Air Pollution

Lesson Objectives

  • Describe the different types of air pollutants.
  • Discuss what conditions lead some cities to become more polluted than others.
  • Describe the sources of air pollutants.


  • photochemical smog
  • slash-and-burn agriculture


Earth’s atmosphere provides living creatures on the planet with the gases they need for photosynthesis and respiration. In addition, the ozone layer protects organisms from the Sun’s ultraviolet radiation. The importance of the atmosphere for Earth’s life cannot be overestimated yet people also use the atmosphere as a dump for waste gases and particles.

Air Quality

Pollutants include materials that are naturally occurring but are added to the atmosphere so that they are there in larger quantities than normal. Pollutants may also be human-made compounds that have never before been found in the atmosphere. Pollutants dirty the air, change natural processes in the atmosphere, and harm living things.

Problems with Air Quality

Air pollution started to be a problem when early people burned wood for heat and cooking fires in enclosed spaces such as caves and small tents or houses. But the problems became more widespread as fossil fuels such as coal began to be burned during the Industrial Revolution (Figure below).

The 2012 Olympic Games in London opening ceremony contained a reenactment of the Industrial Revolution – complete with pollution streaming from smokestacks.

Air pollution became a crisis in the developed nations in the mid-20th century. Coal smoke and auto exhaust combined to create toxic smog that in some places caused lung damage and sometimes death. In Donora, Pennsylvania, in October 1948, 20 people died and 4,000 became ill when coal smoke was trapped by an inversion. In London in December 1952, the “Big Smog” killed 4,000 people over five days. Many thousands more likely died of health complications from the event in the next several months.

Photochemical smog, a different type of air pollution, first became a problem in Southern California after World War II. The abundance of cars and sunshine provided the perfect setting for a chemical reaction between some of the molecules in auto exhaust or oil refinery emissions, and sunshine (Figure below). Photochemical smog consists of more than 100 compounds, most importantly ozone.

Much of the air pollution seen over Los Angeles is photochemical smog.

The Clean Air Act

The terrible events in Pennsylvania and London, plus the recognition of the hazards of photochemical smog, led to the passage of the Clean Air Act in 1970 in the United States. The act now regulates 189 pollutants. The six most important pollutants regulated by the Act are ozone, particulate matter, sulfur dioxide, nitrogen dioxide, carbon monoxide, and the heavy metal lead. Other important regulated pollutants include benzene, perchloroethylene, methylene chloride, dioxin, asbestos, toluene, and metals such as cadmium, mercury, chromium, and lead compounds.

What is the result of the Clean Air Act? In short, the air in the United States is much cleaner. Visibility is better and people are no longer incapacitated by industrial smog. However, despite the Act, industry, power plants, and vehicles put 160 million tons of pollutants into the air each year. Some of this smog is invisible and some contributes to the orange or blue haze that affects many cities.

Regional Air Quality

Air quality in a region is not just affected by the amount of pollutants released into the atmosphere in that location but by other geographical and atmospheric factors. Winds can move pollutants into or out of a region and a mountain range can trap pollutants on its leeward side. Inversions commonly trap pollutants within a cool air mass. If the inversion lasts long enough, pollution can reach dangerous levels.

Pollutants remain over a region until they are transported out of the area by wind, diluted by air blown in from another region, transformed into other compounds, or carried to the ground when mixed with rain or snow.

Table below lists the smoggiest cities in 2012: nine of the 10 are in California. Why do you think California cities are among those with the worst air pollution?

The state has the right conditions for collecting pollutants including mountain ranges that trap smoggy air, arid and sometimes windless conditions, agriculture, industry, and lots and lots of cars.

Smoggiest U.S. Cities, 2012
Rank City, State
1 Los Angeles, California
2 Visalia-Porterville, California
3 Bakersfield, California
4 Fresno, California
5 Hanford, California
6 Sacramento, California
7 San Diego, California
8 Houston, Texas
9 San Luis Obispo, California
10 Merced, California

Types of Air Pollution

The two types of air pollutants are primary pollutants, which enter the atmosphere directly, and secondary pollutants, which form from a chemical reaction.

