Smog is a visible haze of air pollution that forms when emissions from vehicles, factories, and fuel burning react with sunlight and get trapped near the ground. The word itself is a blend of “smoke” and “fog,” coined over a century ago to describe the thick, grimy air that choked industrial cities like London. Today, smog looks different than it did in the coal-burning era, but it remains one of the most widespread environmental health threats worldwide.
Two Types of Smog
Classical smog, sometimes called London-type smog, comes from burning large amounts of high-sulfur coal. Its main ingredient is soot, along with sulfur dioxide and fine ash particles. This type created the infamous “pea soup” fogs of 19th and early 20th century industrial cities. It still occurs in regions that rely heavily on coal for heating and power.
Photochemical smog is the more common variety today. It forms when nitrogen oxides from vehicle exhaust and volatile organic compounds react with sunlight. The process starts during morning rush hour: combustion engines produce nitrogen oxide, which converts to nitrogen dioxide over a few hours. Sunlight then breaks nitrogen dioxide apart, releasing a highly reactive oxygen atom that binds with atmospheric oxygen to create ground-level ozone. This ozone, mixed with dozens of other chemical byproducts, produces the brownish haze visible over many cities on hot, sunny days. The resulting cocktail contains over 100 different chemicals, with ground-level ozone being the most abundant.
Why Smog Gets Trapped Over Cities
Smog episodes become severe when a weather pattern called a temperature inversion settles over an area. Normally, warm air near the ground rises and carries pollutants upward, where they disperse. During an inversion, a layer of warmer air sits above cooler air at ground level, acting like a lid on a pot. Pollutants from traffic, industry, and heating systems accumulate in the trapped layer, growing more concentrated with each passing hour.
This is especially common during extended high-pressure periods in winter. The ground loses heat rapidly at night under clear skies, cooling the air closest to the surface. The warmer air above prevents mixing, and pollution builds until the weather pattern shifts. Cities surrounded by mountains or valleys are particularly vulnerable because the terrain physically contains the stagnant air mass.
Where Smog Pollution Comes From
A global analysis of urban particulate matter found that traffic contributes roughly 25% of fine particle pollution in cities, domestic fuel burning accounts for about 20%, and industrial activities add another 15%. An additional 22% comes from other human sources, while natural dust and salt make up around 18%. These proportions vary significantly by region. In cities with heavy car dependence, transportation dominates. In parts of South Asia and sub-Saharan Africa, household cooking and heating fuels are the largest contributors.
Health Effects of Breathing Smog
Smog triggers inflammation, oxidative stress, and immune suppression in cells throughout the body. The finest particles in smog, those smaller than 2.5 microns in diameter (known as PM2.5), are small enough to pass through lung tissue into the bloodstream and circulate to organs throughout the body. The World Health Organization identifies stroke, heart disease, chronic obstructive pulmonary disease, lung cancer, and pneumonia as the conditions most strongly linked to air pollution exposure.
Short-term exposure on high-smog days can worsen asthma, trigger coughing and chest tightness, and irritate the eyes and throat. Formaldehyde and other compounds produced during smog formation are direct irritants to the eyes and respiratory tract. Long-term exposure over months and years is where the more serious risks emerge: it increases the likelihood of developing heart disease, stroke, COPD, and cancer. There is also growing evidence linking chronic exposure to diabetes, cognitive decline, neurological disease, and adverse pregnancy outcomes like low birth weight.
In 2021, the WHO tightened its recommended air quality limits significantly, advising that annual average PM2.5 concentrations stay below 5 micrograms per cubic meter, half of the previous 2005 guideline of 10. The recommended limit for nitrogen dioxide dropped from 40 to just 10 micrograms per cubic meter. Most major cities in the world exceed these thresholds.
How Air Quality Is Measured
Most countries use an Air Quality Index (AQI) to translate pollution measurements into a simple scale. In the United States, the AQI runs from 0 to 500 and is divided into color-coded categories:
- Green (0 to 50): Good. Air pollution poses little or no risk.
- Yellow (51 to 100): Moderate. Acceptable for most people, though unusually sensitive individuals may notice effects.
- Orange (101 to 150): Unhealthy for sensitive groups, including people with asthma, older adults, and children.
- Red (151 to 200): Unhealthy. The general public may begin experiencing effects.
- Purple (201 to 300): Very unhealthy. Health risk is increased for everyone.
- Maroon (301 and above): Hazardous. Emergency conditions affecting the entire population.
You can check your local AQI in real time through apps and websites like AirNow.gov, which pulls data from monitoring stations across the country. Many weather apps now include AQI alongside temperature and humidity.
Damage Beyond Human Health
Ground-level ozone, the primary component of photochemical smog, is also toxic to plants. It enters leaves through the same pores plants use to absorb carbon dioxide and damages cells from the inside. The agricultural consequences are significant. Research projecting crop losses through 2030 estimates that ozone exposure could reduce global wheat yields by 5 to 26%, soybean yields by 9 to 19%, and maize yields by 2 to 9%, depending on emission trends. In dollar terms, that translates to $12 to $35 billion in annual crop losses worldwide. Forests and natural ecosystems suffer similar damage, with visible leaf injury, reduced growth, and increased vulnerability to disease.
Reducing Your Exposure
On high-smog days, the simplest protective step is limiting time outdoors, especially during afternoon hours when ozone levels peak. If you exercise outside, shifting your workout to early morning can make a meaningful difference, since ozone builds throughout the day as sunlight drives the chemical reactions that produce it.
Indoors, keeping windows closed during poor air quality days and running an air purifier helps. HEPA filters remove at least 99.97% of airborne particles at 0.3 microns, which is actually the hardest particle size to capture. Larger and smaller particles are trapped with even higher efficiency. For central air systems, filters with higher MERV ratings do a better job catching fine particles, though not all HVAC systems can handle the airflow resistance of the highest-rated filters.
At a broader level, smog has declined substantially in cities that have adopted stricter vehicle emission standards, transitioned away from coal, and invested in public transportation. Los Angeles, once the poster city for photochemical smog, has seen ozone levels drop by roughly two-thirds since the 1970s despite a massive increase in population and vehicles. The chemistry of smog is well understood, and the tools to reduce it already exist. The challenge is political will and the speed of adoption.