1. Activate students' prior knowledge about types of pollutants.

Show the 1980 Mount St. Helens Eruption photograph. Tell students that there are two types of pollutants: visible pollutants and invisible pollutants. Ask:

• What kind of pollutants can you see in this picture? (You can see ash emitted, rock fragments, and gas clouds.)

Tell students that there are many different types of invisible, or gaseous, pollutants. Let students know that they will be exploring the sources of visible and invisible pollutants and their effects on human and environmental health in this activity.

2. Discuss the role of uncertainty in the scientific process.

Tell students that science is a process of learning how the world works and that scientists do not know the “right” answers when they start to investigate a question. Let students know that they can see examples of scientists' uncertainty in forecasting air quality. Tell students that air quality is measured by the Air Quality Index. Show students the Air Quality Index, which includes explanations of the ranges used in the air quality index. Then project the Forecast of Air Quality on December 10, 2013 image and Air Quality on December 10, 2013 image. (Click on the link in the media carousel above and download using the arrow in the lower right corner of the window.) Tell students that these are snapshots of the air quality forecast and the real-time air quality in the United States on December 10, 2013. Ask:

• Did the forecast accurately predict which areas would have poor quality air? (The forecast air quality overlaps with many of the poor air quality areas, but it does not cover all of them. The air quality in some areas [Northern California] is much worse than the forecast predicted.)

• Why do you think scientists did not accurately predict the air quality for more of the United States? (Student answers will vary. The air quality forecast is affected by human activities that may not be easily predicted.)

Tell students that they will be asked questions about the certainty of their predictions. Let students know they should think about what scientific data is available as they assess their certainty with their answers, and encourage them to discuss the scientific evidence with each other to better assess their level of certainty with their predictions.

3. Introduce the concept of stocks and flows in a system.

Tell students that materials flow into and out of systems. The flow of the materials over time can change and can be influenced by many different factors and interacting parts. Scientists think about how one part of the system can affect other parts of the system. Give students a simple example of a stock and flow in a system, as described in the scenario below.

There is a bathtub with water flowing in from the faucet and water leaving through the drain. Ask:

• When the drain is plugged, what happens to the level of water in the bathtub? (The water level will increase because the outflow of water is stopped, but water keeps coming in from the faucet.)

• When the faucet is turned off, what happens to the level of water in the bathtub? (The water level will decrease because the inflow of water is stopped, but the water keeps leaving through the drain.)

• How can the level of water in the bathtub be kept at the same level? (The water in the bathtub can be kept at the same level by making the inflow equal to the outflow. Then, the water that comes in through the faucet will be offset by the water that leaves through the drain.)

4. Have students launch the Visible and Invisible Pollutants interactive.

Provide students with the link to the Visible and Invisible Pollutants interactive. Divide students into groups of two or three, with two being the ideal grouping to allow groups to share a computer workstation. Tell students that they will be working through a series of pages of data with questions related to the data. Ask students to work through the interactive in their groups, discussing and responding to questions as they go.

NOTE: You can access the Answer Key for students' questions—and save students' data for online grading—through a free registration on the High-Adventure Science portal page.

Tell students that this is Activity 4 of the Will the Air Be Clean Enough to Breathe? lesson.

5. Discuss the issues.

After students have completed the activity, bring the groups back together and lead a discussion focusing on these questions:

• What types of particulate pollutants are produced naturally, and what types are produced through human actions? (Some natural particulates include sand and salt particles, as well as ash and soot from forest fires. Ash and soot are also produced through human actions as materials are burned for fuel in transportation, heating, and electricity production. Humans cause some forest fires, making those “natural” sources really anthropogenic sources.)

• How do invisible (gaseous) pollutants affect the environment? (Invisible pollutants such as SO₂ and NO_x can combine with water to form acidic precipitation. The acidic precipitation can harm plants and animals. VOCs and CO are harmful to animals' health as well as human health.)

• Why are particulate pollutants hazardous to human health? (Particulates can be inhaled deep into the lungs and cross over into the bloodstream. Then they can travel around the body, causing effects to many organ systems.)

• Show the NO_x Emissions, 2008 pie chart (page 6 of the interactive). How many of the NO_x emissions are from human sources? (Almost all of the NO_x emissions are anthropogenic. Up to 1% [miscellaneous and fires] could be from natural sources.)

• What types of materials can cause indoor air pollution? (Carpets, perfumes, hairsprays, furniture, and poorly ventilated heating/cooking devices can cause indoor air pollution.)

• Why is indoor pollution more hazardous to human health than most outdoor pollution? (Indoor pollution cannot be blown away by the wind or precipitated out of the air by rain or snow. It can be more concentrated because there is less dilution with clean air than is possible outdoors.)

• How can increasing fuel efficiency reduce the emissions of gaseous pollutants (SO₂, NO_x, CO)? (Increasing fuel efficiency can reduce the emissions of gaseous pollutants because less fuel needs to be burned to go the same distance [in a vehicle] or produce electricity [in a power plant]. If less fuel is used, there will be fewer emissions.)