The World Health Organization plans to urge smoking bans worldwide based on a landmark California study that was the first to add breast cancer to a list of diseases caused by secondhand tobacco smoke.

The WHO will announce today at the 13th World Conference on Tobacco or Health in Washington that the study by California's Environmental Protection Agency will be the scientific basis for recommendations due in September.

"It contains the most updated research," says Yumiko Mochizuki, director of the WHO's Tobacco Free Initiative. The CalEPA report and the WHO policy will be published together, Mochizuki said.

The WHO will push for regulations that would make 100 percent of the world's workplaces and public spaces smoke-free. Only a few countries, including Ireland, have done so.

The CalEPA report found that secondhand smoke causes lung cancer, heart disease, adult asthma, premature birth and sudden infant death syndrome. It also determined that exposure causes an average 68 percent increase in breast cancer risk for women under 50 and that some women who had not reached menopause have as much as a 120 percent risk.

A U.S. Surgeon General's report released June 27 said secondhand smoke causes heart disease, lung cancer and other illnesses but found that evidence only suggested a link to breast cancer, which kills 40,000 women a year in the United States.

In January, the CalEPA report led the state to declare secondhand smoke a "toxic air contaminant," a legal designation that allows regulators to enact further restrictions on exposure. California already has the nation's toughest anti-smoking laws, including bans on lighting up in bars, restaurants, workplaces and a growing number of beaches and other outdoor places.

As for the connection with breast cancer, the surgeon general's report "is not saying there's no risk. It's saying there could be a risk," says Terry Pechacek of the Centers for Disease Control and Prevention's office of smoking and health.

Katharine Hammond, a member of a scientific review panel that evaluated the CalEPA report, was also an author on the surgeon general's report. She says they're 95 percent in agreement. "The surgeon general is only off by a shade on breast cancer," she says. "But it's a very important shade."