7.5 Indoor Air Pollutants

TLDR

Indoor air pollutants are harmful gases and particles that build up inside homes and buildings from natural sources, human-made materials, and combustion. The big ones to know for AP Environmental Science are carbon monoxide (an asphyxiant), radon-222 (a radioactive gas linked to lung cancer), particulates like asbestos and smoke, and VOCs like formaldehyde from furniture and building materials.

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Why This Matters for the AP Environmental Science Exam

This topic shows up when the exam asks you to identify specific air pollutants and connect them to their sources and health effects. Students often mix up which pollutants are indoor versus outdoor, or which one causes which illness, so being precise here pays off.

You may be asked to identify a pollutant from a description, sort pollutants by source (natural, human-made, or combustion), or explain a health effect. On free-response questions, you might describe the effects of a pollutant or propose a solution and back it up with reasoning, so knowing why radon or carbon monoxide is dangerous matters more than just memorizing names.

Key Takeaways

• Carbon monoxide (CO) is an asphyxiant produced by incomplete combustion; it is colorless and odorless and blocks your blood from carrying oxygen.
• Radon-222 is a naturally occurring radioactive gas from uranium decay in rocks and soil; it is the second leading cause of lung cancer in America.
• Particulate indoor pollutants include asbestos, dust, and smoke.
• Indoor air pollutants come from three source types: natural, human-made, and combustion.
• VOCs (like formaldehyde) and lead from paint are common human-made indoor pollutants from furniture, carpets, paneling, and building materials.
• Combustion indoor pollutants include carbon monoxide, nitrogen oxides, sulfur dioxide, particulates, and tobacco smoke.

Carbon Monoxide

Carbon monoxide (CO) is classified as an asphyxiant, which means it interferes with your body's ability to use oxygen. It forms during incomplete combustion, such as burning fossil fuels in a car engine, a gas appliance, or a fireplace.

Here is why it is dangerous: your blood cells normally carry oxygen, but they absorb CO even more easily than O2. Once CO binds, your body cannot use it, which can cause you to pass out from asphyxiation. Because CO is colorless and odorless, you cannot detect it without a detector. Never leave a vehicle running in a closed garage, since CO accumulates quickly in enclosed spaces.

Radon-222

Radon-222 is a naturally occurring radioactive gas produced by the decay of uranium found in some rocks and soils. It is a natural-source indoor pollutant, not something people add to a home on purpose.

Radon moves up through the soil and enters homes through the basement or through cracks in walls and the foundation. It can also dissolve in groundwater and enter a home through a well. Because it is radioactive, exposure to radon can lead to radon-induced lung cancer, which is the second leading cause of lung cancer in America.

Particulates: Asbestos, Dust, and Smoke

Some indoor air pollutants are classified as particulates, meaning they are small solid or liquid particles in the air. The ones to know are asbestos, dust, and smoke.

Asbestos was commonly used in ceilings and insulation because it resists fire. The problem is that inhaling small asbestos fibers can cause lung cancer. Removing or sealing asbestos today requires trained professionals and is treated as hazardous material. Dust can come from natural sources, and smoke (including tobacco smoke) is a combustion product.

VOCs and Other Human-Made Pollutants

Volatile Organic Compounds (VOCs) are chemicals that evaporate at room temperature, and many come from products people buy on purpose. Formaldehyde is a common VOC found in building materials, furniture, upholstery, and carpeting. That "new carpet" or "new furniture" smell is often VOC off-gassing.

Other common human-made indoor pollutants include insulation, VOCs from furniture, paneling, and carpets, and lead from paint. Lead-based paint is a concern especially for childhood exposure.

Combustion Pollutants

Burning fuels indoors releases its own set of pollutants. Common combustion indoor air pollutants include carbon monoxide, nitrogen oxides, sulfur dioxide, particulates, and tobacco smoke. These can come from sources like gas stoves, unvented heaters, wood stoves, and cigarettes.

Sick Building Syndrome (Application)

Sick Building Syndrome is a real-world example of what happens when indoor pollutants have nowhere to go. To save on heating and cooling costs, buildings are often sealed tightly with weather stripping and closed windows. That traps indoor pollutants inside, where they can build up and cause illness. One example source is ozone build-up from office copier machines.

This is an application of the concept, not a separate required term. Opening a window to let air circulate increases the air exchange rate and can reduce the problem.

How to Use This on the AP Environmental Science Exam

MCQ

• Read pollutant questions carefully and sort by source type: natural (radon, mold, dust), human-made (VOCs, formaldehyde, lead paint, insulation), or combustion (CO, nitrogen oxides, sulfur dioxide, particulates, tobacco smoke).
• Match the pollutant to its health effect. Common pairings: CO with asphyxiation, radon with lung cancer, asbestos with lung cancer.
• Remember that carbon monoxide is an asphyxiant and that particulates include asbestos, dust, and smoke.

Free Response

• If asked to describe the effects of an indoor pollutant, name the specific health effect (for example, radon causes radon-induced lung cancer).
• If asked how radon enters a home, mention movement up through soil, entry through the basement or cracks in walls and foundation, and dissolved radon in well water.
• When proposing a solution, connect it to the source. Better ventilation increases air exchange and removes trapped pollutants.

Common Trap

• Do not confuse indoor pollutants with outdoor smog. Photochemical smog forms outdoors from nitrogen oxides and VOCs reacting in sunlight, which is a different topic.

Common Misconceptions

• Carbon monoxide (CO) is not the same as carbon dioxide (CO2). CO is the asphyxiant from incomplete combustion that you need a detector to find.
• Radon is not made by people. It is a natural radioactive gas from uranium decay in rocks and soil that seeps into buildings.
• VOCs are not always from obvious pollution. They off-gas from everyday items like furniture, carpets, and paint, which is why "new" smells can be VOCs.
• Asbestos itself does not poison the air with a gas; the danger is inhaling its tiny fibers, which can cause lung cancer.
• A tightly sealed, energy-efficient building is not automatically healthier. Sealing in air can trap pollutants and lead to Sick Building Syndrome without enough ventilation.

Related AP Environmental Science Guides

• 7.1 Introduction to Air Pollution
• 7.2 Photochemical Smog
• 7.3 Thermal Inversion
• 7.6 Reduction of Air Pollutants
• 7.7 Acid Rain
• Unit 7 Overview: Atmospheric Pollution