Smog is a visible haze of 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 particles that hang in the air and reduce visibility. The word itself comes from "smoke" and "fog," which describes what it looks like. When you see a brownish or grayish haze over a city on a hot day, you are looking at smog. It forms when nitrogen oxides and volatile organic compounds (chemicals that evaporate easily) react with sunlight in the atmosphere. This chemical reaction creates ground-level ozone, which is the main ingredient in smog and the reason it can make breathing difficult.

Smog is different from the fog you see on a cool morning. Fog is water vapor that condenses naturally. Smog is pollution that has been chemically altered by sunlight. It tends to be worse in summer and on hot, still days because heat and sunlight speed up the chemical reactions that create it. Cities in valleys or surrounded by mountains often trap smog because the geography prevents wind from blowing it away.

Key Takeaways

  • Smog forms when sunlight causes nitrogen oxides and volatile organic compounds from vehicles and industry to react with each other in the atmosphere.
  • Ground-level ozone is the main pollutant in smog and can irritate your lungs, throat, and eyes, especially during physical activity.
  • Smog is worse on hot, still days and in areas where geography traps air, such as cities in valleys or near mountains.
  • You can reduce your exposure by checking air quality forecasts, staying indoors on high-smog days, and limiting outdoor exercise when smog is visible.

Where smog comes from

The main sources of smog-forming pollution are vehicle exhaust, power plants that burn fossil fuels, and industrial facilities. Cars and trucks emit nitrogen oxides when their engines burn gasoline or diesel. Factories and refineries release both nitrogen oxides and volatile organic compounds. Even some consumer products like paints, solvents, and aerosol sprays release volatile organic compounds into the air.

Once these chemicals are in the atmosphere, they do not stay as they were released. Sunlight acts as a catalyst — it triggers a chain of chemical reactions that transforms them into ground-level ozone. This is why smog is sometimes called photochemical smog. The reactions happen faster on hot days because heat provides additional energy for the chemicals to react. This is why smog is typically worst in the afternoon and early evening, when the sun has been strong all day.

How smog affects your health

Ground-level ozone irritates the respiratory system — your lungs, throat, and airways. When you breathe in smog, ozone can inflame the lining of your lungs and make breathing harder. People with asthma, emphysema, or other chronic lung conditions are more sensitive to smog and may experience symptoms even on days when smog levels are moderate. Children and older adults are also at higher risk because their lungs are still developing or have weakened over time.

Healthy people can also be affected by smog, especially during physical activity. When you exercise or do heavy work outdoors, you breathe more deeply and more air enters your lungs. This means more ozone reaches deeper into your respiratory system. Even short-term exposure to high smog levels can cause coughing, throat irritation, chest pain, and shortness of breath. These symptoms usually go away once you move to cleaner air, but repeated exposure over years can cause lasting damage.

Smog also irritates your eyes and can make them water or feel gritty. Some people experience headaches or fatigue on high-smog days, though these symptoms are less direct than respiratory effects.

The difference between smog and other air pollution

Smog is ground-level ozone pollution, but air quality reports often mention other pollutants too. Particulate matter (PM) is tiny solid particles or liquid droplets suspended in air — things like dust, soot, and pollen. Nitrogen dioxide is a gas that contributes to smog formation but is also harmful on its own. Sulfur dioxide comes mainly from power plants and industrial sources. Carbon monoxide is an odorless gas produced by vehicle exhaust.

All of these pollutants can coexist in the same air, and on a bad air quality day, multiple pollutants may be at unhealthy levels at once. However, when people talk about smog specifically, they are usually referring to the visible haze caused by ground-level ozone and the particles suspended in it. The Air Quality Index (AQI) measures all these pollutants separately and gives each one a number from 0 to 500, so you can see which pollutants are the main problem on any given day.

When smog levels are highest

Smog is worst during summer months in most parts of the country because that is when temperatures are highest and the sun is strongest. The peak hours are usually mid-afternoon through early evening, when the sun has been heating the atmosphere all day. On a hot, windless day in July or August, smog can reach unhealthy levels in cities with heavy traffic or industrial activity.

Geography also plays a major role. Cities in valleys, such as Los Angeles and Salt Lake City, tend to have chronic smog problems because mountains trap polluted air and prevent it from dispersing. Coastal cities sometimes experience smog when sea breezes push polluted air inland. Rural areas usually have lower smog levels because there are fewer emission sources, though smog can drift from cities and affect surrounding regions.

How to check smog and air quality in your area

The EPA publishes real-time air quality data through AirNow.gov, which shows the Air Quality Index for your location. You can enter your zip code and see the current AQI, which tells you whether air quality is good, moderate, unhealthy for sensitive groups, unhealthy, very unhealthy, or hazardous. The same information is available through many weather apps and local news websites.

Most cities also issue air quality forecasts for the next few days, similar to weather forecasts. If a high-smog day is predicted, you can plan outdoor activities for a different day or take precautions like wearing an N95 mask if you must be outside. Local health departments sometimes issue smog alerts or air quality advisories when pollution reaches dangerous levels.

Steps to reduce your exposure to smog

On days when smog is visible or the AQI is in the unhealthy range, limit time outdoors, especially during peak smog hours (mid-afternoon through early evening). If you must be outside, avoid strenuous exercise, which causes you to breathe more deeply and inhale more pollutants. Children, older adults, and people with respiratory conditions should be especially cautious.

Keep windows and doors closed on high-smog days to prevent polluted air from entering your home. If you have air conditioning, use it and set it to recirculate indoor air rather than pulling in outside air. A HEPA filter in your home can remove some particles from indoor air. If you drive, keep your car windows closed and set the ventilation to recirculate mode rather than drawing in outside air.

Longer-term exposure reduction means choosing to exercise indoors on high-smog days, carpooling or using public transit to reduce vehicle emissions, and supporting policies that limit industrial pollution. Even small individual choices add up when many people make them.

Frequently Asked Questions

Is smog the same as air pollution?

Smog is one type of air pollution — specifically, ground-level ozone and the particles that make it visible. Air pollution is a broader category that includes many different pollutants like nitrogen dioxide, sulfur dioxide, and particulate matter. A day can have poor air quality from other pollutants even if there is no visible smog.

Can smog cause permanent lung damage?

Short-term exposure to smog usually causes temporary symptoms that go away once you breathe cleaner air. However, repeated exposure over many years, especially during childhood when lungs are developing, can reduce lung function permanently. People who live in high-smog areas their entire lives have been shown to have lower lung capacity than those in cleaner areas.

Does rain wash smog out of the air?

Rain can remove some particles from the air, which is why visibility often improves after a rainstorm. However, rain does not eliminate ground-level ozone — it only removes the particles suspended in it. Ozone is a gas and does not dissolve in water the way some other pollutants do. Wind is more effective at dispersing smog than rain.

Why do cities with strict emissions rules still have smog?

Smog forms from reactions between nitrogen oxides and volatile organic compounds, so reducing either one helps. However, even with strict vehicle and industrial emissions standards, smog can still form if there are enough emission sources and the weather conditions are right. Additionally, smog can drift from other regions, so a city with good local controls may still experience high pollution from upwind sources.