Nonsmokers who regularly breathe others' tobacco smoke have a one-fifth greater chance of developing lung cancer because of it, said scientists with the International Association for Research on Cancer (IARC) in a recent report.

And smokers have more of an increased risk of cancer developing in many sites throughout the body than earlier thought, the report said. It was written by a panel of recognized experts from around the world.

"This picture of the burden of early deaths among generations shows that the impact of the tobacco epidemic is greater than previously thought," said Kurt Straif, MD, PhD, an IARC epidemiologist specializing in carcinogen identification and evaluation for the organization.

For example, carcinogens (cancer-causing chemicals) from tobacco smoke pass through the bladder on their way out of the body. The IARC report shows the chemicals increase a smoker's risk of bladder cancer by five or six times that of a non-smoker, instead of three or four times as earlier thought, said the authors.

The IARC team also confirmed that smoking causes or helps to cause some cancers that have not always been linked to tobacco. These include cancers of the cervix, liver, stomach, kidney, and nasal sinus, as well as myeloid leukemia.

The report makes it clear that smoking cigars, pipes, bidis, or any other form of tobacco is not less harmful than cigarettes.

And the risk is about the same for females as for males, the scientists noted.

The scientists found no smoking link to breast cancer, prostate cancer, or to cancers that develop during childhood, although Straif said it is too early to rule out the latter.

The report confirms the value of quitting smoking, the authors said.

"If you stop smoking in early adulthood — in your 30s — you can avoid much of the risk that you would incur if you continued smoking, and quitting at any time lowers risk compared to continuing to smoke," Straif said.

A panel of 29 experts from 12 countries drafted the report after looking at all the peer-reviewed literature on tobacco published in the last 25 years — more than 3,000 studies — according to very strict standards of proof.

The IARC is part of the United Nations' World Health Organization (WHO). The new report may bring needed attention to a massive problem, said one expert.

"Each country and region is in a different place in its awareness of the ultimate consequences of tobacco use," said Michael Thun, MD, vice-president of epidemiology and surveillance research for the American Cancer Society (ACS).

In Europe and many other parts of the world, there is not as much awareness of the added cancer risk from involuntary tobacco smoking, Thun said.

In Asia and other still-developing parts of the world, chronic infections in the stomach and liver make cancer in those organs very common. The new findings that tobacco smoke helps cause those cancers may be useful to health policy makers there, Thun said.

As countries struggle to find ways to stem the tide of tobacco-caused diseases, many are beginning to work together to get policies that can be effective in many nations.

"An appreciation of the magnitude of this epidemic is a really important piece that the IARC report tries to communicate," said Thun.