SMOG is an acronym for smoke and fog mixed with air pollution

SMOG stands for smoke and fog. The term describes a visible haze that forms when smoke, fog, and other air pollutants combine in the atmosphere. The word itself was coined in the early 1900s in London to describe the thick, yellowish-brown clouds that hung over the city during winter months. Today, smog is a common air quality problem in cities around the world, especially in areas with heavy traffic, industrial activity, or geographic features that trap pollutants close to the ground.

Smog comes in two main types, each formed by different chemical processes. London smog (also called gray smog or sulfurous smog) forms when coal smoke combines with fog and sulfur dioxide, creating a thick, dark haze. Los Angeles smog (also called photochemical smog or brown smog) forms when sunlight reacts with nitrogen oxides and volatile organic compounds from vehicle exhaust and industrial emissions, creating a reddish-brown haze. Most smog you see in modern cities is photochemical smog, because it forms in warm, sunny climates where cars are the primary pollution source.

Key Takeaways

  • SMOG stands for smoke and fog, and describes visible air pollution that forms when smoke, fog, and other pollutants mix in the atmosphere.
  • London smog forms from coal smoke, fog, and sulfur dioxide, while Los Angeles smog forms when sunlight reacts with vehicle exhaust and industrial emissions.
  • Smog typically forms in cities with heavy traffic, industrial activity, or valleys and basins that trap pollutants near the ground.
  • Air quality alerts warn when smog levels are high enough to affect human health, especially for children, older adults, and people with respiratory conditions.

How photochemical smog forms on sunny days

Photochemical smog requires three ingredients: nitrogen oxides (NOx), volatile organic compounds (VOCs), and sunlight. Nitrogen oxides come primarily from vehicle exhaust and power plants. Volatile organic compounds come from vehicle exhaust, industrial facilities, and even some consumer products like paint and cleaning supplies. When sunlight hits these pollutants, a chemical reaction occurs that creates ground-level ozone and other secondary pollutants, which give smog its characteristic brown or reddish color.

This type of smog typically builds up during the afternoon and early evening, when the sun is strongest and traffic is heaviest. It is worst in cities surrounded by mountains or hills, because the geography traps pollutants close to the ground instead of allowing them to disperse. Los Angeles, Denver, Mexico City, and other cities in valleys or basins experience severe photochemical smog for this reason. On cooler days or when wind patterns change, the smog can clear within hours.

Why London smog was deadlier and less common now

London smog formed differently and was far more dangerous than modern photochemical smog. It occurred when coal smoke combined with fog and sulfur dioxide during cold, windless winter weather. The fog trapped the pollutants at ground level, creating a thick, toxic blanket that could last for days or weeks. The most infamous event was the Great Smog of December 1952, which killed an estimated 12,000 people in London over the course of a few weeks, mostly from respiratory and cardiovascular failure.

London smog is rare today because most developed countries have phased out coal burning for heating and power generation, and have strict regulations on sulfur content in fuel. When it does occur, it is usually in cities that still rely heavily on coal, such as some cities in China and India. The shift away from coal has made this type of smog largely a historical problem in North America and Europe, though air quality in coal-dependent regions remains a serious public health concern.

Geographic and weather conditions that trap smog

Smog forms most easily in cities with certain geographic and weather patterns. A temperature inversion occurs when a layer of warm air sits above a layer of cool air, trapping pollutants near the ground. This is common in valleys and basins, where cool air settles at night and warm air moves in during the day. Cities like Los Angeles, Denver, and Salt Lake City experience frequent inversions because they are surrounded by mountains that prevent air from mixing vertically.

Calm, windless days make smog worse because pollutants cannot disperse. High temperatures accelerate the chemical reactions that create photochemical smog. Coastal cities sometimes experience smog when sea breezes push pollutants inland and trap them against mountains. Urban areas with heavy traffic congestion produce more nitrogen oxides and volatile organic compounds, which means more raw material for smog formation. Industrial zones and refineries add to the problem in cities where they are located near residential areas.

How smog affects air quality and health

Smog reduces visibility and makes the air look hazy or discolored, but the real danger is what you cannot see. Ground-level ozone, the main component of photochemical smog, irritates the lungs and airways. Breathing smog can trigger coughing, wheezing, chest pain, and shortness of breath. People with asthma, emphysema, or other chronic respiratory conditions are most vulnerable, as are children whose lungs are still developing and older adults with weakened immune systems.

Air quality agencies measure smog and other pollutants using the Air Quality Index (AQI), which ranges from 0 to 500. When the AQI reaches unhealthy levels, health departments issue air quality alerts warning people to limit outdoor activity. On very bad days, schools may cancel outdoor sports, construction sites may shut down, and people are advised to stay indoors with windows closed and air filters running. Long-term exposure to smog increases the risk of heart disease, stroke, and premature death, even at levels that do not cause when ready symptoms.

Efforts to reduce smog in cities

Governments have reduced smog through vehicle emission standards, industrial regulations, and fuel quality requirements. The Clean Air Act in the United States set limits on nitrogen oxides and volatile organic compounds, and required catalytic converters on cars to reduce emissions. California has the strictest vehicle emission standards in the country, which is why Los Angeles smog has improved significantly since the 1970s despite population growth. Similar regulations exist in Europe, Canada, and other developed nations.

Cities also manage smog through traffic reduction programs, industrial zoning, and public transportation investment. On high-pollution days, some cities restrict vehicle traffic or offer free public transit to reduce emissions. Planting trees and vegetation helps absorb some pollutants. Switching to renewable energy sources and phasing out coal reduces the baseline pollution that combines with fog and sunlight to form smog. Despite these efforts, smog remains a problem in many cities, particularly in regions with rapid industrial growth and less stringent environmental regulations.

Frequently Asked Questions

Is smog the same as fog?

No. Fog is water vapor that condenses into visible droplets in cool air. Smog is fog mixed with smoke and air pollutants. You can have fog without smog, but smog always contains fog or moisture. Smog looks discolored (brown, yellow, or gray) while fog is usually white or gray.

Can smog happen on cloudy days?

Photochemical smog requires sunlight to form, so it is worst on sunny days. However, London smog can form on cloudy or foggy days because it does not depend on a chemical reaction with sunlight. On cloudy days in cities with heavy traffic, you may still see haze from pollutants, but it is not true photochemical smog.

Does indoor air get contaminated by outdoor smog?

Yes, but less so than outdoor air. Smog and its pollutants can seep indoors through windows, doors, and ventilation systems. Keeping windows closed and using air filters reduces indoor pollution during high-smog days. People with respiratory conditions should monitor air quality forecasts and stay indoors when smog levels are high.

Why is smog brown instead of gray?

Photochemical smog is brown or reddish because of nitrogen dioxide, one of the pollutants created when sunlight reacts with vehicle exhaust. London smog was gray or yellowish-brown because it contained sulfur dioxide and soot from coal smoke. The color depends on which pollutants are present and in what concentrations.