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 hanging over a city or region. Unlike natural fog, which is just water vapor, smog contains harmful chemicals. The most common type is ground-level ozone, which forms when nitrogen oxides and volatile organic compounds (chemicals that evaporate easily) sit in the air on a warm, sunny day. Sunlight acts as a trigger, causing these chemicals to react and create ozone — the same gas that protects us in the upper atmosphere, but harmful when you breathe it at ground level.

Smog is not a single pollutant but a mixture. On any given day in a smoggy city, you are breathing nitrogen dioxide (from vehicle exhaust), sulfur dioxide (from power plants and refineries), particulate matter (tiny solid particles), and ground-level ozone all at once. The exact recipe changes depending on what industries are nearby, how many cars are on the road, the time of year, and the weather. This is why some days are much worse than others, even in the same city.

Key Takeaways

  • Smog forms when sunlight causes vehicle exhaust and industrial emissions to react chemically in the air, creating ground-level ozone and other pollutants.
  • The worst smog days typically occur in summer and early fall when temperatures are high and the air is still, trapping pollutants near the ground.
  • Breathing smog can irritate your lungs and worsen asthma, heart disease, and other respiratory conditions, especially during physical activity.
  • Ground-level ozone is the main ingredient in smog, different from the ozone layer high in the atmosphere that blocks ultraviolet radiation.

Why smog forms on hot, still days

Smog is worst when three conditions line up: warm temperatures, strong sunlight, and stagnant air. Warm air holds more pollutants and speeds up the chemical reactions that create ozone. Sunlight provides the energy those reactions need. Stagnant air — air that is not moving or being blown away by wind — keeps the pollutants trapped in one place instead of dispersing them.

This is why smog peaks in summer and early fall rather than winter. It is also why cities in valleys or basins surrounded by hills tend to have worse smog than cities on open plains — the geography traps air in place. Los Angeles, Denver, and Mexico City all sit in geographic bowls where air naturally stagnates, which is why they are known for smog problems.

Weather patterns matter too. A thermal inversion makes smog much worse. This happens when a layer of warm air sits above a layer of cooler air near the ground, acting like a lid and preventing pollutants from rising and dispersing. When a thermal inversion is in place, smog can build up to dangerous levels in just a few hours.

What smog does to your health

Breathing smog irritates your airways. You might notice a scratchy throat, coughing, or chest tightness, especially if you are exercising outdoors. For people with asthma, smog can trigger an attack. For people with heart disease or chronic lung disease, smog can make their condition worse and increase the risk of a heart attack or stroke.

Children and older adults are more vulnerable because their lungs are still developing or have weakened over time. People who work outdoors — construction workers, landscapers, mail carriers — face higher exposure than people who work indoors. Even healthy people can experience symptoms on a bad smog day if they spend hours outside.

The harm is not just from one day of exposure. Breathing smog regularly over months and years can reduce lung function permanently and increase the risk of developing asthma or heart disease later in life. This is why cities with chronic smog problems track air quality daily and issue health warnings when levels get dangerous.

How smog is measured and reported

The Air Quality Index (AQI) is the standard way to measure and report smog and other air pollution in the United States. The AQI ranges from 0 to 500, with higher numbers meaning worse air quality. An AQI of 0 to 50 is considered "good," 51 to 100 is "moderate," 101 to 150 is "unhealthy for sensitive groups," 151 to 200 is "unhealthy," 201 to 300 is "very unhealthy," and above 300 is "hazardous."

Government agencies measure five main pollutants to calculate the AQI: ground-level ozone, particulate matter (PM2.5 and PM10), nitrogen dioxide, sulfur dioxide, and carbon monoxide. The pollutant with the highest concentration on a given day determines that day's AQI number. Most cities publish their AQI online and update it hourly, so you can check before going outside.

When the AQI reaches unhealthy levels, cities often issue air quality alerts. These alerts recommend that sensitive groups — children, older adults, and people with respiratory or heart conditions — stay indoors or limit outdoor activity. On very bad days, schools may cancel outdoor sports, and people are advised to wear N95 masks if they must go outside.

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 other tiny particles — is a separate pollutant that can exist without smog. You might see particulate matter as a gray haze, especially near highways or industrial areas. Acid rain is another type of air pollution caused by sulfur dioxide and nitrogen oxides, but it falls as precipitation rather than hanging in the air as a visible haze.

Smog specifically refers to the visible haze created by ground-level ozone and the particles that form with it. You can have a day with high particulate matter but low ozone, or vice versa. Both are harmful to breathe, but they form through different chemical processes and require different control strategies.

Where smog is worst in the United States

Smog is not evenly distributed. Cities with heavy vehicle traffic, warm climates, and geography that traps air tend to have the worst problems. Southern California, particularly the Los Angeles area, has historically had the worst smog in the nation because of its combination of millions of cars, year-round warm weather, and mountains that trap air. The San Joaquin Valley in California also ranks high because of agricultural emissions and stagnant air patterns.

Other cities with significant smog problems include Denver, Phoenix, Houston, and parts of the eastern United States during summer months. Even cities that do not have chronic smog can experience unhealthy air quality on particular days when weather conditions align. Smog does not respect city boundaries — pollution from one region can drift downwind and affect neighboring areas.

What causes the emissions that create smog

The main sources of the chemicals that create smog are vehicles, power plants, refineries, and chemical manufacturing facilities. Cars and trucks emit nitrogen oxides and volatile organic compounds. Power plants that burn coal or natural gas emit nitrogen oxides and sulfur dioxide. Refineries and chemical plants emit volatile organic compounds. Even some consumer products like paints, solvents, and air fresheners release volatile organic compounds into the air.

In rural areas, agricultural activities and wildfires can contribute to smog formation. Wildfires in particular can create massive amounts of smoke and particulate matter that drift hundreds of miles downwind, affecting air quality in cities far from the fire itself. Climate change is making wildfire season longer and more intense, which means more smoke-related air quality problems in recent years.

Frequently Asked Questions

Is smog the same as fog?

No. Fog is water vapor that condenses into tiny water droplets, and it is not harmful to breathe. Smog is a mixture of pollutants including ground-level ozone, nitrogen dioxide, and particulate matter. Smog looks similar to fog — a haze in the air — but it contains chemicals that irritate your lungs and airways.

Can you see smog on every bad air quality day?

Not always. High levels of particulate matter or other pollutants can make the air unhealthy to breathe even if you cannot see a visible haze. Conversely, you might see a haze that looks like smog but the AQI is still in the moderate range. Visibility and actual air quality do not always match, so checking the AQI is more reliable than judging by what you see.

Does smog only happen in big cities?

Smog forms most often in cities with heavy traffic and industry, but it can affect rural areas too. Pollution from cities drifts downwind and can create smog in smaller towns and agricultural regions. Wildfires can create smog hundreds of miles away from the fire itself.

Can you get smog indoors?

Ground-level ozone and other smog pollutants can seep indoors, especially if windows and doors are open or if your home does not have good air sealing. However, indoor air quality is usually better than outdoor air during a smog event. Running an air purifier with a HEPA filter can reduce indoor smog exposure further.