Mapping the Most Polluted U.S. Counties Over Two Decades

Air pollution hotspots in the U.S. can change yearly, but certain areas are consistently exposed to high levels of harmful pollutants.

Healthnews examined over 20 years of air quality data from the United States Environmental Protection Agency (EPA) to pinpoint where U.S. residents are regularly exposed to dangerous air pollution levels.

The report covers nearly 1,000 counties, cities, boroughs, and census areas.

How air pollution shortens lifespans

Why is this research necessary? Numerous peer-reviewed studies have shown that long-term exposure to air pollution is linked to increased mortality and a shorter lifespan.

The Harvard Six Cities Study revealed that people in dirtier cities had shorter life expectancies — about two to three years — than those in cleaner cities, primarily due to fine particulate matter (PM2.5).

The American Cancer Society (ACS) Cohort Study followed nearly 500,000 individuals and found that for every 10 μg/m³ increase in PM2.5 exposure, the risk of death rose by 4% for all causes, 6% for cardiopulmonary mortality, and 8% for lung cancer mortality.

The Global Burden of Disease (GBD) Study estimated that long-term exposure to air pollution caused 4.1 million premature deaths globally in 2019.

A study of the European ESCAPE Project found long-term exposure to particulate matter is associated with an increased incidence of coronary events.

A peer-reviewed study by Chen et al. (2013) found that the Chinese government's provision of free coal for heating, which significantly elevated air pollution levels, led to a 5.5-year reduction in life expectancy in the region. An increase in cardiorespiratory-related deaths drove this decline.

Methodology

The Healthnews team analyzed over 20 years of historical air quality data (2003–2023) from the EPA’s annual reports. EPA assessed the number of days each county experienced unhealthy levels of air pollution during this time frame.

Unhealthy air pollution levels are classified into four categories: unhealthy days for sensitive groups, unhealthy days, very unhealthy days, and hazardous days.

To rank the unhealthiest counties, we assigned point values to each pollution category and calculated an overall pollution score as follows:

• Unhealthy for sensitive groups days: 1 point
• Unhealthy days: 2 points
• Very unhealthy days: 3 points
• Hazardous days: 4 points

For example, the most polluted U.S. county in the last 20 years is San Bernardino, which had 1,602 unhealthy for sensitive groups days, 1,126 unhealthy days, 291 very unhealthy days, and one hazardous day, totaling a score of 4,731.

The full dataset can be found here.

Note: In some years, certain counties did not have data in the EPA's annual reports, which may affect their representation in the analysis.

California ranks as the most polluted once again

Previous reports have consistently ranked California as the most polluted state in the nation.

Our study aligns with these findings, revealing that 20 of the 30 most polluted counties over the past 20 years are in California, including San Bernardino County, Riverside County, Los Angeles County, Kern County, and others.

The remaining states represented in the top 30 most polluted counties include Texas with two counties (Harris and Montgomery), Arizona with two counties (Maricopa and Pinal), Colorado with two counties (Jefferson and Douglas), Nevada with one county (Clark), Alaska with one borough (Fairbanks North Star Borough), Ohio with one county (Hamilton), and Utah with one county (Washington).

#1 and #2: San Bernardino County and Riverside County

San Bernardino and Riverside counties are the most and second most polluted areas in the U.S. San Bernardino County recorded 3,020 bad air days over the two-decade period. This includes 1,602 days classified as unhealthy for sensitive groups, 1,126 days classified as unhealthy, 291 days classified as very unhealthy, and one day classified as hazardous. On average, residents faced 151 bad air days per year in the last two decades.

San Bernardino ranks as the number one most polluted county despite Riverside having more total polluted days. This is because San Bernardino experienced a greater number of days with higher pollution levels, specifically in the unhealthy and very unhealthy categories.

Riverside County experienced the most bad air days in the last 20 years, totaling 3,090 days. The breakdown comprises 1,979 days that were unhealthy for sensitive groups, 943 days as unhealthy, 144 days as very unhealthy, and 24 days as hazardous. This results in an average of about 155 bad air days per year.

