Is e-cigarette without nicotine safe? please advise!

Nobody knows: the ingredients in e-cigs have been trade secrets, and up until recently, they have not been under any regulatory control, and don't even have to reveal the ingredients. We do know that the propellant is propylene glycol, which is an oil and is volatilized and inhaled, together with various additives, preservatives, colorings, flavorings etc. Whether these are safe is unknown and will take studies