• 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.


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

    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.

    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.

  • Term Part of Speech Definition Encyclopedic Entry
    ailment Noun

    illness or disease.

    airborne Adjective

    transported by air currents.

    air pollution Noun

    harmful chemicals in the atmosphere.

    Encyclopedic Entry: air pollution
    appliance Noun

    tool used to carry out a specific task.

    asthma Noun

    disease that makes it difficult to breathe.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    avoid Verb

    to stay away from something.

    basin Noun

    a dip or depression in the surface of the land or ocean floor.

    Encyclopedic Entry: basin
    behavior Noun

    anything an organism does involving action or response to stimulation.

    blanket Verb

    to cover entirely.

    burn day Noun

    time designated by a community for burning waste on private land.

    Carlos Fuentes Noun

    (1928-present) Mexican author.

    carpool Noun

    system of transportation where one car transports several riders.

    city Noun

    large settlement with a high population density.

    coal Noun

    dark, solid fossil fuel mined from the earth.

    electricity Noun

    set of physical phenomena associated with the presence and flow of electric charge.

    emission Noun

    discharge or release.

    exhaust Noun

    gases and particles expelled from an engine.

    familiar Adjective


    fog Noun

    clouds at ground level.

    Encyclopedic Entry: fog
    gasoline Noun

    liquid mixture made from oil and used to run many motor vehicles.

    harmful Adjective


    industrial Adjective

    having to do with factories or mechanical production.

    landscape Noun

    the geographic features of a region.

    Encyclopedic Entry: landscape
    law Noun

    public rule.

    lawn mower Noun

    machine that cuts grass to a uniform level.

    lung Noun

    organ in an animal that is necessary for breathing.

    mass transit Noun

    large-scale public transportation, such as buses or trains.

    mileage Noun

    number of miles traveled per specific amount of fuel, usually a gallon.

    mountain Noun

    landmass that forms as tectonic plates interact with each other.

    nitrogen oxide Noun

    one of many chemical compounds made of different combinations of nitrogen and oxygen.

    ozone Noun

    form of oxygen that absorbs ultraviolet radiation.

    ozone layer Noun

    layer in the atmosphere containing the gas ozone, which absorbs most of the sun's ultraviolet radiation.

    Encyclopedic Entry: ozone layer
    paint Noun

    chemical used for color.

    particle Noun

    small piece of material.

    photochemical smog Noun

    air pollution produced when sunlight reacts with automobile exhaust.

    pollution Noun

    introduction of harmful materials into the environment.

    Encyclopedic Entry: pollution
    power plant Noun

    industrial facility for the generation of electric energy.

    reduce Verb

    to lower or lessen.

    respiratory illness Noun

    disease of the lungs.

    restriction Noun

    barrier or prohibition.

    smog Noun

    type of air pollution common in manufacturing areas or areas with high traffic.

    Encyclopedic Entry: smog
    smoke Noun

    gases given off by a burning substance.

    solvent Noun

    substance that dissolves another substance.

    tissue Noun

    cells that form a specific function in a living organism.

    traffic Noun

    movement of many things, often vehicles, in a specific area.

    tune-up Noun

    regularly scheduled maintenance appointment for a car or other vehicle.

    ultraviolet radiation Noun

    powerful light waves that are too short for humans to see, but can penetrate Earth's atmosphere. Ultraviolet is often shortened to UV.

    valley Noun

    depression in the Earth between hills.

    visibility Noun

    the ability to see or be seen with the unaided eye. Also called visual range.

    volatile organic compound (VOC) Noun

    gas released from some solids or liquids that may cause harm to people and the atmosphere.

    wind Noun

    movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.

    yard Noun

    land surrounding a house or building.