Environmental toxicants and fetal development

Environmental toxins and fetal development is the impact of different toxins from the environment on the development of the fetus. This article deals with potential adverse effects of environmental toxins on the prenatal development of both the embryo or fetus, as well as pregnancy complications. The human embryo or fetus is relatively susceptible to impact from adverse conditions within the mother's environment. Sub-par fetal conditions often cause various degrees of developmental delays, both physical and mental, for the growing baby. Although some variables do occur as a result of genetic conditions pertaining to the father, a great many are directly brought about from environmental toxins that the mother is exposed to.

Various toxins pose a significant hazard to fetuses during development. A 2011 study found that virtually all US pregnant women carry multiple chemicals, including some banned since the 1970s, in their bodies. Researchers detected polychlorinated biphenyls, organochlorine pesticides, perfluorinated compounds, phenols, polybrominated diphenyl ethers, phthalates, polycyclic aromatic hydrocarbons, perchlorate PBDEs, compounds used as flame retardants, and dichlorodiphenyltrichloroethane (DDT), a pesticide banned in the United States in 1972, in the bodies of 99 to 100 percent of the pregnant women they tested. Bisphenol A (BPA) was identified in 96 percent of the women surveyed. Several of the chemicals were at the same concentrations that have been associated with negative effects in children from other studies and it is thought that exposure to multiple chemicals can have a greater impact than exposure to only one substance.[1]

Effects

Environmental toxins can be described separately by what effects they have, such as structural abnormalities, altered growth, functional deficiencies, congenital neoplasia, or even death for the fetus.[2]

Preterm birth

One in ten US babies is born preterm and about 5% have low birth weight. Preterm birth, defined as birth at less than 37 weeks of gestation, is a major basis of infant mortality throughout childhood. Exposures to environmental toxins such as lead, tobacco smoke, and DDT have been linked with an increased risk for spontaneous abortion, low birth weight, or preterm birth.[3]

Structural congenital abnormality

Further information: Teratology and Congenital abnormality

Toxic substances that are capable of causing structural congenital abnormalities can be termed teratogens.[4] They are agents extrinsic to embryo or fetus which exert deleterious effects leading to increased risk of malformation, carcinogenesis, mutagenesis, altered function, deficient growth or pregnancy wastage.[5] Teratogens are classified in four main categories:

Teratogens affect the fetus by various mechanism including:

Neurodevelopmental disorder

Neuroplastic effects of pollution can give rise to neurodevelopmental disorders.

Many cases of autism are related to particular geographic locations, implying that something in the environment is complementing an at-risk genotype to cause autism in vulnerable individuals. These findings regarding autism are controversial, however, with many researchers believing that increasing rates in certain areas are a consequence of more accurate screening and diagnostic methods, and are not due to any sort of environmental factor.[6]

Individual substances and toxin classes and their effects

Substances which have been found to be particularly harmful are lead (which is stored in the mother's bones), cigarette smoke, mercury (a neurological toxicant consumed through fish), carbon dioxide, and ionizing radiation.[7]

Tobacco smoke

Main article: Smoking and pregnancy

Children exposed to prenatal tobacco smoke may experience a wide range of behavioral, neurological, and physical difficulties.[8] Adverse effects include stillbirth, placental disruption, prematurity, lower mean birth weight, physical birth defects (cleft palate etc.), decrements in lung function, increased risk of infant mortality.[7]

Mercury

Elemental mercury and methylmercury are two forms of mercury that may pose risks of mercury poisoning in pregnancy. Methylmercury, a worldwide contaminant of seafood and freshwater fish, is known to produce adverse nervous system effects, especially during brain development. Eating fish is the main source of mercury exposure in humans and some fish may contain enough mercury to harm the developing nervous system of an embryo or fetus, sometimes leading to learning disabilities.[9] Mercury is present in many types of fish, but it is mostly found in certain large fish. One well-documented case of widespread mercury ingestion and subsequent fetal development complication took place in the 1950’s in Minimata Bay, Japan. Used by a nearby industrial plant in the manufacture of plastics, methyl mercury was discharged into the waters of Minimata Bay, where it went on to be ingested regularly by many villagers who used the fish living in the bay as a dietary staple. Soon, many of the inhabitants who had been consuming the mercury-laden meat began experiencing negative effects from ingesting the toxin; however, the mercury especially impacted pregnant women and their fetuses, resulting in a high rate of miscarriage. Surviving infants exposed to mercury in-utero had extremely high rates of physical and mental handicaps, as well as physical abnormalities from exposure in the womb during key stages in fetal physical development.[10] The United States Food and Drug Administration and the Environmental Protection Agency advise pregnant women not to eat swordfish, shark, king mackerel and tilefish and limit consumption of albacore tuna to 6 ounces or less a week.[9]

High mercury levels in newborns in Gaza are theorized to originate from war weaponry.[11]

Mercury exposure in pregnancy may also cause limb defects.[3]

Lead

Adverse effects of lead exposure in pregnancy include miscarriage, low birth weight, neurological delays, anemia, encephalopathy, paralysis, blindness,[3][7]

