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
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
In an industry (Procter & Gamble) study, adding the preservative glutaraldehyde to an experimental liquid fabric softener at 550 ppm (and, as a worst case, 5500 ppm) did not trigger allergic contact dermatitis in 16 subjects already known to be allergic to glutaraldehyde, whether exposed via undiluted product on skin, twice-daily open use for 8 days, or 14 days of wearing laundered T-shirts.
Moderate evidence.
Caveats
Funded by, and authored in part by, Procter & Gamble, the product's manufacturer; no independent (non-industry) replication of this specific formulation exists; results are specific to glutaraldehyde at these concentrations, not to laundry allergens generally.
Source: Weaver & Maibach 1977, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In the same study, 706 previously non-allergic subjects underwent repeated-insult patch testing with the glutaraldehyde-containing formulation (220 ppm applied dose for 340 subjects, 22 ppm for 366 subjects) and none developed new allergic contact dermatitis.
Moderate evidence.
Caveats
Industry-funded, single study, no independent replication; a repeated-insult patch test is a screening design meant to detect sensitization potential, not a guarantee that sensitization cannot occur in a much larger real-world population over longer exposure.
Source: Weaver & Maibach 1977, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
The authors calculate that the diagnostic dose used to confirm glutaraldehyde allergy in their sensitized subjects (10,000 ppm) was 18.2 times the glutaraldehyde concentration in their undiluted fabric-softener liquid (550 ppm) and 25,000 times the concentration reaching skin at recommended use (0.4 ppm in the laundry rinse water) — a large margin between the sensitizing/diagnostic dose and the realistic use dose.
Moderate evidence.
Caveats
This margin is specific to glutaraldehyde in this formulation; margins differ by orders of magnitude for other allergens, so it should not be generalized as a universal laundry-product safety margin.
Source: Weaver & Maibach 1977, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In the largest patch-test study of laundry-detergent-caused allergic contact dermatitis (738 referred dermatitis patients across 10 North American clinics), only 0.7% (5 patients) had a positive patch-test reaction to a 0.1% aqueous dilution of a liquid or granular laundry detergent, and the study's own authors believe even that rate is likely too high.
Moderate evidence.
Caveats
Study was funded by, and both detergents were supplied by, Procter & Gamble (the products' manufacturer); no independent (non-industry) replication exists; enrollment was a non-randomized convenience sample of patients already referred for suspected contact dermatitis, not a general population.
Source: Belsito et al. 2002, Journal of the American Academy of Dermatology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Of the only 2 patients whose detergent patch-test reaction could be further evaluated with dose-response testing and a blinded T-shirt wear test, one had a negative repeat test and use-test suggesting the original reaction was likely irritant, not allergic, and the other reacted equally to a fabric swatch laundered without any detergent as to one laundered with detergent.
Moderate evidence.
Caveats
Only 2 of the 5 original phase-I reactors could be re-tested (the other 3 were lost to follow-up, disqualified, or declined); conclusions about mechanism rest on this very small subgroup.
Source: Belsito et al. 2002, Journal of the American Academy of Dermatology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Belief that one's dermatitis is caused by laundry detergent (10.7% of 738 referred dermatitis patients, and 8.4% of consumers in an earlier mail survey) is far more common than allergy confirmed by patch testing (0.7% in this study).
Moderate evidence: Referred-clinic population, not general population; belief and confirmed-allergy rates come from different (though overlapping) study designs.
Source: Belsito et al. 2002, Journal of the American Academy of Dermatology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
None of the 5 patients who reacted to a detergent patch test also reacted to nickel or potassium dichromate, and only 1 of 5 had any fragrance-allergy marker positive (balsam of Peru, but not cinnamic aldehyde or fragrance mix) — among the 74 patients (10.0%) who did react to balsam of Peru, only 1 also reacted to a detergent mix, and that reaction was judged most likely irritant.
Moderate evidence.
Caveats
Small numbers (n=5 detergent reactors); does not rule out other, untested detergent components (e.g. surfactant impurities, enzyme residues) as the culprit in the small subset that may be truly allergic.
Source: Belsito et al. 2002, Journal of the American Academy of Dermatology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
At the 0.1% aqueous test concentration used for detergents, irritant or questionable reactions (11-12 patients, 1.5-1.6%) outnumbered clearly allergic-looking reactions (5 patients, 0.7%), showing these test concentrations already sit close to the irritant threshold, which limits how much higher a concentration could be used without generating false positives.
