Smog is a visible haze of pollution that forms when sunlight reacts with vehicle exhaust and industrial emissions
Smog is not a single substance — it is a mixture of gases and particles suspended in air that makes the sky look hazy, brown, or orange. The word itself is a combination of "smoke" and "fog." When sunlight hits nitrogen oxides and volatile organic compounds (mostly from cars, trucks, and factories), a chemical reaction creates ground-level ozone and other pollutants that hang in the air and reduce visibility.
The two main types are photochemical smog, which forms in the presence of sunlight and is common in warm, sunny cities, and London smog, which is a thicker, greyish fog that forms in cold, stagnant air and contains soot and sulfur dioxide. Most smog you see today in North America is photochemical smog. It does not stay in one place — wind carries it downwind, so a city with strict emissions rules can still experience smog from sources hundreds of miles away.
Key Takeaways
- Smog forms when sunlight chemically reacts with nitrogen oxides and volatile organic compounds, mostly from vehicle exhaust and industrial sources.
- Photochemical smog is the most common type in warm climates and creates a brownish haze that reduces visibility and air quality.
- Temperature inversions — layers of warm air trapping cool air below — trap smog close to the ground and make it worse.
- Smog affects human health by irritating the lungs and airways, and it is worse on hot, still days when pollutants accumulate.
How smog forms step by step
The process begins with emissions. Cars, trucks, power plants, and refineries release nitrogen oxides (NOx) and volatile organic compounds (VOCs) into the air. These are invisible at first. When the sun rises and its ultraviolet rays hit these chemicals, a chain of reactions begins. The nitrogen oxides break apart, and the fragments recombine with VOCs to form ground-level ozone and other secondary pollutants.
This process takes time — smog typically peaks in the afternoon, several hours after rush-hour traffic. The reaction is faster on hot days because heat speeds up the chemical reactions. On cool, cloudy days, smog formation slows. The smog then drifts with the wind; if the air is still or trapped by surrounding hills or mountains, it accumulates and becomes thicker and more visible.
Why temperature inversions make smog worse
Normally, air gets colder as you go higher in the atmosphere, so warm air near the ground rises and carries pollution upward and away. A temperature inversion reverses this: a layer of warm air sits above cooler air near the ground, acting like a lid. The cool air cannot rise through the warm layer, so pollution gets trapped close to where people breathe.
Inversions are common in valleys and coastal areas, especially in early morning or evening. They can last for days, allowing smog to build up to dangerous levels. Los Angeles, Denver, and Salt Lake City experience frequent inversions because of their geography. When a weather forecast mentions an inversion, smog levels typically spike within hours.
The difference between smog and regular air pollution
Air pollution is a broad term for any harmful substance in the air — dust, pollen, soot, gases, or particles. Smog is a specific type of air pollution that is visible and forms through a chemical reaction with sunlight. You can have air pollution without smog (for example, soot from a smokestack on a cloudy day), but smog is always a form of air pollution.
Smog is measured by the concentration of ground-level ozone, which is the main harmful component. The Air Quality Index (AQI) reports ozone levels on a scale from 0 to 500, with higher numbers indicating worse air quality. When the AQI reaches 100 or above, the air is considered unhealthy for sensitive groups like children, older adults, and people with asthma or heart disease.
Health effects of breathing smog
Smog irritates the lungs and airways because ozone is a reactive gas that damages the cells lining your respiratory system. Short-term exposure causes coughing, throat irritation, and difficulty breathing, especially during physical activity. People with asthma are more sensitive and may experience attacks on high-smog days.
Long-term exposure to smog can reduce lung function over time, even in people without diagnosed respiratory disease. Children are at higher risk because their lungs are still developing and they spend more time outdoors. Older adults and people with heart disease are also vulnerable because smog can trigger cardiovascular problems. On days when smog is heavy, health agencies recommend that sensitive groups stay indoors and limit outdoor activity.
Where and when smog is most common
Smog is most common in warm, sunny regions with heavy vehicle traffic and industrial activity. Southern California, the Central Valley of California, the Denver metropolitan area, and parts of Texas and Arizona experience the worst smog. In Canada, the Lower Mainland of British Columbia and parts of Southern Ontario see regular smog events. Internationally, cities like Beijing, Delhi, and Bangkok experience severe smog, though the causes vary.
Smog season typically runs from late spring through early fall, when temperatures are warmest and the sun is strongest. The worst days are usually in July and August. However, smog can occur any time of year if conditions align — a warm day, stagnant air, and high emissions. Winter smog is less common in most places but can occur in areas with temperature inversions and heavy heating fuel use.
What reduces smog formation
Smog levels have fallen in many cities over the past 30 years because of stricter vehicle emissions standards, cleaner fuels, and industrial regulations. Cars built after 2000 emit far less nitrogen oxide than older vehicles. Refineries and power plants now use pollution control equipment. On a day-to-day basis, wind and rain clear smog from the air — a strong breeze or a rainstorm can reduce smog levels dramatically within hours.
Individuals can reduce their contribution to smog by driving less, combining trips, using public transit, and maintaining their vehicle's emissions system. Reducing electricity use during peak afternoon hours also helps, because power plants produce emissions when demand is highest. On high-smog-forecast days, many regions ask people to postpone outdoor activities and avoid driving if possible.
Frequently Asked Questions
Is smog the same as fog?
No. Fog is water droplets suspended in air and forms naturally when moisture condenses. Smog is pollution mixed with air and forms from chemical reactions with sunlight. Fog is usually white or grey and clears when the sun warms the air. Smog is often brown or orange and persists even in sunlight.
Can smog make you sick when ready?
It depends on the concentration and your sensitivity. Healthy people may not notice mild smog. People with asthma, children, and older adults can experience coughing, wheezing, or chest tightness within hours of exposure to heavy smog. Symptoms usually improve once you move to cleaner air or the smog clears.
Why is smog brown or orange?
The brown or orange color comes from nitrogen dioxide, one of the pollutants in smog. Nitrogen dioxide absorbs light in the blue part of the spectrum, so the remaining light appears brown or orange. Thicker smog looks darker. The color is a visual indicator that ozone and other harmful pollutants are present.
Does rain wash smog out of the air?
Yes. Rain removes particles and some gases from the air, clearing smog within hours. However, rain does not remove all pollutants — some gases remain. After rain stops, if emissions continue and conditions are right, smog can form again. Heavy, steady rain is more effective at clearing smog than light drizzle.
Can you see smog at night?
Photochemical smog requires sunlight to form, so it is less visible at night. However, the pollutants remain in the air even after dark. You might see a haze or reduced visibility at night if smog accumulated during the day and wind did not disperse it. Some types of smog, like London smog, are visible at any time of day.