Photochemical smog is a haze created when sunlight reacts with pollution already in the air

Photochemical smog is not the thick, gray industrial smoke of the past. It forms when ultraviolet light from the sun hits nitrogen oxides and volatile organic compounds (VOCs) already present in the air — usually from vehicle exhaust and factory emissions. The chemical reaction creates ground-level ozone and other compounds that turn the air brown or orange and make breathing difficult. Unlike traditional smog, which was mostly soot and sulfur, photochemical smog is a secondary pollutant, meaning it does not come directly from a source but forms in the atmosphere itself.

The process happens fastest on hot, sunny days with little wind, which is why cities in warm climates — Los Angeles, Phoenix, Mexico City — experience the worst episodes. The pollution builds up during the day as the sun gets stronger, peaks in the afternoon, and can linger into evening. On cooler, cloudy, or windy days, the reaction slows or the pollutants disperse, so smog levels drop.

Key Takeaways

  • Photochemical smog forms when sunlight chemically reacts with nitrogen oxides and volatile organic compounds from cars and factories, not from direct emissions.
  • The main harmful component is ground-level ozone, which damages lungs and is especially dangerous for children, older adults, and people with asthma.
  • Smog is worst on hot, sunny, windless days and peaks in the afternoon when UV radiation is strongest.
  • Reducing vehicle emissions and industrial VOC releases are the primary ways to lower photochemical smog in a region.

The chemical reaction that creates photochemical smog

The reaction starts with nitrogen oxides (NOx), which come mainly from vehicle engines and power plants. When sunlight hits these molecules, they break apart and recombine with oxygen and other compounds in the air. One of the products is ground-level ozone (O₃), a gas that is beneficial high in the atmosphere but harmful at ground level where people breathe it.

The reaction also involves volatile organic compounds (VOCs) — chemicals that evaporate easily from gasoline, paint, solvents, and industrial processes. VOCs and nitrogen oxides do not harm people directly at the concentrations found in smog, but when they react together in sunlight, they create ozone and other oxidants that do. The longer the sun shines and the more pollutants present, the more ozone forms. This is why smog worsens throughout the day and why a sunny afternoon produces worse air quality than a cloudy morning, even if the same amount of pollution is in the air.

Why photochemical smog is different from industrial smog

Industrial smog, common in the early 20th century, formed when coal smoke mixed with fog and sulfur dioxide. It was thick, gray, and visible when ready at the source. Photochemical smog is lighter in color — often a tan or reddish-brown haze — and forms downwind of pollution sources, sometimes miles away. A city can have low emissions but still experience bad smog if wind carries pollutants from nearby industrial areas or heavy traffic corridors.

Industrial smog was worst in winter and on calm, cold days when fog trapped pollution near the ground. Photochemical smog is worst in summer and early fall, when the sun is strong and temperatures are high. The two types also require different control strategies: industrial smog was reduced by burning cleaner fuel and removing sulfur from coal, while photochemical smog requires reducing nitrogen oxides and VOCs before they can react.

Health effects of breathing photochemical smog

Ground-level ozone irritates the airways and reduces lung function, even in healthy people. Exposure causes coughing, throat irritation, and chest pain, especially during physical activity. People with asthma, emphysema, or chronic bronchitis are at higher risk for severe symptoms and hospital visits on high-smog days. Children are more vulnerable because their lungs are still developing and they spend more time outdoors and exercising.

Older adults and people with heart disease also face increased risk, because ozone can trigger inflammation that affects both the lungs and the cardiovascular system. Repeated exposure over years may cause permanent damage to lung tissue. On days when air quality is poor, health agencies recommend that at-risk groups stay indoors, keep windows closed, and avoid strenuous outdoor activity.

Where photochemical smog is most common

Photochemical smog occurs in any region with high vehicle traffic, warm temperatures, and strong sunlight. Los Angeles is the most famous example because it sits in a basin surrounded by mountains, which traps air and prevents wind from dispersing pollutants. The city also has year-round sunshine and millions of cars. Other U.S. cities with frequent smog episodes include Phoenix, Houston, Denver, and parts of the San Francisco Bay Area.

Internationally, Mexico City, Beijing, Delhi, and Bangkok experience severe photochemical smog because they combine high vehicle emissions, industrial activity, and geography that traps air. Even smaller cities can have smog problems if they are surrounded by mountains or if pollution from nearby regions blows in. The problem is not limited to hot climates — cities in cooler regions can experience smog during warm spells, though it is usually less severe.

How regulations reduce photochemical smog

The primary approach is to reduce nitrogen oxides and VOCs at the source. Vehicle emission standards require catalytic converters and other technology to lower NOx and hydrocarbon emissions from tailpipes. Industrial facilities must capture or treat VOC emissions from manufacturing, painting, and chemical processes. Gasoline formulations have been changed to reduce evaporative emissions, and gas pumps are equipped with vapor recovery systems.

Ozone standards set by environmental agencies define the maximum concentration allowed on any given day. When forecasts predict that ozone will exceed the standard, some regions implement "smog alerts" that ask people to reduce driving, postpone outdoor activities, and limit industrial operations. Over the past 40 years, these regulations have significantly reduced smog in most U.S. cities, even though vehicle traffic has increased. However, many regions still exceed ozone standards on hot days, and climate change is expected to increase the frequency and severity of smog episodes as temperatures rise.

What you can do on high-smog days

Check your local air quality index (AQI) before planning outdoor activities. The AQI is reported daily by environmental agencies and is available on weather apps and websites. An AQI above 100 indicates unhealthy air; above 150 is very unhealthy. On these days, limit strenuous outdoor exercise, especially if you have asthma, heart disease, or are over 65.

Keep car windows closed and use the recirculate setting on your air conditioning to avoid drawing in outside air. If you must drive, avoid peak traffic hours when emissions are highest. Stay indoors in air-conditioned spaces if possible, and use HEPA filters in home air purifiers. Postpone lawn mowing, refueling, and other activities that release VOCs. If you have a respiratory condition, follow your doctor's information and keep rescue medications on hand during smog season.

Frequently Asked Questions

Is photochemical smog the same as ozone pollution?

Photochemical smog is a mixture of pollutants, but ground-level ozone is its main harmful component. Smog also contains nitrogen dioxide, peroxyacetyl nitrate, and other oxidants. When people talk about ozone pollution or ozone smog, they usually mean the same thing as photochemical smog.

Why does smog smell bad?

Photochemical smog has a sharp, acrid smell caused by ozone and other oxidants. The smell is a sign that harmful chemicals are present, though you cannot always smell smog even when ozone levels are high. A lack of smell does not mean the air is safe.

Can photochemical smog happen at night?

Photochemical smog forms during the day when sunlight is present. At night, ozone breaks down and smog levels drop. However, nitrogen oxides and VOCs remain in the air, so smog can form again quickly the next morning when the sun rises.

Does rain wash photochemical smog out of the air?

Rain can remove some particles and gases from the air, but it does not eliminate photochemical smog. Ozone and other oxidants are gases that do not dissolve easily in water. Rain may temporarily improve air quality, but smog returns as soon as the sun returns and the chemical reaction resumes.

Is photochemical smog getting worse?

In most U.S. cities, smog has improved since the 1970s because of emission regulations. However, climate change is expected to increase smog frequency and severity as temperatures rise and create more favorable conditions for ozone formation. Some regions are seeing smog episodes earlier in the year and lasting longer into fall.