Every fact on this site, with its evidence grade, its caveats and a link to the source.
We start you on the strong ones: findings that have been replicated or come from one large, well-controlled study. Moderate and weak facts are here too, labeled for what they are. Use “Link to this fact” to share a single card.
Fragrance is everywhere / Measured in people or real products
A 2015 survey of 817 unique detergent products from 131 German households found limonene and linalool — the two fragrance chemicals most often linked to skin allergy when oxidized — named on 23.1% and 20.1% of product labels, more than any of the other 24 EU-declarable fragrance allergens.
Moderate evidence: Label-declaration frequency above the EU's 0.01% threshold, not a chemical measurement; Northern Germany, 2015, convenience sample, not representative.
Source: Wieck 2018, Regulatory Toxicology and Pharmacology Link to this fact
Green, natural and unscented / Animal study
'Natural' doesn't mean allergy-free: essential oils oxidize too; in mouse tests lavender oil exposed to air for 45 weeks became about 8 times more potent as a sensitizer (EC3 36% fresh, 4.4% aged), and oxidized alpha-terpinene from tea tree oil was the most potent of all.
Moderate evidence.
Caveats
Mouse LLNA (groups of 3-4 mice; one 45-week sample) from the primary Hagvall 2008, lab open-flask exposure; aged oil reached 'moderate', not 'strong', potency. Human support: 4/4 oxidized-linalool-allergic patients reacted to oxidized lavender oil (elicitation, n=4). Earlier evidence base figure '11% -> 4.4%' was a misreading.
Hidden ingredients / Measured in people or real products
Once in the air, scent chemicals such as linalool, ionones and the woody musk OTNE ('Iso E Super') reacted with ozone to form dozens of new oxidized chemicals, including a hydroperoxide. None of these was in the product.
Moderate evidence: Compound identities are tentative (formula-level mass spectrometry); products unquantified; no hazard testing.
Source: Wu 2024, Environmental Science & Technology Letters Link to this fact
Hidden ingredients / Animal study
The regulatory endocrine dossier on linalool tested 99.7%-pure, freshly handled material — not the air-oxidized linalool that forms in an opened bottle and that accounts for almost all of linalool's known allergy risk. Nobody has tested whether oxidized linalool is hormonally active.
Moderate evidence: This is standard and correct practice for a REACH dossier; it is a limit on what the result covers, not a flaw in the study.
Source: Hareng 2024, Archives of Toxicology Industry-funded Link to this fact
The industry / Measured in people or real products
The study's own authors note that current diagnostic patch-test concentrations for oxidized linalool allergy are more than 500 times higher than the hydroperoxide levels they actually found in consumer products.
Moderate evidence: Industry-funded interpretation; does not itself prove patch tests over-diagnose, since already-sensitized skin can react to much lower doses than the diagnostic test threshold.
Source: Kern 2014, Analytical and Bioanalytical Chemistry Industry-funded Link to this fact
The industry / Measured in people or real products
Despite thousands of positive allergy-test reactions to oxidized limonene and linalool reported worldwide, the fragrance industry's own market-testing program concluded that where people are actually getting exposed to these allergens "remains elusive" — the industry itself has not identified the real-world consumer-product source of a widely documented skin allergy.
Moderate evidence.
Caveats
This is the industry's own conclusion, from an industry-funded study; it does not mean the allergy is not real or not caused by fragrance chemicals — only that the specific consumer-product exposure route has not been chemically confirmed even by the party with the strongest incentive to find an innocuous explanation.
Source: Natsch 2019, Food and Chemical Toxicology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In patch tests on 61 people, fresh limonene and linalool did not irritate skin, but after air exposure (oxidation) both did; oxidized limonene at 20% caused strong redness (average 7.3 of 9 vs near 0 fresh).
Moderate evidence: RIFM-funded; test concentrations far above product levels.
Source: Bråred Christensson 2009, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Review of other studies
In about half of oxidized limonene/linalool allergy cases, doctors could link the allergy to products the patient actually used.
Moderate evidence.
