Don’t take our word for it. Here’s every fact on this site, with its evidence grade, its caveats, and a link to the source, so you can check us.
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
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 / Measured in people or real products
Essential oils are made largely of the same chemicals labeled as fragrance allergens: supplier data sheets give sweet-orange oil about 95% limonene, lavender oil about 40% linalool, and palmarosa oil up to 84% geraniol.
Moderate evidence: Composition from supplier safety data sheets, which vary between suppliers (cypress oil limonene 5-7% vs 14%); not measured in finished products.
Source: Klaschka 2016, Environmental Sciences Europe Link to this fact
Skin allergy / Measured in people or real products
The two commonest fragrance allergens on Danish children's cosmetics, limonene (22% of fragranced products) and linalool (18%), are not in the standard European patch test series; they become strong allergens when they oxidize in air.
Moderate evidence: Frequencies are label presence; oxidation not measured in these products (relayed from hydroperoxide literature); some clinics now add the hydroperoxides to their baseline.
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
Only 15% of people with a borderline ('doubtful') patch-test reaction to limonene hydroperoxides went on to react when exposed repeatedly to a realistic dose, a rate no different from people with no allergy at all.
Moderate evidence: Small subgroup (13 people, 2 positive); underpowered to rule out that some fraction of doubtful reactors are truly allergic.
Skin allergy / Measured in people or real products
A large fragrance-industry study tested 104 real consumer products, including creams, lotions, and antiperspirants, for the allergenic chemicals that form when the scent chemicals limonene and linalool oxidize in air, and found almost none of them: 96% of products had no detectable hydroperoxide, and the one confirmed positive was hundreds of times below the dose known to trigger a reaction.
Moderate evidence: Industry-funded (IDEA consortium: Givaudan, Firmenich, IFF, IFRA), not independently replicated by a non-industry group.
Source: Natsch 2019, Food and Chemical Toxicology Industry-funded Link to this fact
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 / Lab study (cells or chemistry)
For several fragrance chemicals with no allergy-triggering chemical structure in their original (parent) form — including linalool and D-limonene, two of the most widely used fragrance ingredients in the world — the fragrance industry's own computer models predict that their breakdown products (metabolites) can trigger skin allergy, even though the parent chemicals were classified as non-sensitizers overall.
Moderate evidence.
Caveats
This paper's human no-effect-level data for linalool and limonene do not specify the oxidation state of the test material used; independent clinical studies already in this evidence base's contradiction register find that once these two chemicals are exposed to air, their oxidation products are among the commonest fragrance contact allergens in European patch-test clinics.
Source: Na et al. 2022, Dermatitis Industry-funded Link to this fact
Skin allergy / Review of other studies
Limonene and linalool are often weak allergens when fresh, but after a container is opened they oxidize in air into far more potent allergenic hydroperoxides, which are now among the most frequent causes of new fragrance contact allergy (reaction rates of about 2.5%-9.4% for oxidized limonene and 3.9%-11.7% for oxidized linalool across several patch-test studies).
Moderate evidence.
Caveats
Pooled ranges across several studies of varying design in this secondary review; the underlying phenomenon is independently replicated elsewhere in our evidence base at a higher evidence grade.
Source: de Groot 2020, Dermatitis 31(1):13-35, Dermatitis® Link to this fact
Skin allergy / Measured in people or real products
Among 4,192 dermatitis patients tested in Sweden (2015-2020), 10.2% reacted to oxidized linalool and/or oxidized limonene; people who reacted only to oxidized linalool were about 4 times as likely to also have three or more other contact allergies ("multiple contact allergies") as other tested patients.
Moderate evidence: Clinic population referred for suspected contact allergy; a subgroup of the same cohort as Sukakul 2022, not an independent replication.
Source: Sukakul 2024, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
People who reacted only to oxidized limonene mostly had additional allergies to other fragrances specifically, not the broader cosmetic-ingredient pattern seen with linalool, and were not significantly more likely to have multiple contact allergies overall.
Moderate evidence: Odds ratio for multiple contact allergies was 1.51 (95% CI 0.97-2.36), not statistically significant; single-center clinic sample.
Source: Sukakul 2024, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Four EU-listed fragrance allergens (limonene, eugenol, lilial, benzyl salicylate) were missing from the label of at least one of the tested products, although EU rules require naming them above 0.001% in leave-on products.
Moderate evidence: Products were bought in the US and Canada, where this EU rule did not apply, so the omissions were lawful there; counts are 3, 2, 2 and 1 products.
