Table 3.

Challenges of exposure assessment for chemical contaminants in drinking water.

Challenge	Comments
Low exposure levels	Accuracy of analytical measurements in water is particularly important at the low range of exposure. In addition, detailed personal information of water use behavior is convenient.
Chemicals occurring in mixtures	Examples include pharmaceutical residues and disinfection by-products. Depending on the individual constituents of the mixture, chemical-by-chemical exposure assessment may not be feasible or could result in simplistic exposure estimates.
Time–space variability	Repeated measurements and distribution of sampling points covering different water zones is necessary to evaluate geographical and temporal variation during the relevant exposure period.
Long-term exposure windows	Longer exposure periods are likely to result in greater exposure misclassification. In the case of chronic diseases such as cancer, data collection must include accurate location of study participants (residence and workplace) and water use over the duration of an exposure period relevant to disease etiology. Combined with environmental levels, quantitative estimation of exposure can be conducted. An added challenge is the lack of historical monitoring data.
Lack of monitoring data	This is particularly problematic to evaluate some exposures (such as emerging contaminants) and some outcomes (such as cancer because historical records are frequently unavailable). More research is needed to develop validated simulation models that can be used to estimate levels and exposure over the relevant time period.
Lack of validated biomarkers of exposure	Currently available validated biomarkers typically reflect recent exposures and thus may not be useful for outcomes with latency periods longer than the half-life of the biomarker compound. Exceptions may occur if the time between consecutive exposure events is shorter than the elimination half-life or exposure can be regarded as constant within the relevant time window (such as for trichloroacetic acid).
Multiple exposure routes (ingestion, inhalation, dermal absorption)	Exposure to a number of water contaminants can occur through multiple routes. For example, some DBPs can be incorporated through inhalation, dermal absorption and ingestion. For other waterborne contaminants, such as nitrate (at levels in water < 50 mg/L) and per- and polyfluorinated compounds, diet is the main source of exposure (Ericson Jogsten 2011; IARC 2010). For such contaminants, exposure by all plausible routes should be assessed in order to produce the most accurate estimate of disease risk.