Each year in the United States, more boys are born than girls, but a 30-year decline in the percentage of male births is raising concern.

A new study says environmental and other factors may be affecting conception of male embryos and hindering the health of male fetuses.

Devra Lee Davis, director of the Center for Environmental Oncology at the University of Pittsburgh Cancer Institute, said her study, which included researchers from the United States and Japan, shows that "male reproductive health is in trouble."

Sperm determines gender, so problems with male health can reduce the number of male babies conceived.

Since 1970, in the United States, there have been 135,000 fewer white male births than historic patterns would indicate, Dr. Davis said. The birthrate of white males has dropped from an average of 105.5 males to every 100 females in 1970 to 104.6 males per 100 females in 2001.

The decline in male birthrate was even greater in Japan. And while the birthrate of African-American males has not declined in 30 years, the ratio of male births to female births remains significantly lower than the ratio for whites, the study states.

Yet another concern is that the male proportion of fetal deaths has increased in the United States.

Although the study focused on Japanese and U.S. birthrates because the two nations have detailed data on birth rates and deaths, researchers suspect a similar problem in all industrialized nations. Reasons for the trend are still under study, but evidence points to exposure to environmental contaminants of various kinds.

"There has been a syndrome in industrialized nations of increased testicular cancer, reduced testosterone levels and reduced sperm count," said Dr. Davis, an epidemiologist.

Other studies have found that military recruits show a reduction in sperm count, and that testicular cancer has increased by 50 percent and is affecting ever-younger men, she said.

Exposure to carcinogens, estrogen and other environmental risks causes changes in the human endocrine system and disrupts normal reproduction, she said. Changes in parental age at time of conception, obesity, assisted methods of reproduction and nutrition could represent other factors.

Determining the causes of changing gender patterns "merits serious concern," the study concludes.

For now, the overall reduction in the male population is nothing notable, but the causes are "something we need to find out and act upon," Dr. Davis said.

Annie Sasco, a cancer epidemiologist in Bordeaux, France, who has read the study, said people should avoid exposure to environmental contaminants, including growth hormones in American-grown beef.

She also recommended avoiding pesticides and contaminants in plastics that can produce a hormonal effect and have impact on sex ratio.

"This study should tell us that if you don't want to see diseases increase, we should be more stringent with contamination of the environment and with products that are carcinogens or endocrine disrupters," Dr. Sasco said. "There is a need for regulatory aspects to reduce the presence of these contaminants in what we eat, breathe and are exposed to."

The study says one in every four to five couples nowadays reports difficulty with reproduction. There also is mounting evidence that exposure to contaminants on the job, along with other workplace issues, can affect the DNA of sperm, increasing the tendency of men to father children with birth defects.

"We hope this [study] will be a call to look more closely at health problems of young men and protect them at the time before they want to become fathers," Dr. Davis said.