Guides

• Will an Air Purifier Help Where You Live? Smoke vs Ozone — A HEPA purifier removes smoke and dust but not ozone. In 117 of 213 US counties with 5+ bad air days a year, ozone is the main cause.
• What Counts as a Bad Air Day? AQI 100 Explained — A bad air day here means AQI over 100. What each AQI band means, who it affects, and how many of 996 monitored US counties cross each line.
• Is US Air Getting Worse? Bad Air Days Since 2000 — 76% of US counties with a long EPA record have fewer bad air days than in 2000-2009. Where it got worse, it is mostly smoke.
• When Are Bad Air Days Most Likely? The Months That Matter — 57% of US bad air days fall in June, July and August. Ozone fills early summer, smoke peaks in August, and some valleys get theirs in winter.
• How BadAirDays Counts Bad Air Days, County by County — Where each number comes from: EPA AirData annual and daily county files, how missing monitor days are scaled, and why 93 counties get no figure.