Chemical sensitivity: Symptom, syndrome or mechanism for disease?

Several different meanings have been attached to the term ''chemical sensitivity'' by those who use it. Feeling ill from odors is a symptom reported by approximately one-third of the population. The syndrome of chemical sensitivity, frequently called ''Multiple Chemical Sensitivity'' or ''MCS'' has been the subject of three federally-sponsored workshops; at least five different case definitions for research on MCS have been proposed. In contrast, the hypothesis that chemical sensitivity may be a mechanism for disease posits that a broad spectrum of ''recognized'' chronic illnesses, ranging from asthma and migraine to depression and chronic fatigue, may be the consequence of environmental chemical exposures. According to this theory, a two-step process occurs: (1) an initial salient exposure event(s) (for example, a one-time, intermittent, or continuous exposure to pesticides, solvents, or air contaminants in a sick building) interacts with a susceptible individual, causing loss of tolerance for everyday, low level chemical inhalants (car exhaust, fragrances, cleaning agents), as well as for foods, drugs, alcohol, and caffeine; (2) thereafter, such common, formerly well-tolerated substances trigger symptoms, thus perpetuating illness. ''Masking'' (acclimatization, apposition, and addiction) may hide these exposure-symptom relationships, thus obfuscating the environmental etiology of the illness. Accumulating clinical observations lend credence to a view of chemical sensitivity as an emerging theory of disease causation and underscore the need for its testing in a rational, scientific manner. While chemical sensitivity may be the consequence of chemical exposure, the term ''toxicant-induced loss of tolerance'' more fully describes the two-step process under scrutiny.