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.
Children and pregnancy / Measured in people or real products
In 34 California childcare centers, the chemicals at the highest levels in the air children breathed came from cleaning and personal-care products: a silicone ingredient called D5 (median 51 µg/m³) and the citrus-scent chemical limonene (median 33 µg/m³).
Strong evidence: Single-day samples; upper values capped by calibration (underestimates).
Children and pregnancy / Review of other studies
About half of children's cosmetics in a Danish registry (634 of 1,179, 53.8%) listed perfume or a named fragrance allergen, and a US survey of 533 baby/child personal care products found a closely similar share (255/533, 47.8%; Bonchak 2018); in Danish baby-care products the commonest named allergens were linalool, limonene, and benzyl alcohol (the US primary does not report limonene or linalool by name).
Strong evidence.
Caveats
Danish half is primary (Botvid 2023): baby-care products specifically were 38.0% (178/468), the least fragranced category — do not write 'half of baby products' for Denmark. 'Fragranced' there includes products naming only an allergen such as benzyl alcohol (often a preservative). US half is now also primary (Bonchak 2018): 47.8% (255/533) across wipes/diaper care/soap/moisturizer/shampoo/sunscreen — but Bonchak's fragrance count is a ~20-keyword composite that does not report limonene or linalool individually, so the 'top three' allergen list is confirmed only for Denmark. Label counts remain lower bounds.
Source: Sukakul 2024, Acta Dermato-Venereologica Industry-funded Link to this fact
Fragrance is everywhere / Measured in people or real products
Even in Perth, a city with very clean outdoor air, limonene and pinene (scent chemicals typical of citrus- and pine-scented cleaners and air fresheners) were found in 80-100% of homes when a sensitive test was used.
Strong evidence: Low concentrations (median ~1.5-2.4 µg/m³); sources not measured; 64 weekday-daytime visits.
Source: Maisey 2013, Atmospheric Environment Link to this fact
Fragrance is everywhere / Measured in people or real products
In EPA's large 1980s studies of US homes, limonene, the chemical behind lemon scents, was the airborne chemical with the highest average concentration in people's homes.
Strong evidence: 1980-87 TEAM data; no health risk estimated for limonene.
Source: Wallace 1991, Environmental Health Perspectives Link to this fact
Fragrance is everywhere / Measured in people or real products
Scientists can identify which everyday products are polluting a city's outdoor air just by measuring a handful of specific chemical fingerprints: a silicone compound (D5-siloxane) for personal-care products and citrus/pine-scent chemicals (monoterpenes, mostly limonene) for fragranced products, confirmed by mobile-lab measurements across New York City, Pittsburgh, Chicago, and Denver.
Strong evidence: Outdoor air only; four US cities, no LA data; tracer status is based on population-density correlation and inventory apportionment, not a direct source test.
Source: Gkatzelis 2021, Environmental Science & Technology Link to this fact
Indoor air / Review of other studies
Scent chemicals such as limonene and pine oil react with ozone indoors to make new pollutants, including formaldehyde and fine particles, that keep forming for hours after cleaning ends.
Strong evidence: Size of the effect depends on dose and indoor ozone; confirmed by later chamber and field studies.
Source: Nazaroff & Weschler 2004, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
Limonene was detected in every childcare room studied and was more than 1,000 times higher indoors than outdoors, so it came from products used inside.
Strong evidence: Outdoor detected in 1/20 sites.
Indoor air / Measured in people or real products
In week-long air measurements across 11 European cities, the scent chemicals limonene and alpha-pinene were the compounds most concentrated in homes compared with workplaces (about 4x), attributed to perfumes, cleaning and fragranced products.
Strong evidence: Source attribution by the authors, not measured per product.
Source: Geiss 2011, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
Limonene was 13x higher indoors than outdoors in European offices and classrooms, one of the two highest indoor/outdoor ratios of 23 chemicals measured.
Strong evidence: Ratio of medians.
Source: Geiss 2011, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
Most European homes had low limonene (median 9.5 ug/m3 over a week), but the heaviest-use homes reached weekly averages up to 493 ug/m3, the range used in lab studies of indoor ozone chemistry.
Strong evidence: Upper tail only; median home 20-60x below chamber levels.
Source: Geiss 2011, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
In a real test house, using a limonene-scented foam cleaner exactly as the label directed produced a burst of 45,000-150,000 tiny new particles per cubic centimeter within minutes, but only when ozone was in the air.
Strong evidence.
Caveats
One product, one house; particle number not mass. Independently replicated in a second real building (a working primary school; Morawska 2009), with a different detergent and country, at a similar order of magnitude (5.89-6.85×10^4 cm-3 in controlled tests) and the same water-only-no-burst, ozone-dependent pattern.
Source: Rossignol 2013, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
Scent-type chemicals typical of fragranced and cleaning products (limonene, pinene, carene) were the signature of home air: close to zero outdoors but tens to hundreds of times higher inside homes.
Strong evidence.
Caveats
Small original study (n=7); the indoor-source signature (not the specific product-category attribution) is now independently replicated by a much larger 2021 UK study with real paired outdoor sampling (Heeley-Hill 2021, 60 homes: limonene indoor:outdoor ~19x, alpha-pinene ~10x), though that larger study also found cumulative product-use frequency does not predict concentration (only insecticide/plug-in-air-freshener use showed weak covariance with limonene) — so the indoor-source signature is strong, the specific 'cleaning products' attribution stays inferential. Givaudan-funded (COI); analysis independent.
Source: Gokhale 2008, Science of the Total Environment Link to this fact
Indoor air / Measured in people or real products
Limonene was the most prevalent and highest-concentration of 88 volatile organic compounds measured in a national survey of Canadian homes: detected in 99% of homes, with an arithmetic mean of about 45 ppb (roughly 250 ug/m3) and a geometric mean of about 25 ppb (roughly 140 ug/m3), levels well above limonene concentrations reported in Australian, European, and Californian indoor-air studies.
