Occupational aluminum exposure: evidence in support of its neurobehavioral impact

Aluminum is a metal with known neurotoxic properties which are linked to encephalopathy and neurodegenerative diseases. The objectives of the current meta-analysis study were: (1) to summarize neurobehavioral data obtained by epidemiological studies in occupational settings and (2) to analyze confounding within these data. The meta-analysis was based on estimates of effect sizes. Overall effect sizes were obtained by application of a random effects model. The final sample consisted of nine studies examining 449 exposed and 315 control subjects. The mean urinary aluminum concentrations in the exposed groups ranged from 13 to 133 microg/l. Six neuropsychological tests, which yielded 10 performance variables, were analyzed. Nine overall effect sizes indicated an inferior performance for the exposed group. A significant overall effect size (d(RE)=-0.43) was obtained for the digit symbol test measuring speed-related components of cognitive and motor performance. Moreover, the individual effect sizes obtained for this test suggested an exposure-response relationship. Results obtained from either raw or adjusted mean scores revealed that confounding in the data could not be excluded. The results were compared to studies not included here due to a shortage of required data. Similarities were discussed in terms of sensitivity of the tests for detecting aluminum-related changes in brain function. There was concurring evidence from different studies that urinary Al concentrations below 135 microg/l have an impact on cognitive performance. The significant effect for the digit symbol might be related to its multifaceted character which requires functioning in different components of cognitive and motor performance. This feature could possibly turn the test into a screening instrument for neurobehavioral effects. However, additional studies are necessary to verify and to differentiate the effect of aluminum on cognitive performance. From a neuropsychological perspective, implicit and explicit memory, visuo-spatial and central odor processing should be examined. A measure of verbal intelligence should be included in order to address the influence of confounding. Internationally standardized exposure measures would enhance the comparability of studies.