail the disinfection effects and safety of far UV-C.

The white paper cautiously favors “human-friendly” far UV-C light. The research and development of human-friendly far UV-C light are less than 10 years old, and there are only a limited number of studies, as shown in Tables 1 and 2. Only four studies of human safety have been conducted in human subjects, two of which were medical applications for wound disinfection and two of which were direct exposures to human skin. In the latter two, the results showed one safe and one unsafe article each. Two far-violet external ray-related papers were published in Scientific Reports (Nature Publishing Group). Reports (a publication of Nature Publishing Group), both in Table 2 In both cases, the study was on the disinfection effect of far UV-C light, not on the safety of the disinfection process. The two papers published in Scientific Reports (a publication of the Nature Group), both in Table 2, were about the disinfection effect of far UV-C light, not the safety of the disinfection process for humans.

2 Far UV wavelength range

The International Organization of Standardization (ISO) defines far UV-C as 122 to 200 nm. The International Commission on illumination (CIE) considers that the wavelength range of far UV is related to the application, but CIE does not define the wavelength range of far UV specifically. The International Ultraviolet Association (IUVA) published a white paper on far UV, which defines 200 to 230 nm (Far UV). It is clear that the ISO definition of far UV(122-200 nm) is not based on “friendly to humans, producing no or little ozone”. This is because ozone is produced at 122-200 nm. For example, low-pressure mercury lamps containing 185 nm UV light are commonly known as ozone-producing UV lamps and produce some ozone. Some ozone is produced. Therefore, the current term “far-UV” is a vague concept without a clear wavelength range. For example, some scholars use the term “far-UV” directly. For example, some scholars simply use the term “around 200 nm”. The definition of far-UV by the International UV Society, a professional association in the field of UV expertise, appears in a white paper on the topic of far UV disinfection.

1 Far UV disinfection mechanism characteristics

When nucleic acids absorb UV photons, dimers are formed inside them. In the case of viruses or bacteria, the formation of dimers causes the cells to fail to reproduce and be inactivated. For human cells, the same mechanism can cause damage. If this inactivation or damage is not very deep, it is possible to repair it. Otherwise, the cell containing the corresponding nucleic acid will die. Far-UV light is absorbed not only by nucleic acids but also by proteins, and the absorbance in proteins is tens of times higher than in nucleic acids, see Figure 1. Therefore, it is likely that far-UV light disinfection has both pathways of disinfection mechanisms that destroy nucleic acids and proteins. Proteins are organic macromolecules that are impossible to repair once they are destroyed. It has the potential to solve the problem of photo-resurrection that plagues UV disinfection. Because of its ability to attack both nucleic acids and proteins, it is generally considered to have comparable or greater disinfection capacity than conventional 254 nm UV. A study comparing the infectivity and nucleic acid damage to adenovirus 2 were reported to be comparable near 254 nm UV, but the infectivity was much more sensitive than the nucleic acid damage near 222 nm. Kang et al. showed a 1 + 1 > 2 synergistic effect when 222 nm and 254 nm were applied simultaneously. This may be related to protein destruction. UV disinfection experiments conducted for the COVID-2019 showed good disinfection at 222 nm.

3 About far UV photons friendly to the human body

1) Theoretical basis

According to the first law of photochemistry, only photons that are absorbed can induce photochemical reactions. Before a photon can reach a cell, it must travel a certain distance, i.e. through some medium. The higher the photon energy, the more reactive it is and thus the shorter the distance it has to travel through the medium. Compared to conventional 254 nm UV disinfection, far-UV photons are more energetic and more likely to decay in the medium. In the outermost stratum corneum of human skin, the energy of far-UV light is halved at a distance of 0.3 mm. This stratum corneum, which is 5 to 20 mm thick, forms a protective film that prevents far-UV light from entering the body. Between the human eye and the air, there is a corneal layer, about 500 mm thick, through which far-UV rays cannot pass and reach the eye. Bacteria and viruses are usually smaller than 1 μm in geometry, at the nanometer level, so far-UV light can still disinfect. Therefore, theoretically, far UV does not cause harm to humans, but can still inactivate viruses or pathogenic microorganisms with sizes in the micron or nanometer range. Currently, scientists are careful and rigorous in the terminology used to describe human-friendly, e.g.: “no apparent harm”, “safer”, “human-friendly” as used in this paper “.

