Tobacco opponents say that we’ve had too little experience with e-cigarettes to know whether they are safe. While it is true that we don’t yet know the health consequences of long-term use, that should not discourage smokers from switching.
We know that smoke contains high levels of thousands of agents, many of which are toxic or carcinogenic. In contrast, e-cigarette vapor contains water, propylene glycol and/or vegetable glycerin, nicotine, flavors and perhaps a few contaminants at minuscule levels. None of these – with the exception of buttery flavors (here) – are linked to any specific disease. This difference alone justifies encouraging smokers to switch to e-cigarettes.
In the case of cigarettes, the effects of long-term use were not apparent for 20 years.
As I discuss in my book, For Smokers Only, smoking prevalence increased substantially around World War I (1914-1918). The first clinical report of an increase in lung cancer and the suggestion of a link to smoking was published in 1939 by Alton Oschner and Michael Debakey in the journal Surgery, Gynecology & Obstetrics (68: 435-451, 1939). “Until recently,” they wrote, “[cancer] of the lung has been considered a relatively infrequent condition. However, recent studies demonstrate that [lung cancer] is one of the most frequent [cancers] of the body.” But they acknowledged, “…it is controversial whether the increase in [lung cancer] is apparent or real.” Oschner and DeBakey described 79 previous cases and presented seven cases that they had seen.
German pathologist Dietrich Eberhard Schairer and colleague Erich Schöniger published perhaps the first epidemiologic case-control study of smoking and lung cancer in their native language in 1943. Now considered a groundbreaking study, it was republished in English by the International Journal of Epidemiology in 2001 (reference here). They confirmed “the [earlier] report of Müller  that non-smokers rarely get lung cancer whereas heavy smokers get it more frequently than average.”
The smoking-lung cancer link did not appear in mainstream medical literature until 1950, when studies by Ernst Wynder and Evarts Graham (Journal of the American Medical Association, here), and by Richard Doll and Austin Hill in the (British Medical Journal, here) were published.
While the strong link between smoking and lung cancer was not discovered for decades, today’s advanced surveillance techniques may detect a vapor-linked problem sooner. It should be noted, however, that evaluating the effects of vaping will likely be complicated by the fact that most vapers already have smoking histories.
Smokers shouldn’t wait to vape.