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
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 / Other evidence
Healthcare workers (medicine, dentistry, nursing, veterinary) had the highest fragrance-mix allergy rate among the occupations the London authors analyzed, about 10-12%, which they link to frequent hand washing followed by moisturizers and fragranced medicaments.
Weak evidence.
Caveats
Small groups (154 men), clinic patients; the hand-barrier explanation is the authors' suggestion, not tested; occupation subset tables for retired persons and students contain apparent transcription slips.
Source: Buckley 2003, British Journal of Dermatology Link to this fact
Skin allergy / Other evidence
Health care workers appear more often sensitized to BAK: one cited study found 7.5% relevant BAK reactions in health care workers versus 2.3% in other patients, and another 23.8% of 223 nurses sensitized.
Weak evidence: Relayed from other studies of small, selected groups; a third cited study found only a slight excess (2.0% vs 1.6% control).
Skin allergy / Review of other studies
Shorter-chain ceramides make the skin's fat layers pack more loosely and leak more: in lab membranes, swapping a 24-carbon fatty acid for a 16-carbon one raised permeability.
Weak evidence: Artificial membranes and porcine skin, not human skin in use; chemical-specific penetration was not tested.
Source: Berdyshev 2024, Allergy, Asthma & Immunology Research Link to this fact
Skin allergy / Other evidence
The CIR draft records that polysorbate 60 and a polysorbate 60-based cream caused hives (urticaria) within 20 minutes on the forehead of test subjects, but its Summary and Discussion say polysorbate 60 at 100% was 'not dermally irritating'.
Weak evidence: The urticaria entry is a one-line summary of an old study with no n or amount; the conflict is within the draft's own text.
Source: CIR 2015 draft Industry-funded Link to this fact
Skin allergy / Other evidence
If a skin-care product or topical treatment seems to make rosacea worse, ask a dermatologist for patch testing that includes your own products and any eye drops or medicines you use.
Weak evidence: Recommendation from a small study; no rosacea flare occurred in any patch-test-positive patient.
Skin allergy / Measured in people or real products
Wet work of 2 hours or more a day (occupational plus domestic) was not significantly associated with hand eczema after adjustment in a car-industry case-control study (irritant hand eczema OR 1.62, 95% CI 0.78-3.37), although it was more common in irritant cases univariately (49.1% vs 31.7%).
Weak evidence: Underpowered for exposure effects by the authors' own account; workplace had strong skin-protection policy; a null here is not evidence wet work is harmless.
Source: Apfelbacher et al. 2010, Contact Dermatitis, Contact Dermatitis Link to this fact
Skin allergy / Other evidence
36.4% of surveyed UAE women reported an adverse reaction to cosmetics in the past 2 years, mainly skin redness (20.6%), pimples (18.4%) and itching (16.9%); skincare (21.7%) and makeup (14.1%) were blamed far more often than perfume (5.2%).
Weak evidence.
Caveats
Open-link snowball sample of 461 women, mostly young and university-educated; self-report, no exposure measurement; not weighted, so not a UAE prevalence. Mixes fragrance-related and non-fragrance reactions (pimples, hair loss, pigmentation); attribution is the respondent's own.
Source: AlBlooshi 2026, Frontiers in Toxicology Link to this fact
Information, not medical advice. See also: myths we won’t tell you.