2)Experimental data

A) Effects of far-UV light on the skin, etc.

A number of studies have reported that no damage was observed. However, only four of these studies were conducted with humans, the rest were experimental studies with rats or non-real skin. Of these human experiments, two were medical applications of wound disinfection. Substantially only two were studies with direct irradiation of human skin, one reported as safe and one reported as unsafe.
Ponnaiya et al. showed that 222 nm far-UV was as effective as 254 nm UV in killing methicillin-resistant Staphylococcus aureus (MRSA) on skin wounds, but 222 nm far-UV was not harmful to the skin. Another experiment by Narita et al. showed that 222 nm far-UV was as effective as 254 nm UV in killing methicillin-resistant Staphylococcus aureus (MRSA) on rat skin wounds, but 222 nm far-UV was not harmful to the skin. The other experiment by Narita et al. indicates that long Kang et al. studied the disinfection of seeds with 222 nm far-UV light and succeeded in disinfecting pathogenic bacteria on the seed surface as well as 254 nm UV light, but 222 nm far-UV light did not harm the seeds, while 254 nm did. For more studies, see Tables 1 and 2.

B) Effects of far-UV light on the eye

Kaidzu et al. showed no damage to the cornea at radiation doses as high as 600 mJ/cm2 in a study with rats, and Yamano et al. conducted an experimental study using a Kr-Cl excimer lamp with a grating that strictly controlled the transmission of UV light at wavelengths of 200 to 230 nm. The radiation parameters were: 1 mW/cm2 at 30 cm, and for the eye, in vitro experiments showed that less than 0. 001% of the 222 nm UV light could pass through the artificial corneal tissue of the eye.

3) Existing toxicity weighting functions and safety thresholds for radiation

Several international organizations (IEC, IES, EU, ICN IRP, ACG IH) have established their own wavelength weighting functions for toxicity, and the values adopted are the same. ICN IRP and ACG IH have also established thresholds for the maximum acceptable radiation to the human body, and both have the same threshold values. The data are shown in Table 3.

From the perspective of far-UV applications for disinfection, these data need to be viewed with scientific caution and the following points need to be considered together:

• Logically, by setting a threshold value, it is assumed that there may be harm to humans. On the other hand, in addition to UV, these institutions or organizations also have control thresholds for visible light;
• These thresholds are mainly for occupational sites;
• The ACG IH describes the meaning of thresholds as not being a precise dividing line between safety and hazard;
• In its publication on thresholds in 2021, ACG IH proposes a revision (and only a revision) of the threshold values that have remained unchanged for 30 years: the current threshold value of 23 mJ/cm² at 222 nm (without distinction between eye and skin) is revised to 161 mJ/cm² (eye) and 479 mJ/cm²(skin). As a serious occupational safety organization, ACG IH should have a theoretical and experimental basis for making such a substantial correction (although the author has not seen the information yet), and the values are moving in the direction of far-UV friendly to humans. There have also been published papers on the topic, and in view of the COVID-19 epidemic considerations, the thresholds set by some international organizations and agencies have been discussed and relaxation is recommended.

4 Far UV light source

Far-UV light sources are mainly based on dielectric blocking discharges of quasi The history of quasi molecular lamps dates back to the 1960s. Excimer refers to two molecules that cannot or can only weakly bound chemically in the ground state but are bonded together chemically in the excited state. Currently, research and application of far-ultraviolet light sources for disinfection are focused on the krypton-chloride excimer lamp, in which the excited states of krypton and chlorine combine to form excimer molecules. It emits three main wavelengths: 200 nm Cl₂＊＊, 222 nm KrCl*, and 257 nm Cl₂, see Figure 2.

The rest of the wavelengths (about 15% of the total energy radiation) can harm people (257 nm) or produce more ozone (200 nm). Therefore, far-UV disinfection lamps must be equipped with a grating to block the emission of wavelengths other than 222 nm. Structurally, the excimer lamp differs from the mercury lamp in that the electrodes are external, see Figure 3, and the external electrodes generate a corona discharge that produces ozone.Therefore, far-UV light sources used for disinfection require measures to prevent the escape of ozone.

