Multiple Chemical Sensitivity is a perceived abnormal reactivity to common contaminants and chemicals in the environment that adversely affect quality of life, also known as “Environmental Illness”.

By Dr Adrian Morris

Synonyms: Environmental Illness, Multiple Chemical Sensitivity, Idiopathic Environmental Illness, 20th Century Disease, Universal Allergy, Total Allergy Syndrome, Allergic Toxaemia.

Randolph hypothesised that the body was like “a barrel filling up with chemicals” until a critical point is reached after which it reacts to any further chemical exposure. With numerous household chemicals and everyday agents suspected in the aetiology, the label Multiple Chemical Sensitivity (MCS) was coined by Cullen in the 1980’s [1]. MCS enjoyed wide acceptance as a medical condition in the popular lay media, amongst alternative practitioners, and in those individuals with non-specific symptoms who self-diagnosed themselves with MCS.

Up to 16% of individuals report some form of “unusual sensitivity” to common everyday chemicals (a phenomenon which is culturally restricted to North America and Europe).  This prompted the National Institute of Environmental Health in the USA to develop a consensus statement in 1999 [2].  The NIEH defined multiple chemical sensitivity as a “chronic recurring disease caused by a person’s inability to tolerate an environmental chemical or class of foreign chemicals” and proposed the preferred medical term Idiopathic Environmental Intolerance (IEI) with 6 diagnostic criteria (see figure 1).

Symptoms attributed to MCS/IEI are diffuse and include: recurrent headaches, sinus and nasal congestion, itchy eyes and throat, with abdominal symptoms such as nausea and vomiting, diarrhoea or constipation and bloating.  Additional symptoms and signs include non-specific rashes, breathing difficulties, muscle and joint aches, bladder problems, fatigue and flu-like symptoms, irritability, anxiety, depression, malaise, inability to concentrate, stupor (“brain fog”), hypotension, sleepiness, insomnia, mania, tremor and even paraesthesia.  These symptoms are attributed to a chronic prolonged and low dose environmental chemical exposure.

Overlap has been proposed with other environment-linked conditions such as Sick-building syndrome (SBS), food intolerance syndrome (FIS) and even Gulf War Illness (GWI).  It is postulated that trigger chemicals include formaldehyde found in furnishings and plastics, ink in newsprint and printers, volatile organic compounds (VOCs) in cleaning materials, Freon in poorly ventilated buildings, sulphites, pesticides and other chemicals in wine and processed food products.  Those affected may also be prone to chronic fatigue syndrome, fibromyalgia, irritable bowel syndrome, connective tissue disorders after silicone breast implants, reactive hypoglycaemia, drug-induced hepatitis, reactions related to living near toxic waste dumps and electromagnetism from power lines, dental amalgam disease and reactions to the petrol additive MTBE (methyl-tert-butyl ether).

Figure 1: Idiopathic Environmental Intolerance (IEI) diagnostic criteria:

  1. Symptoms recur and are reproducible
  2. The condition is chronic
  3. Low levels of exposure (lower than previously or commonly tolerated) result in manifestations of the syndrome (i.e. increasing sensitivity occurs over time)
  4. The symptoms improve, or resolve when the triggering chemicals are removed from the environment
  5. Responses often occur to multiple unrelated chemicals
  6. Symptoms involve multiple-organs (runny nose, itchy eyes, headache, scratchy throat, nausea and/or diarrhoea, abdominal cramping, aching joints etc.)

Immunological mechanisms (mainly non-IgE) have been proposed but none have been satisfactorily proven.  Terr extensively reviewed the immune theories proposed by the clinical ecologists and found none could be substantiated [3].  Aetiological theories are usually based on empiric hypotheses drawn mostly from poor quality studies published in the fringe medical press.  These have failed to establish any convincing evidential link between ill-health and day-to-day low-level chemical exposure.

