Smog is a type of intense air pollution that forms when emissions from vehicles, factories, and other sources react with sunlight and weather conditions to create a hazy, harmful mixture of gases and fine particles. The word itself is a blend of “smoke” and “fog,” coined in early 20th-century London. Today, smog affects cities on every continent, and over 90% of the world’s population breathes air that exceeds the World Health Organization’s recommended safety limits.
Two Types of Smog
Smog comes in two main forms, each with different chemistry and triggers. The first, sometimes called classical or sulfurous smog, is what choked London in the early 1900s. It forms when coal and heavy fossil fuels are burned, releasing sulfur dioxide and thick particulate smoke into the air. When these pollutants mix with damp, foggy conditions, the result is a yellowish-gray haze that clings to the ground. This type is less common in wealthy nations today but still occurs in regions that rely heavily on coal.
The second type, photochemical smog, is the brown haze most people picture over cities like Los Angeles, Beijing, or Delhi. It forms when nitrogen oxides and volatile organic compounds (VOCs) from tailpipes, power plants, and industrial processes react in the presence of sunlight. That reaction produces ground-level ozone and other irritating compounds. Unlike classical smog, photochemical smog is worst on hot, sunny days and doesn’t require fog at all.
How Ground-Level Ozone Forms
The key ingredient in modern smog is ground-level ozone, which is chemically identical to the ozone in the upper atmosphere but harmful when it forms at street level. The process starts when vehicles and industrial sources release nitrogen dioxide into the air. Sunlight breaks nitrogen dioxide apart, freeing an oxygen atom that quickly attaches to an oxygen molecule to create ozone. Under clean conditions, nitric oxide (another tailpipe emission) would react with that ozone almost immediately, breaking it back down. The two would cycle back and forth without ozone ever building up.
VOCs disrupt that balance. These are carbon-based gases released by gasoline vapors, paints, solvents, and even trees. VOCs generate highly reactive molecules called free radicals, which intercept nitric oxide before it can destroy ozone. With nothing to break it down, ozone accumulates. The more VOCs and nitrogen oxides in the air, and the stronger the sunlight, the higher ozone levels climb. This is why smog peaks in the afternoon on summer days and why cities in sunny climates are especially vulnerable.
Why Smog Gets Trapped Over Cities
Normally, warm air near the ground rises and carries pollutants upward, where wind disperses them. A temperature inversion flips this pattern. A layer of warm air settles above the cooler air near the surface, acting like a lid on a bowl. Pollutants have nowhere to go and concentrate at ground level. High-pressure weather systems make this worse by pushing warm air downward and suppressing vertical mixing.
In winter, valley cities are especially prone. Snow-covered ground reflects heat instead of absorbing it, keeping the surface air cold and stable. In summer, the same lid effect can trap photochemical smog for days. A strong inversion can push air quality readings into dangerous territory quickly, while even a weak inversion slows pollutant dispersal enough to noticeably degrade the air.
Wildfire Smoke Makes Urban Smog Worse
Wildfires have become a major contributor to smog in cities that might otherwise have moderate air quality. Fires release enormous quantities of fine particulate matter, carbon monoxide, nitrogen oxides, and VOCs. When wildfire plumes drift over urban areas already rich in nitrogen oxides from traffic, the extra VOCs supercharge ozone production. Research on wildfire smoke events found that smoke boosted the overall chemical reactivity of the air by 53% compared to clear days, mainly because of the surge in VOCs.
On smoky days, researchers observed ground-level ozone concentrations roughly 8 parts per billion higher than normal, with fine particle levels jumping by about 8 micrograms per cubic meter. At some locations, wildfire smoke has pushed ozone up by as much as 40 parts per billion, enough to exceed U.S. federal air quality standards on its own. The interaction between wildfire emissions and existing urban pollution means that even cities with strong emission controls can experience severe smog episodes during fire season.
Health Effects of Breathing Smog
Smog’s two most harmful components are ground-level ozone and fine particulate matter, particles smaller than 2.5 micrometers in diameter (known as PM2.5). Ozone irritates the airways, triggering coughing, chest tightness, and worsening of asthma. With repeated exposure, it can permanently reduce lung function.
