Look, I'm not going to sit here and say that we know enough to conclusively say that e-cigs are generally "safe". But the studies thus far are mixed at best.

Here's a study which compared (in vitro) the toxicity of e-cigs and tobacco : http://www.mdpi.com/1660-4601/11/11/11325/htm

it finds "no cytotoxic, genotoxic, or inflammatory effects".

Now, the nanoparticles aspect is not something I have delved into deeply, but the reports of metals in your eliquid where attributed to the cartridge, rather than the eliquid itself by one of the authors of one of these studies: https://news.usc.edu/67718/e-cigarette-smoke-found-to-contain-toxic-metals/

Which, of course it would be nice to avoid. I personally would like to see some manufacturing standards brought into action in the U.S. I'd love to hear someone chime in about the nano-particles issue mentioned in the paper. My impression was: very small particulate is very dangerous if it is something the body cannot break it down (ie. coal dust) because it accumulates in the lungs, causes in-elasticity of the lungs, etc. if we are talking about nano-particles of eliquid....well that's the point. Very small droplets of nicotine juice are being deposited in your lungs, and absorbed. I'm sure this is not ideal compared to NOT breathing in vapors, but I also STRONGLY doubt that it is very harmful. And if the metal nanoparticles are from the devices/tanks/cartridges rather than the elquid itself, than just mandate glass interiors to tanks, or some other non-reactive surface, and I think we'll be square.

There's also the accusation that e-ciggs produce formaldehyde. The study which found this was essentially burning e-liquid rather than vaping it, so if used normally and desirably, e-ciggs don't produce that. There's also no incentive to burn your eliquid, as when it happens, it tastes really, really gross. So I don't think there's a danger of people trying to do so:

http://ntr.oxfordjournals.org/content/early/2014/05/14/ntr.ntu078.full

In addition there have been several studies which DO link e-cig use to reduced smoking:

http://www.ecigarette-research.com/web/index.php/2013-04-07-09-50-07/2014/186-ecig-rct http://www.biomedcentral.com/content/pdf/1471-2458-14-1159.pdf http://ntr.oxfordjournals.org/content/early/2014/10/29/ntr.ntu200