The purpose of this study is to determine if subjects diagnosed with 1) Multiple Chemical Sensitivity Syndrome (MCSS) have a lower cognitive efficiency (CE) when compared to medical and normal controls but greater than those subjects suffering from Post Traumatic Stress Disorder (PTSD); 2)after controlling for psychological distress, NCSS subjects will have lower CE than all other subjects; and 3) cacosmia (order intolerance index) of all subjects will correlate inversely with CE, after controlling for psychological distress.