(Undated) -- Secondhand marijuana smoke may be just as bad for you as secondhand cigarette smoke. That's what a new study from researchers at the University of California-San Francisco reveals. They exposed lab rats to second-hand marijuana smoke, and found after 30 minutes the rats' blood vessel function was reduced by 70 percent. Impaired blood vessels can increase the risk of heart attack and heart disease. The researchers also point out that more U.S. states are legalizing marijuana, which makes exposure to secondhand marijuana smoke an increasing public health concern.