Abstract Methylmercury is a neurotoxicant that bio-accumulates in the aquatic food chain and is present in all fish. Little is known about the effects of long-term low-dose exposure to methylmercury in adults. The aim of this study was to determine whether a dose–response relationship exists between long-term methylmercury exposure and neurological abnormalities in aboriginal Quebec Cree adults for whom fish is a dietary staple. We re-analysed data from a 1977 cross-sectional study conducted by Kofman and collaborators on a group of Quebec Cree individuals claiming ill health from local fish consumption. In the original 1977 study, 306 adult participants aged 18–82 years were assessed for methylmercury exposure. Tremor and other neurologic outcomes were assessed with a clinical examination. The investigators did not find clinical evidence of methylmercury intoxication based on an analysis of covariance. We used ordinal regression to obtain odds ratios for the relationship between total hair mercury levels and neurologic abnormalities. Hair mercury concentrations ranged from 0.5 to 46 ppm (parts per million). A 6 ppm increase in hair mercury was associated with increasing levels of tremor (OR, 2.22; 95%CI, 1.15–4.26) in adults under 40 years of age. There was no association with nine other outcomes considered, nor with tremor among older persons. Odds ratios were not influenced by gender, smoking, alcohol use, or co-morbidity.
Conclusion: Dose-dependent effects of methylmercury on tremor may occur below the commonly accepted 50 ppm threshold, particularly in young adults. These effects may be detectable by clinical examination. However, the results should be interpreted with caution given that alcohol use was probably under-reported and that multiple outcomes were studied.