Main indoor air pollutants
EDIAQI Wiki / Indoor air pollutants / Main indoor air pollutants
Classification: Gases, particulate matter, bioaerosols
Key Markers: PM2.5, CO, NO2, TVOC, HCHO, Rn
Regulatory References: WHO 2010, WHO 2021
Standard Units: µg/m³, mg/m³, Bq/m³, ppm
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Jump to: Introduction • Five Pillars • Gases • Particulate Matter • Bioaerosols • Guidelines Table • Related Pages
Introduction
Indoor air contains a wide spectrum of airborne contaminants comprising gases, solid particles, liquid droplets, and biological materials.[1] While outdoor ambient pollutants penetrate into buildings via mechanical ventilation, open windows, and building envelope cracks, internal sources frequently lead to indoor concentrations that exceed outdoor levels by a factor of two to five.[2]
Understanding the physical properties, chemical classifications, and biological dimensions of indoor pollutants is essential for selecting appropriate monitoring equipment and evaluating indoor environmental safety.[3] Detailed generation pathways and release dynamics are explored in Sources of indoor air pollutants, while specific toxicological mechanisms and disease endpoints are addressed in IAQ relationship to human health.
The Five Pillars of Indoor Air Quality
This page forms the second foundational pillar of the introductory IAQ knowledge framework:
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Gaseous Pollutants
Gases diffuse uniformly throughout enclosed spaces. Their airborne quantities are expressed either as mass concentration (µg/m³ or mg/m³) or volume mixing ratios (parts per million, ppm, or parts per billion, ppb).
Inorganic Gases
Inorganic gases lack carbon-hydrogen bonds and represent major combustion byproducts and atmospheric oxidants:[4]
- Carbon Monoxide (CO): An invisible, odorless, non-irritating toxic gas produced by incomplete combustion of carbon-based fuels.[4] It binds to hemoglobin with an affinity over 200 times higher than oxygen, forming carboxyhemoglobin (COHb) and starving body tissues of oxygen.[4]
- Nitrogen Dioxide (NO2): A reddish-brown, highly reactive gas generated during high-temperature fuel burning in gas stoves, boilers, and vehicle engines.[4] It acts as a deep respiratory irritant, induces airway inflammation, and triggers bronchoconstriction in children with asthma.[5]
- Ozone (O3): A strong oxidant gas that infiltrates from ambient photochemical smog or is formed indoors by high-voltage office electronics such as photocopiers and laser printers.[5] Ozone readily reacts with indoor surface waxes, skin lipids, and terpenes, producing secondary ultrafine particles and irritating carbonyls.[2]
- Sulfur Dioxide (SO2): A pungent, water-soluble gas emitted during the burning of sulfur-bearing solid fuels and heavy oils, capable of inducing acute reflex bronchospasm in sensitive individuals.[5]
- Carbon Dioxide (CO2): A natural metabolic product of human respiration. In non-industrial buildings without combustion appliances, CO2 serves as an established surrogate indicator for ventilation adequacy and bioeffluent accumulation rather than a primary toxic agent.[3]
Organic Gases (VVOCs, VOCs, SVOCs)
Organic chemical compounds are classified by boiling point ranges and vapor pressures in accordance with World Health Organization definitions:[2]
- Very Volatile Organic Compounds (VVOCs): Boiling points from below 0 °C to 50–100 °C. These substances exist entirely in the gaseous phase under typical room conditions.
- Key Examples: Formaldehyde (HCHO), acetaldehyde, dichloromethane, and chlorofluorocarbons.
- Volatile Organic Compounds (VOCs): Boiling points from 50–100 °C to 240–260 °C. VOCs evaporate readily at ambient temperatures and represent the most widely quantified organic category in buildings.[2]
- Key Examples: Benzene, toluene, ethylbenzene, xylenes (BTEX), styrene, acetone, ethanol, 2-propanol, and monoterpenes (alpha-pinene, limonene).
- Semi-Volatile Organic Compounds (SVOCs): Boiling points from 240–260 °C to 380–400 °C. Due to lower vapor pressures, SVOCs partition dynamically between the gas phase, airborne particulate matter, and settled interior dust.[2]
- Key Examples: Phthalate plasticizers (DEHP, BBzP), polycyclic aromatic hydrocarbons (PAHs), brominated flame retardants (PBDEs), polychlorinated biphenyls (PCBs), and synthetic pyrethroid pesticides.
