Sources of indoor air pollutants

Indoor air pollutants originate from a wide variety of sources located both inside and outside the building envelope. These contaminants can be either anthropogenic, meaning they are produced by human activities and everyday routines, or natural, arising from the surrounding geological environment. Because modern populations spend up to 90% of their time indoors, understanding where these pollutants come from is essential for maintaining a healthy indoor environment.

Combustion Sources

Combustion processes are among the most significant contributors to indoor air pollution. When fuels or organic materials are burned for heating, cooking, or other activities, they release a complex mixture of gases and fine particles directly into the living space:

• Carbon Monoxide (CO): A colorless and odorless toxic gas produced by the incomplete combustion of carbonaceous fuels. Major indoor sources include unvented or faulty gas stoves, poorly maintained heating appliances, fireplaces, and tobacco smoke.
• Nitrogen Dioxide (NO2): An irritating combustion gas emitted by gas cookers, space heaters, and kerosene appliances, as well as by the infiltration of outdoor traffic exhaust.
• Particulate Matter (PM2.5 and PM10): Fine and coarse microscopic particles generated by cooking activities, wood or pellet stoves, candles, incense, and tobacco smoke.
• Polycyclic Aromatic Hydrocarbons (PAHs): Persistent toxic compounds bound to airborne particles, originating from indoor combustion sources such as wood burning, biomass heating, and smoking.

Building Materials and Furnishings

The physical structure of a building and its interior finishes can continuously release chemical vapors into the air:

• Paints and Varnishes: Freshly applied paints, lacquers, and surface coatings contain organic solvents that release high levels of volatile organic compounds (VOCs) as they dry.
• Adhesives and Sealants: Glues, caulks, and construction sealants used during renovations emit chemical vapors that can persist indoors for extended periods.
• Carpets and Flooring: Synthetic carpets, flooring materials, and associated adhesives can off-gas VOCs, particularly when newly installed.
• Furniture and Pressed Wood Products: Furniture manufactured from particleboard, plywood, and medium-density fiberboard frequently emits formaldehyde, a well-known VOC of health concern.

Household Products

Everyday consumer goods used for home maintenance, hygiene, and convenience contribute significantly to the indoor chemical load:

• Cleaning Agents: Surface cleaners, dishwashing detergents, polishes, and disinfectants release chemical vapors and can interact with other indoor gases to form secondary pollutants.
• Personal Care Products: Perfumes, deodorants, hair sprays, cosmetics, and nail polish removers introduce a variety of volatile organic compounds into indoor air.
• Pesticides and Insecticides: Products used to control household pests release chemical residues that remain suspended in the air or settle into indoor dust.
• Air Fresheners: Scented candles, incense, plug-in air fresheners, and sprays emit numerous VOCs and fine particulate matter.

Biological Sources

Biological pollutants consist of living organisms or organic fragments suspended in the air that can trigger allergic reactions and respiratory illnesses:

• Mold and Fungi: Spores and fragments grow on damp building materials, window frames, and unventilated surfaces, releasing mycotoxins and allergens linked to asthma exacerbation.
• Bacteria and Viruses: Airborne pathogens can spread easily in crowded or poorly ventilated indoor spaces, increasing the risk of infections.
• Dust Mites: Microscopic arachnids that thrive in warm, humid environments and feed on shed human skin cells, producing potent allergens found in house dust.
• Pet Dander: Tiny scales of animal skin, saliva, and protein secretions shed by pets can remain airborne and trigger allergic sensitization.

Indoor Activities

Human routines and daily activities directly influence indoor air quality levels:

• Cooking: Frying, boiling, and roasting food release cooking oil mists, fine particulate matter, and organic gases, especially when cooking ventilation hoods are not used.
• Smoking and Vaping: Tobacco consumption and electronic cigarettes introduce thousands of hazardous chemicals, including nicotine, fine particles, and toxic gases, into the indoor environment.
• Office Equipment: Laser printers, photocopiers, and scanners can emit ozone, ultrafine particles, and volatile organic compounds during operation.

Outdoor Environment and Infiltration

Outdoor pollution sources can significantly impact indoor air quality through natural ventilation or infiltration:

• Outdoor Air Infiltration: Pollutants from nearby road traffic, industrial emissions, and urban smog enter buildings through open windows, doors, and structural cracks.
• Radon: A naturally occurring, radioactive gas formed by the decay of uranium in soil and rock. Radon can seep up through foundations, basement cracks, and building materials, accumulating in indoor spaces as a leading cause of lung cancer.

[+] Click here to view technical details on pollutant behavior and emission dynamics

Indoor pollutant concentrations are governed by complex dynamic processes, including indoor-outdoor air exchange rates, surface deposition, chemical reactions, and emission decay curves over time. For instance, secondary organic aerosols can form indoors via reactions between ozone infiltrating from outdoors and unsaturated organic gases like terpenes emitted from cleaning products or air fresheners. Furthermore, energy-efficient building retrofits that reduce air exchange rates without ensuring adequate mechanical ventilation can lead to the significant accumulation of indoor-generated VOCs and moisture.

Related Project Pages

• What is IAQ?
• Main indoor air pollutants
• IAQ relationship to human health
• Controlling the pollution sources