A small native community living in the shadows of Sarnia's chemical valley has had an unusual distinction: Researchers believe it has one of the most skewed sex ratios in the world.

For reasons that are not known, the percentage of male births in the community, known as the Aamjiwnaang First Nation, began to fall precipitously in the mid-1990s. By 2003, newborn girls outnumbered boys by about 2 to 1. It was a dramatic reversal of what is considered normal in human populations - a modest excess of male births - the trend that had previously prevailed on the reserve.

New figures on the reserve's birth rates presented yesterday at a conference in Sarnia indicate that the extent of the male birth dearth may be diminishing, although only slightly. In 2004, equal numbers of boys and girls were born, the first time since the mid-1990s that the reserve has had a birth ratio approaching normal. The next year, however, the number of baby girls once again exceeded boys. Figures for 2006 are not yet available.

Margaret Keith, one of the researchers who compiled the data, cautioned in an interview that annual figures comparing the number of male to female births in a small population can be volatile, but the longer-term trend to fewer male births seems to be persisting.

"Because we only have a year ... where it doesn't look too far off, I just think it's too early to draw any conclusions" that the area is returning to a normal birth ratio, said Ms. Keith, a researcher at the Sarnia office of the Occupational Health Clinics for Ontario Workers.

She said that over the past five years, only 42 per cent of births have been male, well under the national figure of 51.2 per cent. While she said she hopes the birth data will revert to normal "to me, it looks like the trend is still there."

The unusual birth ratio in the community, which has major petrochemical plants on two sides and is downwind of high-polluting U.S. power plants, has attracted worldwide attention. There is an international trend observed in many industrialized countries, including Canada, the United States, the Netherlands and Japan, to lower-than-expected percentages of male births, although the drop in large populations is very slight.

Nonetheless, the observation that the birth ratio is changing has fuelled speculation that long-term exposure to pollutants from industries might be a factor. Many researchers have been looking to the reserve for clues about the trend because it is a small community that is easy to study where residents are likely to have above-average exposure to many synthetic chemicals.

At the conference yesterday, organized by Aamjiwnaang and funded in part by Health Canada, experts said that the global trend to lower than expected numbers of male births is not fully understood, but is thought to be firmly established because it is being observed simultaneously in so many different places.

"This is a global issue. It's not something that's located either just in North America or for that matter in the Sarnia area," said Warren Foster, director of the Centre for Reproductive Care at McMaster University. "There is a weight of evidence that says that there is something going on."

He said the factors behind the trend "are very poorly understood," but that evidence has emerged linking air pollutants and hormone exposures to these changes, among other factors.

Details on the sex ratio at Aamjiwnaang emerged in 2004, when residents of the community began remarking that they had no trouble fielding girls' sports teams while boys often complained of not having friends of their sex nearby.

A peer-reviewed scientific study on the birth ratio, published the next year in Environmental Health Perspectives and authored in part by Ms. Keith, estimated that there was only a 1-per-cent probability that the long period of low numbers of male births compared with females was a statistical fluke.

The sex ratio at a nearby reserve was found to be normal, suggesting the Sarnia trend wasn't due to cultural or genetic factors.

Scientists have observed that over time, about 106 males are born for every 100 females, a way that nature has compensated for higher rates of mortality among boys.

Separate data presented at the conference from Lambton County's community health services department found that from the early 1980s to 2001, there were no unusual changes in the sex ratio at birth in the non-native community in Sarnia and surrounding areas.