Against In-Duct UV

When I pitch people on far-UVC they often ask about in-duct UV. How about putting UV inside your HVAC ducts, where you can safely blast the air with cheap toxic wavelengths. Unfortunately, it's rarely a good approach. The biggest issue is that most people don't have ducts. They're common in the US, though less so in older construction (radiators) or newer (mini-splits). Outside the US (and Canada, and Australia), however, ducted systems are mostly limited to large modern office buildings. Worldwide, maybe one in ten indoor hours are spent in ducted spaces. [1] Even in spaces that do have ducts, in-duct UV only works when air is flowing. Most HVAC systems only run the blower when they're calling for heat or cooling: a small fraction of the time. To get useful pathogen reduction you need ~constant recirculation, which isn't great. Blower motors draw a lot of power, so running them continuously gets expensive. Plus, during cooling season in a humid climate it will pick up humidity from…