How Los Angeles Smog Forms and Why It Happens Most in Summer

What LA smog is and why it's worse than other cities' air pollution

Los Angeles smog is not the gray industrial haze you see in old photographs. It's a brownish or yellowish haze caused by sunlight reacting with car exhaust and factory emissions — a process called photochemical smog. The reaction creates ground-level ozone, which is the main pollutant that makes LA's air unhealthy to breathe, especially in summer.

LA gets worse smog than most US cities because of its geography. The city sits in a basin surrounded by mountains on three sides. When air pollution gets trapped in this basin, it cannot blow out to sea or disperse inland. On hot, sunny days — which LA has many of — the sunlight intensifies the chemical reaction that turns vehicle emissions into ozone. The result is air that can make your eyes water, your throat hurt, and your lungs work harder than normal.

The pollution does not come from LA alone. Wind patterns push emissions from inland areas and neighboring counties into the LA basin, where they get trapped and concentrated. This is why air quality in LA can be bad even on days when local traffic is light.

Key Takeaways

• LA smog forms when sunlight reacts with nitrogen oxides and volatile organic compounds from cars and factories, creating ground-level ozone that irritates your lungs and eyes.
• The city's geography — a basin surrounded by mountains — traps pollution and prevents it from dispersing, making the problem worse than in cities with better air circulation.
• Summer smog is worst because longer daylight hours and hotter temperatures speed up the chemical reactions that create ozone.
• Air quality in LA is affected by emissions from inland areas and neighboring counties, not just local sources, because wind pushes pollution into the basin.
• Ground-level ozone is different from the ozone layer in the upper atmosphere; ground-level ozone is harmful to breathe, while the upper-atmosphere ozone protects us from UV radiation.

Why summer is smog season in Los Angeles

Smog peaks in Los Angeles from May through October, with the worst days usually in August and September. Heat and sunlight are the two ingredients that make the chemical reaction happen faster. On a cool, cloudy day, the same amount of car exhaust might not turn into much ozone. On a hot, sunny day, it does.

Summer also brings stagnant air patterns. High-pressure systems sit over Southern California and prevent wind from pushing the trapped pollution out of the basin. This is why the South Coast Air Quality Management District (SCAQMD) — the agency that monitors air in the LA area — issues the most ozone alerts in late summer, even though people are not necessarily driving more than they do in winter.

Early morning and late afternoon are when ozone levels peak, because that is when sunlight is strongest and the day's accumulated emissions have had time to react. If you exercise outdoors, doing it in the early morning before 10 a.m. or after 7 p.m. reduces your exposure to peak ozone levels.

The sources of LA smog: where the pollution comes from

Cars and trucks are the largest source of the nitrogen oxides that create smog. Every time an engine burns fuel, it produces these oxides. LA has millions of vehicles, and they run year-round, but the problem gets worse in summer because of the heat and sunlight that speed up the chemical reaction.

Factories, refineries, and power plants also emit nitrogen oxides and volatile organic compounds. Southern California has several oil refineries and industrial facilities, mostly in Long Beach and other coastal areas. These facilities operate continuously, but again, their emissions cause more smog on hot, sunny days.

Consumer products like paint, solvents, and aerosol sprays release volatile organic compounds into the air. These compounds are less visible than car exhaust, but they contribute to the chemical reaction that creates ozone. Even small sources add up across a region of 13 million people.

How the chemical reaction creates ground-level ozone

The process starts with nitrogen oxides from vehicle exhaust. When sunlight hits these oxides, it breaks them apart and causes them to react with volatile organic compounds in the air. This reaction creates ozone — a molecule made of three oxygen atoms bonded together.

Ozone at ground level is harmful because it is a strong oxidant. When you breathe it in, it irritates your airways and can reduce how much oxygen your lungs can absorb. People with asthma, children, and older adults are most affected, but anyone can experience symptoms on a high-ozone day: coughing, throat irritation, chest tightness, or shortness of breath during exercise.

This ground-level ozone is different from the ozone layer high in the atmosphere. The upper-atmosphere ozone protects Earth from ultraviolet radiation. Ground-level ozone is a pollutant that forms from human activities. You cannot see it, but you can feel it.