Strong evidence.
Caveats
The ppb-to-ug/m3 conversion is evidence base arithmetic (limonene MW 136.23, 25C/1atm), not stated by the authors, who reported concentrations only in ppb. Comparator studies used different sampling durations, locations, and methods, so the size of the gap should be read cautiously even though the direction (Canada higher) is clear.
Source: Dales & Cakmak 2019, Environmental Pollution Industry-funded Link to this fact
Indoor air / Other evidence
Of 109 air fresheners bought in the Netherlands in 2009, 20 of the 24 regulated fragrance allergens turned up; linalool was in 87% of products and limonene in 69%, and one product was 15% linalool by weight.
Strong evidence: One survey of one national market in January 2009; weight fractions only, not what is emitted into air.
Source: ter Burg 2014, Inhalation Toxicology Link to this fact
Indoor air / Government agency
When limonene and linalool sit in air they oxidize into allergens: the EU committee said oxidized limonene and oxidized linalool rank among the most common contact allergens in patch testing (about 2-3% of consecutive patients for limonene at 3-5%, and 0.8-7.2% for linalool depending on test strength).
Strong evidence.
Caveats
Clinic patients, not the general population; test concentrations differ between studies (limonene 3-5%, linalool 2-11%); no published data on peroxide content of finished cosmetic products existed when written; transformation chain supported by multiple independent clinics but patch-test groups overlap (several Swedish centers).
Indoor air / Other evidence
The 2018 list printed d-limonene as Group 3 (Vols 56, 73, 1999), acetaldehyde as 2B, and styrene as 2A (Vol 121, listed 'In prep.'), alongside household combustion of biomass fuel (2A) and coal (1).
Strong evidence: 2018 snapshot; Group 3 means not classifiable, not safe; hazard not risk.
Source: IARC 2018, Agents Classified by the IARC Monographs, Vols 1-123 Link to this fact
Indoor air / Other evidence
Terpene ingredients in scented cleaners, air fresheners, perfumes, colognes and essential oils (limonene, pinene, linalool, linalyl acetate) are listed as ozone-reactive indoor sources that make formaldehyde and tiny particles when ozone is present.
Strong evidence.
Caveats
Table 1 is a literature summary; perfumes and essential oils rest on three chamber citations (Chao 2005, Karamalegos 2005, Shu 1997). No product was measured in this paper and it gives no product-specific numbers.
Source: Weschler 2006, Environmental Health Perspectives Link to this fact
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 / 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; the allergens form when these chemicals react with air, which also happens in essential oils and perfumes during storage.
Strong evidence.
Caveats
A '0.1-0.3% of patients' figure was removed because it does not trace to a source in our evidence base. Parent-compound sensitization rates are low but vary by dataset; see the Dittmar 2019 entry for oxidized-terpene positive rates.
Source: Bråred Christensson 2016, Contact Dermatitis Industry-funded 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 form much stronger allergens; products and oils kept open or stored a long time can contain more of them.
Strong evidence.
Caveats
Review of experimental and clinical work; no new data. Hydroperoxide content depends on storage, light, air exposure and antioxidants; no study here shows that a product's age alone predicts content, so do not claim 'older means worse'.
Source: Karlberg & Lepoittevin 2021, Contact Dermatitis Link to this fact
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 / 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 / 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 / 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 / Measured in people or real products
In a Spanish study of 3,639 consecutive dermatitis patients at 22 clinics, 5.1% were allergic to oxidized limonene and 4.9% to oxidized linalool, two of the most common fragrance chemicals, at the recommended patch-test concentrations.
Strong evidence.
Caveats
Figures are from a clinic-referred dermatitis population, not a general-population sample; positivity depends heavily on the concentration tested (see next claim), and one major network (IVDK) disputes whether these top concentrations over-call true allergy.
Source: Deza 2016, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Positive reactions to oxidized limonene and linalool roughly tripled as the patch-test dose increased (limonene hydroperoxides 1.4% to 5.1%; linalool hydroperoxides 1.3% to 4.9% across three concentrations), and about a third to two-fifths of allergic patients would have been missed entirely at the next-lower dose.
Strong evidence.
Caveats
Irritant reactions also rose slightly at the highest doses (up to 1.5-1.9%); whether this dose-response reflects true allergy or growing over-sensitivity of the test material is an open dispute.
Source: Deza 2016, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
Standard fragrance allergy screening tests (the fragrance mixes, HICC, Myroxylon pereirae, colophonium) missed about 70% of patients allergic to oxidized limonene or linalool; only 30-33% of hydroperoxide-allergic patients also reacted to any standard baseline marker.
Strong evidence: None.
Source: Deza 2016, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Measured in people or real products
About a quarter of patients allergic to either oxidized limonene or oxidized linalool reacted to both, while three-quarters reacted to only one, supporting that these are two distinct, specific fragrance allergies rather than a single non-specific reaction to hydroperoxide chemistry in general.
Strong evidence: Replicates an earlier international multicenter finding (25.3%/74.7%) in an independent, larger, single-country cohort.
Source: Deza 2016, Contact Dermatitis Industry-funded Link to this fact
What you can do / Measured in people or real products
If you have unexplained or recurring dermatitis, ask whether patch testing will include oxidized linalool (and oxidized limonene); standard fragrance tests missed 60% of these allergies.
Strong evidence: Hydroperoxide tests now recommended for baseline series in Europe.
Source: Christensson 2012, Contact Dermatitis Link to this fact
Information, not medical advice. See also: myths we won’t tell you.