DEAR DR. ROACH: My son is studying aviation maintenance, and the instructors tell the students to expect random urine tests for drugs when they are employed. Drinking alcohol is fine, but using marijuana and other drugs are cause for losing your license and thus your career. Recently we were walking around Boston for a day and we smelled pot frequently. Recreational and medical marijuana are legal in Massachusetts. He asked if one could fail a urine test by inhaling secondhand pot smoke. I doubt you could just by walking through a park, but what about attending a party or sitting in a car with someone who is smoking? F.L.
ANSWER: Several studies have shown that a person sitting in a car or a closed room where cannabis is being smoked can result in low, but detectable, levels in a persons urine, even if they were not using the cannabis themselves. However, most laboratories choose a threshold for calling a sample positive that is higher than was shown to be possible in the passive smoke experiments.
On the other hand, the concentration of THC in cannabis is much higher now than it was in those experiments, done mostly in the 1980s. In a 2010 study from the Netherlands (looking at a real-life coffee shop), urine levels in nonusers (between 5 and 8 ng/mL) still were well below the thresholds used to detect cannabis use that I found in the published literature (25 to 50). I think it is unlikely that secondhand cannabis smoke will turn a urine test positive.
