That is a thing that's done. The idea here though is that UV lights are a much cheaper retrofit than HVAC systems, so they could be a huge improvement in places that don't have good HVAC.

I didn't think of that. This seems like a very good idea. Put the lights where the people aren't, but where the pathogens will be circulating.

That’s what I did a few years ago. Drill a hole in the side of the air handler and mount it in there. https://mrscrappy.com/products/uv-c-air-sanitizer

I wonder if it would be possible to have a small, battery powered device that could gently blow sterile air back at you for a couple of hours. Or maybe something you could wear around your neck?

This is already a thing, and many companies and government agencies have been spending time and money on it for years. It is, indeed, a viable approach.

the interaction time between the UV and virus particles is too short to deactivate them.