Civilization and Hats (really: hair)
What gets in your hair? With so much surface area, hair picks up more particulate than any other part of your body, especially since, at least in the winter (if you are uncouth) most everything else is covered up.
So, if you had to cover your hair, with a hat or hoodie, half the time, which half would it be? I always find drug stores to be quite smelly. I find health food stores to be quite smelly, but in an OK way. The air in NYC, least so on Monday mornings, very early, are always filled with dust of life (esp. industry, transportation and cooking). So, always when on the street. What about at work? I am thinking of old movies featuring newsrooms, where wearing hats indoors was the norm.
Putting on your hat, you are saying the air could use improvement, taking it off, you are expressing satisfaction.
By the way, I have always thought this concerning tying one's hair up, if it is long. Seeing people with long hair flowing on the subway (where the air seems foulest) makes me wonder... what do they smell when they get home? No, dears, wear it up in a bun, tie it in a pony tail, or, better yet, put it under a hat.