The association of multiple physical symptoms with prolonged or recurrent exposures to low levels of environmental pollutants. Clinical research has failed to establish the precise nature of the syndrome, its causes, the functional limitations it may cause, or the best course of treatment. Many hypotheses have been suggested: some proponents of the syndrome believe that it results from allergic or immune-mediated mechanisms; skeptics have suggested that the symptoms are a form of masked depression, adverse conditioning to unusual odors, or, in some instances, a form of malingering. None of these hypotheses has been definitively proven.