So, I wear a chemical cartridge respirator (gas mask) in most environments due to severe MCS. It has two parts to it: a pair of cartridges that filter out chemicals, and a pair of filters that go over the cartridges, which filter out particulates.
When I lived in the suburbs, the particulate filters would remain white - I never had to change them, ever. As in, years went by. And the air there wasn't always clean, we'd have temperature inversions that would deam it 'unsafe for sensitive people". I wore the filter mainly for cosmetic reasons, as they made the mask look nicer. In theory, they would make the cartridges last longer if there were particulates in the air.
When I moved to Seattle, within 3 months my filters were turning grey. Left longer, and they'd start turning black.
I suspect the dust from brakes and tires had a lot to do with it. *shudders*