Smog is air pollution that reduces visibility

National Geographic


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 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 up 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, 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 on schedule, and inflating tires to the proper level can improve gas mileage and 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.

smogSmog is often worst in the mornings, when there is less air movement.

Photograph by Jodi Cobb

Where the Air Is Not So Clear
During the early 1900s, Mexico City 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.

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.


  • Human Impacts on the Environment1.0KHumans impact the physical environment in many ways: overpopulation, pollution, burning fossil fuels, and deforestation. Changes like these have triggered climate change, soil erosion, poor air quality, and undrinkable water. These negative impacts can affect human behavior and can prompt mass migrations or battles over clean water.   Help your students understand the impact humans have on the physical environment with these classroom resources.VIEW COLLECTION
  • Atmosphere57One of the main components of Earth’s interdependent physical systems is the atmosphere. An atmosphere is the layers of gases surrounding a planet or other celestial body. Earth’s atmosphere is composed of about 78% nitrogen, 21% oxygen, and one percent other gases. These gases are found in layers (troposphere, stratosphere, mesosphere, thermosphere, and exosphere) defined by unique features such as temperature and pressure. The atmosphere protects life on earth by shielding it from incoming ultraviolet (UV) radiation, keeping the planet warm through insulation, and preventing extremes between day and night temperatures. The sun heats layers of the atmosphere causing it to convect driving air movement and weather patterns around the world. Teach your students about the Earth’s atmosphere with the resources in this collection.VIEW COLLECTION
  • ozone layer38Encyclopedic entry. The ozone layer is one layer of the stratosphere, the second layer of the Earth’s atmosphere. The stratosphere is the mass of protective gases clinging to our planet.VIEW ARTICLE
  • Ozone Depletion 101133Far above Earth’s surface, the ozone layer helps to protect life from harmful ultraviolet radiation. Learn what CFCs are, how they have contributed to the ozone hole, and how the 1989 Montreal Protocol sought to put an end to ozone depletion.VIEW VIDEO


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