A genetic predisposition with heightened sensitivity and abnormal olfactory or “odorant” receptors has also been proposed but never confirmed in clinical studies. In studies where the chemical odour was masked or suppressed by menthol, there was no difference between the study and control group symptomatology.

Many physicians believe that the symptoms of IEI are psycho-physiological in nature and those affected are prone to panic responses which enhance their symptoms.  Binckley and Kutcher demonstrated that individuals with self-identified chemical sensitivities responded with typical panic attacks when challenged with intravenous lactate (in a similar manner to individuals with underlying panic disorders) [4]. In Lenzoffs study, all the MCS patients who responded to a challenge with their trigger substances developed symptoms and signs of acute anxiety with hyperventilation and a rapid fall in PCO2, while their lung function remained normal [5].

In addition, IEI prone individuals appear to have a heightened olfactory awareness (hyperosmia) and a profound odour aversion (cacosmia) to otherwise harmless volatile organic compounds found in many household products, aerosols and even from trees. A recent systematic review of provocation studies for Multiple Chemical Sensitivities by Das-Munshi, Rubin and Wessely drew negative conclusions. Thirty seven provocation studies testing 784 persons reporting MCS and 547 controls were assessed in their meta-analysis [6].  Blinding was found to be inadequate in many studies, as most did not conceal the implicated chemical’s odour from their subjects. Only 3 studies used olfactory masking agents and in these none found associations between provocations and response. They concluded that persons with MCS/IEI do react to chemical challenges if they can smell the chemical but do not react when the odour is masked.  In one study participants were less likely to react to an odour described as “natural extracts with relaxing effects” (positive bias) as opposed to another described as “industrial solvents” (negative bias).  Thus suggesting their response was not to the chemical itself, but rather related to their “expectations and prior beliefs”.

Psychological conditioning has been proposed as the underlying abnormality in IEI, but is complicated by a high incidence of pre-morbid psychological trauma (including childhood physical and sexual abuse).  This may lead to profound long-term effects on mood and affect, cognitive processing, hypervigilence and entrenched beliefs of victimisation [7].   It is possible for patients with IEI to react to an olfactory sensation by amplifying the perceived effects into a full-blown panic attack and even trigger off underlying Idiopathic Anaphylaxis.

In the final analysis, after many years of investigation, there appears to be no convincing evidence in the medical literature for the existence of multiple chemical sensitivities or Idiopathic Environmental Intolerance.  The underlying cause for the IEI symptom complex is unlikely to be a direct reaction to everyday chemicals, but rather a masked stress disorder with heightened olfactory awareness (hyperosmia) and associated behavioural conditioning. Some patients respond to a low 10mg dose of the anti-depressant Amitriptyline.


  1. Cullen MR. The worker with multiple chemical sensitivities: an overview. Occup Med State Art Rev 1987;2:655-62
  2. Multiple chemical sensitivity: a 1999 consensus. Arch Environ Health 1999;54:147-9
  3. Terr AL. Unconventional theories and unproven methods of allergy. In: Middleton E, Reed CE, Ellis EF, Adkinson NF, Yunginger JW,  Busse WW editors. Allergy: principles and practice. 4th ed. St Louis: Mosby, 1993; 2: 1767-93
  4. Binckley K, Kutcher S. Panic response to sodium lactate infusion in patients with multiple chemical sensitivity syndrome J Allergy Clin Immunol 1997; 99:570-5
  5. Lenznoff A. Provocation challenges in patients with multiple chemical sensitivity. J Allergy Clin Immunology 1997;99:438-42
  6. Das-Munshi J, Rubin GJ, Wessely S. Multiple chemical sensitivities: A systematic review of provocation studies. J Allergy Clin Immunol; 2006; 118.6:1257-1264
  7. Staudenmayer H Multiple chemical sensitivities or Idiopathic environmentl intolerances: Psychophysiologic foundation of knowledge for a psychogenic explanation. J Allergy Clin Immunol 1997;99:434-7
Written and researched by Dr Adrian Morris
Originally Written 2008 & Last Reviewed 2nd January 2019.