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.
Skin allergy / Measured in people or real products
Common scent chemicals such as linalool hardly cause allergy when fresh, but once exposed to air they form new chemicals that do: in 1,511 dermatitis patients at six European clinics, 1.3% were allergic to air-oxidized linalool (later studies with a stronger test found about 7%).
Strong evidence.
Caveats
Dermatitis patients, not general population; 2005 test concentration (2.0% pet.) was later shown too low. Clinical relevance no longer just inferred: Andersch Björkman 2014 (ROAT) shows patients identified this way go on to develop dermatitis from twice-daily use of a cream/perfume containing oxidized linalool at concentrations as low as 0.3% (0.056% linalool hydroperoxides, ~27 µg/cm²) — inside the 0.1–2% range measured for individual fragrance ingredients in real products.
Skin allergy / Animal study
Common fragrance chemicals such as limonene (citrus) and linalool (lavender) react with air to form far stronger skin allergens; in mouse tests, oxidized limonene was about 10 times more potent than fresh limonene.
Strong evidence: Mouse LLNA potency; review of replicated data.
Skin allergy / Measured in people or real products
In European skin clinics, 2.8-5.2% of patients tested for dermatitis reacted to oxidized limonene.
Strong evidence: Consecutive dermatitis patients, not general population.
Skin allergy / Measured in people or real products
In patch tests of 2,900 dermatitis patients in six countries, 5.2% were allergic to oxidized limonene and 6.9% to oxidized linalool, two of the most common fragrance chemicals.
Strong evidence: Clinic population, not general public.
Source: Bråred Christensson 2016, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In 2,900 dermatitis patients tested at nine clinics in Europe, Asia and Australia, 6.9% were allergic to air-oxidized linalool, the form a common floral scent chemical found in most perfumes, cosmetics and many cleaners takes after contact with air.
Strong evidence: Dermatitis patients, not general population; 72% weak reactions; range 3-14% by clinic.
Source: Christensson 2012, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
60.5% of patients allergic to oxidized linalool would never have learned they had a fragrance allergy from the standard fragrance patch tests.
Strong evidence: Dermatitis patients.
Source: Christensson 2012, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
80% of patients allergic to oxidized linalool whose products were checked were using fragranced products, mainly shampoos, soaps and creams.
Strong evidence.
Caveats
Linalool confirmed in their products in 38%; clinical relevance established in 41%. Andersch Björkman 2014 (ROAT), drawn from this same 2,900-patient cohort's Gothenburg patients, causally confirms the exposure-effect link: repeated open application of oxidized-linalool-containing cream/perfume at real-product concentrations (down to 0.3%) reproduced dermatitis, and participants who had already avoided linalool-containing products mostly reported improvement.
Source: Christensson 2012, Contact Dermatitis Link to this fact
Skin allergy / Review of other studies
Limonene and linalool become skin allergens when they react with air: in European dermatitis clinics, 3.9-20% of patients tested reacted to oxidized linalool and 1.4-13.7% to oxidized limonene, against under 1% for the fresh chemicals.
Strong evidence: Clinic patients referred for patch testing, not general population.
Skin allergy / Review of other studies
Standard allergy tests with fresh limonene and linalool miss most of these allergies, so European experts now recommend adding the oxidized forms to routine patch testing.
Strong evidence.
Skin allergy / Measured in people or real products
Among 5,773 people tested for eczema at a Swedish hospital, 9.4% were allergic to the air-oxidized forms of two of the most widely used scent chemicals: linalool (7.0%) and limonene (5.1%).
Strong evidence: Clinic population referred for suspected contact allergy; single center.
Skin allergy / Measured in people or real products
Among about 4,100 North American patients referred for allergy patch testing in 2019–2020, 12.8% reacted to the standard fragrance mix and 11.1% to oxidized linalool, both among the ten most common skin allergens and both up from 2017–2018.
Strong evidence: Referral population, not the general public; primary is DeKoven 2023 (NACDG), cited by the review.
Skin allergy / Measured in people or real products
The industry's safety panel cleared linalool, a lavender-scented chemical in most fragranced products, as raising 'no safety concerns' after testing the fresh chemical; independent clinics later found that air-exposed (oxidized) linalool causes allergic reactions in about 7% of patients patch-tested for skin allergy (2,900 patients in 9 centers; 5,773 in Sweden, with rates rising).
Strong evidence: Clinic rates are in dermatitis patients, not the general population; test preparations debated; fresh linalool is genuinely a weak sensitizer.
Source: Bickers 2003 (RIFM Expert Panel), Regulatory Toxicology and Pharmacology Industry-funded Link to this fact
Skin allergy / Industry source
The industry once allowed the allergen citral only if mixed with a companion terpene (originally tested at about 20% d-limonene, later mandated at ≥25%), claiming the terpene 'quenched' the allergic reaction; the original 1976 evidence for this was a single unreplicated human test with no subject numbers reported and was already internally inconsistent, later industry HRIPTs could not reproduce quenching for a related aldehyde (cinnamaldehyde), and air-oxidized limonene, the proposed quencher, is itself now one of the commonest fragrance allergens (about 5% of patients tested).
Strong evidence.
