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

The word "smog" is a combination of "smoke" and "fog," and it describes what you actually see: a brownish or grayish haze that reduces visibility and makes the air look thick. Unlike fog, which is water vapor, smog is made of chemical pollutants. The two main types are photochemical smog, which forms in sunlight and is common in warm cities, and London smog (or sulfurous smog), which forms in cold, stagnant air and is less common in modern cities.

Smog forms through a specific chemical chain. Cars, trucks, and factories release nitrogen oxides and volatile organic compounds into the air. When sunlight hits these chemicals, they react and create ground-level ozone and other secondary pollutants. This is why smog is often worse in the afternoon and early evening, when the sun is strongest, and why it tends to build up in valleys or cities surrounded by mountains that trap air.

Key Takeaways

  • Smog is a visible haze caused by sunlight reacting with vehicle and industrial emissions, not just smoke or fog.
  • Photochemical smog forms in warm, sunny conditions and is the type most people encounter in modern cities.
  • Weather patterns like temperature inversions trap smog near the ground, making it worse on still, warm days.
  • Smog reduces visibility, damages lung tissue, and can trigger asthma and other respiratory problems, especially in children and older adults.

How sunlight and emissions create smog

The chemistry behind smog starts with what cars and factories release. Nitrogen oxides (NOx) come from burning fuel at high temperatures. Volatile organic compounds (VOCs) come from gasoline, solvents, and industrial processes. On their own, these chemicals are harmful but invisible.

When ultraviolet light from the sun hits these chemicals, a chain reaction begins. The nitrogen oxides break apart, and the pieces recombine with oxygen and organic compounds to form ground-level ozone (O₃). Ozone at ground level is not the same as the ozone layer high in the atmosphere—ground-level ozone is a pollutant that damages lungs. The reaction also produces other compounds like peroxyacetyl nitrate (PAN) and secondary organic aerosols, which add to the visible haze.

This is why smog gets worse as the day goes on. Morning emissions sit in the air, and as the sun climbs higher, the reaction accelerates. By mid-afternoon, ozone levels peak. This is also why smog is worse in summer than winter—longer days and stronger sun mean more reaction time and more intense ultraviolet light.

Why geography and weather trap smog in place

Not every city with cars and factories has visible smog. The difference is often geography and weather. A temperature inversion is the main culprit. Normally, air gets colder as you go higher in the atmosphere, so warm air near the ground rises and carries pollution away. In an inversion, a layer of warm air sits above cooler air near the ground, acting like a lid. The cool air and its pollutants cannot rise, so they accumulate.

Cities in valleys or surrounded by mountains are especially vulnerable. Los Angeles, Denver, and Salt Lake City all experience frequent inversions because mountains trap air. On a still, warm day with an inversion in place, smog can build to dangerous levels in just a few hours. Wind helps disperse smog, which is why coastal cities with sea breezes often have less smog than inland cities.

Humidity also plays a role. High humidity can increase the formation of secondary aerosols, making smog thicker and more visible. Cold, dry air (like in winter) is less likely to produce visible smog, even if the same emissions are present, because the chemical reactions are slower.

Health effects of breathing smog

Smog damages the respiratory system. Ground-level ozone irritates the airways, causing inflammation and reducing lung function. People who breathe smog often experience coughing, throat irritation, and shortness of breath, especially during physical activity. The damage is not always when ready—some people feel fine during exposure but develop symptoms hours later.

Children, older adults, and people with asthma or other lung diseases are at highest risk. Children's lungs are still developing, and they spend more time outdoors playing, so they inhale more smog. Older adults often have reduced lung capacity already. People with asthma, emphysema, or bronchitis can have severe reactions—wheezing, chest pain, and difficulty breathing that may require emergency care.

Long-term exposure to smog increases the risk of chronic lung disease, even in people without existing conditions. Studies have linked repeated smog exposure to reduced lung growth in children and accelerated lung decline in adults. The effects are cumulative, so living in a chronically smoggy city carries more risk than occasional exposure.

