The CNS is an integrative center that transduces biological and psychosocial experiences into changes in neural activity; in turn, these send biological, psychological, and social behaviors back out into the environment. Many pesticides and solvents, which are putative triggers for MCS, have effects on central nervous system function (Isaacson et al., 1990; Llorens et al, 1990; Tham et al., 1990) that could interact with individual vulnerabilities.

The olfactory system is the usual pathway by which airborne chemicals from the external environment interact with the brain. The limbic system is the phylogenetically older part of the CNS that receives extensive input from the olfactory system and regulates numerous functions including mood and social behaviors, cognitive function, and eating/drinking/ reproductive behaviors (Cain, 1974). Dysfunctions of the limbic system may play a key role in unipolar and bipolar depression, panic and other anxiety disorders, and schizophrenia. Damage to the hippocampus ill the limbic system is an important feature of dementia, especially of the Alzheimer type (Sapolsky, 1990).

At the outset, a model for MCS must be able to account for the clinical phenomenology. Patients with MCS report multiple symptoms in multiple systems. Clinical observations also include initial sensitization with acute high dose exposure, chronic low-dose sensitivity, spreading of number of sensitivities from one to many chemicals, adaptation to chronic exposures, and concomitant multiple food sensitivities (MFS). Little rigorous data are available to characterize the MCS population and their responses to chemical challenges (Tables 1a and 1b).