Effects of Prenatal Exposure to Pollutants on Children's Development: Additional Issues
Environ Health Perspect. doi:10.1289/ehp.11763 available via http://dx.doi.org [Online 25 September 2008]
Referencing: Effects of Prenatal Exposure to Coal-Burning Pollutants on Children's Development in China
Tang et al. (2008) made a significant contribution to understanding the effects of prenatal exposure to coal-burning pollutants:
Decrements in DQs [developmental quotients, measured by the Gesell Developmental Schedules] were significantly associated with cord blood levels of PAH–DNA adducts and lead, but not mercury.
Recent developments compel us to consider Hg sources and attenuating factors in neurodevelopment related to early human exposure. My comments are specifically directed to breast-feeding (neurodevelopment modulator) and uncontrolled sources of Hg: ethylmercury (EtHg) in thimerosal-containing vaccines (TCV), and methylmercury (MeHg) consumed in rice—not fish. Breast-feeding is essential to promote or prime neonatal neurodevelopment, and China is among the countries that use TCV; therefore, controlling for these variables is important in neurodevelopmental studies (Dórea 2007a).
Tang et al. (2008) discussed the literature showing that prenatal Hg exposure is related to adverse neurodevelopmental outcomes at 2 years of age. During the postnatal period, the central nervous system is still vulnerable to Hg exposure; therefore, additional early exposure to Hg may be difficult to disentangle from prenatal events. Newborns in China are immunized with TCVs carrying concentrations of Hg ranging from 12.5 to 17.5 g Hg/dose (Dórea 2008). Furthermore, TCVs, such as the hepatitis B vaccine, are given immediately after birth. Also, some mothers could use products containing thimerosal during pregnancy (e.g., Rh-negative mothers taking anti-RhoD immune globulins). Considering that Tang et al. (2008) reported a 70-day range in gestational age for their cohort, it is reasonable to speculate that if TCV could be taken during the first 10 weeks postnally, EtHg exposure should be normalized. We were not informed of the immunization schedule (or maternal exposure to thimerosal products) of this cohort, but it is possible that in the time interval of gestational-age variation (10 weeks), there would be opportunity for five shots of TCV (Dorea 2007a). Considering the reported 70-day interval of gestational age, we should expect an even wider range of Hg exposure (on a body mass basis) due to variation in birth weight and respective rate of weight gain.
The effects of TCV-EtHg exposure on neurodevelopment are controversial. The most recent epidemiologic studies (Thompson et al. 2007; Young et al. 2008) exemplify current uncertainties related to the U.S. Federal Court compensation claimed on adverse effects triggered by TCVs (Offit et al. 2008). Although the statistical analysis of Tang et al. (2008) was well designed for prenatal events, perinatal TCV-EtHg exposure not evaluated by cord blood measurements could not account for effects (albeit transient) on neurodevelopment at 2 years of age.
Studies that measured neurodevelopmental outcomes as a result of prenatal exposure to neurotoxic substances have shown that breast-feeding, in most cases, can counteract some of the adverse effects (Dórea 2007b); compared with formula feeding, children had better neurobehavioral scores due to prenatal exposure to several classes of environmental pollutants. Because breast-feeding is an important modifier of neurodevelopmental outcome, not controlling for its duration could be a limitation in the Gesell Developmental score (GDS) outcomes related to Hg. Indeed, using principal component analysis, we have found effects of prenatal and postnatal Hg from both EtHg (from vaccines) and MeHg (fish consumption) in exclusively breast-fed children that were also evaluated by GDS (Marques et al. 2008). Tang et al. (2008) showed that cord blood Hg was three times lower than that reported for the Faroe Island whale-eaters, thus attributing the 7.0 µg Hg/L to low fish consumption (only nine mothers consumed fish or shell fish); compared with the high concentrations of Hg in whale meat, the Hg levels in these non–fish-eating mothers are relatively high. In this context, it should be noted that recent studies have indicated that rice can significantly contribute to MeHg exposure in China (Feng et al. 2008; Qiu et al. 2008). A post hoc assessment of these issues can enrich Tang et al.'s study.
The author declares he has no competing financial interests.
José G. Dórea
Faculty of Health Sciences
Universidade de Brasilia
Brasilia, Brazil
E-mail:
dorea@rudah.com.br
References
Dórea JG. 2007a. Exposure to mercury during the first six months via human milk and vaccines: modifying risk factors. Am J Perinatol 24:387–400.
Dórea JG. 2007b. Maternal smoking and infant feeding: breastfeeding is better and safer. Matern Child Health J 11:287–291.
Dórea JG. 2008. Early mercury exposure (with ethylmercury) could include 3-day olds: is that the case in China? Environ Res 106:420–422.
