Multiple Chemical Sensitivity

About MCS (Multiple Chemical Sensitivity) In Nutshell

MCS is a disease process that makes people – over 1.1 million people in Canada in 2020 – very sick when they breathe, touch or ingest many chemicals found in our everyday products – hence, multiple chemical sensitivity. It is multi-causal and can affect multiple body systems, so it is a complex disease, but it always involves chemical injury to the immune and/or nervous systems in onset and chemical triggering thereafter. Examples of common incitants in MCS include pesticides, solvents, adhesives, stains, varnishes, paints, sealants, gasoline, diesel, oil and wood smoke, many cleaning, laundry and personal care products, especially those high in synthetic fragrances, printer cartridge ink, flame retardants, phthalates, certain metals, cigarette smoke, certain foods, certain drugs and medicines, and molds and mycotoxins.

In the absence of competent and effective treatment and chemically-safe housing, MCSers develop symptoms that cause great suffering and can even become life-threatening during flares. These symptoms are disabling in their own right when they occur; and they also disable people by forcing them to withdraw from virtually all social spaces – work, education, recreation, medical facilities and even family homes – which today are full of such chemicals. MCS is definitely responsive to competent and effective care, but our health care system does not provide it, anywhere, and our medical schools do not teach it. That’s why CareNow Ontario is working to bring care and education into being.

Nobody Can See What is Happening in my Body'...

‘When I am exposed to perfumes and other chemicals … nobody can see what is happening in my body, they don’t understand. So, my body is burning. My eyes are burning. My throat is burning. I think I am going to throw up. I get migraines. I get shaky and then those other symptoms that I describe, I am also dealing with as well. It is kind of like a package deal that feels like every system in your body is going into hyper-drive and you don’t feel a sense of control of your body and of your life.’ Hope MCS

The Bigger Picture

In This Section:


The Chemicals To Which MCERS React In Onset And Chronicity

Chronic MCS – The “Triggering Phase”

Risk Factors

MCS – Inciting Chemicals Have Serious Adverse Health Effects On Broad Populations

Is There Help For MCS?

Chemical Avoidance and Safe Housing

Medical Treatment

Disability Needs and Rights

A Short Working Description of MCS


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The Bigger Picture

MCS is both a complex medical condition – a syndrome and disease process – and a disability. As of 2020, more than 1.1 million Canadians had been diagnosed with it, and it affected more than 61 million people across just the US, UK, Sweden and Australia. This means that despite its invisibility and neglect in our own health care system, MCS is widespread, not rare. Other names for it are TILT (for toxicant induced loss of tolerance), EI (for environmental Illness) and IEI (for idiopathic environmental intolerance), or simply chemical intolerance (CI) or chemical sensitivity (CI).

While we prefer the term intolerance to sensitivity, we have chosen to use the name MCS because it is the most widely known. Chemical sensitivity or intolerance is frequently accompanied by EHS (for electrohypersensitivity), though EHS may occur alone as well. EHS involves many similar symptoms to MCS, especially but not only neurological ones. These occur, explain Yael Stein MD and Iris G Udasin MD “following the patient’s acute or chronic exposure to electromagnetic fields in the environment or in occupational settings. Numerous studies have shown biological effects at the cellular level of electromagnetic fields (EMF) at magnetic (ELF) and radio-frequency (RF) frequencies in extremely low intensities. Many of the mechanisms described for Multiple Chemical Sensitivity (MCS) apply with modification to EHS.” “Electromagnetic hypersensitivity (EHS, microwave syndrome) – Review of mechanisms,” Environmental Research, July 2021

Together, MCS and EHS comprise ES (for environmental sensitivities). There is still a great deal to be learned about MCS as well as EHS, as with ME. Over the last fifty years, two broad schools of thought on this “contested” illness have developed: one that considers it to be a complex bio-physical-toxicological disorder, based in neurological and immunological dysfunction due to toxic exposure, and thus an “environmental” disorder. The other school of thought sees its causes and mechanisms as caused by personal psychology, notably by depression and anxiety. CareNow Ontario considers that the overwhelming burden of research, clinical and patient experience supports the biophysical-toxicological view. It sees symptoms of anxiety and depression as only a few of a broader constellation of neurological symptoms during flares that include dysfunction in speech, thinking (“brain fog”) and coordination, so as effects of MCS, not causes; and it sees the constellation of neurological symptoms as only some among a broader set of multi-system impacts. This approach has been validated and adopted by the main phases of study in Ontario, and a number of its key recommendations. For important references, including an in-depth, highly referenced and very current account of the biophysical-toxicological approach and its disagreements with the psychogenic school, scroll all the way down and utilize the link to “Putting the Chemicals Back in MSC.”

