Multiple chemical sensitivity (MCS) is an undiagnosed and contentious condition characterized by chronic symptoms brought on by low-level chemical exposure. Symptoms are often ambiguous and hazy. Fatigue, headaches, nausea, and dizziness are among the symptoms. Despite the severity of these symptoms, the World Health Organization, the American Medical Association, and several other professional medical organizations do not recognize MCS as an organic, chemically-caused illness. Blinded clinical trials reveal that people with MCS react as frequently and strongly to placebos as they do to chemical stimuli; the presence and severity of symptoms appear to be related to the perception that a chemical stimulus is present.