[lucubratory]

Devices like the Aranet4 are mainly useful for telling you how well ventilated a space is relative to how many people are breathing into it, which is a very good proxy for viral load and viral risk. It's pretty much only in spaces with high quality air filtration (aka high CADR for cleaning an enclosed environment) or significant far-UVC protective measures that the proxy relationship breaks down, and the breakdown will always be an overestimation of risk from the CO2 monitor, never an underestimation. For those reasons it's a very useful tool to have to put an upper bound on the risk of the space you're in and help you make informed decisions about whether you can e.g. hold your breath and take a drink under far-UVC cover or not.

PM2.5 sensors or other things like what you're thinking of do have their place, but they also have a bunch of other issues - they'll be detecting VOCs, brake and tire dust that's fine enough, any aerosol whether it's coming out of a mammal or not. I think it's inaccurate to say they're just a generically better solution than CO2 monitors for viral risk.

[rallison]

Exactly. CO2 is an imperfect proxy, but it's a pretty decent one. And, even for spaces that have high quality air filtration (e.g. a strong commercial central air system with good HEPA/MERV filters), the CO2 measurement can also tell you if the system is configured with a good recirculated to outdoor air ratio (residential systems usually have no such thing, unfortunately).

But yeah, as you said, the best aspect of a CO2 monitor is that it gives you a useful upper bound for the general respiratory virus risk in a space.