While smoking is known to be a major risk factor for lung cancer, an estimated 10-15% of lung cancer cases occur in people who have never smoked and this trend appears to be increasing in developed countries, Irish doctors have said.

According to Dr Judith Lyons and Dr Finbarr O'Connell, both of St James's Hospital in Dublin, the cause of lung cancer in people who have never smoked is unclear, however ‘a number of risk factors have been identified'.

These include exposure to radon, which is a naturally occurring radioactive gas present in all rocks and soils. It is classified as a class A carcinogen by the International Agency for Research on Cancer. When it surfaces in the open air, it is quickly diluted to harmless concentrations. However, when it enters an enclosed space, such as a house, it can sometimes build up to high concentrations, leading to a major health risk.

The doctors noted that ‘numerous studies' involving miners who were exposed to increased levels of radon, as well as studies involving the general public in their own homes, ‘show a significant association between lung cancer and radon independent of smoking status'.

They also pointed out that while these people have never smoked themselves, smoking is still a risk factor, in the shape of passive smoking.

"Secondhand smoking is a well-established risk factor for the development of lung cancer in never-smokers. Numerous studies have repeatedly demonstrated the association," they explained.

However, they said that the smoking ban in all workplaces in Ireland ‘should help to reduce the exposure of non-smokers to the harmful effects of cigarettes'.

Meanwhile another risk factor among people who have never smoked is occupational exposure to certain substances, such as paint, paint thinners, smoke soot and exhaust fumes.

Other risk factors include a history of another lung disease, such as asthma, chronic bronchitis and emphysema and a family history of lung cancer.

The doctors pointed out that lung cancer in people who have never smoked tends to occur at a younger age than in smokers. It also appears to affect more women than men. However, the survival rates appears to be better than in cases of lung cancer caused by smoking.

"Tumours in never-smokers tend to be diagnosed at a smaller size than in that of smokers, suggesting that the cancer may grow slowly and metastasise (spread) later," they explained.

Overall, cases of lung cancer in people who have never smoked ‘tend to occur at a younger age and have a better prognosis than lung cancer in smokers'.

The doctors made their comments in the journal, Hospital Doctor of Ireland.