Counties in Pennsylvania by highest daily Air Quality Index (US EPA), 2025

Chester County had the highest highest daily Air Quality Index (US EPA) of 38 counties in 2025, at 157. Elk County had the lowest, at 84. The median was 116. Source: the U.S. Environmental Protection Agency.

Highest157Chester County

Lowest84Elk County

Median11638 counties

All counties, 2025

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

Full ranking

Rank	Counties	2025 index	Change on 2024
1	Chester County	157	+14
2	Delaware County	153	+17
3	Erie County	149	+56
4	Philadelphia County	147	+11
5	Warren County	138	n/a
6	Bucks County	136	-14
7	Montgomery County	133	-7
8	Beaver County	131	+19
9	Allegheny County	129	0
10	Tioga County	127	+43
11	Westmoreland County	126	+38
12	Monroe County	124	+5
13	Berks County	122	+17
13	Indiana County	122	+48
13	Northampton County	122	+17
16	Lancaster County	119	-6
17	Lackawanna County	117	+20
18	Wyoming County	116	+48
18	York County	116	+15
20	Blair County	115	+15
20	Cambria County	115	+28
20	Lehigh County	115	-4
20	Mercer County	115	+18
24	Bradford County	109	+34
25	Lebanon County	108	0
26	Susquehanna County	107	+43
27	Greene County	105	+18
27	Lycoming County	105	-17
29	Centre County	101	+4
29	Dauphin County	101	-4
31	Adams County	100	-31
31	Washington County	100	-5
33	Armstrong County	96	-12
33	Cumberland County	96	+3
35	Fayette County	93	-8
35	Somerset County	93	+3
37	Franklin County	87	-3
38	Elk County	84	+17

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. 6 counties with data in other periods have no value for 2025 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 Pennsylvania by highest daily Air Quality Index (US EPA), 2025) [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 Pennsylvania by highest daily Air Quality Index (US EPA), 2025. 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/pennsylvania-counties. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: highest daily AQI. Counties in Pennsylvania by highest daily Air Quality Index (US EPA), 2025. 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/pennsylvania-counties.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: highest daily AQI, Counties in Pennsylvania by highest daily Air Quality Index (US EPA), 2025 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/highest-daily-air-quality-index/pennsylvania-counties (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_pennsylvania_by_2025,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: highest daily AQI: Counties in Pennsylvania by highest daily Air Quality Index (US EPA), 2025}},
  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/pennsylvania-counties. 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 Pennsylvania by highest daily Air Quality Index (US EPA), 2025
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/pennsylvania-counties
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/pennsylvania-counties.
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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Rank Counties in Pennsylvania by highest daily Air Quality Index (US EPA) using official data, and cite the source.

rank_places {"indicator":"aqi_max","level":"province","within":"Pennsylvania","limit":10}
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Questions

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

Chester County, with 157 in 2025, according to the U.S. Environmental Protection Agency.

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

Elk County, with 84 in 2025.

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. 6 counties have no value for 2025 and are not ranked.