It has a mix of data from home sensors that hobbyists put up at their houses and “official” sensors. If you fine tune to your neighborhood you can see precisely what’s happening with the air quality on your block. (If someone has a sensor.). Sometimes you’ll see one weird reading from someone’s sensor being too close to their BBQ, but not often.

A friend of mine volunteered to host a sensor on his roof, which is how I found out about this. There are other apps and sites that use the same data and make it look prettier, but I like the original .gov site just fine.