The developing nervous system of the fetus is particularly vulnerable to lead toxicity. Neurological toxicity is observed in children of exposed women as a result of the ability of lead to cross the placental barrier. A special concern for pregnant women is that some of the bone lead accumulation is released into the blood during pregnancy. Several studies have provided evidence that even low maternal exposures to lead produce intellectual and behavioral deficits in children.[12]

Dioxin

Dioxins and dioxin-like compounds persists in the environment for a long time and are widespread, so all people have some amount of dioxins in the body. Intrauterine exposure to dioxins and dioxin-like compounds have been associated with subtle developmental changes on the fetus. Effects on the child later in life include changes in liver function, thyroid hormone levels, white blood cell levels, and decreased performance in tests of learning and intelligence.[13]

Air pollution

Air pollution can negatively affect a pregnancy resulting in higher rates of preterm births, growth restriction, and heart and lung problems in the infant.[14]

Compounds such as carbon monoxide, sulfur dioxide and nitrogen dioxide all have the potential to cause serious damage when inhaled by an expecting mother.[15] Low birth weight, preterm birth, intrauterine growth retardation, and congenital abnormalities have all been found to be associated with fetal exposure to air pollution.[16] Although pollution can be found virtually everywhere, there are specific sources that have been known to release toxic substances and should be avoided if possible by those who wish to remain relatively free of toxins. These substances include, but are not limited to: steel mills, waste/water treatment plants, sewage incinerators, automotive fabrication plants, oil refineries, and chemical manufacturing plants.[15]

Control of air pollution can be difficult. For example, in Los Angeles, regulations have been made to control pollution, by putting rules on industrial and vehicle emissions. Improvements have been made to meet these regulations. Despite these improvements, the region still does not meet federal standards for ozone and particulate matter. Approximately 150,000 births occur every year in Los Angeles. Thus, any effects air pollution has on human development in utero are of great concern to those who live in this region.[17]

Pesticides

Pesticides are created for the specific purpose of causing harm (to insects, rodents, and other pests), pesticides have the potential to serious damages to a developing fetus, should they be introduced into the fetal environment. Studies have shown that pesticides, particularly fungicides, have shown up in analyses of infant's cord blood, proving that such toxins are indeed transferred into the baby's body.[18] Overall, the two pesticides most frequently detected in cord blood are diethyltolaumide ( a commonly used repellant) and vinclozolin (a fungicide).[18] Although pesticide toxicity is not as frequently mentioned as some of the other methods of environmental toxicity, such as air pollution, contamination can occur at any time from merely engaging in everyday activities such as walking down a pathway near a contaminated area, or eating foods that have not been washed properly.[18] In 2007 alone, 1.1 billion pounds of pesticides were found present in the environment, causing pesticide exposure to gain notoriety as a new cause of caution to those wishing to preserve their health.[18]

Other

Avoiding relevant environmental toxins in pregnancy

The American College of Nurse-Midwives recommends the following precautions to minimize exposure to relevant environmental toxins in pregnancy:[20]

Role of the placenta

The healthy placenta is a semipermeable membrane that does form a barrier for most pathogens and for certain xenobiotic substances. However, it is by design an imperfect barrier since it must transport substances required for growth and development. Placental transport can be by passive diffusion for smaller molecules that are lipid soluble or by active transport for substances that are larger and/or electrically charged. Some toxic chemicals may be actively transported. The dose of a substance received by the fetus is determined by the amount of the substance transported across the placenta as well as the rate of metabolism and elimination of the substance. As the fetus has an immature metabolism, it is unable to detoxify substances very efficiently; and as the placenta plays such an important role in substance exchange between the mother and the fetus, it goes without saying that any toxic substances that the mother is exposed to are transported to the fetus, where they can then affect development. Carbon-dioxide, lead, ethanol (alcohol), and cigarette smoke in particular are all substances that have a high likelihood of placental transferral.[7]

Identifying potential hazards for fetal development requires a basis of scientific information. In 2004, Brent proposed a set of criteria for identifying causes of congenital malformations that also are applicable to developmental toxicity in general. Those criteria are:

