What smog is and where it comes from

Smog is a visible haze in the air caused by a mix of pollutants — mainly ground-level ozone and fine particles — that form when sunlight reacts with emissions from cars, factories, and power plants. It is not the same as fog, which is water vapor. Smog is pollution that sits in the air and can affect your health, especially on hot days when the air is still.

The word itself comes from "smoke" and "fog," and it describes what you actually see: a brownish or grayish cloud hanging over a city or region. This happens most often in summer and early fall, when heat and sunshine are strongest. Areas with heavy traffic, industrial activity, or geography that traps air — like valleys or areas surrounded by mountains — tend to have worse smog problems.

Ground-level ozone, the main ingredient in smog, forms when nitrogen oxides (from vehicle exhaust and power plants) and volatile organic compounds (from paint, gasoline, and solvents) react in sunlight. This is different from the ozone layer high in the atmosphere, which protects us from UV rays. The ozone near the ground is what harms your lungs.

Key Takeaways

  • Smog is a visible haze made of ground-level ozone and fine particles that form when sunlight reacts with car and factory emissions.
  • Smog is worst on hot, sunny days when air is still, and it affects people with asthma, heart disease, and children more severely than others.
  • Air quality is measured on the Air Quality Index (AQI), which ranges from 0 to 500, with higher numbers meaning worse air and greater health risk.
  • You can check your local air quality through the EPA's AirNow website or your phone's weather app, which often includes an AQI reading.
  • On high-pollution days, people in sensitive groups should limit outdoor activity, especially exercise, and keep windows closed.

How smog affects your health

Smog irritates your lungs and airways. When you breathe it in, it can cause coughing, wheezing, shortness of breath, and chest pain — even in people without lung disease. The effects are worse when you are active outdoors, because you breathe more air and breathe it deeper into your lungs.

Some people are more vulnerable than others. Children, older adults, and people with asthma, emphysema, or heart disease face greater risk. Even a few hours in smoggy air can trigger an asthma attack or make existing heart problems worse. People who work outdoors — construction workers, landscapers, mail carriers — are exposed to higher levels throughout the day.

Long-term exposure to smog is linked to reduced lung function, especially in children whose lungs are still developing. Studies show that people who live in areas with chronic smog problems have higher rates of respiratory disease and earlier death from lung and heart causes.

Understanding the Air Quality Index (AQI)

The Air Quality Index, or AQI, is a number from 0 to 500 that tells you how polluted the air is right now. The EPA calculates it based on five major pollutants: ground-level ozone, particle pollution, nitrogen dioxide, sulfur dioxide, and carbon monoxide. The higher the number, the worse the air quality and the greater the health risk.

The AQI is color-coded to make it straightforward to understand at a glance. Green (0–50) means air quality is good. Yellow (51–100) means it is moderate — acceptable for most people, but members of sensitive groups may feel effects. Orange (101–150) means air is unhealthy for sensitive groups. Red (151–200) means unhealthy for everyone. Purple (201–300) is very unhealthy, and maroon (301+) is hazardous.

The AQI also breaks down which pollutant is the main problem that day. On a smoggy day, you will usually see that ground-level ozone is the "primary pollutant." This tells you what is actually in the air and helps you understand why the air quality is poor.

How to check air quality in your area

The easiest way to check your local air quality is through AirNow.gov, the EPA's official air quality website. You enter your zip code and see the current AQI, a forecast for the next few days, and a map showing pollution levels across your region. The site updates every hour.

Many weather apps and services also display the AQI alongside temperature and forecast. If your phone's weather app shows an air quality reading, that data usually comes from AirNow or a similar EPA-linked source. Some apps let you set alerts so you get a notification when air quality reaches unhealthy levels.

Local news stations often report on air quality during their weather forecast, especially on days when smog is expected. If you see an air quality alert on the news or receive one from your city or county, that is a signal to take precautions if you are in a sensitive group.

What to do on high-pollution days

When the AQI reaches orange (unhealthy for sensitive groups) or higher, people with asthma, heart disease, or other respiratory conditions should limit time outdoors and avoid strenuous activity outside. This includes exercise, yard work, and sports. If you must go outside, do it during cooler parts of the day — early morning or evening — when ozone levels are usually lower.

Keep your windows and doors closed on smoggy days to prevent polluted air from entering your home. If you have air conditioning, use it and keep the filter clean. If you do not have AC, closing windows is still better than letting smog in, even if it makes your home warmer.

If you have asthma or another lung condition, keep your rescue inhaler with you and take your regular medications as prescribed. Do not skip doses because the air is bad — staying on your regular treatment helps your lungs handle pollution better. If you feel chest pain, severe shortness of breath, or other serious symptoms, seek medical care right away.

Long-term solutions and what is being done

Smog is not a permanent problem. The Clean Air Act, passed in 1970 and updated since, set national air quality standards and required states to develop plans to meet them. Because of these rules, many cities that had severe smog in the 1970s and 1980s — like Los Angeles — have seen major improvements, even though they still have pollution problems on hot days.

Improvements come from stricter vehicle emission standards, cleaner power plants, and industrial regulations. Newer cars produce far less pollution than older ones. Power plants have switched to cleaner fuels or added pollution-control equipment. These changes take time and cost money, but they work.

You can reduce smog by driving less, combining trips, using public transit, carpooling, or biking when possible. Avoiding gas-powered lawn equipment and choosing low-VOC paints and products also helps. These individual actions matter most when many people do them, but they are part of how air quality improves over time.

Frequently Asked Questions

Is smog the same as air pollution?

Smog is one type of air pollution — the visible haze you see on bad days. Air pollution is a broader term that includes all harmful substances in the air, including some you cannot see. Smog is what happens when specific pollutants react in sunlight.

Can smog get into my home even if I close the windows?

Some smog can seep in through cracks and gaps, but closing windows and doors blocks most of it. If you have air conditioning with a good filter, that is the best protection. If you do not have AC, closed windows are still much better than open ones, even if your home gets warm.

Why is smog worse in summer?

Heat and strong sunlight are what create ground-level ozone. In summer, temperatures are higher and days are longer and sunnier, so the chemical reaction that forms smog happens faster and more intensely. Winter air is usually cleaner because there is less sunlight to trigger ozone formation.

Do masks protect me from smog?

Regular cloth or surgical masks do not filter out smog effectively. An N95 or P100 respirator mask can block some particles, but it must fit tightly and be used correctly. For most people, staying indoors on bad air days is more practical than wearing a mask all day.

What is the difference between smog and haze?

Haze is often caused by dust, salt, or water droplets and can be natural. Smog is specifically pollution — a chemical haze caused by emissions reacting in sunlight. Smog usually smells bad and causes health effects; haze may not.