Can secondhand exposure to cannabis smoke cause a positive drug test?

By Work Place | Last updated: January 17, 2019

In the mid to late 1980s, there were many studies done investigating this very question. Some people were concerned that secondhand pot smoke would cause a non-user to fail a drug test. The result of those studies was that it is very unlikely to test higher than 20 ng/mL in a urine drug test with passive exposure to THC. These studies were one of the reasons that the initial screening level for THC was raised to 50 ng/mL in the late 1980s. Concern was raised again in more recent years (the last 10 years) due to the fact that marijuana is much more potent that it was in the 1970s and 1980s. A new string of studies have been done in more recent times to explore this concern again.

We are going to look at three studies. The first one looks at urine drug testing of passive THC exposure in a confined space, the second looks at oral fluid drug testing of passive THC exposure in a confined space, and the third study looks at urine and blood testing of passive exposure in “real-life” settings.

In the first study, six drug-free non-smokers were seated alternating with six experienced cannabis smokers in a sealed chamber. The cannabis smokers smoked 5.3% THC cigarettes in session No.1, and 11.3% cigarettes in sessions No.2 and 3. Session No.1 and 2 were conducted with no ventilation and session No.3 was conducted with ventilation. Non-smoking participant specimens were analyzed at four different cut-off levels (20, 50, 75, and 100 ng/mL). No presumptive positives were seen at 100 and 75 ng/mL, and only a single presumptive positive occurred at 50 ng/mL; multiple positives were seem at 20 ng/mL. With ventilation, the exposure levels were substantially reduced. The authors state that positive tests are likely to be rare, limited to the hours immediately post-exposure, and are likely to occur only under environmental circumstances where exposure is obvious (a very confined space with little to no ventilation).

The second study looks at oral fluid drug testing. The researchers performed two studies.

  • Four passive subjects alongside four THC smokers inside a van smoking a 5.4% THC cigarette (THC mixed with tobacco; Study No.1) and a 10.4% joint (just THC; Study No.2). For Study No.1 researchers collected the first samples up to one hour after smoking cessation while still in the van. At one hour, the subjects were allowed to exit the van and continued testing the passive subjects until 72 hours post exposure; they waited eight hours for the active smokers.
  • The exact same timing for collections was followed in the second study. The only difference was that all collections were conducted outside of the van to eliminate any contamination of the collection device with marijuana smoke. In Study No.1, peak THC levels in passive subjects recorded at the end of cannabis smoke exposure were up to 7.5 ng/mL, and quickly declined to negative levels (less than 1.5 ng/mL within 30 minutes. In Study No.2, with the collections occurring outside the van to eliminate environmental exposure of the collection device, all tests were negative, even though the concentration of cannabis was double in this study.

This tells us two things regarding passive THC exposure in oral fluid. First, performing an oral fluid test on an individual while THC smoke is in the air can affect the result of the test within the first 30 minutes after smoking cessation. Second, if the testing is conducted in an area with no THC smoke, there is essentially no chance a passive subject will test positive for THC.

The last study is quite interesting because the researchers looked at real-life passive exposure to THC instead of the laboratory conditions seen in essentially all other passive THC exposure studies (in a small confined space with little or no ventilation and a very significant amount of marijuana smoke present). The researchers wanted to test cannabis concentrations in the blood and urine after passive exposure in real-life conditions. Eight drug-free volunteers were exposed to cannabis smoke for three hours in a “well-attended” coffee shop in the Netherlands. Blood samples were taken at one and a half, three, six, and 14 hours after the start of initial exposure, and urine samples were taken after three and half, six, 14, 36, 60, and 84 hours after initial exposure. It could be demonstrated that all participants absorbed THC, however the detected concentrations were rather small. In urine, no participants tested above the 25ng/mL cutoff with immunoassay screening, and only tested up to a maximum of 7.8 ng/mL with GC-MS (again below the cutoff level). Trace amounts could be detected in the blood for the first two collections, and was undetectable after that. The authors state, “the study clearly demonstrated that all volunteers absorbed THC after passive exposure to cannabis smoke under real-life conditions. However, the resulting blood and urine concentrations were only very small… none of the passive inhalers would be misjudged for cannabis use in a routine drug screening…”

Out of all of the samples in all of the studies, there was only one that tested above the 50 ng/mL cutoff level of initial screening, but was below the confirmation level of 15ng/mL. The researchers went on to clarify that the study conditions were very extreme and very unlikely to happen in real settings.

Like in the research of the 1980s, the research done in the last 10 years looking at more potent marijuana and passive exposure show that it is very unlikely a person will test positive on an initial screen test or a confirmation test from passive exposure.

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