Environmental factors in medically unexplained symptoms and related syndromes: the evidence and the challenge.

Symptoms, and especially those without clear underlying medical explanations, account for a large percentage of clinical encounters. Many unexplained symptoms have been organized by patients and practitioners into syndromes such as chronic fatigue syndrome, multiple chemical sensitivity, sick building syndrome, Gulf War syndrome, and the like. All these syndromes are defined solely on the basis of symptoms rather than by medical signs. Some of the above-described conditions overlap strongly with explained conditions such as asthma. The relationship of such symptoms and syndromes to environmental exposure is often sharply debated, as is the distinction between the various syndromes. This leads to problems of what type of research should be conducted and who should conduct it. It is time to develop a comprehensive research agenda to sort out nomenclature, epidemiology, and environmental causation for these conditions, moving toward comprehensive and effective public health and clinical approaches.