Frequency and Timing of Environmental Exposure

Identifying the temporal dimensions of exposure is a key challenge in the assessment of risk from environmental contaminants. Temporal models of exposure consist of combinations of three elements: duration, frequency, and magnitude. Different temporal models have been developed to characterize exposure and are described: cumulative, time-weighted average, peak, and time-specific. In addition, the influence of different measures of time, also referred to as temporal orientations are also considered. For example, comparing exposure across people will change depending on whether time is oriented in reference to age (i.e., years/months since being born), diagnosis/interview (i.e., years/months before diagnosis/interview), calendar year/date, or other points in time, for example, age at menarche and time since beginning an occupation. Importantly, the appropriate temporal model or temporal orientation may change from one exposure to another, even for the same disease, making it difficult to hypothesize the appropriate temporal model to select. The example of exposure to arsenic in drinking water is presented to illustrate different means of incorporating magnitude, frequency, duration, and timing of exposure.