Indoor Air
Pollutants
Explore the hidden contaminants found inside buildings, their sources, health effects, and strategies for reducing exposure in the built environment.
AP Environmental Science
What Is Indoor Air Pollution?
Indoor air pollution refers to the contamination of air within enclosed spaces by chemical, biological, or physical agents. Because people in industrialized nations spend roughly 90% of their time indoors, exposure to indoor pollutants can exceed outdoor exposure by 2 to 5 times.
Why Indoor Air Matters
Exposure Time
Humans spend approximately 90% of their lives indoors: homes, offices, schools, and vehicles.
Sealed Buildings
Energy-efficient construction reduces ventilation, trapping pollutants and increasing their concentration over time.
Health Impact
Indoor air pollutants cause respiratory disease, cancer, and neurological damage: roughly 3.2 million deaths per year globally (WHO).
Categories of Indoor Pollutants
Key Chemical Pollutants
1
Radon (\( \text{Rn-222} \)): Radioactive gas from uranium decay in soil and rock. Seeps through foundation cracks. Second leading cause of lung cancer in the U.S.
2
Formaldehyde (\( \text{CH}_2\text{O} \)): Off-gasses from plywood, particle board, adhesives, and furniture. Causes eye irritation and is classified as a known human carcinogen.
3
Volatile Organic Compounds (VOCs): Emitted by paints, solvents, cleaning products, and air fresheners. Include benzene and toluene: linked to liver, kidney, and CNS damage.
4
Pesticides: Used indoors for pest control. Residues persist on surfaces and become airborne particulate matter.
Combustion Pollutants
Carbon Monoxide
Colorless, odorless gas from incomplete combustion. Binds to hemoglobin \( \sim\!200\times \) more readily than \( \text{O}_2 \), causing oxygen deprivation.
Nitrogen Dioxide
\( \text{NO}_2 \) produced by gas stoves and heaters. Irritates airways, worsens asthma, and increases susceptibility to respiratory infections.
Biological Pollutants
Biological pollutants are living organisms or their byproducts that contaminate indoor air. They thrive in warm, humid environments and are a primary trigger for allergies and asthma.
Mold
Fungi that grow on damp surfaces. Release spores and mycotoxins into air, causing allergic reactions and respiratory illness.
Dust Mites
Microscopic arachnids found in bedding, carpets, and upholstery. Their fecal pellets are potent allergens.
Pet Dander
Tiny skin flakes shed by animals. Remain airborne for hours and penetrate deep into lungs.
Radon: The Invisible Threat
Asbestos & Lead
Asbestos
Naturally occurring mineral fiber used in insulation, tiles, and roofing before the 1980s. Inhaled fibers cause mesothelioma and asbestosis. Danger is highest when materials are disturbed or deteriorating.
Lead Paint
Homes built before 1978 may contain lead-based paint. Peeling or sanding creates lead dust. Children are especially vulnerable: leads to neurological damage, lowered IQ, and developmental delays.
Indoor vs. Outdoor Pollution
| Indoor | Outdoor | |
|---|---|---|
| Primary Sources | Building materials, combustion appliances, biological agents | Vehicles, factories, power plants |
| Key Pollutants | Radon, \( \text{CO} \), formaldehyde, mold | Ozone, \(\text{SO}_2\), \(\text{PM}_{2.5}\), \(\text{NO}_x\) |
| Regulation | Minimal direct regulation (EPA guidelines only) | Clean Air Act, NAAQS standards |
| Exposure Duration | ~90% of time | ~10% of time |
| Ventilation Effect | Poor ventilation increases concentration | Atmospheric mixing dilutes pollutants |
Sick Building Syndrome
Sick Building Syndrome (SBS) describes a condition where occupants experience acute health effects: headaches, dizziness, nausea, eye and throat irritation: that appear to be linked to time spent in a building but no specific illness or cause can be identified.
Symptoms
Headache, fatigue, difficulty concentrating, irritation of eyes, nose, and throat. Symptoms improve after leaving the building.
Causes
Inadequate ventilation, VOC off-gassing, biological contaminants, and poor HVAC maintenance.
Distinction
Unlike Building-Related Illness (BRI), SBS has no identifiable pathogen. BRI has diagnosable conditions like Legionnaires' disease.
How to Reduce Indoor Pollutants
1
Source Control: Remove or reduce the pollutant at its origin. Use low-VOC paints, ban indoor smoking, fix leaks that promote mold growth.
2
Ventilation Improvement: Increase outdoor air exchange. Open windows, use exhaust fans, maintain HVAC systems to dilute indoor pollutants.
