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, also known as bioaerosols, are a diverse group of airborne materials that originate from living or once-living organisms. These microscopic particles and microorganisms are commonly present in virtually all indoor environments and can significantly influence indoor air quality and occupant health. The presence and concentration of these agents are influenced by various factors including building design, maintenance, occupant activities, and ambient environmental conditions such as humidity and temperature.

This category encompasses a wide array of entities, including microorganisms like fungi (molds), bacteria, and viruses, as well as allergens derived from plants, animals, and humans. Their byproducts, such as toxins and volatile organic compounds, can also contribute to adverse health effects. Exposure to biological contaminants can lead to a range of health issues, broadly categorized as infectious diseases, allergic reactions, and toxic effects.

Understanding the diverse nature and sources of these biological agents is crucial for assessing risks and implementing effective control measures to maintain healthy indoor air.

Microorganisms

Microorganisms are ubiquitous in indoor environments, colonizing damp surfaces, HVAC systems, and even residing on occupants themselves.

Fungi and Mold

Fungi, commonly referred to as mold when growing indoors, are eukaryotic organisms that thrive in damp or humid environments where organic material is available for them to decompose. They are a significant concern for indoor air quality due to their ability to release various particles and compounds into the air. Common indoor locations for mold growth include areas with water damage (e.g., leaky roofs, burst pipes), poorly ventilated bathrooms and kitchens, damp basements and crawl spaces, and within HVAC systems where condensation can accumulate.

Airborne Spores

These are microscopic reproductive units released in vast quantities by mature molds. Spores are designed for dispersal and can remain airborne for extended periods, readily inhaled by occupants. They can trigger allergic reactions and asthma, and in some cases, cause infections in susceptible individuals.

• Hyphal fragments: These are broken pieces of the vegetative mold structure (mycelium). Like spores, they can become airborne and contribute to allergic responses and respiratory irritation.

• Mycotoxins: Certain mold species, under specific growth conditions, produce toxic chemical compounds known as mycotoxins. These can be present on spores or hyphal fragments, or within the materials molds have colonized. Exposure to mycotoxins can occur through inhalation, ingestion, or skin contact, potentially leading to a variety of severe health effects, including carcinogenic (cancer-causing), immunotoxic (damaging to the immune system), cytotoxic (toxic to cells), and mutagenic (causing genetic mutations) effects. Examples of mycotoxin-producing molds include certain species of Aspergillus, Penicillium, and Stachybotrys chartarum (often called "black mold").

• Microbial Volatile Organic Compounds (MVOCs): Actively growing molds release gases known as Microbial Volatile Organic Compounds (MVOCs). These compounds are responsible for the characteristic musty or earthy odors often associated with mold growth. While the direct health effects of many MVOCs at typical indoor concentrations are still being researched, they can cause eye, nose, and throat irritation, headaches, dizziness, and nausea. Their presence is a strong indicator of active mold proliferation.

Bacteria

Bacteria are single-celled prokaryotic microorganisms that, similar to mold, can proliferate in damp indoor conditions and contribute to health problems. They are found on virtually all indoor surfaces and are shed by occupants.

• Airborne bacterial cells: Whole bacterial cells can become aerosolized from various sources, including contaminated water systems (e.g., humidifiers, cooling towers – a source for Legionella pneumophila, the causative agent of Legionnaires' disease), soil tracked indoors, pets, and human activities like coughing and sneezing. Inhalation can lead to respiratory infections and exacerbate allergic conditions.
• Endotoxins: These are lipopolysaccharides, toxic components found in the outer membrane of Gram-negative bacteria (e.g., Escherichia coli, Pseudomonas aeruginosa). Endotoxins are potent inflammatory agents and are released when bacterial cells die and break apart. Inhalation can cause fever, malaise, respiratory distress, and exacerbate asthma. They are common in environments with high bacterial loads, such as buildings with water damage or agricultural settings.
• Exotoxins: Some bacteria, including certain Gram-positive species like Staphylococcus aureus (which can be found on skin and in nasal passages), can produce exotoxins, which are proteins secreted by living bacteria that can also have detrimental health effects if aerosolized, though this is less commonly the primary concern in typical IAQ assessments compared to endotoxins.

