To be clear, those two units are different. 155.8 mW/cm^2, and 0.1 mW/cm^2/nm. So it's not 1,558 times stronger than sunlight overall. Neglecting atmospheric effects, sunlight across all wavelengths is 135 mW/cm^2, so about the same total flux---it's like we compressed the broad spectrum of sunlight to all be a single wavelength. My point is that it's much much more blue-violet light (not much much more light total) than what we usually encounter. Blue-violet from sunlight may be safe for our skin, but that doesn't necessarily imply that blue-violet light at germicidal intensity is safe.

Unless the novel coronavirus has porphyrins in its chemistry, I wouldn't expect to this effect on the virus, as the paper notes.

This is an important point. Germicidal blue-violet would likely be ineffective against airborne viruses.

The reported data indicate that the effect is dependent on other elements of the mixture; something else in the solution may break down under the flux and attack the virus.

This is exactly my concern. What's to say that the reactive chemical species that "attack" the virus won't also be a problem for human cells? The mixture in which blue-violet light is more effective was made of "artificial saliva, artificial faeces and blood plasma," so whatever is breaking down could break down exactly the same way in our bodies.

(To be clear, I'm not saying it's necessarily unsafe for human skin. I'm just saying we shouldn't assume it is safe.)

Right, I will readily believe that for many people with good glove hygiene, it is better to wear gloves than not. But if we're talking about irradiation that encourages everybody to cover up, that's a different question, especially since many people don't own single-use gloves. Would universal use of gloves make spread better or worse? It seems like most experts say worse.

Also, on the question of whether gloves prevent a person from touching their face, it probably can be helpful for some people, but that doesn't mean that it's helpful overall. From the same NPR piece:

But perhaps wearing gloves would keep you from touching your face? Nope, says Landon. "We see people touch their face all the time with gloves on in the hospital."

Adalja concurs. People wearing gloves still want to itch their faces, still want to adjust their glasses. "I know for one that when I'm wearing gloves, I do have a tendency to try to touch my face," he says.

The germicidal flux from the paper is 155.8 mW/cm^2, which is much stronger than sunlight which is about 0.1 mW/cm^2/nm at sea level for short wavelength visible light. I don't know what your threshold for alarming is, but to me that seems high enough that concern is warranted. And I'm not sure that conventional sunscreen would actually help. Based on this overview of sunscreen, it looks like most broad spectrum sunscreens don't do much above 380 nm or so.

Also, It doesn't seem clear to me that the fact that blue light is visible implies it's not harmful. (UV is harmful, and some species of bird and rodent can see ultraviolet light, as can some humans.) What's your reasoning? It is definitively known that UVA light is carcinogenic by an indirect oxidative stress mechanism, and I can't find any discussion of at what wavelength exactly this mechanism stops being relevant. Because the mechanism for germicidal action for 405 nm light (described in the linked paper) is also oxidative stress, it seems very likely that 405 nm light is carcinogenic for exactly the same reason as UVA, although possibly less so.

It's germicidal by an oxidative stress mechanism, so we should expect it to be carcinogenic (via the indirect DNA damage mechanism), at least at germicidal intensities (normal everyday exposure may be harmless to the skin, I don't know).

Wow, I would not have guessed that light within the visible spectrum would have any germicidal action at all (looks like 405 nm light is germicidal at fluences about 10,000 times as high as germidal UVC).

This brings up an important point which is that sunlight (UVA, UVB, and apparently visible blue-violet too) is somewhat germicidal, and open air treatment during the Spanish Flu had positive patient outcomes, as well as less spread, with nurses and doctors less likely to become infected.

This is exactly the sort of concern that needs to be more rigorously addressed before a solution like widespread far-UVC is implemented.

Agreed that some sort of prioritization of the public spaces most in need of sanitization would be good. The threshold for safety should be lower in cases where the potential benefit is high (and I didn't mention this in the post because it's highly speculative, but I suspect that shorter term occasional exposure may be much safer than constant exposure). Making such a priority list should involve empirical evidence. I agree that airplanes seem like a likely culprit given what we know about mechanism of spread, but it would be good to confirm this intuition with actual infection data (maybe this has already been done?). Given the way some countries are closely tracking infections, a data-driven understanding of which types public spaces would benefit most seems very feasible.

Additionally, as you mentioned, germicidal UVC (far-UVC or the conventional spectrum) could be used in public spaces when no humans are present. Of course there would still be some risk, because there will inevitably be some degree of error when it comes to turning UVC lamps on and off, but if implemented carefully, such risk could be negligible.

Regarding the idea that cancer risk could be beneficial, this seems doubtful to me. I don't know much about gloves, but my understanding is that they don't protect much if at all from coronavirus. The virus doesn't pass through the skin, so there's no obvious reason to prefer to have it on gloves than directly on your hands---germ covered gloves are no better for preventing spread than germ covered hands. And skin as a surface has evolved to be pretty good at dispelling pathogens, so cleaning your hands is actually easier than cleaning gloves. From an NPR FAQ:

Using your ungloved hands – and then washing them often – is the best bet for the typical tasks of everyday life.

"Skin is really great because it's like a hydrophobic covering, which means it's kind of waxy and sort of repels moisture," explains Dr. Emily Landon, hospital epidemiologist and infectious diseases specialist at University of Chicago Medicine. "That means when you touch things, they sort of stay on your hands and then you can wash your hands and get rid of them."

Wearing gloves might cause you to practice worse hand hygiene because you keep wearing the now-dirty gloves instead of washing your hands, Landon says. Gloves are only useful when you use them the right way and in a meaningful way.

In hospitals, for example, gloves are used for certain purposes and disposed immediately afterward. "We put them on what we're going to do a task where we might touch something that's going to be wet or icky or contaminated," Landon says. "Then we take them off right away and leave them in the area with contamination and then clean our hands. Because it turns out more than 10-to-15% of the time, people who take off their gloves actually contaminate their hands with whatever was on the gloves."

Plus forcing people to be completely covered up to avoid skin cancer would be quite a burden, at least in some cultures and climates. I know I would have trouble finding comfortable clothes that would keep all of my skin safe from carcinogenic rays.

I spent a while looking into safety for this idea, and far-UVC has a ways to go before it's established safe enough for public settings---it's only been shown to be non-carcinogenic for one of two relevant DNA damage mechanisms. I've posted a more in depth discussion here.