How many bad air days does your county get?

Days a year when the air index tops 100 in your county, from ten years of EPA monitor records. Plus the part purifier ads skip: whether your bad days are smoke, which a filter catches, or ozone, which it does not.

Your county

1,089 counties with an EPA air monitor. Counties without one are not listed, because a number we made up would be worse than none. Looking for today's air? That is AirNow.

Before you buy a purifier: smoke or smog?

A HEPA purifier pulls smoke and dust out of a room. It does nothing for ozone, which is a gas. In 117 of the 213 counties with five or more bad days a year, ozone is the main cause. Every county page tells you which one you have.

Smoke and dust

Filters help. Worst counties, bad days a year:

Ozone

Filters do not. Timing does: outdoors in the morning.

Every bad air day on file, 2016 to 2025, by month. Ozone fills June and July; wildfire smoke makes August the peak and keeps September high.

Half of monitored counties see 2.1 or fewer bad days a year. The worst tenth see 8.8 or more.

Every county, by state