Smoking causes most cases of lung cancer, the leading cause of cancer deaths worldwide.

But 10-20% of lung cancer patients have never smoked.

New research by the National Cancer Institute published in the journal Nature Genetics shows those cases likely stem from a buildup of natural mutations.

Whole genome sequencing of 232 patients has found three subtypes of lung cancer that affect people who have never smoked. The subtypes bear mutation patterns distinct from lung cancers in smokers. They also differ from each other in terms of growth speed, mutation frequencies and survival rates.

The findings could aid in the development of more targeted treatments.

More research is needed to pin down what causes these cancers, although secondhand smoke, radon, air pollution and asbestos are leading candidates.