What smog is and where it comes from

Smog is a visible haze that forms when sunlight reacts with pollution already in the air. It is not fog — fog is water vapor. Smog happens when nitrogen oxides and volatile organic compounds (chemicals that evaporate easily) from car exhaust, factories, and power plants sit in warm air and get cooked by UV rays. The result is ground-level ozone, which is the main ingredient in smog you can see and smell.

Smog forms most easily in warm, stagnant air — which is why it gets worse in summer and in valleys or basins where air does not move much. Los Angeles, Denver, Phoenix, and the Central Valley of California see the heaviest smog because they have mountains that trap air, hot weather, and heavy vehicle traffic all at once. But smog can form anywhere cars and industry are concentrated, especially on hot days when the sun is strongest.

The pollution that creates smog comes from specific sources you can identify. Cars and trucks are the largest single source in most cities. Power plants, refineries, and chemical plants add more. Even lawn mowers, leaf blowers, and grills release volatile organic compounds that contribute. On a smoggy day, you are breathing ozone that was literally created in the air above you a few hours earlier.

Key Takeaways

  • Smog forms when sunlight chemically reacts with car exhaust and industrial pollution, not from fog or dust.
  • Ground-level ozone — the main component of smog — is harmful to breathe and can damage your lungs even during light activity.
  • Smog is worst on hot, windless days in summer, and in geographic areas where air gets trapped by terrain.
  • You can check your local air quality in real time using the EPA's AirNow website or your phone's weather app.
  • On high smog days, people with asthma, heart disease, or lung conditions face the most risk, but healthy people can also be affected.

Why smog damages your health

Breathing smog means breathing ozone, which is a reactive gas that damages the lining of your airways. When you inhale it, ozone irritates your lungs and can trigger coughing, wheezing, and shortness of breath — even in people with no history of lung disease. The damage happens whether you are exercising or sitting still, though exercise makes it worse because you breathe more air in.

People with asthma, emphysema, or chronic bronchitis face the highest risk. For them, a smoggy day can mean an asthma attack or a flare-up that sends them to the hospital. People with heart disease are also vulnerable because ozone stress on the lungs can strain the heart. Children and older adults are at higher risk than middle-aged adults, partly because children's lungs are still developing and older adults often have existing lung or heart conditions.

The damage from ozone exposure is cumulative over a lifetime. A single smoggy day will not cause permanent harm in most people, but repeated exposure over years — especially during childhood — can reduce how well your lungs work as an adult. People who live in chronically smoggy areas and exercise outdoors regularly face more long-term risk than people in cleaner air.

How to check air quality where you live

The EPA publishes real-time air quality data for most of the United States through a website called AirNow (airnow.gov). You can enter your zip code and see the current Air Quality Index (AQI) for your area. The AQI is a number from 0 to 500 — higher numbers mean worse air. The color coding is straightforward: green means good, yellow means moderate, orange means unhealthy for sensitive groups, red means unhealthy, and purple means very unhealthy.

Most weather apps and smartphone weather services now include an air quality forecast alongside temperature and humidity. If your phone's weather app does not show it, you can check AirNow directly or search "[your city] air quality" in a search engine. Many cities also publish their own air quality alerts through local news or city websites.

The AQI focuses on five main pollutants: ground-level ozone, particulate matter (PM2.5 and PM10), nitrogen dioxide, sulfur dioxide, and carbon monoxide. On a smoggy day, ozone is usually the pollutant driving the unhealthy reading. The forecast typically updates every hour during the day, so you can check in the morning to plan outdoor activities.

What to do on high smog days

When the AQI reaches orange (unhealthy for sensitive groups) or higher, people with asthma, heart disease, or lung conditions should limit outdoor activity. This means staying indoors, keeping windows closed, and using air conditioning if you have it. If you must go outside, wear an N95 or P100 mask — a cloth mask or surgical mask will not filter ozone effectively.

