AP® Environmental Science · Unit 7 · Topic 7.5
Indoor Air Pollutants
Indoor air pollutants build up because indoor spaces trap what outdoor air disperses: radon seeping from bedrock, VOCs off-gassing from new materials, and combustion gases like carbon monoxide all concentrate in a small, poorly ventilated volume of air.
Outdoors, a pollutant molecule has the whole atmosphere to get lost in. Indoors, it has a few hundred cubic meters of air and a house that only exchanges that air slowly. Radon is the clearest case: uranium decaying in bedrock produces radon gas, which seeps up through soil and squeezes through foundation cracks or gaps around pipes. Outside, it would disperse into open air within minutes. In a basement with limited air exchange, it keeps arriving faster than it leaves, so concentrations climb far above outdoor background levels.
Volatile organic compounds, or VOCs, work on a different timeline. New carpet, paint, and furniture contain solvents and adhesives that were never meant to stay put. Right after installation the material is saturated with these compounds, so off-gassing is fastest and odors and headaches are worst. As weeks and months pass, the easily released molecules near the surface are used up first, so the rate falls, which is why the smell and symptoms fade even without any cleaning or ventilation.
Both problems get worse, not better, when a house is sealed tightly for energy efficiency. Weatherstripping and insulation that stop heat from escaping also stop stale air from escaping, so radon and VOCs that used to leak out through drafts now stay put. Carbon monoxide fits the same pattern but adds a lethal twist: it is colorless and odorless, produced by incomplete combustion in furnaces or car engines, so it binds to hemoglobin and causes harm long before any of the senses could give a warning.
Key terms
- Radon
- A naturally occurring radioactive gas produced by uranium decay in rock and soil that seeps into buildings through foundation cracks.
- Volatile organic compound (VOC)
- A carbon-based chemical that evaporates easily at room temperature, released by products like paint, carpet, and furniture.
- Off-gassing
- The gradual release of trapped volatile chemicals from a solid material into the surrounding air.
- Air exchange rate
- How quickly indoor air is replaced by outdoor air, which determines how much pollutants can accumulate indoors.
- Carbon monoxide
- A colorless, odorless gas from incomplete combustion that binds to hemoglobin and blocks oxygen transport in blood.
Watch it explained
- How it worksWalk me through radon seeping out of bedrock and through a foundation crack into a basement.
- How it worksHow do volatile organic compounds off-gas from new carpet, paint, and furniture over months?
- Going furtherWhat happens to indoor pollutant levels once a house is sealed tight for energy efficiency?
- Common mistakeWhy isn't carbon monoxide something you can smell before it becomes dangerous?
Check yourself
End of topic.