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 / Review of other studies
EU scientific advisers (SCCS 2012) list 82 fragrance ingredients as established causes of skin allergy in people.
Strong evidence: Cite SCCS 2012 directly; this review is low reliability.
Source: Kumar 2020 (SCCS 2012), Journal of Cosmetic Dermatology Link to this fact
Skin allergy / Measured in people or real products
In 658 Danish/Swedish patients with hand eczema, 67 (10.2%) reacted to fragrance chemicals commonly found in household products, and the standard fragrance-mix patch test alone would have missed more than half of these reactions.
Strong evidence.
Caveats
Clinic-referred hand-eczema population, not the general public. Confirmed directly against the full primary (Heydorn et al. 2003, Contact Dermatitis 48:317-323), not just its PubMed abstract or Kumar 2020's secondary table.
Source: Kumar 2020 (Heydorn 2003), Journal of Cosmetic Dermatology Link to this fact
Skin allergy / Measured in people or real products
The fragrance chemicals most often found in household products were mostly not in the standard fragrance-allergy patch test used at the time: 6 of its 8 chemicals were in 12% of products or fewer.
Strong evidence.
Caveats
Frequency in products is not the same as clinical sensitization rate; FM II and hydroperoxide testing later added. Clinically confirmed in Heydorn et al. 2003 (n=658 hand-eczema patients, 3 centers): only 3 of the 14 chemicals selected from this paper's household-product survey were FM I constituents, and FM I caught only 45.5% of the fragrance-allergic patients (5.6% of all patients tested missed). The exposure-frequency leader, limonene (78% of products), caused only 0.8-0.9% oxidized-limonene reactivity clinically — product frequency of a chemical still does not predict its clinical allergy rate.
Source: Rastogi 2001, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Common scent chemicals such as linalool hardly cause allergy when fresh, but once exposed to air they form new chemicals that do: in 1,511 dermatitis patients at six European clinics, 1.3% were allergic to air-oxidized linalool (later studies with a stronger test found about 7%).
Strong evidence.
Caveats
Dermatitis patients, not general population; 2005 test concentration (2.0% pet.) was later shown too low. Clinical relevance no longer just inferred: Andersch Björkman 2014 (ROAT) shows patients identified this way go on to develop dermatitis from twice-daily use of a cream/perfume containing oxidized linalool at concentrations as low as 0.3% (0.056% linalool hydroperoxides, ~27 µg/cm²) — inside the 0.1–2% range measured for individual fragrance ingredients in real products.
Skin allergy / Animal study
Common fragrance chemicals such as limonene (citrus) and linalool (lavender) react with air to form far stronger skin allergens; in mouse tests, oxidized limonene was about 10 times more potent than fresh limonene.
Strong evidence: Mouse LLNA potency; review of replicated data.
Skin allergy / Measured in people or real products
In European skin clinics, 2.8-5.2% of patients tested for dermatitis reacted to oxidized limonene.
Strong evidence: Consecutive dermatitis patients, not general population.
Skin allergy / Review of other studies
About 1 in 22 European adults (4.5%) tested positive to at least one of six fragrance-related patch-test markers (2008-2011); single fragrance markers pick up 1-3%, and up to 16% of patients referred for patch testing react.
Strong evidence: The 4.5% is a six-marker union including non-fragrance markers (colophonium, sesquiterpene lactones), with no clinical-relevance criterion.
Source: Sukakul 2024, Acta Dermato-Venereologica Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In patch tests of 2,900 dermatitis patients in six countries, 5.2% were allergic to oxidized limonene and 6.9% to oxidized linalool, two of the most common fragrance chemicals.
Strong evidence: Clinic population, not general public.
Source: Bråred Christensson 2016, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Fresh limonene and linalool rarely cause allergy (0.1-0.3% of patients); the allergens form when these chemicals react with air, which also happens in essential oils and perfumes during storage.
Strong evidence: Parent-compound rates cited from IVDK/Danish data.