Moderate evidence.
Caveats
Irritant-vs-allergic classification at the borderline (1+) reaction grade is inherently subjective; the paper does not report a formal irritancy-threshold titration beyond the three dilutions tested.
Source: Belsito et al. 2002, Journal of the American Academy of Dermatology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Fragrance allergy caused by a deodorant correlated with a positive patch test to fragrance mix II or HICC (34.3% and 24.9% of deodorant-linked cases) more than to fragrance mix I or Myroxylon pereirae (28.2% and 12.7%), while scented lotion and shampoo correlated more with fragrance mix I and M. pereirae — suggesting deodorants are enriched for the specific chemicals in fragrance mix II rather than just containing more fragrance overall.
Moderate evidence: Correlational, single retrospective dataset; the paper does not independently verify deodorant formulations chemically, relying on prior surveys for that.
Source: Heisterberg 2011, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
Up to 30% of German deodorants in a given year contained at least one strong (Group I) fragrance allergen above the concentration that legally triggers labeling (0.001%).
Moderate evidence.
Caveats
Label-based count, not chemical analysis; the author cites independent chemical testing (Kantonslabor Basel) and Rastogi 2007a finding under-threshold allergens missed by label counts, so true prevalence may be higher.
Source: Klaschka 2012, International Journal of Hygiene and Environmental Health Link to this fact
Skin allergy / Measured in people or real products
There was no significant difference in fragrance-allergen labeling between deodorants marketed for men and for women in Germany, even though a separate Danish clinical study found men have significantly more deodorant-caused fragrance allergic contact dermatitis than women.
Moderate evidence.
Caveats
Different countries and different measurements (label content vs. clinical outcome); the author explicitly says her data cannot explain the sex gap in allergy rates and only speculates that women may use deodorants, including neutral ones, more often.
Source: Klaschka 2012, International Journal of Hygiene and Environmental Health Link to this fact
Skin allergy / Measured in people or real products
A history of contact dermatitis nearly doubled the odds of having a positive patch test (OR 1.9), but a history of atopic (eczema-type) dermatitis did not raise the odds at all (OR 1.0).
Moderate evidence.
Caveats
Cross-sectional self-report of dermatitis history, not confirmed diagnosis in all cases (about half were physician-confirmed); the OR rose to 2.4 when restricted to physician-confirmed contact dermatitis.
Source: Diepgen 2016 (EDEN), British Journal of Dermatology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In a 2019 survey of 65 best-selling US laundering products, fragrance/essential oils were the single most common potential contact allergen in every category: 66.7% of laundry detergents, 90% of fabric softeners, 75% of dryer sheets, and 58.8% of stain removers.
Moderate evidence.
Caveats
Ingredient-label declaration survey (n=65, non-random 2019 US best-sellers), not a chemical measurement or clinical study; 'fragrance' defined broadly to include 16 named essential oils and fragrance-mix components.
Skin allergy / Measured in people or real products
Methylisothiazolinone (MI), a preservative allergen, was the second most common allergen in laundry detergents overall (40%), and the single most common allergen in detergents labeled 'baby safe' (80%, 4 of 5) and 'free and gentle' (57.1%, 4 of 7).
Moderate evidence: Ingredient-label survey, not measured concentration or patch-test data.
Skin allergy / Review of other studies
Reviewing all available human, animal, and lab-dish evidence on 106 well-studied fragrance chemicals, the fragrance industry's own research institute found that 90 of them (85%) are capable of causing allergic skin sensitization at some dose — 6 strong, 23 moderate, 41 weak, and 26 very weak sensitizers — and only 10 had no evidence of sensitization at all.
Moderate evidence.
Caveats
Industry-authored (RIFM/Givaudan-consultant); the 106 materials were chosen specifically because in vivo data already existed on them, so this is not a random or representative sample of fragrance ingredients in commercial use, and none was rated the most severe ("extreme") category.
Source: Na et al. 2022, Dermatitis Industry-funded Link to this fact
Skin allergy / Review of other studies
Fragrance allergens differ enormously in how little exposure it takes to sensitize someone: prior published data cited in this paper's introduction describe known allergens that sensitize skin at under 1 microgram per square centimeter, while others need up to 10,000 micrograms per square centimeter — roughly a ten-millionfold range.
