QUITTING smoking lowers the risk of heart disease and stroke quickly, but the risk of lung cancer goes down very slowly. A new study may help explain why.

The report, published in the online journal BMC Genomics, compared genes in the lung tissue of eight current smokers, 12 former smokers and four people who’d never smoked. The researchers found that smoking changes the activity of certain genes. In ex-smokers, some of those changes reverse to normal, but others don’t. The irreversible changes may permanently increase the lung cancer risk, the authors say