People often ask about the possible psychoactive effect of exposure to second-hand marijuana smoke and whether a person who has inhaled second-hand marijuana smoke could fail a drug test. Researchers measured the amount of THC in the blood of people who do not smoke marijuana and had spent 3 hours in a well-ventilated space with people casually smoking marijuana; THC was present in the blood of the non-smoking participants, but the amount was well below the level needed to fail a drug test. Another study that varied the levels of ventilation and the potency of the marijuana found that some non-smoking participants exposed for an hour to high-THC marijuana (11.3 percent THC concentration) in an unventilated room showed positive urine assays in the hours directly following exposure; a follow-up study showed that non-smoking people in a confined space with people smoking high-THC marijuana reported mild subjective effects of the drug—a “contact high”—and displayed mild impairments on performance in motor tasks.

The known health risks of second-hand exposure to cigarette smoke—to the heart or lungs, for instance—raise questions about whether second-hand exposure to marijuana smoke poses similar health risks. At this point, very little research on this question has been conducted. A 2016 study in rats found that second-hand exposure to marijuana smoke affected a measure of blood vessel function as much as second-hand tobacco smoke, and the effects lasted longer. One minute of exposure to second-hand marijuana smoke impaired flow-mediated dilation (the extent to which arteries enlarge in response to increased blood flow) of the femoral artery that lasted for at least 90 minutes; impairment from 1 minute of second-hand tobacco exposure was recovered within 30 minutes. The effects of marijuana smoke were independent of THC concentration; i.e., when THC was removed, the impairment was still present. 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 exposure among vulnerable populations, such as children and people with asthma.