(2) Air Pollution

Air Pollution Comes from Natural and Human Sources

Air pollution is the presence of chemicals in the atmosphere in concentrations high enough to harm organisms, ecosystems, or materials, or to alter climate.

The effects of air pollution range from annoying to lethal.

Air pollutants come from natural and human sources. Natural sources include dust blown by wind, pollutants from wildfires and volcanic eruptions (Core Case Study), and volatile organic chemicals released by some plants.

Most natural air pollutants are spread out over the globe or removed by chemical cycles, precipitation, and gravity. However, chemicals emitted from volcanic eruptions and some natural forest fires can reach harmful levels.

Human inputs of outdoor air pollutants occur mostly in industrialized and urban areas where people, motor vehicles, and factories are concentrated. Most of these pollutants enter the atmosphere from the burning of coal in power and industrial plants and gasoline and diesel fuel in motor vehicles (mobile sources).

Scientists classify outdoor air pollutants into two categories. Primary pollutants are harmful chemicals emitted directly into the air from natural processes and human activities. While in the atmosphere, some primary pollutants may react with one another or with the basic components of air to form new harmful chemicals, called secondary pollutants.

Indoor air pollution is considered by experts to be a higher-risk human health problem than outdoor air pollution. Some indoor air pollutants come from outdoors. But chemicals used or produced inside buildings in developed areas can be dangerous pollutants, as can smoke from poorly designed wood or coal stoves used to provide heat and cook food in many developing countries. Let us look more closely at the major air pollutants.

Carbon Oxides

Carbon monoxide (CO) is a colorless, odorless, and highly toxic gas that forms, during the incomplete combustion of carbon-containing materials. Major sources are motor vehicle exhaust, clearing and burning of forests and grasslands, tobacco smoke, and cooking with open fires and inefficient stoves.

CO reacts with hemoglobin in red blood cells and reduces the ability of blood to transport oxygen to body cells and tissues. Chronic exposure can trigger heart attacks and aggravate lung diseases such as asthma and emphysema. At high levels, CO causes headache, nausea, drowsiness, mental impairment, collapse, coma, and death.

Carbon dioxide (CO2) is a colorless, odorless gas. About 93% of the CO2 in the atmosphere is the result of the natural carbon cycle. The rest comes from human activities, mostly burning fossil fuels and clearing forests and grasslands.

Emissions from human activities have been rising sharply since the industrial revolution. Increasing levels of CO2 can contribute to warming of the atmosphere and global climate change.

Nitrogen Oxides and Nitric Acid

Nitrogen oxide (NO) is a colorless gas that forms when nitrogen and oxygen gas in air react at the high-combustion temperatures in automobile engines and coal-burning power plants. Lightning and certain bacteria in soil and water also produce NO as part of the nitrogen cycle. In the air, NO reacts with oxygen to form nitrogen dioxide (NO2), a reddish-brown gas. Collectively, NO and NO2 are called nitrogen oxides (NOx). Some of the NO2 reacts with water vapor in the air to form nitric acid (HNO3) and nitrate salts (NO3-)   components of harmful acid deposition. Both NO and NO2 play a role in the formation of photochemical smog-a mix of chemicals formed under the influence of sunlight in cities with heavy traffic.

 Nitrous oxide (N2O), a greenhouse gas, is emitted from fertilizers and animal wastes and is produced by burning fossil fuels. Nitrogen oxides can irritate the eyes, nose, and throat, aggravate lung ailments such as asthma and bronchitis, suppress plant growth, and reduce visibility.

Sulfur Dioxide and Sulfuric Acid

Sulfur dioxide (SO2) is a colorless gas with an irritating odor. About one-third of the SO2 in the atmosphere comes from natural sources as part of the sulfur cycle. The other two-thirds (and as much as 90% in urban areas) come from human sources, mostly combustion of sulfur-containing coal in electric power and industrial plants and from oil refining and smelting of sulfide ores.

In the atmosphere, SO2 can be converted to microscopic suspended droplets of sulfuric acid (H2SO4) and suspended particles of sulfate (SO42–) salts that return to the earth as a component of acid deposition. These pollutants also reduce visibility and aggravate breathing problems. SO2 and H2SO4 can damage crops, trees, soils, and aquatic life in lakes. They can corrode metals and damage paint, paper, leather, and stone on buildings and statues.


