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 air. The word itself comes from combining "smoke" and "fog," which describes what it looks like: a brownish or grayish haze that reduces how far you can see. When you look at a city on a smoggy day, the skyline disappears into a murky layer.

The pollution that creates smog comes from real sources: vehicle exhaust, industrial emissions, power generation, and chemical reactions between those emissions and sunlight. On hot, still days — especially in summer — smog builds up because the pollutants stay trapped near the ground instead of dispersing into the upper atmosphere.

Key Takeaways

  • Smog forms when sunlight causes a chemical reaction between nitrogen oxides and volatile organic compounds, both released by vehicles and industry.
  • Ground-level ozone, the main component of smog, is different from the ozone layer in the upper atmosphere and is harmful to breathe.
  • Smog is worst on hot, windless days when pollution gets trapped near the ground and cannot disperse.
  • People with asthma, children, older adults, and anyone exercising outdoors are most affected by smog exposure.

How smog forms: the chemical reaction

Smog begins with emissions that come from everyday sources. Cars release nitrogen oxides when fuel burns in the engine. Factories, refineries, and power plants release both nitrogen oxides and volatile organic compounds — chemicals that evaporate into the air. On their own, these pollutants are harmful, but they are not smog yet.

Smog forms when sunlight hits these chemicals and triggers a chain of reactions. The nitrogen oxides and volatile organic compounds break apart and recombine, creating ground-level ozone. This ozone is the main ingredient in smog. The hotter the day and the stronger the sunlight, the faster these reactions happen, which is why smog is often worst in the afternoon on summer days.

Weather patterns trap smog near the ground. When air is still and there is no wind to push pollution away, and when warm air sits above cooler air (a condition called a temperature inversion), pollutants accumulate in a layer close to where people breathe. This is why smog is a problem in valleys and cities surrounded by mountains — the geography traps the air.

Ground-level ozone versus the ozone layer

Many people confuse ground-level ozone with the ozone layer, but they are different and opposite problems. The ozone layer is high in the upper atmosphere and protects Earth from ultraviolet radiation. Losing ozone up there is bad. Ground-level ozone, the kind in smog, is at the surface where people breathe, and having more of it is bad.

Ground-level ozone forms only when sunlight is present, which is why smog is a daytime problem. At night, when the sun sets, the chemical reactions stop, and ozone breaks down. This is why air quality often improves in the evening, even if the same pollutants are still in the air.

What smog does to your health

Breathing smog irritates the lungs and airways. Ground-level ozone damages the cells that line your respiratory system, making it harder to breathe and triggering coughing, wheezing, and chest pain. Repeated exposure over time can reduce how well your lungs work, even after the smog clears.

Some people are more vulnerable than others. Children are at higher risk because their lungs are still developing and they spend more time outdoors and playing. Older adults and people with asthma, emphysema, or other lung diseases face greater harm. People exercising outdoors breathe more deeply and take in more ozone, so athletes and outdoor workers are also at higher risk during smoggy days.

You do not have to see visible smog to be exposed to ground-level ozone. On days when the air looks clear, ozone can still be present at levels that harm health. This is why air quality reports measure ozone separately from what you can see.

Where and when smog is worst

Smog is most common in cities with heavy traffic, warm climates, and geography that traps air. Los Angeles, Phoenix, Houston, and the San Bernardino Valley in California are known for frequent smog because they have all three conditions: many vehicles, hot summers, and mountains or basins that hold pollution in place. But smog forms in many cities across the United States during summer months.

Smog season typically runs from late spring through early fall, peaking in summer. On any given day, whether smog forms depends on temperature, sunlight strength, wind speed, and how much pollution is in the air. A hot, still day with heavy traffic will produce more smog than a cool, windy day with the same number of cars.

How air quality reports measure smog

The Air Quality Index (AQI) is a number from 0 to 500 that tells you how polluted the air is. It measures several pollutants, including ground-level ozone. An AQI of 0 to 50 is "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 301 and above is "Hazardous."

You can check the AQI for your area on websites like AirNow.gov, which is run by the Environmental Protection Agency. The index also tells you which pollutant is the main problem that day — sometimes it is ozone, sometimes it is particulate matter, sometimes it is something else. This helps you understand what is in the air and who should take precautions.

What you can do on smoggy days

When the AQI is high, people in sensitive groups — children, older adults, and those with lung disease — should limit outdoor activity, especially strenuous exercise. Everyone else can usually go about their day, but intense outdoor exercise like running or sports should be moved indoors or rescheduled for a day with better air quality.

If you must be outside on a smoggy day, you can reduce exposure by exercising during early morning or evening when ozone levels are lower, staying in air-conditioned spaces, and keeping windows closed. People with asthma should have their rescue inhaler with them and may need to use it more often on high-ozone days.

Over the long term, reducing vehicle emissions and industrial pollution lowers smog. This happens through fuel standards, emission controls on cars and factories, and power plants switching to cleaner energy sources. Individual choices like using public transit, carpooling, or driving less on hot days also reduce the pollution that forms smog.

Frequently Asked Questions

Is smog the same as air pollution?

No. Air pollution is a broad category that includes many pollutants — dust, pollen, particulate matter, nitrogen oxides, sulfur dioxide, and others. Smog is a specific type of air pollution made of ground-level ozone and other chemicals that form when sunlight reacts with emissions. You can have air pollution without visible smog.

Can smog hurt you if you cannot see it?

Yes. Ground-level ozone can be present at harmful levels even when the air looks clear. The Air Quality Index measures ozone separately from visibility, so check the AQI rather than relying on what you see. A hazy day is obviously polluted, but a clear-looking day can still have unhealthy ozone levels.

Does smog go away on its own?

Smog clears when weather conditions change. Wind disperses the pollutants, cooler temperatures slow the chemical reactions that form ozone, and nightfall stops the sunlight-driven reactions. However, the underlying emissions that cause smog remain until they are reduced at the source through cleaner vehicles and industry.

Why is smog worse in summer?

Smog forms fastest in heat and strong sunlight, both of which are strongest in summer. Additionally, summer weather patterns — high pressure systems and temperature inversions — trap air near the ground. Winter cold and wind disperse pollution more effectively, so smog is less common even in cities with year-round emissions.