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New study plays down risks from passive smoking

As the world votes on the first international treaty to protect public health from smoking, U.S. researchers report controversial findings that passive smoking may not be as harmful as previously thought.

The research by Dr James Enstrom, of the University of California, Los Angeles and Dr Geoffrey Kabat of the State University of New York, Stony Brook is published in today's issue off the British Medical Journal.

Inhaling second-hand cigarette smoke has been linked to a 25 % increase in risk of lung cancer and heart disease, but Enstrom and Kabat's study, which focused on more than 35,500 people who had never smoked but who had spouses who did, found the effects may have been overstated.

"The association between exposure to environmental tobacco smoke and coronary heart disease and lung cancer may be considerably weaker than generally believed," said the researchers, however they did not rule out a small effect.

The authors declare in the article that the cancer prevention study of 118,094 California adults from 1959 to 1998, used in their analysis, was funded by the tobacco industry. Enstrom states he had only received industry funds for his tobacco related epidemiological research "because it had been impossible for him to obtain equivalent funds from other sources". Kabat said he had never received industry funds until last year, when he conducted an epidemiological review for a law firm which had several tobacco companies as clients. Both state they are "lifelong non-nonsmokers whose primary interest is an accurate determination of the health effects of tobacco".

Study design "underestimated" risk

However, an editorial in the same journal criticised the study. Professor George Davey Smith, a clinical epidemiologist at the University of Bristol in the U.K. argued that the researchers had used a measure of exposure to smoke which "underestimated" the risk.

"It is not being married to a smoker - the indicator of exposure to environmental tobacco smoke used in the paper by Enstrom and Kabat - that leads to disease; rather, it is the inhalation of environmental tobacco smoke."

Smith said such misclassification was a key problem in passive smoking studies

"In the field of passive smoking the tobacco industry has eagerly discussed measurement error that would lead to the effect of passive smoking being overestimated....while ignoring misclassification that would lead to underestimation of the strength of the association between environmental tobacco smoke and disease," said Smith.

Enstrom argues his detractors are applying a double standard because several major studies routinely quoted as proving the risks of second-hand smoke also relied on a comparison between smoking and nonsmoking spouses. However Smith argues there are too many problems with epidemiological studies and other methods of investigating the problem are required.

"The considerable problems with measurement imprecision, confounding, and the small predicted excess risks limit the degree to which conventional observational epidemiology can address the effects of exposure to environmental tobacco smoke," he said.

The new study comes in the run-up to a final vote on the World Health Organisation's Framework Convention for Tobacco Control, to be presented next week at the annual conference of the 192 WHO member states in Geneva. Under the agreement, each party will "in accordance with its constitution or constitutional principles, undertake a comprehensive ban of all tobacco advertising, promotion and sponsorship." Countries unable under their constitutions to impose such a ban would seek to restrict tobacco advertising. The treaty will come into effect once it is ratified by 40 countries.

Some 4.9 million people are estimated to have died last year from tobacco use around the world, and without coordinated international action the number of deaths was projected to rise to 10 million a year by 2020, according to the WHO.

Tags: environment, health