Photo: Paramount Pictures

Marijuana, like anything else that’s been stigmatized and driven underground for a long time, has developed a lot of mythology surrounding it. One pot-related issue that has kick-started many arguments is the question of “contact highs” — namely, how smoky a room/car/whatever has to be before nonsmokers can get high simply from sitting there — and, relatedly, whether this can lead to false-positive drug tests. It’s a question with real-world ramifications — talented, but troubled, Cleveland Browns wide receiver Josh Gordon, for example, has blamed a positive drug test on secondhand marijuana smoke.

To find out whether this particular strain of the “There’s a perfectly logical explanation!” excuse has any scientific weight behind it, a team led by Dr. Evan S. Herrmann of the Johns Hopkins University School of Medicine ran what they say is “the first study … to examine the influence of room ventilation on secondhand exposure to cannabis smoke, and the first study to examine the effects of secondhand cannabis smoke exposure on behavioral/cognitive performance.” It was published recently in the journal Drug and Alcohol Dependence.

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This was an unusual experiment: The scientists recruited both regular pot smokers and non-pot-smokers and constructed a special Plexiglas compartment designed to be about the size of a small room. After blood tests were administered both to confirm that the non-pot-smokers hadn’t smoked pot recently and the pot smokers had remained abstinent overnight (for data-collection purposes), and to measure pre- and post-effects of the smoking session, the key part of the experiment began:

After completing baseline assessments, 6 cannabis smokers and 6 nonsmokers entered a specially constructed smoke exposure chamber (approximately 0900 hours). The chamber measured 10 ft. X 13 ft. (3.05 m X 3.96 m) with a 7 ft. (2.13 m) ceiling, was constructed using a combination of Plexiglas and aluminum support beams, and incorporated an adjustable ventilation/exhaust system. Smokers and nonsmokers sat around a table in alternating seats. All participants wore protective clothing over their normal clothes (disposable booties, jumpsuits) during the exposure session, and were provided with swimming goggles to prevent eye irritation from accumulated smoke. … Each smoker was provided with 10 cannabis cigarettes obtained from the National Institute on Drug Abuse (NIDA) Drug Supply Program and a CReSS Pocket smoking topography device (Borgwaldt KC, Richmond, VA, USA). Each cannabis cigarette contained approximately 1 gram of high potency (11.3% THC) cannabis. The door to the chamber was sealed using magnetic strips around the frame of the door at the start of each 60-minute exposure session. Active smokers were instructed to smoke the provided cannabis cigarettes ad libitum through the CReSS device, which measured the number of puffs taken, puff volume, and other parameters of smoking behavior. Participants were instructed to remain seated for the entirety of the session, but were allowed to engage in leisure activities (e.g., talking, reading, using personal electronic devices, etc.). Research staff monitored the exposure session from outside the chamber to ensure nonsmokers did not actively inhale from the cannabis cigarettes and that smokers only consumed cigarettes from their own supply. Pulse oximeters were used to monitor blood oxygen concentration of study participants every 15 minutes to ensure an adequate oxygen supply was maintained in the chamber.

To test how ventilation, or a lack thereof, affected the nonsmokers, the researchers ran two sessions like this, one with the vents open and one with them closed. After their hour in the pot room, the participants had their physiological readings taken, were given tests designed to determine how the pot affected their ability to complete certain tasks, and were asked how they were feeling.

The results:

Exposure to secondhand cannabis smoke in an unventilated chamber the size of a small room produced minor increases in heart rate, mild to moderate subjective drug effects, and minor, but detectable, levels of performance impairment on some behavioral/cognitive assessments … The [THC] concentration in one specimen was sufficient to trigger a positive urine drug screen at a cut-off of >50 [nanograms per milliliter] (i.e., the cut-off recommended for use by the … mandatory guidelines for federal workplace drug testing programs), and multiple positive urine results were observed for four participants using a more stringent cut-off of >20 ng/ml, which is used by some commercial/private workplace drug testing programs, within a day of exposure.

In other words, yes, both “contact highs” and positive drug tests as a result were observed. But the study suggests you’d almost have to intentionally try in order to get snared by a positive drug test without actually smoking or eating pot: Ventilating the pot room, the authors write, “reduced levels of exposure among nonsmokers, evidenced by much lower (in some cases undetectable) levels of cannabinoids in blood and urine and the absence of subjective and behavioral/cognitive effects.”

In fact, the only behavioral effect the researchers noticed among nonsmokers in the ventilated session was an uptick in their responses to a “hungry/have munchies” item. But as the authors point out, there’s a perfectly logical explanation: “Lunch hour was approaching.”