TORONTO (Reuters) - More girls than boys are born in some Canadian communities because airborne pollutants called dioxins can alter normal sex ratios, even if the source of the pollution is many kilometers away, researchers say.

Dioxin exposure has been shown elsewhere to lead to both higher cancer rates and the birth of more females.

Researchers at the IntrAmericas Centre for Environment and Health say their findings, released this month, confirm the phenomenon in Canada.

The study also reveals the health risks of living within 25 km (15.5 miles) of sources of pollution -- a greater distance than previously thought, they said.

Normally, 51 percent of births are boys and 49 percent are girls. But the ratio was reversed -- with as few as 46 males born for every 54 females -- in Canadian cities and towns where parents were exposed to pollutants from sources such as oil refineries, paper mills and metal smelters, according to the study.

"If you find an inverted sex ratio, and want to know what causes it, look for sources of dioxin," said James Argo, a medical geographer who headed the study, which was published in a journal of the American Chemical Society.

"In every one of those cities where those industries are found ... there was a higher probability of female births to male births," Argo said in an interview.

Using birth data and an inventory of pollution sources, the study also concluded that early exposure to dioxins -- even at 25 km away from the source -- increased the risk of cancer later in life in a group of 20,000 people surveyed during the 1990s.

Previous studies that linked dioxins with cancer and a gender imbalance focused on smaller distances, usually about 5 km, Argo said.

Dioxins are toxic chemicals found in very small amounts in the air, water, soil and some foods.

The large-scale burning of municipal and medical waste is the primary source of dioxins in Canada, but they are also created by fuel and wood burning, electrical power generation, and in the production of iron and steel.

Since more females were born in the 90 communities studied, more breast, uterine, cervical and ovarian cancers were observed among them than other forms of cancer, Argo said.