D. Indoor Air Quality: Pollutants, Sources, and Influencing Factors
Indoor air quality is impacted by several pollutants and contaminants that are influenced by a multitude of factors, stemming from both indoor and outdoor sources(50). These sources include human activities such as smoking, burning solid fuels, cooking, and cleaning, as well as emissions from building materials, equipment, furniture, and biological contaminants like mould, viruses, and allergens. Common contaminants include:
• Allergens, substances that can induce allergic reactions by triggering the immune system. They can circulate in the air and persist on furniture and carpets for extended periods as well as emanate from pets and pests(51).
• Asbestos, a fibrous material once used in fireproof building materials, can release microscopic fibres into the air when disturbed, posing a carcinogenic risk(52).
• Carbon monoxide, a toxic and odourless gas, emerges from burning fuels, such as gasoline kerosene, in various appliances and systems, necessitating proper venting to prevent air contamination(53).
• Elevated Carbon dioxide (CO2) levels emerge from both indoor and outdoor sources and hinge upon ventilation adequacy, with human occupancy being a significant contributor(54). Elevated carbon dioxide levels (<5,000 ppm) may pose direct health risks, including inflammation, cognitive decline, bone and kidney issues, oxidative stress, and endothelial dysfunction. Further research is needed to identify sources, mitigation strategies, and effects of chronic or intermittent exposure to higher indoor CO2 concentrations(55).
• Formaldehyde, with a strong smell, is present in pressed wood furniture, cabinets, and coatings, as well as adhesives and paints, and is recognised as a human carcinogen(56).
• Lead, a naturally occurring metal, has been utilised in diverse products like gasoline, paint, plumbing pipes, and cosmetics, posing health hazards(57).
• Legionella bacterium, found in warm, stagnant water. It spreads through aerosolisation from sources like cooling towers and showers. Commonly affecting immuno-weak individuals, Legionella is hard to detect and resists chlorine treatment due to biofilm formation.
• Mould, a type of fungus, thrives in damp environments indoors and outdoors, presenting potential health risks(58).
• Ozone is formed through various sources including sunlight, lightning, and pollution, and it can be harmful to human health and particularly in indoor environments due to its reactivity with common indoor substances and skin oils(59).
• Pesticides, substances used to control pests like plants and insects, can carry health risks to humans(60).
• Radon, an odourless and colourless gas originating from soil decay, can infiltrate indoor spaces through gaps and cracks, contributing to lung cancer cases(61).
• Smoke (indoor combustion), produced from combustion processes like cigarette smoking, cookstoves, and wildfires, contains toxic chemicals such as formaldehyde and lead(62) as well as particulate matter.
• Volatile organic compounds, a wide range of chemicals emitted by various products such as paints, cleaning supplies, and building materials. These compounds can have short- and long-term adverse health effects and are often found at higher concentrations indoors than outdoors(63).
Within indoor environments, various pollutants can profoundly impact well-being and health(64) and these are influenced by the presence of a variety of sources and factors. These include the presence of chemicals, the existence of radon, the prevalence of fine and ultrafine particles, the occurrence of microbial agents, the presence of pets and pests, humidity levels, ventilation effectiveness, and temperature control.
Chemicals: The presence of specific chemicals in indoor environments can lead to irritations of the eyes, nose, and throat, accompanied by unpleasant odours. While these chemicals might trigger short-term discomfort, their long-term health effects remain insufficiently understood, especially with prolonged exposure(65).
Radon: A naturally occurring gas found in soil and rock, radon can infiltrate buildings and heighten the risk of lung cancer when present in indoor air(66). Notably, several member states of the European Union have recorded elevated radon concentrations(67).
Particulate Matter (PM): Coarse (10-2.5 µm), fine (2.5 µm), and ultrafine particles (0.1 µm) in ambient air are known to trigger adverse health effects, impacting the respiratory and cardiovascular systems(68). Although some particles originate from outdoor sources, indoor combustion for heating and cooking, along with chemical reactions between ozone and volatile organic compounds, contribute to particle formation indoors(69). Furthermore, the potential impact on IAQ of man-made nanoparticles (0.1 µm), increasingly employed in consumer products, necessitates further investigation(70).
Microbes: Microorganisms such as fungi and viruses can influence the development of asthma and allergies that affect the airways. Damp indoor environments, especially those with mould, can release allergenic substances from fungi, leading to health complications. Additionally, indoor air can serve as a transmission medium for specific virus infections, some of which are linked to increased asthma and allergy incidence(71).
Pets and Pests: Indoor allergens emanate from pests, house dust mites, cockroaches, and, particularly in urban settings, mice(72). These allergens can trigger respiratory diseases like rhinitis and asthma. The level of exposure varies with the environment(73).
Humidity: Indoor humidity requires an optimal range. Low humidity levels can cause skin dryness, eye irritation, and nasal discomfort, whereas excessive humidity can lead to water damage, mould growth, and dust mite proliferation(74).
Ventilation: Proper ventilation significantly influences IAQ, serving as a crucial measure to mitigate health risks. Inadequate ventilation rates and elevated carbon dioxide concentrations indoors have been associated with significant health consequences(75). Moreover, ventilation rates have been linked to work performance in office environments and the academic performance of school children(76).
Temperature: Appropriate indoor temperatures are vital for human comfort and well-being. Extreme temperatures also pose significant health risks(77). Excessively high temperatures can worsen the effects of inadequate humidity(78).
This compilation is by no means exhaustive, and substantial knowledge gaps persist regarding the primary factors as well as sources contributing to poor IAQ.
Numerous additional complexities intricately influence IAQ, including the age and maintenance of sources(79). For instance, an improperly calibrated stove might emit substantially higher levels of carbon monoxide compared to a well-adjusted one(80). Moreover, specific sources like smoking, cleaning, redecorating, or engaging in hobbies intermittently release pollutants, while unvented or malfunctioning appliances, along with improper product usage, can elevate indoor pollutant levels to potentially hazardous extents(81). Additionally, certain pollutants can be situationally specific, such as chloramines in swimming pools. Beyond these intricacies affecting sources and influencing factors related to IAQ, substantial gaps in knowledge persist regarding sources, factors, contaminants, pollutants, and the broader dynamics shaping IAQ(82).
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