The known health risks of secondhand exposure to cigarette smoke—to the heart and lungs, for instance—raise questions about whether secondhand exposure to marijuana smoke poses similar health risks, but very little research on this question has been conducted at this point.
A 2016 study in rats found that secondhand exposure to marijuana smoke affected a measure of blood vessel function as much as secondhand tobacco smoke, and the effects lasted longer. This research has not yet been conducted with human subjects, but the toxins and tar levels known to be present in marijuana smoke raise concerns about secondhand smoke exposure among vulnerable populations, such as children and people with asthma.