>>1735062Probably a lot. In the U.S. there's something called the Air Quality Index, which measures air quality based on particulate matter, carbon monoxide, and others. For most of the U.S., it's green ("Healthy") at any given time, even urban areas. The "yellow" areas will tend to change within 24 hours. Warnings under "Moderate" say that "If you suffer respiratory issues, reduce prolonged or heavy outdoor exertion". As a typical mask will tend to make breathing a bit more difficult, it's probably a zero sum game, and if you don't like the smell of car exhaust, probably won't do much good to block it.
The only time a mask makes sense is if the air quality is way worse than anything a typical roadway produces, like if you live in an area with active wildfires going on or an industrialized shithole like China or India where pollution controls don't exist.
