Methylmercury and the health of autochthons in northwest Quebec

We conducted a cross-sectional epidemiologic and clinical study to evaluate the nature, magnitude, and frequency of unfavourable health states among residents of northwest Quebec. Of particular interest were the possible occurrence of Minamata disease and any other neurological abnormalities. We also sought to determine whether industrial or naturally-occurring mercury in the region's environment were causally associated with any disorders that might be detected. A total of 321 subjects were studied in four groups with contrasting exposure to point-source mercury from a chlor-alkali plant in the area. Methylmercury in hair served as the marker of subject's ingestion of any organic mercury (natural or industrial). Ten pre-chosen target variables, seven clinical and three neurophysiological, were assessed in all subjects. Relationships between mercury in hair and each target variable were calculated. We determined whether gradients in target variables corresponded to gradients in potential exposure of the subjects to the aquatic emissions of the chlor-alkali plant. Minamata disease was not found. Diabetes and alcohol-related health problems assessed as important clinically plausible confounders, were common. Differences in frequency or magnitude of clinical, toxicological, and physiological findings among the contrast groups were small and not of clinical or statistical significance. There were definite relationships between mercury in hair and four target variables despite the low values of mercury measured and the mild nature of the health problems observed. This would suggest that verifiable biological threshold levels for clinical effects, if such exist, may be substantially lower among Canadians than those currently adopted as policy for adults elsewhere. A gradient was found only for two of the ten target variables. Clinically meaningful alternate hypotheses for observed health effects emerge in the data and agree with observations in the field made by consulting clinical experts.