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Study: Second-Hand Smoke May Cause Positive Drug Test

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Exposure to second-hand marijuana smoke may trigger a positive drug test result under certain conditions. That’s the conclusion of a just-published study in the January/February issue of the Journal of Analytical Toxicology.

Investigators affiliated with John Hopkins University in Baltimore and the Substance Abuse Mental Health Services Administration in Rockville, Maryland assessed the likelihood of second-hand smoke exposure triggering a positive urine test under varying environmental conditions.

Six non-users were seated with experienced cannabis smokers in an alternating manner in each of the three study conditions. Non-smoking participants were exposed to second-hand cannabis smoke for a period of one hour.

In session 1, marijuana smokers consumed cannabis of roughly 5 percent THC in a non-ventilated room. In session 2, smokers consumed cannabis consisting of roughly 11 percent THC under similar circumstances. Smokers in session 3 also consumed higher potency cannabis, but did so in a ventilated environment.

Short-term exposure to second-hand smoke of higher potency cannabis in a non-ventilated setting was significantly more likely to produce elevated carboxy-THC levels in non-users compared to exposure in the other conditions. (Carboxy-THC is the primary inert metabolite of THC.) In one subject, exposure in this setting produced carboxy-THC levels above 50ng/ml, the standard cut-off for a presumptive urine screen. Several other session 2 participants tested positive at levels below 50ng/ml but above 20ng/ml, the standard cut-off for a confirmatory drug screen.

Session 2 participants continued to test positive for the presence of carboxy-THC at levels of 10ng/ml some 30 hours post exposure.

Authors concluded: “Cannabis potency and room ventilation were demonstrated to be two major factors in determining the extent of cannabis smoke exposure to non-smokers residing in close proximity to smokers. … Whether test results for non-smokers would be reported as positive or negative will be highly dependent upon the sensitivity of initial and confirmatory tests and related reporting criteria. Overall, these results indicated that extreme smoke exposure can produce positive tests at lower cutoff concentrations, but not generally at the higher initial test cutoff concentration in general use by SAMHSA's Mandatory Guidelines for Federal Workplace Drug Testing Programs.”

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