text: PFTs are believed non-toxic  and chemically inert, clear, colourless liquids. They are non-flammable and are not radioactive. They do not occur in nature at all, so background levels are very low, but they can be detected at extremely low concentrations.  
  There is a range of PFTs available commercially, allowing the experimenter to release different PFTs at the same time. Cyclic perfluorocarbons, such as perfluoromethylhexane and perfluoro-1,3-dimethylcyclohexane, are generally believed to be better than acyclic ones as they can be detected at lower levels.