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

The main factors that determine indoor air quality are:

• Chemicals: Two common causes of complaint regarding poor indoor air quality are bad smells and irritation of the eyes, nose and throat. Such irritation may be induced by specific chemicals, but also by factors such as dry air. Bad smells are not harmful in themselves but may cause unpleasant effects and increase symptoms such as headache, nausea, and irritation of eyes or throat. The hazards associated with chemicals and their effects on health are not always well known, particularly for long term exposures.
• Radon: Radon is a gas that occurs naturally in soil and rock in some regions and that can get inside buildings by diffusing through the soil. In indoor air, it can lead to lung cancer.
• Suspended particles: Coarse, fine and ultrafine particles in ambient air are known to cause adverse health effects, including on the respiratory and cardiovascular systems. Particles may in part come from outdoor pollution, but can also form indoors by the burning of fuels for heating and cooking, and by reactions between ozone and some volatile organic compounds (VOCs). In addition, man-made nanoparticles, that are increasingly used in consumer products, may have an impact as indoor air pollutants.
• Microbes: Microorganisms such as fungi and viruses may play a role in the development of asthma and allergies involving the airways. This is for instance a problem in damp buildings or indoor environments where there is mould because many fungi release substances that cause allergies. Virus infections may also be transmitted by indoor air and some of them can lead to an increase in asthma and allergies.
• Pets and pests: Indoors, pests, house dust mites, cockroaches, and – in inner cities in particular – mice are important sources of allergens. These allergens can lead to diseases of the airways, rhinitis, and asthma. Exposure varies depending on the type of environment and cultural habits. How much contact with pets affects the development of asthma in people remains unclear. However, allergic people should avoid exposure to substances to which they are allergic.
• Humidity: There is an optimal level of humidity in indoor air. Too low humidity causes eye irritation, dry skin, and rashes, whereas too high humidity results in water damage and mould problems and favours the growth of dust mites.
• Ventilation: It is one of the most important factors determining indoor air quality. Poorly aired offices and schools, for instance can affect health and work or academic performance. Controlled ventilation is especially needed in heavily insulated buildings that allow little air exchange with the outside.

• Temperature: As well as causing discomfort, indoor air that is very cold or hot is highly unhealthy. Air that is too warm, for example, aggravates the effects of insufficient humidity.

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