I lived in the Denver area for about 26 years. I finally had to move because I got a bad sinus infection every winter for the last 15 years there. For some years I lived in the foothills overlooking Denver. I could see the horrible air pollution in winter due to pm 2.5. It was especially bad on warmer, sunny days in winter. Once I moved away (in 2007), my winter sinus infections stopped happening.