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15 July, 2019 00:00 00 AM

Reproductive health and environmental exposure

Reproductive health and 
environmental exposure

The influence of the environment on human reproduction is a concern in all countries and anxiety has been increased following disasters such as those which occurred at Chernobyl and Bhopal with their recorded and potential adverse effects. Reproductive health and the environment focus on exposures to environmental contaminants during critical periods of human development.

These periods are directly related to reproductive health throughout the life course, including the period before conception, at conception, fertility, pregnancy, child and adolescent development, and adult health. Exposures to different environmental contaminants may influence reproductive health status through the process of epigenetics. Environmental toxicants may potentially induce effects in human reproductive processes. This article addresses the environmental exposures and the relationship to reproductive health.

Reproductive health

The WHO defines reproductive health ‘as a state of complete physical, mental and social well-being, and not merely the absence of reproductive disease or infirmity.’ Reproductive health involves all of the reproductive processes, functions and systems at all stages of human life. This definition implies that people are able to have a satisfying and safe sex life and that they have the capability to reproduce and the freedom to decide if, when and how often to do so. Furthermore, men and women should have access to appropriate health care services that will enable women to go safely through pregnancy and childbirth, as well as to provide couples with the best chance of having a healthy infant.

Reproductive health is a universal concern, but is of special importance for women particularly during the reproductive years. However, men also demand specific reproductive health needs and have particular responsibilities in terms of women's reproductive health because of their decision-making powers in some reproductive health matters. Reproductive health is a fundamental component of an individual’s overall health status and a central determinant of quality of life.

The biological process of reproduction involves:

Production of healthy germ cells

Conception

Viable conceptus (embryo)

Growth & development of fetus in favourable maternal environment

Successful delivery of baby

Growth and development of baby into healthy child and a healthy adult and parent!

Any environmental factor that affects one or more of these key stages can result in reproductive failure.

Exposure Pathways of environmental pollutants

Some environmental toxicants, such as pesticides, are intentionally released into the environment. Others, however, are released unintentionally during manufacturing, use, and disposal. A few chemicals are created unintentionally as by-products of industrial processes. This is true for a group of chemicals known as "dioxin-like" pollutants.

Environmental toxicants that may exert adverse effects on reproductive health are present in media such as the water, air, soil, dust, food, and consumer products. Humans are exposed to these contaminants in the home, community, school, or workplace. To potentially cause harm, a toxicant must come into contact with an individual and enter the body, a step referred to as biologic uptake. Biologic uptake is the moment at which exposure occurs.

Toxicants enter the body in one or more of three ways: inhalation, ingestion, or dermal absorption through the skin. Toxicants are then distributed to tissues an organs and subject to metabolism. Toxicants, or their metabolites, travel to target organs, such as the thyroid, ovaries, or testes, where they exert biological effects. Certain toxicants are stored in the body for long periods of time in muscle, bones or adipose tissue. On a daily basis, humans can be exposed a mixture of environmental contaminants that are found in the air, water, and food. Certain chemicals can have a greater adverse effect when other chemicals are present in the body.

Figure 1. Illustrates the links between ambient environmental media and exposure pathways of pollutants

Environmental exposure and reproductive health effect

The reproductive process is particularly vulnerable to adverse environmental conditions including chemical pollutants and physical hazards such as irradiation. They have direct effects resulting in impaired fertility, high rates of abortions, and abnormal pregnancies. Several chemicals, compounds (both synthetic and organic), metals, and other environmental toxicants have been associated with adverse human health effects.

Significant scientific concerns over the potential impact of these environmental hazards on reproductive health have increased research and public debate on this issue. For instance, evidence is arising on relationships between spontaneous abortion as well as reduction of anogenital distance and exposure to dichlorodiphenyltrichloroethane (DDT) during pregnancy.

There are several mechanisms of action that environmental contaminants may have within the human body. However, it is only recently that research has discovered these pathways. Thus, perhaps more mechanisms exist of which we are currently unaware. An environmental contaminant acting directly on gene expression would alter hormone function and influence changes in reproductive processes and systems. This could either increase or decrease levels of endogenous hormones within the body.

Neuroendocrine effects could occur by nervous system monitoring of the environment and neuronal signaling to the endocrine system. The endocrine system would then alter hormonal function in response. The epigenetic route includes the alteration in gene expression by environmental factors without a direct change in DNA sequence.

