Long-term mercury exposure and cognitive functions in a First Nation community in Northern Ontario, Canada

Prenatal, childhood, and current mercury (Hg) exposure through fish consumption have each been associated with cognitive deficits, but little information exists on the consequences of long-term exposure among adults. Since 1962, Grassy Narrows First Nation has been exposed to Hg from an industrial discharge. Average Hair Hg (HHg) concentrations, initially very high, decreased over time and stabilized in the 1990’s. Montreal Cognitive Assessment (MoCA) test outcomes were analyzed in 85 persons aged 32–75 y (median: 53 y) with respect to retrospective year-based HHg measurements between 1970 and 1997 and current blood Hg. Since the MoCA has not been clinically validated for Indigenous populations, residuals of age- and education-adjusted scores were used (MoCA-r scores). Lower MoCA-r scores were observed among persons in the higher quartile of maximum HHg compared to those in the lower quartile (p = 0.007). Clustering of the test items yielded 3 clusters representing verbal fluency and abstraction, cognitive flexibility and working memory, and visuospatial functioning. To model the evolution of HHg over time, longitudinal mixed effect models (LMEM) were performed with persons with ≥ 10 repeated year-based HHg measurements. Higher long-term past HHg was associated with lower MoCA-r and all cluster scores. No association was observed between MoCA-r or cluster scores and blood Hg, which reflects recent exposure. The findings suggest that legacy exposure can affect cognitive functioning decades later, even when average current concentrations have decreased to below recommended guidelines. Prospective studies could provide information on the rate of decline and the possible future impact of current exposure.