If the trend continues, the number of deaths from lung cancer among non-smokers will leapfrog those among nicotine users in a decade. Previously, nine in ten cases of the disease were linked to cigarette use, but this proportion has decreased as more people shun the habit. China has also seen a sharp rise in the incidence of lung cancer in the past 10-15 years, with many blaming heavy smog in cities. Experts at the China Academy of Medical Sciences said they had noticed more cases among non-smokers and women. Its study said there had been a rapid increase in a form of lung cancer that develops deep in the lung and is not associated with tobacco use. China has been waging a battle against air pollution, with concentrations of PM2.5 often topping 300 micrograms per cubic metre. Last year's national average stood at 47 micrograms, with only a quarter of cities meeting the country's official air quality guideline of 35 micrograms. The World Health Organisation recommends levels of no more than 10 micrograms.