The difference between smog and other air pollution

Smog is one type of air pollution, but not the only one. Particulate matter (dust, soot, and tiny particles) is another major pollutant. Smog is chemical pollution created by a reaction; particulate matter is physical pollution—solid or liquid particles suspended in air. A city can have smog without visible particulate matter, or vice versa, or both at once.

Acid rain is related but different. Smog forms near the ground where people breathe it. Acid rain forms higher in the atmosphere when sulfur dioxide and nitrogen oxides dissolve in water droplets, creating acidic clouds and precipitation. The same emissions can contribute to both smog and acid rain.

Air quality indexes (AQI) measure multiple pollutants, not just smog. The AQI includes ozone, particulate matter, nitrogen dioxide, sulfur dioxide, and carbon monoxide. On a smoggy day, the ozone reading will be high, but other readings may be low. This is why an AQI reading of 150 (unhealthy) might look like thick brown haze (smog) or might look like clear air with invisible particulate matter.

Why some cities have worse smog than others

The amount of smog a city experiences depends on three things: emissions, geography, and climate. Cities with heavy traffic, many factories, or coal-fired power plants produce more nitrogen oxides and volatile organic compounds. But emissions alone do not determine smog levels. Phoenix produces fewer emissions than many cities but has frequent smog because of its geography and climate—surrounded by mountains, hot year-round, and prone to inversions.

Regulations have reduced smog in many cities over the past 40 years. The Clean Air Act and state emissions standards have pushed cars to be cleaner and factories to install pollution controls. Los Angeles still has smog, but far less than it did in the 1970s, even though the city has more cars. However, some regions still exceed federal ozone standards regularly, especially in the Southwest and parts of the Midwest.

Climate change is making smog worse in some places. Higher temperatures speed up the chemical reactions that create ozone, and longer summers extend the smog season. Drought and heat waves can create conditions that trap smog for days or weeks.

What you can do on a smoggy day

When smog is visible, the air quality is poor enough to affect health. Check your local air quality index before spending time outdoors. If the AQI is above 100 (unhealthy for sensitive groups) or 150 (unhealthy for everyone), limit outdoor activity, especially strenuous exercise. Children, older adults, and people with lung disease should be more cautious.

If you must go outside on a smoggy day, avoid peak smog hours (usually mid-afternoon through early evening). Stay indoors with windows closed and use an air filter if you have one. N95 masks can reduce inhalation of some pollutants, though they do not filter ozone, which is a gas.

Reducing your own emissions helps over time. Driving less, maintaining your vehicle, and using public transit or biking reduce the nitrogen oxides and volatile organic compounds that create smog. On high-smog days, many cities ask people to avoid driving to help reduce the chemical precursors in the air.

Frequently Asked Questions

Is smog the same as fog?

No. Fog is water vapor that condenses into visible droplets when air cools. Smog is chemical pollution—nitrogen oxides and volatile organic compounds that react in sunlight to form ozone and other pollutants. Fog is harmless; smog damages lungs. You can have fog without smog, smog without fog, or both at once.

Does smog go away on its own?

Smog disperses when wind blows it away or when weather conditions change. A cold front, rain, or strong wind can clear smog in hours. But the chemical reactions that create smog continue as long as sunlight and emissions are present, so smog often returns the next day in the same location.

Can an air purifier remove smog from my home?

A HEPA filter removes particulate matter but not ozone, which is a gas. If smog is visible outside, some ozone will enter your home even with windows closed. A purifier with activated carbon can absorb some gases, but the most effective approach is to keep windows and doors closed and use your home's air conditioning on recirculate mode.

Why is smog worse in summer?

Longer days and stronger ultraviolet light speed up the chemical reactions that create ozone. Higher temperatures also increase the rate of reaction and can create temperature inversions that trap smog. Winter has shorter days, weaker sun, and cooler temperatures, all of which slow smog formation.

Does rain wash smog out of the air?

Rain removes particulate matter and some gases by dissolving them in water droplets. However, ozone is not very soluble in water, so rain removes only a small fraction of the smog. Wind is more effective at dispersing smog than rain is at removing it.