Filters?

Typically only work on particulates past a certain size, which won't deal with many forms of pollution. I've heard of some businesses in Beijing using them but even if one is fortunate enough to work at such a place, that leaves the rest of one's life.

measurably cleaner indoor air

Ah, my old foe: statistical significance. (You can also measure the level of cyanide in your breakfast eggs, doesn't mean you're going to be re-enacting Arsenic and Old Lace anytime soon.)