On September 16th, 2015 the National Park Service banned the used of E-cigarettes in National parks.
E-Cigarettes are a battery powered device that allows the user to vaporize a small quantity of Glycerin (a food additive), flavoring, and sometimes nicotine to inhale.
Evidence of negative consequences to users or 2nd hand vaping is largely unknown due to the newness of the device. What is known is that E-cigarettes contain no tobacco, do not produce any tar, and evidence of other carcinogens show that they produce much less if any at all when compared to traditional tobacco cigarettes.
The ban of cigarettes in National Parks was to prevent forest fires, not as a health measure.
As a health measure, banning a smoking cessation device like E-Cigarettes makes no sense at all.