Qualité de l’air à Los Angeles

What is the air quality in Los Angeles?

Los Angeles air quality averages a US AQI or air quality index rating of “moderate.” Monthly averages in 2019 varied from AQI 32 (“good”) in February to AQI 64 (“moderate”) in November. Despite seemingly optimistic ratings, Los Angeles’s air pollution is among the worst in the United States, both for PM2.5 and ozone.

PM2.5 is airborne particulate matter measuring up to 2.5 microns in size. It is widely regarded as one of the most harmful pollutants to human health for its prevalence at dangerous levels. Exposure to PM2.5 has been linked to health effects such as heart disease, respiratory illness, and premature death.

For PM2.5, the greater Los Angeles county contains 9 of the 15 most polluted cities in the United States, according to the 2019 World Air Quality Report. In this same report, the city of Los Angeles ranked 82nd in the US (out of 1,517 included cities). Its annual average, however, differed by only 4 micrograms from the number one most polluted city in the U.S.: Portola, California.

According to the 2019 State of the Air report, which compared data across 229 metropolitan areas, Los Angeles has the worst ozone air pollution in the United States. Ozone is a gas pollutant formed when sunlight reacts with nitrogen oxides and organic substances. Vehicle exhaust contains both the nitrogen oxides and reactive organic substances needed to form ozone, so traffic is frequently identified as a leading source. Like PM2.5, ozone can cause health effects ranging from respiratory infections and inflammation to premature death.

Together, PM2.5 and ozone form the smog that Los Angeles is often known for. The summer months of June, July, and August tend to be more polluted than other months for both PM2.5 and ozone. This is because of drier conditions, less rainfall, higher temperatures, and a higher frequency of wind-blown dust and wildfires fanned by the Santa Ana winds.

Los Angeles does not currently meet the U.S. EPA’s national air quality standards for both PM2.5 and ozone.¹ The associated health implications are evident in the numbers. According to the County of Los Angeles Public Health Department, 1 in 10 children have been diagnosed with asthma.² Overall risk for cancer, meanwhile, is increased by 900 for every million, according to the South Coast Air Quality Management District.

Air pollution data is an important resource for taking action to mitigate these health effects. Refer to the top of this page for Los Angeles’s forecast air quality data and real-time air quality data.

Has air quality improved in Los Angeles?

Los Angeles air quality has dramatically improved over the last 30 years because of the 1990 amendments to the Clean Air Act. Most recently, year-over-year trends have resulted in reductions in L.A. air pollution of 10.6% from 2017 to 2018, and another 11.8% from 2018 to 2019.

Data collected by EPA governmental monitors and analyzed in the COVID-19 Air Quality Report found that Los Angeles experienced a long stretch of WHO-target air quality (< 10 μg/m³) from March 7-28, 2020, its longest streak since at least 1995. This 18-day stretch of exceptionally clean air is likely the result of lockdown measures put in place to reduce the spread of the COVID-19 pandemic, during which non-essential businesses were ordered to close and residents were urged to stay at home. March 2020 became Los Angeles’s cleanest air quality month on record, averaging PM2.5 levels of 5.6 μg/m³ (US AQI 23).

Yearly averages are often dependent on the wildfire season, which can contribute to greatly elevated periods of city-wide air quality. The last three years have seen fewer state-wide forest fires and burned acreage, though wildfires are expected to increase in frequency over the long term as temperatures rise droughts become longer.

Why does Los Angeles have unhealthy air quality?

Los Angeles is a city notorious for its smog, a combination of particle and ozone pollution. The prevalence of these pollutants result from many factors, including the burning of fossil fuels, especially by vehicles, ships, planes and manufacturing, as well as wildfires.

The large population of 4 million in Los Angeles, with another 6 million in the surrounding Los Angeles county, contributes significantly to the its ‘nonattainment’ air quality status because of heavy vehicular emissions and traffic congestion. It is estimated that there are 6.5 million vehicles in the city of Los Angeles alone. Current mayor Eric Garcetti set forth a sustainability plan that seeks to increase zero-emission vehicles in the city, growing their share from 1.4% in 2018 to 25% by 2025, and 100% by 2050.³ Power consumption, BBQs, and other such personal local emissions are also a major source of air pollution as a result of the city’s population.

Los Angeles’s shipping industry is another key contributor, particularly in recent years as trade with Asia has expanded. The Port of Los Angeles and the Port of Long Beach are the two busiest container ports in the United States. ⁴ Many port operations rely on fossil fuel or diesel to power ships, trucks, and other transportation. Since the implementation of the Clean Air Action Plan of 2006, particulate matter (PM) from these operations has dropped by 87%, while nitrogen oxides, an important precursor pollutant to ozone, are down 58%. Still, the ports remain a significant pollution source, producing an estimated 100 tons of smog daily, with little year-over-year improvement since 2011. The Los Angeles air pollution map often reveals higher AQI in Los Angeles’s port areas. Port authorities are looking for new ways to further decrease these emissions, such as investing cleaner energy transport vehicles.

