The Evidence

TRPV1 and TRPA1 in MCS: the environmental alarm receptors

TRPV1 and TRPA1 are the strongest mechanism in MCS, and they are ahead of the others for a concrete reason rather than a preference: they are the only mechanism tested by provocation in living patients. Capsaicin challenge applies the exposure and measures what the body does instead of asking the person to report it, and it finds the response altered in people with MCS. These receptors sit where environmental irritants meet cough, pain, neurogenic inflammation and sensory signalling, which is the pathway this illness runs along. What the evidence does not carry, here or for any other mechanism, is the word cause: the sensitization is measured, and whether it starts the disorder or is one step inside it is open. The field is small enough that strongest means strongest of a thin field — and the review making the fullest case for these receptors has a published rebuttal in the same journal, which this page cites below rather than hiding.

Why these receptors are interesting

TRP channels are sensory proteins that help the body detect potentially important environmental conditions. TRPV1 responds to heat, acidity, and capsaicin, the compound that gives chili peppers their burn. TRPA1 responds to a wide range of reactive irritants and participates in airway and sensory signaling.

That makes the pair immediately relevant to MCS. People with MCS often report disproportionate responses to airborne irritants, fragrances, smoke, solvents, cleaning products, and other chemically unrelated exposures. A sensory system capable of detecting many environmental irritants is therefore an obvious place to investigate.

The capsaicin cough evidence

One of the clearest objective findings comes from capsaicin challenge. In a randomized, double-blind study, 12 patients with MCS showed greater cough responsiveness and more symptoms during capsaicin inhalation than healthy controls. Capsaicin is a classic TRPV1 activator, so the result supports increased sensory-neural responsiveness in MCS.

This does not prove that TRPV1 is the cause of MCS. Capsaicin challenge tells us that a pathway involving TRPV1-responsive sensory nerves behaves differently. It does not tell us what made that pathway different or whether the abnormality begins in the receptor, the nerve, central processing, or an interacting system.

Capsaicin also points toward central sensitization

Other studies used capsaicin as a probe of pain processing rather than cough. In 2011, 16 patients meeting MCS criteria showed greater capsaicin-evoked pain and larger areas of secondary hyperalgesia than healthy controls. A 2012 study of 15 MCS patients without overlapping disorders again found larger capsaicin-induced secondary hyperalgesia and other evidence compatible with facilitated central sensitization.

These findings matter because secondary hyperalgesia is not simply a stronger local skin reaction. It reflects altered processing beyond the site of the original capsaicin stimulus. That makes MCS difficult to reduce to either “the chemical is toxic at this tiny dose” or “the patient merely believes it is dangerous.” The nervous system itself may be operating at a different gain.

TRPA1 broadens the environmental story

TRPA1 is activated by many electrophilic and reactive chemicals and is heavily involved in sensory irritation and neurogenic inflammation. Because MCS involves reactions to many structurally unrelated environmental chemicals, TRPA1 is attractive as a convergence point: different substances could stimulate overlapping sensory pathways even though the chemicals themselves are unrelated.

The 2023 review by Molot, Sears, and Anisman placed TRPV1 and TRPA1 sensitization at the center of a broader MCS model that also included genetics, gene-environment interaction, oxidative stress, inflammation, cellular dysfunction, brain processing, and psychosocial influences. That broader context is important. The receptors are not being proposed in isolation.

What happens after the receptor opens is well described in general sensory biology, and it is not an allergy. Electrophilic chemicals bind TRPA1 covalently; the channel opens, calcium enters, and the nerve terminal releases Substance P and CGRP into the surrounding tissue through an axon reflex. Substance P then acts on mast cells through the MRGPRX2 receptor, triggering degranulation without any IgE antibody involved, and the released tryptase and nerve growth factor feed back onto the nerve and lower the threshold at which the TRP channels fire next time.5,6 That loop — chemoreception, neuropeptide release, non-IgE mast-cell activation, threshold lowering — is what “neurogenic inflammation” means, and it is the best-measured pathway MCS symptoms run through. It has been shown in tissue and in animal models; it has not been shown to be what starts MCS. The verdict on this page does not move: measured, and not yet proven to be the cause.

Why we say best-supported rather than proven

TRPV1 and TRPA1 are the strongest candidate and not a proven cause. A 2024 critique of the TRP-centered interpretation correctly emphasized that association and biological plausibility are not causation and that altered responsiveness does not establish that TRP sensitization initiates MCS.

That criticism is useful. A good biological hypothesis should survive being challenged. It forces us to ask what evidence would distinguish cause from downstream consequence: receptor expression, functional responsiveness, timing relative to onset, changes during recovery, genetic or epigenetic differences, and whether targeted manipulation of the pathway changes actual chemical tolerance.

