Abstract
The hypothesis is advanced that when a living organism is exposed to a toxic chemical, the appropriate detoxifying reactions will not take place unless the toxic stimulus is above a certain threshold level. It is proposed that exposures below this level give rise to the phenomena of multiple chemical sensitivity (MCS). Supporting arguments are given based on thermodynamics and biological parallels, and it is shown that the idea gives simple explanations for some of the paradoxical symptomatology of MCS. MCS is described as the complement of allergy, and it is suggested that the theory could lead to new approaches to the treatment of MCS.