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
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 / 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
Patients allergic only to oxidized linalool also tended to react to a wide range of other cosmetic ingredients (preservatives such as parabens and MCI/MI, colophony, and most fragrance-mix ingredients), a pattern the researchers link to using multiple scented or cosmetic products rather than the chemicals being related to each other.
Moderate evidence: Patch-test associations, not confirmed shared product exposures; the authors' own interpretation, not chemically verified.
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
Information, not medical advice. See also: myths we won’t tell you.