1) 254 nm is the same part of the spectrum that the ozone layer protects us from, and also the absorption peak for DNA. IIRC non-aquatic life didn’t take off until the ozone layer formed, probably for that reason. UV does bad things to DNA, and I wouldn’t bet on dead skin providing adequate protection from large exposures. The DNA spectrum has another peak around 212 nm (although the dropoff at low energy may be from the limitations of the optics rather than from lower absorption by the molecule).
2) The absorption spectrum for melanin shows its strongest peak between 270 and 220 nm. I’d say it depends on whether the dropoff below 220 nm was due to the absorption characteristics of the molecule or was an artifact of the measurement (spectrometer, cuvette, solvent, etc.). I’m inclined to guess artifact; such a sharp dropoff toward higher energy doesn’t make physical sense (unless I’m missing something).
As I said, Far-UVC from 200-220nm is supposed to be safe to humans because of our layer of dead skin. This is kind of the whole point of the post and is in the references.
254nm has less energy per photon but it penetrates further through skin, meaning that 254nm is definitely dangerous.
I was curious about how much we could rely on that safety, and it turns out there are threshold limit values (see the sixth slide) for UV-C. Between 200 and 220 nm the TLVs are .02 to .08 J/cm^2 (200 to 800 J/m^2), according to the American Conference of Governmental Industrial Hygienists. At 5W/m^2 (your suggested irradiation) that gives you 40 to 160 seconds of reasonably safe human exposure.
The idea of Far-UVC is that it is supposed to be safe due to the dead skin layer. Until we know for sure it’s difficult to make the tradeoffs.
re (2), I suspect that melanin is not effective against UVC.
re (3), it’s an interesting line of thought, but my suspicion is no.
1) 254 nm is the same part of the spectrum that the ozone layer protects us from, and also the absorption peak for DNA. IIRC non-aquatic life didn’t take off until the ozone layer formed, probably for that reason. UV does bad things to DNA, and I wouldn’t bet on dead skin providing adequate protection from large exposures. The DNA spectrum has another peak around 212 nm (although the dropoff at low energy may be from the limitations of the optics rather than from lower absorption by the molecule).
2) The absorption spectrum for melanin shows its strongest peak between 270 and 220 nm. I’d say it depends on whether the dropoff below 220 nm was due to the absorption characteristics of the molecule or was an artifact of the measurement (spectrometer, cuvette, solvent, etc.). I’m inclined to guess artifact; such a sharp dropoff toward higher energy doesn’t make physical sense (unless I’m missing something).
As I said, Far-UVC from 200-220nm is supposed to be safe to humans because of our layer of dead skin. This is kind of the whole point of the post and is in the references.
254nm has less energy per photon but it penetrates further through skin, meaning that 254nm is definitely dangerous.
I was curious about how much we could rely on that safety, and it turns out there are threshold limit values (see the sixth slide) for UV-C. Between 200 and 220 nm the TLVs are .02 to .08 J/cm^2 (200 to 800 J/m^2), according to the American Conference of Governmental Industrial Hygienists. At 5W/m^2 (your suggested irradiation) that gives you 40 to 160 seconds of reasonably safe human exposure.
Apparently those slides contradict the studies cited in my article.
I don’t know which to believe. The rational action is to urgently run more experiments to assess the risk from 200nm-220nm band.
It would be really great if those slides had a references section.