Smoking accounts for about 90 percent of lung cancer cases. In heavy smokers, those smoking more than 40 cigarettes a day, the risk of developing lung cancer is 18 to 24 times higher than that of a nonsmoker.

Lung cancer can also develop from exposure to secondhand smoke, and may account for 2 percent to 3 percent of all lung cancer cases. Other risk factors include: 

• Smoking marijuana cigarettes, which:
  • Contain more tar than tobacco cigarettes
  • Are inhaled very deeply
  • Are smoked all the way to the end where tar content is the highest
    
• Recurring inflammation, such as from tuberculosis and some types of pneumonia
• Exposure to asbestos
• Cancer-causing agents in the workplace, including:
  
  • Radioactive ores such as uranium
  • Arsenic
  • Vinyl chloride
  • Nickel chromates
  • Coal products
  • Mustard gas
  • Chloromethyl ethers
  • Fuels such as gasoline
  • Diesel exhaust
• Radon - a radioactive gas that cannot be seen, tasted, or smelled. It is produced by the natural breakdown of uranium and sometimes seeps up through basements or foundations.
• A family history of lung cancer
• Air pollution and industrial gas emissions