Moderate evidence: (background context the paper cites from earlier literature, refs 1 and 6, not a new measurement of this paper's own).
Source: Na et al. 2022, Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
In this industry dataset, the fragrance chemical methyl-2-nonynoate was rated a strong skin sensitizer with a human no-effect dose of only 24 micrograms per square centimeter of skin — among the lowest doses in the whole 106-chemical set — while a chemical like ethylene brassylate needed over 23,000 micrograms per square centimeter, nearly a thousandfold more, before it showed no allergic reaction.
Moderate evidence: Single industry dataset (RIFM Database plus published literature); NOEL is the maximum dose tested without inducing sensitization, not necessarily the true threshold.
Source: Na et al. 2022, Dermatitis Industry-funded Link to this fact
Skin allergy / Lab study (cells or chemistry)
For several fragrance chemicals with no allergy-triggering chemical structure in their original (parent) form — including linalool and D-limonene, two of the most widely used fragrance ingredients in the world — the fragrance industry's own computer models predict that their breakdown products (metabolites) can trigger skin allergy, even though the parent chemicals were classified as non-sensitizers overall.
Moderate evidence.
Caveats
This paper's human no-effect-level data for linalool and limonene do not specify the oxidation state of the test material used; independent clinical studies already in this evidence base's contradiction register find that once these two chemicals are exposed to air, their oxidation products are among the commonest fragrance contact allergens in European patch-test clinics.
Source: Na et al. 2022, Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Deodorants that fragrance-allergic patients blamed for a rash reproduced that rash on controlled retesting 60% of the time (12 of 20 products), and the deodorants that caused a reaction contained fragrance-mix allergens at 1.3 to 8.6 times higher concentration, on average, than the ones that did not.
Moderate evidence.
Caveats
Small single-study cohort (14 patients, 20 patient-supplied deodorants), independently funded (EU/Danish/Swedish public grants, no industry money); no matched non-allergic control group was tested against the same implicated products, so an irritant (rather than allergic) contribution cannot be fully excluded for any single case.
Source: Johansen et al. 1998, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
How much fragrance-mix allergen a deodorant contained mattered more than how many different allergens it contained: deodorants that caused a reaction and those that did not had a similar number of fragrance-mix constituents present, but the reacting ones had higher concentrations of each.
Moderate evidence: Based on GC/GC-MS analysis of only 19 deodorants (13 use-test-positive/doubtful vs. 6 negative); small group sizes for a concentration comparison.
Source: Johansen et al. 1998, Contact Dermatitis 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
Citral, a fragrance chemical already established as a human skin sensitizer, was detected by GC-MS in 26 of 71 European deodorants tested in 1998 (15 of 22 vapo-sprays, 8 of 22 aerosol sprays, 3 of 27 roll-ons).
Moderate evidence: Presence only, not concentration; a single 1998 market snapshot from 5 European countries.
Source: Rastogi et al. 1998 (Contact Dermatitis 39:293), Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
In a 1996 chemical analysis of 73 European deodorants, the seven allergens used in the standard fragrance-mix patch test were very unevenly present: geraniol was found in 76% of products and eugenol in 57%, but cinnamic aldehyde in only 17% and isoeugenol in 29%.
Moderate evidence.
Caveats
Single study, one product category, purchased 1996 in 5 European countries; not replicated for deodorants specifically, though the pattern matches the same lab's later household-product survey (Rastogi 2001).
Source: Rastogi et al. 1998, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
The two rarest fragrance-mix allergens in this 1996 deodorant survey, cinnamic aldehyde (max 424 ppm) and isoeugenol (max 458 ppm), reached concentrations the study authors judged capable of triggering a reaction in people already sensitized to them, based on published human patch-test and use-test thresholds.
Moderate evidence.
Caveats
The judgment applies only to already-sensitized individuals and only to the single highest-concentration product found for each substance; median concentrations across the sample were more than 10x lower than the level of concern, and the authors state no conclusion can be drawn for the other fragrance-mix constituents because no-effect thresholds have not been studied.
