LONDON (Reuters) - Billions of people around the world who are exposed to secondhand smoke may have an increased risk of developing lung cancer because passive smoking causes the disease, health experts said on Wednesday.

A comprehensive review of medical studies by researchers at the International Agency for Research on Cancer (IARC) showed passive smoking causes cancer and that chemicals and gases in tobacco contribute to cancer of the stomach, liver, kidney, uterine cervix and to myeloid leukaemia.

"Involuntary smoking--breathing in secondhand smoke--is carcinogenic to humans," said Professor Jonathan Samet, of Johns Hopkins University in Baltimore, Maryland, and a member of the IARC group.

Although the concentrations are not as high, passive smokers are breathing in the same carcinogens as smokers.

"There is elegant evidence ranging from what can be measured in air to what can be measured in the body fluids and urine of non-smokers to show that those carcinogens are being breathed in. They are being absorbed into the body," Samet told a news conference.

"To my knowledge it is the first time an organisation with global sweep has reached that conclusion," he added.

IARC, an extension of the World Health Organisation (WHO), is based in Lyons, France. Its findings on smoking are based on an independent analysis of more than 50 medical studies by 29 experts from 12 countries.

The scientists said they found no increased risk from secondhand smoke for childhood cancers but they did not know what impact long-term exposure to tobacco smoke would have on children as they grow older.

ASTOUNDING PROPORTIONS

An estimated 1.2 billion people worldwide smoke cigarettes, cigars, pipes or bidis--tobacco rolled in a leaf--and expose billions more non-smokers to the carcinogenic chemicals, according to Samet.

Marsha Williams, of the British anti-tobacco campaigning group ASH, called for urgent action.

"Passive smoking is quite clearly more than just the nuisance many of the world's tobacco companies would have us believe. People are harmed and killed by it and it is time industry, government and smokers themselves woke up to this fact," she said in a statement.

The scientists also found evidence that in addition to causing 90% of lung cancer cases, smoking also contributes to cancers of the stomach, liver, kidney, uterine cervix and a type of leukaemia--but that it is not linked to breast or prostate cancer.

Samet said scientists are only beginning to see the full picture of what happens when a generation begins to smoke at an early age and continues to smoke throughout their adult lives.

"We're still learning about just how damaging cigarette smoking is. We found that cancers beyond those that we had previously listed as caused by smoking can now be added to the list," he said.

Tobacco smoke contains over 4,000 chemicals in the form of particles and gases. Carbon monoxide, ammonia, formaldehyde and hydrogen cyanide are among the potentially toxic ones.

About one half of persistent smokers will be killed by a tobacco-related disease and half of those deaths will occur in middle age.

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