A Threat to Male Fertility

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 both 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 were phthalate levels correlated 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 lead author of the report, published in February in Fertility and Sterility. “They’re the signal.”

Phthalates belong to a group of industrial compounds known as endocrine disruptors 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.

“I wasn’t surprised at all by this finding,” Andrea Gore, a professor of pharmacology and toxicology at the University of Texas, and editor in chief of the journal Endocrinology, said of the new report. “We see the cell studies, the animal studies and now the human epidemiology work, and they are all showing us a similar picture.”

The focus on male fertility dates back to the early 1990s, when researchers in the United States and Europe published a paper suggesting chemical exposures could be linked to a steady decline in semen quality. One of the authors, Niels Skakkebaek, a reproduction researcher at the University of Copenhagen, has since suggested that an increase in malformations in male reproductive systems, which he calls “testicular dysgenesis syndrome,” may be linked to environmental exposure to compounds including endocrine disruptors like phthalates.

More recent studies in the United States 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,” explained Heather Patisaul, a biology professor at North Carolina State University 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.)

There are plenty of uncertainties in this picture. 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. “We are still pulling the tricky aspects together.”

There are different kinds of phthalates complicating the picture; some seem to have a much larger effect than others. And these are far from the only factors, chemical and otherwise, that influence human fertility. Dr. Buck Louis’s 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 it were not that people are constantly exposed from multiple sources.

These include not only cosmetics and plastics, but also packaging, textiles, detergents and other household products. Phthalates are found 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.

“The W.H.O. called them ‘pseudopersistent’ in one report,” Dr. Woodruff said, because continued exposure keeps phthalates in the body. But here’s the silver lining: the transient nature of these compounds also 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 get transferred to what you consume.

“These compounds leach from plastics,” Dr. Buck Louis said. “You can switch to glass for drinking. You can cook your frozen dinners on paper plates.”

Studies have shown that these kinds of actions do make a difference; experiments have found measurably lower levels within several days in people who make these changes.

“Lifestyle has an important place here,” said Dr. Buck Louis.

By Deborah Blum, The New York Times