When Are Bad Air Days Most Likely? The Months That Matter
Summer, then smoke
Of every bad air day on file in monitored counties from 2016 to 2025, 57% fall in June, July and August, and 13% more in September. December through March together hold 10%.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
Why the season looks like that
Ozone needs sunlight and heat, so it builds from May and peaks in June and July, worst on still, hot afternoons. Wildfire smoke arrives later, usually August and September in the West, and can travel across the country. An ozone county's bad days cluster in midsummer afternoons; a smoke county's come in runs of several days whenever a fire is upwind.
The winter exception
9 monitored counties get half or more of their bad days between November and February:
- Fairbanks North Star, Alaska: 37 bad days a year
- San Joaquin County, California: 18 bad days a year
- Uintah County, Utah: 13 bad days a year
- Duchesne County, Utah: 10 bad days a year
- Platte County, Wyoming: 9 bad days a year
- Cache County, Utah: 8 bad days a year
- Lancaster County, Pennsylvania: 6 bad days a year
- Franklin County, Idaho: 4 bad days a year
These are valleys and basins where cold air gets trapped under warm air (an inversion) and wood smoke, traffic exhaust or oilfield emissions build up for days. A purifier helps with the particle part; the advice to exercise in the morning does not, because inversions are often worst at dawn.
Your county
Every county page has its own month strip. If you are planning around it, for a move, a summer camp or a training schedule, use that strip rather than the national one: Maricopa County, Arizona, for example, spreads its bad days across far more of the year than the national pattern suggests.