I rode the other day with a mask on (was just doing Z2, so it was motivation to stay at the right power level). I am wary of PM2.5 particulates; usually with an AQI of 100+ I'll try to mask, and if was stupidly high I'd just stay in. You could make the argument that I could push to 150 before it'd be worth worrying about; I chose to be a bit conservative. The lovely thing about this issue, compared to, say, contagious diseases, is that individuals' behavior really doesn't impact other folks too much. So if you think PM2.5 particulates aren't a big deal, at least at lower levels, go nuts...they're your lungs.