New research suggests secondhand smoke poses a potentially major health risk.

Researchers from the University of California -San Francisco found that nonsmokers sitting in an automobile with a smoker for one hour will be exposed to significant secondhand smoke, raising their risk of developing cancer.

Those exposed to secondhand smoke also showed elevated levels of carcinogens and other toxins, such as butadiene, acrylonitrile, benzene, methylating agents and ethylene oxide, in their urine. This group of toxins is thought to be the most important among the thousands in tobacco smoke that cause smoking-related disease.

"Ours is the first study to measure exposure to these particular chemicals in people exposed to secondhand smoke," Neal Benowitz, senior investigator of the study, said in a statement. "This indicates that when simply sitting in cars with smokers, nonsmokers breathe in a host of potentially dangerous compounds from tobacco smoke that are associated with cancer, heart disease and lung disease."

For the study, 14 nonsmokers each sat for one hour in the right rear passenger seat of a parked sport utility vehicle behind a smoker in the driver's seat. During that time, the smoker smoked three cigarettes. The front and rear windows were opened 10 centimeters, or almost four inches.

Before being exposed to the smoke and then eight hours afterward, the nonsmokers' urine was analyzed for biomarkers of nine chemical compounds found in cigarette smoke that are associated with cancer, cardiovascular disease and respiratory diseases. Seven biomarkers showed a significant increase following exposure to secondhand smoke.

"This tells us that people, especially children and adults with preexisting health conditions such as asthma or a history of heart disease should be protected from secondhand smoke exposure in cars," Gideon St. Helen, lead author of the study and a postdoctoral researcher in the UCSF Department of Medicine, said in a statement.

Researchers cautioned that the research might not represent smoking situations in most cars because the stationary vehicle used in the research would provide less ventilation than a moving car.

"Nonetheless, the air samples we took were similar in makeup to those seen in previous smoking studies that used closed cars and cars with different ventilation systems in operation," St. Helen said. "And so we believe that the general levels of risk to nonsmokers that we present is realistic."

The findings are detailed in the journal Cancer, Epidemiology, Biomarkers & Prevention published by the American Association for Cancer Research.