3
Air Cleaning: Use HEPA filters and activated carbon filters to remove particulates and some gases. Effective for allergens and fine particles.
4
Testing: Test for radon with home kits. Monitor CO with alarms. Professional testing for asbestos and lead in older buildings.
Source control and ventilation address different indoor pathways
Source removal, appropriate ventilation and pollutant-specific treatment address different indoor exposure pathways. Matching the intervention to the pollutant and its entry route improves control.Control sources; route clean and dirty air
Source removal, appropriate ventilation and pollutant-specific treatment address different indoor exposure pathways. Matching the intervention to the pollutant and its entry route improves control.Example: Radon in a Home
A homeowner in the Appalachian region tests their basement and finds a radon level of \( 8.2 \; \text{pCi/L} \). The EPA action level is \( 4 \; \text{pCi/L} \).
1
Identify the source: Radon seeps from uranium-rich bedrock through foundation cracks and gaps around pipes.
2
Compare to standard: \( 8.2 \; \text{pCi/L} > 4 \; \text{pCi/L} \): this exceeds the EPA action level by more than double.
3
Recommend mitigation: Install a sub-slab depressurization system: a pipe and fan that vents radon from beneath the foundation to the outdoors.
Example: Carbon Monoxide Exposure
A family reports persistent headaches and dizziness during winter. Their home has a gas furnace and an attached garage where a car idles each morning.
1
Identify the source: Incomplete combustion from the gas furnace and vehicle exhaust from the garage. \( \text{CO} \) is colorless and odorless.
2
Explain the mechanism: \( \text{CO} \) binds to hemoglobin forming carboxyhemoglobin, reducing blood oxygen transport. Even low-level chronic exposure causes symptoms.
3
Recommend solutions: Install CO detectors. Service the furnace annually. Never idle vehicles in an attached garage. Ensure proper venting of all combustion appliances.
Explain why an invisible gas can impair oxygen delivery.
Use this illustration to explore the concept described in its caption.
Example: Biomass Burning in Developing Nations
Common Mistakes
✕ Confusing SBS and BRI
Sick Building Syndrome has no identifiable cause. Building-Related Illness (BRI) involves a diagnosed condition like Legionnaires' disease. They are not interchangeable.
✕ Thinking Radon Is Only Outdoor
Radon is the #1 indoor radioactive pollutant. It accumulates inside sealed buildings: outdoor levels are far lower due to atmospheric dilution.
✕ Ignoring Developing Nations
AP exams frequently test biomass burning as the leading indoor air pollutant source globally: not formaldehyde or VOCs (which dominate in industrialized nations).
Summary of Major Pollutants
| Pollutant | Source | Health Effect | Mitigation |
|---|---|---|---|
| Radon | Soil / rock decay | Lung cancer | Sub-slab ventilation |
| \( \text{CO} \) | Combustion appliances | Oxygen deprivation | Detectors, venting |
| Formaldehyde | Pressed wood, adhesives | Cancer, irritation | Low-VOC materials |
| Asbestos | Old insulation, tiles | Mesothelioma | Encapsulation / removal |
| Lead | Pre-1978 paint | Neurological damage | Abatement, enclosure |
| Mold | Moisture / humidity | Allergies, asthma | Dehumidify, fix leaks |
Question 1 of 5
Which indoor air pollutant is the second leading cause of lung cancer in the United States?
Radon is a radioactive gas that seeps into buildings from uranium decay in soil. It is classified by the EPA as the second leading cause of lung cancer after smoking, responsible for roughly 21,000 deaths per year in the U.S.
Question 2 of 5
A building's occupants report headaches and fatigue that resolve when they leave the building, but no specific pathogen is found. This is best described as:
Sick Building Syndrome is characterized by symptoms that cannot be traced to a specific pathogen or illness but improve upon leaving the building. BRI, by contrast, involves a diagnosable disease with a known cause.
Question 3 of 5
Which strategy does the EPA rank as most effective for reducing indoor air pollution?
The EPA considers source control the most effective approach because it addresses the root cause. Air cleaners and ventilation are secondary measures that dilute or filter pollutants but do not eliminate them.
Question 4 of 5
In developing nations, the primary source of indoor air pollution affecting approximately 3 billion people is:
The WHO identifies indoor biomass burning as the leading source of indoor air pollution globally. Roughly 3 billion people rely on solid fuels for cooking, producing dangerous levels of particulate matter and carbon monoxide indoors.
Question 5 of 5
Carbon monoxide is dangerous primarily because it:
CO binds to hemoglobin roughly 200 times more readily than oxygen, forming carboxyhemoglobin. This prevents red blood cells from carrying oxygen, leading to tissue hypoxia and, at high concentrations, death.