On MCS, I am with
@chipmunk1 . Nobody can really explain it. The science is even less developed than for ME. There are a lot of models and hypotheses, and one, several or none of them might be right.
Chemical sensitivity on the other hand has been explained for a range of chemicals for a long time. Our bodies react to chemicals, sometimes benignly, sometimes they cause issues, and sometimes they are catastrophic. We use allergy and anaphylaxis to describe a particular type of immune reaction, but they are not the only possibilities.
Amine intolerance arises out of interaction with the nervous system, as they are analogs of neurotransmitters. Salicylates are enzymatic toxins, inhibiting critical chemical pathways. Many chemicals have direct or indirect effects on the nervous system, hormone system, immune system, or metabolism. This is an area woefully researched, particular in respect to quality studies (which cost a lot of money).
We know our metabolic rate tends to be not only low, but partially invariant. It can easily go lower, but its very hard to raise appropriately, particularly when there is demand on the body. This will impact everything, as normal detox and function requires energy, and the bulk of energy the body makes (unless you are an elite athlete on a marathon, or an arctic explorer in the cold, or similar) goes just to general maintenance of the body. If energy for that is insufficient then weaknesses within the body are likely to be amplified, and formerly sound biochemical systems may become dysfunctional.
MCS and some sensitivities are often thought to be central, that is there is some sensitization of the brain. I am less than convinced. Some of the chemicals, including salicylates and organic solvents, can act on the nervous system in seconds. Now it may be that we are sensitive to that and over-react, but the chemical insult is the trigger.
I am deeply suspicious about the idea that MCS is a single disease entity. I think its probably a syndrome, used to describe multiple problems that can co-occur and overlap.
Let me state something very bluntly though. Anyone who claims, as some detractors of MCS claim, that these natural and even artificial chemicals cannot have an adverse effect on the body, or even if they just imply it, is
inept or deceptive. We need well funded targeted science to figure it out, and to date its yet another area of medical research that is badly funded.
One last thing. Chemical and other sensory sensitivities are very common in ME and related disorders.