What’s the reason behind the incredibly high pollution in these two areas? The Inland Empire, comprising San Bernardino and Riverside counties, has struggled with poor air quality for years, largely due to natural air currents that carry pollutants and particulate matter from the Los Angeles area.

Additionally, the region's booming warehousing and logistics industries have contributed to higher emissions from diesel trucks and trains. By December 2022, the logistics sector employed 284,000 people in the Inland Empire, making it the second-largest employer in the region.

Finally, the problem is compounded by the area's unfavorable geographical conditions — surrounded by mountains, pollutants from traffic become trapped and accumulate, making the air quality even worse.

#3 Kern County

Kern is the third most polluted county in the U.S. in the last two decades. Kern County had 3,083 bad air days from 2003 to 2023. 2,055 days were categorized as unhealthy for sensitive groups, 924 days as unhealthy, 101 days as very unhealthy, and three days as hazardous.

Kern County residents are frequently exposed to poor air quality, averaging approximately 154 bad air days per year.

Before the discovery of ‘black gold’ from a shallow hand-dug oil well on the west bank of the Kern River in 1899, Kern County was primarily known as a quiet agricultural area. The area's economy was modest, based mostly on farming, ranching, and small-scale local businesses.

The oil finding in Kern triggered an oil boom, and a forest of wooden derricks rapidly filled the area. In no time, Kern River's oil production was responsible for seven out of every 10 barrels of oil produced in California. By 1903, the Kern River oil field had propelled California to become the leading oil-producing state in the nation.

Currently, with more than 9,000 wells on just 10,000 acres, the Kern River Oil Field is one of the densest in the nation, and Kern County still accounts for about 70% of California’s oil production.

The oil extraction process releases harmful gasses like volatile organic compounds (VOCs) and nitrogen oxides (NOx) due to leaks and fuel combustion in drilling equipment. These pollutants react with sunlight to form ground-level ozone, a major component of smog.

The use of heavy machinery and increased truck traffic adds more pollutants, including particulate matter, to the air. Additionally, burning off excess natural gas (flaring) releases harmful substances like carbon dioxide, methane, sulfur dioxide, and toxic chemicals such as benzene.

As a result, air pollution in Kern County is among the highest in the country. Kern Oil, the primary oil refinery in the area, is subject to numerous Clean Air Act violations resulting from over-polluting. Since 2015 alone, Kern Oil has amassed 171 formal enforcement actions.

#4 Tulare County

Tulare County, nestled in California's San Joaquin Valley, consistently ranks among the regions with the poorest air quality in the U.S.

Tulare County reported 2,797 bad air days over the last 20 years. This includes 1,921 days that were unhealthy for sensitive groups, 841 days as unhealthy, 34 days as very unhealthy, and one day classified as hazardous.

The average number of bad air days per year stands at about 140.

Tulare County is the number one agriculture-producing county in the U.S. for 2022, reporting $8.6 billion in revenue from agriculture.

Plowing, harvesting, and other farming operations generate large amounts of dust. Additionally, practices like agricultural burning and emissions from dairy farms release harmful pollutants into the atmosphere.

Not to mention that the region is crisscrossed by major highways, including Highway 99 and Interstate 5, which serve as key routes for heavy traffic, especially diesel trucks.

The San Joaquin Valley is flanked by the Sierra Nevada mountains to the east and the Coast Ranges to the west, creating a basin-like effect. This natural topography traps pollutants, preventing their dispersion and causing them to accumulate near the surface.

#5 Los Angeles County

Los Angeles County noted 2,497 bad air days over two decades, placing fifth on the list. The figures include 1,642 days classified as unhealthy for sensitive groups, 735 days as unhealthy, 120 days as very unhealthy, and no days recorded as hazardous.

This amounts to an average of approximately 125 bad air days per year.

Industrial activities, including operations at the Ports of Los Angeles and Long Beach, as well as manufacturing and refinery emissions, significantly impact the region’s pollution.

