Background: Although prenatal methylmercury exposure has been linked to poorer intellectual function in several studies, data from two major prospective, longitudinal studies yielded contradictory results. Associations with cognitive deficits were reported in a Faroe Islands cohort, but few were found in a study in the Seychelles Islands. It has been suggested that co-exposure to another contaminant, polychlorinated biphenyls (PCBs), may be responsible for the positive findings in the former study and that co-exposure to nutrients in methylmercury-contaminated fish may have obscured and/or protected against adverse effects in the latter.

Objectives: We aimed to determine the degree to which co-exposure to PCBs may account for the adverse effects of methylmercury and the degree to which co-exposure to docosahexaenoic acid (DHA) may obscure these effects in a sample of Inuit children in Arctic Québec.

Methods: IQ was estimated in 282 school-age children from whom umbilical cord blood samples had been obtained and analyzed for mercury and other environmental exposures.

Results: Prenatal mercury exposure was related to poorer estimated IQ after adjustment for potential confounding variables. The entry of DHA into the model significantly strengthened the association with mercury, supporting the hypothesis that beneficial effects from DHA intake can obscure adverse effects of mercury exposure. Children with cord mercury ≥ 7.5 μg/L were four times as likely to have an IQ score < 80, the clinical cut-off for borderline intellectual disability. Co-exposure to PCBs did not alter the association of mercury with IQ.

Conclusions: To our knowledge, this is the first study to document an association of prenatal mercury exposure with poorer performance on a school-age assessment of IQ, a measure whose relevance for occupational success in adulthood is well established. This association was seen at levels in the range within which many U.S. children of Asian-American background are exposed.

Citation: Jacobson JL, Muckle G, Ayotte P, Dewailly É, Jacobson SW. 2015. Relation of prenatal methylmercury exposure from environmental sources to childhood IQ. Environ Health Perspect 123:827–833;  http://dx.doi.org/10.1289/ehp.1408554

Address correspondence to J.L. Jacobson, Department of Psychiatry and Behavioral Neurosciences, Wayne State University School of Medicine, 3901 Chrysler Dr., Suite 2-C, Detroit, MI 48201 USA. Telephone: (313) 993-5454. E-mail: joseph.jacobson@wayne.edu .

We thank R. Sun, L. Roy, J. Varin, the late A. Puv, B. Tuttle, and J. Gagnon for their contributions to data collection; and N. Dodge and J. Gagnon, for data management. This paper is dedicated to the memory of Éric Dewailly, an internationally renowned leader in Arctic public health, who played a major role in inspiring this research and in bringing it to fruition.

This work was supported by grants from the National Institute of Environmental Health Sciences/National Institutes of Health (R01-ES007902; J.L.J.); the Northern Contaminants Program, Indian and Northern Affairs Canada (G.M., P.A., and É.D.); and the Joseph Young, Sr., Fund from the State of Michigan (S.W.J.).

The authors declare they have no actual or potential competing financial interests.

Received: 14 April 2014 Accepted: 5 March 2015 Advance Publication: 10 March 2015 Final Publication: 1 August 2015

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