Feng X, Li P, Qiu G, Wang S, Li G, Shang L, et al. 2008. Human exposure to methylmercury through rice intake in mercury mining areas, Guizhou Province, China. Environ Sci Technol 42:326–332.
Marques RC, Bernardi JVE, Dórea JG, Bastos WR, Malm O. 2008. Principal component analysis and discrimination of variables associated with pre- and post-natal exposure to mercury. Int J Hyg Environ Health 211: 606–614.
Offit PA. 2008. Vaccines and autism revisited—the Hannah Poling case. N Engl J Med 358: 2089–2091.
Qiu G, Feng X, Li P, Wang S, Li G, Shang L, et al. 2008. Methylmercury accumulation in rice (Oryza sativa L.) grown at abandoned mercury mines in Guizhou, China. J Agric Food Chem 56:2465–2468.
Tang D, Li TY, Liu JJ, Zhou ZJ, Yuan T, Chen YH, et al. 2008. Effects of prenatal exposure to coal-burning pollutants on children's development in China. Environ Health Perspect 116:674–679.
Thompson WW, Price C, Goodson B, Shay DK, Benson P, Hinrichsen VL, et al. 2007. Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years. N Engl J Med 357:1281–1292.
Young HA, Geier DA, Geier MR. 2008. Thimerosal exposure in infants and neurodevelopmental disorders: an assessment of computerized medical records in the Vaccine Safety Datalink. J Neurol Sci 271:110–118.
Effects of Prenatal Exposure to Pollutants on Children's Development: Tang and Perera Respond
Environ Health Perspect. doi:10.1289/ehp.11763R available via http://dx.doi.org [Online 25 September 2008]
The study we reported in our article "Effects of Prenatal Exposure to Coal-burning Pollutants on Children's Development in China" (Tang et al. 2008) was conducted in the Tongliang County of Chongqing City, an area in western China. In his letter, Dórea suggests that the children who participated in our study may have been exposed to thimerosal in vaccinations. Because the actual levels of thimerosal used in each vaccine are a trade secret, we were not able to directly control for possible exposure to thimerosal in our analysis. We note that in the United States, all routinely recommended vaccines for U.S. infants are available only as thimerosal-free formulations or contain only trace amounts of thimerosal. We do not know whether that has been the case for vaccines used in China. On the other hand, the children's vaccination schedule is well enforced in China, so any thimerosal exposure would have been similar among all the children who participated in our study. Therefore, this exposure would not have been likely to confound the observed relationship between Gesell Developmental scores (GDS) and PAH–DNA adducts and lead concentrations in newborn umbilical cord blood.
We did not observe a significant adverse impact of the Hg levels in cord blood on neurodevelopment at 2 years of age. The main concern for mercury exposure and neurodevelopment concerns prenatal exposures to methylmercury in fish consumed by pregnant women, and the consumption of fish is low in Tongliang. Reports have shown high Hg levels in rice from a mining area of Guizhou, a province located in southwestern China (Qiu et al. 2008; Feng et al. 2008). The Hg levels in the rice from other areas in China did not seem to exceed normal levels (Zhang and Wang 2007). Our study area, Tongliang County, is not within the Hg mining areas of Guizhou Province; thus, it is unlikely that the consumption of rice could expose the study sample to notable Hg levels.
In reference to Dórea's suggestion to account for breast-feeding, we did incorporate a breast-feeding variable in our initial analysis. However, because the effect of breast-feeding was not generally statistically significant, the final model did not include a breastfeeding variable.
The authors declare they have no competing financial interests.
Deliang Tang
Department of Environmental Health Sciences
Columbia University
New York, New York
E-mail:
dt14@columbia.edu
Frederica P. Perera
Columbia Center for Children's Environmental Health
Columbia University
New York, New York
References
Feng X, Li P, Qiu G, Wang S, Li G, Shang L, et al. 2008. Human exposure to methylmercury through rice intake in mercury mining areas, Guizhou province, China. Environ Sci Technol 42:326–323.
Qiu G, Feng X, Li P, Wang S, Li G, Shang L, et al. 2008. Methylmercury accumulation in rice (Oryza sativa L.) grown at abandoned mercury mines in Guizhou, China. J Agric Food Chem 56:2465–2468.
Tang D, Li TY, Liu JJ, Zhou ZJ, Yuan T, Chen YH, et al. 2008. Effects of prenatal exposure to coal-burning pollutants on children's development in China. Environ Health Perspect 116:674–679.
Zhang L, Wong MH. 2007. Environmental mercury contamination in China: sources and impacts. Environ Int 33:108–121.