I don't see anything in your link about 222nm causing skin cancer? It's short enough that it shouldn't be getting through the outermost layer of skin, which is dead.
It cleaves DNA the same as UV-B, and there is no reason to think it isn't cancer causing. There are a number of areas that can get exposed that have very thin epidermis, or none at all (eyes), though eyes would get retinal keratosis not cancer.
That said, hopefully no one is spending enough time close to a high enough power UV-C source for this to REALLY be a problem.
Welding Cite - [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5640727/]
Notably, while having a higher skin cancer risk despite being inside a lot (shop welders, not shipyard welders), the vast majority of welders in that sampled population will have been wearing heavy protective clothing continuously. There was a noticeable increase in risk of skin cancer on the neck, which is one of the few areas that is not always adequately covered.
Anecdotally, I knew folks who didn't wear proper full coverage PPE when welding and welded a lot (auto body repair in one case, farm equipment repair in another), and both died in their early 40's from multiple malignant melanomas. One of them, it was 10+ all at once, and he died in less than a year. No one was surprised, unfortunately. They were ALWAYS sunburned from it, and they didn't spend a huge amount of time outside otherwise. That is a pretty broad spectrum source though.
UV-C As Potentially Mutagenic/Causing Damage not caught by normal replication suppression mechanisms - [https://pubmed.ncbi.nlm.nih.gov/9951833/]
UV-C will also have no problem converting all sorts of organic chemicals into interesting, and often more toxic versions (albeit killing any organisms relying on their original structure in the process), same as UV-B or UV-A.
Presumably what’s going on is that longer wavelength UVC from welding is extremely dangerous.
> UV-C As Potentially Mutagenic/Causing Damage not caught by normal replication suppression mechanisms - [https://pubmed.ncbi.nlm.nih.gov/9951833/]
p53-/- human keratinocyte cell line means that those were in vitro experiments which don't account for the wavelength-dependent penetration depth, which is the point of discussion here.
*Source: https://www.ehime-iinet.or.jp/ehime_e/corp/toyo/ronbun/image...
We do know a lot about how dangerous different frequencies are, and when you combine the "risk to humans" curve with the "emissions by wavelength" curve you get very low risk.
I do still think we should run additional experiments here before rolling this out broadly for hours-a-day usage, but from what we know so far it looks very good.
Here's an example of some of the research here, where they shined 222 nm vs 254 nm into rat eyes and looked for corneal damage. While 254 had some harmful effects 222 was fine: https://www.tandfonline.com/doi/abs/10.1080/10715762.2019.16...
Have an uncle who is / was (recently retired) a welder. He worked everywhere, did all sorts of welding, and even got certified to do underwater stuff -- which he did for 2 years and GTFO out of as soon as he made enough $$$$
He got skin cancer on his next and on his right elbow. Cuz that's where the gloves and other PPE clothing had gaps, esp. when he leaned certain ways or use his right hand to move the torch.