Variability of exposure measurements in environmental epidemiology

In studies in environmental epidemiology, exposure to harmful agents is often highly variable in time and space. It is not usually possible to measure the relevant, personal exposure of study subjects to these agents directly. Instead, exposure measurements are performed at fixed sites and/or for limited periods of time in many cases. Such measurements are imperfect in the sense that they only approximate the "true" personal exposure of study subjects. When measures of exposure are highly variable in time and space, single measurements approximate the true exposure only to a limited extent. The variability of measures of exposure can be investigated by repetition of the measurements in time and space. Analysis of variance techniques can be used to separate the within-subject or "error" variance from the between-subjects or true variance. Computation of the ratio between the error variance and the true variance is a useful technique to evaluate the potential bias in correlation and regression coefficients calculated with these measures of exposure. Using data from a number of different studies, the authors have estimated the variance ratio of lead exposure and nitrogen dioxide exposure variables. The results suggest that these ratios may be large. Empirical illustrations are given of bias in regression coefficients of childhood blood levels on different lead exposure variables. It is recommended that pilot studies be performed more routinely to estimate the magnitude of the variance ratios of exposure variables of interest in studies in environmental epidemiology.