Multiple Chemical Sensitivity (MCS) is sometimes referred to toxicant induced loss of tolerance (TILT) as over exposure to environmental toxic chemicals can sometimes mess with biochemistry (e.g. deplete enzymes necessary for proper detoxification and digestion, over activate the stress response and lead to adrenal fatigue and Limbic-HPA axis dysregulation) resulting in a lower tolerance to chemical exposures and a wide range of unpleasant symptoms from exposure levels not normally problematic for most people. This is a bit similar to having a wide range of allergies and some symptoms of MCS actually overlap with symptoms experienced by people with allergies. But MCS is different as the reactions are primarily due to brain and nerve inflammation and hyper excitability (NMDA, glutamine) and the immune system involvement IgG not IgE mediated like in allergies.
Multiple Chemical Sensitivity is an illness linked to toxicity and central sensitivity syndrome. Most people with MCS also have at least one other central sensitivity syndrome e.g. like chronic fatigue syndrome or fibromylagia. Many people with MCS, due to the enormous bio-psycho-social-environmental stress they are experiencing are also dealing with . No wonder!
A less common name used for central sensitivity syndromes is Dysregulation Spectrum Syndrome.
Many, but not all people with MCS, report that their symptoms started after a heightened exposure to known "neurotoxic" chemicals like solvents, pesticides or carbon monoxide or a prolonged exposure to chemicals like these . MCS is not only a toxicant induced environmental illness but also a stress induced illness that is likely to occur after a trauma (e.g. physical, viral, psychological) in people with a vulnerable genetic profile or people who for various reasons did not have enough reserves for complete recovery. 
“60% of MCS cases report an identifiable chemical initiating event… in addition, approximately 40% of MCS patients cannot identify a specific initiating chemical exposure event.” (Bell – 1)
Once initiated the reactions to chemicals experienced by people with MCS can trigger a stress response and maintain central pain and sensitisation, adrenal fatigue and HPA axis (neuro-endo-immune) dysregulation, particularly if chemical exposures are not reduced and people do not follow an appropriate treatment plan. A spreading response and worsening of sensitivities can also occur most likely due to limbic kindling an neural sensitisation.
“MCS appears to evolve in two stages: (1) initiation, characterized by a profound breakdown in prior, natural tolerance resulting from either acute or chronic exposure to chemicals (pesticides, solvents, indoor air contaminants, etc.), followed by (2) triggering of symptoms by small quantities of previously tolerated chemicals (traffic exhaust, fragrances, gasoline), foods, drugs, and food/drug combinations (alcohol, caffeine).” (Miller -2)
“The number of chemicals that appears to initiate MCS, while diverse, is not as widely varied as the number of chemicals that triggers symptoms in chemically sensitive individuals.” (Sorg -3)
The symptoms experienced by people with MCS vary from person to person and are usually the same as symptoms complained about by people with the co-morbid health conditions chronic fatigue syndrome (CFS), and fibromyalgia (FMS) and sometimes include some symptoms associated with other toxicant induced loss of tolerance (TILT) illness such as autism. The more common symptoms include:
Random Aches and pains
Memory and Concentration difficulties
Joint and muscular pain
MCS, CFS and FMS, as previously mentioned, are all Central Sensitivity Syndromes (CSS) but MCS is the severest of these and also involves quite wicked time depended sensitisation , which leads to a worsening of symptoms and sensitivity from repetitive exposures to triggers. Meaning that it is very important to avoid continuous or at least disrupt exposures to any stimuli that gets breathed in or causes nociception for any extended period of time as repetitive ongoing exposures can turn up the volume on pain, fatigue and sensitivity (neural sensitisation, hyperalgesia, allodynia)
"Hyperalgesia is an increased response to a noxious stimulus. It is caused by sensitization of peripheral nociceptors and/or by sensitization of central neurons that carry nociceptive information.
Allodynia is a painful response to a non-painful stimuli. It is caused by a change in the dorsal horn of the spinal cord that gives non-noxious sensory information access to the nociceptive system causing innocuous stimuli to be perceived as painful." What is Hyperalgesia? What is Allodynia?
If excitoxicity (very heightened NMDA, glutamine) is occurring very frequently (which it is in people with more severe MCS) more than the standard brain cells are lost each day and some brain damage is occurring . This is often in the deeper primitive structures of the brain which can not be seen with standard medical imaging but requires a SPECT scan. These are still mostly used in research, but now offered by a few clinics for example the Amen Clinics.
Brain damage or brain injury is the destruction or degeneration of brain cells.
Brain damage may occur due to a wide range of conditions, illnesses, or injuries.
Possible causes of widespread (diffuse) brain damage include prolonged hypoxia (shortage of oxygen), poisoning, infection, and neurological illness.
