Riddle me this: how can a bus with its windows open smell so strongly of paint?

Currently on a 49 at 14th and Mission and I see no evidence of fresh tagging. I’d ask one of my fellow passengers, but I’m a little scared what I might learn.

This being a 49, however, I must document its standard-fare filth:

Related posts:

1. Tales of the 49
2. A fitting end to a shitty day
3. The Snake-eating-tail Problem of Muni Fare Increases