AP Syllabus focus:
‘Indoor air pollutants can come from natural sources, human-made materials, and combustion indoors.’
Indoor air quality matters because people spend much of their time indoors, where pollutants can accumulate in enclosed spaces. Understanding pollutant categories and their sources helps identify prevention strategies and reduce exposure risk.
What counts as an indoor air pollutant?
Indoor air pollution refers to undesirable chemicals or particles in indoor air that can build up to higher concentrations than outdoors due to limited ventilation and continuous indoor sources.
Indoor air pollutant: A chemical, biological agent, or particle present in indoor air at concentrations that can degrade air quality.
In AP Environmental Science, indoor sources are commonly grouped into three syllabus-aligned categories: natural sources, human-made materials, and combustion indoors.
Category 1: Natural sources (biological and geological)
Biological sources
Living organisms (or their by-products) can introduce bioaerosols and allergens into indoor air, especially in damp or poorly ventilated buildings.
Mold and mildew: grow on damp materials (drywall, carpet, wood), releasing spores and fragments
Dust and dust mites: accumulate in bedding, upholstery, and carpets; can become airborne during cleaning or movement
Pollen: enters through open windows/doors and can be tracked in on clothing or pets
Pet dander: tiny skin flakes and proteins from saliva/urine that become airborne
Key conditions that increase biological pollutants include high indoor humidity, water damage, flooding, and inadequate air circulation.
EPA’s infographic summarizes why moisture is the limiting factor for indoor mold growth and highlights practical controls (humidity reduction, ventilation, rapid drying after wetting). It reinforces the idea that controlling indoor environmental conditions can prevent biological pollutant buildup before remediation is needed. Source
Geological sources
Some pollutants originate from Earth materials beneath or around buildings.
Soil- and rock-derived gases can migrate into structures through small openings
Naturally occurring particulates (fine dust) can enter from outdoor soil disturbance and settle indoors, then resuspend
These sources are strongly influenced by local geology, building foundation integrity, and air pressure differences between indoors and outdoors.
Category 2: Human-made materials (building products and consumer goods)
Many indoor pollutants are released slowly from materials and products via off-gassing and evaporation, particularly in newer or recently renovated spaces.
Volatile organic compounds (VOCs): Carbon-based chemicals that readily evaporate at room temperature, contributing to indoor air pollution.
A brief way to think about this category is “materials and products that emit chemicals,” including:
Pressed-wood products (some composites, cabinetry, flooring) that may emit VOCs
Paints, varnishes, adhesives, and sealants that release VOC vapours during and after application
Cleaning agents and air fresheners that emit fragrances and reactive chemicals
Furniture and textiles (some upholstery, carpets) that can release VOCs and trap dust
Stored chemicals (solvents, hobby supplies, pesticides) that evaporate from containers, especially if poorly sealed
Emission rates often rise with:
Higher temperatures (faster evaporation)
Poor ventilation (less dilution)
Large surface area of new materials (more off-gassing)
Even when emissions are low, constant indoor presence and limited air exchange can allow pollutants to persist.
Category 3: Combustion indoors (burning fuels and smoking)
Combustion is a major indoor source because burning produces a mixture of gases and particulate matter, and indoor spaces may lack adequate exhaust systems.
Common indoor combustion sources include:
Gas stoves and ovens
Fireplaces and wood stoves
Kerosene or gas space heaters
Candles and incense
Tobacco smoke (including second-hand and third-hand residues that can re-enter the air)
Combustion indoors can generate:
Particulate matter (PM), including soot (fine particles that remain airborne)
Nitrogen oxides (NOx) from high-temperature burning
Carbon-containing gases from incomplete combustion
A complex mixture of irritants and reactive chemicals (especially from smoking)
Whether these pollutants accumulate depends on:
Ventilation rate (open windows, mechanical ventilation, exhaust hoods)
Appliance condition and maintenance (poorly adjusted burners increase incomplete combustion)
Indoor air mixing and room volume (small, sealed rooms concentrate pollutants faster)
How pollutants enter and build up indoors (source pathways)
Indoor pollutants typically appear through one or more pathways:
Emission at the source: off-gassing, evaporation, or combustion directly releases pollutants indoors
Infiltration from outdoors: outdoor air brings in pollen, dust, and other contaminants through leaks and openings
Resuspension: settled dust becomes airborne again due to walking, vacuuming, or airflow
Accumulation due to low air exchange: tight building envelopes reduce dilution, allowing concentrations to increase over time
Because indoor environments vary widely (building age, climate, occupant behaviour), identifying the category and source is the first step toward targeted control (source removal, isolation, or improved ventilation).
FAQ
Natural ventilation depends on weather and building leakiness, so pollutant dilution is variable.
Mechanical systems can dilute more consistently, but only if outdoor air intake, filtration, and maintenance are appropriate.
Key influences include:
Product age (newer materials often emit more)
Surface area exposed to air
Air movement across surfaces
How quickly indoor air is replaced (air exchange rate)
Indoor sources may emit continuously in a confined volume. If air exchange is low, pollutants can accumulate faster than they are removed, even when outdoor concentrations are also elevated.
Some chemicals adsorb onto dust particles or settle with them. Later, normal activities can resuspend dust, reintroducing both particles and any attached chemicals back into breathing air.
Kitchens and rooms with heaters/fireplaces are higher risk because combustion devices may be used frequently. Without effective local exhaust (e.g., vented range hood), pollutants can spread and persist indoors.
Practice Questions
State two categories of indoor air pollutant sources and give one example source for each. (2 marks)
1 mark: Any valid category (natural sources / human-made materials / combustion indoors)
1 mark: Correct example matched to category (e.g., mold for natural; paint for human-made; gas stove for combustion)
Explain how indoor air pollutants can originate from (i) natural sources, (ii) human-made materials, and (iii) combustion indoors, and describe one factor that can increase indoor pollutant concentrations. (5 marks)
1 mark: Natural sources explained with valid example (e.g., damp conditions promoting mold; soil-derived gases entering buildings)
1 mark: Human-made materials explained with valid example (e.g., VOC off-gassing from paints/adhesives/furnishings)
1 mark: Combustion indoors explained with valid example (e.g., stoves/heaters/fireplaces producing PM and gases)
1 mark: One concentration-increasing factor identified (e.g., poor ventilation / higher temperature / small enclosed space)
1 mark: Link made between factor and higher indoor concentration (reduced dilution/greater emission rate/greater accumulation)
