Environmental toxicants and fetal development

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Environmental toxicants and fetal development is the impact of different toxic substances from the environment on the development of the fetus. This article deals with potential adverse effects of environmental toxicants 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. Substandard 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.

Contents

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. Among other environmental estrogens, 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 toxicants 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

Toxic substances that are capable of causing structural congenital abnormalities can be termed teratogen s. [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]

Toxicants and their effects

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

Alcohol

Drinking alcohol in pregnancy can result in a range of disorders known as fetal alcohol spectrum disorders. The most severe of these is fetal alcohol syndrome. [8]

Tobacco smoke

Fetal exposure to prenatal tobacco smoke may experience a wide range of behavioral, neurological, and physical difficulties. [9] 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. [10] 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 1950s in Minamata Bay, Japan. Used by a nearby industrial plant in the manufacture of plastics, methyl mercury was discharged into the waters of Minamata 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 intellectual disabilities, as well as physical abnormalities from exposure in the womb during key stages in fetal physical development. [11] 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. [10]

High mercury levels in newborns in Gaza are theorized to originate from war weaponry. [12]

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. [13]

Cadmium

Cadmium is a heavy metal that the United States Environmental Protection Agency classifies as a probable human carcinogen. [14] People are often exposed to cadmium through various industrial and agricultural sources, and diet is considered the primary exposure method. [15] The heavy metal has been shown to cause damage to the kidneys, bones, and neurological systems. [16] Pregnant women are at higher risk of health issues from cadmium due to increased absorption of the metal during pregnancy. [17] Cadmium can also pose health risks to the fetus,  some of which may be lifelong, as it interferes with placental function and fetal development. [18] There has been evidence of changes in birth size associated with high levels of cadmium exposure, particularly in female children. [19] Although studies show that relatively low levels of cadmium exposure can affect pregnancy outcomes, more studies must be done to confirm these effects, establish updated exposure limitation guidelines, and explore ways to decrease exposure, particularly during pregnancy. [20]

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. [21]

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. [22]

Compounds such as carbon monoxide, sulfur dioxide and nitrogen dioxide all have the potential to cause serious damage when inhaled by an expecting mother. [23] Low birth weight, preterm birth, intrauterine growth retardation, and congenital abnormalities have all been found to be associated with fetal exposure to air pollution. [24] 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. [23]

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. [25]

Particulate matter (PM) consist of a mixture of particle pollutants that remain in the air, and vary be region. These particles are very small, ranging from PM10 to PM 2.5, which can easily enter the lungs. Particulate matter has been shown to be associated with acute cardio-respiratory morbidity and mortality. Intrauterine growth has been shown to be affected by particulate matter, leading to unhealthy outcomes for fetal development such as poor or slow fetal growth, and increasing fetal morbidity and mortality. [26] A study from 2012 found that exposures to PM 2.5 differed by race/ethnicity, age, as well as socioeconomic status, leading to certain populations experiencing greater negative health outcomes due to environmental pollution, especially relating to particulate matter. [27]

Pesticides

Pesticides are created for the specific purpose of causing harm (to insects, rodents, and other pests), pesticides have the potential to cause 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 an infant's cord blood, proving that such toxins are indeed transferred into the baby's body. [28] Overall, the two pesticides most frequently detected in cord blood are diethyltoluamide (DEET) and vinclozolin (a fungicide). [28] 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. [28] In 2007 alone, 1.1 billion pounds (500 kt) 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. [28]

A 2013 review of 27 studies on prenatal and early childhood exposures to organophosphate pesticides found all but one showed negative neurodevelopmental outcomes. In the ten studies that assessed prenatal exposure, "cognitive deficits (related to working memory) were found in children at age 7 years, behavioral deficits (related to attention) seen mainly in toddlers, and motor deficits (abnormal reflexes), seen mainly in neonates." [29]

A systematic review of neurodevelopmental effects of prenatal and postnatal organophosphate pesticide exposure was done in 2014. The review found that "Most of the studies evaluating prenatal exposure observed a negative effect on mental development and an increase in attention problems in preschool and school children." [30]

In 2017, a study looked at the possible effects of agricultural pesticides in over 500,000 births in a largely agricultural region of California and compared their findings to birth outcomes in other less agriculturally dominated California areas. Overall, they found that pesticide exposure increased adverse birth outcomes by 5–9%, but only among those mothers exposed to the highest quantities of pesticides. [31]

