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 the coil and have to work harder later. If you do set your system to blow constantly, you don't get much additional benefit from in-duct UV. You can typically use a MERV-13 filter, and this removes the majority of particles, even tiny viral droplets and bacteria. In-duct UV helps some, getting you from >50% to nearly 100%, but at best this doubles your CADR. Another risk with in-duct UV is that it fails invisibly and fails open. If the bulb dies you won't notice, everything will work identically, you'll just stop having cleaned air. In-duct UV is just not that widely applicable, and even if you do have a compatible system you still generally do much better with stand-alone air purifiers (good default choice), far-UVC (especially for larger spaces or where you need minimum noise), or upper-room UVC (especially for high-ceiling spaces). [1] I asked Claude Opus 4.7, ChatGPT 5.5 Thinking, and Gemini 3.1 Pro "Approximately what fraction of indoor hours spent by humans around the world are in spaces with a ducted HVAC system? Can you give me your 50% confidence interval?", and got 9-13%, 10-20%, and 6-11% respectively. Discuss