Air Pollution in the US: The Top 10 Most Polluted Cities

Since the creation of the Environmental Protection Agency (EPA) in 1970, the American public has become increasingly aware of the impact that air pollution can have on a person’s health. We have started to improve outdoor air quality in some parts of the nation, but other parts of the country have seen increased numbers of unhealthy air quality days. What causes this fluctuation in air pollution levels in certain areas? Which parts of the country have the worst air quality? Those are the questions that the American Lung Association (ALA) seeks to answer with their annual “State of the Air” report, which found that 4 in 10 Americans live in areas of polluted air.

The above map shows the top 25 polluted cities according to year-round particle pollution. As you may notice in the below map, California contains the greatest number of most polluted cities.

Before we get to the top 10 most polluted cities, here are some some of the problems associated with air pollution.

Harmful effects of air pollution

Smog as we know it — that ugly, yellow-brown haze — is caused by the chemical reaction of sunlight, nitrogen oxides and volatile organic compounds in the atmosphere that produce airborne particles and ground-level ozone.

Increased exposure to particulate matter and ground-level ozone may lead to cancer, lung problems, asthma attacks and more. The effects are especially harmful in young children and the elderly, in whom asthma attacks can be dangerous if not dealt with promptly. (Asthma attacks can be severe for healthy adults too, but the young and old are especially susceptible.)

Air pollution effects - pyramid

Air pollution is not only a problem for the environment. In the worst-polluted regions, it can have dramatic health and financial impacts. (EPA graphic)

While most populated places suffer from air pollution, areas near ports, airports or industrial areas are particularly bad. Similarly are places like California — one of the world’s largest economies and contains the top six most polluted cities. Here are the worst 10 metros.

Top 10 Most Polluted Cities

The information in the 2019 “State of the Air” report by the American Lung Association focuses on air quality data from 2015 to 2017. Note that the report lists air pollution by ozone, short-term particle pollution (which may be briefly dense but passes) and year-round particle pollution. The list of most polluted cities was compiled based on current, quality-assured air pollution data from official monitoring sites across the United States, maintained by federal, state and local governments and tribes.

Key findings of the “State of the Air” 2019 report include:

  • The effects of climate change—hotter weather, higher ozone days and increases in particle pollution—are increasing the public health risk posed by polluted air.
  • Hotter temperatures contributed to increased ozone formation and played a significant role in the rising number of unhealthy air days.
  • About 43.4 percent of Americans live in counties that have experienced unhealthy levels of ozone, particle pollution or both.
  • Major cities in the West and Southwest were once again found to top the most-ozone-polluted list. Almost half of the 25 most-polluted cities were in California, with Los Angeles remaining the worst city in the United States for ozone pollution—as it has been for 19 of the past 20 years.

The below list explores the rankings according to annual particle pollution.

#1: Fresno-Madera, CA (#5 in 2018)

This part of California experiences persistent, lingering air pollution due to the combination of warm, stable weather fronts (which keeps pollution in place) and valley walls (which collect it over the area). It is ranked fourth for high ozone days out of 220 metropolitan areas.

#2: Bakersfield, CA (#3 in 2018)

You might not associate a farming community with one of the highest levels of pollution, but Bakersfield sits in a giant bowl, which collects soot and ozone, which according to TIME Magazine, turns to smog in warm weather. Plus, it is California’s oil capital, leading to high rates of emissions.

#3: Fairbanks, AK (#1 in 2018)

Fairbanks, Alaska was ranked third in both year-round and short-term particle pollution. The Fairbanks community faces the challenge of several cold inversions that happen each winter. The woodstoves and wood heaters that residents use to heat their homes are some of the biggest contributors to particle pollution in Fairbanks.

#4: Visalia-Porterville-Hanford, CA (Ranked #2 in 2018)

The towns of Visalia, Porterville and Hanford in the San Joaquin Valley experience particularly bad air pollution. The valley is surrounded by high mountain ranges, which trap pollution. In addition, year-round sunny skies (warm temperatures can make ozone more likely to form), the great number of trucks that pass through, and residential wood burning contribute to the region’s poor air quality.

#5: Los Angeles-Long Beach, CA (Ranked #4 in 2018)

Los Angeles pollution is famous for its thick layers of smog that hang over the city. The smog is due to the high rate of car ownership and that a natural pollution trap created by its mountains.

#6: San Jose – San Francisco – Oakland, CA (new for 2019)

California’s Bay Area, home to San Jose, San Francisco and Oakland, experiences high levels of both ozone and particulate matter pollution. During the summertime, vehicle exhaust, combined with warmer weather and longer days, is the primary source of ozone in the air. In the winter, wood smoke from fireplaces causes high levels of particulate matter pollution. Wildfires in the area also have a significant impact on air quality.

#7: Pittsburgh-New Castle-Weirton, PA-OH-WV (Ranked #8 in 2018)

Pollution tends to concentrate in river valleys, where it is funneled along by wind and geography. Unfortunately, Pittsburgh and surrounding areas have several major rivers, which transfer pollution easily. Vehicle emissions contribute as well to particle pollution in the area.

#8: El Centro, CA (Ranked #7 in 2018)

El Centro’s hot, dry climate contributes to the city’s high pollutant levels, as does its proximity to Mexico, which has less stringent standards for air pollution.

#9: Cleveland-Akron-Canton, OH (Ranked #10 in 2018)

Cleveland and surrounding areas take the number 9 spot. Significant contributors to air pollution in this region include cars, trucks, trains and coal-burning plants.

#10: Medford Grants Pass, OR (New for 2019)

The Medford-Grants Pass area is a new addition to the top 10 most polluted cities in 2019’s report, most likely due to wildfires along the west coast. The fires, which happened more frequently and lasted longer than in past years, contributed to high levels of particulate matter pollution in this part of Oregon.

Changes from the Top 10 Most Polluted Cities from 2018

With two new cities added in 2019, there are two cities from 2018 that are no longer in the top 10:

Modesto-Merced, CA (Ranked #6 in 2018)

Here emissions from both cars and industrial agricultural equipment taint the air. High valley walls and warm weather also contribute to poor air quality. Valley fever, a fungal infection, can also be a problem here.

Lancaster, PA (Ranked #9 in 2018)

Lancaster has a combination of factors that contribute to poor air quality. The first is the region’s air pollution and the second is its geographical location downwind of pollution. Major power plants and large cities like Pittsburgh and Harrisburg lie to the west of the city.

What causes pollution

The above list reflects a variety of factors that cause the West and Southwest United States to experience more unhealthy air days than other parts of the country. One reason for this uneven distribution is the increase of oil and gas extraction in the Southwest while eastern states have instead focused on cleaning up power plants. Other significant contributors to particulate matter pollution are wildfires on the West Coast, wood burning stoves, older diesel vehicles and industrial plants. Weather and climate in certain regions can also serve to trap pollutant particles in one area, leading to multiple consecutive unhealthy air days.

Air quality rankings like the list above are not intended to discourage, but to inform present and future efforts to reduce air pollution not only in these metropolitan areas but across the country. The entire United States, not only the communities in the most polluted cities, must face the burden of improving air quality. Even the small steps that you take can make a big difference for you and your neighbors.

According to the EPA, with a little effort, you can make a difference in your city’s air quality. By working together to form carpools and install and utilize bike lanes, sidewalks and public transportation, you can join with your community to help cut down on vehicle emissions. Other ways to cut down on pollution include reducing electricity use, finding alternatives to burning wood or trash, and staying up-to-date on your vehicle maintenance.