Multiple chemical sensitivity (MCS) is characterized by heightened self-reported sensitivity to extremely low concentrations of chemicals. It has numerous symptoms in common with the sick building syndrome, the Gulf War syndrome, and chronic fatigue. Despite much research, reproducible objective findings are lacking for MCS, as is a sound model to explain it. This paper proposes a 2-step model combining the needed epidemiologic terminology with that of psychophysiological activation and sensitization. It is suggested that different environmental stressors act as initiators. After initiation, the limbic system and other parts of the brain become sensitized and hyperrreactive to environmental triggers. Odor acts as one important trigger. Future research should use more biological assessments in combination with environmental and psychosocial data and involve patient groups with similar symptoms, although diagnosed as suffering from different entities. The similarities and differences of patients with such entities need to be understood before the entities themselves can be understood, diagnosed, treated, and prevented.

Key terms electric sensitivity; environmental hyperreactivity; environmental illness