When narcotics police officers or other persons handling drug materials at work are suspected of consuming drugs, hair analysis may be useful to prove or refute such suspicion. However, it is known for many drugs that differentiation between actual drug use and external contamination can be challenging or sometimes impossible. This study evaluated the extent of external contamination caused by handling of synthetic cannabinoidcontaining materials under realistic conditions in a forensic laboratory. Hair samples of laboratory staff were systematically analyzed for synthetic cannabinoids with a validated liquid chromatography-tandem mass spectrometry method after a large quantity of seized ''legal high'' products was analyzed in our laboratory. Furthermore, hair samples of laboratory staff not directly in contact with the drug materials and close relatives of exposed subjects were analyzed to check for cross contamination. All samples of persons who were in direct contact with drug materials tested positive for at least one synthetic cannabinoid. Concentrations ranged from trace amounts up to a maximum of 170 pg/mg (JWH-210), and roughly reflected the duration and intensity of exposure. Unexpectedly, subjects without direct contact with drug material also showed measurable concentrations of synthetic cannabinoids in hair. Concentrations caused by contamination were within the typical range found in known users of these drugs and could lead to false positive results and incorrect conclusions. Therefore, we recommend that body fluids should be simultaneously analyzed to unambiguously prove use of these drugs.