What smog is and why it happens

Smog is a visible haze that forms when sunlight reacts with pollution already in the air. It is not smoke from a fire — it is a chemical reaction between nitrogen oxides (mostly from car exhaust and power plants) and volatile organic compounds (from paint, gasoline, and industrial processes) that creates ground-level ozone and fine particles. On a clear day you can see the horizon. On a smog day, the air itself looks brown or gray, and visibility drops.

The reaction happens fastest on hot, sunny days with little wind. That is why smog is worst in summer and in valleys or basins where air gets trapped. Los Angeles, the San Francisco Bay Area, and parts of the Southwest experience the most persistent smog in the country because they have all three conditions: heavy vehicle traffic, warm weather, and geography that traps air.

Smog is not the same as haze from dust or wildfire smoke, though all three reduce visibility. Smog is specifically the product of pollution plus sunlight. If you see brown air on a sunny day in a city, that is likely smog. If you see gray air during or after a fire, that is smoke.

Key Takeaways

  • Smog forms when sunlight reacts with car exhaust and industrial pollution, and it is worst on hot, sunny days with little wind.
  • The Southwest, California, and parts of the Midwest have the highest smog levels because of traffic, heat, and air that gets trapped by geography.
  • Ground-level ozone — the main ingredient in smog — damages lungs and is especially harmful to children, older adults, and people with asthma.
  • The Clean Air Act set national standards for smog, and the EPA measures air quality daily in most U.S. cities and publishes it on AirNow.gov.
  • You can reduce your exposure by checking the Air Quality Index before outdoor activity and staying indoors on high-smog days.

Where smog is worst across the United States

The EPA divides the country into regions and tracks which areas meet federal air quality standards. Nonattainment areas — places where smog regularly exceeds safe levels — are concentrated in California, the Southwest, and parts of the Midwest and Northeast.

California has the most persistent problem. The South Coast Air Quality Management District (which covers Los Angeles, Orange County, and inland areas) has recorded some of the highest ozone levels in the nation for decades. The San Francisco Bay Area, Sacramento Valley, and San Diego also see regular smog. In the Southwest, parts of Arizona, Nevada, and Utah experience smog from both local traffic and pollution that drifts from California. The Houston area in Texas and parts of the Midwest around Chicago and the Great Lakes also report elevated smog levels.

Smaller cities in mountain valleys — including parts of Colorado, Utah, and Wyoming — can see smog even with less traffic because cold air sinks into valleys at night and traps pollution. When the sun rises, that trapped air heats up and the chemical reaction begins.

How the EPA measures and reports smog

The EPA uses the Air Quality Index (AQI) to measure smog and other air pollution on a scale from 0 to 500. An AQI of 0 to 50 is "Good." 51 to 100 is "Moderate." 101 to 150 is "Unhealthy for Sensitive Groups." 151 to 200 is "Unhealthy." 201 to 300 is "Very Unhealthy." Above 300 is "Hazardous."

The EPA monitors air quality at thousands of stations across the country and updates the data hourly. You can see real-time AQI for your city on AirNow.gov, which also shows forecasts for the next few days. Most weather apps and local news stations also report the AQI. When the index reaches "Unhealthy for Sensitive Groups" or higher, the EPA and local health departments issue alerts.

The index measures ground-level ozone (the main smog ingredient), fine particles, nitrogen dioxide, sulfur dioxide, and carbon monoxide. On a smog day, ozone is usually the pollutant pushing the index highest. The EPA sets the standard for ozone at 70 parts per billion averaged over eight hours — the level below which most people should not experience harm.

Health effects of breathing smog

Smog irritates the lungs and airways. When you breathe it, ozone damages the lining of your respiratory system, making it harder to breathe and triggering coughing, wheezing, and chest pain. The damage is temporary if exposure is brief, but repeated exposure over days or weeks can reduce lung function permanently.

