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 “.
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.