Main indoor air pollutants

Gases

Inorganic gases

Inorganic gaseous air pollutants are a significant group of air contaminants that lack carbon-hydrogen bonds, distinguishing them from organic pollutants.

These gases, including ozone (O3), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and carbon monoxide (CO), originate from various sources such as vehicle emissions, industrial processes, and natural occurrences like volcanic eruptions.

Organic gases

Organic gaseous compounds can be categorized based on their boiling points, which directly relate to their volatility - their tendency to evaporate at room temperature. This causes their different behaviour in indoor air. Also, the health effects can vary depending on the specific compound and its volatility.

Very volatile organic compounds

Very volatile organic compounds (VVOCs) are gases at room temperature and very easily evaporate.

Examples:

• Formaldehyde (although some sources classify it as a VOC)
• Methane
• Butane
• Propane
• Freons (e.g., chlorodifluoromethane)

Volatile organic compounds

Volatile organic compounds (VOCs) are a large group of chemicals that easily evaporate at room temperature and are the most commonly referred to group when discussing VOCs in indoor air. VOCs are also mainly gases but they may condense on colder surfaces.

They are found in many common household products and building materials, and they can have a significant impact on indoor air quality (IAQ).

Examples:

• Benzene
• Toluene
• Xylene
• Acetone
• Ethanol
• Isopropanol
• Many components of fragrances and cleaning products

Semi-volatile organic compounds

Semi-volatile organic compounds (SVOCs) have lower volatility than VOCs and tend to exist both in the gas phase and adsorbed onto surfaces or particles (like dust).

Examples:

• Phthalates (plasticizers)
• Polychlorinated biphenyls (PCBs) - mostly phased out but can be found in older buildings
• Polyaromatic hydrocarbons (PAHs)
• Some pesticides
• Flame retardants (PBDEs)

Radon

Radon is a naturally occurring radioactive gas that is colorless, odorless, and tasteless. It is formed from the decay of uranium, which is found in rocks and soil throughout the world.

Radon gas can seep into homes and buildings through cracks and openings in the foundation, floors, and walls. It can also be released from building materials that contain radium.

Indoor air particles

Particulate matter (PM) is a mixture of solid particles and liquid droplets suspended in the air. These particles vary in size, composition, and origin, and their effects on human health depend on these characteristics.

PM is classified by its aerodynamic diameter, which is the size of a unit-density sphere with the same aerodynamic properties as the particle.

Coarse particles (PM₁₀)

Coarse particles are inhalable particles, with diameters that are generally 10 micrometers and smaller.

Fine particles (PM_2.5)

Fine particles are inhalable particles, with diameters that are generally 2.5 micrometers and smaller. Due to their small size, they can penetrate deep into the respiratory tract, reaching the lungs.

Ultrafine particles (PM_0.1 )

Ultrafine particles (UFP) are a subset of particulate matter with a diameter of less than 0.1 micrometers. Due to their extremely small size, UFP can penetrate deep into the lungs and potentially enter the bloodstream, raising concerns about their health effects.

Biological agents

Indoor air biological contaminants are airborne microorganisms and organic materials originating from living or once-living sources. They are commonly found in indoor environments and can significantly affect occupant health.

This category encompasses living organisms and their byproducts that can negatively affect indoor air quality.

Fungi and mold

Fungi and mold thrive in damp or humid indoor environments and can release mycotoxins, which can have carcinogenic, immunotoxic, cytotoxic, and mutagenic effects.

Airborne spores (for reproduction)

Hyphal fragments (broken pieces of the mold structure)

Mycotoxins (toxic chemical compounds produced by some molds, can be on spores or fragments)

Microbial Volatile Organic Compounds (MVOCs) (gases with musty odors produced by actively growing molds)

Bacteria

Similar to mold, bacteria can grow in damp conditions and cause health issues.

Airborne bacterial cells

Endotoxins (toxic components of the cell walls of certain (Gram-negative) bacteria, released when bacteria die and break apart)

Viruses

These can be transmitted through the air, especially in poorly ventilated spaces.

Airborne viral particles (often attached to dust particles or carried in respiratory droplets)

Pollen and plant material

These substances can trigger the immune system, causing allergic reactions. Common indoor allergens include dust mites, pet dander, and pollen.

Animal-Derived

Pet Dander: Skin flakes (epidermal scales), dried saliva proteins, and urine proteins from common household pets (cats, dogs, rodents, birds, etc.).

Pest Allergens:

Dust Mites: Allergenic proteins found in their fecal pellets and decaying body fragments.

Cockroaches & Other Insects: Allergens from their droppings, saliva, shed skins, and body parts.

Rodents (mice, rats): Allergens from urine, dander, and saliva.

Human-Derived Bioaerosols

Shed skin cells (a major component of household dust and food for dust mites)

Microorganisms (bacteria and viruses) expelled through breathing, talking, coughing, and sneezing.

Concentration limit values for main air pollutants

Categorization	Pollutant	Averaging period	Concentration limit value		Reference
Organic gases	Benzene	-	no safe level		WHO 2010
	Naphthalene	annual	10	μg/m3	WHO 2010
	Polycyclic aromatic hydrocarbons		no safe level		WHO 2010
	Tetrachloroethylene	annual	250	μg/m3	WHO 2010
	Trichloroethylene				WHO 2010
	Formaldehyde	30-minute	100	μg/m3	WHO 2010
Inorganic gases	O3	8-hour peak season	100 60	μg/m3	WHO 2021
	SO2	10-minute 24-hour	500 40	μg/m3	WHO 2021
	NO2	1-hour 24-hour annual	200 25 10	μg/m3	WHO 2021
	CO	15-minute 1-hour 8-hour 24-hour	100 35 10 4	mg/m3	WHO 2021
Particulate matter	PM2.5	24-hour annual	15 5	μg/m3	WHO 2021
	PM10	24-hour annual	45 15	μg/m3	WHO 2021
Radioactive gases	Radon