The decision by the California Air Resources Board puts environmental tobacco smoke in the same category as diesel exhaust, arsenic and benzene.

The unanimous decision 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, 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. He reviewed the science behind Thursday's decision. "This is a seminal, international document. It's impossible to underestimate what a big deal this is."

Greater Breast Cancer Risk

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

The most significant new finding 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 finding conflicts with a 2004 report by the US Surgeon General. Sanford Barsky, a UCLA researcher writing on behalf of the RJ Reynolds tobacco company, told the board the report "either ignores mentioning or does not give the appropriate weight to studies which refute this association" between secondhand smoke and breast cancer.

Review Called Exhaustive

California scientists say their research is more current than the Surgeon General's report. The California 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.

RJ 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.

Next, the air board must 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.

The effect is likely to be greatest outside of California, which already bans smoking in or near most public buildings, including bars and restaurants.

Much of the initial effort in California will focus on public education emphasizing the scientific findings and Thursday's air board decision, said Paul Knepprath, vice president for government relations at the American Lung Association of California.

The association unsuccessfully pushed 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.

Ruling May Alter Building Rules

The association may also push for nonsmoking floors or wings in apartment buildings, much as hotels offer smoke-free areas, Knepprath 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."

Hallett said that could one day force regulations requiring separate ventilation systems for smoking and nonsmoking apartments.