A manmade ingredient of many plastics, cosmetics and other consumer products may be interfering with prenatal male sexual development, new research suggests.

A study of 85 infant boys found a correlation between increased exposure to some forms of the chemical phthalate (search) and smaller penis size and incomplete testicular descent.

It is the first time phthalate has been shown to influence the sexual development of human males.

"This is clearly something that needs to be examined in a larger sample," said Shanna Swan, a professor at the University of Rochester School of Medicine and Dentistry (search) who headed the study.

A paper describing the research will appear in a future issue of Environmental Health Perspectives, a journal of the National Institute for Environmental Health Sciences (search). Swan discussed the findings in an interview Thursday.

Previous experiments in rats indicate that the chemical interferes with testosterone during gestation, producing a condition known as "phthalate syndrome." Rats with the syndrome suffer from genital birth defects, infertility and testicular cancer.

The human study raises concerns because the infants did not experience levels even close to the high doses used in rat experiments. The boys' exposures, measured by analyzing their mothers' urine during pregnancy, were no higher than those found among the general population.

The last few decades have seen a rise in the types of birth defects that would be expected from prenatal testosterone interference, including hypospadia, a defect in which the urethra does not extend to the tip of the penis, and undescended testicles. Testicular cancer has increased as well, although it is a different type than rats experience in phthalate syndrome.

"The results are both groundbreaking and potentially troubling," said Russ Hauser, a professor at the Harvard School of Public Health (search) who was not involved in the research. "A majority of the U.S. population is exposed to phthalates at the level measured in Shanna's study."

The study does not necessarily indicate that the boys were harmed by their exposure to phthalate, however, and none of them exhibited overt birth defects. The researchers found a correlation between exposure to some forms of phthalate and a measurement called the anogenital index (search) — the distance between the genitals and the anus.

Though that measurement has no physiological significance by itself, it is connected to penis size at birth and is "a very good indicator of internal malformations," said Paul Foster, a senior fellow at the National Institute of Environmental Health Sciences. Newborn rats and other lab animals with phthalate syndrome have unusually short anogenital distances.

A growing body of research suggests that some chemicals used in consumer products may cause public health problems by interfering with sex hormones.

A study in the current issue of the journal Endocrinology exposed newborn mice to bisphenol-A (search), a chemical found in plastics and dental sealants, at doses comparable to those found in the human environment. At puberty the mice were more likely to develop cancer-related mammary duct abnormalities.

"In humans this would cause breast cancer," said Tufts University (search) cell biologist Anna Soto, the study's lead author.