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

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.

Undercover
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.

ailment
Noun

illness or disease.

airborne
Adjective

transported by air currents.

Noun

harmful chemicals in the atmosphere.

appliance
Noun

tool used to carry out a specific task.

asthma
Noun

disease that makes it difficult to breathe.

Noun

layers of gases surrounding a planet or other celestial body.

avoid
Verb

to stay away from something.

Noun

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

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

well-known.

Noun

clouds at ground level.

gasoline
Noun

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

harmful
Adjective

damaging.

industrial
Adjective

having to do with factories or mechanical production.

Noun

the geographic features of a region.

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.

Noun

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

paint
Noun

chemical used for color.

particle
Noun

small piece of material.

photochemical smog
Noun

air pollution produced when sunlight reacts with automobile exhaust.

Noun

introduction of harmful materials into the environment.

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.

Noun

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

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.