A study published in the March 1, 1990 issue of the "Proceedings of the National Academy of Sciences" stated that "there are virtually no reports of fatal cannabis overdose in humans" and attributed this safety to the low density of cannabinoid receptors in areas of the brain controlling breathing and the heart. Gettman claims that the discovery of the cannabinoid receptor system in the late 1980s revolutionized scientific understanding of cannabis' effects and provided further evidence that it does not belong in Schedule I.