Actors are smoking significantly less often on the silver screen these days, but more than half of all PG-13 rated movies still show people lighting up — a situation that encourages some young people to take up the habit, a new study reports.

The authors of the study, which was published by the Centers for Disease Control and Prevention (CDC) last Friday, recommend that movies with tobacco imagery be given an R rating. They’d also like to see strong anti-tobacco ads before every movie that depicts smoking, a ban on showing specific tobacco brands in movies, and an assurance (with regulatory teeth, presumably) from movie producers that they’re not taking money from Big Tobacco whenever they do decide to depict tobacco in their productions.

The R-rating requirement is the recommendation that’s most likely to cause shivers down the spines of movie producers. A 2005 report discovered that family-friendly films had a much higher return on investment (94.5 percent for G-rated, 72.6 percent for PG-rated and 43.6 percent for PG-13-rated films) than R-rated ones (28.7 percent).

Consider the financial repercussions if these popular kid-oriented movies — all with tobacco images in them — had received an R rating: “Curious George,” “The Fantastic Mr. Fox, “The Chronicles of Narnia,” “Scooby Doo 2: Monsters,” and “The SpongeBob Square Pants Movie.” How much money would they have made? Among current PG-13 releases that would get R ratings are “Eat, Pray, Love,” “The Other Guys,” “Salt” and “Dinner for Schmucks.”

For the new study, researchers counted the number of incidents of tobacco use in each of the 50 top-grossing movies from 1991 through 2001 and in all movies that were among the 10 top-grossing movies in any calendar week from 2002 to 2009. They found that the peak number of incidents (about 4,000) occurred in 2005. After that year, the incidents dropped steadily, to about 2,000 in 2009.

Interestingly, other research has found that the percentage of high-school students trying cigarettes for the first time also declined during that period, from 54.3 percent in 2005 to 46.3 percent in 2009.  “The reduction in smoking in movies might have been a contributing factor to this decline,” the authors of the CDC study write.

Another possible factor: Pressure from government officials, health advocates and the general public (through such groups as the Smoke Free Movies campaign). Since 2007, several major studies have adopted internal policies for monitoring the smoking content of their movies, the study reports. However, only one company — Paramount (Viacom) — no longer permits tobacco images in its youth-rated movies.

Although the recent drop in cigarette use among teenagers is encouraging, tobacco remains alluring to them. Each day about 4,000 young people between the ages of 12 and 17 smoke a cigarette for the first time in the United States, the CDC reports. And, according to a 2008 report from the National Cancer Institute, young people with the heaviest exposure to onscreen smoking are about two to three times more likely to light up that first cigarette than their peers who have the lightest exposure.

FYI: The lead author in the CDC study is Stanton A. Glantz, director of the Center for Tobacco Control Research and Education at the University of California, San Francisco. He got into a heated dispute with film director James Cameron earlier this year about Cameron’s decision to have Sigourney Weaver’s character, Grace, smoke cigarettes in “Avatar,” which is rated PG-13. Cameron's argument (in brief): "I wanted Grace to be a character who is initially off-putting and even unpleasant. She's rude, she swears, she drinks, she smokes. She is not meant to be an aspirational role model to teeangers." Glantz' response (again, in brief): Showing an environmental scientist languidly enjoying a cigarette is comparable to putting "a bunch of plutonium in the water supply."

If the proposal in the CDC study ever gets implemented, Cameron would have the creative license to have his character smoke as many cigarettes as he'd like. He just wouldn't get a PG-13 rating for the movie.