Counties in Ohio by highest daily Air Quality Index (US EPA)

Lawrence County had the highest highest daily Air Quality Index (US EPA) of 46 counties in 1991, at 228. Hancock County had the lowest, at 44. The median was 194. Source: the U.S. Environmental Protection Agency.

Highest228Lawrence County

Lowest44Hancock County

Median19446 counties

Map of counties, 1991

Under 65
65 to 110
110 to 190
190 to 200
200 to 210
210 and over
No data

index, 1991. Hatched areas have no value for this period. Source: EPA, EPA AirData annual AQI by county: highest daily AQI. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 1991

index. Source: EPA, EPA AirData annual AQI by county: highest daily AQI. Retrieved 2026-10-12.

Full ranking

Rank	Counties	1991 index	Change on 1990
1	Lawrence County	228	+25
1	Stark County	228	-76
3	Jefferson County	222	-18
4	Warren County	220	+5
5	Hamilton County	219	+10
6	Summit County	215	+8
7	Washington County	213	+41
8	Franklin County	210	+18
8	Medina County	210	+54
10	Preble County	209	+62
11	Butler County	204	-9
11	Logan County	204	+14
11	Lucas County	204	+45
11	Mahoning County	204	+32
15	Clark County	203	+3
15	Clinton County	203	+11
15	Union County	203	+34
18	Ashtabula County	202	+30
19	Licking County	201	n/a
20	Crawford County	197	+28
20	Lake County	197	-22
22	Cuyahoga County	195	+10
22	Portage County	195	+10
24	Fayette County	192	-3
25	Montgomery County	185	-18
26	Allen County	182	+21
27	Knox County	174	+8
28	Clermont County	172	+3
29	Lorain County	169	+18
30	Madison County	166	n/a
31	Scioto County	110	+36
32	Noble County	107	+49
33	Pickaway County	88	+44
34	Columbiana County	82	0
35	Seneca County	76	n/a
36	Belmont County	75	-1
37	Trumbull County	66	-2
38	Sandusky County	65	-5
39	Wyandot County	64	+4
40	Richland County	59	+2
41	Jackson County	53	n/a
42	Greene County	51	-8
43	Adams County	49	-8
43	Ottawa County	49	-9
45	Athens County	47	n/a
46	Hancock County	44	n/a

Other periods

About this ranking

Highest daily Air Quality Index (AQI) value recorded in a US county during the year, from the EPA AirData annual summaries. Values above 500 are beyond the AQI scale and are published as EPA computes them.

A single day decides the value, so one wildfire or dust event can set it. Dust from the dry Owens Lake bed puts Inyo and Mono counties in California in the thousands. United States counties with air quality monitors only, about 1,000 of some 3,100 in recent years; a county without monitors has no value. The number of days with an AQI differs between counties because some monitors run only in the ozone season.

Every value in the ranking comes from the same publisher and the same period, so the counties are compared on one definition. 12 counties with data in other periods have no value for 1991 and are left out rather than filled from another source. Equal values share a rank.

How to cite

U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.

Citation styles

The publisher is the author. Invokely is named only as the place the figures were read, where a style asks for it.

APA 7

U.S. Environmental Protection Agency. (2026). EPA AirData annual AQI by county: highest daily AQI (Counties in Ohio by highest daily Air Quality Index (US EPA), 1991) [Data set]. Retrieved October 12, 2026, from https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual

MLA 9

EPA AirData annual AQI by county: highest daily AQI, Counties in Ohio by highest daily Air Quality Index (US EPA), 1991. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/ohio-counties?period=1991. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: highest daily AQI. Counties in Ohio by highest daily Air Quality Index (US EPA), 1991. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/ohio-counties?period=1991.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: highest daily AQI, Counties in Ohio by highest daily Air Quality Index (US EPA), 1991 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/highest-daily-air-quality-index/ohio-counties?period=1991 (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_ohio_by_1991,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: highest daily AQI: Counties in Ohio by highest daily Air Quality Index (US EPA), 1991}},
  year = {2026},
  howpublished = {Data set},
  url = {https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual},
  urldate = {2026-10-12},
  note = {Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/ohio-counties?period=1991. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest daily AQI. https://aqs.epa.gov/aqsweb/airdata/download\_files.html\#Annual. Retrieved 2026-10-12.},
}

RIS

TY  - DATA
AU  - U.S. Environmental Protection Agency
TI  - EPA AirData annual AQI by county: highest daily AQI: Counties in Ohio by highest daily Air Quality Index (US EPA), 1991
PY  - 2026
DA  - 2026/07/13
PB  - U.S. Environmental Protection Agency
DB  - Invokely
UR  - https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual
L2  - https://invokely.ai/rankings/highest-daily-air-quality-index/ohio-counties?period=1991
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/ohio-counties?period=1991.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER  -

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Questions

Which county has the highest highest daily Air Quality Index (US EPA)?

Lawrence County, with 228 in 1991, according to the U.S. Environmental Protection Agency.

Which county has the lowest highest daily Air Quality Index (US EPA)?

Hancock County, with 44 in 1991.

Where does this ranking come from?

Every value comes from one source and one period, so the places are comparable: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: highest daily AQI. Retrieved 2026-10-12. 12 counties have no value for 1991 and are not ranked.