"The more exposed the fishermen were to the chemicals the more Y chromosomes we found," Professor Aleksander Giwercman, of Malmo University in Sweden, told Reuters.

An egg fertilised by a Y chromosome sperm will produce a boy while an X chromosome sperm results in a girl.

Earlier studies have suggested that exposure to different chemicals in the environment could change the ratio of boys to girls being born.

Dioxin, DDT and PCBs, which were used in industrial and commercial applications before being banned, are examples of organochlorine pollutants.

Giwercman and his team studied semen from 149 fishermen who were exposed to persistent organochlorine pollutants (POPs) to determine whether the chemicals had an impact on the number of sperm carrying Y and X chromosomes.

"The answer was 'yes,'" said Giwercman, whose findings were reported online by the journal Human Reproduction.

"To our knowledge, this is the first study to show that the distribution of the sex chromosomes in sperm can be affected by exposure to POPs," he added.

When they compared the 20 percent of fishermen with the highest exposure to the pollutants with the 20 percent with the lowest, they found an increase in Y chromosome sperm in the higher group.

"This is more evidence that chemicals which everyone is exposed to have an effect on the function of the reproductive system," Giwercman said.

He added that the quality of the sperm is also affected by the pollutants.

In a separate study also published by the journal, scientists from Denmark, Lithuania and Finland suggested that a larger than expected number of male babies born with undescended testes in Lithuania could also be linked to environmental factors.