Smog is a visible layer of pollution that forms when sunlight reacts with vehicle exhaust and industrial emissions

The word "smog" is a combination of "smoke" and "fog," and it describes air pollution thick enough to see. Unlike fog, which is water vapor, smog is a mixture of gases and particles that form a haze over cities and regions. The two main types are photochemical smog, which forms when sunlight hits nitrogen oxides and volatile organic compounds in the air, and London smog (or industrial smog), which forms when sulfur dioxide and soot accumulate in cold, stagnant air.

Photochemical smog is the type most people encounter in warm, sunny regions. It appears as a brownish or yellowish haze and typically gets worse in the afternoon when the sun is strongest. London smog, historically common in industrial cities during winter, appears as a thick gray or black fog and is less common today because of stricter emissions rules, though it still occurs in some parts of the world.

Key Takeaways

  • Smog forms when sunlight reacts with pollution from cars, factories, and power plants, creating a visible haze that can harm breathing.
  • Photochemical smog is brown or yellow and forms on warm, sunny days; London smog is gray and forms in cold, stagnant air.
  • Ground-level ozone, a main component of photochemical smog, is created by a chemical reaction between nitrogen oxides and volatile organic compounds.
  • Weather patterns, geography, and the amount of local pollution all affect how thick smog becomes and how long it lingers.

How photochemical smog forms step by step

Photochemical smog begins with emissions from cars, trucks, and factories. These sources release nitrogen oxides (NOx) and volatile organic compounds (VOCs) into the air. When the sun rises and its ultraviolet rays hit these chemicals, a chain reaction starts: the nitrogen oxides break apart, and the pieces recombine with VOCs to form ground-level ozone and other secondary pollutants.

This process takes time, which is why smog often gets worse as the day goes on. In the morning, pollution levels may be moderate. By afternoon, after hours of sunlight exposure, the ozone concentration peaks. This is why air quality warnings are often issued in the afternoon rather than the morning, and why people with asthma or other respiratory conditions are advised to stay indoors during peak smog hours.

The reaction continues until sunset, when ultraviolet radiation decreases. At night, some of the ozone breaks down, but not all of it disappears. If weather conditions trap the air over a region, smog can persist into the next day and accumulate further.

What causes London smog and why it is less common now

London smog forms differently than photochemical smog. It occurs when cold air settles over a region and cannot rise or disperse—a condition called a temperature inversion. Pollution from coal burning, factories, and vehicles gets trapped in this cold air layer and mixes with water vapor to create a thick, dark fog. The sulfur dioxide from coal combines with soot particles to form a toxic mixture that can reduce visibility to just a few feet.

The most famous example is the Great Smog of London in December 1952, which killed thousands of people over several days. That event led to the Clean Air Act in the United Kingdom and similar laws in other countries that restricted coal burning in cities and required cleaner industrial practices. Today, London smog is rare in developed nations but still occurs in regions that rely heavily on coal for heating and power, particularly in parts of Asia and Eastern Europe during winter.

Geography and weather patterns that make smog worse

Certain locations are more prone to smog because of their geography and climate. Cities in valleys or basins, where air cannot easily escape, tend to have worse smog problems. Los Angeles is a classic example: the city sits in a basin surrounded by mountains, and warm, sunny weather is common year-round. These conditions create the perfect environment for photochemical smog to form and linger.

Temperature inversions also play a major role. Normally, air gets cooler as you go higher in the atmosphere, so warm air near the ground rises and carries pollution away. But when a layer of warm air sits above cooler air near the ground, it acts like a lid and traps pollution. This happens most often in winter or in early morning and evening hours. High-pressure weather systems that bring clear, calm days can also trap smog because the stagnant air has nowhere to go.

Wind and rain help clear smog. Strong winds disperse the pollutants, and rain washes particles and gases out of the air. Regions with little wind or rain during certain seasons are more likely to experience persistent smog.

The difference between smog and haze

Smog and haze look similar but are not the same thing. Haze is usually caused by dust, salt particles, or water droplets suspended in the air, and it does not necessarily indicate pollution. You might see haze on a clear day in a desert or near the ocean. Smog, by contrast, is always pollution—it contains harmful chemicals like ozone, nitrogen dioxide, and particulate matter.

The key difference is chemical composition and health risk. Haze may reduce visibility but does not typically trigger respiratory problems. Smog does. If the air looks thick and discolored and you feel irritation in your throat or eyes, you are likely looking at smog, not haze. Air quality indexes in your area will specify whether the pollution is smog-related or caused by other factors like wildfire smoke or dust.

Health effects of breathing smog

Smog irritates the lungs and airways because it contains ground-level ozone and other reactive chemicals. When you breathe smog, these substances can inflame the lining of your respiratory tract, causing coughing, wheezing, and shortness of breath. People with asthma, emphysema, or other chronic lung conditions are at higher risk for severe reactions.

Children and older adults are also more vulnerable because their lungs are still developing or have declined with age. Even healthy people can experience symptoms during heavy smog events—burning eyes, scratchy throat, and chest discomfort are common. Repeated exposure over weeks or months may contribute to long-term lung damage, though the research on this is still developing.

The risk is highest during peak smog hours, usually mid-afternoon through early evening. People in areas with frequent smog are often advised to check the air quality index before exercising outdoors and to limit strenuous activity on high-pollution days.

Frequently Asked Questions

Is smog the same as air pollution?

Smog is a type of air pollution, but not all air pollution is smog. Smog specifically refers to visible haze caused by chemical reactions in the atmosphere. Other air pollutants, like carbon monoxide or particulate matter from dust, may not create visible smog but can still harm air quality.

Can smog form on cloudy or cool days?

Photochemical smog requires sunlight, so it is less likely on heavily cloudy days. However, the chemical reactions can still occur on cool days if the sun is strong enough. London smog, which does not depend on sunlight, can form on any day when a temperature inversion traps cold air over a polluted region.

Does smog go away on its own?

Smog disperses when weather conditions change—wind carries it away, rain washes it out, or the sun sets and stops driving the chemical reactions. However, smog can persist for days if high pressure, calm winds, and temperature inversions keep the air stagnant. Reducing emissions at the source is the only way to prevent smog from forming in the first place.

Why is smog brown or yellow instead of gray?

The color comes from nitrogen dioxide, one of the main chemicals in photochemical smog. Nitrogen dioxide absorbs blue light and reflects yellow and red wavelengths, which is why smog often appears brownish or yellowish. London smog appears gray or black because it contains soot and sulfur compounds that reflect light differently.