PM2.5 is arguably more dangerous because of where it goes. These particles are small enough to travel deep into the lungs, cross into the bloodstream, and reach organs throughout the body. Once in circulation, they trigger widespread inflammation. Long-term exposure is linked to chronic lung disease, heart attacks, strokes, and lung cancer. The WHO estimates that 9 out of 10 people worldwide breathe air exceeding recommended pollution levels, placing billions at elevated risk for these conditions. Children, older adults, and people with existing heart or lung disease are hit hardest, but sustained exposure affects healthy adults too.
Damage to Crops and Ecosystems
Smog doesn’t just harm people. Ground-level ozone enters plants through the tiny pores on their leaves, where it generates destructive molecules that damage the internal machinery plants use for photosynthesis. The result is slower growth, visible leaf injury, and lower yields. Plants effectively choke: ozone forces those leaf pores to close, cutting off the carbon dioxide the plant needs to build sugars and grow.
The agricultural losses are significant. Studies tracking crop performance over decades have found ozone-related yield reductions of 10 to 20% in soybeans and corn under hot, dry conditions. Wheat losses range from 27 to 41% and rice losses from 21 to 26% in areas with elevated ozone. Projections suggest that by 2030, ozone could reduce global wheat yields by 5 to 26% and corn yields by 4 to 9%, depending on emission trends. These numbers translate directly into food prices and supply, particularly in regions of South and East Asia where both smog levels and population density are high.
How Air Quality Is Measured
Most countries use an Air Quality Index (AQI) to translate complex pollution data into a single number that tells you how safe the air is to breathe. In the United States, the AQI runs from 0 to 500 and is divided into six color-coded categories:
- Green (0 to 50): Good. Air quality poses little or no risk.
- Yellow (51 to 100): Moderate. Acceptable for most, though unusually sensitive individuals may notice effects.
- Orange (101 to 150): Unhealthy for sensitive groups, including children, older adults, and people with asthma or heart disease.
- Red (151 to 200): Unhealthy. The general public may begin experiencing effects.
- Purple (201 to 300): Very unhealthy. Health risk is elevated for everyone.
- Maroon (301 and above): Hazardous. Emergency conditions for the entire population.
The AQI tracks several pollutants individually, including ground-level ozone and PM2.5. The highest reading among them becomes the overall AQI for the day. You can check real-time readings for your area through government monitoring sites like AirNow.gov.
Protecting Yourself on Smoggy Days
On high-AQI days, reducing your time outdoors, especially during afternoon hours when ozone peaks, is the simplest and most effective step. If you exercise outside, shifting your workout to early morning helps, since ozone levels are typically lowest before the sun has had time to drive the chemical reactions that produce it.
When air quality is poor enough to warrant a mask, the type matters enormously. N95 respirators filter at least 97% of fine particles across nearly all sizes and reduce overall exposure by a factor of 14 to 18, depending on particle size, when worn with a good seal. Modeling suggests that widespread N95 use during severe pollution events could reduce respiratory hospital admissions by 22 to 39%. Surgical masks, by contrast, have decent filter material (about 90% efficiency) but fit loosely, allowing roughly half the air to leak around the edges. In practice, they only cut exposure by a factor of about 1.9. If you’re choosing between the two during a wildfire smoke event or a heavy smog day, the N95 offers dramatically better protection.
Indoors, keeping windows closed and running air conditioning or a portable air purifier with a HEPA filter can substantially reduce PM2.5 levels in your home. Avoid adding to indoor pollution with candles, gas stoves, or anything that burns during these periods.
The 1952 London Disaster That Changed Policy
The single event that did more than any other to shape modern air quality regulation happened in London in December 1952. On December 4, a high-pressure system settled over the city, creating a temperature inversion that locked cold, still air at ground level under a cap of warm air. Smoke from millions of coal fires, vehicle exhaust, and industrial emissions had nowhere to go. Within a day, a thick smog blanketed the entire city, reducing visibility to near zero in some areas.
Both smoke and sulfur dioxide concentrations reached extreme levels. The immediate death toll was staggering, and mortality remained elevated for months afterward. The British government initially attributed some of the lingering deaths to an influenza epidemic, but later analysis showed that only a small fraction could be explained by flu. The vast majority were caused by pollution exposure. The disaster led directly to the United Kingdom’s Clean Air Act of 1956, one of the first major pieces of air quality legislation in the world, and set a precedent that shaped environmental policy globally.