The US government profiles a variety of air quality sensors (VOCs, particulates, etc) and publishes their findings on the South Coast AQMD. These sensors are generally profiled by both lab and field testing and the statistical ‘r’ values are published.
Some of these sensors are consumer and there are lots of very expensive industrial ones as well. Do note that some sensors may not be “consumer” but are included in consumer products, such as Sensirion sensors being used in consumer products.
I think it’s mostly people trying to improve the indoor air quality in their home, though some also extend this to a hobby of sorts. The quality of air actually varies greatly from indoor and outdoor air. For example, indoor air generally has lower particulates but higher VOCs/gasses whereas outdoor air typically has higher particulates and low VOCs/gasses. Urban areas will have problems with VOCs and other emissions, but the EU is probably going to be better than most industrial nations with respect to those.
From my understanding, there is a pretty wide variance in housing construction throughout Europe in terms of active/passive heating and cooling. Any buildings optimized for energy conservation are going to be well-sealed meaning that contaminants have a hard time escaping, which makes indoor air quality poor. Lots of things indoors generate contaminants, including the house itself as they offgass things like formaldehyde for years after construction. In the US, modern construction is mandated to draw in a small portion of outdoor air to help dillute these contaminants. Without this ventilation, CO2 can easily raise to over 1000ppm which can cause transient cognitive issues.
You would be surprised on what shows up on these sensors and how bad it can get. Cooking is pretty bad if you don’t have a hood to vent it outdoors, for example. All sorts of things create problems in typical homes/cities:
The long story short is using these sensors can help you take control of your indoor air quality. Most homes have some issues with air quality and sensors help identify what, if any, problems may exist. They also allow you to know if your attempts at purifying the air are working and can help you know when to change filters out.
I created a community for air purifiers that goes over that side of filtration (in progress due to Lemmy transition): https://discuss.tchncs.de/c/airpurifiers