I'm guessing you've been smoking so long, you don't know what it's like to not live in polluted air. When you change an air filter, repaint a house, wash the inside of a car, you can immediately tell whether somebody is a smoker. It's fucking gross, and I suspect that it's not 1/5th, but probably 1/20th as often. "What makes me think that" is plain simple experience.

I don't have the full paper handy (and can't be bothered to look this up just for you) but the highlights should be enough for you on this one:

https://www.sciencedirect.com/science/article/abs/pii/S00489...