INMA: “Does exposure to mercury influence neurodevelopment of children?”


An INMA study published in the European Journal of Epidemiology explores the trend in exposure to mercury (Hg) between birth and the age of 4 years, and the association between childhood exposure and neuropsychological development in 4-5 year-old children.

Infants and children are highly susceptible to their environment because of specific behaviors that may increase their absorption rates, and immature detoxification systems that diminish their ability to metabolize and eliminate the toxins. Moreover, the brain is known to have a long development time, from the formation of neurons at embryonic stage, to myelinization pursuing into adolescence. Then, both prenatal and postnatal life exposures may alter the neurodevelopment of school-aged children and lead to learning and behavioral disorders.

Methylmercury (MeHg) is a pollutant known to be neurotoxic to humans. It can bioaccumulate in organic tissues. The main source of exposure in humans is through the consumption of seafood, or predatory fishes, such as swordfish, shark, and fresh tuna. Nevertheless, fish is also an important source of omega-3 polyinsaturated fatty acids (PUFAs), iodine, selenium, and vitamins, which are nutrients of a healthy diet. Therefore, there is a concern that MeHg concentrations in children eating regularly fish may compensate the benefits of fish consumption.

In this study, mercury levels were measured in cord blood collected at birth, and in hair samples when children were 5 years old. The neuropsychological development of the children was assessed by trained psychologists at 4–5 years of age by using the McCarthy Scales of Children’s Abilities, which comprises 18 subtests that yield standardized test scores for six cognitive domains: processing of verbal information; numerical abilities; perceptual information processing; short-term retention of information; and the motor scale for fine and gross abilities.

The Investigators found that INMA newborns had relatively high levels of mercury in cord blood. Indeed, 64% of them were above the equivalent of the US Environmental Protection Agency (US-EPA) reference dose, and the 25% of them had levels above the equivalent to the World Health Organization Provisional Tolerable Weekly Intake (WHO-PTWI). However, prenatal exposure to mercury did not seem associated with neurodevelopmental delays in INMA children at 14 months and 4-5 years of age.

In order to study the Hg concentrations trend from birth to age 4, authors applied the hair:blood ratio factor proposed by the WHO Expert Committee to compare the mercury levels in cord blood and hair samples. The authors found that Hg concentrations decreased from birth to age 4, whereas 50% of the children were above the US-EPA recommendations, and 11% of them above the WHO-PWTI reference. Notably, higher levels of mercury at age 4 were associated with higher cognitive scores, in particular the ability of processing verbal information at the same age.

Overall, authors did not find adverse association between postnatal exposure to mercury and any of the cognitive domains assessed in the study. Considering that there is insufficient evidence about the possible neurotoxic effects of postnatal exposure to mercury at moderate doses, they claim for more research in this field.

Referencia: Llop S, Murcia M, Amorós R, Julvez J, Santa-Marina L, Soler-Blasco R, Rebagliato M, Iñiguez C, Aguinagalde X, Iriarte G, Lopez-Espinosa MJ, Andiarena A, Gonzalez L, Vioque J, Sunyer J, Ballester F. Postnatal exposure to mercury and neuropsychological development among preschooler children. Eur J Epidemiol. 2020 Mar;35(3):259-271.