The researchers attribute the dramatic drop to the "near elimination" of harmful effects of "second-hand" smoke - passive smoking. A smoke-free environment also encourages smokers to reduce smoking or quit altogether, the team adds.

Statistician Stanton Glantz, at the University of California, San Francisco, and colleagues studied diagnoses of heart attacks in the town of Helena, Montana, where the ban was imposed.

"This striking finding suggests that protecting people from toxins in second-hand smoke not only makes life more pleasant, it immediately starts saving lives," Glantz says. The researchers claim the study is the first to show that smoke-free policies rapidly reduce heart attacks, as well as having long-term benefits.

"This clearly shows the great need for controls on smoking in public places," says Amanda Sandford of UK pressure group Action on Smoking and Health. "Passive smoking is a killer. The public certainly underestimates the impact of passive smoking on the heart."

Small dose, large impact

The smoking ban in Helena was introduced in June 2002 but was suspended after six months because of a legal challenge. Glantz and researchers at St Peter's Community Hospital in Helena compared the hospital charts of heart attack patients admitted from the smoke-free town with those from neighbouring areas, as well as with records from Helena in the four years before the ban.

During an average six-month period, heart attack admissions to the hospital had averaged just under seven per month. But this fell to less than four a month during the smoking ban.

The study suggests that although second-hand smoke delivers only a small dose of harmful chemicals, it appears to have a very heavy impact on health. This paradox has puzzled scientists before, says Robert West, an expert on smoking cessation at St George's Medical School, London, "but there are now plausible mechanisms for this".

Immediate and acute

The risk of lung cancer rises steadily with the amount of tobacco a person smokes, he notes, but the risk of heart attack shows a non-linear relationship. Recent studies have shown "there is an immediate and acute effect of passive smoke exposure as a particulate pollutant," West told New Scientist.

The mechanism for this effect is likely to be that the inhaled smoke stimulates the immediate production of macrophages - white blood cells that "clean up the system".

But these break down and lead to the production of blood clotting agents. "So if someone is teetering on the brink of a heart attack, this clotting is likely to tip them over," says West.

Sandford notes that many public smoking bans are becoming more common. New York banned smoking from 30 March, and the Republic of Ireland will introduce a ban on smoking in the workplace - including pubs and restaurants - from January 2004.

The study was presented on Tuesday at the American College of Cardiology's annual meeting in Chicago

Shaoni Bhattacharya