Answer to Question #9450 Submitted to "Ask the Experts"

I have been studying the effect of UVC (ultraviolet C) on humans. There are two different explanations for the effects on humans. An article on the Health Physics Society website states that UVC cannot penetrate human dead skin. But, on the other hand, there are a few sources that mention that UVC is the most dangerous wavelength that can cause skin cancer. Please explain further or provide any theory/calculation why UVC cannot penetrate human skin even though the energy is stronger than UVB and UVA.

The shorter-wavelength, higher-energy UV radiation associated with the UVC category is very strongly absorbed by most organic materials. This is why the common 254 nm sources have become so popular for germicidal/disinfectant applications. The strong absorption by organic molecules, including DNA, leads to severe damage to the molecule and to the organism's reproductive processes, leading to the death of the microbes.

The UVC radiation is sufficiently energetic that individual photons may produce chemical bond breakage and ionization of some atoms and molecules. The preferential absorption of particular energy photons by materials, both organic and inorganic, is evident throughout the electromagnetic spectrum from microwaves through infrared and visible light, ultraviolet, x rays, and gamma rays. The absorption at particular wavelengths may be associated with resonance-type effects in which the gaps between certain energy states in an atom or molecule are nearly matched by the energies of the incoming photons. Atomic or molecular excitation may occur as a result of the absorption, or an electron may be ejected from an atom when the incoming photon energy exceeds the binding energy of the electron in the atom. It is common that photon absorption by particular atoms or molecules may be small at a given energy, increase with increasing energy, and then decrease again at yet higher energies, so it is not surprising that some higher-energy UV radiation may be more strongly absorbed than lower-energy UV.

To my knowledge, UVC radiation has not been shown to be a cause of human skin cancer or other types of human cancer, although some experiments with mice have demonstrated a causative link between squamous cell cancer and UVC irradiation. It is true that the dead skin on most of the human body is sufficient to absorb UVC radiation almost completely. In areas of reduced dead-skin thickness and intense radiation, however, UVC radiation might reach live skin tissue and could produce erythema and possibly other undocumented effects. This would be true especially in an abraded area of skin or a region of a significant flesh wound.

Because UVC is strongly attenuated by atmospheric gases, no significant irradiation of human beings on earth results from natural sources. With increasing germicidal uses of man-made UVC radiation, perhaps especially in hospital settings where there is a strong concern about the spread of harmful bacteria, it is likely that more human exposures will occur, and future studies may show some links with disease that we have not so far identified. As has been recognized for some time, UVC radiation can damage the superficial tissues of the eye, and care must be taken to avoid excessive exposures of the eye. We should note, though, that while eye exposures to UVC may cause extreme discomfort, the symptoms usually subside within a rather short time, and no evidence of any malignant effects has ever been noted.

Good luck in your continuing studies.

George Chabot, PhD, CHP