AP Syllabus focus:
‘Common natural indoor pollutants include radon, mold, and dust.’
Indoor air can contain naturally occurring pollutants that build up because homes are enclosed and people spend long periods indoors. Radon, mold, and dust are widespread and can affect health, comfort, and indoor environmental quality.
What makes these pollutants “natural” indoors?
Natural indoor pollutants originate from geologic materials, biological growth, or outdoor particles that enter and accumulate indoors. Their concentrations often increase when ventilation is limited and when indoor conditions (especially moisture) favour persistence.
Radon
Radon is a naturally occurring gas that can accumulate indoors, particularly in lower levels of buildings, because it has no smell or colour and may not be noticed without testing.
This cross-section diagram shows radon migrating from geologic materials in the ground and entering a building through common openings such as cracks, joints, and utility penetrations. The arrows emphasize that radon can accumulate indoors—especially in basements—when soil gas is drawn into the structure and ventilation is limited. Source
Radon: A naturally occurring radioactive gas that can be present in indoor air.
Why radon can be an indoor problem
Source: Natural geologic materials (released from the ground into air)
Indoor buildup: More likely where air exchange is low
Key concern: Long-term exposure is associated with increased health risk, so identification typically depends on radon testing
Practical risk reduction (general strategies)
Increase ventilation where appropriate and safe
Use building and maintenance practices that reduce gas entry and accumulation
Test indoor air to determine whether mitigation is needed
Mold
Mold becomes an indoor pollutant when it grows on damp materials and releases spores and fragments into the air.
This photograph shows visible mold colonization along a wall surface near tile, a common pattern in areas with chronic dampness or water intrusion. It provides a concrete example of how moisture-damaged building materials can become a source of indoor airborne spores and fragments. Source
Mold problems are strongly linked to water intrusion and persistently high humidity.
Mold: A group of fungi that can grow indoors on moist surfaces and release airborne spores or fragments.
Conditions that promote indoor mold
Moisture availability is the limiting factor
Common contributors:
Leaks, flooding, or condensation
Poorly ventilated bathrooms or kitchens
Damp basements or crawlspaces
Health and comfort relevance
Airborne spores can contribute to allergic reactions and respiratory irritation, especially for sensitive individuals.
Mold also signals an indoor moisture imbalance that can damage building materials.
Control and prevention
Keep indoor relative humidity lower (often with dehumidification and ventilation)
Fix water sources promptly (leaks, seepage, condensation)
Remove or clean moldy porous materials when growth is established
Dust
Indoor dust is a complex mixture of tiny solid particles that can come from outdoors (tracked in or blown in) and from indoor sources such as human activity, fibres, and biological material. Dust is important because it can be inhaled and can also resuspend into the air during movement.
Dust: A mixture of fine particles indoors, including soil/mineral fragments, fibres, skin cells, pollen, and other debris.
Why dust accumulates indoors
Homes act as particle “reservoirs” because:
Surfaces trap particles over time
Movement (walking, cleaning) can resuspend settled material
Some spaces have limited filtration or air exchange
Reducing dust exposure
Use regular damp dusting and vacuuming (filters can reduce particle recirculation)
Control entry from outdoors (doormats, shoe removal, cleaning high-traffic areas)
Maintain HVAC/portable air cleaner filters as recommended to limit airborne particles
FAQ
Differences in building design and maintenance can change how easily soil gas enters and how quickly indoor air is exchanged.
Small structural differences and occupant ventilation habits can lead to large concentration differences.
Visible growth indicates a colony on a surface, while spores are reproductive particles that can become airborne.
Spores may be present even when growth is hidden behind walls, under flooring, or inside HVAC components.
Dust particles provide surface area that can hold other substances.
Some chemicals can sorb onto dust
Dust can transport biological material (e.g., pollen, dander)
When disturbed, contaminated dust can become airborne again.
No. Performance depends on particle size and the device’s filter design and airflow rate.
High-efficiency particle filters can reduce fine airborne particles, but they do not remove settled dust unless it is resuspended and captured.
Ventilation can bring in outdoor particles (e.g., pollen or smoke) depending on outdoor air quality.
Using filtration alongside ventilation, and timing ventilation when outdoor air is cleaner, can reduce this trade-off.
Practice Questions
State two common natural indoor pollutants. (2 marks)
Any two of: radon; mould; dust. (1 mark each)
Explain how indoor conditions can increase exposure to natural pollutants such as radon, mould, and dust, and describe one practical way to reduce exposure for each pollutant. (5 marks)
Enclosed indoor spaces/low ventilation can allow pollutants to build up. (1)
Radon: accumulates indoors; reduction method such as testing and mitigation/greater ventilation. (1+1)
Mould: increased by moisture/humidity; reduction method such as fixing leaks/dehumidifying. (1+1)
Dust: accumulates on surfaces and can resuspend; reduction method such as filtration/vacuuming with suitable filter. (1)
