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

The word "smog" is a combination of "smoke" and "fog," and that blend describes what you actually see: a brownish or grayish haze hanging over a city or region. Unlike natural fog, which is water vapor, smog is made of harmful chemicals and particles that form when sunlight hits pollution already in the air. The main culprits are nitrogen oxides from vehicle exhaust and volatile organic compounds from industrial sources. When these chemicals react in sunlight, they create ground-level ozone — the primary ingredient in smog.

Smog is not the same everywhere. In some places it looks like a thick brown cloud; in others it is a hazy veil that reduces visibility. What matters is that you are breathing in pollutants that can irritate your lungs and airways, whether you can see them clearly or not.

Key Takeaways

  • Smog forms when sunlight chemically reacts with pollution from vehicles, factories, and power plants, creating ground-level ozone and other harmful compounds.
  • The two main types are photochemical smog (formed by sunlight) and London smog (formed by coal burning and fog), though photochemical smog is more common in warm climates today.
  • Smog is worse on hot, sunny days with little wind, because heat and sunlight speed up the chemical reactions that create it.
  • Breathing smog can cause coughing, wheezing, and chest pain, especially for children, older adults, and people with asthma or heart disease.

How smog actually forms: the chemical process

Smog does not just appear — it is created by a chain of chemical reactions. It starts with emissions: nitrogen dioxide from car tailpipes, sulfur dioxide from power plants, and volatile organic compounds from paint, solvents, and gasoline. These chemicals sit in the air, invisible. When the sun rises and its ultraviolet rays hit these pollutants, a reaction begins. The nitrogen dioxide breaks apart, and the pieces recombine with oxygen and other chemicals to form ground-level ozone, which is the main toxic ingredient in smog.

This is why smog gets worse as the day goes on. Morning rush hour puts pollution into the air, but the smog does not peak until afternoon, when the sun is strongest and has had hours to cook the chemicals together. On a hot, sunny day with little wind to blow the pollution away, smog can become thick enough to see and smell.

Two types of smog: photochemical and London smog

Photochemical smog is what most people encounter today. It forms in warm, sunny climates — Los Angeles, Phoenix, and other cities with intense sun and heavy traffic are frequent examples. The sunlight is essential; without it, the chemical reactions do not happen. This is why photochemical smog is worst in summer and in the afternoon.

London smog is a different beast, named after the deadly smog events that blanketed London in the 1950s. It forms when coal smoke and sulfur dioxide mix with fog and cold air, creating a thick, dark haze. London smog is less common now because most developed countries have moved away from coal heating and power, but it can still occur in places that burn coal heavily or in regions with cold, stagnant air and industrial pollution.

Why some days have worse smog than others

Weather is the biggest factor. Smog is worst when the air is hot, sunny, and still. Heat speeds up the chemical reactions that create ozone. Sunlight provides the energy that drives those reactions. And calm air — no wind — means pollution stays in one place instead of being blown away. This is why smog alerts often come on hot summer afternoons in cities surrounded by mountains or valleys, where geography traps air and prevents it from dispersing.

Seasonal patterns matter too. In warm climates, smog season runs from late spring through early fall. In colder regions, smog is less common because the chemical reactions slow down in winter, even if pollution levels are high. Time of day also plays a role: smog typically peaks in the afternoon and early evening, when the sun has been working on pollutants all day.

Health effects of breathing smog

Smog irritates the lungs and airways because ozone and other smog chemicals damage the cells lining your respiratory system. Short-term exposure can cause coughing, wheezing, shortness of breath, and chest pain, especially during physical activity. These symptoms usually fade once you leave the smog, but repeated exposure can cause lasting damage.

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 exercising. Older adults face greater risk because their lungs are less resilient. People with asthma, emphysema, or chronic bronchitis are hit harder because their airways are already inflamed. People with heart disease can experience chest pain or irregular heartbeats from smog exposure. Even healthy adults can feel effects on high-smog days, especially if they exercise outdoors.

How smog is measured and reported

Air quality is measured using the Air Quality Index, or AQI, which rates pollution on a scale from 0 to 500. The index focuses on five major pollutants: ground-level ozone, particulate matter, nitrogen dioxide, sulfur dioxide, and carbon monoxide. Each pollutant gets its own score, and the highest score becomes the overall AQI for that day.

The AQI is color-coded to make it straightforward to understand: green means good air quality, yellow means moderate (sensitive groups may feel effects), orange means unhealthy for sensitive groups, red means unhealthy for everyone, and purple or maroon means hazardous. You can check the AQI for your area through the EPA's website or through local news and weather services. Many cities also issue smog alerts or air quality warnings when the AQI reaches unhealthy levels.

What reduces smog: regulations and individual choices

Smog has decreased in many U.S. cities over the past few decades, even as population and traffic have grown. This happened largely because of the Clean Air Act and its amendments, which set limits on emissions from vehicles and factories. Catalytic converters on cars, cleaner fuel standards, and industrial emission controls all reduce the pollution that becomes smog. States with the worst smog problems, like California, have stricter vehicle emission standards than the federal government requires.

Individual choices matter too, though they are small compared to regulatory changes. Driving less, combining trips, using public transit, and avoiding unnecessary idling all reduce nitrogen oxides and volatile organic compounds. Choosing low-VOC paint and cleaning products reduces emissions from consumer products. On high-smog days, limiting outdoor exercise — especially for children and older adults — reduces the amount of polluted air you breathe in.

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, industrial chemicals, and more. Smog is one specific type of air pollution made of ozone and other chemicals formed when sunlight reacts with emissions. You can have polluted air without visible smog, and smog is always a form of pollution.

Can smog make you sick permanently?

Repeated exposure to smog can cause lasting lung damage, especially in children whose lungs are still developing. Long-term studies show that people who live in high-smog areas have lower lung function as adults. However, short-term smog exposure usually causes temporary symptoms that go away once you leave the polluted air.

Does a mask protect you from smog?

A regular cloth or surgical mask does not filter out ozone or the smallest particles in smog. An N95 or P100 respirator mask can block some particles, but it does not filter gases like ozone. On high-smog days, the best protection is to stay indoors with windows closed and use an air purifier with a HEPA filter if you have one.

Why is smog brown or gray?

The color comes from nitrogen dioxide, which is reddish-brown, and from tiny particles suspended in the air. When you see a thick brown haze, you are seeing nitrogen dioxide and particles of soot and dust. Gray smog usually means the pollution is thinner or farther away. The color does not indicate how harmful the smog is — invisible ozone can be just as damaging as visible haze.