Chemical sensitivity disorder or Multiple Chemical Sensitivity (MCS) is defined as "an acquired disorder characterized by recurrent symptoms, referable to multiple organ systems, occurring in response to exposure to many chemically unrelated compounds at doses below those established in the general population to cause harmful effects."

Most of us need little convincing that our urban environments are polluted. The belief that the food chain is contaminated with toxic chemicals is universal and readily validated. The exact number of persons suffering from chemical sensitivity is unclear, but the rise in immune and nervous system disorders indicates that this problem is becoming more common, and somewhat more widely recognized. Everyone knows someone who is sensitive to particular household products. We all recognize that people vary widely in their response to a room full of cigarette smoke - some people can ignore it easily while others react with stuffy nose, teary eyes, coughing, sneezing, shortness of breath. It should be no surprise, therefore, to learn that scientists are now confirming that different people react differently to modern compounds such as dry cleaning solvents, perfumes, detergents, glues, waxes, pesticides and other common household and industrial chemicals.

It has been suggested that MCS is not the best name for this family of ailments because it fails to reflect the importance of the initiating chemical exposure. Toxicant Induced Loss of Tolerance (TILT) better describes the true nature of the illness(es) initiated by a toxic exposure which leads to the loss of tolerance of common chemicals. Different initiating events may give rise to somewhat different ailments, all of which cause sensitivity to chemicals - just as different infectious diseases can all cause a fever.

Some people react violently to chemicals, to the point of being made ill by very low doses of common substances. Typical symptoms include fatigue, severe migraine-like headaches, nausea, that "run down" feeling, rashes, itching, swelling, pain, stuffiness, disorientation, and dizziness. This group of symptoms goes by various names: ecological illness, total allergy syndrome, environmental illness, and MCS.

There are over 70,000 chemicals commercially produced in the United States today. The long-term, low dose effects of many of these chemicals have never been investigated. With the construction of closed buildings, the rise in use of toxic construction materials, and an increase in the use of office machines, an environment of indoor air pollution has been created. Sixty per cent of indoor chemicals are generated by products or machines used indoors.

MCS is thought to afflict somewhere between 2% and 15% of the American public, and appears to be increasing, according to a publication of the American Chemical Society.

The prevailing rational and scientific viewpoint is that although some people are sensitive to small amounts of one or a few specific chemicals, there is no general hypersensitivity to chemicals. Scientifically oriented allergists, psychiatrists and occupational health clinicians suspect that the majority of "MCS" (Multiple Chemical Sensitivity) patients suffer from psychological disorders such as depression, anxiety reactions and somatization (bodily reactions to stress).

On the other hand, clinical ecologists (doctors who specialize in environmental illness), believe that MCS is a widespread condition caused by exposure to common foods, chemicals and other stressors that can sensitize people, causing them to react adversely to even tiny amounts of these substances.

Both inorganic compounds (such as carbon monoxide, nitrous oxide and heavy metals) and organic compounds (pesticides, formaldehyde, phenol, etc.) are involved. A major indicator of chemical sensitivity is multi-system disease: this is because once the chemicals enter the body, they enter the bloodstream and circulate to all parts of the body, even the brain.

Potential chemical toxins include:

• Formaldehyde which can be found in foam insulation, plywood, particleboard and press cabinets, fabric finishes, new carpet, polyurethane foam rubber (used in pillows, cushions, mattresses and rug padding), mobile homes, adhesives, synthetic clothes that crease resistant, wrinkle resistant.
• Oil vapors: from oil furnaces, motor-oil air-conditioning filters, electric kitchen appliances such as food processors, blenders, can openers.
• Polyethylene plastics: fake leather, artificial flowers, shower curtains.
• Household chemicals such as dry cleaning chemicals in clothes, mothballs, rug-cleaning products, paints, solvents, stain removers, air fresheners, window washing compounds.
• Polyesters in clothing, upholstery, drapery, furniture and stuffing for pillows and quilts.
• Pesticides residue on cottons and woolens; residues from exterminators.
• Epoxy adhesives on plastics, electronic equipment (TVs, microwaves,) which release gases when heated up.
• Common school paraphernalia such as carbon paper, ink, mimeographic and duplicating chemicals, glue.

Two important phenomena to understand regarding the symptoms are spreading and switching. Spreading occurs when additional organ systems are involved, or when a patient additionally becomes sensitive to inhalants, foods, dust, animal danders, or other environmental exposures. Switching occurs when the same exposure produces entirely different organ involvement e.g. photocopier fumes initially caused headache, and subsequently caused no headache but wheezing.

The main mechanism for chemical sensitivity seems to be the failure of the body’s enzyme detoxification pathways to adequately clear chemical compounds. Both immune and non-immune processes have been involved. Chemical sensitivity can develop after a massive chemical exposure, after specific non-chemical events such as massive trauma or surgery, after severe infections (viral, bacterial, parasitic) or with no identifiable cause.

Because most of the toxic chemicals involved are fat-soluble, they become stored in the body’s fat, resistant to metabolism and excretion. In fact, chemical levels measured from fat biopsies are sometimes 300 times greater than circulating serum levels.

The timing of symptoms after exposure, reproducibility of the symptoms, spreading, and switching must all be investigated. Serum levels of suspected chemicals confirm the diagnosis. If serum levels are negative and suspicion is high, a fat biopsy must be performed to complete the diagnosis. Additionally, chemical challenge tests can demonstrate the cause-and-effect relationship between a chemical exposure and symptoms.