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Outdoor Air Pollution

Introduces outdoor air pollution, acid rain, and global warming.

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Outdoor Air Pollution

What is this haze?

This picture shows a thick layer of smog and dust over a very polluted city (Singapore). Smog in the air is a serious health hazard for people living in many big cities around the globe. Smog is one example of air pollution.

Outdoor Air Pollution

Air is all around us. Air is essential for life. Sometimes, humans can pollute the air. For example, releasing smoke and dust from factories and cars can cause air pollution. Air pollution is due to chemical substances and particles released into the air mainly by human actions. This pollution affects entire ecosystems around the world. Pollution can also cause many human health problems, and it can also cause death. Air pollution can be found both outdoors and indoors.

Outdoor air pollution is made of chemical particles. When smoke or other pollutants enter the air, the particles found in the pollution mix with the air. Air is polluted when it contains many large toxic particles. Outdoor air pollution changes the natural characteristics of the atmosphere. Primary pollutants are added directly to the atmosphere. Fires add primary pollutants to the air. Particles released from the fire directly enter the air and cause pollution (Figure below). Burning of fossil fuels such as oil and coal is a major source of primary pollutants (Figure below).

Secondary pollutants are formed when primary pollutants interact with sunlight, air, or each other. They do not directly cause pollution. However, when they interact with other parts of the air, they do cause pollution. For example, ozone is created when some pollutants interact with sunlight. High levels of ozone in the atmosphere can cause problems for humans.

Wildfires generate particles that contribute to air pollution

Wildfires, either natural or human-caused, release particles into the air, one of the many causes of air pollution.

A major source of air pollution is the burning of fossil fuels in factories, power plants, and motor vehicles

A major source of air pollution is the burning of fossil fuels from factories, power plants, and motor vehicles.

Sources of Outdoor Air Pollution

Most air pollutants can be traced to the burning of fossil fuels. Fossil fuels are burned during many processes, including in power plants to create electricity, in factories to make machinery run, in power stoves and furnaces for heating, and in waste facilities. Perhaps one of the biggest uses of fossil fuels is in transportation. Fossil fuels are used in cars, trains, and planes.

Air pollution can also be caused by agriculture, such as cattle ranching and the use of fertilizers and pesticides. Other sources of air pollution include the production of plastics, refrigerants, and aerosols, in nuclear power and defense, from landfills and mining, and from biological warfare.

Acid Rain

One result of air pollution is acid rain. Acid rain is precipitation with a low (acidic) pH. This rain can be very destructive to wildlife. When acid rain falls in forests, freshwater habitats, or soils, it can kill insects and aquatic life. It causes this damage because of its very low pH. Sulfur oxides and nitrogen oxides in the air both cause acid rain to form (Figure below). Sulfur oxides are chemicals that are released from coal-fired power plants. Nitrogen oxides are released from motor vehicle exhaust.

This forest has been severely damaged by acid rain

A forest in the Jizera Mountains of the Czech Republic shows effects caused by acid rain. What do you observe?

Global Warming

Pollutants also affect the atmosphere through their contribution to global warming. Global warming is an increase in the Earth’s temperature. It is thought to be caused mostly by the increase of greenhouse gases like carbon dioxide. Greenhouse gases can be released by factories that burn fossil fuels. Over the past 20 years, burning fossil fuels has produced about three-quarters of the carbon dioxide from human activity. The rest of the carbon dioxide in the atmosphere is there because of deforestation, or cutting down trees (Figure below). Trees absorb carbon dioxide during cellular respiration, so when trees are cut down, they cannot remove carbon dioxide from the air.

Deforestation creates significant increases in carbon dioxide levels around the world

Deforestation, shown here as a result of burning for agriculture in southern Mexico, has produced significant increases in carbon dioxide emissions over the past 20 years.

This increase in global temperature will cause the sea level to rise. It is also expected to produce an increase in extreme weather events and change the amount of precipitation. Global warming may also cause food shortages and species extinction.


  • Air pollution is caused by chemical substances and particles released into the air, mainly by human activities.
  • The major cause of outdoor air pollution is the burning of fossil fuels.
  • Problems caused by the burning of fossil fuels include acid rain and global warming.

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Use the resource below to answer the questions that follow.

  1. What are pollutants?
  2. What are three sources of pollutants?
  3. What are the two main categories of pollutants?


  1. What is air pollution?
  2. What's the difference between primary and secondary pollutants? Give examples of each.
  3. What are three ways that polluting fossil fuels are burned?
  4. Why is acid rain dangerous.

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acid rain Low-pH precipitation that forms when air pollution combines with water.
air pollution Pollution caused by chemical substances and particles released into the air mainly by human actions, such as the burning of fossil fuels.
fossil fuels Remains of long-dead organisms that now serve as an energy source, such as as oil, coal, or natural gas.
global warming Global rise in Earth’s temperature due to increases of greenhouse gasses in the atmosphere.
greenhouse gases Gas that lets in light energy but does not let heat escape, contributing to global warming.
outdoor air pollution Chemical substances and particles released into the outside air.
ozone Pollutant in the lower atmosphere; created when some pollutants react with sunlight.
primary pollutants Harmful chemical substances and particles added directly to the air.
secondary pollutants Harmful chemical substances formed when primary pollutants interact with sunlight, air, or each other.

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