Understanding Multiple Chemical Sensitivity (MCS)
A Chronic Condition Triggered by Chemical Exposure
1999 Consensus Criteria
- Reactions are reproducible with repeated chemical exposure
- The condition is chronic
- Symptoms occur at low levels of exposure
- Symptoms improve when exposure is removed
- Reactions occur to multiple, unrelated substances
- Multiple organ systems are affected
Nervous System
Recognizing MCS: Triggers and Symptoms
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.
Common Triggers
Symptoms by Body System
Central Nervous
Heightened sense of smell, migraines, brain fog, difficulty concentrating, memory challenges, dizziness, fatigue, insomnia, anxiety
Respiratory & Immune
Coughing, wheezing, shortness of breath, sinus pain, sore throat, swollen glands, flu-like symptoms
Cardiovascular
Palpitations, rapid or irregular heartbeat, blood pressure changes, chest pain or pressure, cold extremities
Gastrointestinal
Nausea, heartburn, bloating, abdominal pain, constipation and/or diarrhea
Musculoskeletal
Joint pain, muscle pain, twitching, spasms, weakness
Skin
Flushing, hives, eczema, rashes, itching
Eyes
Red or watery eyes, eye discomfort or pain, disturbed vision
Genitourinary
Urinary urgency or frequency, bladder spasms
Navigating Diagnosis and Treatment
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.
Care Pathway
Ask about chemical exposures and triggers
Use BREESI/QEESI tools, document history
Record MCS, triggers, and functional impact
Source control and fragrance-free practices
Accommodation, referrals, ongoing assessment
MCS is not rare. It is under-recognized. When the evidence is followed, the path forward becomes clear.
MCS by the Numbers
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.
Rising Prevalence: 2000–2020
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.
How MCS Changes Everyday Living
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.
Employment Impact
not working
not working
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.
Areas of Life Commonly Affected
Work & Income
Workplace access, employment, meetings
Housing
Finding safe, low-chemical living spaces
Healthcare
Accessing clinics, hospitals, appointments
Education
Schools, libraries, community centres
Relationships
Stigma, misunderstanding, social isolation
Financial
Costs of safer products and environments
The Air Inside Matters
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.
Key Indoor Exposure Sources
Cleaning Products
Bleach, sprays, disinfectants
Fragranced Products
Air fresheners, candles, plug-ins
Building Materials
Paint, adhesives, flooring
Furniture
Off-gassing from new items
Combustion
Gas stoves, fireplaces, heaters
Mould
Moisture-related contaminants
The Pandemic's Impact on People with MCS
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.
Increased Chemical Exposure
Healthcare Barriers
Reduced PPE Stigma
Social Isolation and Understanding
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.
Lessons for the Future
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.
Challenging What People Think They Know
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.
Myth "MCS is psychological."
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.
Myth "People with MCS are just sensitive to smells."
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.
Myth "MCS only affects a small number of people."
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.
Myth "Accommodations for MCS are unreasonable or too costly."
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.
Myth "If the air smells clean, it is clean."
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.
Continue Learning
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.