Here's an interesting study.
Three nonsmokers were placed in a closed, non-ventilated 10 X 10 X 8 room with eight marijuana smokers who each smoked 4 cigarette sized joints.
None of the nonsmokers tested positive for marijuana on a urine drug screen.
Other studies have also confirmed that although it's true that passive inhalation of marijuana smoke results in absorption of THC (the active ingredient in marijuana) in the body, none of the THC levels were high enough, in the nonsmokers, to cause a positive result using the current screening and confirmation levels mandated by the Substance Abuse and Mental Health Services Administration.
It's good to have these studies.
I can't tell you how many times patients have told me the only reason they tested positive for THC was because they were in the same room with someone smoking a joint.