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.

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.