Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
as closely as possible, with respect to matrix type and concentrations
of the samples and analytes of interest.
The availability of certified reference materials is still
limited, however. In addition, owing to their relatively high cost,
these materials can only occasionally be used for QC. Alternatively, a
laboratory may prepare control materials calibrated against and,
therefore, traceable to certified reference materials, or a
conventional true value may be established by one or more reference
laboratories, preferably using a validated definitive method (ISO,
1994).
11.6 Quality assurance and control issues in population-based
studies
Population-based studies present specific challenges to QA/QC
programmes that are not common to more conventional exposure
investigations. A description of the potential problems and
alternative approaches to address them, avoid them and estimate the
biases that they may introduce in the results should therefore be part
of the study plan. Study design, including QA/QC procedures, should
consider methodologies that address configurations as they occur in
the field, and provide for modifications that will not compromise the
stated objectives.
Among the most important considerations in human population
studies are the potential differences in cultural, geographical or
social class framework between the exposure assessment study team and
the target population. The investigators should be cognizant of local
culture and conditions and include those considerations in the study
design. For example, perceptions and customs with regard to the
importance of time in daily life varies across cultures and even among
subpopulations within the same country. As a result, appointments for
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