The Facts About Multiple Chemical Sensitivity

History of Multiple Chemical Sensitivity

Chemical intolerance was first described as a medical condition in the 1950’s. Since that time, it has been referred to as multiple chemical sensitivity (MCS). In those early days, there was no body of scientific evidence that could support the existence of such a phenomenon. It didn’t make sense according to our traditional understanding of toxicology. Conventional physicians assumed that it must be psychological, although there was not much evidence to support that either.


In 1996, the World Health Organization (WHO) developed the term, ‘idiopathic environmental intolerance’ (IEI) to label patients with MCS.  According to opponents of the existence of MCS as a biological entity, IEI is a better term because it does not imply a chemical cause or susceptibility. Idiopathic means that we don’t know the cause. Using the term intolerance presumes no particular biological mechanism.

Those that support the use of the term IEI suggest that these patients are under the false belief that sensitivity to low-level exposures to multiple chemicals are the cause of their physical and mental symptoms. These opponents to the existence of MCS as a biological condition say that these beliefs are instilled and reinforced by speculation that common chemical exposures must be toxic, by the influence of misguided physicians, by misleading patient support/advocacy networks, and by social contagion. They say the beliefs must be bogus because they cannot be substantiated by science.

False Beliefs?  No Science?

The term idiopathic is defined as arising spontaneously from an unknown cause. But, in the 18 years since WHO came up with the label IEI, there has been an abundance of studies that have proven that MCS is a biological phenomenon. For example, several papers have been published, which demonstrate abnormal brain scans in these patients. The most recently published study showed an abnormality in the brain, described as limbic system hyper-reactivity. The limbic system is the part of the brain responsible for survival and adaptation to the environment.

There are other studies concluding that MCS involves mechanisms of limbic system hyperactivity, plus studies showing abnormal genotypes for detoxification, oxidative stress, and upregulation of TRPV1 receptors on neurons. We know the mechanisms. MCS is more likely to occur in people who are genetically predisposed to be poor detoxifiers. Less efficient or over-burdened detoxification systems leads to measurable changes in the ability of nerve cells, to not just be more sensitive to chemical exposures, but to also react adversely causing a variety of neurological symptoms. All these abnormalities have been amply demonstrated in well-designed animal and human studies published in peer reviewed journals. Sensitization to chemicals can and does occur.

It might be understandable that in 1996, before research could prove that these people were reacting to chemical exposures, that WHO could come up with this term. However, the fact that it is still being used by many physicians points to their lack of awareness that the science now validates the biological existence of MCS. To call the condition idiopathic is just wrong.  

Why make a big fuss over the name?

MCS patients have challenges. They can have problems accessing common community resources that most persons take for granted, including grocery stores, shopping centres, community meetings, public libraries, restaurants, movies, use of public transportation, the homes of extended family members and friends, offices of dentists and medical doctors, public parks, classes at universities, communities of worship, and most significantly, attending the workplace. Continuing to argue without substantiation that this condition is idiopathic feeds others’ false belief,  lack of understanding, and refusal to provide proper accommodation in the workplace. People with MCS are still stigmatized by their condition, creating difficulty accessing appropriate, effective health care. 

Many doctors are still either uneducated or misinformed about MCS. There is no education regarding the disorder in medical schools. Often, medical students, residents and practicing professionals will turn to the time-honoured Merck Manual. As accurately stated on their website,

Healthcare professionals have consulted the Merck Manual for over a century, for trusted, concise and correct discussions of diagnosis and therapy. It is the best, first place to go for clinical decision support.”

Merck Manual and MCS

This manual, which was last updated/revised in May 2014, refers to MCS as IEI. These are some of the key points emphasized:

  • Based on current evidence, idiopathic environmental intolerance cannot be explained by nonpsychologic factors.
  • Encourage psychologic therapies such as graded exposure, and drug treatment of coexisting psychiatric disorders.

We now know that the first key point is wrong.

Even worse, the Merck Manual states that psychological desensitization and graded exposure is a useful technique for therapy, an opinion that is quite weak, given that it is based on just a few cases described in the medical literature. In fact, the most recent case is only a description in a letter to the editor,  published by the author of this section of the Manual, and it is already 12 years oldThe published literature to date actually points out that MCS patients obtain the most help by living in a chemically-free space and avoiding chemicals.

So why does the Merck Manual state that avoidance behaviors should be discouraged? This advice certainly contradicts the Canadian Human Rights Commission policy, which clearly states that people with MCS have the legal right to accommodation in the workplace, including minimizing or eliminating exposures to chemical triggers.

What’s the harm?

The medical condition we are referring to should no longer be called idiopathic environmental intolerance; it is multiple chemical sensitivity. There is documented neurological dysfunction that is not psychological. Attempts at psychological desensitization by gradually increasing chemical exposures is more likely to cause harm than help.

This potential for harm has more ramifications than it appears because some insurance carriers are denying disability to people with MCS, and instead are forcing them to endure increased exposures as a treatment. These patients have no choice but to do so because they do not have the right to refuse therapy and still expect to be compensated by the insurance carrier. Most significant is the fact that the providers of this ‘insured treatment’ do not inform their clients that they could get worse from the recommended therapy. This is enforced, not informed, consent. And if the patients do get worse and stop the prescribed program, they are denied disability because they refused the recommended treatment – the medical insurance version of Catch 22.

Advances in education in environmental medicine

Medical students, family practice residents, and practicing medical professionals can access proper, evidence-based education in environmental health. They just need to look in the right places. The Canadian College of Family Physicians provides continuing professional development/medical education programs for family physicians and other health care professionals. There are five accredited (Mainpro-C), evidence-based workshops on environmental health, including one entitled, Chronic Pain, Fatigue, and Chemical Intolerance Linked to Environment Exposures: Office Assessment and Management. There is an Environmental Health Clinic at Women’s College Hospital in Toronto, which is a teaching hospital fully affiliated with the University of Toronto. The clinic was established in 1996 by the Ministry of Health and Long-Term Care (MoHLTC) to improve health care for people with environment-linked conditions such as MCS. Medical students and family practice residents from various medical schools have obtained elective training in evidence-based environmental health. This fall, two young doctors have begun a new Fellowship program at the clinic, funded for the next three years by the MoHLTC, and provided in collaboration with the University of Toronto’s Department of Family and Community Medicine and Dalla Lana School of Public Health. 

As one of the physicians on staff at this clinic, and a writer and collaborator of the environmental health workshops, I can attest to the fact that the content of these teaching programs, which are evidence-based, is in clear contrast to that provided by the Merck Manual.

The name of this condition is multiple chemical sensitivity and the Merck Manual needs to be better informed.

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