A Meta-Analysis of the Neuropsychological Effects of Occupational Exposure to Manganese

ABSTRACT

This article reports a meta-analysis of 25 samples in 20 peer-reviewed published neuropsychological studies of the cognitive, psychological, motor, and sensory/perceptual effects of exposure to manganese. These studies included 1,410 exposed participants and 1,322 controls, for a total N = 2,732. Studies were excluded from this analysis if they were unpublished, had uncodeable data, were based on fewer than four participants, failed to have a comparison group, or reported on manganese effects other than cognitive or sensory/motor (e.g., liver functioning). Because the independent variables defining manganese exposure varied across studies, effect sizes were calculated for exposed versus non-exposed workers. Dose–response relations were considered for measures of manganese levels in air/dust (84% of studies reported), blood (MnB; 76% reported), urine (MnU; 52% reported), and hair samples (4% reported). Level of exposure was also estimated by reported years of exposure (M = 13.1 years). Cohen's d statistic yielded a statistically significant weighted mean effect size of − .17, p < .0001 for manganese exposure. However, an effect this small is typically undetectable when evaluating individuals because it is smaller (about 1/6 SD) than the confidence intervals of most neuropsychological measures. Because the effect is so slight and the overlap so great between exposed and unexposed participants (87%), the error rate would exceed the hit rate if causal conclusions were rendered for occupational exposure to manganese as the source of an individual's cognitive, sensory, or motor impairments based on neuropsychological testing or symptom reports.

Acknowledgments

Dr. Lees-Haley has consulted with attorneys representing current and former manufacturers of welding consumables. Dr. Rohling and Dr. Langhinrichsen-Rohling have not previously consulted with any attorneys regarding welding claims. The preparation of this paper was not funded.

Notes

^a Number of exposed participants in study.

^b Number of control participants in study.

^c Number of dependent variables in study.

^d Cohen's d for unweighted effect size. The statistics listed are based on Cohen's (1988) recommendation that the beta-to-alpha ratio equals 4.0, which results in a beta of .20 when alpha is set at .05. We set beta at .20, used each study's actual sample size and mean-study effect size, and generated the t and p values listed in the Table 2.

^e T values.

^f Probability of difference in means based on t test. Also see footnote d above.

^g Note: A negative effect size means the exposed group scored lower than controls or “worse” than controls.

^a Type of moderator variable.

^b Cohen's d for weighted effect size.

^c Standard error of estimate of Cohen's d.

^d Number of studies used to calculate effect size.

^e Lower limit of the 95% confidence interval for the effect size.

^f Upper limit of the 95% confidence interval for the effect size.

^g One sample z statistic (weighted effect size/standard error of the mean).

^h Probability that z is significantly different than zero.

ⁱ Q statistic—a test of homogeneity of variance, distributed about the X ² -distribution.

^j Probability of heterogeneity in variances.

^a Type of dependent variable.

^b Cohen's d for weighted effect size.

^c Standard error of estimate of Cohen's d.

^d Number of studies used to calculate effect size.

^e Lower limit of the 95% confidence interval for the effect size.

^f Upper limit of the 95% confidence interval for the effect size.

^g One sample z statistic (weighted effect size/standard error of the mean).

^h Probability that z is significantly different than zero.

ⁱ Q statistic—a test of homogeneity of variance, distributed about the X ² distribution.

^j Probability of heterogeneity in variances.

[Note: References with asterisks indicates studies included in the meta-analystic review.]