See also

References

  1. ↑ Woodruff, T. J.; Zota, A. R.; Schwartz, J. M. (2011). "Environmental Chemicals in Pregnant Women in the United States: NHANES 2003–2004". Environmental Health Perspectives 119 (6): 878–885. doi:10.1289/ehp.1002727. PMC 3114826. PMID 21233055.
  2. ↑ Pohl, Hana R.; Smith-Simon, Cassandra; Hicks, Heraline (1998). "Health Effects Classification and Its Role in the Derivation of Minimal Risk Levels: Developmental Effects". Regulatory Toxicology and Pharmacology 28 (1): 55–60. doi:10.1006/rtph.1998.1232. PMID 9784433.
  3. 1 2 3 Lanphear, Bruce P.; Vorhees, Charles V.; Bellinger, David C. (2005). "Protecting Children from Environmental Toxins". PLoS Medicine 2 (3): e61. doi:10.1371/journal.pmed.0020061. PMC 1069659. PMID 15783252.
  4. ↑ "teratogen". dictionary.com. Retrieved 4 October 2013.
  5. ↑ Daftary, Shirish; Chakravarti, Sudip (2011). Manual of Obstetrics, 3rd Edition. Elsevier. pp. 38-41. ISBN 9788131225561.
  6. ↑ Wing L., Potter D. (2002). "The epidemiology of autistic spectrum disorders: is the prevalence rising?". Mental Retardation and Developmental Disabilities Research Reviews 8: 151–161. doi:10.1002/mrdd.1002.
  7. 1 2 3 4 5 6 7 ATSDR (January 17, 2013). "Principles of Pediatric Environmental Health: How Can Parents' Preconception Exposures and In Utero Exposures Affect a Developing Child?". Centers for Disease Control and Prevention.
  8. ↑ Hackshaw, A; Rodeck, C; Boniface, S (Sep–Oct 2011). "Maternal smoking in pregnancy and birth defects: a systematic review based on 173 687 malformed cases and 11.7 million controls.". Human Reproduction Update 17 (5): 589–604. doi:10.1093/humupd/dmr022. PMC 3156888. PMID 21747128.
  9. 1 2 Abelsohn, A; Vanderlinden, LD; Scott, F; Archbold, JA; Brown, TL (January 2011). "Healthy fish consumption and reduced mercury exposure: counseling women in their reproductive years.". Canadian Family Physician 57 (1): 26–30. PMC 3024155. PMID 21322285.
  10. ↑ Burt Susan D (1986). "Mercury Toxicity, An Overview". AAOHN Journal 34 (11): 543–546.
  11. ↑ Manduca, Paola, Awny Naim, and Simona Signoriello. "Specific Association of Teratogen and Toxicant Metals in Hair of Newborns with Congenital Birth Defects or Developmentally Premature Birth in a Cohort of Couples with Documented Parental Exposure to Military Attacks: Observational Study at Al Shifa Hospital, Gaza, Palestine." International Journal of Environmental Research and Public Health. N.p., 14 May 2014. Web. 25 July 2014. <http://www.mdpi.com/journal/ijerph>.
  12. ↑ "Chapter 1, Lead-based Paint Hazards, 98–112". Cdc.gov. Retrieved 25 November 2011.
  13. ↑ Facts about Dioxins. from Minnesota Department of Health. Updated October 2006
  14. ↑ Backes, CH; Nelin, T; Gorr, MW; Wold, LE (Jan 10, 2013). "Early life exposure to air pollution: how bad is it?". Toxicology letters 216 (1): 47–53. doi:10.1016/j.toxlet.2012.11.007. PMC 3527658. PMID 23164674.
  15. 1 2 Le, Hien Q.; Batterman, Stuart A.; Wirth, Julia J.; Wahl, Robert L.; Hoggatt, Katherine J.; Sadeghnejad, Alireza; Hultin, Mary Lee; Depa, Michael (2012). "Air pollutant exposure and preterm and term small-for-gestational-age births in Detroit, Michigan: Long-term trends and associations". Environment International 44: 7–17. doi:10.1016/j.envint.2012.01.003. PMID 22314199.
  16. ↑ Minguillón, M.C.; Schembari, A.; Triguero-Mas, M.; de Nazelle, A.; Dadvand, P.; Figueras, F.; Salvado, J.A.; Grimalt, J.O.; Nieuwenhuijsen, M.; Querol, X. (2012). "Source apportionment of indoor, outdoor and personal PM2.5 exposure of pregnant women in Barcelona, Spain". Atmospheric Environment 59: 426–36. Bibcode:2012AtmEn..59..426M. doi:10.1016/j.atmosenv.2012.04.052.
  17. ↑ Ritz, Beate; Wilhelm, Michelle (2008). "Air Pollution Impacts on Infants and Children". Southern California Environmental Report Card. UCLA Institute of the Environment and Sustainability.
  18. 1 2 3 4 Wickerham, Erin L.; Lozoff, Betsy; Shao, Jie; Kaciroti, Niko; Xia, Yankai; Meeker, John D. (2012). "Reduced birth weight in relation to pesticide mixtures detected in cord blood of full-term infants". Environment International 47: 80–5. doi:10.1016/j.envint.2012.06.007. PMC 3410737. PMID 22796478.
  19. ↑ Lazarus, J. H. "Controlled Antenatal Thyroid Screening study". doi:10.1186/ISRCTN46178175.. Also described in Medscape article: Perchlorate Levels in Pregnancy Linked to Low Childhood IQ, by Nancy A. Melville, October 22, 2013
  20. ↑ Environmental Hazards During Pregnancy Volume 51, No. 1, January/February 2006.
  21. ↑ Brent, Robert L. (2004). "Environmental causes of human congenital malformations: The pediatrician's role in dealing with these complex clinical problems caused by a multiplicity of environmental and genetic factors". Pediatrics 113 (4 Suppl): 957–68. PMID 15060188.

Further reading

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