How LA's geography traps pollution

Los Angeles sits in a coastal basin with the San Gabriel and San Bernardino Mountains to the north and east, and the Santa Monica Mountains to the north. This geography creates a natural bowl that traps air. On days when wind is weak or comes from inland, pollution gets pushed into the basin and cannot escape easily.

The Pacific Ocean to the west usually brings cool, moist air that pushes inland during the day. This sea breeze can help disperse pollution if it is strong enough. But in summer, when high-pressure systems dominate, the sea breeze weakens and the air becomes stagnant. Pollution that would normally blow out to sea or disperse inland instead accumulates in the basin.

Pollution from the Inland Empire — the region east of Los Angeles with cities like San Bernardino and Riverside — also gets pushed westward into the LA basin by afternoon winds. This means LA's air quality depends partly on what is happening 50 or 100 miles away. On bad air days, the entire South Coast region is affected.

How air quality is measured and reported in LA

The SCAQMD monitors air quality at dozens of stations across the LA region and reports it using the Air Quality Index, or AQI. The AQI ranges from 0 to 500, with higher numbers meaning worse air. An AQI below 50 is considered good; 51 to 100 is moderate; 101 to 150 is unhealthy for sensitive groups; 151 to 200 is unhealthy; 201 to 300 is very unhealthy; and above 300 is hazardous.

You can check the current AQI for your area on the SCAQMD website or through apps like AirNow. The index focuses on ozone in summer and particulate matter in winter, because those are the main pollutants that affect LA at different times of year. When the AQI reaches unhealthy levels, SCAQMD issues alerts and recommends that people limit outdoor activity.

The AQI is reported for different areas of LA County and neighboring counties, so you can see whether your neighborhood has worse air than another part of the region. Coastal areas and the beaches usually have better air than inland valleys, because the ocean breeze helps disperse pollution.

Steps you can take to reduce your exposure to smog

Check the AQI before you plan outdoor activities. If the index is above 100, limit strenuous exercise outdoors, especially for children and older adults. If it is above 150, stay indoors if possible and keep windows closed.

Exercise early in the morning or after sunset, when ozone levels are lowest. Avoid exercising near busy roads, where nitrogen oxide concentrations are highest. If you have asthma or another respiratory condition, carry your rescue inhaler with you on high-ozone days.

Reduce your own vehicle emissions by carpooling, using public transit, or biking when possible. Every car trip avoided means fewer nitrogen oxides released into the air. On Spare the Air days — when SCAQMD asks people to avoid driving — using transit or staying home helps reduce the chemical reactions that create smog.

Keep your car well-maintained. A poorly tuned engine emits more nitrogen oxides than one in good condition. If you are buying a vehicle, choose one with lower emissions or consider an electric vehicle, which produces zero tailpipe emissions.

Frequently Asked Questions

Is LA smog getting better or worse?

LA's air quality has improved significantly since the 1970s, when smog was so thick that visibility was often less than a mile. Stricter vehicle emission standards and industrial regulations have reduced pollution. However, the region still has more high-ozone days than most of the country, and climate change may make summer smog worse in the future because hotter temperatures speed up the chemical reactions that create ozone.

Can I see smog, or is it invisible?

Ground-level ozone is invisible, but you can see the brownish or yellowish haze it creates when it accumulates in the air. On very bad days, visibility drops and distant mountains disappear. On moderate days, you might not see anything but still experience irritated eyes or throat from breathing ozone.

Does smog only happen in Los Angeles?

No. Other cities with similar geography, hot summers, and heavy vehicle traffic — like Phoenix, Denver, and the San Bernardino Valley — also experience photochemical smog. However, LA's combination of geography, population size, and climate makes it one of the worst smog regions in the United States.

What is the difference between smog and wildfire smoke?

Smog is created by sunlight reacting with vehicle and industrial emissions. Wildfire smoke is particulate matter — tiny ash and soot particles — that blow into the region from fires. Both reduce air quality and can irritate your lungs, but they have different sources and require different monitoring. During fire season, LA can experience both at the same time.

Does rain help clear smog?

Yes. Rain washes pollutants out of the air and also breaks up the stagnant air patterns that trap smog in the basin. This is why air quality usually improves after a rainstorm. However, LA gets very little rain in summer, which is when smog is worst, so rain does not provide relief during peak smog season.