California became the first state to declare secondhand smoke a toxic air pollutant Thursday, putting tobacco fumes in the same category as diesel exhaust, arsenic and benzene because of its link to breast cancer.

The unanimous decision by the state Air Resources Board relied on a September report that found a sharply increased risk of breast cancer in young women exposed to secondhand smoke. It also links drifting smoke to premature births, asthma and heart disease, as well as other cancers and numerous health problems in children.

"If people are serious about breast cancer, they have to deal with secondhand smoke. That's what this is all about," said Dr. Stanton Glantz, director of the Center for Tobacco Control, Research and Education at the University of California, San Francisco.

"This is a seminal, international document," Glantz said. "It's impossible to underestimate what a big deal this is."

The report by scientists at California's Office of Environmental Health Hazard Assessment drew on more than 1,000 other studies of secondhand smoke and blamed the fumes for 4,000 deaths each year in California from lung cancer or heart disease alone.

The most significant new finding cited by state officials is that young women exposed to secondhand smoke increase their risk of developing breast cancer between 68 percent and 120 percent. The disease kills about 40,000 women in the United States each year.

That conclusion conflicts with a 2004 report by the U.S. Surgeon General. Sanford Barsky, a researcher writing on behalf of the R.J. Reynolds tobacco company, told the board the state report "either ignores mentioning or does not give the appropriate weight" to studies refuting a link between secondhand smoke and breast cancer.

California scientists say their research is more current than the Surgeon General's report. The state report went through an exhaustive review that delayed its release for nearly a year but ensures it is based on sound research, said Dr. John Froines, director of UCLA's Center for Occupational and Environmental Health and head of the scientific review panel.

R.J. Reynolds spokesman David Howard said regardless of the dangers from passive smoke indoors, no research supports regulators' decision to declare it an air pollutant.

"No studies exist that show that exposure outdoors leads to any increased risk of tobacco-associated illness," he said.

The air board must next consider regulatory steps to reduce exposure, a process that could take years.

"This is no longer some crazy, California, Left Coast way of thinking," said Cynthia Hallett, executive director of Berkeley-based Americans for Nonsmokers' Rights. She cited smoking bans that have been enacted or are being considered across the nation and in other countries.

Much of the initial effort in California will focus on public education, said Paul Knepprath, vice president for government relations at the American Lung Association of California.

The association unsuccessfully sought legislation in 2003 that would have banned smoking in motor vehicles containing young children, and could try for a similar law next year, Knepprath said.

The group may also seek nonsmoking floors or wings in apartment buildings, much as hotels offer smoke-free areas, he said.

"People live in apartments all across California who are exposed to secondhand smoke on a daily basis," Knepprath said. "It drifts from a common area or another apartment."

That could one day force regulations requiring separate ventilation systems for smoking and nonsmoking apartments, he said.