Qualité de l’air à California

Why does California have the worst air quality?

Across 112 cities, California averaged a PM2.5 concentration of 12.1 μg/m³ (“moderate”) in 2018. Only 35.7 percent of cities met the World Health Organization (WHO) target for annual PM2.5 exposure of 10 μg/m³, as compared to the national average of 81.7 percent.

According to the American Lung Association's recent "State of the Air 2019" report, California leads the charts for cities with the worst air pollution.¹

The top five cities in the country with the worst annual PM2.5 levels are all located in the state, including Bakersfield, Fresno-Madera-Hanford, Visalia, Los Angeles-Long Beach, and San Jose-San Francisco- Oakland.

California also holds all of the top five cities in the country for worst ozone, including Los Angeles-Long Beach, Visalia, Bakersfield, Fresno-Madera-Hanford, and Sacramento-Roseville.

California’s unhealthy air quality, relative to other US states, is the result of a combination of factors. The state’s large population of 39 million, significant port industry, and growing economy create significant emissions by way of traffic, diesel trucks, construction, agriculture, and domestic emissions. Environmental conditions, which are prone to frequent and severe wildfires, with mountainous terrain that traps pollution, and a warm climate that contributes to ozone formation, present additional challenges to California's air quality.

In any given year, California’s most polluted cities tend to be cities most affected by that year’s wildfire season. While wildfires represent a temporary emission source, their impact on monthly and yearly air pollution averages can be severe. Human-driven climate change is expected to further aggravate the intensity of wildfires in the future by creating warmer and drier conditions. Such a progression is likely to worsen California air quality levels.

The wildfire season of 2020 constituted some of the most severe wildfires in recent years, raising pollution levels far above typical local ranges. Cities from Los Angeles to San Francisco experienced numerous days of sustained unhealthy air quality levels.

Breathing air pollution can be fatal, and higher air pollution levels increase the risk of adverse health effects. A 2010 study conducted by the California Air Resources Board estimates that PM2.5 pollution causes over 9,200 deaths in California annually, with a statistical range from 7,300 to 11,000 deaths.²

Use the IQAir California air quality map to discover air pollution levels across the state and help attribute city air quality to the emission source. Follow health recommendations when air quality levels exceed “good” standards.

Why do parts of California have such bad air pollution?

Sources of air pollution in California include vehicular emissions, demand for energy production through fossil fuels, and household energy consumption as well as wildfires as a prominent natural cause. Areas of relatively dense populations often experience higher daily pollution levels as a result of increased traffic, industry, and domestic emissions. Locations in or near the Klamath, Sierra-Nevada, and Coastal mountains, among numerous other mountainous areas, are more likely to experience elevated pollution levels as a result of wildfires, despite having less pollution most of the year.

California’s most populous cities, including Los Angeles, San Diego, San Jose, and San Francisco, are all located near California’s coastal mountain ranges. Here, westerly sea breezes can create a pollution-trapping effect in which emissions blown inland become trapped by the mountains, inversion layers, and stagnant air. Without a means for dispersion, pollution accumulates in the coastal valleys.³

Warm temperatures and abundant sunshine, for which California’s coast is known, bake nitrogen oxides and volatile organic compounds, creating ozone. As summer temperatures rise, as they have in recent years, so too do California’s ozone levels.

In addition to ozone, warming temperatures have also contributed to California’s rising PM2.5 levels. Increasingly hot and dry conditions have drastically increased the threat of California wildfires. Since 1970, the number of annual burned acreage has risen eightfold, while over the same time period summer temperatures have increased by 2.5 degrees.⁴

In 2019, 19 of the 20 most polluted cities in the United States for PM2.5 pollution were in California, where wildfires contributed to heightened monthly averages. Notably, while wildfires in 2019 were less severe and less frequent than in the previous 5 years, they still comprised a major source of statewide air pollution. 2020, meanwhile, was a record-setting year for wildfires in the Pacific West, including California. By the first week of September, more than 5.2 million acres across the western US had burned, 3.2 million of which were in California (a state record).⁵

In an effort to combat the trend of increasingly severe and frequent wildfires, The California Department of Forestry and Fire Prevention (CAL FIRE) is working on the arduous task of clearing volatile forest undergrowth by setting low-intensity “prescribed fires”.⁶ It is estimated that an estimated 8-10 million acres currently require ‘prescribed burning.’ At the current clearance rate of a million acres a year, it will be some time before the situation is under control. Until then, California’s wildfires season is likely to continue to bring smoky skies to much of the state.

How to reduce air pollution in California?

California’s unhealthy air pollution levels were first noted in 1943 when residents complained that smog was causing side effects such as burning lungs, irritated eyes, coughing, and nausea.⁷ It wasn’t until 1967, however, that state legislatures addressed growing concerns with the Mulford-Carrell Air Resources Act, which established the California Air Resources Board (CARB). CARB unites local governments, businesses, and residents to address California's high air pollution levels with research and proposed legislation.

