It was into this mix that e-cigarettes arrived about a decade ago. Unlike ordinary cigarettes, which rely on burning tobacco to deliver their payload, e-cigarettes use an electric charge to vaporise a dose of nicotine (accompanied, often, by various flavouring chemicals). They have proved extremely popular, particularly in America, Britain and Japan. Public-health officials have been quick to conclude that they are much better than smoking. Consumers, says Robert West, a professor of health psychology at University College London, are “voting with their lungs”.

Still, not everyone is happy. E-cigarettes are new, so information about their effects is still scarce. Others worry about who is using them. The Food and Drug Administration, an American regulator, says it has data showing an “epidemic” of vaping among teenagers which it will release in the coming months. Earlier this month it put e-cigarette firms on notice that they must try to combat underage use of their products or face sanction. How worried should vapers—or their parents—be?

The chemistry is the best place to start. Cigarette smoke is genuinely nasty stuff. It contains about 70 carcinogens, as well as carbon monoxide (a poison), particulates, toxic heavy metals such as cadmium and arsenic, oxidising chemicals and assorted other organic compounds.

The composition of e-cigarette vapour varies between brands. A best guess suggests that, instead of the thousands of different compounds in cigarette smoke, it contains merely hundreds. Its main ingredients—propylene glycol and glycerol—are thought to be mostly harmless when inhaled. But that is not certain. People with chronic exposure to special-effect fogs used in theatres—which contain propylene glycol—have reported respiratory problems. Nitrosamines, a carcinogenic family of chemicals, have been found in e-cigarette vapour, albeit at levels low enough to be deemed insignificant. Metallic particles from the device’s heating element, such as nickel and cadmium, are also a concern.

Some studies have found that e-cigarette vapour can contain high levels of unambiguously nasty chemicals such as formaldehyde, acetaldehyde and acrolein, all derived from other ingredients that have been exposed to high temperatures. The vapour also contains free radicals, highly oxidising substances which can damage tissue or DNA, and which are thought to come mostly from flavourings. According to work published this January flavourings such as cinnamon, vanilla and butter generate the most.

Several studies in mice have confirmed that the vapour can induce an inflammatory response in the lungs. In June, for example, Laura Crotty Alexander at the University of California San Diego and her colleagues published results which showed that e-cigarette vapour has a variety of unpleasant effects, inducing kidney dysfunction and a thickening and scarring of connective tissue in their hearts called fibrosis. Her data suggest that the vapour may also be disrupting the epithelial barrier that lines the lungs, triggering inflammation. They speculate that this could make it easier for pathogens like bacteria to take hold. That would fit with recent work by Lisa Miyashita at Queen Mary University of London, which found that vaping makes cells lining the airways stickier and more susceptible to bacterial colonisation.

Puffed up
It all sounds worrying. But a dose of scepticism is useful too. One alarming study in August said that e-cigarette users are more likely to have been diagnosed with cardiovascular disease. But many vapers have smoked in the past, or still do. The paper may have been picking up old harms from smoking, rather than new ones from vaping. Many think that the toxic nature of e-cigarette vapour may have been exaggerated by unrealistic laboratory conditions. Overheating the fluid creates an unpleasant taste that users actively avoid. Lab tests may heat the fluid more vigorously than real vapers do, for example.

The last piece of the puzzle is the nicotine. Besides being addictive, it is known to have an adverse affect all around the body. But the current source of concern is its effects on children. For instance, work in animals suggests that exposure to nicotine could be bad for adolescent brains, making users more susceptible to other addictive substances later in life. This could be one reason why human smokers who start young have higher rates of addiction as adults. It might also mean that children who vape risk a lifelong addiction to nicotine, and may even start smoking. But, says Dr West, these concerns have not yet been borne out by epidemiological studies.

Smoking during adolescence has also been associated with lasting cognitive and behavioural impairments, including on working memory and attention. Animal tests suggest that exposure to nicotine specifically could explain at least some of that effect. All this forms the scientific backdrop to the FDA’s worries about the effects of vaping among the young.

Getting definitive answers will take time. Epidemiology is a tricky business. All sorts of confounding factors and overlooked connections can skew conclusions. Smoking stands out in medical history as a pastime which is so unambiguously bad for you that the signal cuts through almost any amount of noise. The truth about e-cigarettes will take longer to tease out.

That may sound frustratingly vague. But it points to at least one clear conclusion—whether it is harmless or only moderately bad for you, vaping is almost certainly safer than smoking. That is a message which needs spreading. In Britain about a third of smokers say they have not tried vaping because they are worried about its safety and addictiveness. This attachment to a known evil is self-defeating. At least for now, the e-cigarette looks like a useful innovation in public health.