You don't get fresh air in cities. It's different to deeply breathe in air on a trail surrounded by trees with no cars in sight than to stand on the balcony and breathe in the contaminated air of even smaller cities.

I used to only go out to do grocery shopping, but then I started exercising by cycling, and now, when I don't get out near the woods for three days, I get a real crisis. After breathing in fresh air for 30 mins or so I feel much, much better.