Suspended particulate matter (SPM) consists of a variety of solid particles and liquid droplets small and light enough to remain suspended in the air for long periods. About 62% of the SPM in outdoor air comes from natural sources such as dust, wild fires, and sea salt. The remaining 38% comes from human sources such as plowed fields, road construction, unpaved roads, tobacco smoke, coal-burning electric power and industrial plants, and motor vehicles.

These particles can irritate the nose and throat, damage the lungs, aggravate asthma and bronchitis, and shorten life.

Toxic particulates (such as lead, cadmium, and PCBs) can cause mutations, reproductive problems, and cancer. Toxic lead particles mostly from burning coal and leaded gasoline and smelting lead ores can accumulate in the body and cause nervous system damage, mental retardation (especially in children), and digestive and other health problems. In the United States, particulate air pollution is responsible for about 60,000 premature deaths a year, according to the EPA and the Harvard School of Public Health. Particulates also reduce visibility, corrode metals, and discolor clothes and paints.


Ozone (O3), a colorless and highly reactive gas, is a major component of photochemical smog. It can cause coughing and breathing problems aggravate lung and heart diseases, reduce resistance to colds and pneumonia, and irritate the eyes, nose, and throat. It also damages plants, rubber in tires, fabrics, and paints.

Volatile Organic Compounds (VOCs)

Organic compounds that exist as gases in the atmosphere are called volatile organic compounds (VOCs). Most are hydrocarbons, such as isoprene (C3H8) and terpenes like C10H15 emitted by the leaves of many plants, and methane (CH4, a greenhouse gas). About a third of global methane emissions come from natural sources, mostly plants, wetlands, and termites. The rest comes from human sources-primarily rice paddies, landfills, and oil and natural gas wells-and from cows (from belching and flatulence). Other VOCs, including benzene, vinyl chloride, and trichloroethylene (TCE), are used as industrial solvents, dry-cleaning fluids, and components of gasoline, plastics, drugs, synthetic rubber, and other products.

Radioactive Radon (Rn)

Radon-222 is a naturally occurring colorless and odorless radioactive gas found in some types of soil and rock. It can seep into homes and buildings sitting above such deposits. Long-term exposure can cause lung cancer, especially among smokers.

Biological indicators such as lichens, at left), chemical instruments, and satellites can detect levels of air pollutants. In addition, scientists are trying to develop inexpensive Nan detectors for various air pollutants.

Sources and types of air pollutants. Human inputs of air pollutants may come from mobile sources (such as cars) and stationary sources (such as industrial and power plants). Some primary air pollutants may react with one another or with other chemicals in the air to form secondary air pollutants.


Air Pollutants

Explain how your daily lifestyle produces some of each of the primary air pollutants.

Using Lichens to Detect Air Pollution

Nineteenth-century coal miners took canaries with them into the mines-not to enjoy their songs but for the moment when they stopped singing. Then the miners knew it was time to get out of the mine because the air contained methane, which could ignite and explode.

Today we use sophisticated equipment and satellites to monitor air quality, but living things such as lichens can also warn us of bad air. Lichens consist of a fungus and an alga living together, usually in a mutually beneficial (mutualistic) relationship.

These hardy pioneer species are good biological indicators of air pollution because they continually absorb air as a source of nourishment. A highly polluted area around an industrial plant may have only gray-green crusty lichens or none at all. An area with moderate air pollution may have orange crusty lichens on outdoor walls. Walls and trees in areas with fairly clean air may support leafy lichens.

Because lichens are widespread, long lived, and anchored in place, they can also help track pollution to its source. Isle Royale, Michigan (USA) in Lake Superior is a place where no car or smokestack has ever intruded. The scientists who discovered sulfur dioxide pollution there used Evernia lichens to point the finger northward to coal burning facilities in Thunder Bay, Ontario, Canada.

Critical Thinking

Investigate rocks, trees, and buildings in the area where you live for the presence of various lichens. Use their presence, colors, and types to identify high- and low-pollution areas. Red and yellow crusted lichens growing on slate rock in the foothills of the Sierra Nevada near Merced, California (USA). The vulnerability of various lichen species to specific air pollutants can help researchers detect levels of these pollutants and track down their sources.

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