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 a 2015 survey of 187 US pediatric personal-care products marketed as hypoallergenic, dermatologist-recommended, fragrance-free, or paraben-free, 89% (167/187) contained at least one contact allergen from the standard NACDG screening tray, with a mean of 2.4 allergens per product.
Strong evidence.
Caveats
Single-metro US convenience sample (six unnamed Redlands, CA retailers, 2013), label-based (no chemical verification, no concentrations), and allergen matching by an unaudited custom algorithm; the four marketing-term categories overlap (a product can carry more than one), so subgroup percentages are not independent comparisons.
Source: Hamann 2015, Journal of Allergy and Clinical Immunology Link to this fact
Skin allergy / Measured in people or real products
In a clinical study of 658 hand-eczema patients across 3 European centers, 67 (10.2%) reacted on patch testing to at least one fragrance chemical selected because it is common in household cleaning and personal-care products used on the hands.
Strong evidence: Clinic-referred hand-eczema population (3 tertiary centers), not a random sample of the general public.
Source: Heydorn 2003, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In a survey of 187 US pediatric personal-care products, preservatives (57.8% of products) were a more common source of contact allergens than fragrance (29.4%), even though the products were being screened specifically for hypoallergenic/fragrance-related marketing claims.
Moderate evidence.
Caveats
Category-level result from one survey; the two categories are not mutually exclusive at the product level (a product can carry both preservative and fragrance allergens), and the finding has not been replicated in a different pediatric sample.
Source: Hamann 2015, Journal of Allergy and Clinical Immunology Link to this fact
Skin allergy / Other evidence
Of 226 chemicals identified in 71 deodorants, 84 (37%) carried at least one chemical-structure 'alert' for skin-protein reactivity, and 70 of those were aldehydes, ketones, or alpha,beta-unsaturated aldehydes/ketones/esters – the reactive chemical classes already established as the dominant fragrance-allergen chemotype.
Moderate evidence.
Caveats
A structural alert is a hazard flag based on molecular structure only; the paper explicitly states it gives no information on potency (strong/moderate/weak) and is not a substitute for patch testing or animal studies.
Source: Rastogi et al. 1998 (Contact Dermatitis 39:293), Contact Dermatitis Link to this fact
Skin allergy / Other evidence
Fine fragrances (perfume, cologne) carry more contact-allergy risk per use than the same allergen amount spread over a larger, more dilute area, because sensitization risk depends on allergen dose per unit skin area, not total allergen delivered.
Moderate evidence: General dose-response principle from cited elicitation studies, not this review's own new experiment.
Source: Johansen 2003, Am J Clin Dermatol 4(11):789-798, American Journal of Clinical Dermatology Link to this fact
Skin allergy / Measured in people or real products
Hydroxyisohexyl 3-cyclohexene carboxaldehyde (HICC/Lyral) was labeled on 13.5% of 632 deodorants in this 2015-16 Danish sample, significantly lower than the 33.0% found in an earlier 2007 Danish deodorant survey of 88 products (chi-square p<0.0001).
Moderate evidence.
Caveats
Different sampling methods and sample sizes between the two surveys (app-based self-selection vs. market survey); the comparison is suggestive of declining HICC use ahead of the EU ban, not a controlled before/after study.
Source: Bennike 2017/2018 Link to this fact
Skin allergy / Measured in people or real products
In a post hoc analysis of the same trial, infants with AD/eczema by 32 weeks had significantly higher egg-white IgE sensitization than those without (56%, 24/43, vs 33%, 16/49; P=.043; OR 2.86, 95% CI 1.22-6.73), and any skin lesion (AD/eczema or skin rash) carried an even larger OR of 3.73 (95% CI 1.49-9.36) for sensitization versus no skin lesion, consistent across 20 of 25 different IgE cutoff values tested.
Moderate evidence: Post hoc, not a prespecified comparison; association, not proof that eczematous skin causes sensitization rather than the reverse.
Source: Horimukai et al. 2014, Journal of Allergy and Clinical Immunology Industry-funded Link to this fact
Skin allergy / Other evidence
Hairdressers' hands carried measurable hair-dye chemicals even before they started mixing dye (PPD in 3 of 33, 3-aminophenol in about 20 of 33), pointing to carryover from earlier clients and contaminated trolleys, dryers, and phones.
Moderate evidence: Nine hairdressers had dyed earlier the same day; surface contamination data were not shown; background and task exposure can't be fully separated.
Source: Lind et al. 2005, The Annals of Occupational Hygiene Link to this fact
Skin allergy / Other evidence
Povidone-iodine reactions are often due to the povidone carrier or to irritation rather than iodine; the review says iodine itself has not been shown to be the allergen in reactions to povidone-iodine, contrast media, or amiodarone.
Moderate evidence: Relayed reasoning from reviews; false-positive patch tests are also possible, so ROAT is advised.
Source: Lauriola 2025, Italian Journal of Dermatology and Venereology Link to this fact
Skin allergy / Other evidence
Hairdressers carry a higher hair-dye allergy burden: hand dermatitis risk 1.7-2.5 times the general population, and 15.3% of 300 Spanish hairdressers patch-tested positive to PTDS.
Moderate evidence: Selected, older hairdresser series (1994-2016) with different test panels; rates vary widely between series.
Skin allergy / Review of other studies
In eczema, skin ceramides are shorter and scarcer, partly because the immune signals IL-4 and IL-13 switch off the enzymes (ELOVL3/ELOVL6) that lengthen fatty acids, and they also suppress the barrier protein filaggrin.
Moderate evidence: Mechanism drawn from keratinocyte and skin-equivalent work plus clinical lipidomics; author discloses grants from Sanofi-Genzyme and LEO Pharma.
