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

The word "smog" comes from combining "smoke" and "fog," and that describes what you actually see: a brownish or grayish haze that reduces how far you can see and makes the air look thick. Unlike natural fog, which is just water vapor, smog contains harmful chemicals. The two most common types are photochemical smog, which forms when sunlight hits nitrogen oxides and volatile organic compounds in the air, and industrial smog, which is mostly sulfur dioxide and soot from burning coal and oil.

Smog does not just look bad—it irritates your lungs and eyes, and breathing it regularly can damage your respiratory system over time. Children, older adults, and people with asthma or heart disease are hit hardest. On high-smog days, you may notice your eyes watering, your throat feeling scratchy, or shortness of breath during exercise.

Key Takeaways

  • Smog is pollution made visible, created when sunlight reacts with emissions from vehicles, factories, and power plants.
  • Photochemical smog forms on warm, sunny days and is the type most common in cities; industrial smog forms from burning coal and oil.
  • Smog irritates your lungs and eyes and can cause lasting damage to your respiratory system with repeated exposure.
  • Weather patterns like temperature inversions trap smog close to the ground, making it worse in valleys and during calm, warm periods.
  • Air quality indexes published by local health departments tell you when smog levels are high enough to limit outdoor activity.

How photochemical smog forms on sunny days

Photochemical smog is the type you see most often in cities. It starts with nitrogen oxides—mostly from car exhaust—and volatile organic compounds (VOCs), which come from paint, gasoline, and industrial processes. When sunlight hits these chemicals, a chain reaction happens. The sun's energy breaks apart nitrogen dioxide molecules, which then recombine with oxygen and other compounds to create ozone and other harmful substances.

This process takes hours, which is why smog is often worst in the afternoon and evening, not right at rush hour. The chemicals have had time to cook in the sun. Warm, sunny days make it worse because more sunlight means more reactions. Cities in valleys or surrounded by mountains are especially vulnerable because the geography traps the polluted air and prevents wind from blowing it away.

Industrial smog and what causes it

Industrial smog forms differently and is less common in most developed countries now, but still occurs in areas with heavy coal burning or oil refining. When coal or oil burns, it releases sulfur dioxide and particulate matter—tiny solid particles and liquid droplets. These mix with water vapor in the air to create a thick, gray haze that smells like rotten eggs (the sulfur dioxide smell).

Industrial smog was the dominant type in cities like London and Pittsburgh in the mid-1900s, before pollution controls were put in place. It is still a major problem in parts of Asia and other regions where coal power plants and heavy industry are concentrated. Unlike photochemical smog, industrial smog can form on cool, cloudy days and does not require sunlight to develop.

Temperature inversions trap smog near the ground

Normally, air gets colder as you go higher in the atmosphere, so warm air near the ground rises and carries pollution upward and away. A temperature inversion reverses this: a layer of warm air sits above cooler air near the ground, acting like a lid. The cool air cannot rise, so pollution gets trapped close to where people breathe it.

Inversions happen most often in winter mornings or in valleys surrounded by hills. They can last for days, especially if the weather is calm and there is no wind to break them up. Cities like Los Angeles and Salt Lake City experience regular inversions because of their geography, which is why they have chronic smog problems even though they have strict emissions rules.

What ozone is and why it matters in smog

Ozone is a molecule made of three oxygen atoms bonded together. In the upper atmosphere, ozone protects us from ultraviolet radiation. But near the ground, where we breathe it, ozone is a pollutant. It forms when nitrogen oxides and volatile organic compounds react in sunlight, and it is the main harmful ingredient in photochemical smog.

Breathing ozone irritates your airways and can trigger asthma attacks. It also damages the lining of your lungs, making them more vulnerable to infection and reducing how much oxygen your blood can pick up. People who exercise outdoors on high-ozone days are at higher risk because they breathe more deeply and take in more of the pollutant.

Reading air quality reports and what the numbers mean

Most cities publish an Air Quality Index (AQI) daily, usually on the local health department or environmental agency website. The AQI ranges from 0 to 500, with color codes: green (good), yellow (moderate), orange (unhealthy for sensitive groups), red (unhealthy), purple (very unhealthy), and maroon (hazardous). The number is based on measurements of ozone, nitrogen dioxide, sulfur dioxide, particulate matter, and carbon monoxide.

When the AQI is orange or higher, people with asthma, heart disease, or respiratory problems should limit outdoor activity. When it is red or purple, everyone should reduce time outside and strenuous exercise. The AQI for your area updates throughout the day, so you can check it before planning outdoor activities. Many weather apps and local news stations also report the AQI alongside the regular forecast.

Pollution sources that create smog

Vehicle exhaust is the largest source of nitrogen oxides in most cities. Trucks, buses, and cars all emit these gases, and the problem gets worse during rush hour. Power plants that burn fossil fuels are another major source, as are refineries, chemical plants, and other industrial facilities. Smaller sources include gas stations, dry cleaners, and even lawn mowers and leaf blowers—anything that burns fuel or uses solvents releases VOCs or nitrogen oxides.

Natural sources also play a role. Trees and plants release VOCs, especially on hot days, and wildfires can create massive amounts of smoke and pollutants. However, human-caused emissions are the main driver of smog in populated areas. Reducing vehicle use, supporting cleaner energy sources, and maintaining emissions controls on industrial facilities all help lower smog levels.

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. Smog is one visible form of air pollution—you can see it. Other air pollutants like carbon monoxide or some particulates may not be visible but are still harmful.

Can smog damage my health permanently?

Repeated exposure to high smog levels can cause lasting damage to your lungs and airways, especially in children whose lungs are still developing. Short-term exposure causes irritation that goes away, but years of breathing polluted air can reduce lung function permanently and increase the risk of asthma and other respiratory diseases.

Why is smog worse in summer than winter?

Photochemical smog requires sunlight to form, so it is worst on warm, sunny days. Winter has shorter days and weaker sunlight, so less ozone forms. However, industrial smog and pollution from heating can make winter air quality poor in some areas despite lower photochemical smog.

Can I reduce my exposure to smog?

Yes. Check the AQI before going outside, and limit outdoor activity on high-smog days. Keep car windows closed during rush hour, use air filters in your home, and avoid strenuous exercise outdoors when the AQI is orange or higher. Staying indoors with filtered air is the most effective way to reduce exposure.

Do air purifiers remove smog?

HEPA filters and activated carbon filters can remove some smog particles and gases from indoor air, but they work best in sealed rooms. An air purifier cannot protect you outdoors, and it is not a substitute for checking the AQI and limiting outdoor time on high-pollution days.