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Nine controllable risk factors are responsible for more than one-third of cancer deaths worldwide, according to a recent estimate from researchers at the Harvard School of Public Health and other institutions. Of these, smoking and drinking alcohol are the most damaging, they reported in The Lancet.

The other risk factors they looked at include: overweight/obesity, physical inactivity, eating few fruits and vegetables, unsafe sex, urban air pollution, indoor smoke from household fuels, and contaminated injections in health care settings. The researchers looked at each factor's impact on 12 different types of cancer based on age, gender, and region of the world. The data came from several sources, including the World Health Organization's Comparative Risk Assessment project, an initiative to determine which risk factors play a role in the global burden of disease.

Of the 7 million global cancer deaths in 2001, the team estimated that 2.43 million (35%) were caused by the combined effect of these 9 risk factors. Smoking was by far the biggest culprit, causing 21% of those deaths overall (29% of cancer deaths in high-income countries and 18% in low-/middle-income regions). Alcohol use caused about 5% of global cancer deaths, as did low fruit and vegetable intake.

"These results clearly show that many globally important types of cancer are preventable by changes in lifestyle behaviors and environmental interventions," said senior study author Majid Ezzati, assistant professor of international health at the Harvard School of Public Health.

Other experts agree. "What's interesting is that even without the potential benefits of early detection and treatment, at least one-third of cancer deaths are preventable," said Michael Thun, MD, MS, vice president of epidemiology and surveillance research at the American Cancer Society.

The findings highlight areas where the United States has made considerable progress in reducing the toll cancer takes. For instance, the impact of unsafe sex on cancer is lower in the US than it is in poorer parts of the world. One way unsafe sex can lead to cancer is by spreading human papilloma virus (HPV), the virus responsible for nearly all cases of cervical cancer. But in the US, cervical cancer screening through Pap tests is widely available. That means pre-cancerous problems can be caught earlier, and fewer women go on to develop -- and die from -- cervical cancer.

Likewise, contaminated injections in the health care setting remain a problem in low-income nations that have poor sanitation, but are not a significant problem in the US.

But the report also shows the tremendous toll that lifestyle factors like excess weight and lack of exercise are taking in the US. After smoking and alcohol use, overweight/obesity was the third most important cause of cancer death in high-income nations, the researchers found. It was an especially important factor in colorectal, breast, and uterine (endometrial) cancers. Thun noted that the impact of excess weight on cancer may be even greater than reflected in the Lancet report because the rate of overweight and obesity is increasing so rapidly, particularly in the US and other developed nations.

In low-/middle-income nations, low fruit and vegetable intake was the third most common risk factor for cancer deaths.

The study shows ways that public policy and medical technology can be used to reduce the terrible burden of cancer worldwide, Thun said. For instance, tobacco control -- through the Framework Convention on Tobacco Control and other means -- could greatly reduce the deadly impact of smoking. The development of vaccines for HPV holds great promise for reducing the number of cervical cancer cases and deaths, especially in regions of the world where screening is not widely available.

These types of measures are crucial in regions of the world that lack advanced medical facilities that can provide top-notch cancer treatment, Ezzati said.

"To win the war against cancer we must focus not just on advances in biomedical technologies, but also on technologies and policies that change the behaviors and environments that cause those cancers," he said.

Citation: "Causes of cancer in the world: comparative risk assessment of nine behavioural and environmental risk factors." Published in the Nov. 19, 2005 issue of The Lancet (Vol. 366; No. 9499:1784-1793). First author: Goodarz Danaei, MD, Harvard School of Public Health.