Caveats
The failed replication documented in our evidence base is for cinnamaldehyde + eugenol/limonene, not citral + limonene itself; IFRA's formal withdrawal of the quenching rule is not documented in our evidence base. The founding 1976 test (Opdyke) also showed the effect failing at a different ratio of the same ingredients (cinnamic aldehyde:eugenol 2.5:1 sensitized where 1:1 had not), and its own essential-oil control (cinnamon bark oil) itself sensitized, contradicting the paper's own summary.
Source: Ford 1994 (RIFM) Industry-funded Link to this fact
Skin allergy / Measured in people or real products
For people already allergic to oxidized linalool (the air-exposed form of a common floral fragrance chemical), using a scented cream or perfume containing it at concentrations similar to real products (as low as 0.3% oxidized linalool, 0.056% linalool hydroperoxides) was enough to trigger eczema within three weeks of twice-daily use in a repeated-use trial.
Strong evidence.
Caveats
Small N (6 completers); participants were pre-selected as already patch-test-positive, not a general-population sample; lowest concentration tested (0.1% in perfume) did not produce a formal positive.
Source: Andersch Björkman 2014, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
A positive patch test for oxidized linalool is not just a lab artifact: patients who test positive also develop real dermatitis from everyday-strength fragranced products used the way a consumer would use them, not only from the strong test doses used in the clinic.
Strong evidence: No irritant reactions occurred in vehicle controls, so the effect is allergic rather than irritant; the reverse case (a general population screen) was not tested here.
Source: Andersch Björkman 2014, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
In a controlled study, everyone already allergic to limonene hydroperoxides (11/11) developed a skin reaction within about a week of daily exposure to fine-fragrance-level doses of oxidized limonene (140 ppm and up); healthy people (0/17) had no reaction even at the highest dose tested.
Strong evidence: Single multicenter study; 11 allergic subjects and 17 controls.
Skin allergy / Review of other studies
Oxidized forms of limonene and linalool, formed when these common fragrance terpenes react with air, are now treated as a standard, distinct fragrance-allergen category in mainstream dermatology, alongside fragrance mix I/II and balsam of Peru.
Strong evidence.
Caveats
This 2025 general-ACD review reflects consensus classification, not new prevalence data; primary prevalence numbers should be cited from Sukakul 2022, Bråred Christensson 2016/2012, or the EECDRG review already in our evidence base, not this source.
Source: Aristizabal-Torres 2025, JAAD Reviews 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
Skin allergy / Measured in people or real products
Most patients who reacted to oxidized scent chemicals (58%) also had other fragrance allergies or a history of reacting to fragranced products.
Weak evidence: 31 positive patients.
Skin allergy / Measured in people or real products
People without skin disease reacted to oxidized limonene and linalool as strongly as dermatitis patients.
Weak evidence: Small groups (9 controls per comparison after exclusions).
Source: Bråred Christensson 2009, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Lab study (cells or chemistry)
In lab tests on human skin cells, the oxidized forms of limonene and linalool reshaped the cells' fats in the same way as a classic potent skin allergen (DNFB), a pattern consistent with oxidative stress.
Weak evidence: One cell line, one dose.
Skin allergy / Lab study (cells or chemistry)
In lab tests on reconstructed human skin, oxidized linalool broke down into reactive radicals that bind to skin proteins, which helps explain why linalool becomes an allergen only after it reacts with air.
Weak evidence: Skin model and model thiols; mechanism, not prevalence.
Source: Kuresepi 2020, Chemical Research in Toxicology Link to this fact
Skin allergy / Lab study (cells or chemistry)
In lab tests on human skin cells, the air-oxidized forms of limonene and linalool (the forms that cause fragrance allergy) damaged the fats in cell membranes at concentrations from 100 µM, with oxidized limonene strongest; a possible second route to skin inflammation not yet shown in people.
Weak evidence: Cell line and liposomes only; added iron; real skin doses unknown.
Skin allergy / Measured in people or real products
Once someone is sensitized, patch tests can trigger reactions at low ppm concentrations of oxidized terpene — but the specific 'as low as 1 ppm' figure is for Δ³-carene in older, narratively-reviewed occupational turpentine literature; for the two terpenes that dominate consumer exposure, modern peer-reviewed dose-response studies put the threshold one to two orders of magnitude higher (limonene hydroperoxides: ~24 ppm single patch application, ~140 ppm repeated realistic exposure, ED₁₀ ≈85 ppm; linalool hydroperoxides: ~560 ppm repeated realistic exposure).
Weak evidence.
Caveats
The 1 ppm figure applies to Δ³-carene in historical occupational patch testing, not to limonene or linalool, and its own study design (occlusion, participant numbers) is not given in the citing review. No limonene- or linalool-specific study supports reactions at ~1 ppm.
Source: Karlberg & Lepoittevin 2021, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
All four patients already allergic to oxidized linalool reacted to air-exposed lavender oil on patch testing.
Weak evidence: N=4 pre-selected sensitized patients; fresh oil not tested alongside; elicitation, not sensitization.
Source: Hagvall 2008, Contact Dermatitis Industry-funded Link to this fact
Information, not medical advice. See also: myths we won’t tell you.