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5. Why are the combined effects of indoor air pollutants hard to measure?

Indoor air is a complex mixture of pollutants (chemical substances, allergens and microbes) from various sources that changes with time. At present, however, relevant data and established methods to evaluate the health effects of such mixture of air pollutants are rare and risk assessments of air pollution mostly deal with exposure to single substances. In addition, the few findings available on interactions among more than two chemicals usually do not address issues of long-term toxicity at realistic concentrations.

Data from studies on individual chemicals can be used directly if the components of the mixture act independently. However, some of the chemicals in the air may influence each other. Indeed, several chemicals acting together may cause more (or less) harmful effects than the sum of the effects caused by each chemical separately. For example, potentially harmful combined effects of mixtures of pesticides have been observed at concentrations of each single component at which, individually, they do not affect health. In these cases, data from exposure to single chemicals cannot be used directly to determine health risks.

A number of studies have dealt with the combined effects of indoor air pollutants, including effects of fine particles and gases in ambient air. For example, particles in the air may help some pollutants reach the deeper parts of the lungs (bronchioles, alveoli), so exposure to particulate matter could enhance the allergic responses of susceptible individuals. In asthmatics, nitrogen dioxide increases the inflammatory effect of exposure to allergens in the air.

There is no generally accepted strategy for evaluating health risks from exposure to a mixture of pollutants. The approach consisting in adding up the doses of the individual pollutants can be useful for some mixtures of chemicals. In other cases, it is possible to use data from the combined effects of two chemicals and extrapolate the results to more complicated mixtures. In general however, there are not enough relevant results on the mechanisms of actions to estimate the toxicity of a chemical mixture, and the available methods used may not be adequate.

Risk assessments which take into account the combined exposure and cumulative effects of the pollutants in indoor environments are seldom possible. However, the possibility of combined effects should be considered in the risk assessment and evaluated case-by-case. Interactions between chemicals and other factors such as microbes are insufficiently known to provide guidance. More...