Primary Pollutants

Some primary pollutants are natural, such as volcanic ash. Dust is natural but exacerbated by human activities; for example, when the ground is torn up for agriculture or development. Most primary pollutants are the result of human activities, the direct emissions from vehicles and smokestacks. Primary pollutants include:

  • Carbon oxides include carbon monoxide (CO) and carbon dioxide (CO2) (Figure below). Both are colorless, odorless gases. CO is toxic to both plants and animals. CO and CO2 are both greenhouse gases.

High CO2 levels are found in major metropolitan areas and along the major interstate highways.

  • Nitrogen oxides are produced when nitrogen and oxygen from the atmosphere come together at high temperatures. This occurs in hot exhaust gas from vehicles, power plants, or factories. Nitrogen oxide (NO) and nitrogen dioxide (NO2) are greenhouse gases. Nitrogen oxides contribute to acid rain.
  • Sulfur oxides include sulfur dioxide (SO2) and sulfur trioxide (SO3). These form when sulfur from burning coal reaches the air. Sulfur oxides are components of acid rain.
  • Particulates are solid particles, such as ash, dust and fecal matter (Figure below). They are commonly formed from combustion of fossil fuels, and can produce smog. Particulates can contribute to asthma, heart disease, and some types of cancers.

Particulates from a brush fire give the sky a strange glow in Arizona.

  • Lead was once widely used in automobile fuels, paint, and pipes. This heavy metal can cause brain damage or blood poisoning.
  • Volatile organic compounds (VOCs) are mostly hydrocarbons. Important VOCs include methane (a naturally occurring greenhouse gas that is increasing because of human activities), chlorofluorocarbons (human-made compounds that are being phased out because of their effect on the ozone layer), and dioxin (a byproduct of chemical production that serves no useful purpose, but is harmful to humans and other organisms).

Secondary Pollutants

Any city can have photochemical smog, but it is most common in sunny, dry locations. A rise in the number of vehicles in cities worldwide has increased photochemical smog. Nitrogen oxides, ozone, and several other compounds are some of the components of this type of air pollution.

Photochemical smog forms when car exhaust is exposed to sunlight. Nitrogen oxides is created by gas combustion in cars and then into the air (Figure below). In the presence of sunshine, the NO2 splits and releases an oxygen ion (O). The O then combines with an oxygen molecule (O2) to form ozone (O3). This reaction can also go in reverse: Nitric oxide (NO) removes an oxygen atom from ozone to make it O2. The direction the reaction goes depends on how much NO2 and NO there is. If NO2 is three times more abundant than NO, ozone will be produced. If nitrous oxide levels are high, ozone will not be created.

The brown color of the air behind the Golden Gate Bridge is typical of California cities, because of nitrogen oxides.

Ozone is one of the major secondary pollutants. It is created by a chemical reaction that takes place in exhaust and in the presence of sunlight. The gas is acrid-smelling and whitish. Warm, dry cities surrounded by mountains, such as Los Angeles, Phoenix, and Denver, are especially prone to photochemical smog (Figure below). Photochemical smog peaks at midday on the hottest days of summer. Ozone is also a greenhouse gas.

Counties with such high ozone levels that they do not attain federal air quality standards.

Causes of Air Pollution

Most air pollutants come from burning fossil fuels or plant material. Some are the result of evaporation from human-made materials. Nearly half (49%) of air pollution comes from transportation, 28% from factories and power plants, and the remaining pollution from a variety of other sources.

Fossil Fuels

Fossil fuels are burned in most motor vehicles and power plants. These nonrenewable resources are the power for nearly all manufacturing and other industries. Pure coal and petroleum can burn cleanly and emit only carbon dioxide and water, but most of the time, these fossil fuels do not burn completely and the incomplete chemical reactions produce pollutants. Few sources of these fossil fuels are pure, so other pollutants are usually released. These pollutants include carbon monoxide, nitrogen dioxide, sulfur dioxide, and hydrocarbons.

In large car-dependent cities such as Los Angeles and Mexico City, 80% to 85% of air pollution is from motor vehicles (Figure below). Ozone, carbon monoxide, and nitrous oxides come from vehicle exhaust.

See the relative amounts of CO2 released by different fossil fuels in this animation

Auto exhaust like this means that the fuels is not burning efficiently.

