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Indoor Air Quality

2. What are the main factors in indoor air quality?

    The SCHER opinion states: Hazard identification

    A number of factors in the indoor environment can affect well-being and health. The main factors related to indoor air are

    • Chemicals for intended use or unintentional emissions from different sources
    • Radon
    • Particles
    • Microbes
    • Pets and pests
    • Humidity
    • Ventilation
    • Temperature


    Data requirement for hazard identification of chemicals is described in the TGD [Technical Guidance Document]. Information on the acute and sub-acute health effects can be obtained from toxicological data-bases and sources. However, information on possible health effects after long term exposures (carcinogenic, mutagenic, reproductive effects) is not available for many of the chemicals reported to be present in indoor air. The relevance of such data gaps needs to be discussed on case-by-case.

    Sensory irritation of the eyes and upper airways is a specific effect often related to exposure to indoor air pollutants (Alarie 1973; Alarie et al. 1998, Nielsen 1991; Doty et al. 2004) and needs particular attention. Sensory irritation may be induced by specific chemicals, but also by factors such as too low relative humidity of air.

    Malodours are generally undesirable in indoor environments (Wolkoff et al. 2006a). Odours per se do not cause toxicological effects but may increase the reporting of symptoms (for example, headache, nausea, eye and throat irritation) (Wolkoff et al. 2006a). Other symptoms of odour exposures are hyperventilation or conditioned responses (Shusterman 2002).

    Odours and sensory irritation and conditioned responses due to these challenges are common causes of complaints regarding poor indoor air quality.


    Radon in indoor air has been associated with lung cancer (WHO 1998, Darby et al., 2005). Radon gas diffuses through soil into residences in areas where bedrock contains in excess uranium. Radon evaporates also from household water into air upon warming. High radon concentrations have been measured indoors locally in several countries.


    Particles (PM10, PM2.5, fine particles, ultrafine particles) in ambient air have been associated with adverse health effects, including respiratory and cardiovascular effects (WHO 2003, WHO 2005b). Particles from outdoor air may contribute to particle load in indoor air, but there are also indoor sources such as combustion (Lam et al., 2006), cooking, and particles may be formed by reactions between ozone and some VOCs (Wainman et al., 2000, Sarwar et al., 2004, Afsari et al., 2005). Man-made nanoparticles are increasingly used in consumer products but their impact as indoor air pollutants is not yet known and finally assessed.


    Microbial agents may play a role in the development of asthma and allergic airway diseases. Many fungi contain allergens, and sensitization is possible by indoor exposure to fungi due to dampness and mould growth (see answer to Question 4c).

    Additionally, virus infections may be transmitted by indoor air. In the first years of life, virus infections are common causes of wheezing. Some viruses are associated with an increase in asthma and allergy incidence (Schaub et al. 2006).

    Pets and pests

    Pests, house dust mites and cockroaches are important sources of indoor allergens (aeroallergens) (D’Amato et al. 1998; Platts-Mills et al. 2000; Nielsen et al. 2002). Exposure may be different due to cultural habits, and cockroach and mouse allergens may be more important in inner cities (Eggleston 2001, Phipatanakul et al. 2000a; 2000b). Exposures to aeroallergens may cause sensitization with production of IgE antibodies. The IgE antibodies promote development of the allergic airway diseases, rhinitis and asthma (Beasley et al. 2000). The scientific literature on contact with pets and their effects on development of asthma is contradicting (for review e.g. Chan-Yeung and Becker 2006). However, allergic individuals should avoid exposures to allergens that elicit the allergic reactions.


    Humidity in indoor air has an optimal window. Too low humidity results in increased reporting of skin symptoms (dryness, rash), eye irritation and nasal dryness (Reinikainen and Jaakkola, 2003, Wolkoff et al., 2006b). In excessive humidity, water condenses onto (cold) surfaces, causing water damage and mould problems. High humidity also favours the growth of dust mites.


    Ventilation is one of the most important contributing factors to indoor air quality. Low ventilation rates, as well as the carbon dioxide (CO2) concentrations in indoor air, have been associated with health and perceived air quality outcomes (Seppänen et al., 1999). Ventilation rate has been associated also with work performance in office work and academic performance of school children (Seppänen et al., 2005, Schaughnessy et al., 2006, Wargocki et al. 2007). Thermal insulation contributes to building tightness which usually increases the need of controlled ventilation.


    Proper temperature is the basic indoor air requirement. Extreme indoor temperatures are a serious health hazard (Healy 2003, Kosatsky 2005) and too high or low temperatures perceived unpleasant. Too high temperature e.g. exacerbates the effects of insufficient humidity (Reinikainen and Jaakkola, 2001).

    Source & ©: SCHER,  Opinion on risk assessment on indoor air quality (2007), Hazard identification, p. 7-9

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