Putting the Chemicals Back
into "Multiple Chemical Sensitivity"

The big-picture report on multiple chemical sensitivity today.

Putting the Chemicals Back into Multiple Chemical Sensitivity: Ontario Environmental Health Advocates Address  Syndrome de sensibilité chimique multiple, une approche intégrative pour identifier les mécanismes physiopathologiques/ Multiple chemical sensitivity syndrome, an integrative approach to identifying the pathophysiological mechanisms. Lead author, Varda Burstyn. Collaborating author, Maureen MacQuarrie. 2022. Ontario Environmental Health Advocates Group.

French summary/sommaire Francaise. 


Many people have never heard of MCS (multiple chemical sensitivity) and even fewer understand this contested illness, yet it afflicts more than 62 million people across the U.K., the U.S., Sweden, Australia, and Canada alone. This 2022 report aggregates, analyzes and presents in lay language recent scientific research on MCS that shows its biophysical and toxicological dimensions. It addresses its growing prevalence, provides references relevant to toxicological and other  environmental health research findings, summarizes the lessons of decades of clinical experience and makes policy recommendations. In short, it provides the big, up-to-date picture on chemical sensitivity today.

Multiple Chemical Sensitivity, a disease process that produces great physical suffering, and often leads to social isolation, impoverishment and disability at moderate to severe levels was first identified in the 1950s but remains a highly contested medical disorder. It is without doubt a product of the chemical age - roughly the decades since the end of World War 2. Yet after all this time, there are still two warring accounts of it, including of its causes. For example, from the report:  


The quote below is from Masri, S., Miller, C. S., Palmer, R. F., and Ashford, N., (2021), “Toxicant-induced loss of tolerance for chemicals, foods, and drugs: assessing patterns of exposure behind a global phenomenon,” Environmental Sciences Europe. Background and evolution of chemical intolerance, paragraph 1.

The sharp growth in reports of TILT [“toxic-induced loss of tolerance,” a synonym for MCS], appears to coincide with the post-WWII expansion of the petrochemical industry and widespread growth in the production of petrochemicals such as organophosphate pesticides, solvents, dyes, and fragrances. U.S. production of the so-called “synthetic organics,” which had been less than 1 billion pounds per year, soared to over 460 billion pounds per year by 1994 (of note, while the term “synthetic” can be interpreted differently, its use in this paper is in reference to compounds whose chemical structures do not appear in nature). The same pattern can be seen for pesticide use in U.S. agriculture, which grew from 200 million pounds of active ingredient in 1960 to over 600 million pounds by 1980. Assuming that exposure to synthetic pesticides and other chemicals is a function of their production and use in everyday society, it is reasonable to assume that these trends have led to increased human exposure over time. Importantly, given their absence prior to modern history, such chemicals can be considered evolutionarily novel and may present particular challenges as [they] relate to the body’s ability to process them through detoxification or elimination pathways. Furthermore, while the human toxicity of pesticides is widely recognized, regulations to safeguard the public are likely insufficient given their focus on the toxicity of individual chemical ingredients . . . as opposed to complex mixtures of multiple chemicals, the latter being more reflective of commercial chemical products and other environmental exposures.


The quote below has been translated from The Institut National de Santé Publique du Québec report (2021), Syndrome de sensibilité chimique multiple, une approche intégrative pour identifier les mécanismes physiopathologiques, p. 811.


The authors of this report conclude that MCS . . . is due to fear conditioning accompanied by chronic anxiety resulting from the constant desire to avoid exposure to odours that cause these people to develop or exacerbate symptoms because they consider this exposure to be threatening to their health.

These two accounts reflect two schools of thought about MCS. The chemical industry, for which the reality of MCS is a major threat , along with a small group of physicians and  psychologists, insist against decades of mounting evidence that it is a psychological disorder based in anxiety.

The patients, on the other hand - scores of millions estimated in 2019 across the UK, Sweden,  Canada, the US and Australia alone - along with the few and courageous physicians who have chosen to care for them, along with a growing list of environmental health researchers all understand MCS as a biophysical-toxicological phenomenon, recognition of which is being pro-actively suppressed. 

The 2022 report was written to dispute and refute the psychological school with the best recent research. It is relevant for anyone interested in population and personal health. For as long as MCS can be dismissed as an anxiety disorder, the chemicals that first cause, then subsequently trigger it at low levels will be given a free pass as “harmless” and the tens, perhaps hundreds of millions of sufferers on a global scale, will continue to be denied insured, effective health care as they and their families stagger and often collapse under the burden of this terrible affliction.




The year 2021 was, for the most part, a good year in multiple chemical sensitivity (MCS) studies. Several major research articles that we substantially draw on in this commentary were published. An extensive literature review from Alberta Health was released. In a field so terribly underfunded, these important additions were very welcome.

However, the INSPQ report, Syndrome de sensibilité chimique multiple, une approche intégrative pour identifier les mécanismes physiopathologiques, came to our attention in the fall of 2021, and although we looked forward to reading and learning from it, as soon as we began, the alarm bells went off. For we saw that it had taken an approach and arrived at conclusions highly divergent from the other new pieces, and that, it soon became clear, were both wrong and dangerous. The Association pour la santé environnemental du Québec - Environmental Health Association of Québec (ASEQ-EHAQ), similarly concerned about the INSPQ report’s conclusions, asked their Minister of Health and Social Services to remove the report from the institute’s website and update it. The ASEQ-EHAQ letter of appeal, which we support, is included as an Appendix to our commentary. 