Benzenes

Benzene exposure in mothers has been linked to fetal brain defects especially neural tube defects. In one study, BTEX (Benzene, toluene, ethylbenzene, xylenes) exposure during the first trimester of pregnancy has been clearly indicating negative association with biparietal brain diameter between 20 and 32 weeks of pregnancy. Women with high exposure to toluene had three to five times the miscarriage rate of those with low exposure, and women with occupational benzene exposure have been shown to have an increased rate of miscarriages. Paternal occupational exposure to toluene and formaldehyde has also been linked to miscarriage in their partners. Normal development is highly controlled by hormones, and disruption by man made chemicals can permanently change the course of development. Ambient ozone has been negatively associated with sperm concentration in men, chemicals associated with UOG operations (e.g., benzene, toluene, formaldehyde, ethylene glycol and ozone) have been associated with negative impacts on semen quality, particularly reduced sperm counts. [32]

A 2011 study found a relationship between Neural Tube Defects and maternal exposure to benzene, a compound associated with natural gas extraction. The study found that mothers living in Texas census tracts with higher ambient benzene levels were more likely to have offspring with neural tube defects, such as spina bifida, than mothers living in areas with lower benzene levels. [33]

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: [35]

Natural gas development

In a rural Colorado study of natural gas development, maternal residence within a 10-mile radius of natural gas wells was found to have a positive association to the prevalence of congenital heart defects (CHDs) and neural tube defects (NTDs). Along with this finding, a small association was found between mean birth weight and the density and proximity to the natural gas wells. Maternal exposure through natural gas wells may come in the form of benzene, solvents, polycyclic aromatic hydrocarbons (PAHs), and other air pollutants such as toluene, nitrogen dioxide, and sulfur dioxide. [36]

In Pennsylvania, unconventional natural gas producing wells increased from zero in 2005 to 3689 in 2013. A 2016 study of 9384 mothers and 10946 neonates in the Geisinger Health System in Pennsylvania found prenatal residential exposure to unconventional natural gas development activity was associated with preterm birth and physician-recorded high-risk pregnancy. [37] In Southwest Pennsylvania, maternal proximity to unconventional gas drilling has been found to be associated with decreased birth weight. It was unclear which route of exposure: air, soil or water could be attributed to the association. Further research and larger studies on this topic are needed. [38]

Endocrine disruptors are compounds that can disrupt the normal development and normal hormone levels in humans. Endocrine-disrupting chemicals (EDCs) can interact with hormone receptors, as well as change hormone concentrations within the body, leading to incorrect hormone responses in the body as well as disrupt normal enzyme functioning. Oil and gas extraction has been known to contribute to EDCs in the environment, largely due to the high risk of ground and surface water contamination that comes with these extractions. In addition to water contamination, oil and gas extraction also lead to higher levels of air pollution, creating another route of exposure for these endocrine disruptors. This problem often goes under-reported, and therefore, the true magnitude of the impact is underestimated. In 2016, a study Archived 2017-05-06 at the Wayback Machine was conducted to assess the need for an endocrine component to health assessments for drilling and extraction of oil and gas in densely populated areas. With the high potential for release of oil and gas chemicals with extraction, specifically chemicals that have been shown to disrupt normal hormone production and function, the authors highly emphasized the need for a component centering around endocrine function and overall health with health assessments, and how this in turn impacts the environment. [39]

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

Related Research Articles

Teratology is the study of abnormalities of physiological development in organisms during their life span. It is a sub-discipline in medical genetics which focuses on the classification of congenital abnormalities in dysmorphology caused by teratogens. Teratogens are substances that may cause non-heritable birth defects via a toxic effect on an embryo or fetus. Defects include malformations, disruptions, deformations, and dysplasia that may cause stunted growth, delayed mental development, or other congenital disorders that lack structural malformations. The related term developmental toxicity includes all manifestations of abnormal development that are caused by environmental insult. The extent to which teratogens will impact an embryo is dependent on several factors, such as how long the embryo has been exposed, the stage of development the embryo was in when exposed, the genetic makeup of the embryo, and the transfer rate of the teratogen.