A few pollutants come primarily from power plants or industrial plants that burn coal or oil (Figure below). Sulfur dioxide (SO2) is a major component of industrial air pollution that is released whenever coal and petroleum are burned. SO2 mixes with H2O in the air to produce sulfuric acid (H2SO4).

Mercury is released when coal and some types of wastes are burned. Mercury is emitted as a gas, but as it cools, it becomes a droplet. Mercury droplets eventually fall to the ground. If they fall into sediments, bacteria convert them to the most dangerous form of mercury: methyl mercury. Highly toxic, methyl mercury is one of the metal’s organic forms.

A power plant and its emissions before emission control equipment was added.

Biomass Burning

Fossil fuels are ancient plants and animals that have been converted into usable hydrocarbons. Burning plant and animal material directly also produces pollutants. Biomass is the total amount of living material found in an environment. The biomass of a rainforest is the amount of living material found in that rainforest.

The primary way biomass is burned is for slash-and-burn agriculture (Figure below). The rainforest is slashed down and then the waste is burned to clear the land for farming. Biomass from other biomes, such as the savannah, is also burned to clear farmland. The pollutants are much the same as from burning fossil fuels: CO2, carbon monoxide, methane, particulates, nitrous oxide, hydrocarbons, and organic and elemental carbon. Burning forests increases greenhouse gases in the atmosphere by releasing the CO2 stored in the biomass and also by removing the forest so that it cannot store CO2 in the future. As with all forms of air pollution, the smoke from biomass burning often spreads far and pollutants can plague neighboring states or countries.

A forest that has been slash-and-burned to make new farmland.

Particulates result when anything is burned. About 40% of the particulates that enter the atmosphere above the United States are from industry and about 17% are from vehicles. Particulates also occur naturally from volcanic eruptions or windblown dust. Like other pollutants, they travel all around the world on atmospheric currents.


Volatile organic compounds (VOCs) enter the atmosphere by evaporation. VOCs evaporate from human-made substances, such as paint thinners, dry cleaning solvents, petroleum, wood preservatives, and other liquids. Naturally occurring VOCs evaporate off of pine and citrus trees. The atmosphere contains tens of thousands of different VOCs, nearly 100 of which are monitored. The most common is methane, a greenhouse gas (Figure below). Methane occurs naturally, but human agriculture is increasing the amount of methane in the atmosphere.

Methane forms when organic material decomposes in an oxygen-poor environment. In the top image, surface methane production is shown. Stratospheric methane concentrations in the bottom image show that methane is carried up into the stratosphere by the upward flow of air in the tropics.

Lesson Summary

  • Industrial pollution causes health problems, though the Clean Air Act has decreased these health problems in the United States by forcing industry to clean their emissions.
  • The increase in motor vehicles in arid cities has increased ozone and other secondary pollutants.
  • Burning fossil fuels is the greatest source of air pollution.
  • Biomass burning is also a large source of air pollution, especially in places where slash-and-burn agriculture is practiced.

Review Questions

  1. What is the difference between the type of smog experienced by cities in the eastern United States and that found in Southern California?
  2. London has suffered from terrible air pollution for at least seven centuries. Why is the city so prone to its famous “London fog?” What did London do to get rid of its air pollution?
  3. Imagine two cities of the same size with the same amount of industrialization and the same number of motor vehicles. City A is incredibly smoggy most of the time and City B usually has very little air pollution. What factors might go into creating these two different situations?
  4. What might be a reason why the city of San Francisco and its metropolitan area is not on the list of smoggiest cities for 2012?
  5. Why are naturally occurring substances, such as particulates or carbon dioxide, sometimes considered pollutants?
  6. How does ozone form from vehicle exhaust?
  7. What are the necessary ingredients for ozone creation, excluding those that are readily available in the atmosphere? Why could there be a city with a lot of cars but relatively little ozone pollution?
  8. Some people say that we need to phase out fossil fuel use and replace it with clean energy. Why is fossil fuel use becoming undesirable?
  9. Mercury is not particularly toxic as a metal but it is very dangerous in its organic form. How does mercury convert from the metal to the organic form?
  10. In what two ways does deforestation contribute to air pollution?

Points to Consider

  • Despite the Clean Air Act, the air over many regions in the United States is still not clean. Why?
  • How do pollutants damage human health?
  • In what ways does air pollution harm the environment?