1. UVA/UVB is more efficient in terms of $/watt than 222nm KrCl*, but they're FAR less efficient in terms of germicidal dose than UVC (both 254nm and 222nm). You have to output a LOT of UVA/UVB to kill pathogens at anywhere near the same rate as UVC. I would consider UVA to basically be only "technically" germicidal--it's just not in the same league at all.

2. They're technically considered a cancer risk because UVC is considered "actinic" and so has been grandfathered into that definition, but it actually seems like commonly used UVC wavelengths generally don't penetrate deeply enough into skin to plausibly cause cancer--and you have to keep in mind the difference between 222nm and 254nm. There's definitely no evidence that 222nm is a cancer risk,** but I'm even suspicious of some of the older 254nm studies that report tumors in hairless mice exposed to 254nm. 254nm UVC is produced by low-pressure mercury lamps, and they're *almost* monochromatic, but not quite--they also produce low levels of UVB, which is a serious skin cancer risk even at very low levels (see figure 6 in https://www.tandfonline.com/doi/full/10.1080/10643389.2022.2...). My lab is currently trying to reproduce that result with a filtered/true-monochromatic 254nm lamp and we are just not seeing any tumors in those mice. Those poor bastards are being exposed to such high doses of 254nm that their skin looks super gross, cracked and bleeding, WAY higher doses than anything you'd encounter in a real-world installation--and they're still just not developing tumors. It looks like 254nm is "actinic" in that it's quite unpleasant to experience an overexposure, but it probably doesn't cause cancer, and 222nm seems to not be particularly actinic at all. I get overexposures all the time and basically never notice.

It's also definitely not true that KrCl lamps don't produce continuous illumination. That's exactly what they do!

However, it is true that a typical 222nm installation won't kill covid in 60 seconds. It only reduces it by about 40% in that time period--it'll take about 5 minutes to get to 90% reduction, and 10 minutes to get to 99% reduction. But that is WAY faster than basically any other tech that won't be blowing your hair back, including in-duct UV.

In-duct UV works well for some stuff but not really for person-to-person airborne disease transmission. If it's in the ducts, it's hard to maintain and verify, if the lamps get dusty they stop working (and ducts are super dusty), and anything that goes in the duct only works while your HVAC system is running--and your HVAC system just doesn't run very fast, or else it would be loud as hell. If you're going to put something in your ducts, it should be a MERV-13 filter. Way cheaper, way more maintainable. Then if you want extra air cleaning but don't want to use UV, get portable air filters. I vouch for the reviews at https://housefresh.com/

*although 254nm low pressure mercury lamps are also quite power efficient--about 40% WPE **to be fair, it hasn't been around long enough for long term studies to be done, so if that's your bar for evidence, you'll have to wait--but I think there's other forms of reasonable evidence that can give us information about what's likely a cancer risk and what isn't. The fact that 222nm seems to be >99% absorbed in dead skin cells, and the remaining <1% in non-dividing cells, is pretty strong evidence to me that it's unlikely to cause cancer. Certainly compared to UVB-containing sunlight!*