Source: Berdyshev 2024, Allergy, Asthma & Immunology Research Link to this fact
Skin allergy / Measured in people or real products
In this prospective nurse cohort, carrying a filaggrin (FLG) loss-of-function mutation without a history of atopic dermatitis did not raise hand eczema risk (adjusted OR 0.7, 90% CI 0.3-1.7), whereas FLG carriers who also had atopic dermatitis had the highest odds (OR 3.6, 90% CI 1.7-7.5).
Moderate evidence.
Caveats
Only 28 FLG+/AD+ and 38 FLG+/AD- subjects, so CIs are wide; the interaction test was not significant overall (OR 2.1, 90% CI 0.6-7.1); the authors' own case-control study found an FLG effect (OR 1.6), see the register entry.
Source: Visser et al. 2014, Contact Dermatitis, Contact Dermatitis Link to this fact
Skin allergy / Other evidence
In healthcare worker studies, hand dermatitis rose with the number of handwashes per day (for example US workers washing 10 or more times a day had about 1.95 times the incidence, and Korean nurses washing more than 30 times a day had an odds ratio of 13), while alcohol-based hand rub was not associated with hand eczema in several studies.
Moderate evidence.
Caveats
Individual cohort and cross-sectional studies summarized by the review; Korean figure is from self-reported hand eczema with wide confidence interval; two follow-up studies found non-significant ratios; some healthcare studies found no association with duration of washing.
Source: Jacobsen 2022 Link to this fact
Skin allergy / Other evidence
More than 1 in 5 patch-tested patients (640 of 3,056, 21.1%) had a clinically relevant reaction to something outside the standard 80-allergen series, most often a personal care product or cosmetic (53.8% of those reactions), so a negative standard panel does not rule out a product allergy.
Moderate evidence: Relevance judged by the treating clinician; the specific allergens were not recorded; referral population.
Skin allergy / Measured in people or real products
Skin care products caused the most cosmetic reactions (26%), then non-color hair preparations (16%) and facial makeup (12%); perfumes, colognes and toilet waters were only 7%.
Moderate evidence: Selected referral series of contact dermatitis specialists, 1977-1980 (40 months); not general-population rates.
Source: Eiermann 1982, Journal of the American Academy of Dermatology Link to this fact
Skin allergy / Review of other studies
Benzyl alcohol was named the American Contact Dermatitis Society's 2026 Allergen of the Year; US dermatologists describe it as an underrecognized contact allergen that is a preservative, solvent and fragrance ingredient in personal care products and skin medicines.
Moderate evidence: Narrative review; selection by a dermatology society that promotes the award; no new data.
Source: Zheng et al. 2026, ACDS Allergen of the Year (Cutis), Cutis Link to this fact
Skin allergy / Measured in people or real products
In a German car-industry cohort followed from apprenticeship, people with atopic skin diathesis had about 2.5 times the odds of current hand eczema (adjusted OR 2.47, 95% CI 1.71-3.55; 110 cases vs 120 controls), and it was the only significant factor measured.
Moderate evidence.
Caveats
Single-company cohort with an active skin-protection program; small sample; prevalent cases; controls deliberately enriched with people of unhealthy skin; the narrow CI looks unusually tight versus the crude prevalences (38.5% vs 10.9%).
Source: Apfelbacher et al. 2010, Contact Dermatitis, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
Among 57 cases of current irritant hand eczema in the same car-industry cohort, atopic skin diathesis was again the only significant factor (adjusted OR 1.84, 95% CI 1.20-2.80), somewhat lower than for all hand eczema.
Moderate evidence: Same small single-cohort dataset as the all-hand-eczema analysis, not an independent replication.
Source: Apfelbacher et al. 2010, Contact Dermatitis, Contact Dermatitis Link to this fact
Skin allergy / Other evidence
Benzalkonium chloride, a disinfectant and preservative in hand sanitizers, eye products, cleaners and personal care products, gave allergic-pattern patch reactions in 6.9% of 2,546 patients tested (sixth highest), most of them weak.
Moderate evidence: Single clinic; weak reactions can be irritant, relevance not reported, and benzalkonium chloride is a known irritant.
Source: [[src-2018-trends-patch-testing-mayo-clinic-standard-series|Veverka 2018]], Dermatitis® Link to this fact
Skin allergy / Measured in people or real products
Stasis dermatitis patients also reacted more often to several preservatives and moisturizer ingredients: quaternium-15 (11.0%), diazolidinyl urea (6.0%) and propylene glycol (5.0%), with moisturizers and other personal care products the most common recorded sources.
Moderate evidence.
Caveats
Quaternium-15 odds ratio 1.48 had P = 0.038 against the paper's own cutoff of 0.039; propylene glycol is also an irritant at the 30% test dose; source data cover only positives with a recorded source (n = 19 to 33 per allergen).
Source: Silverberg et al. 2021, Arch Dermatol Res, Archives of Dermatological Research Link to this fact
Skin allergy / Other evidence
About 1 in 5 (22.5%) patients in 13 North American contact-dermatitis clinics had a relevant allergic reaction to something outside the group's 70-allergen standard panel, and about half of those came from cosmetics, beauty and skin-care products (47.4%).
Moderate evidence: Referral patients; the specific non-series allergens were not recorded.
Skin allergy / Other evidence
Parabens and benzophenones are personal-care ingredients (preservatives and UV filters), not fragrance ingredients; this pregnancy cohort did not measure fragrance compounds or product use, so it says nothing directly about fragrance and eczema.
Moderate evidence: Scope statement. Source of exposure (lotions, cosmetics, sunscreen) was not recorded.
Source: Miller 2025 (ECHO), Environment International Link to this fact
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.