Source: Rastogi et al. 1998, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
In a controlled study, deodorants containing hydroxycitronellal at 0.032% to 0.32% triggered allergic armpit rashes within a few weeks in all 7 people already allergic to the chemical, while an identical unscented deodorant on the other armpit, and the same deodorants used by 7 non-allergic people, caused no reactions (P<0.001).
Moderate evidence.
Caveats
Single-center study (Malmö only), unlike the 5-center Bruze 2003 cinnamic-aldehyde companion study graded strong; small cohort (7 test + 7 control patients), though statistically significant with a concurrent negative control group and blinded, randomized contralateral-axilla design.
Source: Svedman et al. 2003, Contact Dermatitis Link to this fact
Skin allergy / Measured in people or real products
Sensitivity to hydroxycitronellal varies enormously between allergic people: this study measured roughly a 33,000-fold range in the patch-test threshold concentration needed to trigger a reaction, from 0.00012% in the most sensitive patient to 4.0% in the least sensitive.
Moderate evidence.
Caveats
Range measured in 7 patients only; the low-end figure (0.00012%, 1.2 p.p.m.) was checked against the original page image because the PDF's extracted text drops decimal points throughout and could otherwise be misread as 12 p.p.m.
Source: Svedman et al. 2003, Contact Dermatitis Link to this fact
Skin allergy / Other evidence
A standard fragrance-mix patch test misses a substantial share of true fragrance allergy: only 50-80% of people with a real fragrance allergy are picked up by it.
Moderate evidence: A review-level synthesis of other groups' clinic populations, not a general-population estimate; figures are for patch-tested eczema/dermatitis patients.
Source: Johansen 2003, Am J Clin Dermatol 4(11):789-798, American Journal of Clinical Dermatology 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
Sensitivity to isoeugenol varies enormously between allergic people: this study measured a 4000-fold range in the patch-test concentration needed to trigger a reaction, with the most sensitive patient reacting at 0.0005%, below the EU's 0.001% fragrance-labeling threshold.
Moderate evidence: Range measured in 13 patients only; the 0.0005% threshold is a single patient's patch-test result in ethanol, not the deodorant vehicle.
Source: Bruze et al. 2005, Contact Dermatitis Link to this fact
Skin allergy / Review of other studies
'Black henna' tattoos can contain added paraphenylenediamine (used to darken the color), which has triggered severe bullous (blistering) contact dermatitis in some users.
Moderate evidence: Relayed from a single case report (Jung et al. 2006).
Source: Brasch 2007, Current Opinion in Allergy and Clinical Immunology 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 / Other evidence
162 individual fragrance chemicals have documented reports of causing contact allergy or allergic contact dermatitis, but routine patch testing screens for only a handful of markers (fragrance mix I, fragrance mix II, balsam of Peru, HICC, and the EU's 26 labeled allergens).
Moderate evidence: A single author's own literature tally, not a systematic review with stated search criteria; the same paper's abstract rounds this down to "more than 150".
Source: de Groot 2020, Dermatitis 31(1):13-35, Dermatitis® 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 / Other evidence
The standard fragrance patch-test screen (fragrance mix I, fragrance mix II, balsam of Peru) misses a large share of true fragrance allergy: this review's own cited figures range from about 40% to 70% missed depending on the comparison, and "approximately half" of relevant reactions to perfumes, deodorants, and shaving lotions go undetected by baseline testing.
Moderate evidence.
Caveats
Different underlying comparators (26-fragrance panel vs hydroperoxides vs own products) give different miss rates; a specific percentage should always name what it was compared against.
Source: de Groot 2020, Dermatitis 31(1):13-35, Dermatitis® Link to this fact
Skin allergy / Review of other studies
Propylene glycol, a solvent and humectant used in many deodorants, can make up as much as 73% of the product and acts as both a skin irritant and an allergen, which makes its allergy patch tests hard to interpret (positivity ranges 0.1%-3.8% across studies).
Moderate evidence.
Caveats
Concentration and positivity-range figures are secondary citations (to Fiume 2012 and other cited surveys, not independently verified here) relayed by a 2024 review, not this review's own data.
Source: Musicante 2024, Cutis Industry-funded Link to this fact
Skin allergy / Review of other studies
As of 2022, US dermatology guidance (American Contact Dermatitis Society) recommends patch testing for aluminum allergy with a 10% aluminum chloride hexahydrate concentration rather than the older 2%, to improve detection of this uncommon but real cause of antiperspirant-related axillary dermatitis.
