An emerging issue in environmental health is the phenomenon of multiple chemical sensitivity (MCS). Multiple chemical sensitivity is a controversial disorder characterized by multiorgan symptoms in response to low-level chemical exposures that are considered safe for the general population. The onset of MCS is often attributed to prior repeated chemical exposures in the home and/or workplace, and, once initiated, symptoms are triggered by extremely low levels of many chemicals/foods. No single case definition exists for MCS due to several issues that call into question its validity as a distinct illness induced by prior chemical exposure. Hypotheses regarding the etiological basis for MCS range from direct toxicological effects of chemicals to the notion that MCS is purely a psychological "belief system". One leading hypothesis suggests that MCS represents a neural sensitization phenomenon, wherein susceptible individuals demonstrate extreme sensitivity to chemicals and odor intolerance due to central nervous system (CNS) sensitization processes. The recent development of an animal model for MCS provides some support for the sensitization hypothesis and may offer evidence for behavioral changes observed in at least a subset of those reporting MCS.