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Multiple chemical sensitivity (MCS) is the name given to a very wide range of symptoms, related to a large number of chemicals. MCS has been called by various names, including "environmental illness" and, most recently, "toxicant-induced loss of tolerance" (TILT). MCS has been much talked about since the 1980s but research so far has not yet uncovered the underlying mechanisms.

Some doctors believe that the symptoms of MCS sufferers are "all in the head." This is a view which is far from helpful for patients. Dr Joseph LaDou, Professor of Occupational Medicine at the University of California says, "Although the aetiology of MCS is controversial, the patient may be suffering from disabling symptoms, frustrated by the lack of definitive answers from clinicians, and is sometimes desperately seeking advice and counsel regarding treatment. Approaching the history with the suspicion that the patient with MCS is suffering from a psychiatric disorder, is malingering, or seeking monetary benefits is not helpful."

Although a psychological cause for MCS has not been ruled out, evidence from most research on MCS points to a physical cause or causes. What puzzles doctors, and frustrates sufferers, is that the symptoms vary widely between individuals, and the substances associated with it are also numerous. Although there are several definitions, most agree that MCS affects one or many organ systems, symptoms come and go, and symptoms are brought on by very low exposures to many different chemicals.

Although they vary a great deal, the symptoms of MCS often involve the upper respiratory tract (blocked nose, dryness or burning), central nervous system (problems with memory or concentration, insomnia, drowsiness, irritability or depression), and gut, as well as muscle or joint pain, headaches and tiredness. These symptoms are triggered by low-level exposure to a large range of substances, but the initial cause will usually have been exposure to one of a smaller number of initiators. In other words, MCS is a two-stage process, involving initiation and triggering.

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