Moderate evidence: A practice-guidance update relayed from a 2022 Allergen-of-the-Year designation; not itself new patch-test data.
Source: Musicante 2024, Cutis Industry-funded Link to this fact
Skin allergy / Review of other studies
Allergic contact dermatitis from textile dyes in clothing characteristically affects the borders/rim of the armpit while sparing the central vault, the opposite pattern from deodorant- or antiperspirant-caused dermatitis, which typically affects the vault where the product is applied.
Moderate evidence: Distribution pattern is a clinical observation relayed from case reports and small case series (Ryberg, Seidenari), not a controlled comparison.
Source: Musicante 2024, Cutis Industry-funded Link to this fact
Skin allergy / Review of other studies
Contact dermatitis from deodorants and antiperspirants is overwhelmingly caused by fragrance ingredients (isoeugenol, cinnamic aldehyde, hydroxycitronellal, lyral, oak moss, ylang-ylang oil), not by the antiseptic or antiperspirant active ingredients.
Moderate evidence: Synthesis judgment by a single reviewer across multiple studies, not a head-to-head comparison of allergen rates.
Source: Martini 2020, Annales de Dermatologie et de Vénéréologie Link to this fact
Skin allergy / Other evidence
In a 4-year German patch-test study of 1,468 patients tested with their own deodorants and perfumes, 129 had positive reactions to a specific product, and 58 of them (45%) were missed by the standard fragrance-mix, balsam-of-Peru, and lyral patch tests.
Moderate evidence.
Caveats
Numbers relayed secondhand via this review, citing Uter et al.; the primary IVDK dataset is not yet in our evidence base, and the specific allergens responsible for the 58 missed reactions are not given.
Source: Martini 2020, Annales de Dermatologie et de Vénéréologie 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
Skin allergy / Review of other studies
Nearly all common contact allergens in the standard European patch-test series (ICDRG) have a molecular weight under 500 Dalton; the one exception, neomycin sulfate (712 Da), is a dimer of two 322-Da units, and the smaller monomer may be the true sensitizer.
Moderate evidence: A tabulation of one standard allergen panel (34 compounds), not a systematic survey of all known contact allergens, and not a measured penetration study.
Source: Bos & Meinardi 2000, Experimental Dermatology Link to this fact
Skin allergy / Review of other studies
The individual fragrance-mix allergens in the standard patch-test series (eugenol, geraniol, cinnamaldehyde, cinnamyl alcohol, isoeugenol, hydroxycitronellal, alpha-amyl-cinnamaldehyde) all have molecular weights between 132 and 202 Dalton, among the smallest molecules in the entire panel.
Moderate evidence: Direct data from the paper's own Table 1; small molecular size predicts penetration is possible, not that a given exposure causes sensitization.
Source: Bos & Meinardi 2000, Experimental Dermatology Link to this fact
Skin allergy / Review of other studies
Atopic dermatitis skin has a weakened barrier that allows topical drugs up to about 800 Dalton (tacrolimus 822 Da, the ascomycin derivative SDZ ASM 981 811 Da) to work, whereas the same molecules and larger ones (cyclosporine, 1202 Da) are not effective applied topically to normal skin.
Moderate evidence: Narrative synthesis of separately published clinical trials, not a new experiment; treated by the authors as the named exception to their own 500 Dalton rule.
Source: Bos & Meinardi 2000, Experimental Dermatology Link to this fact
Skin allergy / Lab study (cells or chemistry)
At least six substances on the EU's official list of 26 named fragrance allergens (geraniol, cinnamaldehyde, eugenol, isoeugenol, cinnamyl alcohol, and anisyl alcohol) were included among the 56 chemicals tested for standardized 24-hour human-skin penetration in this 2020 industry dataset, but the paper's main text and figures report only their relative ranking (not exact absorption percentages) among the 56 chemicals; the individual percentages exist only in a supplementary table not available in this ingest.
Moderate evidence.
Caveats
Exact per-chemical dermal-delivery percentages for these fragrance ingredients were not accessible in the extracted text; only their approximate rank position among the 56 tested chemicals is reported here.
