Physically, Dr. Shanna Swan is tiny. But she may become a thorn in the side of one of this country's biggest industries.

An epidemiologist at the University of Rochester Medical Center, Swan's research examines the effects of chemicals on the male reproductive system. Specifically, Swan studied a group of chemicals called phthalates, which are found in a wide range of soft and flexible plastic products like shower curtains.

Her findings not only spell problems for the male sex, but pose profound questions about the effects everyday items have on human health.

Swan began her work by looking at an earlier study that determined baby male rats are born with lowered testosterone levels when the mother rats are exposed to phthalates.

"When that male rat is ready to mate, his ability to reproduce is lowered," Swan says.

She then examined blood and urine samples from pregnant women. Phthalates are common in women of reproductive age, she says, because of our environment.

"The samples turned out to be golden," Swan says. "It was like looking through windows into the bodies of these women, including their fetuses."

Swan designed a study that revisited the women and their male children. It turns out, Swan says, that the mothers with higher levels of a particular form of phthalates, commonly referred to as DEHP, produced boys with higher incidences of what she calls incomplete masculinization: penises that are smaller in size and sometimes malformed, and testicles that have not descended.

Another sign of incomplete masculinization caused by high levels of mother and male fetus DEAP exposure is a shortened distance between the anus and the male genitals.

Swan is getting ready to launch a new study to replicate and expand on some of her findings. The study will involve 200 pregnant women from Rochester, Seattle, San Francisco, and Minneapolis.

The public's concern over exposure to chemicals in plastics has risen in recent years, Swan says, and with good reason. The Environmental Protection Agency recently issued a warning about exposure to phthalates, saying that the chemical may present health risks to humans. And Congress has banned the use of phthalates in toys.

But Swan is concerned because the change in the reproductive system occurs in the male fetus. So, banning the use of phthalates in toys does nothing to prevent the damage to from occurring. And the damage, when it occurs in rats, Swan says, is permanent.

Another finding: boys with higher levels of DEHP exposure also display less masculine play behavior.

But a larger question about the effects of multiple and repetitive chemical exposures needs to be answered, Swan says. There are 80,000 chemicals in commercial use in the US at this time, with about 7,000 referred to as high-use chemicals. Few chemicals undergo rigorous testing to determine if they pose health risks to humans, Swan says, despite how universally they're used.

"This is only one study about one chemical," she says. "We've unwittingly entered into this grand experiment over the last 50 years with the chemical industry. And we don't know what any of it will mean."