Smog forms when sunlight reacts with pollution already in the air
Smog is not a single pollutant—it is a visible haze that forms when nitrogen oxides and volatile organic compounds (VOCs) already released into the air meet sunlight and heat. The sun's ultraviolet rays trigger a chemical reaction between these pollutants, creating ground-level ozone and other secondary pollutants that make the air thick, brown, or gray. This process happens most visibly on hot, still days when air does not disperse and sunlight is strongest.
The word "smog" itself comes from combining "smoke" and "fog," and the term describes two distinct types. Photochemical smog, the kind most people see in cities on summer afternoons, forms through the sun-driven reaction described above. Industrial smog, which is less common in modern cities, forms when coal or heavy fuel oil burns and creates sulfur dioxide and soot—this type does not require sunlight to form and was the dominant type in industrial cities during the 19th and 20th centuries.
Key Takeaways
- Smog forms when sunlight reacts with nitrogen oxides and volatile organic compounds already in the air, not from smoke alone.
- Vehicle exhaust is the largest source of nitrogen oxides in most cities, while paints, solvents, and fuel vapors release volatile organic compounds.
- Heat and stagnant air make smog worse because they slow the dispersal of pollutants and increase the speed of the chemical reaction that creates it.
- Industrial facilities, power plants, and wildfires also contribute nitrogen oxides and VOCs, but transportation is the dominant source in urban areas.
Vehicle exhaust is the primary source of nitrogen oxides in cities
Cars, trucks, and buses burning gasoline or diesel release nitrogen oxides (NOx) as a byproduct of combustion. The hotter the engine burns, the more nitrogen oxides form. In most metropolitan areas, transportation accounts for 40 to 60 percent of the nitrogen oxides in the air—the exact share varies by city size, traffic volume, and how many older vehicles are still in use. Newer vehicles with catalytic converters emit far less than older ones, but the sheer number of vehicles on the road means transportation remains the dominant source.
Diesel engines produce more nitrogen oxides than gasoline engines, which is why cities with heavy truck traffic or large bus fleets often see worse smog. Idling vehicles contribute too—a truck or bus sitting in traffic with the engine running releases pollutants without moving anyone anywhere. Rush hour traffic, congestion on highways, and delivery trucks making stops throughout the day all add to the nitrogen oxide load in the air.
Volatile organic compounds come from paints, solvents, fuels, and refineries
VOCs are carbon-containing chemicals that evaporate into the air at room temperature. Paint, varnish, and lacquer release VOCs as they dry. Gasoline vapors escape during refueling at gas stations and from fuel tanks in vehicles. Dry cleaning solvents, adhesives, printing inks, and cleaning products all release VOCs. Industrial facilities—particularly oil refineries, chemical plants, and manufacturing operations—emit large quantities of VOCs during production and storage.
Unlike nitrogen oxides, which come almost entirely from burning fuel, VOCs come from both combustion and evaporation. A gallon of gasoline sitting in a tank on a hot day releases VOCs into the air even if the vehicle never starts. This is why smog is often worse in summer: higher temperatures mean more evaporation from paints, solvents, and fuel. Some states and cities have reduced VOC emissions by requiring lower-volatility paints and by installing vapor recovery systems at gas pumps, but the sources remain widespread.
Heat and stagnant air allow smog to accumulate and intensify
Smog is worst on hot, calm days because two things happen simultaneously. First, heat speeds up the chemical reaction between nitrogen oxides and VOCs, so more ground-level ozone forms per hour. Second, calm air means pollutants do not disperse—they stay in the same place and accumulate. A city surrounded by hills or mountains can trap air in a basin, preventing wind from carrying pollutants away. Los Angeles, Denver, and Mexico City all sit in geographic basins where air naturally stagnates, which is why they experience chronic smog problems.
Temperature inversions make this worse. Normally, air is warmer near the ground and cooler higher up, so warm air rises and carries pollutants away. During an inversion, a layer of warm air sits above cooler air near the ground, acting like a lid. Pollutants cannot rise through this warm layer, so they accumulate at ground level. Inversions are common in fall and winter mornings in many cities and can trap smog for days.
Power plants and industrial facilities contribute significant nitrogen oxides
Coal-fired and natural gas power plants burn fuel to generate electricity and release nitrogen oxides in the process. The amount varies by the plant's age, efficiency, and pollution controls. Older plants without modern emission controls release far more than newer ones. Steel mills, cement plants, refineries, and chemical manufacturers all burn fuel or use high-temperature processes that produce nitrogen oxides. In regions with heavy industry, these sources can account for 20 to 40 percent of total nitrogen oxide emissions, though in most cities transportation still dominates.
Power plants and industrial facilities are often located outside city centers, but their emissions travel downwind. A coal plant 50 miles upwind of a city can contribute to that city's smog problem. This is why smog is sometimes a regional problem rather than a local one—pollution from multiple sources across a wide area can accumulate in a single city or region.
Wildfires and agricultural burning release large amounts of pollutants
Wildfires release nitrogen oxides, VOCs, and particulate matter directly into the air. During fire season, smoke from wildfires hundreds of miles away can drift into cities and create or worsen smog. In the western United States, wildfire smoke has become a major contributor to poor air quality in late summer and fall. Agricultural burning—the practice of burning crop residue or clearing land—also releases these pollutants, though it is less common in developed countries due to air quality regulations.
Wildfires are not a controllable source the way vehicle emissions or industrial output are, but they are a significant contributor to smog in regions prone to fire. Climate change and drought have extended fire seasons in many areas, meaning wildfire smoke is affecting air quality for longer periods each year.
Frequently Asked Questions
Why is smog brown or orange instead of just gray?
The brown or orange color comes from nitrogen dioxide, one of the nitrogen oxides released by vehicles and power plants. Nitrogen dioxide absorbs blue light, which is why smog appears brown or reddish rather than white or gray. The intensity of the color roughly indicates how much nitrogen dioxide is present.
Can smog form on cold days?
Photochemical smog requires sunlight, so it forms most readily on warm, sunny days. Industrial smog from coal burning can form on cold days because it does not depend on sunlight. In modern cities, photochemical smog is far more common, but cold-weather smog can still occur in regions with heavy coal use or during temperature inversions that trap pollutants.
Does rain wash smog out of the air?
Rain removes some pollutants from the air, particularly particulate matter and some gases, which is why air quality often improves after rainfall. However, rain does not eliminate smog permanently—as long as sources like vehicle traffic continue, new pollutants are released and new smog forms once the sun returns and conditions warm up.
Is smog the same as air pollution?
Smog is a visible form of air pollution, but air pollution is broader. Air pollution includes gases and particles you cannot see, like carbon monoxide and fine particulate matter. Smog is specifically the haze that forms when sunlight reacts with certain pollutants. You can have poor air quality without visible smog, and you can see smog on days when overall air quality measurements are not as bad.