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Air quality in Ohio

EPA monitors report from 39 of Ohio’s 88 counties. That is the honest limit of this page: the other 49 are not measured, not clean. In 2025 the most affected monitored county was Hamilton County, with 13 days above the moderate category.

39
Counties with monitors
49
Counties without
105
Days above moderate, 2025
Summed across monitored counties
11.1%
Adults with asthma
Population-weighted county estimate

County by county

Monitored Ohio counties

GoodModerateUnhealthy for sensitive groupsUnhealthyVery unhealthyHazardous
County90th %ile AQIAbove moderateMax AQI2025 days by category
Hamilton County7113154
Lucas County729154
Warren County677126
Butler County775151
Franklin County745156
Lake County715133
Madison County715112
Trumbull County745130
Cuyahoga County784150
Montgomery County674129
Ashtabula County613122
Clermont County583119
Clinton County613112
Delaware County613119
Summit County673126
Clark County652126
Greene County582105
Knox County542159
Mahoning County652122
Medina County672126
Portage County712150
Preble County642115
Scioto County642238
Stark County702124
Washington County542105
Allen County611129
Belmont County651123
Geauga County611129
Jefferson County681101
Licking County611119
Lorain County541119
Miami County611101
Wood County581108
Athens County54075
Columbiana County22031
Fayette County46064
Harrison County53071
Lawrence County59090
Noble County54090

EPA Air Quality System annual summaries by county, 2025. The 90th percentile AQI describes a day worse than nine out of ten, which is a steadier measure than the annual maximum a single wildfire can set.

Not measured

49 counties without a monitor

No EPA monitor reported from these counties, so there is no air quality record to publish. Their radon zone is known, and radon is measured in the building rather than the air outside it.

Other states

Air quality elsewhere