WASHINGTON (AP) - Smoking cigarettes for 20 years or more can increase by more than 40 percent the risk of dying of colorectal cancer, according to a study that blames tobacco use for nearly one in eight such deaths in the United States.

A team of American Cancer Society researchers report Wednesday in the Journal of the National Cancer Institute that a study of 781,351 people over 14 years showed a strong statistical linkage between cigarettes and death from colon or rectal cancer. "It is clear that cigarette smoking is associated with colorectal cancer mortality for both men and women," Ann Chao, a researcher with the American Cancer Society and first author of the study, said Tuesday. Chao said earlier lab studies showed that carcinogens in cigarette smoke may cause tumors in the colon and rectum and may damage the DNA in cells. The new study is the first to link cigarettes and colorectal cancer death among such a large number people followed for such a long period of time, she said. Based on the study, Chao concluded that of the approximately 56,000 Americans who die annually of colorectal cancer, about 6,800 of the deaths, some 12 percent, are associated with cigarette smoking. Colorectal cancer is diagnosed in more than 780,000 people worldwide annually, according to the World Health Organization. There about 129,400 cases of colorectal cancer identified in the U.S. annually, according to 1999 figures from the American Cancer Society. Cigarette smoking is identified by the Centers for Disease Control and Prevention as the cause of 160,000 deaths from eight kinds of cancer, but colorectal cancer is not included on that list as a disease linked to tobacco use, said Chao. Chao and her co-authors suggest in the study that colorectal cancer now should be classified as a "smoking-related cancer." Dr. Bernard Levin of the University of Texas M.D. Anderson Cancer Center in Houston said the study gives strong support for classifying colorectal cancer as a smoking-related illness. "This study and others show that there is a high risk (of colorectal cancer) after 20 years or more of exposure (smoking)," he said. Levin, the vice president for cancer prevention at M.D. Anderson, said the new study shows a clear dose-related effect from smoking: The more one smokes, the greater the risk of cancer. Smoking also has been linked to death from heart and pulmonary disease. The CDC estimates that cigarette smoking causes more than 400,000 premature deaths annually. In the research, Chao and her colleagues found that 4,432 people in the study group died of colon or rectal cancer over the 14-year period. An analysis of the smoking habits of the 781,351 people in the study, said Chao, showed the risk of colorectal cancer death increased steeply among 20-year smokers. The risk of death from the disease, she said, was directly linked to the number of cigarettes smoked and to the number of years that a person smoked. For instance, people in the study who smoked more than 40 cigarettes a day were 54 percent more likely to die of the cancer than were those who never smoked. Smokers who have puffed for more than 60 years were 48 percent more likely to die of colorectal cancer than those who never smoked. The age when smoking started also played a role, said Chao. People who started the habit before the age of 15 had a 47 percent greater risk of dying from colorectal cancer than did those who never smoked. Quitting cigarettes lowered the risk, but not until 20 years after quitting, when the risk became similar to that of those who never smoked.

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