AbstractBackground: The deposition of cannabinoids on/into hair from environmental smoke can be considered as a potential source of drug findings in hair. We studied external uptake of cannabinoids from marijuana smoke, investigating possible influencing factors on drug uptake and the efficiency of decontamination procedures.Methods: Strands of a natural hair sample were moistened with water, greased with sebum or sebum/sweat, or bleached or permed. Treated and untreated samples were exposed to marijuana smoke for 60 min. Aliquots of each hair strand were either kept unwashed or were washed with methanol, dichloromethane, or 5 g/L dodecyl sulfate in water. Cannabinoid concentrations in unwashed and washed hair samples, as well as in air samples collected from the exposure chamber and in the marijuana sample being combusted, were quantified by gas chromatography–mass spectrometry or gas chromatography.Results: Cannabinoids were deposited on the hair fibers from marijuana smoke. Cannabinoid concentrations were dependent on air concentration and hair pretreatment. Uptake was less in untreated than in pretreated hair. Concentrations were increased in damp hair, but were even higher in greased hair. There was no significant difference in concentration between bleached and permed strands. External contaminants were completely removed by washing with methanol and dichloromethane in untreated hair only. Washing with dodecyl sulfate in water was insufficient in all cases.Conclusions: Exposures of hair to marijuana smoke yields detectable cannabinoids depending on concentrations in the air, hair care habits, and cosmetic treatment. Environmental marijuana smoke exposure may produce false-positive or falsely increased test results in hair.