A new study shows that one-third of all cancer cases in the world could be avoided by lifestyle and environmental changes. The research finds that tobacco and alcohol use are leading causes of preventable cancers. So are indoor fires for cooking.

U.S., Australian, and New Zealand scientists report in the journal Lancet that nine factors that people and societies can control cause nearly 2.5 million cancer cases each year. That is about one-third of the seven million annual global total.

These include cancers of the lung, breast, bladder, cervix, liver, pancreas, mouth, and digestive tract.

Leading the list of causes is smoking, which the researchers say is responsible for 20 percent of preventable cancers. Alcohol use and low consumption of fruits and vegetables cause another five percent each. Also significant are overweight and obesity, physical inactivity, urban air pollution, indoor smoke from household cooking, contaminated injections in health clinics, and unsafe sex. This last one is the transmission route for the human papilloma virus, the leading cause of cervical cancer in women in developing nations.

One of the study's co-authors, Harvard University public health researcher Majid Ezzati, says the statistics reflect data on cancer deaths in 2001. He expects an increase in the proportion of deaths from these risk factors.

"The estimates that we have are looking at what happens today as a result of past exposure," said Majid Ezzati. "So one-third of cancer deaths today would have been avoided had these lifestyle and environmental exposures not been present. If we were to speculate about the future, we could actually say more than one in every three cancer deaths could be avoided because things such as smoking actually have been going up in many countries in the world, so we actually haven't seen the full effects of it yet."

The scientists base their conclusions on cancer data from the World Health Organization, government reports, and other studies. Several previous studies have made such estimates, but most were restricted to just one risk factor, one type of cancer, or one population.

The new study finds that developing countries had more than twice as many preventable cancer deaths as rich nations. Among the developing regions, Eastern Europe and Central Asia had the highest proportion of cancer deaths attributable to the nine risk factors - 39 percent compared to the global average of 35 percent. Furthermore, Mr. Ezzati says twice as many men as women died from such cancers.

"Some of the risk factors, in particular tobacco and alcohol, in most developing countries are much more common among men than they are among women," he said.

Mr. Ezzati and his colleagues say that the best option for reducing the increasing global burden of cancer is through lifestyle and environmental modifications because medical science is not close to controlling it. Despite the drive to seek cancer cures, they point out that advances in treatment have not been as effective as for other chronic diseases, and effective screening methods are available for only a few cancers.

"There is a huge amount of resources going in to the war on cancer and to biomedical technologies," explained Majid Ezzati. "It is certainly not matched by equivalent research and application of risk factor reduction. So I think this should be a reminder of how large the role of prevention can be for reducing cancer deaths."

The Iranian-born researcher says that while individuals can modify many of the risky behaviors that cause cancer, public policy can have a large influence by taxing tobacco and alcohol, banning smoking in public places, promoting healthy diets and behaviors, and other interventions.