Shisha pens only contain propylene glycol, glycerine and flavouring, and they work by vaporisation as opposed to combustion, so they are completely safe to use in relation to shisha and tobacco - which both contain nicotine and create harmful toxins as they burn. Propylene glycol and glycerine are common benign food additives that are used to create the vapour which is inhaled.
It should be noted that whilst using a shisha pen is much safer than smoking a cigarette or hookah tobacco, scientists have not fully explored the long term effects of vaporising propylene glycol. Due to the fairly recent creation and rise of the shisha pen, not much research has been done into the subject. However, anecdotal reports of using a shisha pen have described experiencing improved breathing, less coughing, higher levels of fitness and less disease than when using tobacco.