Source: Bråred Christensson 2016, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In 2,900 dermatitis patients tested at nine clinics in Europe, Asia and Australia, 6.9% were allergic to air-oxidized linalool, the form a common floral scent chemical found in most perfumes, cosmetics and many cleaners takes after contact with air.
Strong evidence: Dermatitis patients, not general population; 72% weak reactions; range 3-14% by clinic.
Source: Christensson 2012, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
60.5% of patients allergic to oxidized linalool would never have learned they had a fragrance allergy from the standard fragrance patch tests.
Strong evidence: Dermatitis patients.
Source: Christensson 2012, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
80% of patients allergic to oxidized linalool whose products were checked were using fragranced products, mainly shampoos, soaps and creams.
Strong evidence.
Caveats
Linalool confirmed in their products in 38%; clinical relevance established in 41%. Andersch Björkman 2014 (ROAT), drawn from this same 2,900-patient cohort's Gothenburg patients, causally confirms the exposure-effect link: repeated open application of oxidized-linalool-containing cream/perfume at real-product concentrations (down to 0.3%) reproduced dermatitis, and participants who had already avoided linalool-containing products mostly reported improvement.
Source: Christensson 2012, Contact Dermatitis Link to this fact
Skin allergy / Review of other studies
Limonene and linalool become skin allergens when they react with air: in European dermatitis clinics, 3.9-20% of patients tested reacted to oxidized linalool and 1.4-13.7% to oxidized limonene, against under 1% for the fresh chemicals.
Strong evidence: Clinic patients referred for patch testing, not general population.
Skin allergy / Review of other studies
Standard allergy tests with fresh limonene and linalool miss most of these allergies, so European experts now recommend adding the oxidized forms to routine patch testing.
Strong evidence.
Skin allergy / Measured in people or real products
Among 5,773 people tested for eczema at a Swedish hospital, 9.4% were allergic to the air-oxidized forms of two of the most widely used scent chemicals: linalool (7.0%) and limonene (5.1%).
Strong evidence: Clinic population referred for suspected contact allergy; single center.
Skin allergy / Review of other studies
Scent chemicals such as limonene and linalool rarely cause skin allergy when fresh, but once exposed to air they turn into much stronger allergens; the older the product, the more of them it contains.
Strong evidence: Review of experimental and clinical work; no new data.
Source: Karlberg & Lepoittevin 2021, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
Even the fragrance industry's own safety institute classes the cinnamon-scent chemicals cinnamal and cinnamyl alcohol as human skin sensitizers: in its volunteer tests, 29 of 451 people became allergic to cinnamal at 1% or more.
Strong evidence: Industry-run HRIPT/maximization panels, mostly unpublished RIFM reports; 0/≈200 sensitized at ≤0.5%.
Source: RIFM Expert Panel 2005, Food and Chemical Toxicology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In people already allergic to cinnamal, a deodorant with just 0.01% of it triggered a reaction in 1 of 9, and 0.032% in 4 of 9; the industry limit is 0.05%, and it was set to prevent new allergies, not reactions in people already sensitized.
Strong evidence.
Caveats
Confirmed by the primary source, now in our evidence base: Bruze et al. 2003, an independent (non-industry, no COI declared), double-blind provocation study with a 20-patient non-allergic control group (0/20 reacted) and P<.001 statistics tying the reaction to both cinnamal-sensitization and the actively-scented axilla. The study's own authors recommend a deodorant limit below 0.01%, stricter than the 0.05% IFRA cap.
Source: RIFM Expert Panel 2005 (citing Bruze 2003), Food and Chemical Toxicology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
The fragrance industry's own safety review of the rose ketone cis-beta-damascone records that 10 of 50 human volunteers became sensitized when it was patch tested at 5%, and that the test had to be stopped early because of the number of skin reactions.
Strong evidence: 5% is roughly 100x a fine-fragrance use level (max reported 0.02%); at 0.05% two panels gave 0/53 and 0/28.
Source: Lalko 2007 (RIFM), Food and Chemical Toxicology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Rose ketones are a real but uncommon allergen: 0.5% of 1606 consecutive European patch-test patients reacted to a 0.2% damascone mixture, and none of 202 Japanese dermatitis patients reacted at 2%.
