What smog is and where it comes from

Smog is a visible haze that forms when air pollution builds up near the ground. The word itself comes from "smoke" and "fog" — it looks like a thick, dirty cloud hanging over a city or region. Smog forms when pollutants from cars, factories, power plants, and other sources get trapped in the air close to Earth's surface instead of dispersing into the upper atmosphere.

Two main weather conditions make smog worse. Temperature inversions occur when a layer of warm air sits above cooler air near the ground, trapping pollutants underneath like a lid on a pot. Still air or light winds also trap pollution in one place instead of blowing it away. This is why smog is often worst in valleys, coastal cities, and areas surrounded by mountains that block wind.

The pollutants that create smog come from burning fuel. Cars emit nitrogen oxides and volatile organic compounds. Coal-fired power plants release sulfur dioxide. Refineries, chemical plants, and even gas stations release vapors. On their own, these chemicals would still be harmful, but they behave very differently depending on whether sunlight is involved.

Key Takeaways

  • Smog is trapped air pollution that forms when pollutants from vehicles and industry cannot disperse due to weather patterns or geography.
  • Classical smog, also called sulfurous smog, forms in cold, still conditions and contains sulfur dioxide and soot from burning coal.
  • Photochemical smog forms when sunlight chemically transforms nitrogen oxides and volatile organic compounds into new, more harmful pollutants like ozone.
  • Photochemical smog is worse in warm, sunny climates and typically peaks in the afternoon when sunlight is strongest.
  • Both types damage human health, reduce visibility, and harm crops and ecosystems, but they require different control strategies.

Classical smog versus photochemical smog

The two main types of smog form through completely different chemical processes. Classical smog, also called sulfurous smog or London-type smog, forms when sulfur dioxide and soot from burning coal accumulate in cold, damp, still air. This type was common in industrial cities during the 19th and early 20th centuries. The 1952 Great Smog of London killed thousands of people in just a few days because the pollution was so thick and toxic.

Photochemical smog is a modern problem that forms when sunlight triggers a chemical reaction between nitrogen oxides and volatile organic compounds. The sun's ultraviolet rays break apart nitrogen dioxide molecules, releasing oxygen atoms that then combine with oxygen gas to form ozone. This ground-level ozone is the main ingredient in photochemical smog, along with other secondary pollutants like peroxyacetyl nitrate (PAN) and formaldehyde. Unlike classical smog, photochemical smog does not require cold weather or coal smoke — it thrives in warm, sunny conditions.

The timing of each type is different too. Classical smog can form any time of year when conditions are right, and it often gets worse in winter. Photochemical smog typically peaks in late spring through early fall and is worst in the afternoon when the sun is strongest and has had hours to drive the chemical reactions.

How photochemical smog forms step by step

Photochemical smog starts with emissions from traffic and industry. Vehicles emit nitrogen monoxide (NO) and volatile organic compounds (VOCs) like benzene and formaldehyde. These chemicals are invisible — you cannot see them in the air. The transformation begins when sunlight hits nitrogen dioxide (NO₂), a reddish-brown gas that is already present or forms quickly from nitrogen monoxide.

Ultraviolet light breaks apart the NO₂ molecule into nitrogen monoxide and a free oxygen atom. That oxygen atom when ready reacts with oxygen gas (O₂) in the air to form ozone (O₃). At the same time, the volatile organic compounds are also being broken down by sunlight and reacting with other chemicals in the air. These reactions produce secondary pollutants — new chemicals that were not directly emitted but formed in the atmosphere. The most important secondary pollutant is ground-level ozone, which is the main component of photochemical smog.

This process is not instantaneous. It takes hours of sunlight and chemical reactions to build up enough ozone and other secondary pollutants to create visible smog. This is why photochemical smog typically appears in the afternoon rather than the morning, even though rush-hour traffic releases the most emissions in the early morning. The smog is worst when the sun has been strong for several hours and the pollutants have had time to accumulate and transform.

Health effects of both types of smog

Both classical and photochemical smog damage human health, but in slightly different ways. Classical smog irritates the respiratory system directly — sulfur dioxide dissolves in moisture in the lungs and forms sulfuric acid, which damages airways and makes breathing painful. People with asthma, bronchitis, or heart disease are at highest risk during classical smog events.