Source: Sarantis et al. 2010 (Not So Sexy, Canadian edition) Link to this fact
Skin allergy / Measured in people or real products
In one tertiary referral clinic's 821 consecutive dermatitis patients, 9.4% reacted to limonene hydroperoxides and 11.7% to linalool hydroperoxides, the allergens that form when these scent chemicals react with air; most reactions were weak.
Moderate evidence.
Caveats
Single tertiary referral center, so higher than the 2.5-5.4% and 3.9-7.7% the authors cite from other studies; the patients were already suspected of contact allergy, not the general public; a further 17% and 22% of readings were doubtful. Test preparations were bought from Chemotechnique and the authors declare no conflicts.
Source: Dittmar & Schuttelaar 2019, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
About 40% of the hydroperoxide patch-test reactions (limonene 40%, linalool 47%) were judged probably or certainly relevant to the patient's dermatitis, and about 20% certainly.
Moderate evidence: Relevance was judged from history and from patients checking labels for limonene or linalool; the hydroperoxides were never measured in the products and no use test was done.
Source: Dittmar & Schuttelaar 2019, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
The most common single fragrance allergens among 6,004 consecutive dermatitis patients were oxidized linalool (3.9%), tree moss (3.0%), oxidized limonene (2.5%) and the synthetic HICC (2.1%); three of the four are not part of either standard fragrance mix.
Moderate evidence: One clinic; oxidized terpenes were tested only from 2012 (4,194 patients); other series report higher oxidized-terpene rates, so the ranking is clinic-specific.
Skin allergy / Measured in people or real products
The two standard fragrance mixes pick up only about 30% of patients allergic to oxidized linalool or oxidized limonene and about half of those allergic to tree moss, so a mixes-only screen misses most of them.
Moderate evidence: Single clinic; mixes compared with a clinic-chosen set of allergens; doubtful readings counted as not positive.
Skin allergy / Measured in people or real products
Oxidized linalool and oxidized limonene stayed well above the 0.5-1% positive rate that justifies routine testing in this clinic, even after falling from their first-year peak.
Moderate evidence: Single center; 20.9% and 13.7% of readings were doubtful, and some may be irritant rather than allergic.
Skin allergy / Other evidence
The main US dermatology society's extended allergy-test panel now uses air-oxidized limonene (0.2%) and linalool (0.5%) rather than the fresh chemicals, because testing the oxidized form finds more relevant reactions.
Moderate evidence: Committee consensus relaying two de Groot 2019 reviews, not new data; the US doses are lower than the European EECDRG doses (1% and 0.3%) because of irritant-reaction risk.
Skin allergy / Other evidence
About 1 in 10 people referred to North American skin-allergy clinics and patch tested in 2021-2022 (308 of 3,040, 10.1%) reacted to hydroperoxides of linalool, the air-oxidized form of a very common scent chemical; 5.7% (173 of 3,047) reacted to oxidized limonene.
Moderate evidence.
Caveats
Referral clinic patients, not the general public; most reactions were weak (+) or doubtful, and the authors say it is unclear whether these reflect true allergy or irritation (irritant reactions 2.0% and 0.9%). Single network, one two-year cycle.
Skin allergy / Other evidence
Fragrance mix I (7.9%), balsam of Peru (7.0%), oxidized linalool (10.1%) and oxidized limonene (5.7%) are all among the 11 most common patch-test allergens in a 3,056-patient North American clinic series, and fragrance markers hold 5 of the 15 highest significance-prevalence scores.
Moderate evidence: Referral population; SPIN weights relevance judged by each clinician, mostly 'possible' relevance, without confirmatory use tests.
Skin allergy / Other evidence
Positive reactions to the two older fragrance-screening mixes fell to record lows in North America between 2019-2020 and 2021-2022 (fragrance mix I 12.8% to 7.9%, fragrance mix II 5.2% to 3.0%), while oxidized limonene reactions rose from 3.5% to 5.7% and balsam of Peru stayed flat (7.4% to 7.0%).
Moderate evidence.
Caveats
Same referral network but membership changed (6 centers left, 5 joined, about 25% fewer patients); 2019-2020 was an unusually high FM I cycle (12.8% vs 9.2% in 2017-2018); no multiple-comparison correction. The authors' explanations (atranol/chloroatranol limits, lyral ban) are untested.
Information, not medical advice. See also: myths we won’t tell you.