Strong evidence: Clinic populations, elicitation prevalence not induction risk; original data are Frosch 2002 and Kozuka 1996, reported via the RIFM review.
Source: Lalko 2007 (RIFM), Food and Chemical Toxicology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
About 1 in 5 people in the general population (20.1% of 20,107 people patch tested in 28 studies) are allergic to at least one common chemical that touches the skin, such as nickel, fragrance, preservatives or hair dye.
Strong evidence: Meta-analysis, very high heterogeneity (I2 97%); mostly nickel; publication bias detected.
Skin allergy / Measured in people or real products
After nickel, the standard fragrance allergy test (Fragrance Mix I) was the most common positive in the general population: about 3.5% reacted, across 19 studies and 19,440 people.
Strong evidence: Pooled 1966-2017 (CI 2.1-5.4%); the largest modern study found 1.8-2.6% for the same test.
Skin allergy / Review of other studies
The standard fragrance patch test covers only 8 of the 82 fragrance chemicals known to cause allergy in people, so any single test undercounts fragrance allergy.
Strong evidence: Authors' discussion; oxidized limonene/linalool never tested in a general-population sample.
Skin allergy / Measured in people or real products
Among about 4,100 North American patients referred for allergy patch testing in 2019–2020, 12.8% reacted to the standard fragrance mix and 11.1% to oxidized linalool, both among the ten most common skin allergens and both up from 2017–2018.
Strong evidence: Referral population, not the general public; primary is DeKoven 2023 (NACDG), cited by the review.
Skin allergy / Review of other studies
Common fragrance ingredients such as limonene and geraniol turn into stronger skin allergens when they react with air in an opened bottle.
Strong evidence: Review relay only; the numbers come from the primary autoxidation studies on the transformation-products hub.
Source: Khalid & Abdollahi 2021, Iranian Journal of Pharmaceutical Research Link to this fact
Skin allergy / Measured in people or real products
The industry's safety panel cleared linalool, a lavender-scented chemical in most fragranced products, as raising 'no safety concerns' after testing the fresh chemical; independent clinics later found that air-exposed (oxidized) linalool causes allergic reactions in about 7% of patients patch-tested for skin allergy (2,900 patients in 9 centers; 5,773 in Sweden, with rates rising).
Strong evidence: Clinic rates are in dermatitis patients, not the general population; test preparations debated; fresh linalool is genuinely a weak sensitizer.
Source: Bickers 2003 (RIFM Expert Panel), Regulatory Toxicology and Pharmacology Industry-funded Link to this fact
Skin allergy / Industry source
The industry once allowed the allergen citral only if mixed with a companion terpene (originally tested at about 20% d-limonene, later mandated at ≥25%), claiming the terpene 'quenched' the allergic reaction; the original 1976 evidence for this was a single unreplicated human test with no subject numbers reported and was already internally inconsistent, later industry HRIPTs could not reproduce quenching for a related aldehyde (cinnamaldehyde), and air-oxidized limonene, the proposed quencher, is itself now one of the commonest fragrance allergens (about 5% of patients tested).
Strong evidence.
Caveats
The failed replication documented in our evidence base is for cinnamaldehyde + eugenol/limonene, not citral + limonene itself; IFRA's formal withdrawal of the quenching rule is not documented in our evidence base. The founding 1976 test (Opdyke) also showed the effect failing at a different ratio of the same ingredients (cinnamic aldehyde:eugenol 2.5:1 sensitized where 1:1 had not), and its own essential-oil control (cinnamon bark oil) itself sensitized, contradicting the paper's own summary.
Source: Ford 1994 (RIFM) Industry-funded Link to this fact
Skin allergy / Animal study
Many fragrance chemicals become stronger skin allergens after the bottle is opened: limonene, linalool and geraniol react with air at room temperature to form hydroperoxides that, in mouse tests, sensitize at 5-10 times lower concentrations than the fresh chemical.
Strong evidence: Potency from mouse LLNA; cite Karlberg 2013 as primary; clinic data (Bråred Christensson, Sukakul) confirm oxidized forms are common human allergens.