Children are at higher risk because their lungs are still developing and they spend more time outdoors. Older adults are at risk because lung function naturally declines with age. People with asthma, emphysema, or other chronic lung disease are at risk because their airways are already inflamed. People exercising outdoors — runners, cyclists, outdoor workers — inhale more air and thus more ozone, so they are at higher risk even if they are young and healthy.

On days when the AQI reaches "Unhealthy for Sensitive Groups" (101 to 150), people in those categories should limit outdoor activity. At "Unhealthy" (151 to 200), everyone should reduce time outside. At "Very Unhealthy" or "Hazardous," most people should stay indoors.

What causes smog to get worse or better

Smog gets worse when the weather is hot and sunny with high pressure and little wind. High pressure systems trap air near the ground instead of letting it rise and disperse. No wind means pollution stays in one place instead of blowing away. Temperature inversions — when a layer of warm air sits above cooler air near the ground — trap pollution like a lid on a pot. These conditions are most common in summer and in enclosed valleys.

Smog gets better when it rains (rain washes pollution out of the air), when wind picks up and disperses the pollution, or when cooler weather arrives in fall and winter. It also improves when fewer cars are on the road — during the COVID-19 lockdowns in 2020, smog levels dropped noticeably in major cities because traffic fell.

Long-term, smog has improved nationwide since the Clean Air Act was passed in 1970 and strengthened in 1990. The EPA tightened ozone standards multiple times, and states were required to reduce emissions. Cars became cleaner, power plants installed pollution controls, and industry shifted away from the dirtiest processes. Despite improvement, many areas still exceed the standard on summer days.

Steps to reduce your exposure on high-smog days

Check the AQI before you plan outdoor activity. Visit AirNow.gov or your local news weather forecast and look at the index for your city. If it is in the "Unhealthy for Sensitive Groups" range or higher, adjust your plans.

If you are in a sensitive group (child, older adult, person with lung disease), limit outdoor activity when the AQI is above 100. If you must go outside, avoid strenuous exercise — walking is safer than running because you breathe less air. Wear an N95 mask if you have one; it filters out fine particles and some ozone, though no mask blocks all pollution.

Keep windows closed on high-smog days and use air conditioning with a clean filter. If you do not have air conditioning, spend time in a public place that does — a library, mall, or community center. Stay indoors during the afternoon and early evening, when ozone levels peak (usually 2 p.m. to 6 p.m.). Ozone is lower in early morning and after sunset.

Frequently Asked Questions

Is smog the same as air pollution?

No. Smog is one type of air pollution — specifically, the haze that forms when sunlight reacts with nitrogen oxides and volatile organic compounds. Air pollution is a broader category that includes smog, dust, wildfire smoke, and other particles or gases in the air. You can have air pollution without smog (like dust from a construction site), and you can have smog without other types of pollution.

Can I see the AQI in real time on my phone?

Yes. AirNow.gov has a mobile version, and most weather apps (including Apple Weather, Google Weather, and Weather.com) display the AQI for your location. Many local news stations also post it on their websites and apps. You can also search "air quality [your city]" to find the current index.

Does smog only happen in cities?

Smog is worst in cities because that is where most cars and factories are, but it can travel. Pollution from Los Angeles drifts inland and affects air quality in Nevada and Arizona. Pollution from the Midwest drifts east and affects the Northeast. Rural areas downwind of cities often see smog even if they have little local traffic.

Will my car contribute to smog if I drive on a high-smog day?

Yes, but the effect is small compared to the overall pollution already in the air. If you can avoid driving on a high-smog day — by working from home, using public transit, or postponing a trip — that helps. But the smog that day was mostly created by pollution from previous days and weeks, not just that day's traffic.

What is the difference between smog and haze?

Haze is any visible reduction in clarity caused by particles in the air — dust, salt spray, or water droplets. Smog is specifically the brown or gray haze from chemical pollution. On a dusty day you might see haze but not smog. On a sunny, polluted day in a city, you see smog. The distinction matters because smog is harmful to breathe and haze usually is not.