It is important to note that epigenetic changes may sometimes confer developmental advantages, enabling the growing organism to modify development of organs and systems in response to downstream requirements.

The genetic mechanism of action, however, directly changes the DNA sequence. This may include mutations of the DNA in the female egg cell, the male sperm cell, or in the developing fetus. Any of these direct genetic changes may impact reproductive processes. Finally, systemic toxicity indicates that an environmental exposure may result in widespread effects on many systems.

The first way that an environmental agent can affect normal female reproductive function is through direct gene expression. Direct gene expression means that an environmental contaminant, once it enters the human body, will directly change the normal function of naturally occurring human hormones.

This environmental contaminant will change normal hormonal functioning by acting directly on the gene responsible for this process. For example, a specific environmental toxin may enter the body, mimic a naturally occurring hormone, like estrogen, and bind to a cell’s receptor for estrogen. This binding process may directly change the normal hormonal functioning of a specific system and lead to augmentation in gene expression.

In addition, an environmental contaminant may directly alter gene expression that regulates hormone production or secretion. This action may result in an increase or decrease in the levels of naturally occurring hormones in the body, leading to an imbalance of the endocrine system. Such an imbalance may have significant effects on the proper functioning of the reproductive system.

Some environmental contaminants may alter estrogen, androgen, and thyroid signaling, essential for normal reproductive activity and embryonic development. Xenohormones (a group of either naturally occurring or artificially created compounds showing hormone-like properties) interact with steroid hormones receptors, in particular those for estrogens and androgens.

Xenohormones could mimic estrogen action, antagonize testosterone action, or alter the secretion of follicle- stimulating hormone (FSH) and luteinizing hormone (LH). These actions could have an effect on reproductive health. Many documented incidents of decreased reproductive capacity in wildlife population are strongly associated with exposure to chemicals in the environment. Reproductive disorders in wildlife have included eggshell thinning of birds, widespread population declines, morphologic abnormalities, sex reversal, impaired viability of offspring, altered hormone concentration and changes in socio-sexual behavior.

Endocrine disruptors

The endocrine system is a complex network of hormones that regulates various bodily functions such as growth and development. The endocrine glands include the pituitary, thyroid, adrenal, thymus, pancreas, ovaries, and testes. These glands or organs release carefully-measured levels of hormones into the bloodstream that act as natural chemical messengers to control important processes of the body.

Specific environmental toxicants directly affect the endocrine system. Endocrine disruptors are

exogenous agents that interfere with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body that are responsible for the maintenance of homeostasis, reproduction, development, and/or behavior. Endocrine disruptors can change normal hormone levels, stimulate or halt the production of certain hormones, or change the way hormones move through the body.

An endocrine disruptor is an exogenous substance or mixture that alters the function of the endocrine system and consequently causes adverse health effects in an intact organism, or its progeny, or subpopulations. A potential endocrine disruptor is an exogenous substance or mixture that possesses properties that might be expected to lead to endocrine disruption in an intact organism, or its progeny, or subpopulations.

Key issues in understanding the role and action of endocrine disruptors and potential endocrine

disruptors are the mechanisms of action and consequences of exposure to endocrine disrupting

chemicals, including mixture of various chemical compounds, dose response relationships, latent

effects, and age of exposure.

 Figure 2. Endocrine disruptors (green) disrupt the normal binding process of hormones (orange) to their receptors (purple)

More than 850 chemicals directly impact the nervous system and may cause adverse health effects. This includes some metals, organic solvents, agrochemicals, poly-halogenated aromatic

hydrocarbons, and pharmaceuticals. Some of these environmental contaminants may be endocrine disrupting compounds because the reproductive endocrine system is primarily regulated by the neuroendocrine system.

Endocrine disruptors may activate specific properties in adults and produce transient changes in the nervous system, or, exposure to endocrine disruptors during neural development may induce changes in neurobehavioral function, specifically sex-related behaviours. Also, there is direct toxic action on neuronal cells with effects seen via the immune system, hormonal effects (thyroid) and systemic diseases. Specific chemicals may alter neurotransmitter concentrations, thus influencing neuroendocrine function and eventually reproduction. Some studies have indicated that poly-chlorinated biphenyls (PCBs) may act through this mechanism, but greater research in necessary.

