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 it describes the brownish or grayish haze you see hanging over cities on certain days. Unlike natural fog, smog is made of harmful chemicals and particles that form through a chemical reaction between sunlight and pollutants already in the air. The reaction happens fastest on hot, still days when the sun is strong and air doesn't move much — which is why smog is worse in summer and in cities surrounded by mountains or valleys that trap air.
Smog comes from two main sources: the exhaust from millions of cars and trucks, and emissions from factories, power plants, and refineries. When these sources release nitrogen oxides and volatile organic compounds into the air, sunlight breaks them down and recombines them into new chemicals, primarily ground-level ozone. This ozone is not the protective layer high in the atmosphere — it is a harmful pollutant at ground level that irritates your lungs and eyes.
Key Takeaways
- Smog forms when sunlight chemically reacts with car exhaust and industrial emissions, creating ground-level ozone that damages lung tissue.
- Smog is worse on hot, still days and in cities surrounded by geography that traps air, such as Los Angeles, Houston, and the San Bernardino Valley.
- Breathing smog can trigger asthma attacks, reduce lung function over time, and cause when ready symptoms like coughing and throat irritation.
- The U.S. Environmental Protection Agency sets air quality standards and issues daily air quality forecasts so you know when to limit outdoor activity.
Why certain cities have worse smog than others
Geography plays a huge role in how much smog accumulates. Cities in valleys or surrounded by mountains — like Los Angeles, Denver, and Salt Lake City — trap air and prevent it from dispersing. When pollutants cannot blow away, they stay in place and react with sunlight for longer periods, building up to dangerous levels. Coastal cities and cities in flat, open areas have better air circulation and less smog buildup.
Climate also matters. Cities in hot regions with strong summer sun produce more smog because the chemical reaction that creates ozone happens faster in heat. Southern California, Texas, and Arizona have historically had the worst smog problems in the United States. Cities in cooler climates or at higher altitudes tend to have less smog, though they are not immune — any city with heavy traffic and industry can develop smog under the right conditions.
Population density and industrial activity determine how many pollutants enter the air in the first place. A sprawling city with millions of cars produces far more nitrogen oxides and volatile organic compounds than a smaller city, even if both have similar geography. Cities with oil refineries, chemical plants, or coal-fired power plants add industrial emissions on top of vehicle exhaust, making the problem worse.
How smog affects your health
Smog irritates your respiratory system — your lungs, throat, and airways — because ground-level ozone is a corrosive chemical. On high-smog days, people often report coughing, throat soreness, and watery eyes. These symptoms usually fade once you move to cleaner air, but repeated exposure over years can cause lasting damage.
People with asthma, emphysema, or other chronic lung diseases are at highest risk. Smog can trigger asthma attacks and make breathing harder during physical activity. Children and older adults are also more vulnerable because their lungs are still developing or have already weakened with age. Even healthy people can experience reduced lung function and shortness of breath during high-smog events, especially if they exercise outdoors.
Long-term exposure to smog increases the risk of developing chronic respiratory disease and may contribute to heart disease. Studies have shown that people who live in high-smog areas for decades have lower lung function than people in cleaner areas, even if they never smoked. The damage is not always reversible once it occurs.
Reading the Air Quality Index and smog forecasts
The U.S. Environmental Protection Agency publishes the Air Quality Index, or AQI, which rates air pollution on a scale from 0 to 500. The scale uses color codes: green (0–50) means good air, yellow (51–100) means moderate, orange (101–150) means unhealthy for sensitive groups, red (151–200) means unhealthy for everyone, and purple or maroon (201+) means hazardous. Most weather services and air quality websites display the AQI for your city, updated hourly or several times per day.
When the AQI reaches orange or higher, smog is visible and you should limit outdoor activity, especially strenuous exercise. Children, older adults, and people with lung or heart conditions should stay indoors on red or purple days. On yellow days, sensitive people may notice symptoms but the general population is usually unaffected. You can check the AQI on websites like AirNow.gov, which covers most U.S. cities, or through your local weather forecast.
Many cities issue smog alerts or air quality forecasts the day before, so you can plan outdoor activities accordingly. If you have asthma or another respiratory condition, keeping an eye on the AQI helps you know when to take extra medication or avoid outdoor exercise.
What you can do to reduce smog
Individual actions add up when millions of people do them. Driving less — by carpooling, using public transit, biking, or walking — directly reduces the nitrogen oxides that form smog. On high-smog days, some cities ask residents to avoid unnecessary driving, and following that request helps keep pollution levels from spiking further. Keeping your car well-maintained and using the recommended fuel grade also reduces emissions.
At home, you can reduce volatile organic compounds by choosing low-VOC paints, cleaners, and solvents. Avoiding unnecessary use of lawn equipment on high-smog days helps too, since gas-powered mowers and leaf blowers emit pollutants. Supporting policies that limit industrial emissions and promote clean energy — such as voting for candidates who prioritize air quality — affects smog levels at a larger scale.
On high-smog days, staying indoors with windows closed and using air conditioning with a clean filter reduces the smog you breathe. If you must go outside, avoid strenuous exercise during peak smog hours, which are usually late afternoon when the sun has had all day to react with pollutants.
The difference between smog and other air pollution
Smog is one type of air pollution, but not the only one. Particulate matter — tiny solid particles and liquid droplets suspended in air — is a separate pollutant that comes from dust, pollen, soot, and smoke. You can see particulate matter as haze or dust, and it damages your lungs when you breathe it. Smog and particulate matter often occur together in cities, making the air look even hazier.
Acid rain is another form of pollution caused by sulfur dioxide and nitrogen oxides, but it falls as precipitation rather than hanging in the air as smog. Ground-level ozone — the main component of smog — is different from the ozone layer high in the atmosphere, which protects Earth from ultraviolet radiation. Protecting the ozone layer and reducing ground-level ozone are two separate environmental goals.
Frequently Asked Questions
Is smog the same as fog?
No. Fog is water vapor that condenses into visible droplets when air cools, and it is not harmful to breathe. Smog is a chemical haze made of ozone and other pollutants created by sunlight reacting with car and factory emissions. Smog irritates your lungs and eyes; fog does not.
Can smog travel between cities?
Yes. Wind can carry smog and ozone hundreds of miles from where it formed. A city downwind of a major industrial area or a region with heavy traffic can experience smog even if it has relatively low local emissions. This is why air quality problems sometimes require regional cooperation to solve.
Does smog happen in winter?
Smog is less common in winter because the sun is weaker and days are shorter, so the chemical reaction that creates ozone happens more slowly. However, winter can bring other air pollution problems, such as particulate matter from heating and inversions that trap cold air and pollutants near the ground.
Can air filters in my home remove smog?
HEPA filters and activated carbon filters can remove some particles and gases from indoor air, but they work best when windows and doors are closed. On high-smog days, running an air purifier indoors helps, but the most effective step is to stay inside and avoid outdoor activity until air quality improves.
What is a temperature inversion and how does it make smog worse?
A temperature inversion occurs when a layer of warm air sits above cooler air near the ground, trapping pollutants below like a lid on a pot. This prevents smog from rising and dispersing into the upper atmosphere. Inversions are common in valleys and can last for days, causing smog to accumulate to dangerous levels even if emissions do not increase.