Common causes of focal or localized brain damage are physical trauma (traumatic brain injury), stroke, aneurysm, or neurological illness.
The extent and effect of brain injury is often assessed by the use of neurological examination, neuroimaging, and neuropsychological assessment.
Brain injury does not necessarily result in long-term impairment or disability, although the location and extent of damage both have a significant effect on the likely outcome - (http://www.sciencedaily.com/articles/b/brain_damage.htm).
Under these conditions sensory overload occurs very easily and heightened pain and symptoms are not just a result of exposures to chemicals and other environmental toxins (e.g. mold) but other things affecting the somatosensory system e.g. noise, scents, touch, too much visual stimuli (e.g. bright flashing colours), hot and cold. Due to the nature of limbic kindling and neural sensitisation, if exposures are repetitive and ongoing and no breaks are taken from the "hurting" environments to calm the stress response pain can sensitivity can continue to amplify and cause emotional distress.
“MCS patients often acknowledge hyper-reactivity in various other sensory modalities, including noise, light and touch.” (Bell -7)
To help repair injury and improve blood flow to the brain a comprehensive Wellness Plan is needed that includes the reduction of chemical, food, psychological and stimuli triggers, good stress management and lifestyle and dietary changes that support brain, nervous, system, gut and liver health.
For many people a wellness plan like the one developed by Dr Amen called
BRAIN FIT LIFE provides a great general treatment framework and provides lots of helpful resources and support, but for people with more severe multiple chemical sensitivity (MCS) and Chronic Fatigue Syndrome (CFS) these types of lifestyle, dietary and neural retraining programs can be too aggressive and need modification with the help of a trusted health professional. Some useful guidelines for practitioners are provided by the Centers for Disease Control and Prevention (CDC) and the MINDD.org. (see treatment)
For some people reducing exposures to chemicals and following an appropriate treatment plan can result in quite quick symptom relief and improvements, particularly in the early stages of MCS, however for people who have more disabling MCS, progress is generally slow and recovery (not fully achievable for everyone) can take between 2 and 10 years.
For me following the information shared at this site has resulted in 50 symptoms gone and I am still improving (I was extremely disabled and toxic encephalopathy when started). There were noticeable small improvements at a rate of 1 to 2 a month.
Photo Source: Article Extreme Chemical Sensitivity Makes Sufferers Allergic to Life by Jill Neimark Discovery Magazine
 Bell I.R., Schwartz G.E., Baldwin C.M., Hardin E.E., Neural Sensitization and Physiological Markers in Multiple Chemical Sensitivity, Regulatory Toxicology and Pharmacology, 24: S39 – S47 (1996)
 Miller C.S., The Compelling Anomaly of Chemical Intolerance, Annals of the New York Academy of Sciences, 933 (1): 1-23 (2001)
 Sorg B.A., Multiple Chemical Sensitivity: Potential Role for Neural Sensitization, Critical Reviews in Neurobiology 13 (3) (1999)
 Pall M.L.(2007) Explaining “Unexplained Illnesses”: Disease Paradigm for Chronic Fatigue Syndrome, Multiple Chemical Sensitivity, Fibromylagia, Post-Traumatic Stress Disorder, Gulf War Syndrome and Others. Harrington Park (Haworth) Press, New York.
 Yunus M Fibromyalgia and Overlapping Disorders: The Unifying Concept of Central Sensitivity Syndromes
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 Hooper M Engaging with Multiple Chemical Sensitivity (MCS)- London (2003) Malcolm Hooper PhD, B Pharm, C Chem, MRIC Chief Scientific Advisor to the Gulf War Veterans
 What Causes Multiple Chemical Sensitivity – MCS Research from James Madison University www.mcsresearch.net/causes.htm
 Heuser, G., Mena, I., Alamous, F. "Neurospect Findings in Patients Exposed to Neurotoxic Chemicals." Toxicology and Industrial Health 10, nos. 4-5 (1994):461-571.
 Heuser, G., Mena, I. "Neurospect in Neurotoxic Chemical Exposure. Demonstration of Long-Term Functional Abnormalities." Toxicology and Industrial Health 14, no. 6 (1998):813-827.
 Dr Sarah Myhill Chemical Poisoing – general principles of diagnosis and treatment http://drmyhill.co.uk/wiki/Chemical_poisoning_-_general_principles_of_di...
 The Search for Reliable Biomarkers of Disease in Multiple Chemical Sensitivity and Other Environmental Intolerances Chiara De Luca,1,* Desanka Raskovic,1 Valeria Pacifico,1 Jeffrey Chung Sheun Thai,2 and Liudmila Korkina1.
 Donohue M. Report # 4 Neural Sensitisation - Toxipedia