Image provided by Cedric Letsch on Unsplash
TL;DR: Los Angeles, long associated with photochemical smog, faces persistent air quality challenges due to its geography, climate, heavy reliance on vehicles, and industrial history. While the city has made significant strides—from pioneering emissions standards to adopting advanced air monitoring networks—air pollution remains a concern, especially in low-income and high-traffic areas. Summer worsens smog levels, and climate change is compounding the problem. Progress and policy efforts continue, but protecting vulnerable communities and meeting federal standards will require sustained action.
Los Angeles is often associated with high levels of photochemical smog. Scott Epstein, the assessment manager of the South Coast Air Quality Management District, stated about smog in LA, "We have the highest levels in the U.S." With the summer fast approaching and smog becoming more prevalent, it’s worth examining the city’s historical and current relationship to the ambient haze.
What is photochemical smog?
Photochemical smog refers to a combination of ground-level ozone (O3) and particulate matter (PM2.5). Nitrogen oxides (NOx), predominantly from vehicle emissions, react with volatile organic compounds (VOCs) from various consumer products and fuel combustion, as well as sunlight, to form harmful tropospheric ozone. This type of smog, which has a bluish-brown tint, was first noticed in Los Angeles in the 1940s and has been associated with the city ever since.
Photochemical smog is harmful to both the environment and public health. Ozone harms plant health and damages human lungs, causing chest irritation, coughing, and exacerbating asthma and bronchitis. It also irritates the eyes. These effects can become more concerning with repeated and long-term exposure.
How does summer affect photochemical smog?
Summer can have a stagnating effect on city air quality. Weather that is hot, still, and dry can form stationary domes over cities that trap air pollutants in the lower atmosphere, where they can build up and further harm public health. Summer’s extra sunlight and high temperatures can also stimulate ozone production, increasing the levels of this harmful air pollutant, especially during morning rush hours.
Smog season in the Greater Los Angeles Area begins in May and continues through September. LA sees strong temperature inversions during the summer months, which inhibit vertical air mixing and trap air pollution near the ground.
Unfortunately, global warming has been increasing summer stagnant days in cities all over the United States and similarly increasing the risk of wildfires, negatively affecting air quality in California and Los Angeles.
What makes Los Angeles particularly susceptible to smog?
LA exists in a basin, surrounded by mountains on one side and the beach on the other, trapping air pollution between them. Los Angeles weather frequently sees temperature inversions that confine pollutants, along with hot, sunny days that increase ozone production.
LA once had an efficient system of electric trolleys for public transportation, but, since the 1930s, Los Angeles’ significant population has largely relied on automobiles, increasing the city’s air pollution emissions.
The history of LA’s summer smog
In 1903, industrial fumes and smoke were already affecting the city. However, air quality rapidly declined between 1939-1943 due to an increase in motor vehicles and the city’s industrial base following the start of World War II.
The summer of 1943 saw particularly bad smog in Los Angeles. From June until September, poor air quality plagued the city and surrounding areas for a total of 87 days straight. The smog was so intense that some believed they were experiencing a chemical attack from the Japanese. Visibility was reduced to about three blocks and many residents suffered from burning lungs, eye irritation, and nausea. Eventually, blame fell on the nearby butadiene plant, which was subsequently shut down. Yet the plant was not the main source of the smog.
In 1947, the Los Angeles County Air Pollution Control District was formed, becoming the first such body in the nation. This district regulated stationary industrial sources. But it still failed to address the main emission source.
Early progress in LA
In the early 1950s, Dr. Arie Haagen-Smit identified photochemical smog from a laboratory in Los Angeles. He, along with other Caltech researchers, finally discovered the main source of the smog: automobile emissions. This discovery allowed California to begin forming agencies and creating policies to regulate vehicle emissions in the 50s and 60s.
In 1966, California created the first tailpipe emissions standards in the nation, and in 1967 the state established the California Air Resources Board. Over the subsequent decades, progress included the federal Clean Air Act and the establishment of NOx emission standards, and restrictions on burning garbage dumps. Stripping lead out of gasoline, setting manufacturer quotas, and developing the catalytic converter—along with other technologies and policies—have also reduced smog in LA. In 1977, the South Coast Air Quality Management District went into effect.
By the 1990s, California’s cars and fuel became some of the cleanest in the world. Today, air pollution policies and programs, along with investment in clean technology, have continued to reduce smog. In fact, every smog season in LA sees a slight improvement.
Yet, LA and California still lag behind the rest of the nation, and work still needs to be done. The city's growing population requires new regulations to combat ever-increasing vehicle activity.
Current local government and school efforts
The Environmental Protection Agency tightened the federal standard for ozone back in 2015, at which point the South Coast Air Basin (including Los Angeles) was classified as an “extreme” nonattainment area. However, in 2022, the South Coast Air Quality Management District adopted a plan to meet the new federal standard for ground-level ozone. To meet this new standard by 2037, Southern California will have to reduce emissions of NOx (a precursor to the formation of ozone) by more than two-thirds from what they were when the plan went into effect.
Despite a long way to go, the local government is taking an aggressive role in tackling air pollution. The California Air Resources Board has a zero-emission vehicle program with requirements aimed at ensuring that new vehicles sold in California are zero-emission and clean plug-in hybrid-electric by 2035.
Los Angeles Unified School District (LAUSD), in partnership with the Coalition for Clean Air, has built the nation’s largest school-based outdoor air quality sensor network. This network allows LAUSD to monitor and protect children from air pollution, responding to real-time events and safeguarding a particularly vulnerable group.
Disparities in exposure
Poor air quality does not affect all parts of LA equally. Communities of color and low-income residents are disproportionately impacted by polluted air. In certain areas, traffic-related emissions, including nitrogen oxides (NOx), carbon monoxide (CO), and benzene concentrations, are up to 60% higher.
A study led by UCLA found that the air in disadvantaged neighborhoods contained not only more fine particulate matter, but also more toxic particulates as well. Places facing the most socioeconomic disadvantages “experience about 65% higher toxicity than people in the most advantaged group,” according to Suzanne Paulson, UCLA professor of atmospheric and oceanic sciences and the senior author of the study.
These same groups often have less access to health care and good nutrition, putting them at an even greater health risk. Everyone deserves to breathe clean air, and communities of color and low-income residents are unfortunately facing the worst of LA’s notorious smog.
How can residents protect themselves?
With summer’s smog season fast approaching, there are some things that individuals can do to protect themselves and their families. South Coast Air Quality Management’s Scott Epstein suggests residents get most of their activities done early in the day, stay inside in the afternoon, and, if possible, run an air purifier in the home. He recommends residents keep an eye on the air quality measurements for their area and modify their actions accordingly. Clarity’s OpenMap, which provides real-time air quality data for Los Angeles and other locations around the world, can be found here.
Residents should additionally avoid strenuous outdoor activities during traffic rush hours or near any major emission source, such as freeways and airports. Children, older adults, and people with preexisting health conditions are the most vulnerable and should take extra precautions.
Looking Forward
The first step in tackling air pollution is understanding it. Partner with Clarity Movement to monitor air quality and build a solution.