Bacteria can be found in biofilms within plumbing, on damp building materials, in HVAC condensate pans, and are constantly introduced by human occupants.

Viruses

Viruses are sub-microscopic infectious agents that require a living host cell to replicate. They are a major cause of communicable diseases and can be readily transmitted through indoor air, particularly in densely occupied or poorly ventilated spaces.

• Airborne viral particles: Viruses can become airborne through several mechanisms. They are often encapsulated within respiratory droplets (larger particles >5-10 µm) generated during coughing, sneezing, or talking, which tend to settle relatively quickly. They can also be present in smaller aerosolized particles (<5 µm) that can remain suspended in the air for longer periods and travel greater distances. Furthermore, viral particles can attach to dust particles, which then act as carriers.
• Common indoor viruses: Examples include rhinoviruses (common cold), influenza viruses (flu), respiratory syncytial virus (RSV), measles virus, varicella-zoster virus (chickenpox), and various coronaviruses (e.g., SARS-CoV-2).
• Survival and transmission: The viability of airborne viruses is influenced by environmental factors such as temperature, relative humidity (intermediate humidity levels of 40-60% are often less favorable for many viruses compared to very dry or very humid conditions), and ultraviolet (UV) radiation (sunlight or UVGI systems can inactivate viruses).

Effective ventilation and air filtration are key strategies in reducing the airborne concentration of viruses indoors.

Allergens and Other Biological Materials

Beyond live microorganisms, various biological materials can trigger allergic reactions or other health issues when present in indoor air.

Pollen and Plant Material

While primarily an outdoor allergen source, pollen from trees, grasses, and weeds can easily infiltrate indoor environments through open windows, doors, and ventilation systems. Some indoor plants can also produce pollen or release spores (e.g., ferns) or other plant fragments that may act as allergens or irritants for sensitive individuals. These substances contain proteins that can trigger the immune system in sensitized individuals, leading to allergic rhinitis (hay fever), conjunctivitis, and asthma exacerbations.

Animal-Derived Allergens

Proteins from animals are significant sources of indoor allergens.

• Pet dander: This is a primary source of allergens from common household pets such as cats, dogs, rodents (hamsters, guinea pigs), and birds. Dander consists of tiny skin flakes (epidermal scales) that are constantly shed. Allergens are also found in pet saliva (which adheres to fur during grooming and then dries and flakes off), urine (especially from rodents), and sebaceous gland secretions. These allergens are very small and light, remaining airborne for long periods and settling on surfaces throughout the home.
• Pest allergens:
  • Dust mites: These microscopic arachnids thrive in warm, humid environments and feed on shed human skin cells found in dust. The primary allergens are potent digestive enzymes present in their fecal pellets and also in their decaying body fragments. Dust mites are abundant in bedding, upholstered furniture, carpets, and curtains.
  • Insects: Allergens are derived from their droppings (feces), saliva, shed exoskeletons (skins), and decomposing body parts. Cockroach allergens are particularly problematic in multi-unit dwellings and can be a significant asthma trigger, especially in children.
  • Rodents (mice, rats): Allergens are primarily found in their urine (especially proteins that become airborne as the urine dries), but also in their dander and saliva. Rodent infestations can lead to high levels of these potent allergens.

Human-Derived Bioaerosols

Humans themselves are a significant source of indoor biological particulate matter and microorganisms.

• Shed skin cells: Humans continuously shed skin cells (squames). These cells form a major component of household dust and serve as a primary food source for dust mites.
• Microorganisms from occupants: Bacteria (e.g., Staphylococci, Streptococci) and viruses are expelled into the air through normal physiological activities such as breathing and talking, and in much greater quantities during coughing and sneezing. These bioaerosols can contribute directly to disease transmission between occupants and can also settle on surfaces, potentially leading to indirect transmission or becoming part of the general microbial load of the indoor environment.

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