A recent clinical review has now summarized the latest evidence concerning the use of e-cigarettes as aids to smoking cessation. Do electronic cigarettes cause less harm than smoking, and will they help me quit? These are the key questions that people who smoke but wish to quit raise with their health-care professionals. They are also hot topics in the ongoing debate about the potential benefits and harms of e-cigarettes and their regulation.

The authors of the new review, who work at Aberdeen Royal Infirmary in the United Kingdom, say that their aim is to inform this discussion. The Journal of the Royal College of Physicians of Edinburgh has now published a paper on their findings.

"Fewer people," says corresponding study author Abhi Mathur, of the Department of Respiratory Medicine, "are smoking conventional tobacco cigarettes and more people are vaping."

E-cigarettes are battery-operated devices that people use to inhale, or vape, substances—one of which is nicotine. There are several types across hundreds of brands, and the market is growing. Conventional cigarettes also deliver nicotine into the lungs through inhalation of tobacco smoke. However, they also deliver toxins such as tar and carbon monoxide deep inside the lungs. Vaping does introduce some of the harmful substances that accompany cigarette smoke into the body, but research suggests that the levels present in e-cigarettes are much lower.

Smoking in decline, vaping on the rise

Figures from the World Health Organization (WHO) confirm that the number of people worldwide who smoke tobacco is falling. However, while the trend may be in decline, large numbers of people continue to smoke, and the impact on public health is still huge. In 2015, more than 1.1 billion people smoked tobacco products, and the habit "remains the leading preventable cause of illness and premature death," note the review authors.

Against this backdrop, the trend in use of e-cigarettes, or vaping, is on the rise, with millions of people using a range of products. In the US, the Centers for Disease Control and Prevention (CDC) estimate that 6.9 million adults, or 2.8% of all adults, were using e-cigarettes in 2017. This was the same year in which use of conventional cigarettes fell to its lowest level.

In the UK, about 6% of the population, or 2.9 million adults, used e-cigarettes in 2017. The vast majority of people who use e-cigarettes in the UK either smoke or used to smoke, with the latter now outstripping the former. The rate of e-cigarette use among people who currently smoke in the UK stopped rising by 2017, while that among people who used to smoke continued to rise. In that year, 52% of vapers used to smoke, compared with 45% who were using both e-cigarettes and conventional tobacco cigarettes.

Only 3% of e-cigarette users have never smoked, note the authors. An independent review by an English public health body concluded that while people who have never smoked appear to be trying e-cigarettes, it was unlikely that e-cigarettes were "undermining the long-term decline in cigarette smoking" among young people in the UK.

Vaping and smoking

People who smoke who switch to e-cigarettes can expect to reduce their cancer risk because they are reducing their exposure to more than 70 known carcinogens in tobacco smoke, note the authors. They also cite research that puts the "cancer potency" of vaping at less than 0.5% of that of tobacco smoking. Conventional smoking also raises the risk of developing heart problems and of death associated with them. In fact, more people who smoke die of cardiovascular disease than of cancer. Smoking just one conventional cigarette per day can increase the risk of heart disease to half that of smoking 20 per day.

The main contributor to this risk is the presence of ultrafine particles that enter can the bloodstream from inhaled cigarette smoke. These can trigger inflammation that harms the heart and circulatory system. Research suggests that vaping can also introduce ultrafine particles into the bloodstream, and the authors cite evidence from several studies about their effects. Those results may explain why a recent survey of nearly 70,000 people in the US has tied vaping to heart disease. That study suggests that people who vaped every day had a higher risk of heart attack than those who vape occasionally or those who used to vape. This risk persisted when the researchers ruled out the possible effects of also smoking conventional cigarettes. Another study of cell cultures also revealed that e-cigarette vapor can make a type of immune cell in the lung more likely to promote inflammation and potentially block the clearance of bacteria.

Vaping and smoking cessation

The authors note that while it is highly addictive, at typical inhalation doses, nicotine doesn't cause clinical harm. The substances that accompany nicotine into the body are what make smoking harmful to health, giving rise to the saying that people "smoke for the nicotine but die from the tar."

The authors say that manufacturers originally designed e-cigarettes as a way to help people quit smoking conventional cigarettes, and the devices have even formed part of national guidelines on smoking cessation.

They note that "[i]t is plausible that e-cigarette use has contributed" to the fact that quit rates reached their highest levels in 2017. They summarize a number of studies that support this.

For example, a study that tracked take-up of e-cigarettes saw that it was strongly linked to quitting rate success. Another revealed that quitting smoking was the most common reason that people gave for taking up e-cigarettes.

A third study said that there was evidence to suggest that quit rates were higher when people took up vaping. In addition, a fourth study found that vaping was more effective at helping people stay off cigarettes for 1 year or longer than other quitting aids or giving up without aids.

It appears that some experts in public health believe that the rise in vaping is a good thing, as long as it is due to people who smoke exchanging a harmful habit for a less harmful one. They can claim, note the authors, that there is potential for significant "harm reduction."

Emerging concerns and limited evidence

Other researchers, however, do not agree with this view and point to concerns such as people who have never smoked taking up vaping, and people continuing to both smoke and use e-cigarettes. They suggest that e-cigarettes may be a vehicle for "renormalizing smoking in a society that should ideally be smoke-free." In addition, taking up e-cigarettes could also undermine "complete abstinence."

Given the limited amount of evidence on the benefits and harms of e-cigarettes, it is very difficult to say which view is most valid. One review of smoking cessation studies that included e-cigarettes concluded that vaping nicotine can help people quit smoking conventional cigarettes for up to 1 year. Two others came to similar conclusions.

However, the authors point out that in terms of hard evidence, all three reviews have relied on the results of just two randomized controlled trials.

Evidence from observational studies—that is, those that followed people who smoke over time—is mixed. Their results have differed from those of controlled trials, which researchers put down to small sample sizes, range of devices used, and other factors.

Most observation studies have examined quit rates among people who smoke who did and did not vape. Some showed no benefit from e-cigarettes, whereas others concluded that e-cigarette use actually reduced quitting rates. A follow-up analysis came to the same conclusion: People who smoke and vape "are less likely to quit."

However, differences in vaping patterns may account for such a result. For instance, there could be differences in quit rates among those who vaped on a daily basis compared with those who only vaped occasionally. Another topic of concern is the increasing number of young people who have never smoked who take up vaping. In the UK, e-cigarette use in this group rose 18%–29% during 2014–2016. There is growing evidence, such as from the US, that vaping among people aged 14–30 years is associated with a higher likelihood of taking up tobacco-smoking.

The debate continues

However, as long as millions of people continue to smoke conventional cigarettes, it seems that the main public health focus on e-cigarettes is likely to remain on their use as an aid to help people quit smoking tobacco. Statements such as that recently put out by NHS Health Scotland, and signed by the Royal College of Physicians of Edinburgh, reflect this view.

Compared with how long tobacco smoking has been around and the huge amount of evidence on its harms, research on e-cigarettes is very much in its infancy. It could be many years before there is enough evidence to make an absolute judgement about the benefits and harms of vaping.

"Debate continues regarding safety of e-cigarettes, but NHS Scotland and England have concluded that vaping e-cigarettes [is] less harmful than smoking tobacco."

—Abhi Mathur