No one disputes the fact that cigarette smoking kills, although many people might not realize just how lethal it really is. The World Health Organization estimates that tobacco kills up to one half of its regular users via cardiovascular disease, lung and other cancers, and respiratory illnesses. About 30 percent of current U.S. cancer deaths result from tobacco use.

Electronic cigarettes, however, are taking over at an astonishing pace. They were introduced in the early 2000s, and according to some experts, sales could exceed those of traditional tobacco products within a few years. In reaction, the U.S. Food and Drug Administration announced in May that it would ban sales to individuals younger than 18; it would also require manufacturers to register their products with the agency and submit ingredients for safety testing. Some countries, such as New Zealand, ban e-cigarettes that contain nicotine entirely.

Policies that restrict e-smoking are reasonable. We don't know yet what kind of health risks it carries, so we don't want people who don't smoke—especially kids—to take it up. And yet we should keep in mind that such vaping is almost certainly safer than conventional smoking: some experts have suggested that e-cigarettes carry only between 3 and 5 percent of the health risks of tobacco smoking. We don't want to ban vaping or make it too hard to do, because if smokers can't quit, they should switch to the less harmful habit.

Those safety estimates are just that: nobody knows for sure how dangerous electronic cigarettes really are. Like tobacco products, they contain nicotine. But there is an old saying that nicotine addicts, but tar kills, and there is no carcinogenic tar—a by-product of burning tobacco and paper—in e-cigarettes. It is always possible that e-cigarettes could trigger lung cancer by some currently unknown process, but the disease takes many years to develop, so it is too early to know. Besides, many users combine e-cigarettes with the use of tobacco, thereby complicating the analysis.

Any real danger in e-cigarettes most likely comes from flavorings and other additives, which are big unknowns. Manufacturers use thousands of different chemicals to simulate tastes such as strawberry and cheesecake. It is no wonder that youngsters would be tempted to try them, even before trying regular cigarettes. The new FDA regulations will make it harder for those under 18 to experiment, which is a move in the right direction. Because the rules do not restrict chemical additives, however—which have been ruled safe for eating but not for inhaling—they do not go far enough.

If current assumptions about safety are correct, the risk from e-cigarettes is about on a par with the risk of living with a smoker and inhaling secondhand smoke. That is still significant, and the risk needs to be quantified more precisely: estimates aren't good enough. Unfortunately, that would be prohibitively expensive with current techniques, although emerging testing methods could speed things up over the next several years. In the meantime, however, we cannot ignore the fact that as many as one billion people could be killed by tobacco use during this century. Bringing this number down substantially is an opportunity we cannot miss.

Purists will never be happy with a product that is probably more dangerous than anything we would allow to come to the market in any other industry. They are right. But pragmatic health professionals will see the chance to get many smokers to quit smoking tobacco as a big step in the right direction. They are right, too. My father died from lung cancer caused by cigarettes at the age of 63, after nearly half a century of smoking. If he had switched to e-cigarettes, he might have lived much longer.