Source: Pastor-Nieto 2021 (Karlberg 2013), Current Treatment Options in Allergy Link to this fact
Skin allergy / Lab study (cells or chemistry)
Chemicals that only become allergens after they are changed by air or by the skin's own enzymes are the worst-predicted group in the industry's own analysis (4.5- to 6.5-fold error, versus about 2-fold for directly reactive chemicals), because the dish assays cannot oxidize or metabolize anything.
Strong evidence: N=32 chemicals in the worst domain; TIMES-software-assigned mechanistic class, not exhaustively confirmed in vivo.
Source: Natsch 2015, Toxicological Sciences Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In a double-blind study, deodorants containing as little as 0.01% cinnamic aldehyde (cinnamal) triggered allergic skin reactions within weeks in people already allergic to it, while an identical unscented deodorant on the other armpit, and the same deodorants used by 20 non-allergic people, caused no reactions.
Strong evidence.
Caveats
Small cohort (9 sensitized test patients, double-blind arm), but statistically significant (P<.001) with a concurrent negative control group and randomized/blinded contralateral-axilla design.
Source: Bruze et al. 2003, Journal of the American Academy of Dermatology Link to this fact
Skin allergy / Measured in people or real products
For people already allergic to oxidized linalool (the air-exposed form of a common floral fragrance chemical), using a scented cream or perfume containing it at concentrations similar to real products (as low as 0.3% oxidized linalool, 0.056% linalool hydroperoxides) was enough to trigger eczema within three weeks of twice-daily use in a repeated-use trial.
Strong evidence.
Caveats
Small N (6 completers); participants were pre-selected as already patch-test-positive, not a general-population sample; lowest concentration tested (0.1% in perfume) did not produce a formal positive.
Source: Andersch Björkman 2014, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
A positive patch test for oxidized linalool is not just a lab artifact: patients who test positive also develop real dermatitis from everyday-strength fragranced products used the way a consumer would use them, not only from the strong test doses used in the clinic.
Strong evidence: No irritant reactions occurred in vehicle controls, so the effect is allergic rather than irritant; the reverse case (a general population screen) was not tested here.
Source: Andersch Björkman 2014, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
In a controlled study, everyone already allergic to limonene hydroperoxides (11/11) developed a skin reaction within about a week of daily exposure to fine-fragrance-level doses of oxidized limonene (140 ppm and up); healthy people (0/17) had no reaction even at the highest dose tested.
Strong evidence: Single multicenter study; 11 allergic subjects and 17 controls.
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 patch test of 3,119 people randomly sampled from the general population of five European countries, 2.6% tested positive to fragrance mix I and 1.9% to fragrance mix II, and 1.9% had a fragrance allergy judged clinically relevant (skin symptoms plus product avoidance).
Strong evidence.
Caveats
The largest, best-controlled general-population fragrance patch-test study to date (n=3,119 of 12,377 interviewed, harmonized protocol across 5 countries); the raw marker-positive rate is higher (4.1%) before the clinical-relevance filter, and about half of marker-positive people do not meet the stricter definition.
Source: Diepgen 2015 (EDEN), British Journal of Dermatology Industry-funded 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
More than half (54.5%) of the hand-eczema patients allergic to household-product fragrance chemicals would not have been identified by the standard Fragrance Mix I patch test alone.
Strong evidence: Applies to this hand-eczema clinic population and this specific 14-chemical panel; FM II (2008) and hydroperoxide testing have since narrowed but not closed the gap.
Source: Heydorn 2003, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Review of other studies
Oxidized forms of limonene and linalool, formed when these common fragrance terpenes react with air, are now treated as a standard, distinct fragrance-allergen category in mainstream dermatology, alongside fragrance mix I/II and balsam of Peru.
Strong evidence.
Caveats
This 2025 general-ACD review reflects consensus classification, not new prevalence data; primary prevalence numbers should be cited from Sukakul 2022, Bråred Christensson 2016/2012, or the EECDRG review already in our evidence base, not this source.