Radon (²²²Rn)
Radon is a naturally occurring, chemically inert radioactive noble gas produced by the radioactive decay chain of uranium-238 present in soil, bedrock, and certain mineral construction components.[4] It is odorless, colorless, and tasteless. Radon infiltrates into buildings through foundation fractures, sump pits, and perimeter joints driven by indoor-outdoor air pressure differences. When inhaled, its short-lived alpha-emitting progeny deposit along bronchial airways, delivering localized radiation doses and constituting the leading cause of lung cancer among non-smokers.[4]
Particulate Matter and Suspended Aerosols
Airborne Particulate Matter (PM) consists of a complex suspension of microscopic solid particles and liquid droplets.[6] Particles are classified according to their aerodynamic equivalent diameter, which governs atmospheric residence time and respiratory penetration depth:[5]
- Coarse Particles (PM10): Particles with an aerodynamic diameter of 10 µm or less. This fraction includes mechanically generated dust, soil particles, pollen grains, and construction debris. They deposit predominantly in the extra-thoracic and upper tracheobronchial regions.[5]
- Fine Particles (PM2.5): Particles with an aerodynamic diameter of 2.5 µm or less. Fine particles penetrate deep into terminal bronchioles and alveolar gas-exchange regions, causing localized pulmonary inflammation and systemic cardiovascular strain.[5]
- Sub-Micron Particles (PM1): Particles smaller than 1.0 µm in diameter. Derived largely from combustion processes, this fraction accounts for the bulk of airborne particle surface area and carries toxic heavy metals and condensed organic species.[6]
- Ultrafine Particles (UFPs / PM0.1): Nanoscale particles with diameters below 0.1 µm (100 nm). Due to their high mobility and negligible mass, UFPs are characterized by particle number concentrations rather than mass. They can translocate across the air-blood tissue barrier directly into systemic circulation.[7]
- Black Carbon (BC): The strongly light-absorbing carbonaceous core formed during incomplete combustion of fuels and biomass.[5] Black carbon acts as a reliable tracer for primary combustion exhaust and serves as an effective carrier matrix for adsorbed toxic compounds.[5]
Biological Agents and Bioaerosols
Bioaerosols encompass airborne microorganisms, cellular fragments, and biological metabolic products suspended in indoor air.[8]
Microorganisms
- Fungi and Mould: Microscopic fungi colonize building materials exposed to elevated moisture (relative humidity consistently above 70%).[8] Growing mycelia release airborne reproductive spores, hyphal fragments, and microbial volatile organic compounds (MVOCs) responsible for musty odors.[8] Under specific substrate conditions, certain fungal genera (such as Aspergillus, Penicillium, and Stachybotrys chartarum) synthesize secondary metabolites termed mycotoxins, which possess cytotoxic and immunotoxic properties.[8]
- Bacteria and Endotoxins: Indoor bacteria originate from occupant shedding, damp ventilation sumps, humidifiers, and domestic pets. The disintegration of outer membranes from Gram-negative bacteria releases lipopolyssacharides (endotoxins), which are potent triggers of acute fever, bronchial hyperreactivity, and systemic inflammation.[8]
- Viruses: Pathogenic respiratory viruses (including influenza, respiratory syncytial virus, and coronaviruses) are emitted in exhaled respiratory aerosols during breathing, talking, and coughing.[9] In poorly ventilated rooms, fine infectious droplets remain suspended for extended periods, facilitating aerosol transmission.[9]
Allergens and Organic Dust
- House Dust Mite Allergens: Microscopic arachnids (Dermatophagoides pteronyssinus, Dermatophagoides farinae) thrive in upholstered furniture, mattresses, and carpets with relative humidity levels above 50%.[8] The principal allergens are digestive enzymes concentrated in excreted fecal pellets, which trigger perennial allergic rhinitis and asthma.[8]
