An emerging issue in environmental health is the phenomenon of multiple che
mical sensitivity (MCS). Multiple chemical sensitivity is a controversial d
isorder characterized by multiorgan symptoms in response to low-level chemi
cal exposures that are considered safe for the general population. The onse
t of MCS is often attributed to prior repeated chemical exposures in the ho
me and/or workplace, and, once initiated, symptoms are triggered by extreme
ly 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 distin
ct illness induced by prior chemical exposure. Hypotheses regarding the eti
ological basis for MCS range from direct toxicological effects of chemicals
to the notion that MCS is purely a psychological "belief system". One lead
ing hypothesis suggests that MCS represents a neural sensitization phenomen
on, wherein susceptible individuals demonstrate extreme sensitivity to chem
icals and odor intolerance due to central nervous system (CNS) sensitizatio
n processes. The recent development of an animal model for MCS provides som
e support for the sensitization hypothesis and may offer evidence for behav
ioral changes observed in at least a subset of those reporting MCS.