Cabin air comes from the "bleed air", which is air from the compressor stage of the engine. Very rarely this air is noticeably contaminated due to engine issues

Cabin air comes from the "bleed air", which is air from the compressor stage of the engine. Very rarely this air is noticeably contaminated due to engine issues, but I think it's valid to wonder if it's contaminated in a sub-perceptible way more often than that.