Multiple Chemical Sensitivities

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Multiple chemical sensitivities (MCS), alternatively referred to as idiopathic environmental intolerances (IEI), is a term used to distinguish persons with medically unexplained symptomssuch as fatigue, concentration problems, headaches, and respiratory symptomswhen those symptoms are attributed to and/or triggered by environmental exposures. There is no other diagnostic label that concisely describes individuals who report such symptoms. The term sometimes encompasses individuals with pathologic (medically explained) conditions who attribute their diseases to chemical exposures, although mainstream academic opinion does not support this point of view.

The most widely cited definition for MCS (Cullen, 1987) includes the following four criteria:(1) the condition is acquired following a documented environmental exposure; (2) symptoms involve more than one organ system, and predictably recur and abate in response to environmental stimuli; (3) symptoms are elicited by chemical exposures that are demonstrable, but very low; and (4) the manifestations of MCS are subjective and without clinical evidence of explanatory organ system dysfunction. A common modification of these criteria does not require that the condition be acquired at a specific instance or from a specific putative exposure, allowing it to develop more gradually.

The classification and natural history of MCS is based to a large extent on the presence or absence of a diagnosable psychiatric condition such as affective disorders and anxiety disorders. Much higher rates (5070%) of such diagnoses are present in those who meet MCS criteria than in controls, although not necessarily higher than in those with other medically unexplained-symptom syndromes, such as chronic fatigue syndrome (CFS). Lower rates of psychiatric conditions appear to be seen in those patients who report a clear defining episode at the onset.

The exact relationship between environmental exposures and symptoms of MCS has not been defined and has been difficult to study. During attempts at clinical trials, the specific odor of the inciting agent has interfered with the usual experimental design, in which the subject should be unaware of the exposure. A clinical follow-up study suggested that exposure reduction through avoidance improved well-being, although paradoxically did not reduce levels of symptoms. To date, no studies have shown MCS to be progressive in terms of physical dysfunction or medical complications.

The extent of MCS in the general population is unknown, as no population-based studies using the MCS clinical definition have been published. However, significant amounts of various MCS correlates, such as self-reported sensitivity to chemicals (15%) and self-reported receipt of a physician diagnosis of MCS (6%) have been reported in a rigorous population-based study from California, suggesting the problem is of appreciable magnitude. The presence of MCS symptoms in those diagnosed or labeled with CFS, fibromyalgia, and Gulf War syndrome is well established. No controlled epidemiologic studies have addressed the issue of etiologic exposures, although pesticide exposures, solvent exposures, and constructionrelated exposures are cited in some surveys and case discussions.

Etiologic theories embrace four major categories: pathologic and toxicologic; psychophysiologic; psychiatric; and belief systems. The majority of scientific support currently is for mechanisms primarily focused in the control nervous systems, such as conditioned responses to odors or variations of panic disorders.

How best to prevent MCS is unclear, as it depends upon knowing the mechanisms by which MCS originates and is exacerbated. If exposures act primarily through psychiatric, psychophysiologic, or perceptual mechanisms, then control of exposure may not be an appropriate paradigm for prevention. On the other hand, if certain exposures clearly lead to an excess risk for such symptoms, then prevention of exposure would be beneficial.

Howard M. Kipen

Nancy Fiedler

(see also: Environmental Determinants of Health; Gulf War Syndrome; Risk Assessment, Risk Evaluation. )


Cullen, M. R. (1987). "The Worker with Multiple Chemical Sensitivities: An Overview." In Occupational Medicine: State of the Art Reviews, Vol. 2, ed. M. Cullen. Philadelphia, PA: Hanley and Belfus.

Kipen, H. M., and Fiedler, N. (1999). "Invited Commentary: Sensitivities to ChemicalsContext and Implications." American Journal of Epidemiology 150:1316.

(2000). "A 37-Year-Old Mechanic with Multiple Chemical Sensitivities." Environmental Health Perspective 108(4):377381.

Kreutzer, R.; Neutra, R. R.; and Lashuay, N. (1999). "Prevalence of People Reporting Sensitivities to Chemicals in a Population-Based Survey." American Journal of Epidemiology 150:112.