Marijuana Research Report
What are the effects of secondhand exposure to marijuana smoke?
People often ask about the possible psychoactive effect of exposure to secondhand marijuana smoke and whether a person who has inhaled secondhand 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 nonsmoking 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 nonsmoking participants exposed for an hour to high-THC marijuana (11.3% THC concentration) in an unventilated room showed positive urine assays in the hours directly following exposure 80 ; a follow-up study showed that nonsmoking 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. 81
Toxins and tar levels known to be present in marijuana smoke (see What are marijuana's effects on lung health?) raise concerns about exposure among vulnerable populations, such as children and people with asthma.