The Ports of Los Angeles and Long Beach play a critical role in the nation's economy, handling approximately 40% of the United States' containerized imports. However, their significant cargo volume comes with environmental costs. The ports are Southern California's largest fixed sources of air pollution, contributing around 100 tons of nitrogen oxide (NOx) emissions daily. This accounts for about 10% of the total NOx emissions in the South Coast Air Basin.

The environmental impact is evident in air quality rankings, with the American Lung Association's 2023 State of the Air report identifying the Los Angeles-Long Beach region as the worst in the nation for ozone pollution and among the worst for particle pollution.

The city is also surrounded by the San Gabriel Mountains to the north and east and the Santa Monica Mountains to the west, making it difficult for pollutants to disperse.

Top 6–10 most polluted counties in the nation

At 6th place, Jefferson County noted 2,852 bad air days over two decades. The figures include 2,516 days classified as unhealthy for sensitive groups, 305 days as unhealthy, 26 days as very unhealthy, and five days recorded as hazardous. This poor air quality is largely due to industrial emissions, heavy vehicular traffic, and geographical factors that trap pollutants in the area.

Fresno County, ranked 7th, experienced 2,381 bad air days over two decades. The figures include 1,718 days classified as unhealthy for sensitive groups, 625 days as unhealthy, 37 days as very unhealthy, and one day recorded as hazardous. The main contributors to Fresno's air pollution are vehicle emissions, extensive agricultural activities, and geographic conditions in the Central Valley that trap pollutants.

In 8th place, Pinal County noted 1,695 bad air days over two decades. The figures include 1,148 days classified as unhealthy for sensitive groups, 316 days as unhealthy, 91 days as very unhealthy, and 140 days recorded as hazardous — the highest among these counties — indicating frequent exposure to dangerously poor air conditions that threaten public health. The severe pollution levels in Pinal County are primarily caused by dust storms, agricultural dust, and emissions from mining and industrial operations.

Maricopa County, ranked 9th, reported 1,543 bad air days over two decades. The figures include 1,143 days classified as unhealthy for sensitive groups, 229 days as unhealthy, 165 days as very unhealthy, and six days recorded as hazardous. The poor air quality in Maricopa County can be attributed to heavy vehicle traffic, industrial emissions, and frequent dust storms exacerbated by the desert climate.

Finally, Kings County, in 10th place, had 1,526 bad air days over two decades. The figures include 1,263 days classified as unhealthy for sensitive groups, 247 days as unhealthy, 12 days as very unhealthy, and four days recorded as hazardous. The air quality issues in Kings County stem from agricultural emissions, particularly from pesticide use and soil tilling, combined with stagnant air conditions that prevent the dispersion of pollutants.

Commonalities between the 100 least polluted counties

The 100 least polluted counties in the U.S. are spread across many states, which suggests that no single geographic or political factor dictates pollution levels. However, certain commonalities likely contribute to their cleaner environments.

Many are rural and sparsely populated, such as Musselshell and Sweet Grass in Montana or Wyandot in Ohio. Fewer residents often mean less industrial activity, traffic, and urban development — all major contributors to pollution in more densely populated areas.

These counties also tend to have limited heavy industry. Places like McCone (Montana) and Garden (Nebraska) lack large factories or refineries, reducing industrial emissions and keeping the air and water cleaner.

Geographic isolation and nature also play a significant role. Counties like Boundary (Idaho) and Taos (New Mexico) are located near protected natural areas such as parks and forests, where industrial development is discouraged and cleaner air flows naturally.

Many of these counties, such as Pawnee (Kansas) and Wrangell Petersburg (Alaska), have economies centered on agriculture or tourism. While agriculture can lead to certain types of pollution, it generally produces fewer emissions than urban and industrial centers. Additionally, areas reliant on tourism often prioritize environmental preservation.

Of course, geographically remote areas, like Aleutians East in Alaska, are naturally shielded from urban pollution sources.

How polluted is your county?

Curious about where your county stands in terms of pollution levels nationwide?

Explore the complete dataset, featuring rankings for around 1,000 counties across the United States, evaluated over the past 20 years.

Access the full dataset here for detailed insights and scores.