From:  Indoor air pollution and atopic diseases: a comprehensive framework
 

Characteristics of main indoor pollutants and their sources

PollutantsCharacteristicsSources
PM

The mixture of small particles and droplets consisting of organic chemicals, metals, and dust particles

Categorized by its size:

  • Coarse: 2.5–10 μm

  • Fine: 2.4–0.1 μm

  • Ultrafine: < 0.1 μm

≥ 2.5 μm PM affects mainly high airways; < 0.1 μm PM affects small airways and is eventually trans-located into circulation

  • Tobacco smoke (the major source of PM2.5)

  • Cooking and heating(burning of biomass fuel, liquefied petroleum gas fuel, oil, wood, and food; release of DEPs)

  • Machine operations

  • Cleaning (re-suspending of dust particles, facilitation of outdoor particles penetration inside)

VOCsOrganic chemicals that easily vaporize and spread in the environment under normal conditions (high vapor pressure at room temperature and standard atmospheric pressure; initial boiling point < 250°C)
Highly volatile, mobile, and resistant to degradation (able to be transported to long distances in the environment)
Most abundant: formaldehyde, benzenes, toluene, ethylbenzene, and xylenes

  • Natural: volcanic eruptions, forest fires, etc.

  • Anthropogenic:

    • Exploitation, storage, refining, transport, and usage of fossil fuels

    • Cooking

    • Heating

    • Building materials (particleboard, fiberboard, plywood, paint, and wallpaper)

    • Furniture

Heavy metalsThey can enter the human body through inhalation, ingestion, or dermal contact

  • Infiltration of outdoor pollutants (dust and soil)

  • Tobacco smoke

  • Fuel consumption products

  • Building materials

Chemical gasesSO2, NO2, and CO are products of combustion
O3 is produced in the atmosphere by oxidation of hydrocarbons and CO catalyzed by hydrogen oxide radicals and nitrogen oxide radicals

  • Tobacco and e-cigarette smoke

  • Gas-, wood-, oil-, kerosene-, and coal-burning appliances such as stoves and fireplaces

  • Building materials

  • Air-freshening or air-cleaning devices

  • Copy machines

Biological contaminantsBacteria, viruses, unicellular organisms, molds, algae, HDMs, pests, and animal epithelia
Temperature and relative humidity are important factors that favor the growth and survival (fungi and dust mites)		Humans, carpets, stuffed toys, cushions, and pets
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CO: carbon monoxide; DEPs: diesel exhaust particles; e-cigarettes: electronic cigarettes; HDMs: house dust mites; NO2: nitrogen dioxide; O3: ozone; PM: particulate matter; SO2: sulfur dioxide; VOCs: volatile organic compounds