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In the United States, cigarette smoking causes about 90% of lung cancers. Tobacco smoke is a toxic mix of more than 7,000 chemicals. Many are poisons. At least 70 are known to cause cancer in people or animals. People who smoke are 15 to 30 times more likely to get lung cancer or die from lung cancer than people who do not smoke.

http://www.cdc.gov/cancer/lung/basic_info/risk_factors.htm