Multiple chemical sensitivity (MCS), also known as idiopathic environmental intolerances (IEI), is an unrecognized and controversial diagnosis characterized by chronic symptoms attributed to exposure to low levels of commonly used chemicals. Symptoms are typically vague and non-specific. They may include fatigue, headaches, nausea, and dizziness.
Although these symptoms can be debilitating, MCS is not recognized as an organic, chemical-caused illness by the World Health Organization, American Medical Association, nor any of several other professional medical organizations. Blinded clinical trials show that people with MCS react as often and as strongly to placebos as they do to chemical stimuli; the existence and severity of symptoms is seemingly related to the perception that a chemical stimulus is present.
Commonly attributed substances include scented products (e.g. perfumes), pesticides, plastics, synthetic fabrics, smoke, petroleum products, and paint fumes.
Symptoms
Symptoms are typically vague and non-specific, such as fatigue or headaches. These symptoms, although they can be disabling, are called non-specific because they are not associated with any single specific medical condition.
A 2010 review of MCS literature said that the following symptoms, in this order, were the most reported in the condition: headache, fatigue, confusion, depression, shortness of breath, arthralgia, myalgia, nausea, dizziness, memory problems, gastrointestinal symptoms, respiratory symptoms.
Symptoms mainly arise from the autonomic nervous system (such as nausea or dizziness) or have psychiatric or psychological aspects (such as difficulty concentrating).
Possible causes
Various different causes for MCS have been hypothesized.
There is a general agreement among most MCS researchers that the cause is not specifically related to sensitivity to chemicals, but this does not preclude the possibility that symptoms are caused by other known or unknown factors. Various health care professionals and government agencies are working on giving those who report the symptoms proper care while searching for a cause.
In 2017, a Canadian government Task Force on Environmental Health said that there had been very little rigorous peer-reviewed research into MCS and almost a complete lack of funding for such research in North America. "Most recently," it said, "some peer-reviewed clinical research has emerged from centres in Italy, Denmark and Japan suggesting that there are fundamental neurobiologic, metabolic, and genetic susceptibility factors that underlie ES/MCS."
The US Occupational Safety and Health Administration (OSHA) says that MCS is highly controversial and that there is insufficient scientific evidence to explain the relationship between any of the suggested causes of MCS – it lists "allergy, dysfunction of the immune system, neurobiological sensitization, and various psychological theories" as the suggested causes – and its symptoms.
Immunological
Researchers have studied immunity biomarkers in people with MCS to determine whether MCS could be an autoimmune disorder or allergic response, but the results have been inconclusive. Some people with MCS appear to have excess production of inflammatory cytokines, but this phenomenon is not specific to MCS and overall there is no evidence that low-level chemical exposure causes an immune response.
Genetic
It has been hypothesized that there is a heritable genetic trait which pre-disposes people to be hypersensitive to low-level chemical exposure and so develop MCS. To investigate, researchers compared the genetic makeup of people with MCS, to people without. The results were generally inconclusive and contradictory, thus failing to support the hypothesis.
Gaétan Carrier and colleagues write that the genetic hypothesis appears implausible when the evidence around it is judged by the Bradford Hill criteria.