There’s cornfields next to airports where avgas still contains lead. The lead concentration in the soil around airports is very high. There’s also fields next to old Air Force bases where they used firefighting foam that contains PFAS. They would run drills with the foam, so it was used fairly regularly.
Air conditioning. All kinds of bacteria, dust, mold, algae, who knows what else is going through the evaporator, through the air ducts and blasting right into your lungs. Yeah, there’s a filter, but it’s on the intake side. It doesn’t account for what has built up in there over the years and it doesn’t catch everything. The one in your car can be equally as gross.