Where predictive coding and conditioning might fit

TRP biology also helps explain why “brain versus body” is the wrong argument. Peripheral sensory receptors send signals into the nervous system. Central processing can amplify, dampen, predict, and learn from those signals. Autonomic and immune responses can feed back into the same network.

Conditioning and predictive processing could therefore become part of an established high-gain system without being the original cause of the receptor or sensory abnormality. Conversely, persistent central amplification could increase the impact of otherwise modest peripheral input. Those possibilities can coexist and should be tested rather than turned into competing ideologies.

What the registry and future studies are answering

The registry cannot measure TRP channels directly, but it can identify phenotypes that make laboratory testing more informative: cough hypersensitivity, heat intolerance, pain amplification, airway irritation, migraine, asthma, sensory triggers, dysautonomia, and patterns of rapid versus delayed reaction.

Future phenotype studies could combine capsaicin challenge, quantitative sensory testing, autonomic measures, and other objective endpoints in carefully characterized MCS participants and exposure-matched controls. Recovery studies are especially important: if TRP-related hyperresponsiveness decreases as chemical tolerance returns, that would be a powerful clue.

Where we stand

TRPV1 and TRPA1 are leading biological candidates in MCS because they provide a plausible bridge between environmental chemicals and an overresponsive sensory system, and because capsaicin studies have demonstrated measurable differences in MCS participants.

They are not yet a diagnostic test, a proven initiating mechanism, or the entire explanation. The strongest working model is larger: susceptible people may develop an acquired environmental hyperresponsiveness in which peripheral sensory receptors, central processing, autonomic regulation, immune signaling, and other systems interact. TRPV1 and TRPA1 may be important parts of that network. The task now is to measure exactly how important.

Sources and notes

References support the specific claims above. Study populations and limitations are described in the article; inclusion does not mean every study used the 1999 consensus case definition.

Ternesten-Hasséus E, Bende M, Millqvist E. Increased capsaicin cough sensitivity in patients with multiple chemical sensitivity. J Occup Environ Med. 2002;44(11):1012-1017. PMID: 12448352.

Holst H et al. Increased capsaicin-induced secondary hyperalgesia in patients with multiple chemical sensitivity. Clin J Pain. 2011. PMID: 21268302.

Tran MT, Arendt-Nielsen L, Kupers R, Elberling J. Multiple chemical sensitivity: on the scent of central sensitization. Int J Hyg Environ Health. 2013;216(2):202-210. PMID: 22487274.

Molot J, Sears M, Anisman H. Multiple chemical sensitivity: It’s time to catch up to the science. Neurosci Biobehav Rev. 2023;151:105227. PMID: 37172924.

Pitron V et al. Multiple Chemical Sensitivity: Catching up to what kind of science? Neurosci Biobehav Rev. 2024;157:105524. PMID: 38154656.

We followed the evidence. Here’s where it leads →

Sources

Sources

1Molot J, Sears M, Anisman H. Multiple chemical sensitivity: It's time to catch up to the science. Neuroscience & Biobehavioral Reviews. 2023;151:105227. DOI: 10.1016/j.neubiorev.2023.105227.
2Ternesten-Hasséus E, Bende M, Millqvist E. Increased capsaicin cough sensitivity in patients with multiple chemical sensitivity. Journal of Occupational and Environmental Medicine. 2002;44:1012-1017.
3Pitron V, Köteles FG, Nordin S, Haanes JV, Hillert L, Léger D, Lemogne C, Szemerszky R, van Kamp I, van Thriel C, Witthöft M, Van den Bergh O. Multiple Chemical Sensitivity: Catching up to what kind of science? Neuroscience & Biobehavioral Reviews. 2024;157:105524. PMID: 38154656. DOI: 10.1016/j.neubiorev.2023.105524.
4Rossi S, Pitidis A. Multiple Chemical Sensitivity: Review of the State of the Art in Epidemiology, Diagnosis, and Future Perspectives. Journal of Occupational and Environmental Medicine. 2017;60(2):138-146. PMID: 29111991. DOI: 10.1097/JOM.0000000000001215.
5Chiu IM, von Hehn CA, Woolf CJ. Neurogenic inflammation and the peripheral nervous system in host defense and immunopathology. Nat Neurosci 2012;15(8):1063-7. PMID: 22837035. DOI: 10.1038/nn.3144
6Green DP, Limjunyawong N, Gour N, Pundir P, Dong X. A Mast-Cell-Specific Receptor Mediates Neurogenic Inflammation and Pain. Neuron 2019;101(3):412-420.e3. PMID: 30686732. DOI: 10.1016/j.neuron.2019.01.012
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