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What Causes Air Pollution

Updated on September 6, 2012

According to the World Health Organization, air pollution contributes to more than three million premature deaths per year, many of these among children under five. Air pollution has been linked with higher rates of stroke, heart disease, cystic fibrosis, asthma, chronic bronchitis, emphysema, and multiple forms of cancer.

Outdoor air pollution from industry, transportation, and agriculture are only part of the problem. Indoor air pollution, caused by common household products and activities, is also a significant risk to human health.

The negative impacts of air pollution go well beyond human health, as they affect numerous other plant and animal species and can disrupt entire ecosystems. In the most extreme examples, such as the depletion of the ozone layer due to chlorofluorocarbons or climate change due to greenhouse gas emissions, the impacts of air pollution can be global.

A smoggy sunrise in Beijing
A smoggy sunrise in Beijing | Source

Outdoor Air Pollution

Outdoor air pollution can be grouped into two classifications: primary and secondary pollutants. Primary pollutants are harmful substances emitted into the air as a direct result of an artificial or natural process. Secondary pollutants are harmful substances created in the atmosphere as primary pollutants react with each other and with other chemicals and substances in the environment.

Some of the most significant primary pollutants include:

  • Oxides such as sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), and carbon dioxide (CO2). While all of these are produced naturally in the environment in small amounts, they are produced in far greater amounts by humans as by-products from the burning of fossil fuels, wood, and waste in power plants, factories, vehicles, and homes. While nitrogen dioxide and carbon monoxide are immediately toxic to humans and other animals, carbon dioxide and sulfur dioxide are of concern in the longer term as contributors to global warming and acid rain, respectively.
  • Volatile organic compounds (VOCs) such as methane, benzene, isoprene, and formaldehyde are also produced by natural causes and human activities. The term refers to organic compounds with very low boiling points, allowing them to evaporate or sublimate at room temperature. While methane is a critical greenhouse gas, other volatile organic compounds can have dangerous long-term effects on human health. Plants, molds, and microbes produce most VOCs in the environment, but human activity is a significant contributor through production of paints and solvents as well as the burning of fossil fuels.
  • Particulate matter (PM) refers to fine particles of solid or liquid matter suspended in a gas. The resulting combination of gas and particulate matter is known as an aerosol. While particulate matter is commonly produced by natural forces such as volcanic eruptions, forest fires, and dust storms, human activity is also a significant contributor via burning of fossil fuels and industrial processes. Particulate matter is grouped according to its size, with particles under 10 microns classified as PM10 and particles under 2.5 microns classified as PM2.5. While both categories have significant health impacts on the cardiopulmonary and respiratory systems, PM2.5 substances are considered more dangerous as they can pass through the lungs and enter the bloodstream.
  • Chlorofluorocarbons (CFCs) were commonly in use as refrigerants and aerosol propellants during the 20th century. Their use has been phased out since it was discovered in the late 1970s that CFCs cause destruction of the ozone (O3) layer of the upper stratosphere that protects us from much of the sun's ultraviolet radiation. Though their use in new products is now banned worldwide, existing CFCs already in the atmosphere and in existing refrigeration and air conditioning systems will continue to have negative effects on this critical atmospheric layer for decades to come.

These and other primary pollutants are only the first step in the air pollution equation. Their effects are compounded when these primary pollutants combine with each other and react with other environmental factors to create secondary pollutants:

  • Smog is the combination of fog and pollutants such as smoke or sulfur dioxide. Ultraviolet light can react with some of the pollutants suspended in smog to create secondary pollutants such as formaldehyde, the irritant peroxyacetyl nitrate, and other volatile organic compounds.
  • Ground-level ozone is a secondary pollutant by-product of nitrous oxides, carbon monoxide, and some volatile organic compounds reacting with sunlight. While ozone in the upper stratosphere offers vital protection against harmful radiation, ozone at ground-level is a harmful irritant that can interfere with the respiratory system and worsen conditions such as asthma and allergies.

Indoor cookstoves powered by burning firewood, charcoal, and dried dung are a major source of indoor air pollution in developing countries.
Indoor cookstoves powered by burning firewood, charcoal, and dried dung are a major source of indoor air pollution in developing countries. | Source

Indoor Air Pollution

While natural and human activity can create air pollution hazards in the outdoor environment, we are no safer from inside our homes and work spaces. In fact, the indoor environment can be even more hazardous to human health than outdoor air pollution. The World Health Organization estimates that two million of the worldwide annual premature deaths from air pollution result from poor indoor air quality. As with external air pollution, there are a mixture of natural causes and human causes that contribute to indoor air pollution.

  • Radon (Rn) is a natural indoor air pollutant resulting from radioactive decay of elements in the Earth's crust. It is a dangerous carcinogen that can accumulate in homes and can often go undetected.
  • Solvents, cleaners, and building materials can give off volatile organic compounds that can accumulate in poorly-ventilated homes and offices, causing respiratory conditions and other health effects.
  • Smoke, VOCs, and carbon monoxide from indoor cookstoves fueled by wood, charcoal, and dried dung are a major health hazard in the developing world, contributing to high rates of under-five mortality from pneumonia and other diseases. The concentration of toxic substances and particulate matter found in many homes in developing countries has been found to be more than ten times the guideline limit set by the U.S. Environmental Protection Agency.


Though outdoor and indoor air pollution can seem like a daunting problem, there have been successes in much of the developed world. The worldwide ban on chlorofluorocarbons, which has begun to reach even the largely unregulated areas of the developing world, has been largely successful. The U.S. Acid Rain Program - a cap-and-trade system to reduce emissions of NO2 and SO2 - has reduced acid rain levels in the U.S. by 65% since 1976. Similar programs and regulations in the European Union have also shown great success in reducing air pollution.

These examples demonstrate that the problem of air pollution - at least, the portion contributed by human causes - is potentially solvable. The problem can only be solved, however, when government, industry, and the general public are willing to take the necessary steps to reduce emissions.


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    • nifwlseirff profile image

      Kymberly Fergusson 5 years ago from Villingen Schwenningen, Germany

      It's always surprised me how much noxious glue is used to build homes and furniture, and even used to bind plastic sleeves on hard back books in libraries. Air pollution to many is completely invisible...