Canadian controversy over male birth rate
Study and media report spark debate over whether pollution is altering sex ratios in Ontario
Scientists remain skeptical ? but intrigued ? after both a study and news article suggested that pollution is skewing the sex ratio in communities around Sarnia, Ontario, where girls outnumber boys.
Salim Daya, an expert in reproductive medicine from McMaster University, who was not involved in the study, said that this issue was raised years before, when experts proposed that pollution may be depressing the rate of male births in Canada. But, "we have to wonder if we are making too much out of nothing," Daya told The Scientist
. "It's a dilemma for Canada ? is it an artifact or truly a decline?"
The link between pollution and declining male birth rates, often referred to as 'missing boys', is a controversial issue in the scientific literature. Some studies have found decreases in the proportion of male offspring upon exposure to environmental pollutants ? for example, after the industrial accident in Seveso, Italy, males exposed to high levels of dioxins were more likely to father daughters than sons. Still, other studies have shown that pollution may cause fewer female births, or have no effect on the sex ratio.
The most recent data on this question comes from a report in the October issue of Environmental Health Perspectives
by Constanze Mackenzie and colleagues from the University of Ottawa, which examined the sex ratio in the Aamjiwnaang First Nation community near Sarnia, Ontario. This community is situated adjacent to the Sarina-Lambton Chemical Valley, which has a large concentration of petrochemical and polymer industrial plants.
"The community noticed there were more girl softball teams and less boy softball teams under age ten," Mackenzie told The Scientist
. The researchers examined the proportion of live male births in the Aamjiwnaang First Nation community between 1984 and 2003. Canada's sex ratio is normally quoted as 105 males to 100 females, or 0.512, a ratio of male births divided by total births. The researchers found the Aamjiwnaang sex ratio was fairly constant at 0.545 between the years 1984-1993, after which it declined, to as low as 0.348 for the years 1999-2003.
Mackenzie said it's unclear what happened in 1993 to precipitate such a marked change in sex ratios, but noted that the community has often been told to evacuate their homes due to chemical emissions, suggesting pollution may be involved.
The study "is very interesting," said Daniel Smith, a research scientist at California's Environmental Health Branch, who was not involved. "It's based on a very small number of births, but the data in the latest time period looks persuasive," he said. Still, the study lacks data on the characteristics of the births and the parents, he noted, a limitation that should be followed-up.
Following the study, a news story in Canada's paper of record, the Globe and Mail
, alleged that other communities are also experiencing a decline in the male birth rate. The article noted that federal census data show that areas most affected have about 100 fewer boys under 5 than expected.
However, according to Statistics Canada, the difference in the sex ratio in the Sarina-Lambton County area is not significant. "For example, for Lambton County there are 103 men for 100 women," Stéphane Gilbert, Head of Subprovincial Estimates for Statistics Canada, told The Scientist
in an email. "If we consider that the sex ratio at birth is around 105 males for 100 females, the case for Lambton County is not exceptional." Mackenzie herself also disagreed with allegations that the entire Sarina-Lambton County area is being affected by a decline in male births.
"The literature provides many indications that other factors could modify the sex ratio -- for example, stress, smoking, malnutrition, and diabetes can all affect the sex ratio," Daniel Desaulniers, a toxicologist from Health Canada, told The Scientist
. Nevertheless, the debate "raises some interesting scientific questions that certainly warrant further investigation," he said.
Links within this article
Mackenzie et al., "Declining sex ratio in a first nation community," Environ Health Perspect 113(10):1295-8, October, 2005.
Mittelstaedt, "Pollution debate born of Chemical Valley's baby boom," Globe and Mail, November 15, 2005.
Allen et al., "Declining sex ratios in Canada," Can Med Assoc J 156(1):37-41, January 1, 1997.
Mocarelli et al., "Paternal concentrations of dioxin and sex ratio of offspring," Lancet 355(9218):1858-63, May 27, 2000.