HELSINKI -- The number of people expected to die globally from smoking could be greater than earlier thought as more girls take up the habit than their mothers a generation ago, a new study released on Thursday showed.

The authors of the Global Youth Tobacco Survey said the basis of World Health Organization (WHO) estimates, that 10-11 million people will die annually from tobacco by 2030, could be flawed.

The study is the largest of its kind ever produced and covered over a million adolescents from over 150 countries. It was put together by a number of organizations including the U.S. Centers For Disease Control and Prevention.

The study found for example that in Africa, for every woman using tobacco there were just over seven men but for girls to boys the ratio was 2.2 to one. The ratio also narrowed in all other global regions.

"These findings suggest that projections of future tobacco-related deaths worldwide might be underestimated because they are based on current patterns of tobacco use among adults where women are only about one-fourth as likely as men to smoke cigarettes," the report's authors said.

The WHO's Vera Luiza Da Costa E Silva told a news conference at the World Conference on Tobacco or Health in Helsinki it was not clear why more girls would be turning to tobacco, but they saw the product differently from boys and anti-tobacco campaigns had to take that factor into account.