Although they’re temporary and sporadic, wildfires often impact yearly average air pollution in Los Angeles. A combination of dry conditions, highly flammable fuels (such as the volatile Douglas fir and ponderosa pine tree species), increasingly hot summers, steep mountains, and strong Santa Ana winds combine to make the area highly susceptible to large and severe wildfires.

The geography of Los Angeles, in a basin surrounded by mountains, is further conducive to trapping air pollution. While many locations around the world commonly experience temperature inversions in the winter, Los Angeles often experiences a similar effect in warmer months. This happens when relatively warm air from the Great Basin or inland Los Angeles traps cooler, ocean air close to the Earth’s surface, preventing polluted air from rising and dispersing. This type of temperature inversion is called a marine inversion. The Los Angeles marine inversion can often be attributed to the layer of haze shrouding downtown buildings in pictures from the famous Hollywood sign.

How to reduce air pollution in Los Angeles?

While air pollution levels in Los Angeles still far exceed federal standards, significant improvements have been made in the last two decades. California began addressing air pollution in 1967 when it established the California Air Resources Board (CARB), an organization whose mission is to unite local governments, businesses, and residents with the goal of researching and legislating effective air pollution policy.⁵ This board was later empowered by the legislative action of the federal government in 1970, that established the Clean Air Act. The Clean Air Act, along with the later 1977 and 1990 amendments, created federal air quality monitoring, emission control, and enforcement. The Clean Air Act has been deemed the most effective federal regulations for clean air, estimated to have prevented as many as 237,000 deaths from 1970 to 2020.⁶

Current efforts have been led primarily by local and state regulations, furthering an agenda to reduce emissions across the transportation sector along with a number of high-polluting industries.

Among the most promising efforts is the shift towards electric vehicles (EVs). Currently, motor vehicles represent the leading source of city-wide PM2.5 and nitrogen dioxide, an ozone precursor pollutant. Other improvements, such as increasing the percentage of the power grid that relies on renewable energy, will be a critical step.

Long-standing trends of improving air quality in Los Angeles, coupled with promising new legislation and shifts in purchasing behavior, provide optimism for the future of air quality in Los Angeles.

Why is the air quality bad today in Los Angeles?

Changes in weather can impact local air quality. It is weather that often helps to explain a city’s air quality variations day to day when emissions remain relatively constant. Sunlight, wind speed and direction, temperature, and pressure conditions all shape current conditions. Examples of this include:

• Sunlight and heat can cause some gas pollutants (nitrogen dioxide and volatile organic compounds) to chemically react and form ground-level ozone, a secondary pollutant. Higher temperatures accelerate this reaction, increasing ozone levels.⁷
• Pressure cells and temperature inversions (a reversal of normal temperature conditions, in which hot air blankets cooler surface air, acting like a lid) can prevent emissions from dispersing naturally, causing a buildup of ground-level pollution.
• Rain can tamp down emissions, improving air quality.
• Wind direction and speed are perhaps the most impactful weather condition on city emissions. Stagnant air can cause emissions to become trapped in the L.A. Basin, while strong winds can move emissions out of the narrow outlets of the San Gabriel Mountains.

In addition to weather, extreme pollution events such as wildfires can cause significant spikes in L.A. air quality. Such pollution events have become increasingly common in the area over the last few decades as a result of anthropogenic climate change. When smoke fills the L.A. Basin, the region's geography and weather conditions can make it difficult for the air pollution to disperse, resulting in slowly accumulating pollution levels until the emissions stop or weather changes. Use the IQAir website and app to follow changing AQI levels in Los Angeles. When air is forecast to be unhealthy, reduce your exposure by limiting strenuous outdoor activity, closing your doors and windows, and running an air purifier and wearing an air pollution mask if available.

+ Article Resources

[1] California non-attainment/maintenance status for each county by year for all criteria pollutants. (2020).
[2] Mazza S. (2018, August 7). Investigation finds LA Harbor-area smog challenges grow as new health threats emerge.
[3] Roth S. (2019, May 6). Los Angeles sets dramatic new goals for electric cars.
[4] Vock DC. (2019, June). Can America’s biggest ports go green?
[5] California Air Resources Board. (2020). History. CARB.
[6] United States Environmental Protection Agency (EPA). (2020). Benefits and costs of the Clean Air Act 1990-2020, the second prospective study.
[7] National Oceanic and Atmospheric Administration. (2020). Clearing the air on weather and air quality.