5 Ozone issues

1) Ozone concentration thresholds for human exposure

Many governmental and non-governmental organizations have set maximum ozone concentration thresholds for human exposure, see Table 4. GB 28235-2020 “Sanitary Requirements for Ultraviolet Disinfectors” is the most stringent, with a threshold of 0. 05 ppm. It is generally accepted that 0. 1 ppm is detectable to humans. Therefore, for the control of ozone concentration, it cannot rely on human smell.

2) Ozone sources for far UV light sources

A) Ozone from corona discharge

Structurally, the excimer lamp as a far-UV source has a high The high-pressure discharge part of the excimer lamp is external and generates ozone around the lamp by corona discharge. Ozone is generated around the lamp by corona discharge. The ozone concentration is related to the power and structure of the light source. In theory, Theoretically, new technologies have been developed to contain this ozone within the structure of the light source or within the disinfection reactor. It is theoretically feasible to develop new technologies to control this ozone inside the structure of the light source, or inside the disinfection reactor.

B) Ozone generated by far UV photons

1) Photochemical mechanism

According to the third law of photochemistry, it is possible (not always possible) to break a chemical bond in a molecule only when the

energy of the photon is greater than or equal to the bond energy of the bond. The bond energy of the O=O double bond of oxygen molecule is 498 kJ/mol, which corresponds to the ultraviolet photon of 240 nm wavelength according to the energy formula of photon.The wavelength of far-ultraviolet light is less than 240 nm, and from the energy point of view, far-ultraviolet light has the theoretical necessary conditions for ozone production. UV photons acting on oxygen molecules can produce ozone, and ozone will be decomposed by the action of photons. This is a cyclic photochemical process :

O2 + hγ → O + O
O + O2 + M → O3 + M
O3 + hγ → O + O2
O + O3 → O2 + O2
O + O + M → O2 + M

B) Research reports/experimental data

Two well-known manufacturers of far-UV lamps have provided measurements of ozone production rates: 0. 0094, 0. 0011, and 0. 17 g O3 /kW-h. The source documents for the data do not describe the test methods. Since there is no standardized method for ozone production from far-UV lamps abroad, the large differences between these data should be due to differences in test methods and test environments. It is likely that the difference lies in the control of the ozone generated by the corona discharge. GB 19258-2012 “UVC Germicidal Lamps” specifies both the test method for ozone and the threshold for ozone production rate, which is 0. 05 g O3 /kW-h. Welch et al. conducted a study on far-UV air disinfection Welch et al. conducted a study on far-ultraviolet air disinfection and measured the ozone concentration in the experimental environment in order to exclude errors caused by ozone participation in disinfection. This test result may be closer to the actual application of <0. 005 ppm ozone. This test result may be closer to the actual application environment. The measurement result of < 0.005 ppm is much lower than the threshold values specified in the four national standards (one of which has been discontinued) in China. The results are much lower than the thresholds set by the four national standards (one of which has been discontinued), see Table 4.

6 Some points of contention about far UV disinfection

In terms of disinfection effectiveness, quantitative comparisons with conventional 254 nm UV disinfection have been made, and it has been suggested that far-UV disinfection is superior to conventional 254 nm UV. The accuracy of this comparison is difficult to control, especially between different studies, due to different experimental conditions and methods of determining radiation dose. In theory, it is possible that far-UV is better than conventional 254 nm UV for disinfection because of its greater absorption not only in nucleic acids but also in proteins.

The vast majority of current studies report no harm to the skin or eyes from far-UV light, but there are isolated reports of harmful effects on human skin, see Table 1. Overall, the number of relevant studies is small and insufficient to give conclusive results, especially for the eyes. There are some reports of applied studies of far-UV disinfection in the medical field. However, there is a difference in the connotation of “human friendly” between the medical field, where “medicine is poisonous”, and the field, where high-quality work and living environments are pursued. This may also be the reason why there is no authoritative institution or organization to give a definite conclusion on this issue. It is the pursuit of professionals in the field of UV to promote the use of UV. Therefore, it is worthwhile to avoid unconsciousness (if there is any, it is beyond the scope of this paper to discuss scientific theory). Theoretical discussion) The psychological expectations of far-UV and the objective science of far-UV are placed in the same frame of mind. This is a subject of human health and safety that spans the multidisciplinary intersection of medicine, UV, and industrial safety. There is no reason to ignore information outside the field of UV. For example, the ACG IH and ICN IRP give a threshold value for maximum radiation dose, 23 mJ/cm2 (222 nm). In the promotion and dissemination of far-UV applications, acceptance or non-acceptance of this threshold value requires cooperation between the two fields of UV and industrial safety to give a sufficient basis for scientific validation.

