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from the general population, often referred to as community controls, or from
hospitals where cases were identified. Three criteria have been proposed for
the selection of controls:
Controls should come from the same registry (e.g., hospital, mortality
records) as the index cases.
Control illness should be unrelated to the risk factors under study (i.e.,
a given risk factor under study should not predispose one to getting the
control illness).
Control illness should be similar to the illness under study, bearing in
mind the factors influencing its appearance (or referral) in the registry
One might want to add a second control if these criteria are not met.
If farmers are under study, then ideally, the control group should also be
farmers. Farming is a unique way of life, not only in terms of chemical
exposure, but also with respect to diet, physical activity, exposure to viruses,
habits, and life-style. Opportunity for exposure to viruses, for example, may
be affected by the presence of intermediate hosts, including livestock or even
particular crops.
In carrying out epidemiological studies, accurate measurement of exposure is
important because errors may reduce estimates of relative risk and dampen
dose-response gradients. Misclassification of toxic substance increases the
chance of attributing elevated cancer risk among exposed people to the wrong
agent. Employment records are commonly used to assess exposure in cohort
studies and interviews are used to determine exposure in case-control studies.
However, these techniques are of use in both types of study and should be
used, when possible, to test and improve the reliability of exposure evaluations.
Information can be obtained from different sources to increase confidence in
exposure estimates. Sources include suppliers who sell pesticides to the subjects
under study, records of applicator companies, interviews about the type
of equipment used for application, and duration and frequency of application
coupled with biochemical monitoring data. For populations, estimates of
exposure may be developed by a panel of experts, including agricultural
scientists, entomologists, and industrial hygienists familiar with patterns of
pesticide use. Biochemical factors can provide better measures of delivered
dose than estimates based on concentration of substances in patches and
ambient air or those derived from job descriptions or interviews. However,
biochemical measures are typically only available for a sample of study
subjects, for only a few pesticides, and at only specific times.
Ideally, to allow epidemiological surveillance of a farming population using
pesticides, it would be necessary to implement a system of registration that
would identify the exposed subjects and specify and quantify exposure. This
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