BadAirDays

Will an Air Purifier Help Where You Live? Smoke vs Ozone

Two different problems behind the same number

A bad air day is any day the AQI goes above 100, and the AQI is set by whichever pollutant is worst that day. Across every bad day in the 996 counties with reliable records from 2016 to 2025, ozone set the number 58% of the time, fine particles (PM2.5, mostly smoke) 37%, and coarse dust (PM10) 5%. Everything else is rounding.

What a filter can and cannot catch

HEPA filters and MERV 13 furnace filters trap particles. That is what they are rated on. Ozone is a gas; it passes straight through a particle filter. Some products sold as ozone fixes use activated carbon, which can take up some ozone while it is fresh, but no home purifier is rated for it the way HEPA is rated for particles, and devices that make ozone ("ionizers", "activated oxygen") add to the problem. If your county's bad days are ozone, a purifier is mostly money spent on the wrong pollutant.

Where particles drive it

Counties with 5 or more bad days a year where particles cause most of them, worst first:

  1. Pinal County, Arizona: 53 bad days a year, 75% particles
  2. Fairbanks North Star, Alaska: 37 bad days a year, 100% particles
  3. Inyo County, California: 29 bad days a year, 80% particles
  4. Plumas County, California: 26 bad days a year, 100% particles
  5. Mariposa County, California: 25 bad days a year, 62% particles

Here a HEPA purifier in the bedroom is worth it, and with 10 or more particle days a MERV 13 furnace filter as well. 168 monitored counties have enough particle days for the bedroom purifier advice; 146 for the furnace filter too.

Where ozone drives it

  1. San Bernardino County, California: 138 bad days a year, 92% ozone
  2. Riverside County, California: 132 bad days a year, 81% ozone
  3. Kern County, California: 109 bad days a year, 71% ozone
  4. Tulare County, California: 107 bad days a year, 77% ozone
  5. Los Angeles County, California: 106 bad days a year, 87% ozone

Ozone forms from traffic and industrial exhaust in sunlight, so it peaks on hot afternoons and falls off overnight. What helps is timing: exercise and outdoor play in the morning, windows shut in the late afternoon on alert days.

Both at once

A county can be mostly ozone and still get weeks of smoke. Fresno County, California averages 81 bad days a year; about 27 of them are particles and 54 ozone, so both a filter and timing matter there. Each county page splits the two in absolute days, not just by which is bigger, for exactly this reason.

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