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 smok morph kit 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 United States, the Centers for Disease Control and Prevention (CDC) estimate that 6.9 million adults, or 2.8 percent 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 U.K., about 6 percent of the population, or 2.9 million adults, used e-cigarettes in 2017. The vast majority of people who use e-cigarettes in the U.K. 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 U.K. stopped rising by 2017, while that among people who used to smoke continued to rise.

In that year, 52 percent of vapers used to smoke, compared with 45 percent who were using both e-cigarettes and conventional tobacco cigarettes.

Only 3 percent 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 U.K.