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

Chester County had the highest highest daily Air Quality Index (US EPA) of 37 counties in 2015, at 166. Somerset County had the lowest, at 77. The median was 115. Source: the U.S. Environmental Protection Agency.

Highest166Chester County

Lowest77Somerset County

Median11537 counties

Map of counties, 2015

Under 100
100 to 110
110 to 120
120 to 150
150 and over
No data

index, 2015. 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, 2015

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

Full ranking

Rank	Counties	2015 index	Change on 2014
1	Chester County	166	+37
2	Bucks County	164	-8
3	Philadelphia County	161	+10
4	Allegheny County	157	0
5	Mercer County	153	+38
6	Delaware County	150	+7
7	Westmoreland County	147	+42
8	Lancaster County	140	-15
9	Lebanon County	133	+12
9	Montgomery County	133	+11
11	York County	126	+18
12	Dauphin County	124	+1
13	Beaver County	122	+3
14	Cumberland County	121	-7
15	Berks County	119	-8
15	Indiana County	119	+4
15	Northampton County	119	+1
15	Washington County	119	+18
19	Armstrong County	115	-4
19	Erie County	115	+14
19	Greene County	115	+7
22	Monroe County	112	+33
23	Blair County	108	0
23	Centre County	108	+18
23	Lackawanna County	108	+8
23	Lehigh County	108	-4
27	Luzerne County	105	+18
28	Adams County	101	-1
28	Elk County	101	-14
28	Lawrence County	101	-18
31	Clearfield County	100	+7
32	Cambria County	97	-11
33	Tioga County	90	+16
34	Lycoming County	87	-6
35	Bradford County	77	+27
35	Franklin County	77	-10
35	Somerset County	77	-20

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. 7 counties with data in other periods have no value for 2015 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), 2015) [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), 2015. 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?period=2015. 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), 2015. 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?period=2015.

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), 2015 [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?period=2015 (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_pennsylvania_by_2015,
  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), 2015}},
  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?period=2015. 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), 2015
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?period=2015
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/pennsylvania-counties?period=2015.
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 166 in 2015, according to the U.S. Environmental Protection Agency.

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

Somerset County, with 77 in 2015.

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. 7 counties have no value for 2015 and are not ranked.