Source: Aristizabal-Torres 2025, JAAD Reviews Link to this fact
Skin allergy / Lab study (cells or chemistry)
In the fragrance industry's own reference set of 62 fragrance ingredients, 19 are rated moderate or strong skin sensitizers, meaning chemicals capable of causing allergy.
Moderate evidence: Industry (RIFM) weight-of-evidence categories; lab and animal data, not human exposure.
Source: Lee 2024, Food and Chemical Toxicology Industry-funded Link to this fact
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
Allergy to these two scent chemicals rose significantly over the eight years studied (2013-2020).
Moderate evidence: Patch-test chamber system changed in 2018, which may affect the trend.
Skin allergy / Measured in people or real products
Women were affected about 1.5 to 2 times as often as men, and these patients were younger on average than people allergic to other fragrance ingredients.
Moderate evidence: Likely reflects product-use patterns; clinic population.
Skin allergy / Review of other studies
Benzyl alcohol, a fragrance allergen, is also a preservative in common prescription creams and injections, including steroid, antifungal (clotrimazole) and antibiotic (mupirocin) creams, so fragrance-allergic patients can meet it in their medicines.
Moderate evidence: Based on case reports; no prevalence of benzyl alcohol allergy given.
Skin allergy / Lab study (cells or chemistry)
Skin converts part of cinnamyl alcohol into cinnamal, the stronger allergen: on human skin samples, about 4% of applied cinnamyl alcohol became cinnamal within a day.
Moderate evidence: Excised human skin, neat material, occluded, 24 h (Smith 2000); single study.
Source: RIFM Expert Panel 2005, Food and Chemical Toxicology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Allergy to one rose ketone tends to mean allergy to its relatives: all 3 volunteers sensitized to delta-damascone reacted to 0.1% cis-beta-damascone on cross-challenge.
Moderate evidence: N=3; from an unpublished RIFM report.
Source: Lalko 2007 (RIFM), Food and Chemical Toxicology Industry-funded Link to this fact
Skin allergy / Review of other studies
Benzyl salicylate, one of the most widely used fragrance chemicals (about 2,500 tonnes a year worldwide in 2007), caused skin allergy in the fragrance industry's own tests, and the industry's standards body IFRA restricts how much may be used.
Moderate evidence: Human maximization 3/50 positive at 20%; repeat-insult patch tests were negative (0/101 at 15%); LLNA EC3 1.5-2.9% in mice. Industry-funded review of largely unpublished reports.
Source: RIFM Expert Panel / Belsito 2007, Food and Chemical Toxicology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Standard allergy screening misses a large share of fragrance allergy: in one UK clinic 42% of patients allergic to at least one of the 26 EU-labeled fragrance chemicals tested negative on the standard fragrance markers, and about 4 in 10 people who reacted to their own perfume or deodorant were negative on the screen.
Moderate evidence.
Caveats
Comparator-dependent: Sukakul 2022 found ~10% missed when comparing fragrance mixes with their own ingredients (abstract-sourced). The Mann 2014 primary, now read in full (not just its abstract), confirms 203 patients reacted to any of the 26 EU fragrances and only 117 (57.6%) also reacted to a baseline marker = 42.4% missed; Mann's own Discussion states this as "40.6%" in one sentence, a small internal inconsistency in the primary (Results math gives 42.4%). More importantly, Mann's own paper reports a similarly-designed Danish study (Heisterberg 2011, n=1,508, also routine testing of all patients with the same 26 fragrances) that found only 15.6% missed — a ~2.5-fold discrepancy the authors attribute only partly to higher test concentrations and a different patient population, not fully resolved. The own-product figure (41%/45%) is from a different study (Uter 2007), not Mann.
Skin allergy / Measured in people or real products
Some fragrance chemicals now banned from EU cosmetics sensitized almost every volunteer in the industry's own tests: diethyl maleate 25 of 25, dihydrocoumarin 25 of 25, alantroot oil 23 of 25.
Moderate evidence: Maximization test is deliberately exaggerated (irritant pre-treatment, occlusion, ~10x use level); substances are banned and mostly left perfumery before 2000.
