Cocaine has been shown to accumulate in hair of admitted users. Before using this test to verify cocaine use, however, it is crucial to differentiate between systemic exposure and external contamination from being in contact with crack smoke. In the present studies, the authors document that pyrolysis of crack results in hair accumulation of cocaine, but not its benzoylecgonine metabolite, whereas after admitted cocaine use both species are detectable in hair. External contamination with crack smoke is washable, whereas systemic exposure is not. The authors suggest these two criteria to distinguish systemic exposure from external contamination.