Despite continuing pressure from health groups for Hollywood to quit the nicotine habit, studies show that cigarette smoking is featured in more films than at any time since the classic black and white romance epics of the 1950s when a swirl of smoke did not suggest heart disease.

Research recently submitted by the University of California, for instance, suggests that as many as three-quarters of films produced in Hollywood feature smoking by one or more characters and the figure drops only to 36% for films aimed at children.

Hollywood's addiction has apparently not been eased by the 1998 settlement barring studios from entering into product placement deals with tobacco companies.

A survey by the Harvard School of Public Health asserts that the top 50 grossing films over a 12-month period showed 12.8 incidents of smoking per hour - the highest rate in 10 years. The statistics have spurred the Harvard School to renew a campaign to lobby studios and the Motion Picture Association of America to take steps to exorcise smoking from scripts, particularly when it comes to films accessible to young audiences. It is using as additional ammunition studies published recently by the Lancet and Pediatricsmedical journals, suggesting that children as young as 10 who see people lighting up on screen are 2.7 times more likely to begin smoking themselves.

In a presentation to the studio heads, Harry Bloom, the Harvard School dean, questioned the current rating system that takes account of bad language but not smoking, which has long been linked to fatal disease.

"No one has died from hearing the f-word, " Bloom said. "But 438,000 people in US, and five million worldwide, die each year from tobaccorelated illness. If you are honest I think you will admit that most smoking in movies is both unnecessary and clich�d."

The Harvard School has a track record of effective public health campaigns and is remembered most for introducing in the 1980s the notion of "designated drivers" among friends who go out drinking. It persuaded television producers to incorporate the new slogan in prime-time shows - including the iconic Cheers - and drunk-driving fatalities fell 25% in the US in three years.

Getting Hollywood similarly to promote non-smoking lifestyles in films has proved a tougher sell. But there have been notable exceptions. In the 2006 hit The Devil Wears Prada, centred on the high-stress fashion industry, almost incredibly, no one smoked.

Producers tried to apply a similar policy to last year's Stranger than Fiction until being persuaded that a tortured novelist played by Emma Thompson simply had to have the habit.

Sometimes, however, it isn't just the audience members that are at risk from on-screen puffing. The actress Salma Hayek confessed last week that she is only now managing to kick a smoking habit that she says she picked up portraying the Mexican painter Frida Kahlo.

"I used to have a terrible judgement of smokers, " she revealed. "I was like, 'Why do they do this? It tastes bad, it's going to kill them, and it doesn't even get them high. It's the sh**iest vice you could possible pick.' Then I got hooked during Frida. I've tried to quit before. But this time I'm done with it."