What should a smart air quality monitor actually measure?

It depends on your concern. For general indoor air quality, look for a unit that tracks particulate matter (PM2.5), CO2, VOCs, temperature and humidity. If your worry is specifically radon, a dedicated radon detector is more accurate than a general monitor that adds radon as one of many sensors. Verified-purchase reviewers consistently note that buyers are happiest when they match the sensor set to the actual problem they want to solve.