Exposure to Secondhand Weed Smoke Can Make You Fail a Drug Test, Study Finds

By May Wilkerson 05/21/15

Being subjected to "extreme" amounts of pot smoke could impair non-users.

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Hanging out in close proximity to pot smoke could get you high or cause you to fail a blood test, a new study finds.

Researchers say this only occurs in “extreme” conditions, like an enclosed space with severely low ventilation, such as a small bedroom, closet, or car. Incidentally, these are all popular places to smoke pot.

"If you're going to breathe in enough passive cannabis smoke to feel high and potentially be slightly impaired, you could fail a drug test," said the study’s lead author, Evan S. Herrmann, of Johns Hopkins School of Medicine. "But this only happens under a very extreme situation."

Exposure to secondhand weed smoke has been linked to positive drug tests in the past. But this is the first study to look at factors like the ventilation of the room and to examine whether smoke exposure can actually have mind-altering effects.

Researchers at Johns Hopkins had six pot smokers and six non-smokers aged 18 to 45 spend two separate hour-long sessions in a plexiglas and aluminum smoke chamber about the size of a dorm room. The six smokers were each given 10 marijuana cigarettes to smoke “leisurely” for the hour while the non-smokers sat beside them in the chamber.

During the first session, the room was ventilated, with air flowing in-and-out at about the rate it would in a typical office building. In the second session, the airflow was restricted, creating a “hot box” environment.

After the “hot box” session, the non-smokers reported feeling high, showed slight cognitive impairment on tests, and had detectable levels of THC in their blood and urine for up to 22 hours afterwards. In contrast, the nonsmokers in the ventilated condition did not feel impaired or high, did not test positive for THC in their urine, and had significantly lower levels of THC in their blood.

However, the scenario in the study is pretty extreme, researchers explained. In the real world, a person would have to remain in a badly ventilated room or car for a full hour while about 15 grams of weed are smoked, in order to replicate these effects.

"Our results are pretty consistent with what we expected," said Herrmann. “It's really hard to get a positive [drug test result] from passive smoke unless you're in an extreme scenario.”

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May Wilkerson is a writer, comic and Managing Editor @someecards. Co-host of the podcast Crazy; In Bed w/alyssalimp. She is also the top Google result for "insufferable lunatic." Follow this insufferable lunatic on Twitter.

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