The discovery should give further motivation to smokers to stop, say researchers from the Vanderbilt University Medical Center; "if you smoke, now is a good time to quit," said lead researcher Hilary Tindle.
The researchers analysed the results of 8,907 participants in the Framingham Heart Study, who had been tracked for up to 34 years. In that time, 284 people developed lung cancer, 93 per cent of whom were heavy smokers, and had smoked at least 20 cigarettes a day for 21 years or more.
But the risk dropped 39 per cent five years after quitting, and the risk continued to fall in following years—although the risk of lung cancer was still three times higher after 25 years compared to people who had never smoked. Four out of every 10 cases of lung cancer occurred in heavy smokers who had quit 15 years earlier.