Smog is a visible haze of pollution that forms when sunlight reacts with emissions from cars, factories, and power plants

Smog is not a single pollutant — it is a mixture of gases and particles that become visible when they accumulate in the air above a city or region. The word itself comes from combining "smoke" and "fog." Unlike natural fog, which forms from water vapor, smog forms when nitrogen oxides and volatile organic compounds (chemicals that evaporate easily) react with sunlight. This chemical reaction creates ozone and other harmful particles that hang in the air, making it look hazy or brownish.

The pollution that creates smog comes from everyday sources: car exhaust, trucks, buses, power plants that burn coal or natural gas, refineries, and even some consumer products like paint and cleaning supplies. On hot, sunny days with little wind, these emissions get trapped near the ground instead of dispersing into the atmosphere. The longer they stay trapped, the more the sunlight cooks them into smog.

Key Takeaways

  • Smog forms when sunlight reacts with nitrogen oxides and volatile organic compounds from vehicles, factories, and power plants.
  • Ground-level ozone, the main ingredient in smog, is different from the ozone layer in the upper atmosphere and is harmful to breathe.
  • Smog is worse on hot, sunny days with little wind, because pollution gets trapped near the ground instead of dispersing.
  • Breathing smog can irritate your lungs and worsen asthma, heart disease, and other respiratory conditions, especially during outdoor activity.

The difference between ground-level ozone and the ozone layer

Ozone is a molecule made of three oxygen atoms. It exists in two very different places, and the distinction matters for understanding smog. High in the atmosphere, the ozone layer protects Earth from ultraviolet radiation. That ozone is beneficial — we want it there.

Ground-level ozone, which forms smog, is the same molecule but in the wrong place. When it forms near the ground where we breathe, it irritates lung tissue and airways. This is why smog is harmful even though ozone itself is just oxygen atoms rearranged. The problem is not the molecule; it is where it is and whether you are breathing it.

Why smog gets worse on certain days

Smog does not form at a steady rate. Several weather conditions make it worse. Heat is the main one — higher temperatures speed up the chemical reactions that turn emissions into ozone. Sunlight is the energy source for those reactions, so sunny days produce more smog than cloudy ones. Wind matters too: when air moves, pollution disperses and concentrations drop. On still, hot, sunny days, smog builds up fastest.

Geography also plays a role. Valleys and basins trap air more easily than flat terrain. Cities surrounded by mountains or hills often experience worse smog because pollution cannot blow away. Los Angeles, Denver, and Mexico City all sit in basins where smog accumulates more readily than in cities on plains or coasts.

What smog does to your health

Breathing smog irritates your lungs and airways. For most people on a smoggy day, this might mean a scratchy throat, coughing, or mild chest discomfort, especially if you are exercising outdoors. For people with asthma, chronic obstructive pulmonary disease (COPD), or heart disease, smog can trigger serious symptoms — wheezing, shortness of breath, or chest pain — even at lower concentrations.

Children and older adults are more vulnerable because their lungs are still developing or have weakened over time. Repeated exposure to smog over years may reduce lung function permanently. People who work outdoors — construction workers, landscapers, delivery drivers — face higher exposure and higher risk. On days when smog is heavy, doctors recommend that people in these groups limit outdoor activity or wear a mask rated for particle filtration.

How smog is measured and reported

Air quality is measured using the Air Quality Index (AQI), a number from 0 to 500 that tells you how polluted the air is. The EPA (Environmental Protection Agency) calculates it based on five major pollutants: ground-level ozone, particulate matter, nitrogen dioxide, sulfur dioxide, and carbon monoxide. Ozone is usually the main driver of the AQI in summer.

The AQI is divided into color-coded categories: green (good), yellow (moderate), orange (unhealthy for sensitive groups), red (unhealthy), purple (very unhealthy), and maroon (hazardous). You can check your local AQI on the EPA website or through weather apps. When the AQI reaches orange or higher, people with respiratory or heart conditions should avoid outdoor exercise, and everyone should limit strenuous activity outside.

Where smog is most common in the United States

Smog is not evenly distributed. It is worst in areas with heavy vehicle traffic, industrial activity, and geography that traps air. Southern California, particularly the Los Angeles basin, has historically had the worst smog in the country because of its size, car dependence, and surrounding mountains. The San Joaquin Valley in California also experiences severe smog, especially in summer and fall.

Other regions with frequent smog include the Houston area (refineries and petrochemical plants), Denver (high altitude and a basin), and the eastern United States during summer when heat and humidity peak. Even cities without heavy industry can experience smog if they sit downwind of pollution sources elsewhere — smog can travel hundreds of miles on wind currents.

What is being done to reduce smog

The Clean Air Act, passed in 1970 and updated since, set national standards for air quality and required states to meet them. The EPA regulates emissions from cars, trucks, power plants, and factories. Cars sold in the United States must meet strict emissions standards, which is why modern cars produce far less pollution than older ones. Power plants have been required to install pollution control equipment or switch to cleaner fuels.

States with persistent smog problems have adopted their own rules. California, for example, has stricter vehicle emissions standards than the federal government requires. Some cities have promoted public transit, bike lanes, and electric vehicles to reduce car emissions. Refineries and chemical plants have upgraded equipment to leak less. These efforts have worked: despite population growth, smog in most U.S. cities is less severe than it was in the 1970s and 1980s.

Frequently Asked Questions

Is smog the same as air pollution?

No. Air pollution is a broad category that includes all harmful substances in the air — dust, pollen, industrial chemicals, and more. Smog is one type of air pollution, specifically the visible haze created when sunlight reacts with emissions. You can have air pollution without visible smog, and smog is always a form of air pollution.

Can smog make you sick permanently?

Repeated exposure over years, especially during childhood or in people with existing lung disease, can reduce lung function permanently. However, short-term exposure to smog — even on very smoggy days — typically causes temporary irritation that goes away once the air clears. The risk of permanent harm is highest for people who live in areas with chronic smog and spend time outdoors regularly.

Does rain get rid of smog?

Yes. Rain washes pollutants out of the air and deposits them on the ground. After rain, the air usually clears noticeably and the AQI drops. However, rain does not solve the underlying problem — the emissions that created the smog are still being released. Once the rain stops and the sun returns, smog can build up again if conditions are right.

Why does smog smell bad?

Smog itself does not have a strong smell, but the pollutants that make it up do. Nitrogen dioxide has a sharp, acrid odor. Volatile organic compounds from paint, solvents, and gasoline have various smells. When these mix together in smog, you may notice an unpleasant or chemical-like odor, especially on hot days when the reactions are most active.