For people living with MCS, exposure to triggering substances reliably produces symptoms across multiple body systems. While profiles vary, continued exposure typically increases sensitization — meaning lower levels of exposure provoke reactions, and the range of triggering substances widens.
Heightened sense of smell, migraines, brain fog, difficulty concentrating, memory challenges, dizziness, fatigue, insomnia, anxiety
Coughing, wheezing, shortness of breath, sinus pain, sore throat, swollen glands, flu-like symptoms
Palpitations, rapid or irregular heartbeat, blood pressure changes, chest pain or pressure, cold extremities
Nausea, heartburn, bloating, abdominal pain, constipation and/or diarrhea
Joint pain, muscle pain, twitching, spasms, weakness
Flushing, hives, eczema, rashes, itching
Red or watery eyes, eye discomfort or pain, disturbed vision
Urinary urgency or frequency, bladder spasms
Validated screening tools exist, including the BREESI (Brief Environmental Exposure and Sensitivity Inventory) and the QEESI (Quick Environmental Exposure and Sensitivity Inventory), both widely used for clinical screening and case identification.
There is currently no cure. The most effective approach is ongoing management through exposure reduction — systematically identifying and minimizing contact with triggering substances across the environments where people spend the most time.
MCS is not rare. It is under-recognized. When the evidence is followed, the path forward becomes clear.
Canada is one of the few countries that systematically tracks MCS prevalence through national surveys. The Canadian Community Health Survey (CCHS), conducted by Statistics Canada, provides two decades of data on this growing public health concern.
Note: MCS prevalence data was not collected nationally for 2023 and 2024, as several provinces opted out of the relevant survey questions. Starting in 2025, Statistics Canada has confirmed that MCS questions will be included annually in every province — ensuring uninterrupted national data going forward.
MCS creates barriers to environments most people take for granted — workplaces, schools, healthcare settings, and public spaces. Without accommodations, exclusion can be severe and far-reaching.
People with MCS also face higher barriers to healthcare access — including service unavailability, high costs, lack of a regular provider, and difficulty accessing specialist services.
Workplace access, employment, meetings
Finding safe, low-chemical living spaces
Accessing clinics, hospitals, appointments
Schools, libraries, community centres
Stigma, misunderstanding, social isolation
Costs of safer products and environments
Source control — choosing fragrance-free, lowest VOC emission, and least toxic products — is the most effective strategy. This eliminates chemical exposures at their origin, rather than relying on ventilation after chemicals have been released.
Bleach, sprays, disinfectants
Air fresheners, candles, plug-ins
Paint, adhesives, flooring
Off-gassing from new items
Gas stoves, fireplaces, heaters
Moisture-related contaminants
COVID-19 created both new barriers and unexpected insights for people living with MCS. A cross-sectional study by ASEQ-EHAQ, published in JMIR Formative Research (2024), surveyed 119 Canadians with MCS during the first year of the pandemic.
People with MCS experienced increased social isolation during the pandemic. However, there was an unexpected benefit: participants reported greater understanding from family members and reduced stigma around protective measures.
The pandemic demonstrated that when environments are modified and accommodations offered widely — remote work, improved ventilation, awareness of cleaning practices — they benefit not only people with MCS but the broader population. These lessons should inform future accessibility policy.
Despite growing evidence and international recognition, MCS faces significant stigma. A 2025 qualitative study involving ASEQ-EHAQ identified four key drivers: psychological misattribution, healthcare gaps, policy resistance, and misleading product labelling. Here are the facts.
MCS is not a psychological condition. Research has identified neurobiological, immunological, and toxicological mechanisms. A 2023 comprehensive review in Neuroscience and Biobehavioral Reviews documented evidence across multiple biological domains. Attributing MCS to psychological causes is not supported by the current evidence base and contributes to dismissal and delayed care.
MCS is a medical condition in which chemical exposures trigger physiological reactions across multiple body systems. It is not a preference or a heightened sense of smell. The distinction between disliking an odour and experiencing measurable, reproducible symptoms is critical to understanding and respecting this disability.
Over 1.13 million Canadians have been diagnosed with MCS — that is approximately 1 in 34 people. Self-reported chemical sensitivity rates range from 6.5% to 9% internationally. Prevalence in Canada has been steadily increasing for over two decades.
Many effective accommodations are simple and low-cost: switching to fragrance-free products, adjusting cleaning practices, improving ventilation, and offering scheduling flexibility. These changes benefit indoor air quality for everyone — not only people with MCS.
Many harmful VOCs are odourless or present below the threshold of smell. The absence of a noticeable odour does not mean air quality is safe. Source control — eliminating chemical emissions at their origin — is the most reliable approach to healthier indoor air.
Each topic above has a full dedicated page with in-depth information, resources, and references. As new evidence emerges, content is updated to reflect the latest developments.
Everyone deserves the right to know.