On the issue of ozone, far-UV lamps have two sources of ozone generation: an external corona discharge mechanism, where far-UV light reacts with oxygen molecules in the air to produce ozone. In theory, the ozone produced by the corona discharge can be prevented from escaping through innovative new technology. If the leakage can be 100% prevented, theoretically “the light source does not release ozone” is valid. The ozone produced by the reaction between far-UV light and oxygen while entering the environment for disinfection is not produced directly by the light source, but by far-UV photons, and this part of ozone has nothing to do with the structure of the light source itself and cannot be avoided unless far-UV light is prevented from entering the environment.

7 Summary & Outlook

1) The concept of “far-UV” used in the field of disinfection is expected to have the connotation of “effective disinfection and human friendliness”. There is a lack of clear thresholds for the upper and lower wavelength ranges, but the 200-230 nm wavelength range used in the IUA white paper is a reasonable reference.

2) Current theories and studies on the human-friendliness of far-UV show great potential in the field of disinfection, but they are only the opinions of scholars, and the IUVA published a white paper on far-UV without endorsing the content (emphasis on the opinions of scholars, not the IUVA). It fully illustrates that the human-friendly characteristics of far-UV are highly valued in the international UV field, but are still in the academic and technical research stage.

3) Five international or U.S. agencies and organizations were searched that have published wavelength weighting functions and/or dose thresholds for radiation received by the human body, all covering far-UV. This is a category of information that cannot be ignored. While there are some voices emerging to relax the thresholds, they are all still in the proposal or discussion stage.

4) Far UV is very friendly to humans compared to the 254 nm UV disinfection that is generally used. At this stage, from the perspective of “low harm” instead of “high harm”, the use of far-UV instead of 254 nm UV for disinfection in certain locations where the leakage is likely to occur and human exposure time (disinfection targets are environmental, not human) is short, provided that appropriate control measures are taken for ozone. The use of far UV instead of 254 nm UV for disinfection is meaningful.

5) Whether the mechanism of far-UV disinfection is the destruction of nucleic acids or proteins or both. It is important to study this in-depth. If the latter is confirmed, far-UV will not only be friendly to humans but will also open up a new field in solving the problem of UV disinfection light resurrection.

6) There is an international opinion (IUVA’s white paper) that it is incorrect to consider the possibility of skin cancer in the application of far-UV light (it should be repeated that IUVA’s publication emphasizes that the content is the opinion of the authors and not the opinion of IUVA). The authors of this paper are cautious about this view of the white paper because a) the current evidence is a scattering of scholarly publications of research findings that have not been supported or rejected by authoritative bodies or organizations; IUVA has published a white paper on the subject but is unwilling to endorse the content, and b) carcinogenicity is strictly a medical specialty-based subject. UV scholars only collect and cite relevant information. It is expected that the medical field will give sufficient experimental or clinical objective evidence, either positive or negative. The author’s outlook: Since it is far more difficult to prove “nothing” scientifically than to prove “something”, whether it is harmless to humans will remain inconclusive for a long time. Perhaps there is a threshold value, which may be large, but is less likely to be infinite.

7) When developing far-UV disinfection equipment, there are four factors that must be considered: a) Krypton-chlorine excimer lamps emit radiation over a wide wavelength range and must be matched with gratings to obtain the narrow wave peak centered at 222 nm that is needed; b) The ozone produced by the corona discharge must be strictly controlled and not exceeded; c) Since far-UV photons react with oxygen in the air to produce small amounts of ozone, the disinfection equipment/ The power of the lamp in the system, the installation environment, ventilation, safety, and other factors are very important. End-users do not have the relevant knowledge or testing equipment. The relevant manufacturers should take the responsibility to solve the ozone problem at the time of application; d) Be careful in dealing with the issue of far UV light being friendly to the human body, and the relevant manufacturers should guide the users scientifically

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