Chemical sensitivities, also known as multiple chemical sensitivity (MCS), is a particular variant of heightened responsiveness to sensory input. Many of the patients that I see are sensitive to light, sound, touch and taste, but for the majority of the symptoms triggered by exposure to even very low concentrations of solvents, pesticides and other volatile chemicals is disabling and a never-ending.
MCS is a medical condition was Identified by US Occupational Physician, Mark Cullen in the late 1980s, and defined in 1999 as  a chronic condition  with symptoms that recur reproducibly  in response to low levels of exposure  to multiple unrelated chemicals and  improve or resolve when incitants are removed with  symptoms occur in multiple organ systems. (Arch Environ Health 1999 May-Jun;54(3):147-9)
I was involved in the MCS Consensus Conference in San Francisco in 2006 where an alternative diagnostic checklist was considered and agreed upon in principle. The checklist was tested by clinicians in four countries and shown to have specificity and sensitivity above 85%. The draft of the MCS Diagnostic Checklist remains a useful determinant of both the presence of MCS and the severity of its impact on a person's life.
More than a decade ago, I wrote a simple book, Killing Us Softly, addressing multiple chemical sensitivity, likely causes, and the lessons we have learned from our controlled-environment inpatient-hospital unit, the Special Environment Allergy Clinic, in Sydney between 1989 and 1994. In that period we treated over 300 people with chemical sensitivities, and learned valuable lessons about chemical detoxification ("depuration"), and the necessity to protect the person nutritionally and biochemically prior to detoxification.
My book is available here under Creative Commons free access, and while I have not updated it in some years, many people continue to provide positive feedback, suggestions, and support from my writing. (Creative Commons Restrictions: This publication is free to be copied, transmitted or distributed without charge or cost to any recipient.)
I think what is now clear is that neuroinflammation and glutamate excitation our impact triggered by chemicals entering the central nervous system through the olfactory bulb, and that these contribute in part to the inflammatory process. This should not be surprising, given that the chemicals used as pesticides, solvents, petrochemicals, antiseptics and plasticisers (to nominate just a few) are toxic and largely untested in humans.
My approach with chemical sensitivities is to minimise exposure to chemicals that trigger reactions, to use inexpensive alkali salts ("bi-salts") to control symptoms following chemical exposure, to use medical oxygen when necessary, and to focus efforts on the reduction of responsiveness of both the olfactory and the trigeminal nerves and the reduction of neuroinflammation and the countering of the glutamate "storm".
Using a variety of methods such as these, it is usually possible to bring a person back to a level of responsiveness compatible with normal day-to-day life, although great care needs to be taken to not ignite the symptoms by accidental and unnecessary exposure to the same types of chemicals in the future.