Movies can receive Adults-Only or Adult-Accompaniment ratings for a number of reasons, although quite often it's due to scenes that are too upsetting for younger viewers. According to a new study, it may soon be possible to arrive at such ratings simply by testing the air in a theater.

As it is, film ratings are typically determined by a panel of people who watch a film before it's released in a given market. Needless to say, it's a highly subjective approach, with an Adult rating potentially limiting the box office revenues of a film.

To make things more objective, scientists at Germany's Max Planck Institute for Chemistry embarked on a study in which a total of over 13,000 test subjects attended 135 screenings of 11 different previously-released movies. As the participants watched the films, a mass spectrometer in the cinema's ventilation system measured concentrations of 60 chemical compounds within the air, once every 30 seconds.

It was discovered that concentrations of isoprene, given off by the viewers, consistently rose in direct correlation to the "Adultness" of a movie's existing rating. Isoprene is stored in muscle tissue, and is given off either via the circulatory system, through the skin, or in exhaled breath as we move. The scientists believe that its higher levels during child-unfriendly films may be due to audience members nervously squirming in their seats or tensing their muscles during particularly intense scenes.

While it may not replace traditional film-classification methods, it is hoped that the system might find use as a deciding factor in cases where the rating of a movie is disputed.

"Isoprene appears to be a good indicator of emotional tension within a group," says research group leader Jonathan Williams. "Our approach could therefore provide an objective criterion for deciding how movies should be classified."