A measurable criterion now exists for determining the age rating of films. A group of scientists at the Max Planck Institute for Chemistry in Mainz has found that the concentration of isoprene in cinema air correlates with the cinema industry’s voluntary classification of films. Evidently, the more nervous and tense people are, the more variable is the isoprene they emit. This can be used to deduce how “stressful” a film might be for children and adolescents.