Smog is a visible haze made of air pollution that forms when sunlight reacts with emissions from cars, factories, and power plants

Smog is not a single pollutant — it is a mixture of gases and tiny particles suspended in the air that you can actually see. The word itself comes from "smoke" and "fog." When sunlight hits nitrogen oxides and volatile organic compounds (VOCs) released by vehicles and industry, a chemical reaction creates ground-level ozone and other secondary pollutants. This visible haze is what you see hanging over cities on hot days, and it forms most readily when the air is still and warm.

The two main types are photochemical smog, which forms from that sunlight-driven reaction and is most common in warm climates, and London smog, which is thicker and grayer and forms when sulfur dioxide from coal burning gets trapped near the ground in cold, stagnant air. Most smog you encounter today in the United States is photochemical smog.

Key Takeaways

  • Smog forms when sunlight reacts with nitrogen oxides and volatile organic compounds from cars, factories, and power plants, creating ground-level ozone and other pollutants.
  • Smog is thickest and most visible on hot, still days when air cannot disperse the pollutants, and it concentrates in valleys and areas surrounded by mountains.
  • Breathing smog can irritate your lungs and airways, and people with asthma, children, and older adults face higher health risks on high-pollution days.
  • Air quality indexes (AQI) published by the EPA and local agencies tell you when smog levels are unhealthy and when you should limit outdoor activity.

How smog forms and why it gets worse on certain days

Smog does not form when ready. The process starts when vehicles, factories, and power plants release nitrogen oxides (NOx) and volatile organic compounds (VOCs) into the air. On a hot, sunny day, ultraviolet radiation from the sun breaks apart nitrogen dioxide molecules, which then recombine with oxygen and other compounds to form ground-level ozone — the main component of photochemical smog. This is why smog is often called an "ozone problem" in air quality reports.

Weather conditions determine whether smog will build up or disperse. On a calm, hot day with little wind, pollutants stay in one place and accumulate. Temperature inversions — when a layer of warm air sits above cooler air near the ground — trap pollutants underneath like a lid, preventing them from rising and dispersing. Geography also matters: cities in valleys or surrounded by mountains experience worse smog because the terrain prevents wind from carrying pollutants away. Los Angeles, Denver, and the San Francisco Bay Area are notorious for smog partly because their geography creates these trapping conditions.

Health effects of breathing smog

Smog irritates your respiratory system — your nose, throat, and lungs. When you breathe it in, ozone and other pollutants inflame the airways, causing coughing, wheezing, and chest tightness. For people with asthma, smog can trigger attacks. Children are at higher risk because their lungs are still developing and they spend more time outdoors playing. Older adults and people with heart or lung disease also face greater harm.

Short-term exposure on a single bad air day may cause temporary irritation that goes away once you move to cleaner air. Long-term exposure — living in a chronically smoggy area for years — is linked to reduced lung function, increased asthma rates, and cardiovascular problems. Research shows that children who grow up in high-smog areas have smaller lung capacity as adults than children in cleaner areas, even if they move away later.

Reading air quality reports and understanding the AQI

The Environmental Protection Agency (EPA) publishes the Air Quality Index (AQI), a scale from 0 to 500 that measures how polluted the air is. Local air quality agencies report the AQI for your area daily, usually broken down by pollutant type. An AQI of 0 to 50 is "Good" — air pollution poses little to no risk. 51 to 100 is "Moderate" — sensitive groups like children and people with asthma should consider limiting prolonged outdoor exertion. 101 to 150 is "Unhealthy for Sensitive Groups" — these people should avoid outdoor activity. 151 to 200 is "Unhealthy" — the general public begins to experience effects. Above 200, air quality is "Very Unhealthy" or "Hazardous," and everyone should stay indoors.

You can check the AQI on the EPA's website (airnow.gov), through your local air quality agency, or on weather apps that include air quality data. Many cities also send alerts on high-pollution days. On days when the AQI is in the unhealthy range, people in sensitive groups should keep outdoor activity short and light, and everyone should consider wearing an N95 mask if they must go outside.

What causes smog in different regions

The sources of smog vary by region and season. In cities with heavy vehicle traffic, cars and trucks are the largest source of nitrogen oxides. In areas with refineries, power plants, or manufacturing, industrial emissions dominate. Wildfires also contribute significantly to smog, especially in the West during fire season — smoke from distant fires can travel hundreds of miles and degrade air quality in areas far from the fire itself.

Seasonal patterns also shift the problem. Summer smog is most common because heat and sunlight drive the chemical reactions that create ozone. Winter smog in some regions comes from heating systems burning fuel and from temperature inversions that trap cold-air pollution. Some areas experience both: Los Angeles has summer ozone smog and winter particulate smog from vehicle exhaust and heating.

Steps to protect yourself on high-smog days

On days when the AQI is unhealthy, limit time outdoors, especially during peak hours (usually late afternoon when the sun is strongest and ozone concentrations peak). Keep windows and doors closed to prevent outdoor air from entering your home. If you have an air conditioner with a clean filter, run it on recirculate mode to filter indoor air. HEPA filters and portable air purifiers can further reduce indoor pollution.

If you must go outside, wear an N95 or P100 mask — cloth and surgical masks do not filter ozone effectively. People with asthma should keep their rescue inhalers with them. Check the AQI before planning outdoor exercise; on high-pollution days, move workouts indoors or postpone them. If you have a chronic lung or heart condition, talk with your doctor about what AQI level should trigger you to stay indoors.

Long-term efforts to reduce smog

Smog has decreased significantly in the United States since the 1970s, when the Clean Air Act was passed and emissions standards were put in place. Cars now have catalytic converters that reduce nitrogen oxide emissions. Power plants have installed pollution control equipment. Refineries face stricter regulations. Despite this progress, many U.S. cities still exceed the EPA's ozone standard on some days, particularly in summer.

Reducing smog requires continued action: tighter vehicle emissions standards, cleaner electricity generation (moving away from coal and natural gas toward renewables), and industrial pollution controls. On an individual level, driving less, using public transit, and supporting policies that reduce emissions all contribute to cleaner air over time.

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 — dust, pollen, particulates, gases. Smog is one visible form of air pollution made specifically from ozone and other pollutants created when sunlight reacts with vehicle and industrial emissions. You can have air pollution without visible smog.

Can smog travel between cities?

Yes. Ozone and other smog pollutants can drift hundreds of miles downwind from their source. A city with strict emissions controls can still experience smog from pollution created upwind. This is why regional air quality management is important — one city's pollution affects neighboring areas.

Does smog go away at night?

Photochemical smog decreases at night because sunlight is no longer driving the chemical reactions that create ozone. However, the pollutants that form smog — nitrogen oxides and VOCs — remain in the air and can reform the next day if conditions are right. Overnight, some pollutants settle or are carried away by wind.

Is an air purifier enough to protect me on high-smog days?

An air purifier with a HEPA filter can reduce indoor pollution, but it works best in a sealed room with the door closed. It cannot protect you outdoors. If you must go outside on a high-smog day, a properly fitted N95 mask is more effective for personal protection than relying on an indoor purifier alone.

Why is smog worse in summer?

Heat and strong sunlight in summer accelerate the chemical reactions that create ground-level ozone. Warm air also rises more easily, but temperature inversions can still trap pollutants. Additionally, summer driving increases as people travel, adding more vehicle emissions to the air.