In one of the most famous scenes in American film history, actor Paul Henreid, playing a dapper married man on an ocean cruise in the 1942 movie Now, Voyager, places two cigarettes between his lips, lights them both, and passes one to a young, idealistic, and unmarried passenger, played by Bette Davis.

This provocative encounter quickly spread the allure of on-screen smoking. Humphrey Bogart and Marlon Brando were habitual smokers, on- and off-screen. Bogart died of throat cancer. Audrey Hepburn smoked several packs a day. For 20 years, Lucille Ball enthusiastically advertised Philip Morris cigarettes. Al Pacino gave up smoking following throat disorders that produced his famous deep, raspy voice. Yet young moviegoers, in particular, learned from these celebrities that smoking was cool.

Glamorized smoking, however, is not a film noir relic. Today, according to the University of California, San Francisco, characters light up in 35 percent of American films intended for young audiences. Pediatrics medical journal found that children as young as ten, exposed to on-screen smoking, are almost three times more likely to take up the habit.

That's why the Harvard School of Public Health has been pressuring Hollywood to eliminate images of smoking in films accessible to children and teens.

At first, Hollywood would not budge. It argued that showing lit cigarettes does not imply industry approval of the habit. But earlier this month, the motion picture association president called smoking "unacceptable behavior in our society." Now new U.S. films depicting smoking -- except those with historical scenes in which realism requires a smoke-filled room -- will carry at least an "R" rating. That means that, in theaters anyway, most on-screen puffing will be seen by adult audiences only.