Multiple Chemical Sensitivities

“MCS” stands for Multiple Chemical Sensitivities (MCS). On this page, you will learn about the triggers, symptoms, diagnostic tools, and treatment available for individuals with MCS.


All life on earth is made up of chemicals, including plants, soil, water, air, bacteria, fungi, animals, and humans. However, over the past several generations, humans have synthesized thousands of synthetic chemicals, and now, over 350 000 industrial chemicals and mixtures are registered globally. The problem is that not all of these chemicals have been studied for low-level, long-term effects. Many of these chemicals can be harmful–if their effects aren’t felt acutely, they may be felt long-term, contributing to chronic illness and disease, and increasing morbidity. Chemical production is connected to global fossil fuel extraction, and larger issues of climate change and the future sustainability of green energy.

When a person thinks of chemicals entering the atmosphere, they might imagine pollution spewing from tall industry stacks, or billowing from a car’s tail pipe, but recent research suggests that chemicals cumulatively released from consumer products (especially in residential housing and workplaces) may presently be one of the largest urban sources of emissions in North America. Some household chemicals, including, but not limited to, those used in scented personal care products, cleaners, nail polish, perfume, carpets, air fresheners, furniture, pesticides, and renovation materials, have the potential to contribute to air pollution much like industrial sources and traffic. This is in part because their chemical composition is similarly derived from the basic building blocks of crude oil and natural gas, so they react with other chemicals in the air, water and soil during their everyday use. For example, many consumer products such as those listed above need solvents, which are petrochemical (meaning, a chemical that comes from petroleum or natural gas) building blocks that help hold chemical blends together. Together, the hundreds to thousands of chemicals we interact with everyday are part of the wider set of exposures our bodies encounter over our lifetimes, also known as the “exposome:”

the extensive range of specific external exposures which include radiation, infectious agents, chemical contaminants and environmental pollutants, diet, lifestyle factors (e.g. tobacco, alcohol), occupation and medical interventions” (Wild, 2012).

While scientific research into consumer product’s contributions to indoor air quality is still evolving, we do know that overall, in Canada, they do have a significant impact, that it is very complex, that it involves many ongoing chemical reactions, and that is dependent on many factors, such as ventilation, and meteorological variables like heat and humidity.

Chemical presences indoors have become ordinary and quotidian. Much of the time, chemical consumer products are used to render spaces and those who occupy them more functional, elegant, comfortable, aesthetic, or enjoyable, such as when used for personal care and “beauty,” cleaning and sanitizing, renovating and repairing buildings, and for aesthetic purposes such as creating decorative ambience. These activities have become important forms of domestic ritual and routine. However, changing the culture surrounding consumer products to prioritize collective accessibility, and changing which products are used, is necessary in order to remove barriers for people who are impacted by chemical exposure and air quality. Changing this culture also supports increased safety and prevention for everyone.


Over one million Canadians have been diagnosed with MCS. MCS is a serious chronic illness and disability that impacts everyday functioning when a person experiences significant reactions to environmental factors, such as chemicals, foods, and biological agents. People with MCS find that after repeated exposures, they become increasingly sensitive to a range of chemicals. There is a large and growing body of scientific research supporting a biological basis for MCS which involves many complex processes, including inflammation, oxidative stress, and sensitization that often take place at the cellular level. While many people have some degree of scent/fragrance sensitivity, not everyone has Multiple Chemical Sensitivities (MCS). International prevalence of fragrance sensitivity and MCS are estimated to be 32.2% and 7.4% respectively.

Possible symptoms of MCS include:

  • Nervous system – heightened sense of smell; difficulty concentrating and/or remembering; variability in mental processing ; feeling dull, groggy or spacey; “brain fog” headache, pain, restlessness, hyperactivity, agitation, insomnia, depression, lack of coordination or balance, anxiety, seizures, tinnitus, fatigue, tension, confusion, memory loss, dizziness.
  • Upper Respiratory System – stuffy nose, itchy nose (the “allergic salute”), blocked ears, sinus stuffiness, pain, chronic infections.
  • Lower Respiratory System – cough, wheezing, shortness of breath, heavy chest, asthma, frequent bronchitis or pneumonia.
  • Eyes – red, watery eyes; dark circles under eyes; pain in eyes, disturbed vision.
  • Gastrointestinal System – excessive salivation, heartburn, nausea, bloating, constipation, abdominal pain, diarrhea.
  • Endocrine System – fatigue, lethargy, blood sugar fluctuations.
  • Musculoskeletal System – joint and muscle pain in the extremities and/or back, muscle twitching, spasm or weakness, swollen limbs.
  • Cardiovascular System – Rapid or irregular heartbeat, cold extremities, high or low blood pressure.
  • Skin – flushing (whole body, or isolated, such as ears, nose or cheeks), hives, eczema, other rashes, itching.
  • Genitourinary System – frequency and urgency to urinate, painful bladder spasms.

