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To study the impact of everyday chemicals on fertility, federal researchers recently spent four years tracking 501 couples as they tried to have children. One of the findings stood out: While men and women were exposed to known toxic chemicals, men seemed much more likely to suffer fertility problems as a result.
The gender gap was particularly wide when it came to phthalates, those ubiquitous compounds used to make plastics more flexible and cosmetic lotions slide on more smoothly. Women who wore cosmetics often had higher levels of phthalates in their bodies, as measured by urinalysis. But only in their male partners did phthalate levels correlate with infertility.
“It’s the males in the study that are driving the effect,” said Germaine Buck Louis, an epidemiologist at the National Institute of Child Health and Human Development and the lead author of the report, published in February in Fertility and Sterility.
Phthalates belong to a group of industrial compounds known as endocrine disrupters because they interfere with the endocrine system, which governs the production and distribution of hormones in the body. The chemicals have been implicated in a range of health problems, including birth defects, cancers and diabetes.
But it is their effect on the human reproductive system that has most worried researchers. A growing body of work over the last two decades suggests that phthalates can rewire the male reproductive system, interfering with the operation of androgenic hormones, such as testosterone, that play key roles in male development. That mechanism, some experts believe, explains findings that link phthalate exposure to changes in everything from testicular development to sperm quality.
The focus on male fertility dates to the early 1990s, when researchers published a paper suggesting chemical exposures could be linked to a steady decline in semen quality. One of the authors, Niels Skakkebaek of the University of Copenhagen, has since suggested that an increase in malformations in male reproductive systems, may be linked to environmental exposure to compounds including endocrine disrupters like phthalates.
More recent studies in the U.S. have also suggested links between phthalate exposure and apparent sperm damage in men. The findings are supported by a host of animal studies, particularly in rats, which have shown that the compounds can interfere with masculinization of young animals and result in odd physical changes to male reproductive tracts.
“They interfere with how testosterone is made,” said Heather Patisaul, a North Carolina State University biology professor who is studying the effect of endocrine-disrupting compounds during puberty. “Anything you can think of that’s testosterone-dependent is likely to be affected.”
Women also have androgenic hormones, but to a lesser degree, and according to some theories, this accounts for the smaller but still observable effects of phthalates on female fertility. (Testosterone, for instance, is part of the cascade of hormones that leads to egg production.)
The Centers for Disease Control and Prevention notes that while studies suggest that phthalate exposure is “widespread in the U.S. population,” it’s difficult to know what those levels are. Health effects from very low levels are still not well understood.
While the "evidence for an effect on male fertility is compelling," said Tracey Woodruff, director of the program on reproductive health and the environment at the University of California, San Francisco, it's still difficult to gauge the impact.
There are different kinds of phthalates complicating the picture; some seem to have a much greater effect than others. And these are far from the only factors, chemical and otherwise, that influence human fertility. Buck Louis' group is looking at a broad range of industrial compounds, including heavy metals like lead and cadmium, that tend to accumulate in the body.
Phthalates, by contrast, tend to be metabolized within a few hours. Their impact would not be so profound if people were not constantly exposed from multiple sources. These include not only cosmetics and plastics, but packaging, textiles, detergents and other household products. Phthalates are found in PVC pipes carrying water and in the tubing used in hospitals to deliver medications; enteric coatings on pills, including some aspirin; materials used to create time-release capsules; and countless other products. In 2008, the government banned them in children's toys, and the European Union is also moving forward on restrictions.
But here's the silver lining: The transient nature of these compounds means that consumers can take fairly simple measures to reduce their phthalate levels. One is to read the labels on cosmetics and other personal care products and to choose those without phthalates. Another is to be cautious with plastic food containers, and to avoid using them to heat food and drink, as the phthalates in them may be transferred to what you consume.
"These compounds leach from plastics," Buck Louis said. "You can switch to a glass for drinking. You can cook your frozen dinners on paper plates."