Table Info

Table 2.

Characteristics of main indoor pollutants and their sources

Pollutants	Characteristics	Sources
PM	The mixture of small particles and droplets consisting of organic chemicals, metals, and dust particles Categorized by its size: (1) Coarse: 2.5–10 μm (2) Fine: 2.4–0.1 μm (3) 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	(1) Tobacco smoke (the major source of PM2.5) (2) Cooking and heating(burning of biomass fuel, liquefied petroleum gas fuel, oil, wood, and food; release of DEPs) (3) Machine operations (4) Cleaning (re-suspending of dust particles, facilitation of outdoor particles penetration inside)
VOCs	Organic 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	(1) Natural: volcanic eruptions, forest fires, etc. (2) Anthropogenic: (a) Exploitation, storage, refining, transport, and usage of fossil fuels (b) Cooking (c) Heating (d) Building materials (particleboard, fiberboard, plywood, paint, and wallpaper) (e) Furniture
Heavy metals	They can enter the human body through inhalation, ingestion, or dermal contact	(1) Infiltration of outdoor pollutants (dust and soil) (2) Tobacco smoke (3) Fuel consumption products (4) Building materials
Chemical gases	SO₂, 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	(1) Tobacco and e-cigarette smoke (2) Gas-, wood-, oil-, kerosene-, and coal-burning appliances such as stoves and fireplaces (3) Building materials (4) Air-freshening or air-cleaning devices (5) Copy machines
Biological contaminants	Bacteria, 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

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

Declarations

Author contributions

ER: Conceptualization, Supervision. AP: Investigation, Writing—original draft. AB, MO, and MC: Investigation. FN: Conceptualization, Investigation, Writing—original draft, Writing—review & editing, Visualization. All authors read and approved the submitted version.

Conflicts of interest

ER is a member of the editorial board of Exploration of Asthma & Allergy journal and had no involvement in the decision-making or the review process of this manuscript. All the other authors declare that they have no conflicts of interest.

Ethical approval

Not applicable.

Consent to participate

Not applicable.

Consent to publication

Not applicable.

Availability of data and materials

Not applicable.

Funding

Not applicable.

Copyright

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