While there are many symptoms of MCS, it is of note that some of the most disabling symptoms—including neurological symptoms and respiratory symptoms—can improve with accessible air, and the use of the lowest-emission and least-toxic consumer products.

Statistics show that the prevalence of MCS is on the rise in Canada.

Multiple Chemical Sensitivity (MCS)Number, 2020%, 2020Number, 2016%, 2016Number, 2015%, 2015

Source: Canadian Community Health Survey, Statistics Canada

Between 2005 and 2010, there was a 34% increase in diagnosed cases of MCS. These increasing numbers are a call for action to efficiently recognize, diagnose and treat MCS.

Diagnosis of MCS

MCS can develop in people of all ages. It may follow an acute, large-scale chemical exposure, or appear after repeated low-level exposures to common consumer products and other chemicals released into the environment through the air, water, or soil.

It is important to understand that while MCS symptoms often appear at levels of exposure tolerated by many people, this can mean, but is not limited to, low-levels of exposure. Why? Because many everyday indoor spaces can have poor air quality. From this perspective, what a “low-level” of exposure is becomes more complicated. It becomes even more complicated when we think about exposure cumulatively. MCS is impacted both by spaces that have poor air quality, which able-bodied people do not react to, and by cumulative or repeated exposures that able-bodied people are able to tolerate.

MCS Diagnostic Criteria:

Initiation and subsequent sensitization observed in cases of multiple chemical sensitivities:

  1. Symptoms are reproducible following repeated chemical exposure;
  2. The condition is chronic;
  3. Lower exposure levels than those previously tolerated, or tolerated by the general population, lead to the manifestation of symptoms;
  4. Symptoms improve or disappear when the incitants are removed;
  5. Responses occur to multiple chemically unrelated substances;
  6. Symptoms involve multiple organ systems;
  • Still not refuted in the literature published almost two decades later
  • Definition used both in clinical practice and in research
  • Recently (2018) confirmed by the MOHLTC of Ontario by a group of experts using the Delphi process

Getting a diagnosis of MCS is necessary to get the appropriate support, accommodations, and accessibility for the disability. However, individuals do not need to have a formal diagnosis in order to receive accommodations in the workplace or housing. Please see our pages “everyday life” and “Work Accommodation” for more information.

To help with a diagnosis, to identify and measure exposure and symptoms and as a record for other professionals to take action, the person with MCS is encouraged to keep a record of exposures and symptoms to help identify sources of exposure, for example, in the workplace.

Other useful tools to share with your doctor are the questionnaire titled ‘Taking an Exposure History.’ and the Quick Environmental Exposure and Sensitivity Inventory (QEESI) .


The main principle of treatment is the avoidance of triggers, so as to create an accessible environment and support the body’s healing:

  • Recognize the current or potential triggers, by taking the exposure history of the person: community, housing, hobby (leisure), occupation, personal, diet, drugs (drugs) (CH2OPD2);
  • Consider current or potential exposures as well as stresses that may contribute to the condition, reducing or eliminating them as much as possible;
  • Identify the toxic load of the body, such as heavy metals or chlorine compounds, using appropriate tests, and intervene in a targeted manner to reduce this overload.
  • Make up for nutritional deficiencies such as vitamins and minerals, which make it more difficult for the person to metabolize the triggering agents;
  • Eat organic food.
  • Consider the use of assistive devices such as an N95 mask with an activated carbon filter, or a HEPA indoor air filter with activated carbon

Product choice is important for the management of MCS and for promoting accessible shared environments

Our partners, the Environmental Health Association of Quebec, have built a complete guide for a healthier lifestyle. This tool will help you implement changes in consumer product choices for your daily life, and support the removal of accessibility barriers for those with MCS and other disabilities impacted by the environment, especially air quality. You can find the complete guide on the

To learn more about product choice in relation to workplace accommodation and legislative action, please see the Resources section of our website in the top menu.

Sources and further information on diagnosis

Bartha, L., et al., Multiple chemical sensitivity: a 1999 consensus. Archives of Environmental Health: An International Journal, 1999. 54 (3): p. 147-149.

McKeown-Eyssen, G.E., et al., Multiple chemical sensitivity: discriminant validity of case definitions. Archives of Environmental Health: An International Journal, 2001. 56 (5): p. 406-412.

Nethercott, J.R., et al., Multiple chemical sensitivities syndrome: toward a working case definition. Archives of Environmental Health: An International Journal, 1993. 48 (1): p. 19-26.

Ontario Ministry of Health and Long-Term Care, Guiding Principles for the Diagnosis of Environmental Sensitivities/Multiple Chemical Sensitivity. Center for Effective Practice, 2018.

We thank our partner, l’Association pour la santé environnementale du Québec – the Environmental Health Association of Québec for putting their research and findings at our disposal. For more information, please visit their website.