Effects of Prenatal Exposure to Mercury on Cognitive and Psychomotor Function in One-Year-Old Infants: Epidemiologic Cohort Study in Poland

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Purpose

The aim of the study is to assess the cognitive and psychomotor status of 1-year-old infants whose mothers were exposed to low, but varying, amounts of mercury during pregnancy.

Methods

Mercury levels in cord and maternal blood at delivery were used to assess prenatal environmental exposure to mercury. Bayley Scales of Infant Development were used to assess neurobehavioral health outcomes. The cohort consisted of 233 infants who were born at 33 to 42 weeks of gestation between January 2001 and March 2003 to mothers attending ambulatory prenatal clinics in the first and second trimesters of pregnancy. Enrollment included only nonsmoking women with singleton pregnancies between the ages of 18 and 35 years who were free from chronic diseases.

Results

The geometric mean (GM) for maternal blood mercury level for the group of infants with normal neurocognitive performance was lower (GM = 0.52 μg/L; 95% confidence interval [CI], 0.46–0.58) than that observed in the group with delayed performance (GM = 0.75 μg/L; 95% CI, 0.59–0.94), and this difference was significant (p = 0.010). The GM of cord blood mercury level in the normal group also was lower (GM = 0.85 μg/L; 95% CI, 0.78–0.93) than that observed in the group with delayed performance (GM = 1.05 μg/L; 95% CI, 0.87–1.27), and this difference was of borderline significance (p = 0.070). The relative risk (RR) for delayed performance increased more than threefold (RR = 3.58; 95% CI, 1.40–9.14) if cord blood mercury level was greater than 0.80 μg/L. Risk for delayed performance in the group of infants with greater maternal mercury levels (>0.50 μg/L) also was significantly greater (RR = 2.82; 95% CI, 1.17–6.79) compared with children whose mothers had mercury levels less than 0.50 μg/L.

Conclusions

The results may be of public health importance because delayed psychomotor or mental performance in infants is assumed to be an indicator of later neurocognitive development in children, which may persist into adult life.

Introduction

Mercury is present in the environment in three different forms: organic, metallic, and inorganic. Chemical industry and coal-burning power plants are the major source of metallic and inorganic mercury, which, after being emitted into the air, enter water during precipitation and are converted to methylmercury by microorganisms 1, 2, 3, 4, 5. Methylmercury exposure is usually the result of consumption of contaminated fish foods 6, 7. Other sources of mercury exposure may be ethylmercury (thimerosal, a preservative used in some pediatric vaccines 8, 9), mercury vapor exposure (from accidents or occupations), inorganic mercury (present in mercury-based skin creams and infant teething powders), and metallic mercury from dental amalgams 9, 10, 11, 12, 13. Mercury exposure also can occur from latex paints (14).

Mercury vapor and mercury compounds are absorbed easily by inhalation, digestion, and skin contact. Absorbed mercury compounds rapidly bind to erythrocytes and are carried throughout the body, eventually secreted slowly in urine, feces, saliva, sweat, and breast milk. Both metallic mercury vapor and methylmercury are lipophilic and pass through the blood–brain barrier in adults and children, which may result in deposition in the central nervous system 15, 16, 17, 18, 19, 20, 21, 22, 23. High levels of exposure may lead to a loss of neurons in the brain lobes and such developmental effects as hyperactive reflexes, deafness, blindness, cerebral palsy, mental retardation, and general paralysis 24, 25, 26, 27, 28, 29, 30. Mercury compounds also can pass through the placenta, placing the developing fetus at potential risk 31, 32, 33, 34, 35.

The neurodevelopmental effect of prenatal exposure to mercury compounds in humans was shown first after two episodes of severe environmental mercury contamination. One episode occurred in Iraq, and the other, in Japan 24, 25, 26, 27, 28. Although recent data show that extremely high mercury exposure seldom occurs now, moderate exposure may occur not only in occupational, but also in community and domestic, settings. Because fetuses and infants are very susceptible to neurotoxic effects of mercury and its compounds, prior studies have assessed subclinical effects in children whose mothers' diets include large amounts of fish or marine mammals containing methylmercury and who have blood mercury levels greater than those commonly seen.

Although adverse health effects caused by mercury exposure have been shown clearly in poisoning incidents, implications of lower-level exposures have been controversial. Studies performed on health effects of lower exposure to organic mercury from fish or whale meat consumption produced conflicting results. A cohort of children in the Faeroe Islands was followed up until 7 years of age to document mercury levels and neurobehavioral effects of methylmercury exposure from maternal consumption of pilot whale meat (36). Neuropsychologic tests found pronounced dysfunction in the domains of language, attention, and memory at exposure levels less than what is considered safe. Conversely, a study of mother–child pairs from the ocean-fish–consuming population of the Seychelles Islands found no adverse developmental outcomes associated with prenatal or postnatal methylmercury exposure 29, 37, 38). The divergent data from these studies may result from the different biomarkers of exposure (maternal hair in the Seychelles Islands versus cord blood in the Faeroe Islands) and different health end points used, and this may have some bearing on the statistical results.

The purpose of the present study is to assess cognitive and psychomotor status of 1-year-old infants whose mothers were exposed to low, but varying, amounts of mercury during pregnancy. Whole-blood mercury levels in cord and maternal blood at delivery measured prenatal environmental exposure to mercury, and the Bayley Scales of Infant Development (BSID) were used to assess neurobehavioral health outcomes in 1-year-olds (39).

Section snippets

Study Subjects

The cohort consisted of 233 infants who were born at 33 to 42 weeks of gestation between January 2001 and March 2003 to mothers participating in an ongoing prospective cohort study. Most of the infants (91%) were born after 38 weeks of gestation. The design of this cohort prospective study and population selection were described previously (40). Women who were residents of Krakow, one of the major cities in Poland, and attended ambulatory prenatal clinics in the first and second trimesters of

Results

Table 1 lists characteristics of the study population grouped by results of the BSID-II Motor and Mental Scales. As listed in Table 1, there were no significant differences in demographic characteristics of mothers and newborns in the perinatal period between infants scoring high and low, except that mothers of children in the group with delayed performance were older. The groups did not differ in respect to breast-feeding practices, ETS exposure, or lead levels measured in both maternal and

Discussion

Results of our study show that total mean mercury concentration (GM) in blood measured in pregnant women at delivery was 0.55 μg/L (95% CI, 0.50–0.61 μg/L); in 75% of women, it was less than 1 μg/L, and in 90%, it was not greater than 2 μg/L. In cord blood, mean total blood mercury concentration was 0.88 μg/L (95% CI, 0.81–0.95); most infants (60%) had levels less than 1 μg/L, and only 10% had a mercury level greater than 2 μg/L. Because study subjects were chosen from an urban Polish

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  • Cited by (0)

    The study received funding from grant no. 5 RO1 ES10165 from the National Institute of Environmental Health Services and the Gladys T. and Roland Harriman Foundation.

    This is part of an ongoing comparative longitudinal investigation on the health impact of prenatal exposure to outdoor/indoor air pollution in infants and children being conducted in New York City and Krakow. Principal investigator: F.P.

    Mercury level in blood samples was measured by CDC Environmental Health Laboratory by Robert Jones, PhD and Kathleen Caldwell, PhD.

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