E-cigarettes, e-Hookahs, and vaporizers are completely unregulated by any agency, so their safety and effectiveness has not been tested. No one can say that they are safer than other tobacco products (such as non-combusted tobacco products). This is one major reason why public health officials are reluctant to tell people to use them.
Since the quality of each device may vary, and the nicotine dose that an individual user may receive varies with the brand, battery charge level, “e-juice,” and experience of the user, it is hard to know how well any particular device works or what the user is inhaling. We have little information about the safety of vapors like propylene glycol when they are heated and directly inhaled, rather than eaten or used on the skin. The safety of many of the inhaled flavorings in the e-cigarette liquid is also not known; heating these flavorings could create new chemicals and byproducts. We don’t know how safe it is to breathe “second hand vapor.”
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Keep in mind that these things were invented and are almost exclusively produced by the Chinese, who are not noted for their concern about public health, and there’s no particular reason to think that the few American manufacturers are, either.