If people avoided major risk factors for cancer, more than a third of the 7 million cancer deaths each year could be prevented, scientists said on Friday.
In a report in The Lancet medical journal, the researchers estimated how many deaths from 12 types of cancer were caused by exposure to nine risk factors.
They calculated that smoking, alcohol, obesity, poor diet, unsafe sex, lack of exercise and other factors contributed to 2.43 million cancer deaths worldwide in 2001.
"A third of cancer deaths could have been avoided had those risks been reduced," said Dr Majid Ezzati of the Harvard School of Public Health in the United States.
"Prevention is probably still our best bet for reducing cancer deaths. It is by far larger than what we may be able to achieve using medical technology."
Smoking, which is linked to lung, mouth, stomach, pancreatic and bladder cancers, is the biggest avoidable risk factor, followed by alcohol and not eating enough fruits and vegetables.
"Of the 2.43 million deaths, 37 percent of them are from lung cancer," said Ezzati. "The total lung cancer deaths in the world are 1.23 million and of those 900,000 of them are caused by these risk factors."
Smoking has increased in developing countries in the past few decades so the number of avoidable deaths could grow, he added.
Obesity also plays a role in colorectal and breast cancer in high income countries, according to the research.
Infection with the human papillomavirus (HPV) through unsafe sex is a contributing cause of cervical cancer in women in sub-Saharan Africa and parts of Asia, mainly because of a lack of screening and clinical services.
Urban air pollution is a risk factor for cancer in eastern and southern Asia, while indoor smoke from burning coal is a particular problem in China.
Ezzati said hepatitis infection, which is linked to liver cancer, is sometimes spread by the use of contaminated syringes in health centers in poor countries.
More than 100 scientists around the world contributed data for the study and reviewed medical evidence.
