Chemical Exposures and Human Fertility

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People are exposed, daily, to a vast number of chemicals through food, air and water. Wanted exposures include those chemicals that make up our food and medications. For the most part these chemicals are regarded as safe. However, even with dietary components excessive exposure can be associated with negative health impacts. For example, consumption of larger amounts of vegetables and fruits containing b-carotene (carrots, sweet potatoes, squash, spinach, broccoli, mangos, and cantaloupe) has been linked with golden ovary syndrome and impaired fertility. Thus, although not normally thought to be a source of potential problems, even natural constituents of our diet, if present at high enough concentration or consumed consistently for protracted periods of time, can pose a risk to human reproductive function. 

In recent years natural or complementary (alternative) medicines (black cohosh, Echinacea, evening primrose oil, and pregnancy tea) have emerged as major commercial health products. However, their efficacy in many cases has not been proven nor has their safety been examined experimentally. While use of alternative medicines are generally thought to be safe and pose no threat to human health, studies are starting to appear in the scientific and medical literature that are causing many scientists concern. The isoflavones present in foods such as tofu, soybeans, and chickpeas have been shown to have estrogen like activity that is thought to have beneficial effects for older women entering menopause and to provide protection against breast cancer. 

And, recent studies suggest that exposure to these chemicals at earlier life stages may have adverse health consequences for estrogen dependent diseases later in life such as breast cancer and endometriosis. Scientists are becoming aware that just because a chemical occurs naturally in the environment, it cannot be taken as an endorsement of its safety. For example, copper has long been known to interfere with fertilization. The metal lead has been linked to decreased testosterone levels in exposed men, impotence, decreased semen quality and spontaneous abortion. Therefore, even with naturally occurring substances and health products caution must be exercised. If uncertainty remains, then consulting a physician, pharmacist or registered dietician may be helpful.

Exposure to tobacco smoke represents a unique set of circum-stances since exposure can be both intentional (smoker) and unintentional (second hand smoke). Epidemiological studies have demonstrated that female smokers enter menopause approximately two years ahead of non-smokers. In men that smoke there is a greater risk of sexual dysfunction and decreased semen quality. Tobacco smoke is composed of a vast number of chemicals and only a few have been studied for their effects on the reproductive system. Animal experiments have shown that some chemicals present in tobacco smoke are toxic to the development of sperm, eggs, and embryos. There is also evidence that these chemicals adversely affect the transport of the egg along the fallopian tube. There-fore, there is abundant evidence that direct exposure to tobacco smoke has a negative impact on human fertility and there is concern that this will extend to second hand smoke exposure as well. 

Unwanted or inadvertent exposures describe exposures to chemicals that contaminate our food, our work environments or, are present without our knowledge in our living environment. We will refer to chemical exposures that include solvents, polyhalogentated aromatic hydrocarbons, pesticides, metals, and plasticizers. Of the human-made chemicals, recent concern has focused on those chemicals that possess estrogen like activity (Table 1). However, we now know that chemicals can also:

• affect reproductive and develop-ment through direct toxicity to cells. 

While these compounds have been associated with adverse health effects in wildlife, fish, and experimental animals, there is much less certainty about their effects in the general human population.

Numerous studies of semen quality have appeared in the literature in recent years but there is no conclusive evidence that semen quality has changed over time. In Canada we have shown that there are some regions where semen quality is lower than for other centers but no studies have been undertaken to explore potential causal relationships. In addition, there is very little information in the literature relating changes in semen quality with direct measurements of exposure to chemical contaminants and thus it is not possible to draw any conclusions about the potential role of environmental contaminants in changes in human semen quality.

There is a general perception that the prevalence of infertile couples has increased in recent years but several studies conducted in the United States and Canada have been unable to show an increase in infertility rates. A single study conducted in New York State has shown that the risk of infertility and endometriosis is greater for couples living in heavily contaminated regions compared to couples living in other regions of the State. Unfortunately, no measurements of chemical contaminants were performed in this study and thus there is no way to know what chemicals may or may not be involved. Although the literature does not support the conclusion that infertility rates are increasing, there is the potential for regional differences that may be related to exposure to environmental contaminants.

Several studies have demonstrated that exposure to environmental toxicants from several different chemical classes are associated with adverse reproductive outcomes (Table 1). Occupational exposure to pesticides has been linked with decreased semen quality, infertility, and spontaneous abortion. The pesticide Dibromochloropropane was convincingly linked to decreased semen quality and infertility in a population of workers exposed to this compound during manufacture. In a Canadian study, exposure to a mixture of pesticides was associated with spontaneous abortion. However, other studies exploring the association between exposure to pesticides and reproductive function have failed to find a positive association. Hence, the effect of pesticides on human reproduction remains controversial. Reasons for failure to detect any relationship may be related to the small sample size of the studies as well as the low frequency of the adverse outcomes being studied. Conclusive results have been demonstrated for occupational exposure to some chemicals where the exposure is well defined.

Despite popular opinion, there is no evidence that there has been any increase in the number of infertile couples in Canada. Continued investigation of the links between exposure to chemical contaminants and new chemicals, and reproductive function will be required to characterize hazards and define health risks associated with these exposures. A small group of Canadian scientists from government and university laboratories continue to study the reproductive effects of human-made and dietary chemicals. Regulatory bodies within Health Canada and Environment Canada review the published literature and where necessary take appropriate steps to restrict or eliminate the use of chemicals with the potential to harm human health.

ABOUT THE AUTHORS: Warren G. Foster, Ph.D. is Director, Reproductive Biology Division & Associate Professor, Department of Obstetrics & Gynecology, McMaster University in Hamilton, Ontario. Mary Louise Beecroft is Research Nurse in the Department of Obstetrics & Gynecology working with Dr. Foster on several projects related to endometriosis and women’s health.