The symptoms experienced by patients with MCS make up a long list. They range from mild to severe and affect just about every major body system. While the lack of similarity between patient experiences is a major point of criticism among those who purport that MCS does not deserve its own diagnosis, the research challenges this argument.

A survey of 191 patients achieved good reproducibility regarding self-report of symptoms described in published case definitions of environmental sensitivity (1). This study revealed that, while chemical sensitivity can manifest in a wide variety of ways, the following 6 basic features of MCS are found to be consistent in 80-90% of cases: nature of onset, chronicity, symptom provocation by multiple substances, symptom provocation by an escalating number of exposures, involvement of multiple body systems including the nervous system, provocation by unrelated substances, and addictive behaviors. So, even though the symptoms are varied and non-specific, these unifying characteristics are what makes MCS/IE a separate, unique medical condition.