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This legend is technically true but highly misleading. According to a U.S. Army study, the amount of secondhand cannabis smoke needed to cause a false positive result (failure) is quite large indeed, and would require being sealed in an unventilated car or small room filled with marijuana being actively smoked (often referred to as a "hotbox") for several hours. Hair testing, however, is a different matter, particularly with passive exposure to crack/cocaine, which can deposit onto hair and be readily incorporated into it. With regards to cannabis, however, typically only metabolites (produced by the body and thus not found in smoke) are tested rather than THC, so failure is unlikely to result from non-extreme passive exposure.[104][105]

Legend (1994) full crack [License]