Condensation of marijuana smoke on the hair surface can be a source of an external contamination in hair analysis and may have serious consequences for the person under investigation. Δ9-tetrahydrocannabinolic acid A (THCA-A) is found in marijuana smoke and in hair analysis, but is not incorporated into the hair through the bloodstream. Therefore it might be a promising marker for external contamination of hair and could facilitate a more accurate interpretation of analytical results. In this study, three participants were exposed to the smoke of one joint every weekday over three weeks. Inhalation was excluded by an alternative breathing source. Hair samples were obtained up to seven weeks after the last exposure and analyzed for THCA-A, Δ9-tetrahydrocannabinol (THC) and cannabinol (CBN) by liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. Additionally 30 hair samples from various regions of the head were obtained seven weeks after the exposure from one participant. The obtained results show that the degree of contamination depends on the hair length, with longer hair resulting in higher THC and CBN concentrations (1300 pg/mg and 530 pg/mg at the end of the exposure period) similar to the ones typically found after daily cannabis consumption. THCA-A could be detected in relatively low concentrations. Analysis of the distribution of the contamination showed that the posterior vertex region was affected most. The relatively low THCA-A concentrations in the samples suggest that most of the THCA-A found in forensic hair samples is not caused by sidestream marijuana smoke, but by other sources. Copyright © 2013 John Wiley & Sons, Ltd.