I've tried 3 different filters so far for heavy smoke pollution in Chiang Mai and so far this one has been the best. 4/5 only because these filters are too expensive IMO, and I get a maximum of 4 hours out of them, lifetime (1-2hr/session). Once the sweat soaks in, it becomes difficult to breathe. After 4-5 hours, even dry, the airflow isn't as good. To be fair I use these filters in the most extreme conditions. Up to 38C temps and 80-400ug/m3 PM2.5 pollution. I was able to do a 23km run non-stop on a single filter this week. The filter was visibly blackened after this single run!