Cigarette smoking is responsible for 90 percent of male and 80 percent of female lung cancer deaths, according to the National Center for Biotechnology Information. The length of time a person smokes and the number of cigarettes smoked per day impacts the likelihood of developing cancer. These factors can be calculated in “pack years.”

A pack year is a method to measure the amount a person has smoked over a long period of time. It is calculated by multiplying the number of packs of cigarettes smoked per day by the number of years the person has smoked. For example, one pack year is equal to smoking one pack per day for one year, or two packs per day for half a year, and so on. A person who smokes two packs a day for 15 years is said to have 30 pack years.

However, not everyone who smokes will get lung cancer. Studies by the National Institutes of Health estimate that about 20 percent of people who smoke will develop lung cancer. Scientists are researching whether some people are genetically predisposed to developing lung cancer, but the data is inconclusive thus far.

Other risk factors may or may not be related to smoking. For instance, age by itself is a risk factor of cancer, but it may also relate to the length of time a person smokes. Lung cancer is rarely seen in patients under age 40. Cases begin to increase after age 45 and the number of cases is even greater after age 65.

Race also plays a role. African Americans have a 50 percent greater chance than whites of developing lung cancer. They are also more likely to get it at a younger age and are less likely to survive. However, these risks are more likely due to smoking and reduced access to health care than to genetic factors. African nations have some of the lowest rates of lung cancer.

Men have a slightly greater risk than women of developing lung cancer. Men have a 1 in 12 of developing the disease in their lifetime, while women have a 1 in 17 chance.  The American Cancer Society (ACS) notes that death rates in men have declined from 1991 to 2002 by nearly 2 percent. In women, mortality rates are approaching a plateau after a steady increase for several decades. Overall, death rates in men and women combined have decreased by 0.6 percent since 1991.

Other risk factors for lung cancer include the following:

• Passive smoking or second-hand smoke (also called environmental tobacco smoke). Those who breathe in smoke daily whether at home or in the workplace may have a greater chance of developing lung cancer. The link between lung cancer and second-hand smoke is likely but still somewhat controversial.
  
  
• Marijuana. The content of tar in marijuana is much higher than in cigarettes.  Other factors adding to the cancer risk include deep inhalation, holding smoke in the lungs for a long time, smoking down to the area of highest tar content and other unknown elements such as possible fungus, pesticides or additives.
  
  
• Occupational or environmental exposure to toxins. These include:
  
  
• • Radon. A colorless, odorless gas released from the natural breakdown of uranium. Not normally dangerous outdoors, this gas can collect in homes built over soil with natural uranium deposits.
    
    
  • Asbestos. People who have worked with this mineral fiber are seven times more likely to die of lung cancer, according to the ACS.
    
    
  • Other carcinogens found in the workplace. Arsenic, coal products, gasoline, diesel exhaust, nickel chromates, vinyl chloride and talc dust are associated risk factors for lung cancer.
    
    
  • Air pollution. A contributing factor to lung cancer. This risk is very slight as compared to smoking, however.
    
    
• Previous lung cancer. Patients with a history of lung cancer have a high risk of recurrence, especially if they continue to smoke.
  
  
• Other lung diseases. At higher risk are patients with tuberculosis, emphysema, etc. These diseases cause scarring and/or abnormalities in the lung tissue that predispose it to lung cancer.
  
  
• Family history of lung cancer. A slightly higher risk exists for patients with family members with lung cancer but this genetic link is still unclear.
  
  
• Diet. Some researchers are finding that a diet low in fruits and vegetables may increase the risk of developing certain cancers. The reverse is true – a diet high in these nutrients may offer some protection.