Multiple Chemical Sensitivity.

This is a BIG care of people one.  Multiple Chemical Sensitivity.  It isn't in and of itself a particularly Permacultural topic, but for the fact that a) Permaculture tends to have very low "synthetic chemical" inputs, b) Care of People includes doing what we can to reduce harm and c) I only met my wife through a Permaculture event, and she only became interested in Permaculture due to her MCS.

What is "Multiple Chemical Sensitivity?  Well, the name pretty much covers it.  I wrote a similar post to this as an answer to someone else's blog probably almost 12 years ago.  His contention was that MCS doesn't exist, because people are saying they are sensitive to chemicals, and "everything is made up of chemicals."  And to prove his point, he included an image of the Periodic Table of the Elements.  I found that idea interesting, because, although everything, everywhere that we know of, is made up of the chemicals in the Periodic Table, there are virtually NONE that I would be willing to consume in a pure form.  Some of the inert metals are safe enough, but things like oxygen, which is essential to life, is quite dangerous to life if you are ONLY breathing pure oxygen.
So, because each person has many potential triggers, I think the name is close enough.  It has other names: Environmental Illness, Total Allergy, 20th century Sickness. . . 
But I will stick with Multiple Chemical Sensitivity (MCS) for this post, because it will do.

What are the symptoms?  Well. . . The body is reacting to toxins, so many of the initial symptoms are similar to those of intoxication: slurred speech, loss of co-ordination, inability to concentrate, aggressiveness, mood swings: all the sorts of things you would expect to see from a drunk person.
And it goes further: the inability to remove these toxins from the body, or repeated exposure can lead to temporary loss of muscle function, unconsciousness, organ failure, brain damage: all things you see in long term alcohol abuse, and long term exposure to industrial chemicals.

What causes it? Truly, no-one knows.  There are theories about toxic load, various viruses, compromised gut walls, compromised blood/brain barriers. . . it could be all of these things, or it could just be normal human variation.  The actual incitants, the things that cause reactions, can be almost anything (which is why the name holds) but they are most likely to be things that are toxic.  If it says on the label "Not to be taken" or "Poison" or "Use in a well ventilated area" or "Deliberate misuse by concentrating and inhaling may be harmful or fatal", there is a very good chance that it will cause a reaction in most MCS sufferers.  Unsurprising really, because these things ARE poisons!  The main difference being, in these instances, that at level at which most people wouldn't even be aware of these chemicals in their environment, MCSers are already likely to be reacting.
But it doesn't stop there.  Anything that is conventionally considered a possible allergen might also be a problem. 
And some things for which you will never see the packet that has any of the warnings above: the fire retardant in your carpet, the formaldehyde in you r new clothes, the lingering residue of the termite extermination from 10 years ago.  Even some gases from untreated timbers, the perfume of some flowers (the perfumes "we" wear are often greatly concentrated from the original flower: perfumes are a MASSIVE trigger for MCS.)

So. . . all those things have the potential to be dangerous to all of us.  What makes a sufferer of MCS any different?  Well, it could be the things mentioned at the start of the paragraph "What causes it?" but as I stated there, it could also be normal human variation. 
Normally, when I get stung by a bee, I get a little lump, similar to what I normally get from a mosquito bite.  Recently, I got a few more than my normal number of stings, and ended up in hospital being treated for anaphylaxis.  Why?  as it the increased dose?  Was it the placement of the stings?  Was it the interaction of the venom with something else already in my system?  I don't know.  But I do know, normal human variation has some people not react to bee stings at all, and some who are always anaphylactic.
Do you know someone who suffers from hay fever?  I do.  I also know lots of people who DON'T have any hay fever symptoms.  They are exposed to the same potential incitants, but have completely different reactions.
Reacting to a bee sting makes sense: a toxin, intended to cause great pain in order to dissuade people, bears, monkeys, badgers, mice. .  . anything and anyone, from attacking a hive, and stealing their hard won honey.  The venom is supposed to be poisonous, and pain is supposed to be a result.  Reacting to pollen makes less sense.  Pollen is not evolved to cause discomfort, there is no evolutionary advantage to a plant it it makes a passing mammal uncomfortable.  Plants that don't cope well with being eaten produce spikes, or irritating hairs, or foul flavours, or poisons that prevent them being ingested by educable creatures.  So pollen allergy makes no evolutionary sense from the point of view of the plant, or the person.
So, reacting to "things that are ..."Not to be taken" or "Poison" or "Use in a well ventilated area" or "Deliberate misuse by concentrating and inhaling may be harmful or fatal"" also makes sense.  We want these things out of our systems, therefore, our system lets us know they are bad.
Oh, and as I said, most of these things are toxic.  They are designed to be deadly to some life form, whether it is parasites or weeds or fungus.  A small dose might do most people no harm.  Being bitten by a rattlesnake, or a cobra WILL do most people some harm, even though a bee sting might not.  But you know what?  Bee venom, and rattlesnake and cobra venom are chemically very similar.