The teacher should construct the testing apparatus as follows:

1. Cut a hole in the end of a shoebox. The hole should be sized so that the narrow end of a hairdryer fits into it.
2. Cut out the opposite end of the shoebox so it is completely open.
3. Tape a piece of string so that it extends horizontally across the top of the cut out end.
4. Fold a piece of aluminum foil in half and hang it over the string. The deflection of the foil when the hairdryer is on will be used to indicate the amount of air flow. This will be measured with a protractor.
5. Fold four index cards into an "l" (L) shape. Tape these halfway down the length of the shoebox on the inside to hold the students' filters (see Figure 1).

Figure 1. The filter testing apparatus. click for copyright

6. Set up the testing apparatus in a location to which students have easy access. Position the hairdryer through the first hole cut in the shoebox (a ring stand may be useful for holding the hairdryer). Set a large container on its side at other end of apparatus to catch particles. If you are using a long container, such as a trash can, you may need to place the entire apparatus inside the container so that students will be able to capture particles with a pollution collector (see Figure 2).

Figure 2. Pictures of the testing apparatus confined within a large plastic container. click for copyright

7. Make enough copies of the Cleaning the Air Worksheet so that each student has one.

1. Divide students into groups of two or three depending on how your class works best.
2. Pass out the Cleaning the Air Worksheets to the students.
3. (Optional: Pass out Air Pollution Background Information for students to read.)
4. Review particulate matter with students. Remind them that particulate matter is the "stuff" they collected on their pollution detectors in the I Breathe What? Activity from this unit. It is all the small particles like fly ash, dust and pollen that pollute the air. Challenge students to design an air filter that will filter out the most particulate matter with out blocking the air flow. Tell them that black pepper will be used as the particulate matter, and show them the apparatus you will use for testing.
5. Write the following challenge on the board: Design an air filter that filters out the most particulate matter without blocking the air flow.
6. Show students the testing apparatus. Explain to students that there are two slots that will hold a filter and so students can design two filters if they want, but it must fit into the testing apparatus' filter slots.
7. Ask students if they know what an ammeter is? (Answer: An ammeter is a device that is used to measure air flow.) Show students the ammeter on the testing apparatus — the aluminum foil. Turn the hair dryer on and tell students this is full air flow. Put an index card into the testing apparatus that fully blocks the air flow. This is no air flow.
8. Explain to the students that their air filters cannot block more then 50% of the air flow. Ask students what the ammeter will look like when 50% of the air is blocked. (Answer: the ammeter — i.e., the foil — will come up to half of the angle it was at when air blew through the box with no filter installed.)
9. Explain that there are four criteria for the air filters that the students build (These are also indicated on the Air Pollution Background Sheet.)

• They can only use materials that are provided by the teacher.
• The filter cannot block more than 50% of the air.
• The filter must be designed to fit into the testing apparatus filter slots.
• All students must test the filter at least once.

10. Review the Cleaning the Air Worksheet with the students. Make sure students understand their challenge and the four filter criteria; ask students to copy the four criteria and challenge down on their worksheets.
11. Give the students the remainder of the period to work in groups and design their filter. There is no building on day one; they should just list their ideas and draw a picture of their filter design.

1. Give the groups one entire period to build their air filters and time to test them at least once. Students will probably have to test them several times to optimize and adjust the filter, so use two class periods to build if necessary. If needed, students are allowed to redesign their filter. Remind them that engineers often redesign something several times before they test and finally use the finished product. See Figure 3 for a sample filter.

Figure 3. Example filter to be used in the filter testing apparatus. click for copyright

2. After everyone has optimized their filters, begin final testing. For the final test students should build a pollution collector as in the I Breathe What? Activity from this unit. If the class did not do this activity, explain to students how to make the pollution collector and how to analyze the data. Tell students that they must put a piece of graph paper under the pollution collector and count how many particles are in 4 squares and then take the average (for more information, please refer to the I Breathe What? Activity and accompanying worksheet).
3. The pollution collector is placed outside of the air pollution apparatus just beyond the ammeter. It is best to tape the collector to the bin that is being used to contain the escaping pepper.
4. After all groups have tested their filters, compare the results and filter designs. Ask students what was the most challenging part of the design process. Ask them if they would do it differently a second time.

Group Discussion: Review air pollution causes and filters that engineers use to clean the air by asking the following questions.

1. What are the different types of pollution? (Answer: particulate matter, such as fly ash; toxic gases like sulfur oxides (SO_x), nitrogen dioxide (NO₂) and carbon monoxide (CO); and secondary pollutants, such as ozone (O₃)).
2. What are the major causes of pollution? (Answer: The major man made sources of pollution are automobiles and burning coal; the major natural sources are forest fires.)
3. What can environmental engineers do to prevent pollution? (Answer: The best way to stop air pollution is by not creating it in the first lace. In addition, environmental engineers can engineer filters such as gravity settlers, cyclones, electrostatic precipitators, scrubbers and fabric or fiber filters.)

Cleaning the Air Worksheet: Have the students record measurements and follow along with the activity on their worksheet. After students have finished their worksheet, have them compare answers with their peers.

Sales Pitch!: Have the students pretend to be salespeople who are trying to sell their indoor air filter to a manufacturer or a consumer. Have student teams create a persuasive poster or flyer, as well as a 10-minute sales pitch of their filter design for presentation during the next class. Have them incorporate into their sales pitch particulate matter count, the parts and features of the filter and how it works.