Ranking 100 U.S. Counties With the Worst Air Quality

High air quality is an essential component of health. The United States Environmental Protection Agency links air quality improvements to increased life expectancy. Yet, the World Health Organization (WHO) warns that 99% of the world’s population lives where the air quality guidelines are unmet.

To help U.S. citizens identify and avoid areas with poor air quality, Healthnews analyzed the air quality in close to a thousand counties. We created a list of the top 100 counties with the worst air cleanliness and ranked them based on how bad the air quality in that county is.

Methodology

This report analyzes air quality data of 950 U.S. counties from 2023 provided by the United States Environmental Protection Agency (EPA).

The data includes the number of days each county experienced different air quality levels, categorized as "Unhealthy for Sensitive Groups," "Unhealthy," "Very Unhealthy," and "Hazardous."

The days are categorized based on the Air Quality Index (AQI). The AQI is a nationally uniform color-coded index for reporting and forecasting daily air quality.

It is used to report on the most common ambient air pollutants that are regulated under the Clean Air Act: ground-level ozone, particle pollution (PM10 and PM2.5), carbon monoxide (CO), nitrogen dioxide (NO₂), and sulfur dioxide (SO₂).

For a more detailed description of the AQI, visit the EPA's page on Patient Exposure and Air Quality Index.

A scoring system was applied to rank the counties from worst to best in terms of air quality:

• Unhealthy for Sensitive Groups Days: 1 point
• Unhealthy Days: 2 points
• Very Unhealthy Days: 3 points
• Hazardous Days: 4 points

Higher scores indicate worse air quality.

