It is recommended that you use a 50/50 solution of distilled water and propylene glycol (PG), usually referred to as activator solution.  PG is a hydroscopic substance that absorbs moisture from the environment (like a salt shaker does in humid weather).  The distilled water evaporates until the ambient humidity approaches 70%.  At that point the PG won't allow any more moisture in the air.  Conversely, if there is too much moisture in the air, the PG solution absorbs the excess, bringing the system down to 70% as well.