London ? Tobacco smoke is even more cancerous than previously thought, for both smokers and nonsmokers who breathe in the fumes, causing cancer in many more parts of the body than previously believed, a panel of experts has concluded.

Although smoking has been established as a leading cause of cancer, scientists have only now been able to track more than one generation of smokers to develop a clear picture of the dangers of tobacco.

The scientists, convened by the International Agency for Research on Cancer, a branch of the World Health Organization, said Wednesday that for types of cancer already known to be caused by smoking, the risk of tumors is even higher than previously noted. The research also definitively proves that secondhand smoke causes cancer.

The analysis is the first major examination of the accumulated research on tobacco smoke and cancer since 1986. A full report of the findings will be published later this year.

The scientists combined the results of more than 3,000 studies involving millions of people, which allowed them to draw conclusions not possible in smaller studies.

“We are still learning about just how damaging cigarette smoking is,” said the panel’s chairman, Dr. Jonathan Samet, head of epidemiology at the Johns Hopkins School of Public Health. “Only now are we beginning to see the full picture of what happens when a generation begins to smoke at an early age, as youth do, and then smoke across their whole lifetime. Before, we only had snapshots.

“The full picture is more disturbing than what we saw when we only had the smaller pieces.”

There are about 1.2 billion smokers worldwide, half of whom will die prematurely from cancer, heart disease, emphysema or other smoking-related diseases, research has shown.

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The best way to prevent those deaths is to get smokers to quit, the scientists said.

“Our group concluded that any possible public health gains from changes in cigarette characteristics or composition would be minimal by comparison. Changes in cigarettes are not the way to prevent cancer,” Samet said.

The 29 experts from 12 countries found that in types of cancer already linked to smoking, the risk is even higher than previously believed.

“For example, for tumors of the bladder and the renal pelvis, previously we thought the elevated risk was maybe three to four times that of a nonsmoker. Today, it looks like the risk is elevated five to six times,” said Dr. Paul Kleihues, director of the U.N. cancer research agency.

Two cancer types under suspicion were cleared those of the breast and endometrium, or lining of the womb. Prostate cancer has been less studied, but the group did not believe it is caused by smoking.

Types of cancer newly declared to be caused by smoking were cancers of the stomach, liver, cervix, uterus, kidney, nasal sinus and myeloid leukemia.

“It does look as if it’s the cancers that are principally caused by hormones that are not affected by smoking. Most of the other cancers throughout the body are induced by exposure to chemicals, often environmental ones,” said Sir Richard Doll, an Oxford University professor who was on the panel.

“Practically all the cancers of tissues that are exposed to the environment in one way or another are affected by the chemicals distributed throughout the body when you inhale tobacco smoke,” Doll said.

Cancers already identified as being caused by smoking include lung, oral cavity, gullet, larynx, pharynx, pancreas and bladder.

The panel also analyzed 20 or 30 years of research on cancer and secondhand smoke, and concluded secondhand tobacco smoke increases the risk of lung cancer by 20 percent.

While some national governments have concluded secondhand smoke causes cancer, this is the first time the U.N. cancer agency which has strict criteria for evaluating evidence or any organization with a global sweep has reached this conclusion.

The group found no clear evidence that children exposed to their parent’s tobacco smoke in the womb or after birth have an elevated risk of developing childhood cancers. Whether they face an increased risk of lung or other cancer in adulthood remains unclear, the panel said.