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
Half of best-selling US body moisturizers (87 of 174) list "fragrance" on the label, making it the most common potential skin allergen in them, ahead of parabens (75) and vitamin E/tocopherol (74).
Moderate evidence: Literal-word count, a lower bound for fragranced products; 2016 US online best-sellers.
Skin allergy / Animal study
Left open to air, lavender oil became about 8 times more potent as a skin sensitizer in mouse tests (EC3 36% fresh, 4.4% after 45 weeks).
Moderate evidence: Mouse LLNA, groups of 3-4 mice, one 45-week sample; aged oil reached 'moderate', not 'strong', potency; open-flask lab exposure.
Source: Hagvall 2008, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
After 10 weeks of air exposure, lavender oil contained about 3.3% linalyl acetate hydroperoxides and 0.5% linalool hydroperoxides, the kind of oxidation products behind the commonest fragrance allergies in European clinics.
Moderate evidence: Open-flask lab exposure (upper bound); retail bottles not measured.
Source: Hagvall 2008, Contact Dermatitis Industry-funded 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
Sensitivity to cinnamal varies enormously between allergic people: this study measured a 1000-fold range in the concentration needed to trigger a reaction.
Moderate evidence: Range measured in 9 patients only.
Source: Bruze et al. 2003, Journal of the American Academy of Dermatology 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
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 / Measured in people or real products
A standard fragrance patch-test panel (Fragrance Mix I/II) misses about 1 in 5 cases of medication-caused fragrance allergy, because the reaction is to an individual essential oil or ingredient not covered by the mixes.
Moderate evidence.
Caveats
24/127 (19%) in one clinic's iatrogenic-ACD case series (Belgium, 1978-2008); denominator is a clinic-referred population, not a general-population estimate; different populations give different "missed" rates (Mann 2014 ~42% vs 26 EU allergens; Sukakul-derived ~10% vs FM ingredients).
Skin allergy / Measured in people or real products
For two common fragrance chemicals, trans-2-hexenal and methyl-2-nonanoate, the mouse test underestimated true human skin-sensitizing potency by 26- to 42-fold — the error direction that could make an industry safety limit too permissive rather than too strict.
Moderate evidence.
Caveats
Only 2 of 57 chemicals in the dataset show this specific magnitude of underestimation; industry-funded (RIFM/DABMEB); the human NOEL itself is a floor, not a measured breakpoint, since dosing was not escalated to a positive result for ethical reasons.
Source: Api, Basketter & Lalko 2014, Cutaneous and Ocular Toxicology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Women's fragrance contact allergy rates are roughly double men's.
Moderate evidence.
Caveats
Ratio varies by marker from about 1.5x (fragrance mix I) to 2.1x (HICC), not a uniform 2x; a pooled meta-analysis (Alinaghi 2018) finds fragrance allergy closer to sex-neutral overall.
Source: Diepgen 2015 (EDEN), British Journal of Dermatology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
A Givaudan-funded study that chemically measured 39 real aged fragrance products found linalool hydroperoxide levels far below the dose shown to trigger a reaction in already-allergic patients, roughly 180 times lower on average.
Moderate evidence: Industry-funded (Givaudan), single study, not independently replicated; only two product categories tested (hydroalcoholic fine fragrance, antiperspirant/deodorant).
Source: Kern 2014, Analytical and Bioanalytical Chemistry Industry-funded Link to this fact
Skin allergy / Other evidence
The fragrance industry's original 1976 evidence for aldehyde 'quenching' (that a companion terpene or alcohol stops citral, cinnamic aldehyde or phenylacetaldehyde from causing skin sensitization) was a single, unreplicated human maximization test per mixture with no subject numbers reported, and the paper's own data are internally inconsistent: cinnamon bark oil sensitized despite being one of the paper's three 'protective natural oil' examples, and cinnamic aldehyde + eugenol sensitized at a 2.5:1 ratio where 1:1 had not.
Moderate evidence.
Caveats
No subject number (n) is stated anywhere in the paper for any test; no statistics, blinding or control arm distinct from the aldehyde-alone condition; the author calls the findings preliminary and says further study was 'now in the second year.'.
