If you have MCS, you have probably been told what it “really” is. A detox problem. Mast cells. Mold. Genes. Anxiety. Most of these ideas come from people working hard to make sense of a real illness, and most of them contain something true. None of them is the whole explanation.
MCS is a physical illness. It begins with an exposure, in a person who was susceptible. The best-tested account of what happens in the body is that the nerve receptors which detect chemicals have become over-sensitive. Other body systems are involved in some people. No single cause has been proven.
Below, each common explanation gets the same three things: what is true in it, what has not been shown, and what you can do with it. For what starts MCS, see What causes MCS to begin? For what happens in the body, see How MCS Works.
Lowering your load helps. A failed liver has not been shown to be the cause.
What is true. Reducing what your body has to deal with is the best-supported thing you can do for MCS. In a survey of 917 people with self-reported MCS, about 95 percent of those who had tried avoiding chemicals or creating a chemical-free living space rated them helpful.[11] Glutathione, the molecule your body uses to process chemicals, also measures lower in the blood of people with MCS in more than one study.[1][2]
What has not been shown. That MCS is caused by a liver that cannot clear chemicals. Studies of detox-enzyme genes found a link once and then failed to confirm it three times.[1][3][4][5]
What to do with it. Lower your total load, and look for sources you have not noticed. For some people that includes the outdoor air where they live, even on days when the air quality index looks fine.
Does detox help MCS? → · Does avoiding chemicals help MCS? → · Oxidative stress, glutathione and metabolism in MCS →
Worth testing for. Not shown to be the cause.
What is true. Metal poisoning can produce fatigue, brain fog and nerve symptoms that look like MCS. One small study found raised mercury in many MCS patients, mostly people with amalgam fillings.[6]
What has not been shown. That metals cause MCS, or that removing them restores tolerance to chemicals.
What to do with it. If your history suggests metal exposure, ask for standard blood and urine tests.
Inherited susceptibility is probably real. No gene explains MCS.
What is true. Not everyone exposed gets sick, so something about the person matters. MCS sometimes runs in families.
What has not been shown. A gene for MCS. The genes most often named (CYP2D6, NAT2, PON1, MTHFR and the GST family) looked promising in one early study, and larger studies did not confirm them.[3][4][5] The early study pointed to faster chemical processing, not slower.
What to do with it. A gene report can be interesting. It cannot diagnose MCS or rule it out.
A real overlap. Not the same illness.
What is true. MCAS is its own condition, with lab tests and treatments that work. It overlaps with MCS, and some people have both.
What has not been shown. That MCAS causes most MCS, or that most people with MCS have MCAS. Many people with MCS have no mast cell findings at all.
What to do with it. If you have flushing, hives or sudden reactions that look allergic, ask to be tested.
The immune system is involved. Current evidence has not established MCS as an autoimmune disease.
What is true. People with MCS show measurable inflammatory and immune changes in blood.
What has not been shown. A self-targeting antibody, or any marker that defines MCS as autoimmune.
One of the most common starting points. Not the explanation for every case.
What is true. Water-damaged buildings appear again and again in the stories of how MCS began and why it came back.
What has not been shown. That every case of MCS is mold illness. Pesticides, solvents, new construction and smoke start it too.
What to do with it. If your illness began or worsens in a particular building, take that seriously and have the building checked.
Mold, water-damaged buildings and VOCs → · What causes MCS to begin? →
Calming the nervous system helps you live with MCS. It has not been shown to treat the illness.
What is true. A nervous system on high alert makes every reaction harder, and bringing it down is worthwhile. Some people report real improvement.
What has not been shown. That MCS is a vagus nerve disorder. A large Danish study found no link between MCS and heart rate variability, a standard measure of how the vagus nerve is working.[7] We found no study that has tested vagus nerve treatment for MCS.
What to do with it. Use these tools for coping, sleep and recovery time. Keep reducing your exposures.
No.
What is true. Reactions can look allergic, and a person can have both allergies and MCS.
What has not been shown. An allergic mechanism. Standard allergy tests are negative in MCS. That result is expected, and it is not a reason to dismiss the diagnosis.
No.
What is true. Living with MCS can cause anxiety, fear and depression, and those can change how strong a reaction feels.
What has not been shown. That psychology causes MCS. A physical illness with emotional consequences is still a physical illness.
No. You do not have to smell a chemical to react to it.
What is true. Many triggers have an odor, and odor is often the first warning.
What has not been shown. That smell is the mechanism. Smell and chemical irritation run on two separate nerve systems. In Swedish studies, patients wearing a nose clip could not smell perfume and still developed cough and breathing symptoms, and exposing only their eyes did the same.[8][9] In a blinded study, people with chemical intolerance felt more eye, nose and throat irritation than others from a low dose of an irritant hidden under another odor.[10]
What to do with it. Covering a chemical with a scent you tolerate does not remove it. Protect your eyes as well as your breathing.
The strongest explanation so far is that two receptors on nerve endings, called TRPV1 and TRPA1, have become over-sensitive.
It leads for one reason: it is the only explanation that has been tested by exposing people with MCS and measuring how their bodies respond. It is still not proven to be the cause.
Because more than one process is involved. The first step is a nerve signal, which is why some reactions begin within a breath. Other symptoms, such as stomach and bowel trouble, can follow many minutes later. How fast you react, and whether you react at all, also depends on the state you are in. After one or two reactions, the next one comes more easily and more quickly. That is what a sensitized system does. It is not inconsistency, and it is not imagination.
Because MCS probably does not break the same way in every person, and because it travels with other conditions that do have tests: mast cell disease, mold illness, autonomic disorders and nerve damage.
Test what can be tested. Treat what is found. MCS is what is still there after that work is done, and it is real.
12,000 Canaries Can’t Be Wrong, by John Molot, MD. A plain-language book by the physician whose research this site cites most. A good, valid read. It was written some years ago and research has moved on since, so where the book and this site differ, the site reflects the newer findings.
The Chemical Sensitivity Podcast, hosted by Aaron Goodman, PhD. Interviews with researchers, clinicians and people living with MCS.
MCS is physical, it begins with an exposure, and no single cause has been proven. Each explanation on this page holds part of the picture for some people. None of them is the whole of it.
Lowering your load helps. Testing for the conditions that travel with MCS is worthwhile. Neither one requires anyone to settle what MCS “really” is first.
No. The AMA has issued nothing on MCS this century; its cited documents are from 1992 and 2000 and address clinical ecol…
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Free TILT Self-Assessment including the BREESI screener and QEESI — the gold standard for identifying chemical intoler…
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