Photochemical smog is dangerous primarily because of ground-level ozone. Ozone is a strong oxidant that damages the cells lining the lungs, throat, and eyes. Repeated exposure reduces lung function over time, even in healthy people. Children, older adults, and people with asthma or other lung disease are most vulnerable. Ozone also triggers inflammation in the airways, making it harder to breathe during exercise or physical activity.

Both types of smog reduce visibility, which creates hazards for drivers and can disrupt air traffic. Both also damage crops, forests, and ecosystems by depositing acids and oxidants on plants and soil. Classical smog leaves visible soot and sulfate deposits; photochemical smog's damage is often invisible but just as real.

Where photochemical smog is most common

Photochemical smog is worst in warm, sunny regions with heavy vehicle traffic and industrial activity. Southern California, particularly the Los Angeles area, experiences some of the most severe photochemical smog in the United States because of its geography, climate, and traffic volume. The region is surrounded by mountains that trap air, has year-round sunshine, and millions of vehicles that emit nitrogen oxides and volatile organic compounds daily.

Other regions prone to photochemical smog include the eastern United States during summer months, parts of Europe, and rapidly developing areas in Asia where vehicle use is increasing. Mexico City, Bangkok, and other cities in warm climates with heavy traffic also experience regular photochemical smog events. Classical smog is now rare in developed countries because coal burning has declined, but it remains a problem in regions that still rely heavily on coal for electricity and heating.

Strategies to reduce each type of smog

Controlling classical smog requires reducing sulfur dioxide emissions. This means switching away from high-sulfur coal, using scrubbers on power plant smokestacks to remove sulfur dioxide before it enters the air, and enforcing emissions standards on industrial facilities. Many developed countries have largely eliminated classical smog by doing exactly this.

Photochemical smog is harder to control because it requires reducing both nitrogen oxides and volatile organic compounds — and these come from many sources. Vehicle emissions standards have reduced the amount of nitrogen oxides that cars emit, but the number of vehicles on the road keeps growing. Refineries and chemical plants must capture and control volatile organic compound emissions. Paint, solvents, and gasoline vapors are also sources, so regulations on consumer products help too. Some regions use ozone action days to ask people to reduce driving and industrial activity when conditions favor smog formation.

The most effective approach combines multiple strategies: stricter emissions standards on vehicles and industry, cleaner fuels, better public transportation to reduce vehicle use, and land-use planning that keeps residential areas away from major pollution sources.

Monitoring and forecasting smog events

Air quality monitoring stations measure the concentration of pollutants in the air, including nitrogen dioxide, ozone, sulfur dioxide, and particulate matter. These measurements are reported as an Air Quality Index (AQI) that tells the public how polluted the air is on any given day. The AQI ranges from 0 to 500, with higher numbers indicating worse air quality and greater health risk.

Forecasters use weather models to predict when smog will form. For photochemical smog, they look at the forecast for temperature, wind speed, and cloud cover. Warm, sunny, still days with light winds are ideal for smog formation. When forecasters predict these conditions, they issue air quality forecasts warning the public that smog is likely. People with respiratory or heart conditions can then plan to stay indoors or limit outdoor activity.

Frequently Asked Questions

Is ground-level ozone the same as the ozone layer?

No. The ozone layer is in the upper atmosphere and protects Earth from harmful ultraviolet radiation. Ground-level ozone forms near the surface and is a pollutant that damages health. They are the same chemical (O₃) but in completely different places with opposite effects on human health.

Why does photochemical smog smell bad?

Photochemical smog contains ozone and other secondary pollutants that have a sharp, acrid smell — some people describe it as bleach-like. The smell comes from the reactive chemicals themselves, not from soot or sulfur compounds like in classical smog.

Can photochemical smog form on cloudy days?

Photochemical smog requires ultraviolet light, so it forms much more slowly or not at all on heavily cloudy days. Cloud cover blocks sunlight and stops the chemical reactions that create ozone. This is why photochemical smog is almost always a problem on clear, sunny days.

What should I do if there is a smog warning in my area?

Check the Air Quality Index for your location. If it is in the unhealthy range, limit outdoor activity, especially strenuous exercise. Keep windows closed to reduce indoor pollution. People with asthma, heart disease, or other respiratory conditions should follow their doctor's guidance and consider staying indoors entirely during severe smog events.

Has photochemical smog gotten worse over time?

In developed countries with strict emissions standards, photochemical smog has improved since the 1970s despite more vehicles on the road. However, in rapidly developing regions where vehicle use is growing faster than emissions controls, photochemical smog has worsened significantly.