Some smokers never develop cancer despite accruing thousands of mutations, but this is purely down to luck, Alexandrov says. “Smoking is like playing Russian roulette: the more you play, the higher the chance the mutations will hit the right genes and you will develop cancer,” he says. “However, there will always be people who smoke a lot but the mutations do not hit the right genes.”
The team hopes their findings will deter people from taking up smoking and debunk the myth that social smoking is harmless. Every cigarette has the potential to cause genetic mutations, Alexandrov says.
Quitting smoking will not reverse these mutations – they leave permanent scars on DNA – but it will prevent the added risk of more mutations, he says.
There is good evidence that people who stop smoking have a significantly lower risk of premature death than those who continue, says Simon Chapman at the University of Sydney, Australia.
For example, a UK study that followed 35,000 men for half a century found that smoking shaved 10 years off average life expectancy. But quitting at age 30 mostly erased the risk of premature death, and giving up at 50 halved it.
“Many smokers believe there’s no point in quitting because the damage is already done,” says Chapman. “But if smokers quit by middle age, they can avoid nearly all the excess risk of tobacco-caused deaths.”