Our fears were deepened when, in early 2022, a member of our community, “Sophia” (a pseudonym) ended her unbearable MCS-induced pain and hardship with MAiD (medical assistance in dying). After years of desperately seeking a safe place to live where, on a limited budget, she could be free of the fumes of her neighbours' cleaning products and cigarette smoke, her suffering became unbearable and she chose to end her life. Some of us knew her and had worked directly with her, so her death was particularly difficult. Despite the advocacy of doctors and disability professionals, every level of government refused her help. Except for six units created long ago in Ottawa, no dedicated safe housing units have ever been built for people with MCS, finding an affordable safe residence is extremely difficult and there are no programs to assist people like her to find safer places anywhere. We have learned since that a number of others facing a similarly dire situation have also applied for MAiD.  


Our fear is that if the conclusions about the nature and mechanisms of MCS in the INSPQ report attain acceptance by any government or medical association, they will have extremely deleterious consequences. Because these conclusions are wrong, authorities will treat Sophia’s physical suffering as a mental illness, deny appropriate medical care, leave disability needs unmet and thereby doom many more people to the same fate. Out of this profound concern, we decided that the erroneous and dangerous conclusions of the INSPQ report had to be disputed and refuted substantively and piece by piece. Thus, this critique and counterargument was born.


Though we find the INSPQ report’s conclusions frightening, we used the opportunity that its critique presents to showcase some of the exciting work and top-tier researchers in MCS studies, environmental studies and myalgic encephalomyelitis (ME) studies, not included in the INSPQ report. This is work that policy makers, health providers, those working in the disability field, and many others really need to know about. It will help to explain what MCS really is, and, to a certain extent, also ME (myalgic encephalomyelitis/chronic fatigue syndrome). This knowledge is critical in assisting these groups to understand and to help modernize health care in general to address complex, environmentally-linked diseases and to develop healthier public policy on chemical use – a modernization that is very badly overdue.


The patient perspective is essential for any illness, and its incorporation has become common practice. It is l needed in any process that seeks to identify any or all of the nature, mechanisms and definitions of MCS, and it is also essential to the creation of clinical programs and sites, disability needs, population health and prevention strategies and research priorities. But it is entirely missing from the INSPQ report. It is a perspective we have used to frame our critique, and included it very explicitly at key junctures within it.


We are an Ontario-based group of advocates who have worked together for the recognition and inclusion of the medical conditions ES/MCS, ME and FM, with which about one million Ontarians live and struggle. These are often devastating and disabling conditions, but have little to zero care and support from our provincial health and social services systems, with ES/MCS the most excluded of the conditions. Our group includes environmental health consultants and educators, writers, health and social policy planners, participants in national research efforts (ME), senior health system administrators, health system change experts, human and disability rights advocates, educators, patient organization leaders, a lawyer and caregivers. Some of us live with one or more of the conditions, some of us do not. More details about us can be found in Appendix 1, “Information about the Signatories” [Or click HERE].



MCS is a multi-system, recurrent, environmental syndrome and disease process that flares in response to different exposures (i.e., pesticides, solvents, toxic metals, fragrances, cleaning products, cigarette smoke, certain foods, drugs/medicine, mold and other vehicles of exposure) at concentrations that do not provoke such symptoms in other people. It is characterized by neurological, immunological, cutaneous, allergic, gastrointestinal, rheumatological, cardiological and endocrinological signs and symptoms. MCS is a widespread condition and the majority of those who live with it (approximately 70 percent) are women, though a significant minority are men.

Onset, which may happen slowly over time or rapidly, begins on exposure to a particular chemical or mixture of chemicals (including bio and well as synthetic toxicants) that commonly affect the immune system and/or nervous system, such that MCS appears to be primarily a neuroimmune disease process. This chemical exposure interacts with one (or both) of these systems in a way that renders individuals intolerant to subsequent exposures, which are then experienced as triggering or flaring events. After the initial onset, some new triggering events may result in “crashes” - additional worsening to qualitatively greater degrees of severity that are not easily reversible without intervention.

Affected individuals no longer tolerate everyday exposures to a wide range of structurally diverse substances at levels that never bothered them previously, including ingestants, inhalants, implants, and skin contactants. Many previously tolerated foods and drugs may trigger symptoms. At times, onset is not observed or reported immediately, and the phenomenon of "masking" can obscure MCS and delay diagnosis.

MCS ranges in severity. Early, milder stages are often erroneously perceived to be allergies, require adjustments and avoidance, but go undiagnosed. Moderate to severe MCS involves greater intensity and duration of symptoms. Severe MCS brings intense reactions, great physical suffering and can be life-threatening for some people when exposed to some chemicals. Major efforts to avoid triggers are required, making life in the ambient air of chemically-laden everyday environments unsustainable. This is how MCS disables those affected. When co-morbidities are present – often the case – overall health is further compromised, and additional barriers are encountered.

MCS is usually responsive to appropriate measures and treatments, but becomes worse without these.