Source: Hewitt 2020, Journal of Applied Toxicology Industry-funded Link to this fact
Skin allergy / Lab study (cells or chemistry)
Three chemicals (propylparaben, geraniol, and benzophenone) penetrated human skin markedly less when dissolved in ethanol than when dissolved in a water-based buffer (PBS) in this standardized 24-hour skin-penetration study, apparently because ethanol evaporates faster and can let the chemical precipitate on the skin surface before it is absorbed; a fourth chemical, hydrocortisone, showed no such difference between the two solvents.
Moderate evidence.
Caveats
Single industry-funded lab; n=12 replicates per condition (3 skin discs x 4 donors); the exact percentage difference for geraniol specifically is not given as a number in the accessible text, only described qualitatively as lower in ethanol.
Source: Hewitt 2020, Journal of Applied Toxicology Industry-funded Link to this fact
Skin allergy / Lab study (cells or chemistry)
In lab tests on human skin, how much of the lavender-scented chemical linalool actually soaked in ranged from under 2% to nearly 15% of the amount applied, depending only on what solvent it was dissolved in and whether the skin was covered.
Moderate evidence: Single in-vitro study, one donor pool, fresh (non-oxidized) linalool at a fixed 4% test concentration in laboratory solvents, not any real product formula.
Source: Brain et al. 2022 (An-eX/RIFM), International Journal of Pharmaceutics Industry-funded Link to this fact
Skin allergy / Lab study (cells or chemistry)
Covering the skin after applying linalool (occlusion, as under a bandage or clothing) always increased how much was absorbed, by 2 to 6 times depending on the solvent used.
Moderate evidence: Single in-vitro study; occlusion effect size was vehicle-dependent and not tested on any real consumer product.
Source: Brain et al. 2022 (An-eX/RIFM), International Journal of Pharmaceutics Industry-funded Link to this fact
Skin allergy / Review of other studies
Most fragrance allergens are protein-reactive haptens that bind skin proteins directly to trigger delayed-type allergic contact dermatitis; a minority require activation first, either by air (prehaptens) or by skin enzymes (prohaptens).
Moderate evidence.
Caveats
Narrative review synthesis of established hapten chemistry, not new data; our evidence base has no source making a direct poison-ivy comparison. A separate existing claim (c-69edda7901, Lee 2024/RIFM) shows 19 of 62 industry reference fragrance ingredients rate moderate-or-strong sensitizers, so 'most fragrance compounds are weaker sensitizers' is not true across the board.
Skin allergy / Measured in people or real products
Fragrance-allergic hand-eczema patients were far more likely to also test positive to other standard contact allergens than fragrance-negative patients (80.3% vs 33.7%), a pattern the study authors attribute to possible cross-reactivity or simultaneous exposure to multiple allergens rather than proof that fragrance itself causes new sensitization to other ingredients.
Moderate evidence.
Caveats
Correlational finding from one clinical population (658 hand-eczema patients); does not establish that fragrance exposure causally increases sensitization to other allergens ('co-sensitizing'), only that the two associate strongly, which the authors themselves attribute to cross-reactivity or simultaneous exposure rather than a co-sensitizing mechanism.
Source: Heydorn 2003, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Review of other studies
Standard allergy patch testing keeps allergens on the skin under occlusion for 48 hours, with readings at about day 3 and a final reading at 72-96 hours; a reaction present at day 3 but faded by day 5 suggests irritation rather than true allergy.
Moderate evidence.
Caveats
Narrative (non-systematic) clinical review describing established, consensus patch-test methodology rather than new data; not independently re-verified against a primary methodology paper in this pass. The 'about 70 ingredients' figure in the video is not confirmed anywhere in our evidence base — our evidence base only documents a 50-allergen research panel (Diepgen 2015) and single-marker/mix testing; allergen count varies by which standard series is used (e.g., NACDG ~70-80 allergens, European baseline series ~30). A positive patch test also indicates sensitization, not necessarily current active dermatitis, and standard fragrance mixes are known to miss roughly half of true fragrance allergy (Diepgen 2015; Mann 2014, both already in our evidence base).
Source: Aristizabal-Torres 2025, JAAD Reviews Link to this fact
Skin allergy / Measured in people or real products
In a randomized trial of 20 healthy adults, using a fragrance-free lipid moisturizer three times a day for four weeks on one forearm raised skin hydration but did not reduce, and instead significantly increased, transepidermal water loss after a standardized sodium lauryl sulfate irritant patch test, compared with the untreated forearm on the same person.
