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Even though smoking appears far less frequently in U.S. television shows than it used to, its portrayal may still be triggering the urge in adult smokers, according to a new study.

Researchers reviewed patterns in TV smoking over more than 50 years and found that they tracked with changes in adult tobacco use, suggesting that even established smokers are influenced to light up by seeing it done on the small screen.

"Movie tobacco cues promoting smoking initiation in teens have been extensively covered in the literature, but this paper emphasizes that TV programming-promoted tobacco has been understudied and may be important as well," said lead study author Patrick E. Jamieson of the Annenberg Public Policy Center at the University of Pennsylvania in Philadelphia.

An effective cue "should promote smoking initiation, reinforce a current smoking habit and make it harder for smokers to quit," he told Reuters Health in an email, and smoking on television appears to fit that bill.

For the new study, Jamieson and a colleague noted instances of tobacco use in prime-time TV dramas between 1955 and 2010. They compared these numbers to U.S. adult smoking rates over the same period.

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Over the 55 years studied, smoking in real life and on TV both declined. Between 1955 and 1964 there was an average of almost three tobacco appearances per hour in the primetime dramas, which declined to less than one every three hours between 2001 and 2010.

For every additional tobacco appearance per hour of TV, the researchers found, each U.S. adult smoker consumed about two more packs of cigarettes per year.

Using an economic model, the authors determined that it was more likely TV smoking instances were influencing real-life smoking levels, rather than the other way around.

"The same-year TV tobacco portrayal predicted the same-year and the following-year adult cigarette consumption," but there was no year lag in the other direction, Jamieson said.

The declining appearance of tobacco on TV has happened alongside a similar decline in movies, the authors note.

Cigarette prices increased steadily over the study period, which also probably deterred some smokers. Decreasing TV portrayals of tobacco use seemed to have about half the power of increasing cigarette prices in discouraging smoking, according to the results published in the journal Tobacco Control.

Cigarettes haven't been advertised on television since a 1971 ban, but they continue to appear in the programming itself at producers' discretion.

Although the current study did not test whether showing cigarettes on TV would directly influence viewers, the authors do theorize that seeing tobacco on TV could trigger cravings in adult smokers.

Experts suggested that TV could be an important trigger, of which both smokers and former smokers should be aware.

"There is a concern that seeing smoking on TV may cause adults to both re-start smoking after they have quit and keep them from quitting," said Kristin Carson, senior medical research scientist for respiratory medicine at The Queen Elizabeth Hospital in Adelaide, South Australia.

"Seeing someone else smoke has been reported as a trigger for many smokers and ex-smokers," Carson, who was not involved in the new research, told Reuters Health in an email.

Dr. Roger Thomas of the Department of Family Medicine at the University of Calgary in Alberta, Canada, agreed.

"This is a very strong study methodologically," he told Reuters Health.

Quitting is often a long process with many relapses, since cravings can be triggered by stress, said Thomas, who was not involved in the study but administers tobacco counseling to his patients regularly.

"We advise people not to go to places where people smoke, to avoid the stimulus," he said. "We know there are very strong triggers."

Of course TV viewers don't know when tobacco is going to show up in their favorite programs, he said, but when it does, they should avert their gaze, and always be aware of subliminal messaging.

"Given how powerful a tobacco control tool raising tobacco prices is, I would like to see the tobacco prices continue to rise and producers and writers continue to voluntarily portray less tobacco, to not romanticize it, or if it is portrayed, show more of the problematic consequences of using it," Jamieson said.

"From a public health perspective the complete elimination of cigarettes from society altogether would be ideal," Carson said. "However the reality, at least in the short-term, is that tobacco use will continue to be a part of our society for as long as we continue to allow it to be."