MCS has two identifiable stages: onset or initiation, and chronicity or triggering.


Onset can develop either rapidly or gradually, when a person with certain risk factors is exposed to a particular chemical or, more frequently, a mixture of chemicals. Often these are synthetic toxins but the biologically produced (bio)toxins of molds or certain infections (e.g. chronic Lyme disease) can also injure and sensitize the immune and/or the nervous systems. This chemical exposure interacts with one or both of these systems ways that render individuals intolerant to subsequent exposures of those chemicals at concentrations that don‘t bother most people in the same way.


It all happened as a result of exposure to incorrectly mixed adhesive chemical to repair a windshield in my vehicle. I started getting symptoms within about fifteen minutes of being in my vehicle. Things started to go weird and life … went to hell in a hand basket …It wasn’t until I saw Dr. B. that she diagnosed the FM and chemical sensitivity and the toxic brain injury. Sandra MCS FM p. 69

It’s been about half my life that I have had chemical sensitivities, multiple chemical sensitivities. … I think what happened was when I was young I had a major inner ear infection. Then I went to the hospital; I had surgery; I had tubes put in and I had them taken out. Then I started having the asthma attacks in the fall. Then, when I was in my early 20s – I was in nursing school actually – that’s where it started. I started to have reactions to chemicals and perfumes and things around – the cleaners and stuff. I couldn’t even continue in the class. Petra MCS p. 69

The Chemicals To Which MCSERS React

The sensitization first experienced in onset usually spreads to include other chemicals that are very common in the products used in everyday life, and these chemicals are then referred to as “incitants.” These can be inhaled, ingested, implanted and/or come into contact with skin, and – the nightmare reality for those with MCS – they are everywhere in the social and built environment.

People with MCS are known to have adverse reactions to fragrances – this is a very common problem, and one that creates great difficulty in navigating daily life in our highly fragranced society. World renowned toxics researcher and scholar, Ann Steinemen, writes:

A “fragrance” is a scent and, despite its singular name, is a formulation of dozens, sometimes even hundreds of chemicals. Fragranced products cover hundreds of everyday items, such as air fresheners, deodorizers, cleaning supplies, laundry detergents, fabric softeners, essential oils, candles, soaps, personal care products, colognes, and hand sanitizers. Among these thousands of chemicals in these hundreds of everyday products we find chemicals such as acetone, ethyl acetate, benzaldehyde, formaldehyde, methylene chloride, and phthalates, all neurotoxic and identified as chemicals likely to initiate MCS in a predictable subgroup of people (With thanks to Pinkas et al., 2017; Masri et al. 2021). (Steinemann, 2016, 2019a).

But of course, synthetic fragrances are only one group of inciting chemicals. There are many others, most recently catalogued by University of San Antonio researchers from a major new study.

Medical Researchers Identify Chemical Culprits In MCS Onset and Beyond

In an important new study of TILT (toxicant induced loss of tolerance – another term for MCS), University of San Antonio Health Sciences Centre TILT program researchers reviewed the exposure and health experiences of eight groups of people over time and location, including workers at the US Environmental Protection Agency (EPA) headquarters during renovations, Gulf War veterans, casino workers exposed to pesticides, a group of workers exposed to aircraft oil fumes, people directly involved in the World Trade Center tragedy, people with surgical implants, people who live in moldy environments, and tunnel workers exposed to solvents. They published their finding in 2021. Their research gave them a broad overview, and enabled them to specify the most common chemical culprits they found in onset, and to develop a chemicals-most-likely-to-harm list, an invaluable contribution. They write:

“Mixed volatile and semi-volatile organic compounds (VOCs and SVOCs), followed by pesticides and combustion products were most prevalent across TILT/[MCS] initiation events. As a broader category, synthetic organic chemicals and their combustion products were the primary exposures associated with chemical intolerance. Such chemicals included pesticides, peroxides, nerve agents, anti-nerve agent drugs, lubricants and additives, xylene, benzene, and acetone.” Even more precisely they note, “The mixed VOC/SVOC group of chemicals included such VOCs as benzene, acetone, toluene, and xylene as well as SVOCs including BFRs, PCBs, dioxin, phthalates, and triphenyl and tricresyl phosphates. Among this group of compounds, xylene was identified most frequently across exposure events, followed by both benzene and acetone. Pesticides included carbamates, organophosphates, and organochlorides …

Masri, S., Miller, C.S., Palmer, R. F. & Ashford, N. (2021). Toxicant-induced loss of tolerance for chemicals, foods, and drugs: assessing patterns of exposure behind a global phenomenon. Environmental Sciences Europe, 33, 2021.

