Passive inhalation of marijuana smoke is sometimes put forward as a reason for a positive drug test result. Research has shown that this is improbable. As an example, in one study volunteers were exposed to smoke from cigarettes containing a known concentration of THC (the active ingredient of marijuana) over six one hour periods. The marijuana smoke was so intense that the volunteers had to wear goggles. The highest level detected by GC/MS analysis was only one third of the internationally accepted cut-off level for marijuana.