The number of U.S. movies showing people smoking has declined since 2005, but cigarettes still feature in far too many films and could be influencing young people to take up the habit, according to a report released on Thursday.

The report's authors recommended that movie ratings also consider whether the film depicts smoking and suggested strong advertisements about the dangers of smoking precede movies that show tobacco use.

The results of this analysis indicate that the number of tobacco incidents peaked in 2005, then declined by approximately half through 2009, representing the first time a decline of that duration and magnitude has been observed, the team at the U.S. Centers for Disease Control and Prevention, the University of California San Francisco and elsewhere wrote.

However, nearly half of popular movies still contained tobacco imagery in 2009, including 54 percent of those rated PG-13, and the number of incidents remained higher in 2009 than in 1998, they added in the CDC's weekly report on death and illness.

Two members of the U.S. House of Representatives, Democrat Edward Markey and Republican Joseph Pitts, who both serve on the Energy and Commerce Committee, wrote the Motion Picture Association of America encouraging the industry to adopt stronger anti-smoking measures.

Exposure to onscreen smoking in movies increases the probability that youths will start smoking. Youths who are heavily exposed to onscreen smoking are approximately two to three times more likely to begin smoking than youths who are lightly exposed, the CDC report reads.

The researchers counted each time tobacco use was shown in the biggest-grossing films of 1991 to 2009.

This analysis shows that the number of tobacco incidents increased steadily after the 1998 Master Settlement Agreement between the state attorneys general and the major cigarette companies, in which the companies agreed to end brand placement, they wrote.

They said the Motion Picture Association of America had done little to make changes but noted some studios had made voluntary changes and said Viacom was the first company whose movies rated for youth showed no use of tobacco in 2009.

They suggested more policies could encourage filmmakers to do better.

Such policies could include having a mature content (R) rating for movies with smoking, requiring strong antitobacco ads preceding movies that depict smoking, not allowing tobacco brand displays in movies, and requiring producers of movies depicting tobacco use to certify that no person or company associated with the production received any consideration for that depiction, they wrote.