#	State	County	Unhealthy for Sensitive Groups Days	Unhealthy Days	Very Unhealthy Days	Hazardous Days	Unhealthy Air Index Score
1	Idaho	Bannock	1	0	0	31	125
2	California	Riverside	46	19	2	3	102
3	California	San Bernardino	48	7	1	0	65
4	Utah	Uintah	16	18	3	0	61
5	Arizona	Maricopa	51	4	0	0	59
6	Utah	Duchesne	22	11	1	0	47
7	California	Tulare	35	4	0	0	43
8	Illinois	Cook	27	7	0	0	41
9	California	Inyo	12	2	0	5	36
10	California	Imperial	11	1	2	3	31
11	Michigan	Wayne	15	7	0	0	29
12	Illinois	McHenry	16	4	1	0	27
13	Iowa	Scott	19	4	0	0	27
14	South Dakota	Minnehaha	22	2	0	0	26
15	Arizona	Pinal	22	2	0	0	26
16	Illinois	Kane	16	5	0	0	26
17	Wisconsin	Sauk	15	4	1	0	26
18	Nebraska	Douglas	22	2	0	0	26
19	Nevada	Clark	24	1	0	0	26
20	Illinois	DuPage	11	4	2	0	25
21	Illinois	Will	14	4	1	0	25
22	Wisconsin	Dane	18	2	1	0	25
23	Illinois	Rock Island	16	3	1	0	25
24	Texas	Harris	16	3	1	0	25
25	Illinois	Winnebago	17	2	1	0	24
26	Wisconsin	Waukesha	17	2	1	0	24
27	Illinois	Sangamon	17	2	1	0	24
28	Wisconsin	Rock	19	2	0	0	23
29	Indiana	Porter	19	2	0	0	23
30	Illinois	Peoria	21	1	0	0	23
31	California	Los Angeles	18	2	0	0	22
32	Minnesota	Wright	16	3	0	0	22
33	Indiana	Marion	13	3	1	0	22
34	Iowa	Linn	14	4	0	0	22
35	Iowa	Palo Alto	14	4	0	0	22
36	Wisconsin	Dodge	13	3	1	0	22
37	Iowa	Clinton	13	3	1	0	22
38	California	Fresno	19	1	0	0	21
39	Wisconsin	Kenosha	15	3	0	0	21
40	Ohio	Hamilton	17	2	0	0	21
41	Minnesota	Scott	11	5	0	0	21
42	Wisconsin	Eau Claire	10	5	0	0	20
43	Oregon	Lane	6	7	0	0	20
44	Indiana	Lake	16	2	0	0	20
45	Michigan	Lenawee	9	4	1	0	20
46	Wisconsin	Columbia	16	2	0	0	20
47	Pennsylvania	Lancaster	8	3	2	0	20
48	Illinois	Jersey	17	1	0	0	19
49	Ohio	Butler	11	4	0	0	19
50	Illinois	Macon	14	1	1	0	19
51	Wisconsin	Milwaukee	12	2	1	0	19
52	Minnesota	Hennepin	12	3	0	0	18
53	California	Humboldt	6	3	2	0	18
54	Ohio	Cuyahoga	9	3	1	0	18
55	North Dakota	Burleigh	11	2	1	0	18
56	Iowa	Polk	12	3	0	0	18
57	Missouri	Saint Louis	14	2	0	0	18
58	Illinois	Madison	14	2	0	0	18
59	Utah	Salt Lake	15	1	0	0	17
60	Minnesota	Anoka	13	2	0	0	17
61	Missouri	Jefferson	13	2	0	0	17
62	New York	Queens	6	4	1	0	17
63	Alabama	Jefferson	8	3	1	0	17
64	Wisconsin	Brown	7	5	0	0	17
65	Oklahoma	Tulsa	13	2	0	0	17
66	Michigan	Allegan	11	3	0	0	17
67	Michigan	Washtenaw	9	4	0	0	17
68	Pennsylvania	Dauphin	5	3	2	0	17
69	Ohio	Lucas	13	2	0	0	17
70	Wisconsin	Outagamie	7	5	0	0	17
71	Illinois	McLean	10	2	1	0	17
72	Colorado	Jefferson	17	0	0	0	17
73	Wisconsin	Walworth	16	0	0	0	16
74	Arizona	Yuma	6	3	0	1	16
75	Illinois	Saint Clair	12	2	0	0	16
76	Pennsylvania	Berks	4	3	2	0	16
77	New Jersey	Mercer	5	4	1	0	16
78	Pennsylvania	Allegheny	11	1	1	0	16
79	Iowa	Harrison	16	0	0	0	16
80	Pennsylvania	Lebanon	4	3	2	0	16
81	Indiana	St. Joseph	10	3	0	0	16
82	Illinois	Champaign	11	1	1	0	16
83	Colorado	Douglas	13	1	0	0	15
84	New Mexico	Bernalillo	4	4	1	0	15
85	New Jersey	Middlesex	4	4	1	0	15
86	Michigan	St. Clair	7	4	0	0	15
87	New York	Suffolk	11	2	0	0	15
88	Ohio	Franklin	7	4	0	0	15
89	Wisconsin	La Crosse	5	5	0	0	15
90	Pennsylvania	Philadelphia	3	3	2	0	15
91	Wisconsin	Taylor	5	5	0	0	15
92	Indiana	Madison	10	1	1	0	15
93	Missouri	Saint Charles	15	0	0	0	15
94	Missouri	Lincoln	15	0	0	0	15
95	Wisconsin	Ozaukee	9	3	0	0	15
96	California	San Diego	15	0	0	0	15
97	Illinois	Lake	13	1	0	0	15
98	Nebraska	Knox	13	1	0	0	15
99	Illinois	Randolph	11	2	0	0	15
100	Minnesota	Olmsted	13	1	0	0	15

Top 10 worst counties for air quality

Understanding the regions with the most significant air purity challenges can drive efforts to improve living conditions.

Did you know that since the United States passed the Clean Air Act, pollution has decreased by 64.9% — extending the average American lifespan by 1.4 years?

#1 Bannock County, Idaho

Bannock County experienced 1 "Unhealthy for Sensitive Groups" day and 31 "Hazardous" days in 2023, mainly due to wildfire smoke.

The Idaho Department of Environmental Quality highlights that large wildfires, which have become more frequent, significantly impact air quality by contributing high levels of PM2.5 particles. These particles pose serious health risks.

In addition, the region's topography exacerbates the problem by trapping pollutants.

