Smog is a visible layer of air pollution made from vehicle exhaust, factory emissions, and sunlight reacting together

The word "smog" is short for "smoke" and "fog." It describes air pollution thick enough to see — usually a brownish or yellowish haze that reduces how far you can see and can make breathing harder. Smog forms when nitrogen oxides and volatile organic compounds (chemicals that evaporate easily) from cars, trucks, power plants, and factories mix with sunlight. The sun's ultraviolet rays trigger a chemical reaction that creates ground-level ozone, which is the main ingredient in smog.

Smog is different from regular fog. Fog is water vapor that condenses into tiny droplets when air cools down. Smog is pollution — it contains ozone and other harmful chemicals. You can't wash it away or blow it away. It sits in the air until weather patterns change or the sun sets and stops triggering the chemical reactions that create it.

Key Takeaways

  • Smog forms when vehicle and factory emissions react with sunlight, creating ground-level ozone that you can see as a haze.
  • The worst smog typically occurs on hot, sunny days when there is little wind to disperse the pollution.
  • Breathing smog can irritate your lungs and airways, and people with asthma or heart disease are at higher risk.
  • Smog is a ground-level problem caused by local and regional pollution sources, not the same as the ozone layer high in the atmosphere.

Why smog forms on certain days and not others

Smog requires three things: emissions from vehicles and industry, sunlight, and stagnant air. On hot, sunny days with little wind, smog builds up because the pollution stays in one place instead of being carried away. Temperature inversions make this worse — this happens when a layer of warm air sits above cooler air near the ground, trapping pollution underneath like a lid on a pot.

Summer afternoons typically have the worst smog because the sun is strongest and more people are driving. Morning rush hour produces emissions, but the sun hasn't yet triggered the chemical reaction that creates ozone. By afternoon, the ozone levels peak. Winter smog is less common in most places because the sun is weaker and people drive less, though some regions still see pollution problems year-round.

What smog does to your health

Breathing smog irritates your lungs and airways. You might notice a scratchy throat, coughing, or chest tightness. Your eyes can water and feel irritated too. For most people, these symptoms go away once you move to cleaner air or stay indoors. But for people with asthma, emphysema, or heart disease, smog can trigger serious symptoms — wheezing, shortness of breath, or chest pain — that may require medical attention.

Children and older adults are also more vulnerable to smog because their lungs are still developing or have weakened over time. Outdoor workers and athletes who breathe hard during exercise take in more smog and face higher risk. On high-smog days, health departments often issue air quality warnings suggesting that sensitive groups stay indoors or limit outdoor activity.

The difference between smog and the ozone layer

Ground-level ozone — the kind in smog — is harmful pollution you breathe in. The ozone layer is a different thing entirely. It sits 10 to 30 miles above Earth and blocks ultraviolet radiation from the sun. Damage to the ozone layer (from chemicals called CFCs) lets more UV radiation reach Earth's surface, increasing skin cancer risk. Smog is a local air quality problem. Ozone layer damage is a global atmospheric problem. They are separate issues with different causes and different solutions.

This confusion matters because some people think that reducing smog helps the ozone layer, or that ozone layer damage causes smog. Neither is true. You can have bad smog in a city with a healthy ozone layer above it. Conversely, ozone layer damage happens whether or not a region has smog problems.

Where smog is worst and why

Smog is worst in areas with heavy vehicle traffic, many factories, or both — especially in regions with geography that traps air. Los Angeles, for example, sits in a basin surrounded by mountains. Pollution from millions of cars and trucks gets trapped there instead of being blown away by wind. Cities in valleys or near coasts can have the same problem. Rural areas and regions with fewer cars and factories typically have much less smog.

Smog also travels. Pollution from one city can drift downwind and create smog in another region. A city with strict emissions rules can still experience smog from pollution sources hundreds of miles away. This is why smog is sometimes a regional problem, not just a local one.

How air quality is measured and reported

The Air Quality Index (AQI) is a number from 0 to 500 that tells you how polluted the air is. The higher the number, the worse the air quality and the greater the health risk. An AQI below 50 is considered good. Between 50 and 100 is moderate — sensitive groups should consider limiting outdoor activity. Above 100 is unhealthy, and above 150 is very unhealthy. Most weather services, news stations, and weather apps show the AQI for your area. You can also check AirNow.gov, which is run by the Environmental Protection Agency and shows real-time air quality data for locations across the United States.

The AQI measures several pollutants, but ground-level ozone is usually the main one driving the number on smoggy days. When you see an air quality alert on your phone or in the news, it is usually because ozone levels are high.

What you can do on high-smog days

On days when smog is heavy, limit outdoor activity, especially if you have asthma, heart disease, or are over 65. Keep windows closed and use air conditioning with a clean filter if you have it. If you must go outside, avoid strenuous exercise that makes you breathe harder and take in more pollution. Wear an N95 mask if you have one — it filters out some of the particles and ozone, though it is not a perfect solution.

If you have a respiratory condition, keep your rescue inhaler with you and follow your doctor's instructions about when to use it. If you experience chest pain, severe shortness of breath, or confusion during high-smog days, seek medical attention. These can be signs of a serious reaction.

Frequently Asked Questions

Is smog the same as air pollution?

Smog is a type of air pollution — specifically, pollution thick enough to see as a haze. Air pollution is a broader term that includes gases and particles you cannot see, like carbon monoxide and fine dust. All smog is air pollution, but not all air pollution is smog.

Can smog make you permanently sick?

Short-term exposure to smog causes temporary irritation that goes away once you breathe cleaner air. Long-term exposure — living in a smoggy area for years — can contribute to chronic lung disease and heart problems. Children who grow up in high-smog areas may have reduced lung function as adults.

Does rain wash smog out of the air?

Rain can remove some particles and ozone from the air, but it does not eliminate smog completely. After rain, the air usually feels clearer because the rain has knocked down some pollution, but smog can return quickly if emissions continue and conditions are right for it to form again.

Why do cities with strict car emissions rules still have smog?

Modern cars produce fewer emissions than older ones, but millions of vehicles still release nitrogen oxides and other chemicals every day. It takes time for the fleet to turn over to cleaner vehicles. Additionally, smog can drift in from other regions, and some industrial sources still emit pollution even in cities with strong environmental rules.