If you walked into a cafe or pub in the UK a few years ago, chances were you'd enter a room filled with cigarette smoke. The aroma of burning tobacco lingered on your clothes for the rest of day and your health took a battering from passive smoking. It's no wonder that in many countries smoking in public places has now been banned, and those who choose to do it have to inhale on the pavements outside.

Laws restricting where people can light up and repeated health warnings have seen many people quit the habit. But despite a decline in smoking rates worldwide, a report published in the medical journal The Lancet in 2017 found that smoking causes one in ten deaths worldwide, half of them in just four countries - China, India, the US and Russia. When population growth is taken into account, there is actually an increase in the overall number of smokers. So why - despite the warning signs - are these people still doing it?

Much of it seems to be connected to people's cultural, economic and social background. In the UK, for example, the Office for National Statistics found that people living on a low income are disproportionately likely to smoke. And one in four manual workers smokes, compared with one in ten of those in professional or managerial jobs. Dr Leonie Brose from King's College London, writing about this for the BBC, says there are "startling" regional variations with many more pregnant women smoking in deprived areas. And people with mental health problems are "50% more likely to smoke than the rest of the population". She suggests these groups can have higher levels of dependence, making it harder to give up and are also more likely to be around other smokers, making it seem like normal behaviour.

Increasing the price of cigarettes and making packaging plainer are two ways to discourage smoking; and lower-risk nicotine patches and e-cigarettes are available as an alternative, though they can be just as addictive. It's obvious something needs to be done and recently the UK government pledged to end smoking in England by 2030 as part of a range of measures to tackle the causes of preventable ill health. But as Dr Leonie Brose writes, "with more than 200 deaths in England per day [caused by smoking-related diseases,] that's the equivalent to a plane crashing every day." Imagine what the number must be globally.