Cumulative exposure to inorganic lead and neurobehavioural test performance in adults: an epidemiological review.

OBJECTIVES To evaluate the current evidence that cumulative exposure to inorganic lead is associated with decreased performance in neurobehavioural tests in adults. METHODS 21 unique studies were reviewed from 28 published manuscripts. An algorithm was developed to determine the usefulness of each study on the basis of exposure assessment, control of confounding variables, methods of subject selection, test conditions, and data analysis. Highest emphasis was placed on the use of cumulative measures of exposure or absorption. RESULTS Only three studies used a measure of cumulative exposure to or absorption of lead, and two others used duration of exposure as a surrogate for cumulative exposure. All other studies used a measure that did not adequately estimate cumulative exposure to lead, most often current blood lead concentration. 20 of the studies controlled for age as a confounding variable, although in several studies the possibility for residual confounding by age remained. 16 studies controlled for intellectual ability before exposure; all of them used educational level for this purpose. Of the five studies that used direct or surrogate measures of cumulative exposure to or absorption of lead, two were thought to be of low usefulness because of inadequate duration of exposure. The three remaining studies found stronger associations of neurobehavioural performance with recent exposure measures than with those of cumulative exposure. CONCLUSION The current scientific literature provides inadequate evidence to conclude whether or not cumulative exposure to or absorption of lead adversely affects performance in neurobehavioural tests in adults. The current evidence is flawed because of inadequate estimation of cumulative exposure to or absorption of lead and inadequate control for age and intellectual ability before exposure.