Multiple Chemical Sensitivity (MCS) disorder, also known as environmental illness (EI), is characterized by immediate or delayed reactions to various environmental chemicals. Chemical intolerance (CI) is a symptom of MCS, although the terms are often used interchangeably. Sick Building Syndrome (SBS), a subset of MCS, is characterized by fatigue, upper respiratory infections, and difficulty concentrating.
Various studies suggest that MCS is present in 40 to 80 percent of people with CFS, FMS, and GWS, and the majority of people with a primary diagnosis of MCS have concurrent CFS, FMS, GWS, and allergies. MCS is more prevalent in women than men (except in older populations), with age of onset typically in the thirties and forties. Although there is no specific test for MCS, abnormalities have been detected on immune and brain imaging tests.
(Source: Chronic Fatigue Syndrome, Fibromyalgia and Other Invisible Illnesses, by Katrina Berne, Ph.D.)