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3. How can scientists determine whether indoor air pollutants pose a health risk?

To determine the potential of a pollutant to cause health effects, it is necessary to consider several factors:

• Toxicity of pollutants and their concentrations in indoor air. Indoor air may contain more than 900 different organic compounds in addition to particles, microbes, and allergens. These pollutants are emitted during cooking, cleaning, and heating and from other sources such as building materials. The concentrations of pollutants in indoor air depend on the types of pollutants, the rate at which they are emitted, how they are taken up by different materials and how much the interior is ventilated. Cultural habits may also be an important factor determining indoor air quality.
• Exposure. People can be exposed to pollutants directly by breathing them in, and indirectly by ingesting them, so all possible routes of exposure should be considered. Exposure to dust, for example, can entail exposure to other, less volatile pollutants that stick to dust particles. Young children may ingest more dust than they breathe in, because they spend more time on the floor and put their hands in their mouths. Dust particles are relatively large and do not stay suspended in the air for long. When assessing exposure it is important not to neglect a possible impact of compounds that are present in low concentrations but are very toxic, or of compounds which are difficult to detect. Assessments should use realistic exposure scenarios. Since exposures can vary tremendously, the whole exposure range, including the exposures of the most sensitive and most exposed subgroups, should be considered and not only average or median values.
• Exposure-response relationship. To assess the risk posed by any substance, it is crucial to know how the body responds to different amounts of the substance, both in the short and long term. To evaluate local health effects such as irritation of the nose, skin or eyes, the concentration of pollutants in the air provides relevant information about exposure. However, to assess the effects of a pollutant in the body as a whole, it is necessary to calculate how much of the substance gets inside the body, for instance taking into account breathing volumes, and how much stays in the alveoli or is eliminated when breathing out. To determine whether pollutants pose a health risk and cause illness, scientists sometimes use data on the health of people who have been exposed to pollutants at work. When using these data to draw conclusions it is necessary to consider in the calculations that people usually spend a lot more time at home than at work. It is also important to realize that these results are representative of the working population and may not be directly applicable to the general public.
• Risk characterisation. In the final step of the risk assessment process, all the collected scientific evidence is analysed to determine the probability that a specific pollutant will cause illness.

In some cases, particularly those involving microbes, it is difficult to establish a direct link between a particular agent and a health effect. In those cases the apparent association may be strengthened by eliminating confounding factors.

For some indoor air pollutants, national and international guidelines or limits of exposure have been set, based on known health effects. More...