Caveats
Whether a given product actually reaches the reaction-triggering hydroperoxide level depends heavily on its age/storage: industry data found Lim-2-OOH below 5 ppm in most recalled fine fragrances tested, versus ~2900 ppm in an experimentally aged orange oil after 1 year of dark storage (Bennike 2019). The strongest direct test of "linked to products used" now undercuts it rather than supporting it: Natsch 2019 chemically tested 28 products donated by dermatology patients via the same GEIDAC network this review draws on, including 11 from patients already confirmed patch-test-positive to oxidized limonene/linalool who specifically named the donated product as their suspected trigger, and found zero detectable hydroperoxide in any of them (confirmed by two independent LC-MS methods). Clinical "linkage" judgments based on patient history and declared product content do not survive chemical confirmation in this sample, so the claim moves from a plausible clinical inference to one now directly (though not exhaustively) tested and not supported. Natsch 2019 (industry-funded) found no hydroperoxide in 28 products donated by patients, including 11 from patients allergic to oxidized limonene/linalool. That questions whether the named product is the actual source, but it can't falsify the clinicians' relevance assessments; the exposure route remains unexplained. Kern 2014 and Natsch 2019 are both industry studies (Givaudan / IDEA consortium) with Natsch as a co-author on both, so they are not independent replications of each other.
Skin allergy / Lab study (cells or chemistry)
Oxidized linalool and oxidized limonene, among the most common fragrance allergens found in European skin clinics, are not in the 312-chemical dataset these safety models were built on.
Moderate evidence: Clinic prevalence figures for oxidized terpenes come from a separate paper (Sukakul 2022), not this one.
Source: Natsch 2015, Toxicological Sciences Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Whether a specific limonene-scented product is likely to trigger a reaction in an allergic person depends heavily on how oxidized/aged it is: an experimentally aged orange oil's allergenic hydroperoxide content rose nearly 6-fold (500 to 2900 ppm) after a year in storage, while some real aged fine fragrances tested by industry stayed below the reaction-triggering range.
Moderate evidence: The storage/aging data come from a separate cited experimental study, not this paper's own clinical subjects, and cover only one product type (orange oil).
Skin allergy / Measured in people or real products
Even when scientists chemically tested the actual products that people with a confirmed allergy to oxidized limonene or linalool blamed for their skin reaction, none of the 28 products tested (11 from confirmed allergy patients) contained a detectable amount of the allergenic chemical.
Moderate evidence.
Caveats
Small sample (28 products, 11 from confirmed patch-test-positive patients) relative to the >22,000-patient clinical literature on this allergy; industry-funded, not independently replicated; other exposure routes (skin-surface formation after application, airborne deposition) are untested.
Source: Natsch 2019, Food and Chemical Toxicology Industry-funded Link to this fact
What you can do / Review of other studies
People with suspected fragrance dermatitis should ask for patch testing that includes oxidized limonene and linalool, because standard fragrance screens miss many cases.
Moderate evidence.
Source: Sukakul 2024, Acta Dermato-Venereologica Industry-funded Link to this fact
What you can do / Measured in people or real products
People with unexplained dermatitis can ask for patch testing with oxidized limonene and linalool; standard screens can miss these allergies.
Moderate evidence.
Source: Bråred Christensson 2016, Contact Dermatitis Industry-funded Link to this fact
What you can do / Review of other studies
If you have unexplained eczema, ask for patch testing that includes oxidized limonene and linalool, now recommended for the European baseline series.
Moderate evidence.
What you can do / Measured in people or real products
If you have eczema, ask for patch testing that includes oxidized linalool and limonene: they are not in every standard test series, so the allergy can be missed.
Moderate evidence: The authors say more clinical-relevance data are needed before routine screening.
What you can do / Measured in people or real products
If you are diagnosed with oxidized-linalool contact allergy, avoiding products listing linalool or fragrance/parfum is worth doing even at low concentrations, since repeated use at product-realistic doses reproduced dermatitis in a clinical trial, and most participants who had already stopped using linalool-containing products reported their dermatitis improved.
Moderate evidence: Self-reported improvement on avoidance (4 of 5) is not blinded or controlled; the ROAT elicitation data (the stronger evidence) show reactions down to 0.3% oxidized linalool.
Source: Andersch Björkman 2014, Contact Dermatitis Link to this fact
What you can do / Measured in people or real products
If you have a confirmed (not just borderline) positive patch test to limonene hydroperoxides, avoiding products with appreciable oxidized/aged limonene is well supported: realistic product-level doses reliably triggered a reaction in this group.
Moderate evidence: Applies to confirmed reactors; a borderline patch-test result is much weaker evidence of true allergy.
What you can do / Measured in people or real products
People with chronic, treatment-resistant hand eczema and a history of reacting to scented products can ask their dermatologist for patch testing beyond the standard fragrance mix, including citral, Lyral, hydroxycitronellal and oxidized limonene/linalool.
Moderate evidence: Based on one clinical population's reaction pattern (Heydorn 2003); a dermatologist should tailor markers to local product exposure.
Source: Heydorn 2003, Contact Dermatitis Industry-funded Link to this fact
Information, not medical advice. See also: myths we won’t tell you.