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:
Do all the other particulates, e.g. viruses and bacteria, drop by 80%?
No? So what makes you think the filter needs to be changed 1/5th as often?