Bannock County has been implementing "no-burn" days during high pollution periods to help reduce particulate matter in the air.

#2 Riverside County, California

Riverside County recorded 46 "Unhealthy for Sensitive Groups" days, 19 "Unhealthy" days, 2 "Very Unhealthy" days, and 3 "Hazardous" days throughout last year.

In Riverside, heavy traffic and industrial pollution emit pollutants, which are then trapped due to unfavorable geographical factors.

According to the American Lung Association, Riverside County consistently ranks among the top five worst counties in the U.S. for ozone pollution.

The Energy Policy Institute of Chicago released a report in 2022 showing that California has 29 of the top 30 most polluted counties.

California's counties are highly polluted primarily due to a combination of wildfire smoke, vehicle emissions, and industrial activities.

#3 San Bernardino County, California

In 2023, San Bernardino County had 48 "Unhealthy for Sensitive Groups" days, 7 "Unhealthy" days, and 1 "Very Unhealthy" day.

Significant vehicular emissions and industrial activities, combined with basin-like topography that traps pollutants, contribute to poor air quality.

Frequent wildfires also add to high levels of particulate matter.'

There is a significant number of days in San Bernardino County when the air quality is so poor that it triggers "Spare the Air" alerts, which advise residents to limit outdoor activities.

#4 Uintah County, Utah

Uintah County experienced 16 "Unhealthy for Sensitive Groups" days, 18 "Unhealthy" days, and 3 "Very Unhealthy" days.

Oil and gas extraction activities, which release various pollutants, are the main sources of pollution.

Natural dust events also contribute to elevated pollution levels.

Uintah County is unique for its use of natural gas from local wells to power public transportation, helping reduce local air pollution levels.

#5 Maricopa County, Arizona

Maricopa County recorded 51 "Unhealthy for Sensitive Groups" days and 4 "Unhealthy" days in 2023.

Vehicle emissions, construction dust, and industrial activities are major contributors to poor air quality.

The region's arid climate and frequent dust storms exacerbate particulate pollution.

Maricopa County has been using "dust control" teams to reduce particulate matter from construction sites and unpaved roads.

#6 Duchesne County, Utah

Duchesne County had 22 "Unhealthy for Sensitive Groups" days, 11 "Unhealthy" days, and 1 "Very Unhealthy" day.

Duchesne County experiences air quality challenges primarily due to emissions from oil and gas extraction and natural events like wildfires.

To counteract the pollution issue, Duchesne County has been working on monitoring and regulating emissions from local industries.

#7 Tulare County, California

Last year, Tulare County had 35 "Unhealthy for Sensitive Groups" days and 4 "Unhealthy" days.

Tulare County has one of the worst air qualities in the nation, primarily due to agricultural activities, industrial emissions, and vehicle pollution.

However, Tulare is now a part of a regional effort to reduce agricultural emissions through sustainable farming practices.

#8 Cook County, Illinois

Cook County recorded 27 "Unhealthy for Sensitive Groups" days and 7 "Unhealthy" days.

Air quality in Cook County is significantly affected by pollution from outside the state, particularly from Wisconsin. This cross-state pollution and local sources such as vehicle emissions and industrial activities exacerbate the smog levels.

To somewhat counteract the effects, Cook County has been implementing green infrastructure projects to improve air quality.

#9 Inyo County, California

Throughout 2023, Inyo County experienced 12 "Unhealthy for Sensitive Groups" days, 2 "Unhealthy" days, and 5 "Hazardous" days.

Natural events like dust storms and wildfires affect the area's air quality.

Inyo County has been studying the impact of dust storms on air quality and implementing measures to mitigate them.

#10 Imperial County, California

Imperial County had 11 "Unhealthy for Sensitive Groups" days, 1 "Unhealthy" day, 2 "Very Unhealthy" days, and 3 "Hazardous" days.

The region faces challenges from pollution originating across the border in Mexico, as well as local agricultural emissions.

Imperial County participates in cross-border air quality monitoring initiatives to address pollution from neighboring regions.