If you think e-cigarettes are completely safe, you may be wrong! Even a single exposure to e-cigarette vapour could be harmful to blood vessels. This is a fact concluded by researchers from West Virginia University. In their study, researchers looked at the effect of short- and long-term exposure to flavoured e-cigarette vapour on artery diameter, the blood vessel’s ability to widen (vasodilation) and stiffness of the aorta, the body’s main artery. Aortic stiffness is an age-related complication that can be an indicator of cardiovascular disease. 
The effects of e-cigarette vapour were assessed on female mice. The study showed that just five minutes of e-cigarette exposure caused a 30 percent narrowing of the arteries within an hour! Vasodilation decreased by 9 percent, as well! 
Long-term exposure to e-cigarette vapour also produced negative effects. Exposing mice to e-cigarette vapour for 20 hours per week over an eight-month period resulted in aortic stiffness, which was two and a half times higher compared with the control group, which was exposed to filtered air only.
The authors conclude that e-cigarettes have serious adverse consequences on cardiovascular health. Specifically, exposure to e-cigarettes seems to bring about premature aging of blood vessels. 
This just shows that e-cigarette users may have to think twice before smoking their next e-cig!
References: https://www.sciencedaily.com/releases/2017/08/170812191405.htm  http://www.medicalnewstoday.com/articles/318922.php