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
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 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 / Review of other studies
A 2007 clinical review reports that oxidized limonene caused positive patch-test reactions in about 2.6% of dermatitis patients in a European study, leading to a call to add oxidized limonene to routine allergy screening.
Weak evidence.
Caveats
Secondary relay of Matura et al. 2006 (not independently read in this evidence base); this is an early statement of a position later found by other groups (e.g. the IVDK) to over-call weak/doubtful reactions as true allergy.
Source: Brasch 2007, Current Opinion in Allergy and Clinical Immunology Link to this fact
Skin allergy / Measured in people or real products
Reading a patch test a second time on day 7 revealed extra hydroperoxide allergies: 8% of limonene-hydroperoxide and 4% of linalool-hydroperoxide positives were negative or doubtful on day 3.
Weak evidence: One-clinic count (6 of 77 and 4 of 96); a day-7 reading is a clinic practice, not something consumers control.
Source: Dittmar & Schuttelaar 2019, Contact Dermatitis Link to this fact
Information, not medical advice. See also: myths we won’t tell you.