What smog is and why it happens

Smog is a visible haze that forms when sunlight reacts with pollution already in the air. It is not a single pollutant but a mixture, most commonly ground-level ozone and fine particles. Ground-level ozone forms when nitrogen oxides and volatile organic compounds — released by cars, factories, and power plants — sit in warm air and get hit by ultraviolet light. The particles come from the same sources plus dust, wildfire smoke, and sea salt.

The word itself comes from "smoke" and "fog," which describes what it looks like: a brownish or grayish haze that reduces visibility and makes the air smell sharp or acrid. Unlike smog in London in the 1950s, which was mostly sulfur dioxide from coal burning, modern American smog is primarily an ozone problem. That matters because ozone damages lungs even when you cannot see it.

Smog gets worse in summer and early fall because heat and sunlight speed up the chemical reactions that create it. It also gets trapped in valleys and near coasts where geography keeps air from moving. Cities surrounded by mountains — like Los Angeles, Salt Lake City, and Denver — see the worst buildup because pollution gets funneled in and cannot escape easily.

Key Takeaways

  • Smog forms when sunlight reacts with emissions from vehicles and industry, and it is worst on hot, sunny days in summer and early fall.
  • Ground-level ozone, the main ingredient in smog, damages lungs and can make breathing harder for anyone, especially children and people with asthma.
  • Geography matters: cities in valleys or surrounded by mountains trap smog, so Los Angeles, Salt Lake City, and Denver experience it most often.
  • The EPA measures smog using the Air Quality Index, which tells you whether the air is safe to breathe on any given day.
  • You can reduce your exposure by checking air quality forecasts, staying indoors on bad days, and using air filters in your home.

Where smog is most common in the United States

The worst smog in the country is concentrated in a few regions. Southern California, particularly the Los Angeles basin, has the most persistent problem because of the combination of heavy traffic, warm weather year-round, and mountains that trap air. The San Joaquin Valley in central California also ranks among the worst because of agriculture, oil refining, and the same geographic trap.

The Houston area sees significant smog from petrochemical refineries and port traffic. Salt Lake City experiences severe smog in winter when cold air settles over the valley and traps emissions — this is called an inversion. Denver, Phoenix, and Las Vegas also report frequent high ozone days. The Northeast corridor from Boston to Washington, D.C., sees smog during summer heat waves, though it is usually less persistent than in the West.

Smog is not confined to cities. Rural areas downwind of industrial regions can experience high ozone levels even though they have no local sources. Wind patterns carry pollution from the Midwest into the Northeast, and from California into Nevada and Utah.

How the EPA measures and reports smog

The Environmental Protection Agency uses the Air Quality Index, or AQI, to measure smog and other air pollution on a scale from 0 to 500. An AQI below 50 is "Good." From 51 to 100 is "Moderate." From 101 to 150 is "Unhealthy for Sensitive Groups" — meaning children, older adults, and people with lung or heart disease should limit outdoor activity. From 151 to 200 is "Unhealthy," and above 200 is "Very Unhealthy" or "Hazardous."

The AQI focuses on five main pollutants: ground-level ozone, particulate matter, nitrogen dioxide, sulfur dioxide, and carbon monoxide. On any given day, the EPA reports whichever pollutant is worst. You can check the AQI for your area on AirNow.gov, which updates hourly and includes forecasts for the next few days.

States are required to meet National Ambient Air Quality Standards set by the EPA. If a region fails to meet the standard for ozone, it is designated a "nonattainment area," which triggers stricter rules on emissions from vehicles and industry. California has its own, stricter standards and has been granted a waiver to set rules tougher than federal law.

Health effects of smog exposure

Ground-level ozone irritates the airways and reduces lung function, even in healthy people. Repeated exposure can cause permanent damage. Children are at higher risk because their lungs are still developing and they spend more time outdoors. People with asthma, emphysema, or chronic bronchitis face worse symptoms — coughing, wheezing, and shortness of breath — on high-ozone days.

Older adults and people with heart disease are also vulnerable. Ozone can trigger heart attacks and strokes, possibly by causing inflammation or by affecting how the heart regulates its rhythm. The fine particles in smog can lodge deep in the lungs and enter the bloodstream, carrying toxic metals and chemicals directly into organs.

Short-term exposure on a single bad-air day causes temporary symptoms that usually go away once the air clears. Long-term exposure — living in a smoggy area year after year — is linked to reduced lung function, asthma development, and earlier death. Children who grow up in high-ozone areas have smaller lungs than children in cleaner areas, even if they move away later.

Steps to reduce your smog exposure

Check the AQI before you plan outdoor activities. On days when the AQI is above 100, limit time outside, especially strenuous exercise. Children, older adults, and people with respiratory or heart conditions should stay indoors on those days. If you must go out, an N95 or P100 mask can filter out some particles, though it does not filter ozone.

Keep windows closed on high-ozone days and use air conditioning with a clean filter. A HEPA filter in your home can remove fine particles from indoor air. Avoid outdoor exercise during peak ozone hours, which are usually late afternoon and early evening when the sun is strongest. Morning air is typically cleaner.

If you have asthma or another lung condition, talk to your doctor about how to adjust your medication on high-ozone days. Some people need to use their rescue inhaler more often or take preventive doses before going outside. Keep your car well-maintained and avoid idling, since vehicle emissions are a major source of smog precursors.

Regulations and efforts to reduce smog

The Clean Air Act, passed in 1970 and updated in 1990, gives the EPA authority to set air quality standards and requires states to develop plans to meet them. States must reduce emissions from power plants, refineries, and vehicles. The EPA has tightened the ozone standard three times since 1997, most recently in 2015, pushing states to cut emissions further.

California leads the nation in vehicle emissions rules. Cars sold in California must meet stricter tailpipe standards than federal law requires, and the state has banned the sale of new gasoline-powered cars starting in 2035. Several other states have adopted California's standards instead of federal ones.

Power plants have shifted away from coal toward natural gas and renewables, which has reduced nitrogen oxide emissions. Refineries have upgraded equipment to capture vapors. Vehicles have gotten cleaner with catalytic converters and better fuel injection. Despite these improvements, many areas still fail to meet the EPA standard, especially during summer heat waves when ozone formation accelerates.

Frequently Asked Questions

Is smog the same as air pollution?

No. Air pollution is a broad category that includes all harmful substances in the air. Smog is one type of air pollution — specifically, the visible haze caused by ozone and particles reacting with sunlight. You can have air pollution without visible smog, and you can have smog without other forms of pollution.

Can I see smog on the AQI even if the air looks clear?

Yes. Ground-level ozone is invisible, so you cannot see it even when the AQI is high. Fine particles can also be too small to see. Always check the AQI forecast rather than relying on what your eyes tell you, especially if you have asthma or heart disease.

Does an air purifier in my home remove ozone?

Most standard HEPA filters do not remove ozone. Some activated carbon filters can absorb ozone, but they need to be replaced frequently. The best approach is to keep windows and doors closed on high-ozone days and use air conditioning with a clean filter.

Why is smog worse in summer than winter?

Heat and sunlight speed up the chemical reactions that create ozone. Winter air is cooler and the sun is weaker, so less ozone forms. However, some areas like Salt Lake City experience winter smog from temperature inversions, where cold air traps pollution near the ground.

Can I drive less to help reduce smog?

Yes. Vehicle emissions are a major source of the nitrogen oxides that form ozone. Carpooling, using public transit, biking, or working from home on high-ozone days reduces your contribution to the problem. On a regional scale, widespread behavior change can lower peak ozone levels.