<span class="mw-page-title-main">Birth defect</span> Condition present at birth regardless of cause

A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can range from mild to severe. Birth defects are divided into two main types: structural disorders in which problems are seen with the shape of a body part and functional disorders in which problems exist with how a body part works. Functional disorders include metabolic and degenerative disorders. Some birth defects include both structural and functional disorders.

<span class="mw-page-title-main">Perfluorooctanesulfonic acid</span> Fluorosurfactant and persistent organic pollutant

Perfluorooctanesulfonic acid (PFOS) is a chemical compound having an eight-carbon fluorocarbon chain and a sulfonic acid functional group and thus a perfluorosulfonic acid. It is an anthropogenic (man-made) fluorosurfactant, now regarded as a global pollutant. PFOS was the key ingredient in Scotchgard, a fabric protector made by 3M, and related stain repellents. The acronym "PFOS" refers to the parent sulfonic acid and to various salts of perfluorooctanesulfonate. These are all colorless or white, water-soluble solids. Although of low acute toxicity, PFOS has attracted much attention for its pervasiveness and environmental impact. It was added to Annex B of the Stockholm Convention on Persistent Organic Pollutants in May 2009.

<span class="mw-page-title-main">Fetal alcohol spectrum disorder</span> Group of conditions resulting from maternal alcohol consumption during pregnancy

Fetal alcohol spectrum disorders (FASDs) are a group of conditions that can occur in a person who is exposed to alcohol during gestation, as a result of their mother drinking alcohol during pregnancy. The several forms of the condition are: fetal alcohol syndrome (FAS), partial fetal alcohol syndrome (pFAS), alcohol-related neurodevelopmental disorder (ARND), and neurobehavioral disorder associated with prenatal alcohol exposure (ND-PAE). Other terms used are fetal alcohol effects (FAE), partial fetal alcohol effects (PFAE), alcohol-related birth defects (ARBD), and static encephalopathy, but these terms have fallen out of favor and are no longer considered part of the spectrum.

<span class="mw-page-title-main">Persistent organic pollutant</span> Organic compounds that are resistant to environmental degradation

Persistent organic pollutants (POPs) are organic compounds that are resistant to degradation through chemical, biological, and photolytic processes. They are toxic and adversely affect human health and the environment around the world. Because they can be transported by wind and water, most POPs generated in one country can and do affect people and wildlife far from where they are used and released.

<span class="mw-page-title-main">Low birth weight</span>

Low birth weight (LBW) is defined by the World Health Organization as a birth weight of an infant of 2,499 g or less, regardless of gestational age. Infants born with LBW have added health risks which require close management, often in a neonatal intensive care unit (NICU). They are also at increased risk for long-term health conditions which require follow-up over time.

Prenatal development includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal development until birth.

<span class="mw-page-title-main">Soil contamination</span> Pollution of land by human-made chemicals or other alteration

Soil contamination, soil pollution, or land pollution as a part of land degradation is caused by the presence of xenobiotic (human-made) chemicals or other alteration in the natural soil environment. It is typically caused by industrial activity, agricultural chemicals or improper disposal of waste. The most common chemicals involved are petroleum hydrocarbons, polynuclear aromatic hydrocarbons, solvents, pesticides, lead, and other heavy metals. Contamination is correlated with the degree of industrialization and intensity of chemical substance. The concern over soil contamination stems primarily from health risks, from direct contact with the contaminated soil, vapour from the contaminants, or from secondary contamination of water supplies within and underlying the soil. Mapping of contaminated soil sites and the resulting clean ups are time-consuming and expensive tasks, and require expertise in geology, hydrology, chemistry, computer modelling, and GIS in Environmental Contamination, as well as an appreciation of the history of industrial chemistry.

<span class="mw-page-title-main">Environmental toxicology</span>

Environmental toxicology is a multidisciplinary field of science concerned with the study of the harmful effects of various chemical, biological and physical agents on living organisms. Ecotoxicology is a subdiscipline of environmental toxicology concerned with studying the harmful effects of toxicants at the population and ecosystem levels.

