People reporting MCS-like symptoms may have other health issues, ranging from common conditions, such as depression or asthma, to less common circumstances, such a documented chemical exposure during a work accident. These other conditions may or may not have any relationship to MCS symptoms, but they should be diagnosed and treated appropriately, whenever the patient history, physical examination, or routine medical tests indicates their presence. The differential diagnosis list includes solvent exposure, occupational asthma, and allergies. Definitions
Different researchers and proponents use different definitions, which complicates research and can affect diagnosis. For example, the 1987 definition that requires symptoms to begin suddenly after an identifiable, documented exposure to a chemical, but the 1996 definition by the WHO/ICPS says that the cause can be anything, including other medical conditions or psychological factors.In 1996, an expert panel at WHO/ICPS was set up to examine MCS.
The panel accepted the existence of "a disease of unclear pathogenesis", rejected the claim that MCS was caused by chemical exposure, and proposed these three diagnostic requirements for what they re-named idiopathic environmental intolerances (IEI):

the disease was acquired (not present from birth) and must produce multiple relapsing symptoms;
the symptoms must be closely related to "multiple environmental influences, which are well tolerated by the majority of the population"; and
it could not be explained by any other medical condition.In Japan, MCS is called chemical hypersensitivity or chemical intolerance (化学物質過敏症; kagaku bushitsu kabinsho), and the 1999 Japanese definition requires one or more of four major symptoms – headaches; malaise and fatigue; muscle pain; joint pain – combined with laboratory findings and/or some minor symptoms, such as mental effects or skin conditions. The defined lab findings are abnormalities in parasympathetic nerves, cerebral cortical dysfunction diagnosed by SPECT testing, visuospatial abnormalities, abnormalities of eye movement, or a positive provocation test. International Statistical Classification of Diseases
The International Statistical Classification of Diseases and Related Health Problems (ICD), maintained by the World Health Organization, is a medical coding system used for medical billing and statistical purposes – not for deciding whether any person is sick, or whether any collection of symptoms constitutes a single disease. The ICD does not list MCS as a discrete disease. However, this does not mean that people with MCS-related symptoms cannot be treated or billed for medical services.