Exposure to marijuana smoke is three times more harmful than exposure to tobacco smoke, new research suggests.

Matthew Springer, a professor at University of California, San Francisco School of Medicine, studied the effects of smoke on rats and found exposure to secondhand marijuana smoke makes it harder for arteries to expand and allow a healthy flow of blood.

Springer’s research involved putting a lit cigarette or marijuana joint in a plexiglass box with anesthetized rats. When exposed to tobacco smoke, the rats’ arteries had difficulty expanding for about 30 minutes. However, when exposed to marijuana smoke, the arteries took about 90 minutes to return to their normal function.