The amount of smoking depicted in movies will play a bigger role in how a film is rated by the Motion Picture Association of America, the industry group announced on Thursday.

Underage smoking is already one of the factors the MPAA takes into consideration when it determines a movie's rating. Others include sex, violence and strong language.

If reviewed under the new rules, Good Night, and Good Luck, starring David Strathairn as Edward R. Murrow, would likely carry a warning about 'pervasive smoking.' If reviewed under the new rules, Good Night, and Good Luck, starring David Strathairn as Edward R. Murrow, would likely carry a warning about 'pervasive smoking.'
(Warner Independent Films)

However, MPAA chair Dan Glickman said Thursday that his group's ratings board will now also take into account smoking by adults and factors like the context in which characters light up, how pervasive the smoking is and whether the act is glamorized.

"Clearly, smoking is increasingly an unacceptable behaviour in our society," Glickman said in a statement.

"The appropriate response of the rating system is to give more information to parents on this issue."

For instance, under the new rules, the 2005 film Good Night, and Good Luck would be flagged for "pervasive smoking."

However, the celebrated portrait of journalist Edward R. Murrow would likely still retain the PG rating it was originally granted because of its historical context (that of a newsroom in the 1950s), according to Joan Graves, head of the MPAA's ratings board.

The board also released figures indicating that an MPAA study of films released from 2004 to 2006 showed that films that included even a glimpse of smoking fell from 60 per cent to 52 per cent.

The MPAA decision received support from a range of groups, including filmmakers, the American Cancer Society and the Harvard School of Public Health.

However, other critics are calling for the group to enact stronger measures, like a mandatory R rating (restricted to viewers over 17) for all films that depict smoking.

A study released in February by the American Medical Association Alliance indicated that 70 per cent of the U.S. adults in their poll would support an R rating for movies that show smoking.

With files from the Associated Press