A new US report that says passive smoking might not greatly increase the risk of heart disease and lung cancer has angered health authorities.
Previous research has suggested that exposure to environmental tobacco smoke could increase the risk of heart disease by 30 per cent.
But the authors of the new study said their research showed exposure to environmental tobacco smoke could not "plausibly" cause a 30 per cent increase in risk of coronary heart disease.
"It seems premature to conclude that environmental tobacco smoke causes death from coronary heart disease and lung cancer," the report said.
The researchers used a Californian study of 118,094 people and focused on 35,561 who had never smoked but were married to someone who was a current or former smoker.
Mr James Enstrom of the University of California and Mr Geoffrey Kabat of New Rochelle in New York, found that, among smokers, there was a strong risk of heart disease, lung cancer and chronic obstructive pulmonary disease.
But exposure to passive smoke, through being married to a smoker, was not significantly linked with death from heart disease or lung cancer.
The British Medical Journalsaid Mr Enstrom has received funds from the tobacco industry for research because it was impossible to get the money from other sources.
Dr Vivienne Nathanson, the British Medical Association's head of science and ethics, said: "It would be wrong to be swayed by one flawed study funded by the tobacco industry".
AP