Science Daily [sciencedaily.com]:
Researchers at the Max Planck Institute for Chemistry in Mainz have now developed a method that can objectively evaluate the age at which children and adolescents can safely watch a movie. They measured the composition of air in cinemas as well as levels of volatile organic compounds (VOCs) during 135 screenings of eleven different movies. Over 13,000 audience members were involved. For a variety of film genres and age groups, the researchers found that isoprene levels reliably correlate with the age rating of a film. "Isoprene appears to be a good indicator of emotional tension within a group," says Jonathan Williams, group leader at the Max Planck Institute for Chemistry. "Our approach could therefore provide an objective criterion for deciding how movies should be classified."
They couldn't already objectively measure fear by the amount of urine-soaked seat cushions?