Moderate evidence.
Caveats
Single small trial (n=20), single lab, one moisturizer formulation, healthy non-diseased skin only; does not test moisturizer used shortly before an irritant challenge (a separately documented protective use-case) or barrier-impaired/atopic skin.
Source: Held 1999, Acta Dermato-Venereologica Link to this fact
Skin allergy / Measured in people or real products
Four weeks of daily moisturizer use increased skin hydration (electrical capacitance) but caused no measurable change in transepidermal water loss on intact, non-irritated skin, showing that a standard TEWL barrier-function test can look normal even while the skin's response to a subsequent irritant challenge is already altered.
Moderate evidence: Only one hydration metric (capacitance) and one barrier metric (TEWL) were used; effect may not generalize to other moisturizer formulations or longer/shorter durations of use.
Source: Held 1999, Acta Dermato-Venereologica Link to this fact
Skin allergy / Measured in people or real products
In industry's own comparative testing of 24 household cleaning-product formulations, liquid and powder laundry and dish detergents ranked among the least skin-irritating categories tested, below a standard 20% SDS irritant benchmark (liquid laundry detergent TR50 average 3.48 h; powder laundry and dish detergents >16 h, vs. SDS's 1.81 h).
Moderate evidence.
Caveats
Irritation (erythema from a single undiluted, fully occluded 4-h exposure), not allergy; single industry-funded study series with no independent replication; authors state results should not be extrapolated to real-use exposure conditions.
Source: Robinson et al. 2005, Food and Chemical Toxicology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Mold/mildew removers (TR50 0.37 h), hospital disinfectants and food-contact sanitizers (0.58-0.69 h), and some hard-surface cleaners and concentrated fabric softener (1.09-2.51 h) were classified as more skin-irritating than a 20% SDS benchmark in this industry testing, while liquid/powder laundry and dish detergents were not.
Moderate evidence: Single industry-funded study series; classification is relative to the SDS benchmark under exaggerated (undiluted, occluded) lab exposure, not real-use risk.
Source: Robinson et al. 2005, Food and Chemical Toxicology Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Increasing infant moisturization frequency at 3 months did not reduce the later development of visible eczema at 12 months (p=.98) or 36 months (p=.20) among infants with no history of eczema or dry skin at enrollment; the most-frequently-moisturized group had numerically the highest eczema rate at 36 months.
Moderate evidence.
Caveats
Null/underpowered comparison in a single cohort; the >daily subgroup at 36 months was small, so the numerically highest rate should not be read as a significant harm signal on its own.
Source: Perkin 2021 (EAT cohort), Journal of Allergy and Clinical Immunology Link to this fact
Skin allergy / Measured in people or real products
Moisturization frequency at 3 months also showed a dose-response relationship with food and aeroallergen sensitization (skin prick test/specific IgE) at 36 months, and with transepidermal water loss (TEWL) at 3 months (0.63 g/m2*h higher per unit increase in weekly moisturization, 95% CI 0.53-0.73, p<.0005), a steeper gradient than the same cohort's previously reported bathing-frequency/TEWL relationship.
Moderate evidence: Sensitization and TEWL are surrogate/mechanistic markers, not the clinical outcome itself; single cohort.
Source: Perkin 2021 (EAT cohort), Journal of Allergy and Clinical Immunology Link to this fact
Skin allergy / Measured in people or real products
In a controlled trial of 12 healthy adults, applying a barrier moisturizer 15 minutes before each of four sodium lauryl sulfate (0.375%) hand immersions over two days fully protected the treated hand: transepidermal water loss, blood flow, and skin hydration all stayed at baseline, while the untreated control hand on the same person showed significant deterioration in all three (TEWL p=0.001, blood flow p=0.008, electrical capacitance p=0.002).
Moderate evidence.
Caveats
Single small trial (n=12), single lab, one moisturizer formulation (Locobase), one irritant (SLS), healthy non-diseased skin only; moisturizer present at the moment of irritant exposure, not withdrawn beforehand.
Source: Ramsing & Agner 1997, Acta Dermato-Venereologica Link to this fact
Information, not medical advice. See also: myths we won’t tell you.