"We do not wish to say that having a baby boy is bad. It's just that there were more of them," Wilfried Karmaus, a epidemiologist who led the study, said in a statement.

"A change in the proportion of boys to girls, however, indicates that environmental contaminants may play a role in human reproduction."

PCBs are endocrine disruptors - chemicals that act like hormones. They and related chemicals known as dioxins are reported to have caused deformities of sexual organs in fish and alligators.

In humans both dioxins and PCBs can cause cancer, infertility and perhaps other sexual changes. Vietnam veterans exposed to dioxin in Agent Orange, used to strip foliage in the jungle, claim it has caused a variety of ills including cancer and birth defects in their children.

Karmaus used information from studies of men and women who liked to fish in the Great Lakes, which are polluted with a range of chemicals.

"They were recruited by the Michigan Department of Community Health,' Karmaus said in a telephone interview. Their blood levels of PCBs and other chemicals were measured.

The 101 families they studied had children born after 1963, and both mother and father had detectable PCBs in their blood. There was a total of 208 children.

Of these, 57 percent were boys, which is a slight increase of the normal trend of about 51 percent. Usually, 105 boys are born for every 100 girls.

"However, we did not detect that the PCB levels of mothers affected the number of boys or girls," Karmaus said.

Writing in the January issue of the Journal of Occupational and Environmental Medicine, Karmaus stressed that his sample was small, meaning he worked with too few people to have a good idea of what the numbers meant.

He noted that other studies had showed conflicting findings. One after a dioxin accident in Italy in 1976 found people who were exposed had more girls than boys. "Then there are the Army servicemen who were exposed to Agent Orange during the Vietnam war. They found fewer girls and more boys," Karmaus said.