Women should speak to their doctor or healthcare professional before starting or stopping any medications while pregnant. Non-essential drugs and medications should be avoided while pregnant. Tobacco, alcohol, marijuana, and illicit drug use while pregnant may be dangerous for the unborn baby and may lead to severe health problems and/or birth defects. Even small amounts of alcohol, tobacco, and marijuana have not been proven to be safe when taken while pregnant. In some cases, for example, if the mother has epilepsy or diabetes, the risk of stopping a medication may be worse than risks associated with taking the medication while pregnant. The mother's healthcare professional will help make these decisions about the safest way to protect the health of both the mother and unborn child. In addition to medications and substances, some dietary supplements are important for a healthy pregnancy, however, others may cause harm to the unborn child.

<span class="mw-page-title-main">Reproductive toxicity</span> A hazard associated with chemical substances

Reproductive toxicity refers to the potential risk from a given chemical, physical or biologic agent to adversely affect both male and female fertility as well as offspring development. Reproductive toxicants may adversely affect sexual function, ovarian failure, fertility as well as causing developmental toxicity in the offspring. Lowered effective fertility related to reproductive toxicity relates to both male and female effects alike and is reflected in decreased sperm counts, semen quality and ovarian failure. Infertility is medically defined as a failure of a couple to conceive over the course of one year of unprotected intercourse. As many as 20% of couples experience infertility. Among men, oligospermia is defined as a paucity of viable spermatozoa in the semen, whereas azoospermia refers to the complete absence of viable spermatozoa in the semen.

Birth injury refers to damage or injury to the child before, during, or just after the birthing process. "Birth trauma" refers specifically to mechanical damage sustained during delivery.

<span class="mw-page-title-main">Health effects of pesticides</span> How pesticides affect human health

Health effects of pesticides may be acute or delayed in those who are exposed. Acute effects can include pesticide poisoning, which may be a medical emergency. Strong evidence exists for other, long-term negative health outcomes from pesticide exposure including birth defects, fetal death, neurodevelopmental disorder, cancer, and neurologic illness including Parkinson's disease. Toxicity of pesticides depend on the type of chemical, route of exposure, dosage, and timing of exposure.

Prenatal memory, also called fetal memory, is important for the development of memory in humans. Many factors can impair fetal memory and its functions, primarily maternal actions. There are multiple techniques available not only to demonstrate the existence of fetal memory but to measure it. Fetal memory is vulnerable to certain diseases so much so that exposure can permanently damage the development of the fetus and even terminate the pregnancy by aborting the fetus. Maternal nutrition and the avoidance of drugs, alcohol and other substances during all nine months of pregnancy is important to the development of the fetus and its memory systems. The use of certain substances can entail long-term permanent effects on the fetus that can carry on throughout their lifespan.

<span class="mw-page-title-main">Developmental toxicity</span>

Developmental toxicity is any developmental malformation that is caused by the toxicity of a chemical or pathogen. It is the structural or functional alteration, reversible or irreversible, which interferes with homeostasis, normal growth, differentiation, development or behavior. Developmental toxicity is caused by environmental insult, which includes drugs, alcohol, diet, toxic chemicals, and physical factors.

The health of a mother directly affects the fetus during pregnancy. High levels of vehicle pollution where pregnant women reside can have adverse health effects on fetuses.

Toxic abortion is a medical phenomenon of spontaneous abortion, miscarriage, or stillbirth caused by toxins in the environment of the mother during pregnancy, especially as caused by toxic environmental pollutants, though sometimes reported as caused by naturally occurring plant toxins

The fetal origins hypothesis proposes that the period of gestation has significant impacts on the developmental health and wellbeing outcomes for an individual ranging from infancy to adulthood. The effects of fetal origin are marked by three characteristics: latency, wherein effects may not be apparent until much later in life; persistency, whereby conditions resulting from a fetal effect continue to exist for a given individual; and genetic programming, which describes the 'switching on' of a specific gene due to prenatal environment. Research in the areas of economics, epidemiology, and epigenetics offer support for the hypothesis.

Fetal programming, also known as prenatal programming, is the theory that environmental cues experienced during fetal development play a seminal role in determining health trajectories across the lifespan.

Frederica Perera is an American environmental health scientist and the founder of the Columbia Center for Children's Environmental Health at the Columbia University Mailman School of Public Health. Her research career has focused on identifying and preventing harm to children from prenatal and early childhood exposure to environmental chemicals and pollutants. She is internationally recognized for pioneering the field of molecular epidemiology, incorporating molecular techniques into epidemiological studies to measure biologic doses, preclinical responses and susceptibility to toxic exposure.

References

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