Manganese exposure and cognitive performance: A meta-analytical approach

Occupational manganese exposure is associated with serious health concerns, ultimately leading to an illness called manganism. Competing meta-analytic results were published over a decade ago, ranging from undetectable to serious effects on cognitive performance among working adults. Novel studies and findings about the relationship between occupational manganese and cognitive functions have been proposed since. First of all, a systematic literature search was carried out until October 2022 via multiple electronic databases investigating the relationship between occupational manganese exposure and cognitive functions. Differences between the exposure and control groups in cognitive testing were synthesized by effect size Hedge's g. A random effects model was deployed with a restricted likelihood estimator using Hedges' invariance weighting. Publication bias, p-hacking and exposure-effect relationships were investigated. We included 18 studies with 75 effect sizes comparing n = 888 controls and 1092 exposed participants. After exclusion of outliers, we found significantly lower performances in processing speed, attention, working memory, reaction time, cognitive control and visual attention in workers exposed to manganese. Regression analysis revealed an indication of exposure-effect relationships between manganese exposure and cognitive functioning in exposed workers. We provide results of impaired cognitive functions for working adults exposed to manganese in processing speed, attention, working memory, reaction time and visual attention. Indications of quadratic exposure-effect relationships are discussed. We provide several recommendations for further studies to investigate possible exposure effects in the context of occupational health and safety.