Chronic MCS – The “Triggering” Phase

In chronic MCS, which can last a lifetime without the right environmental and medical supports, post-onset exposures are experienced as triggering or flaring events. Because virtually all of our social spaces – work, educational, transport, recreational, medical, worship and residential – are full of the culprit chemicals from everyday products, MCSers discover that they must find a chemically safe place to live and must isolate away from social spaces to maintain health and well-being. If they are not able to find or create a safe home, and do not receive effective and competent treatment – currently such treatment is not available in any public health care system service – MCSers do not get better, almost always becoming more sick. It is common, then, that after the initial onset, new triggering events can result in “crashes” – additional worsening to qualitatively greater degrees of severity that are not easily reversible without a safe residence and medical support.

From the point of view of those who develop it – and as researchers have shown – while MCS results from injury to the immune and nervous systems (and has characteristics of neurodegenerative disease), through these “master” systems it can also affect multiple other body systems. So MCS has neurological, immunological, cutaneous, allergic, gastrointestinal, rheumatological, cardiological and endocrinological signs and symptoms. This multi-system expression can make MCS very severe, and demonstrates a complexity similar to that of ME, with which it not infrequently co-occurs. Our health care system has not yet learned to understand or treat this complexity, which demands integrative and functional approaches that are not yet taught in Canadian medical schools or practised by our health care providers. As a result, early stages of MCS are very often misdiagnosed as “allergies.” Late detection, however, greatly increases the chances of severity and difficulty in improvement. And both family physicians and specialists are puzzled and sometimes incredulous when presented with it, and many dismiss patient accounts, or erroneously ascribe them to depression, anxiety or even hypochondria.

MCS ranges in severity. Early, milder stages frequently go undiagnosed. Moderate to severe MCS involves greater intensity and duration of symptoms and demand greater modifications of everyday activities. 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.

Risk Factors

At this time, with a severe enough exposure, it seems that anyone can get MCS. However, of those who have it, certain characteristics stand out. First, the majority of those who live with MCS (approximately 70 percent) are women, though a significant minority are men. Women are more vulnerable on a daily basis because of certain sex-associated physical characteristics (e.g. more adipose tissue, poorer elimination capacities, different central nervous system wiring) interacting with gender-associated work exposures (e.g. as cleaners, hospital staff, nail salon, automotive plastics and certain food industry workers) whether in employment or at home. (Unfortunately, gender bias in medicine has been part of the dismissal of MCS). Other risk factors include a history of chemical exposures, a high body burden of chemicals, mold and mycotoxin exposure, certain types of chronic infections such as Lyme, and certain genetic characteristics that lead to elimination pathways that are not well-equipped for metabolizing modern chemicals. These have been found in a number of environmentally-linked conditions.

Certain Genetic Profile A Risk Factor in Gulf War Illness - and MCS

A very recent detailed genetic study on Gulf War Illness, which includes MCS, examined the presence of the PON1 gene (one that has featured in MCS-related work as well). Led by a senior researcher at the division of epidemiology, Department of Internal Medicine, University of Texas Southwestern Medical Center in Dallas, the study found “strong evidence for an etiologic role of low-level nerve agent in GWI. … Troops who had genes that help metabolize sarin were less likely to develop symptoms.”

Robert W. Haley. “Evaluation of a Gene–Environment Interaction of PON1 and Low-Level Nerve Agent Exposure with Gulf War Illness: A Prevalence Case–Control Study Drawn from the U.S. Military Health Survey’s National Population Sample.” <em>Environmental Health Perspectives</em> May 2022.

But MCS is fundamentally a toxic injury that develops in people with certain risk factors. And as the mass incidence of MCS among US military veterans who developed it as part of Gulf War Illness shows, if the toxics are is strong enough, even healthy young males can succumb to this form of disease.

MCS – Inciting Chemicals Have Serious Adverse Health Effects on Broad Populations

It’s important to point out that the need to reduce chemical pollution is not at all only an MCS issue. For what affects an MCSer in the moment, does, over years, have serious and widespread adverse health effects on broad numbers of people, as modern toxicology and environmental health studies have shown. This is why MCSers have often been called “canaries in a chemical coalmine.” Toxics-related disease is now a massive global burden. And remember, MCS is one disease outcome among many that different people develop when exposed to similar chemicals.