Shortly thereafter, the federal Clean Air Act of 1970 marked a landmark national effort to mitigate air pollution emissions to improve public health. Most significant were the 1977 and 1990 amendments, which identified 174 common emission sources and developed unique standards for each. The Act and subsequent amendments additionally established a permit program for high emitters, and an enforcement program to ensure compliance. It is estimated that the success of the 1990 amendments have prevented 237,000 deaths up to 2020, as well as improving the health of countless more.⁸

Current pollution reducing efforts in California are largely focused on transportation emissions from both personal and commercial (diesel) motor vehicles, which account for a majority of California’s air pollution. To target commercial emissions, the EPA helped to fund a grant program in 2005 to retrofit or replace older diesel engines that were most likely to pose the greatest health risk to residents.⁹ To target personal vehicular emissions, many Californian cities are increasing public transportation infrastructure and encouraging residents to transition to electric vehicles by offering tax breaks and other financial incentives as well as enlarging the network of electric vehicle (EV) charging stations.

Despite progress, many cities in California have recently experienced stagnated or worsening air quality levels. While wildfires are in part to blame, others point to the Trump administration’s EPA regulatory rollbacks. As of 2019, 16 regulatory rollbacks on air pollution and emissions have been completed, with dozens more in progress.¹⁰ Rollbacks include the weakening of governmental emission standards, oversight, and enforcement of polluting industries. Such actions are expected to delay further progress in tackling the dangerous pollution levels that exist across the United States.

Where is the best air quality in California?

The best air quality in California is found in the state’s more sparsely populated interior cities, where vehicular and industrial emissions are relatively sparse and wildfires are infrequent. In 2019, California’s 5 cleanest cities for PM2.5 pollution were, Yucca Valley (3.4 μg/m³), Lee Vining (3.6 μg/m³), Twentynine Palms (3.9 μg/m³), Los Gatos (3.9 μg/m³), and Blythe (3.9 μg/m³) respectively. This ranking differs from 2018, in which California’s 5 cleanest cities for PM2.5 pollution were Tahoe City (5.2 μg/m³), Martinez (6.2 μg/m³), Carmel Valley (6.4 μg/m³), Salinas (6.5 μg/m³), and Grass Valley (6.8 μg/m³).

Air quality can vary drastically from year to year and day to day. Stay updated on real-time air quality data with the IQAir app and website.

Which cities in California have the worst air quality?

Air quality across California varies by year, particularly in relation to the severity of each year’s wildfire season. The most polluted cities in the state tend to be the cities most affected by wildfires that year. 2019 was a relatively mild year for wildfires, with the fewest annual acres burned and fires started since before 2015. This had an impact in lowering air quality levels across the state. 2017 and 2018 were significantly more polluted than 2019.

In 2019, 11 of the 15 most polluted US cities were located within 50 miles of Los Angeles. The 5 most polluted cities were Portola (16.9 μg/m³), Maywood (16.5 μg/m³), Walnut Park (16.3 μg/m³), Eastvale (16.2 μg/m³), and Colton (16.1 μg/m³), respectively.

In 2018, California’s most polluted cities were located in mountainous locations that suffered from wildfires, including Anderson (27.8 μg/m³), Three Rivers (20.8 μg/m³), Yosemite Valley (20.4 μg/m³), Portola (20.4 μg/m³), and Yuba City (12.6 μg/m³), respectively.

Use the live California cities ranking on this page to discover which locations in the state are currently experiencing the highest pollution levels. This ranking is updated hourly to show a comparative level of pollution across California cities in real-time.

+ Article Resources

[1] American Lung Association. (2020). State of the air – 2020.
[2] Green Car Congress. (2010, September 10). California ARB report finds fine particle air pollution responsible for 9,000 premature deaths in state each year; based on US EPA peer-reviewed study. Green Car Congress.
[3] Sharip M, et al. (2017, April 14). Air quality in California and steps to help reduce air pollution. Loma Linda University.
[4] Ray S, Miller B, and Jones J. (2020, August 25). California’s new normal: How the climate crisis is fueling wildfires and changing life in the Golden State. East Bay Times.
[5] Arthur D. (2020, September 14). Trump blames California, but here's how much federal land fires have burned this summer. Redding Record Searchlight.
[6] Helvarg D. (2019, December 20). How will California prevent more mega-wildfire disasters? National Geographic
[7] California Air Resources Board. (2020). History.
[8] United States Environmental Protection Agency (EPA). (2020). Benefits and costs of the Clean Air Act 1990-2020, the second prospective study.
[9] EPA. (2020). Diesel Emissions Reduction Act (DERA) funding.
[10] Popovich N, et al. (2019, December 21). 95 Environmental Rules Being Rolled Back Under Trump. The New York Times.