I don’t think it’s just particulates, Denver and the front range are routinely in the news for ozone issues [0]. I think that might be altitude related as well, though it may just be pollution. Den doesn’t quite make the top ten list of US cities with ozone problems, but it’s gotta be close [1].
[0] https://www.govinfo.gov/app/details/FR-2019-08-15/2019-17405 [1] https://www.usnews.com/news/cities/slideshows/the-10-us-citi...
I got a few in CA because of the fires, but I run them all of the time and they work. I also have some AQI sensors and I can see the pm2.5 drop rapidly when the air filters kick in.