- Pet and Pest Allergens: Dander, dried saliva, and urinary proteins from domestic pets (cats, dogs) and residential pests (cockroaches, mice). Cat allergen proteins (Fel d 1) are small and lightweight, remaining suspended in indoor air for hours following mechanical disturbance.[8]
- Pollen: Seasonal anemophilous pollen grains from grasses, trees, and weeds that infiltrate from ambient air through open windows and unfiltered mechanical ventilation intakes.[6]
Concentration Limit Values and Health Benchmarks
The following table synthesises official health-based guideline values established by the World Health Organization for priority airborne contaminants in indoor and ambient environments:
| Pollutant Category | Pollutant Parameter | Averaging Time | Guideline Value / Reference | Source & Toxicological Basis |
|---|---|---|---|---|
| Organic Gases (VOCs) | Benzene (C6H6) | Lifetime exposure | No safe threshold | WHO 2010; Unit risk: 6 × 10⁻⁶ per 1 µg/m³ exposure[4] |
| Formaldehyde (HCHO) | 30 minutes | 100 µg/m³ (0.08 ppm) | WHO 2010; Prevention of sensory irritation[4] | |
| Naphthalene (C10H8) | 1 year | 10 µg/m³ | WHO 2010; Prevention of respiratory tract lesions[4] | |
| PAHs (as Benzo[a]pyrene) | Lifetime exposure | No safe threshold | WHO 2010; Unit risk: 8.7 × 10⁻⁵ per 1 ng/m³ BaP[4] | |
| Tetrachloroethylene (PCE) | 1 year | 250 µg/m³ | WHO 2010; Prevention of renal and hepatic toxicity[4] | |
| Trichloroethylene (TCE) | Lifetime exposure | No safe threshold | WHO 2010; Unit risk: 4.3 × 10⁻⁷ per 1 µg/m³ exposure[4] | |
| Inorganic Gases | Carbon Monoxide (CO) | 15 minutes 1 hour 8 hours 24 hours |
100 mg/m³ 35 mg/m³ 10 mg/m³ 4 mg/m³ (WHO 2021) / 7 mg/m³ (WHO 2010) |
WHO 2010 / WHO 2021; Maintains COHb below 2% in blood[4][5] |
| Nitrogen Dioxide (NO2) | 1 hour 24 hours 1 year |
200 µg/m³ 25 µg/m³ 10 µg/m³ |
WHO 2021; Prevention of acute bronchospasm and chronic asthma[5] | |
| Ozone (O3) | 8 hours (peak season) | 100 µg/m³ 60 µg/m³ |
WHO 2021; Prevention of lung function decline[5] | |
| Sulfur Dioxide (SO2) | 10 minutes 24 hours |
500 µg/m³ 40 µg/m³ |
WHO 2021; Prevention of acute asthmatic exacerbation[5] | |
| Particulate Matter | Fine Particles (PM2.5) | 24 hours 1 year |
15 µg/m³ 5 µg/m³ |
WHO 2021; Mitigation of cardiovascular and pulmonary morbidity[5] |
| Coarse Particles (PM10) | 24 hours 1 year |
45 µg/m³ 15 µg/m³ |
WHO 2021; Protection against respiratory symptoms and mortality[5] | |
| Radioactive Gas | Radon (²²²Rn) | 1 year (annual mean) | 100 Bq/m³ (national action level up to 300 Bq/m³) | WHO 2010; Minimisation of lifetime lung cancer risk[4] |
Related Wiki Pages
References
- ↑ Cincinelli, A., & Martellini, T. (2017). Indoor air quality and health. International Journal of Environmental Research and Public Health, 14(11), 1286.
- ↑ 2.0 2.1 2.2 2.3 2.4 Halios, C. H., Landeg-Cox, C., Lowther, S. D., Middleton, A., Marczylo, T., & Dimitroulopoulou, S. (2022). Chemicals in European residences: Part I: A review of emissions, concentrations and health effects of volatile organic compounds (VOCs). Science of the Total Environment, 839, 156201.
- ↑ 3.0 3.1 Settimo, G., Manigrasso, M., & Avino, P. (2020). Indoor air quality: A focus on the European legislation and state-of-the-art research in Italy. Atmosphere, 11(4), 370.
- ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 World Health Organization. (2010). WHO guidelines for indoor air quality: selected pollutants.
- ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13 World Health Organization. (2021). WHO global air quality guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide.
- ↑ 6.0 6.1 6.2 Cheek, E., Guercio, V., Shrubsole, C., & Dimitroulopoulou, S. (2021). Portable air purification: Review of impacts on indoor air quality and health. Science of The Total Environment, 766, 142585.
- ↑ Fuller, R., et al. (2022). Pollution and health: a progress update. The Lancet Planetary Health, 6(6), e535-e547.
- ↑ 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 World Health Organization. (2009). WHO guidelines for indoor air quality: dampness and mould.
- ↑ 9.0 9.1 Sadrizadeh, S., et al. (2022). Indoor air quality and health in schools: A critical review for developing the roadmap for the future school environment. Journal of Building Engineering, 57, 104908.