Like outdoor air , indoor air contains a complex mixture of pollutants (chemical substances, allergens and microbes) from different sources that changes with time. Findings on the health effects of single air pollutants cannot necessarily be extended to mixtures. Indeed, different chemicals may interact with each other and cause more (or less) harmful effects than the sum of the effects caused by each chemical separately. Very little is known about the combined effects of indoor air pollutants. Risk assessments which take into account the combined exposure and cumulative effects of the pollutants in indoor air are seldom possible. Nonetheless, the possibility of combined effects should be considered in.
