A review of multiple chemical sensitivity

Objective-To review critically the scientific literature on multiple chemic al sensitivity (MCS). Definitions of MCS vary but, for this review, a broad definition of MCS was adopted as symptoms in more than one organ system el icited by various unrelated chemicals at very low levels of exposure. Methods-A systematic literature search identified several hundred reference s from which key papers were selected. Two questions are considered, does M CS exist and what causes MCS. Results and Conclusions-Despite extensive literature on the existence of MC S, there is no unequivocal epidemiological evidence; quantitative exposure data are singularly lacking; and qualitative exposure data are, at best, pa tchy. There is also some evidence to suggest that MCS is sometimes used as an indiscriminate diagnosis for undiagnosed disorders. Despite this, the co llated evidence suggests that MCS does exist although its prevalence genera lly seems to be exaggerated. Many causal mechanisms have been proposed, som e suggesting a physical origin-such as MCS reflecting an immunological over load (total body load)-others favouring a psychological basis-such as MCS s ymptoms being evoked as part of a conditioned response to previous trauma. The available evidence seems most strongly to support a physical mechanism involving sensitisation of part of the midbrain known as the limbic system. However, it is increasingly being recognised that the psychological milieu of a person can considerably influence physical illness, either through ge nerating a predisposition to disease or in the subsequent prognosis. Work i s needed to establish the prevalence of MCS and to confirm or refute select ed causal mechanisms.