The first and the most popular cause of lung cancer is smoking cigarettes. By numerous estimates, smoking cigarettes causes approximately 86% of lung cancer cases, including cases caused by passive exposure to smoke exhaled by other smokers. These chances even increase if a person had started smoking tobacco at a young age. Passive smoking poses a lesser threat, but is still dangerous – it is known that passive smokers (who are usually exposed to smoke at work or at home) have a 25% higher risk of lung cancer compared to people who are not exposed to the smoke of cigarettes. Regular heavy exposure to environmental tobacco smoke can increase the risk of lung cancer by 50% (CancerResearch UK).