Source: Opdyke 1976 (RIFM), Food and Cosmetic Toxicology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In a UK study that routinely patch tested 1,951 eczema patients with both the standard fragrance-allergy screen and all 26 individually EU-labeled fragrance chemicals, 42% of the patients allergic to at least one of those 26 chemicals would have been missed if only the standard screen had been used.
Moderate evidence.
Caveats
Single UK tertiary-referral clinic (n=1,951), not a general-population figure. A comparably-designed Danish study using nearly the same routine-testing protocol found only 15.6% missed, so this exact percentage does not generalize across clinics without qualification.
Skin allergy / Measured in people or real products
The two fragrance mixes used in standard allergy testing (FM I and FM II) each miss roughly half of the people who are allergic to their own listed ingredients: only 58% of those reacting to an FM I ingredient, and only 52% of those reacting to an FM II ingredient, reacted to the mix itself.
Moderate evidence: Single UK clinic (n=1,951), not independently replicated at this exact figure.
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 / Measured in people or real products
Concentrations of hydroxycitronellal found in some 'natural' perfumes (up to 6.04%) exceeded the concentration shown to sensitize 36% of healthy volunteers in repeated-insult patch testing (5%), and were also well above the fragrance industry's own 1996 recommended maximum (equivalent to about 1% in the finished product).
Moderate evidence.
Caveats
Single-study 1996 patch-test-sensitization citations (Ford & Api 1988; Suskind 1992) for the 5%/36% figure, not independently verified in this evidence base; product concentration is not the same as measured consumer exposure dose.
Skin allergy / Review of other studies
Patch testing is the diagnostic gold standard for allergic contact dermatitis, including fragrance allergy, but access to it is uneven: in the US, African American and lower-income patients are referred for and receive patch testing less often than White and higher-income patients, and rural patients have reduced access and longer symptom duration.
Moderate evidence.
Caveats
The disparity claims are cited from secondary/administrative-claims sources within a narrative review, with no effect size given by this paper itself; a distinct, upstream complement to the post-diagnosis label-comprehension gap in Noiesen 2007 (see contradictions C284 extension).
Source: Aristizabal-Torres 2025, JAAD Reviews Link to this fact
Skin allergy / Measured in people or real products
Among all detergent types studied, laundry (textile) detergents had both the highest share of products naming a fragrance allergen (50.0% of 270 products) and the highest average number of named allergens per product (1.7) — ahead of dish soap, glass cleaner, toilet cleaner, and dishwasher detergent.
Moderate evidence: Named-on-label only, not measured concentration or skin dose; German 2015 sample.
Source: Wieck 2018, Regulatory Toxicology and Pharmacology Link to this fact
Skin allergy / Review of other studies
Benzalkonium chloride was the sixth highest-reacting allergen in a large retrospective patch-testing cohort, and caused contact dermatitis in 7-18.6% of patients using BAC-containing topical medicine for chronic leg ulcers.
Moderate evidence: Figures are from BAC used as a medical/topical antiseptic preservative, not from laundry or fabric-softener exposure.
Source: Peyneau 2022, Frontiers in Toxicology Link to this fact
Skin allergy / Measured in people or real products
Most patients who reacted to oxidized scent chemicals (58%) also had other fragrance allergies or a history of reacting to fragranced products.
Weak evidence: 31 positive patients.
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 reconstructed human skin, oxidized linalool broke down into reactive radicals that bind to skin proteins, which helps explain why linalool becomes an allergen only after it reacts with air.
Weak evidence: Skin model and model thiols; mechanism, not prevalence.
Source: Kuresepi 2020, Chemical Research in Toxicology Link to this fact
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 / Measured in people or real products
All four patients already allergic to oxidized linalool reacted to air-exposed lavender oil on patch testing.
Weak evidence: N=4 pre-selected sensitized patients; fresh oil not tested alongside; elicitation, not sensitization.
Source: Hagvall 2008, Contact Dermatitis Industry-funded Link to this fact
Information, not medical advice. See also: myths we won’t tell you.