What smog is and why it forms over American cities
Smog is a visible haze that forms when sunlight reacts with pollution already in the air. It is not a single pollutant but a mixture — mainly ground-level ozone and fine particles suspended in the atmosphere. The process starts when cars, power plants, and factories release nitrogen oxides and volatile organic compounds. Sunlight then triggers a chemical reaction that turns these emissions into ozone, the main ingredient in smog.
Smog forms most readily in warm weather and in areas where air does not move easily. Cities surrounded by mountains or near coasts often trap air in place, allowing pollutants to build up. Los Angeles, Denver, and the San Francisco Bay Area are chronic smog zones for this reason. The problem worsens in summer, when heat accelerates the chemical reactions that create ozone.
Unlike fog, which is water vapor, smog does not dissipate on its own. It persists until wind pushes the air mass away or rain washes pollutants out of the atmosphere. On stagnant days, smog can linger for hours or days, making breathing difficult for people with asthma, emphysema, or heart disease.
Key Takeaways
- Smog forms when sunlight reacts with emissions from vehicles and industry, creating ground-level ozone and fine particles that hang in the air.
- Geography matters: cities in valleys or near coasts trap air and smog more easily than cities in open terrain.
- Smog is worst in summer and on hot, windless days when chemical reactions accelerate and air does not move.
- The worst smog in the United States occurs in Southern California, the San Francisco Bay Area, Denver, and parts of the Southwest.
- People with lung or heart conditions, children, and older adults face the greatest health risk on high-smog days.
Where smog is most common in the United States
Southern California has the worst smog problem in the country. The Los Angeles basin is surrounded by mountains that trap air, and the region has millions of vehicles and port operations that emit nitrogen oxides continuously. Even with decades of pollution controls, the area still exceeds federal air quality standards on many summer days.
The San Francisco Bay Area, Denver, and Phoenix also rank among the smoggiest regions. Denver's location at high altitude in a valley makes it prone to smog formation. Phoenix's rapid growth and heat create ideal conditions for ozone to form. The Inland Empire east of Los Angeles, the Central Valley of California, and parts of Texas and Arizona round out the list of chronic problem areas.
Smog is not confined to the West. The Northeast Corridor from Boston to Washington, D.C., and parts of the Midwest experience significant smog, especially in summer. However, the West generally has more days per year when smog exceeds safe levels, partly because of geography and partly because of the region's growth and vehicle dependence.
How the EPA measures and reports smog
The Environmental Protection Agency (EPA) monitors ground-level ozone using the Air Quality Index (AQI), a scale from 0 to 500 that tells the public how polluted the air is on any given day. An AQI below 50 is considered good. An AQI of 51 to 100 is moderate. Above 100, air quality is unhealthy for sensitive groups — children, older adults, and people with lung or heart disease. Above 150, the air is unhealthy for everyone.
The EPA also tracks fine particulate matter (PM2.5), tiny particles that lodge deep in the lungs and contribute to smog's haze. These particles come from vehicle exhaust, power plants, and wildfires. On high-smog days, PM2.5 levels often rise alongside ozone.
Local air quality agencies report the AQI daily, usually by mid-morning. Most cities and counties post this information on their websites and through apps. The EPA's AirNow website (airnow.gov) shows real-time AQI readings for any zip code in the country.
Health effects of breathing smog
Smog irritates the lungs and airways. On high-smog days, people may experience coughing, throat irritation, chest tightness, and shortness of breath, even during light activity. These symptoms usually fade once air quality improves, but repeated exposure over years can cause lasting damage.
People with asthma face the greatest risk. Smog can trigger asthma attacks, sometimes severe enough to require emergency care. People with chronic obstructive pulmonary disease (COPD), emphysema, and heart disease are also at high risk. Children are vulnerable because their lungs are still developing and they spend more time outdoors. Older adults are vulnerable because their lungs and hearts are less resilient.
Research shows that long-term exposure to smog increases the risk of premature death from respiratory and cardiovascular causes. People who live in chronically smoggier areas have higher rates of asthma, bronchitis, and reduced lung function compared to those in cleaner areas.
What to do on high-smog days
When the AQI reaches unhealthy levels, people in sensitive groups should limit outdoor activity. This means staying indoors, keeping windows closed, and using air conditioning with a clean filter. If you must go outside, an N95 or P100 mask can reduce the amount of ozone and particles you inhale, though it does not eliminate exposure entirely.
People with asthma or other lung conditions should keep their rescue inhalers with them and be prepared to use them. If you have heart disease, pay attention to how you feel during high-smog days — chest pain, unusual shortness of breath, or palpitations warrant a call to your doctor.
Avoid strenuous outdoor exercise on high-smog days, especially in the afternoon and early evening when ozone levels peak. Morning air is usually cleaner. If you exercise outdoors regularly, check the AQI before you go out and adjust your plans accordingly.
Regulations and efforts to reduce smog
The Clean Air Act, passed in 1970 and updated in 1990, gives the EPA authority to set national air quality standards for ozone and other pollutants. States must develop plans to meet these standards. California has the strictest vehicle emission standards in the country and has pushed automakers to produce cleaner cars for decades.
Power plants and refineries are required to install pollution control equipment. Vehicle emission standards have tightened repeatedly, and newer cars emit far less nitrogen oxide than older ones. Electric vehicles produce zero tailpipe emissions, though the electricity grid still relies partly on fossil fuels.
Despite these efforts, many areas still fail to meet EPA standards. Climate change is making the problem harder to solve because higher temperatures accelerate ozone formation. Wildfires, which have grown more frequent and intense in the West, also add particles and precursor chemicals to the air.
Frequently Asked Questions
Is smog the same as air pollution?
Smog is a type of air pollution, but not all air pollution is smog. Smog specifically refers to the visible haze created when sunlight reacts with emissions. Other forms of air pollution include carbon monoxide, sulfur dioxide, and lead, which may not be visible.
Can an air purifier protect me from smog?
A HEPA air purifier can remove fine particles from indoor air, but it does not remove ozone. If you spend most of your time indoors with the windows closed and the air purifier running, you reduce your exposure. However, you still need to go outside, and no purifier protects you then.
Why is smog worse in summer?
Heat accelerates the chemical reactions that turn nitrogen oxides and volatile organic compounds into ozone. Summer also brings more sunlight, which drives these reactions. In winter, cooler temperatures slow ozone formation, and shorter days mean less sunlight.
Does smog affect people who live far from cities?
Yes. Smog and its precursor chemicals can travel hundreds of miles downwind. Rural areas downwind of major cities sometimes experience high ozone levels even though they have little local traffic or industry. Wind patterns determine whether a rural area is affected.
What is the difference between smog and haze?
Haze is a general term for reduced visibility caused by particles in the air. Smog is a specific type of haze caused by ozone and fine particles from pollution. Haze can also come from dust, sea salt, or water vapor. On a high-smog day, the haze you see is smog.