Comment by dskhatri
1 month ago
I am not convinced a single lamp placed over the dining table would provide sufficient irradiance to inactivate viruses and other pathogens in the large room. Consider having an infected individual sneeze or cough at the table. The expiration jet would spread so rapidly. This lamp is like a tiny flashlight placed above the dining table. Furthermore, far UV gets absorbed by oxygen in the air. The net irradiance of far uv is worse than 254 nm at a fixed distance from two equivalent (in power and form) sources.
The lamp would be effective if you were able to quickly circulate the air in the room past the lamp.
Yeah, what responsibly deployed far-UV is definitely not is an instant kill beam that immediately inactivates exhaled pathogens. I estimate it takes 100 mW of far-UV about 5 minutes to inactivate 90% of the flu/covid in a 250 sqft room (10 minutes to get to 99%), assuming the air is well-mixed. Far-UV should be thought of as a super-powerful quiet air filter more than anything else--it prevents infectious aerosols from building up in a space, which can matter a lot. You don't necessarily immediately get infected just because someone infected coughed near you--you might only get infected if you spend a long time near them, sharing their air. Continuously cleaning the air prevents these kinds of infections, and can make the infections that do still happen less severe.
You're totally right that circulating the air is important though! Definitely don't use far-UV in totally still air. But you don't necessarily need a LOT of air movement to achieve "good" mixing. Often just an air-change or so of ventilation is enough--just think about what would happen if you started smoking indoors. The smoke would be ~everywhere pretty fast.
The oxygen absorption at 222nm is not significant however--it's enough to produce a bit of ozone (less than you'd get from just opening a window), but not enough to actually absorb a significant amount of radiation. 222nm lamps are generally less powerful than 254nm lamps anyway, but because 254nm is much less safe, those systems have to be deployed in ways that often reduce their efficacy--in special louvred fixtures that kill most of their output or in portable units/ducts that constrain their effectiveness.
222nm seems to be more effective per-photon at inactivating viruses than 254nm too: https://pubs.acs.org/doi/10.1021/acs.est.3c08675