[galago]

The EPA Air Quality Index isn't everything, but it is a standard value displayed in a phone weather app. I'll notice in times and places when I suspect its bad. If you live outside a city in place without forest fires or other environmental issues you might not notice its there. Some people with respiratory issues monitor this stuff constantly.

[seadan83]

IIRC, pollution from the tail pipe of a car falls off as a cubic function as you get further away from it. Similar for other sources of pollution. Pollution is very akin to smoke, hard to predict, and very localized. It can depend on wind, terrain, can get caught in places, underneath temperature inversions, etc.. All that said, the pollution you experience on a sidewalk next to a road, would be significantly higher than at the weather app meter station.

Determining pollution exposure can be done anecdotally. I've done a number of long distance bike trips, the few times where I was next to a highway for upwards of 8 hours - having a nose bleed by the end of that is pretty common for me. In that vein, recognizing air pollution effects is not necessarily that difficult. Symptoms include: sore throat, headache, burning eyes, etc.. The other side, people do get used to low level irritants, and yeah - you don't really notice its there until you go somewhere else and realize "the air smells different."