Air pollution comes from many different sources. Every time something is combusted — like coal, gasoline or wood — pollutants are produced. Sources of pollution can be either natural or man-made. Some naturally caused air pollution includes large amounts of small pieces of pollen and dirt called particulate matter that is naturally stirred up into the sky during windy periods. Our bodies are designed to filter out a small amount of this type of air pollution, so it is not dangerous, therefore, unless you breathe in a lot of it. For example, forest fires are a natural event that produces a lot of particulate matter in the form of ash. People who live in areas typically plagued by forest fires can be in danger of respiratory problems. Environmental engineers design indoor air filters for people who live in forest fire-prone areas.

The major man made sources of air pollution include coal-burning energy plants and cars (see Figure 1). Coal plants and cars produce most of the major pollutants of concern including, toxic gases like sulfur oxides (SO_x), nitrogen dioxide (NO₂) and carbon monoxide (CO) as well as particulate matter, like ash. Another major pollutant is ozone (O3). Ozone is a little different than other pollutants because it does not come directly from the source, but rather from when other chemicals react with the oxygen in the sky. Toxic gases are the cause of many global problems, such as acid rain or acid deposition and global warming.

Figure 1. The photograph on the right illustrates smoke billowing from a factory. The photograph on the right traffic congestion on a busy highway. click for copyright

The best way to prevent air pollution is to not create it, which means burning less coal and driving fewer cars and even making fewer trips in cars. Unfortunately, it takes a lot of energy to support the American life style, and most people are quite dependant on their cars. Engineers work to reduce pollution by developing renewable energy sources and designing more efficient cars. They also design filters to remove air pollution before it is released into the atmosphere. To remove particulate matter, commonly engineered systems include gravity settlers, cyclones, electrostatic precipitators, scrubbers and fabric or fiber filters.

Figure 2. The air over Denver, Colorado on October 27, 2005. click for copyright

Settling chambers rely on gravity to settle coarse particles out of the air stream and are normally costly, so are not often used. Cyclones and scrubbers are commonly used to filter out coarse particulate matter. Cyclones, as the name implies, uses centripetal force to separate particles out while the air stream spins round and round in a cyclone. A scrubber passes the polluted air up through water. Coarse particulate matter in the air stream will dissolve into the water as it passes through the device. Electrostatic precipitators are able to remove smaller particles from the air by passing the polluted air over electrically charged plates. Because particulate matter often has a slightly positive or negative charge, the pollutant particles will stick (and subsequently collect on) the electrically charged plates. They are commonly used after a cyclone or a scrubber.

Finally, fabric or fiber filters send the air through a physical filter that does not let certain particles through.

Pair-Share Brainstorm: Ask students to think about what causes air pollution. Have them discuss answers with the student sitting next to them and write down one thing that causes the most pollution down on a piece of paper. Have students volunteer their answers and write their answers on the board. Group answers into general categories.

Question/Answer: Solicit and discuss the answers to the following questions.

• How is air pollution created? (Answer: Sources of pollution can be either natural or man-made, including: ash, particulate matter, dust/dirt, emissions from industries, whenever anything is burned or combusted.)
• What happened in London in 1952? (Answer: The air was stagnant for four days, and pollutants in the air made it difficult to breath. It is believed that breathing this toxic air pollution caused the deaths of approximately 4,000 people.)
• What are the major causes of pollution? (Answer: The major man-made sources of pollution are automobiles and burning coal; the major natural sources of pollution are forest fires.)
• What can environmental engineers do to prevent pollution? (Answer: The best way to stop air pollution is by not creating it. In addition, environmental engineers can engineer filters such as gravity settlers, cyclones, electrostatic precipitators, scrubbers and fabric or fiber filters.)

Economic Decisions: Have student groups of 3 or 4 discuss air pollution and the ways to prevent and stop it. Tell students that they work for the EPA and have been granted an extra 1 million dollars to help fight air pollution (the president decided to balance the budget and allocated more money to help the environment and education). Tell the students they need to decide what to do with the 1 million dollars to help reduce air pollution in the United States. They can do one thing and one thing only. Have students discuss (for a set amount of time: 5 or 10 minutes) what they will do and write down two reasons why they think that will help stop air pollution. Each group should present their decision to the class.

Class Debate: Split the class into two groups. Have one group argue for (agree with) the topic and one group talk against (disagree with) the topic. Give the groups a few minutes to come up with their arguments before the class debate. They should debate the following topic:

• Should we require citizens to carpool or use alternative transportation methods for commuting to work or school?

Concept Reflections / Journal Writing: Have the students reflect on the air pollution around them, and write a journal entry on their thoughts. Suggest the following to them:

Sit outside somewhere this evening and observe your community. Can you see examples of pollutant transport around you? How will you know if invisible pollutants are involved? What is the likelihood that pollutants are being transported inside your school? Are the pollutants traveling outside of your state? How does this make you feel?