Smog is air pollution that reduces visibility


5 - 8


Health, Earth Science, Experiential Learning, Geography, Human Geography

NGS Resource Carousel Loading Logo
Loading ...

Smog is air pollution that reduces visibility. The term "smog" was first used in the early 1900s to describe a mix of smoke and fog. The smoke usually came from burning coal. Smog was common in industrial areas, and remains a familiar sight in some cities today.

Today, most of the smog we see is photochemical smog. Photochemical smog is produced when sunlight reacts with nitrogen oxides and at least one volatile organic compound (VOC) in the atmosphere. Nitrogen oxides come from car exhaust, coal power plants, and factory emissions. VOCs are released from gasoline, paints, and many cleaning solvents. When sunlight hits these chemicals, they form airborne particles and ground-level ozone—or smog.

Ozone can be helpful or harmful. The ozone layer high in the atmosphere protects us from the sun’s dangerous ultraviolet radiation. But when ozone is close to the ground, it is bad for human health. Ozone can damage lung tissue, and it is especially dangerous to people with respiratory illnesses like asthma. Ozone can also cause itchy, burning eyes.

Smog is unhealthy to humans and animals, and it can kill plants. Smog is also ugly. It makes the sky brown or gray. Smog is common in big cities with a lot of industry and traffic. Cities located in basins surrounded by mountains may have smog problems because the smog is trapped in the valley and cannot be carried away by wind. Los Angeles, California, United States, and Mexico City, Mexico, both have high smog levels partly because of this kind of landscape.

Many countries, including the United States, have created laws to reduce smog. Some laws include restrictions on what chemicals a factory can release into the atmosphere, or when the factory can release them. Some communities have "burn days" when residents can burn waste such as leaves in their yard. These limits on chemicals released into the air reduce the amount of smog.

Smog is still a problem in many places. Everyone can do their part to reduce smog by changing a few behaviors, such as:

  • Drive less. Walk, bike, carpool, and use public transportation whenever possible.
  • Take care of cars. Getting regular tune-ups, changing oil when scheduled, and inflating tires to the proper level can improve gas mileage and thus reduce emissions.
  • Fuel up during the cooler hours of the day—night or early morning. This prevents gas fumes from heating up and producing ozone.
  • Avoid products that release high levels of VOCs. For example, use low-VOC paints.
  • Avoid gas-powered yard equipment, like lawn mowers. Use electric appliances instead.

Fast Fact

Where the Air Is Not So Clear
During the early 1900s, Mexico City, Mexico, was known for having some of the cleanest air in the world. Author Carlos Fuentes wrote a novel about the city in 1959 and called it Where the Air is Clear. Today, however, Mexico City is one of the smoggiest places on Earth.

Fast Fact

During the Great Smog of 1952, coal pollution blanketed the city of London, England. More than 4,000 people died from respiratory ailments as a result. The smog was so thick that the city had to shut down roads, railways, and the airport. Robbers used the cover of smog to break into houses and shops.

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Hilary Costa
Erin Sprout
Santani Teng
Melissa McDaniel
Jeff Hunt
Diane Boudreau
Tara Ramroop
Kim Rutledge
Hilary Hall
Mary Crooks, National Geographic Society
Tim Gunther
Jeannie Evers, Emdash Editing, Emdash Editing
Kara West
Educator Reviewer
Nancy Wynne
National Geographic Society
Last Updated

October 19, 2023

For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher. They will best know the preferred format. When you reach out to them, you will need the page title, URL, and the date you accessed the resource.


If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media.


Text on this page is printable and can be used according to our Terms of Service.


Any interactives on this page can only be played while you are visiting our website. You cannot download interactives.

Related Resources