Smoking is responsible for more than 90 percent of lung cancers. But traces of tobacco carcinogens have been found in other organs as well. For example, pieces of DNA bound to carcinogens have been found in breast tissue and breast milk, according to the report authors, who reviewed new research over the recent years. [Never too Late: 5 Bad Habits You Can Still Quit]
"These carcinogens are absorbed systemically. They don't just stay in the lungs. They are carried through the blood to many organs," said Stanton Glantz, director of the Center for Tobacco Control Research and Education at the University of California at San Francisco, who was not involved in compiling the report.
In colorectal cancer, tumors often originate in the glands and the cells that cover the inside of the bowel. Carcinogens in tobacco smoke can reach the large bowel through the blood supply and disrupt regular functioning of the cells. These cells then might form polyps, which can progress into malignant, or cancerous, tumors.
Reviewing large previous studies, the researchers found an increased risk of colon and rectal cancer, particularly after smoking for two or more decades. In some studies, smokers were up to twice as likely to develop colorectal cancer as nonsmokers.