New research suggests video games are not adequately rated for tobacco content.

Researchers at the University of California-San Francisco found that video gamers are being widely exposed to tobacco imagery. They concluded that a national ratings board set up more than 20 years ago is not a reliable source for learning whether video games contain tobacco imagery.

"Parents should stop relying on the ratings to screen for tobacco use in buying video games for their kids," Susan Forsyth, first author of the study, said in a statement.

Video games are ever-present in adolescent life in many countries. In the United States, 88 percent of youth between 8 and 18 years old play video games at least occasionally, according to a national study. Despite the widespread practice, there has been little research on whether smoking content is present in the games and whether games are being adequately rated for tobacco content.

Previous studies have found that smoking imagery in movies can lead youths to begin smoking. In the video game study, the researchers sought to assess whether tobacco content was appropriately labeled.

"The presence of tobacco imagery in (video) games exposes players to products and behaviors within an immersive...environment, with unknown effects on real-world smoking behavior," the authors wrote.

For the study, the researchers interviewed 65 gamers between 13 and 50 years old on their favorite games and whether the games contained smoking imagery. Tobacco content included visible smoking equipment, characters mentioning smoking or characters smoking a cigarette, pipe, cigar, or e-cigarette.

The authors found that game ratings do not accurately reflect tobacco content. While 8 percent of the games received tobacco warnings, 42 percent actually had tobacco content. Among games rated "M" for "mature," 75 percent contained verified tobacco content -- but the Entertainment Software Ratings Board (ESRB), which was established in 1994, provided warnings on only 4 percent of the games.

In one popular series, Metal Gear, the ratings board gave the game no tobacco content descriptor, yet it had extensive smoking throughout.

As a result, adolescents are being exposed to significantly more tobacco imagery than previously thought, the researchers said.

"The ratings board needs to stop pretending that it's providing accurate ratings," senior author Ruth E. Malone said in a statement. "And it should more thoroughly and consistently screen material for content and accurately report it."

The findings are detailed in the journal Tobacco Control.