What Researchers Say About the Global Extent of Chemically-Linked Disease

A 2017 article by distinguished researchers Philippe Grandjean, Harvard environmental epidemiologist, and Martine Bellanger, professor of health economics at l’École des Grands Études en Santé Publique, estimated the worldwide extent and costs of disease linked to chemical exposures. “Our economic estimates based on available exposure information and dose-response data on environmental risk factors … suggest that environmental chemical exposures contribute costs that may exceed 10% of the global domestic product and that current calculations [of disability-adjusted life years] substantially underestimate the economic costs associated with preventable environmental risk factors. By including toxicological and epidemiological information and data on exposure distributions, more representative results can be obtained from utilizing health economic analyses of the adverse effects associated with environmental chemicals.”

Grandjean, P. and Bellanger, M. Calculation of the disease burden associated with environmental chemical exposures: Application of toxicological information in health economic estimation. Environmental Health 16. 2017.

MCS Is One Disease Outcome Among Many From Prolonged Chemical Exposures

Exposed to the same chemicals at various concentrations, some people develop MCS, other people a host of other diseases. Here is one story that tells the tale.

I worked in a building for four years in a loft office over a chlorine pool with a glass atrium dome, no ventilation at all; our office had plate glass windows. They had high school kids putting chemicals in the pool, and on many occasions, we had to evacuate the building because of the toxic fumes. The building was also being turned from a hotel into vacation condos, so everything was gutted, including drywall, and redone. So, for four years we not only breathed maximum strength chlorine, but drywall dust, epoxy paint, carpet glue, bug spray, solvents etc. Everyone that worked there was sick all the time and we all, about 14 people, complained daily that we needed clean air to breath, but no one cared. One person had a heart attack and quit because he couldn’t take it anymore. One has hepatitis, one tumours cropping up all over his body, one chronic pain all over her body, one breast cancer, one chronic heartburn and headaches, one skin rashes that won’t go away. I have all the classic symptoms of MCS including chronic fatigue, pain everywhere, sense of smell and taste seem broken, flu-like symptoms all the time, vision problems, memory problems etc. I am all messed up and don’t feel like I can work because I am too tired and I get sick when I go in some buildings or stores and I never know when it will happen and sometimes my throat closes and I can’t breathe or swallow and it’s terrifying. (A.R. to V. Burstyn, consultative correspondence, Sept 24, 2009, cited in Putting Chemicals Back into Multiple Chemical Sensitivity, 2022)

Is There Help for MCS?

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

Chemical Avoidance and Safe Housing

Without any question, the first and most important aspect of treating MCS is to find or create a safe or place to live – one where the individual is not having MCS reactions – and to reduce triggering exposures as much as possible in daily life. This is a massive challenge, and must be recognized as such. It may well require renovation or moving, and accommodation at work or, even more often, leaving the workplace and working from home if possible, or cessation of work if illness is too severe. Chemical avoidance cannot be avoided if improvement is sought. Getting help to achieve it is one of the issues advocates have identified as key for government to help address.

Medical Help

Functional medical doctors and, especially, those who have done special training in environmental medicine, if they are able to practice at full scope, can make a great difference. Normally, they will do much more extensive tests that Ontario doctors, seeking the underlying causes in each individual. Among other things, they usually seek to understand and test for high body burden of chemicals, molds and mycotoxins, dysfunctional components of the immune system, genetic polymorphisms affecting elimination pathways, hormonal and neurotransmitter imbalances, GI issues, parasites and the health of the gut-brain axis. The result of these tests, as well as all the conventional tests involved in a general check-up, then determine what factors will be treated. This complex approach to a complex disease requires special training, new medical codes and compensation mechanisms, and changes in both education and colleges of physicians and surgeon’s guidelines – all issues addressed in the plan CareNow Ontario is fighting for.

Disability Rights

MCS is recognized as a disability, with its rights and entitlements, by both federal and provincial law. If chemical-trigger avoidance is the base line for being able to function, that means disability accommodation “beyond a reasonable hardship” is required, and it involves a low-chemical, safe space for an individual, child or adult, to live, work and study. To date, this “right” has been very difficult to win in the majority, though not all, of the cases where people have tried. It requires being able to identify the incitants, and being able to remove them from the air stream. So, for example, some of the measures might be establishing a no scents and no pesticides policy, placing printers under containers and venting fumes to the outside, using VOC-free building or renovation materials, maintaining a mold-free environment, ensuring that ventilation systems do not bring in the substances from one area of a building to another. In a hospital, accommodation would likely involve sequestration in a private room, since ambient chemicals cannot be easily removed. Because these accommodations require effort on the part of others, efforts many don’t understand or want to make in the absence of awareness of MCS as a medical condition, these rights have been more theoretical than practical. This is another issue that the plan CareNow Ontario is advancing speaks to with practical proposals, so that MCSers can have 1st class human rights in Canada, including to safe medical care.