Reproductive disorder

However, it is important to note that female reproductive disorders may also develop during various life phases of the female. Alterations in proper reproductive functioning may be the result of various occurrences and experiences throughout childhood, adolescence, or adulthood.  

While much is known about the female reproductive system, its development, and many causes of specific disorders, the research pertaining to the mechanisms of action for certain pathologies is still largely unknown. However, exposure to environmental contaminants has been proposed in recent years to potentially contribute to female reproductive disorders.

As increasingly more women enter the workforce, they may be exposed to a variety of occupational chemicals and hazards that may lead to adverse health and reproductive effects. There is now abundant evidence that environmental factors may contribute to many of the disease. Some examples of likely environmental impact on women's health include the following:

Among the most widespread and persistent environmental toxicants are chlorinated hydrocarbons (such as dichlorodiphenyltrichloroethane (DDT) and polychlorinated biphenyls), which are known to possess estrogenic potential, i.e., the ability to mimic the biological effects of estrogens. Imbalanced or unopposed estrogen exposure is a leading risk factor for many gynecologic malignancies, as well as benign proliferative disorders such as endometriosis and leiomyoma.

The potential impact of these compounds on hormone-dependent physiological processes such as conception and fetal development, as well as on disease processes such as osteoporosis and cardiovascular disease, demands further exploration.

Developmental toxicants

Developmental toxicants are agents that adversely affect the developing embryo or fetus. Developmental toxicants include methylmercury, lead, ionizing radiations, polychlorinated biphenyls, polycyclic aromatic compounds, other air contaminants, organic solvents, some pesticides, alcohol etc. Some mothers may be exposed to these in the occupational setting.

In addition to highly sensitive windows for morphological abnormalities (birth defects), there are also time windows important for the development of physiological defects and morphological changes at the tissue, cellular and subcellular levels. Postnatal exposures have been examined in detail for only a few environmental agents, including lead, mercury, some pesticides, and radiation.

Developmental exposures may result in health effects observed: prenatally and at birth, such as spontaneous abortion, stillbirth, low birth weight, small size for gestational age, infant mortality, and malformation; in childhood, such as asthma, cancer, neurological and behavioural effects; at puberty, such as alterations in normal development and impaired reproductive capacity; in adults, such as cancer, heart disease, and degenerative neurological and behavioural disorders.

Dichlorodiphenyl dichloroethylene (DDE) exposure through breast milk may alter duration of lactation by acting through the estrogen receptor; DDE exposures prenatally may have potential effects on adolescent growth and sexual maturation. Dichlorodiphenyl trichloroethane (DDT) might be connected to breast cancer; Polybrominated biphenyls (PBBs) and accelerated onset of puberty

To address the concerns about the impact of the environment on reproductive health, there is a need to generate more public awareness and political will. In life, we are all exposed to a combination of environmental risk factors and mixtures of chemicals. We must learn more about low level exposures. To reduce exposure of environmental contaminant following measure can be taken-

Eat fewer processed foods (which contain additives)

Eat organic food (without pesticides and preservatives)

Don't microwave in plastic containers

Use a home filter for tap water.

Use less cosmetics and personal care products

Avoid artificial fragrances

Avoid breathing gasoline fumes

Avoid aerosols

Don't allow smoking in your household.

Rtn. Md. Nakibul Hasan Khan

Assistant Professor

Department of Environmental Science and Engineering

Jatiya Kabi Kazi Nazrul Islam University

Email: hasannakibk@gmail.com

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Editor : M. Shamsur Rahman

Published by the Editor on behalf of Independent Publications Limited at Media Printers, 446/H, Tejgaon I/A, Dhaka-1215.
Editorial, News & Commercial Offices : Beximco Media Complex, 149-150 Tejgaon I/A, Dhaka-1208, Bangladesh. GPO Box No. 934, Dhaka-1000.

Editor : M. Shamsur Rahman
Published by the Editor on behalf of Independent Publications Limited at Media Printers, 446/H, Tejgaon I/A, Dhaka-1215.
Editorial, News & Commercial Offices : Beximco Media Complex, 149-150 Tejgaon I/A, Dhaka-1208, Bangladesh. GPO Box No. 934, Dhaka-1000.

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