I think understanding all three of the following papers (which are predecessors to the 2018 Nature paper) is important to guessing the efficacy and safety of the intervention. I’ll add to this comment as I gain more understanding.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3797730/
https://www.rrjournal.org/doi/pdf/10.1667/RR0010CC.1
EDIT: I found thinking about this very difficult, so I made progress very slowly and have mostly stopped. Here’s my current story.
The biophysical reason to believe that 207nm light preferentially harms viruses and bacteria over mammalian cells:
Mammalian cells are big, like >50 wavelengths long, while bacteria and viruses are <5 wavelengths long. 207nm light activates certain peptide bonds that are found in ~ all proteins. So energy transfers at a large constant rate, let’s call it lambda (I feel pretty confident about the argument if lambda > 10% per wavelength. I think it can be estimated from the papers but I just haven’t tried hard enough) from light to organic material. When you shine light at a virus, you get something like lambda * (1 - lambda)^3 fraction of the energy deposited in the squishy RNA bits. But when you shine it at a mammal cell, you get like lambda * (1 - lambda)^30 fraction of the energy in the squishy DNA bits.
Thing that’s empirically shown in these papers:
There is a dosage at which 207nm is clearly quite toxic (here toxicity means “causes cell death / virus inactivation”) to bacteria/viruses but is clearly not toxic to mammalian cells.
Thing that’s not empirically shown in any paper that I know of:
There is a power level of 207nm such that constant exposure keeps a space disinfected but which does not cause cancer in humans.
An incomplete argument for the above claim:
Cell-killing and cancer-causing from radiation are both caused by excess energy induced into the nucleus of mammal cells, in such a way that total risk scales linearly with the integral over time of power of radiation exposure.
The toxicity study yields an upper bound on how much radiation power makes it to the nucleus given a certain power of exposure to the cell.
The cancer literature yields “safe exposure” levels that can be cast in terms of “radiation power that reaches the nucleus”.
Combining 2 and 3 gives a safe exposure power level for 207nm lamps, and I conjecture (or maybe just hope?) that this level is greater than the level used in the toxicity study. That level of power is already shown effective at killing viruses and bacteria, and does so in a way that only depends on them being small and made of replicating nucleic acid.
One of Eliezer’s points is that most people’s judgements about adding 1e-5 odds (I assume you mean log odds and not additive probability?) are wrong, and even systematically have the wrong sign.