Healthy adults can usually tolerate moderate smog (yellow AQI), but should avoid strenuous outdoor exercise on orange or red days. Strenuous means running, cycling hard, or playing competitive sports — activities where you breathe deeply and take in more air. Walking, light gardening, and casual play are lower-risk activities. Children should follow the same guidance as adults with lung conditions, since their lungs are still developing.

If you exercise outdoors regularly, check the air quality forecast before you plan your workout. On high smog days, move your exercise indoors or to a time of day when the AQI is lower — smog is usually worst in the afternoon and early evening, so early morning air is often cleaner. If you start to feel chest tightness, shortness of breath, or throat irritation during outdoor activity on a smoggy day, stop and go inside.

Long-term smog exposure and where it is worst

Some parts of the United States have chronic smog problems because of geography, climate, and population density. The South Coast Air Quality Management District (which covers Los Angeles and surrounding areas) has the worst air quality in the nation, with dozens of days per year when the AQI reaches unhealthy levels. The Central Valley of California, the Denver metro area, Phoenix, and parts of the Inland Empire also see frequent smog.

If you live in or are considering moving to one of these areas and have asthma or heart disease, air quality should factor into your decision. Children who grow up in chronically smoggy areas show measurable reductions in lung function compared to children in cleaner air, even if they have no symptoms. The effect is largest in children who play outdoors frequently.

Air quality has improved significantly since the 1970s due to the Clean Air Act and stricter emissions standards for cars and factories. But many areas still exceed federal ozone standards on summer days. If you are moving to a new city, you can check its historical air quality data on AirNow or through the EPA's website to understand what to expect.

How smog is regulated and what is being done

The EPA sets National Ambient Air Quality Standards (NAAQS) for ground-level ozone and other pollutants. States and cities that do not meet these standards are required to develop plans to reduce emissions. This is why California has stricter car emission standards than the rest of the country — it was the first state to exceed federal ozone limits and had to act.

Regulations require power plants and refineries to install pollution control equipment, and car manufacturers must meet fuel economy and emissions standards. Many cities have also implemented programs to reduce vehicle traffic during high smog days, encourage public transit, and restrict the use of gas-powered lawn equipment on smoggy days.

Despite these efforts, climate change is making smog worse in some regions. Warmer temperatures speed up the chemical reactions that create ozone, and changing weather patterns can trap air in place longer. Scientists expect ground-level ozone to remain a public health problem in many parts of the country for decades.

Frequently Asked Questions

Is smog the same as air pollution?

No. Air pollution is a broad category that includes many things — dust, pollen, particulate matter, and various gases. Smog is a specific type of air pollution made when sunlight reacts with nitrogen oxides and volatile organic compounds. You can have polluted air without visible smog, and you can have smog without other forms of pollution being high.

Can an air purifier protect me from smog?

An air purifier with a HEPA filter can remove some particulate matter from indoor air, but it will not remove ozone gas. If you want to reduce ozone indoors on a smoggy day, keep windows and doors closed and use air conditioning with a clean filter. A portable air purifier is less effective than straightforward staying indoors and keeping air sealed.

Does smog only happen in summer?

Smog is worst in summer because it requires warm temperatures and strong sunlight. However, it can form in spring and fall on warm days, and in rare cases even in winter in warm climates like Southern California. Winter smog is usually made of different pollutants (like particulate matter from heating) rather than ozone.

Will my symptoms go away if I move to a place with better air quality?

If you have asthma or other lung conditions made worse by ozone, moving to an area with cleaner air will likely reduce your symptoms and flare-ups. However, if your condition was caused by years of exposure to smog, some lung damage may be permanent. Children who move to cleaner air show improvement in lung function, especially if they move before their lungs finish developing.

How do I know if the smog I see is dangerous?

Check the AQI number for your area — do not rely on how much smog you can see. Sometimes air looks very hazy but the AQI is still moderate, and sometimes the air looks clear but ozone levels are high (because ozone is invisible). The AQI is the only reliable way to know whether the air is safe to breathe.