A Working Description For Your Doctors and Others

Derived from several current medical sources, this is a short description you can use for your doctor, your family, friends, employer and others. Excerpted from Putting the Chemicals Back into Chemical Sensitivity”. V. Burstyn and M. MacQuarrie for Ontario Advocates for Environmental Health, 2022

Multiple Chemical Sensitivity – A Working Description

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.

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.

Useful References and Links


For a full discussion of current research on MCS and on why the biophysical-toxicological approach of MCS is superior to the psychological, see the major new report:

Putting the Chemicals Back in “Multiple Chemical Sensitivity”: Ontario Advocates Address Syndrome de sensibilité chimique multiple, une approche Intégrative pou iIdentifier les mécanismes physiopathologiques/ Multiple Chemical Sensitivity Syndrome, an Integrative Approach to Identifying the Pathophysiological Mechanisms. Lead author Varda Burstyn, collaborating author Maureen MacQuarrie for the Ontario Ad Hoc Environmental Health Advocates. June 27, 2022. Podcast: Putting the Chemicals back in MCS: A Podcast with Varda Burstyn.

For a real-life, Ontario-based qualitative study of the lived experience and health needs of Ontarians with MCS (and ME and FM), 2013:

“Part 3: Community Voices” and “Part 4: Special Issues”. Ann Phillips, Erika Halapy and Varda Burstyn Excerpted into a stand-alone document excerpted from Recognition, Inclusion and Equity – The Time Is Now: Perspectives of Ontarians Living with ES/MCS, ME/CFS and FM, Varda Burstyn and MEAO. Supporting research report for the Steering Committee of the Ontario Centre of Excellence in Environmental Health Business Case Proposal Project.

Read Community Voices and Special Issues.

For the larger report from which the “Community Voices” is extracted, and which discusses how to address disability as well as medical issues:

Recognition, inclusion and equity: Solutions for people living in Ontario with ES/MCS, ME/CFS and FM – The Business Case Proposal, Steering Committee of the OCEEH Business Case project, November 2013.

Additional research reports from multiple components of 2011-2013 study project funded by the Ontario Ministry of Health and Ontario Trillium Foundation, to prepare a business case for a three tiered network of care called the Ontario Centre of Excellence in Environmental Health (OCEEH).

For a 2021 literature review on MCS released by the Alberta government:

Multiple chemical sensitivity: Literature review and state of the science. Intrinsik Consultancy commissioned by Alberta Health. May 2021.

Recent Peer-Reviewed Research References For Your Physicians or Colleagues:

Damiani, G., Alessandrini, M., Caccamo, D., Cormano, A., Guzzi, G., Mazzatenta, A., Micarelli, A., Migliore, A., Piroli, A., Bianca, M., Tapparo, O., & Pigatto, P. D. M. (2021). Italian Expert Consensus on Clinical and Therapeutic Management of Multiple Chemical Sensitivity (MCS). International Journal of Environmental Research and Public Heath, 18 (21) 11294. Available at

Barrett, E. S., & Padula, A. M. (2019). Joint Impact of Synthetic Chemical and Non-chemical Stressors on Children’s Health. Current environmental health reports, 6(4), 225–235. Available at

Masri, S., Miller, C.S., Palmer, R. F. & Ashford, N. (2021). Toxicant-induced loss of tolerance for chemicals, foods, and drugs: assessing patterns of exposure behind a global phenomenon. Environ Sci Eur, 33, 65. Available at: https//

Miller, C.S., Palmer, R.F., Dempsey, T.T., Ashford, N.A., and Afrin, L.B. (2021). Mast cell activation may explain many cases of chemical intolerance. Environmental Sciences Europe, 33. Article 129. Available at

Molot, J. (2021). Response to the report of the National Institute of Public Health Québec (INSPQ) on Multiple Chemical Sensitivity (MCS) – Summary. October 15, 2021. Submitted by ASEQ-EHAQ. Available at Accessed April 15, 2022.

Molot, J., Sears, M., Marshall, L., & Bray, R. I. (2021). Neurological susceptibility to environmental exposures: pathophysiological mechanisms in neurodegeneration and multiple chemical sensitivity. Reviews on Environmental Health. Published online September 16, 2021. Available at

Additional Links

University-Affiliated MCS Research Centres
EHS and MCS Research and Treatment European Group. Clinical Sites, Associations and Education
Ontario Task Force on Environmental Health 2016-2018

L’Association pour la santé environnementale du Québec / Environmental Health Association of Québec

United States

The Chemical Sensitivity Foundation.