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
Children and pregnancy / Measured in people or real products
Childcare air had about three times more limonene than new California homes (33 vs 11 µg/m³), which the authors link to frequent cleaning.
Moderate evidence: Comparison with a different study (Offermann 2009); daytime vs 24-h sampling.
Children and pregnancy / Measured in people or real products
The limonene-asthma association was substantially stronger in children than adults in this Canadian national survey: children (6-16y) had an adjusted odds ratio of 1.46 (95% CI 1.45-1.47) for asthma per 100% increase in indoor limonene versus 1.16 (95% CI 1.15-1.16) in adults (17-79y), and the FeNO increase was about twice as large in children (2.89%, 95% CI 1.88-4.43) as in adults (1.44%, 95% CI 1.16-1.79).
Moderate evidence.
Caveats
Cross-sectional; the authors' proposed explanations (more eosinophilic airway inflammation in children, more time spent at home, higher measured limonene in the younger age group) are speculative, not tested directly in this study.
Source: Dales & Cakmak 2019, Environmental Pollution Industry-funded Link to this fact
Fragrance is everywhere / Measured in people or real products
A lab analysis of 59 soaps, surface cleaners, laundry and dish-washing products from four European countries found the citrus-scent chemical limonene in 78% of them, linalool in 61%, and citronellol in 47%; all three are fragrance chemicals linked to skin allergy.
Moderate evidence: Products c. 2000, EU only; parent terpenes measured, not the allergenic oxidized forms; Unilever co-author.
Source: Rastogi 2001, Contact Dermatitis Industry-funded Link to this fact
Fragrance is everywhere / Measured in people or real products
In a test of 59 best-selling Korean household products, the citrus-scent chemical limonene was the chemical most often given off: it came from 44% of products, including bleach, dish soap, laundry detergent, disinfectant, and even a liquid pesticide.
Moderate evidence: Heated headspace, library-match ID, presence not exposure.
Source: Kwon 2007, Journal of Hazardous Materials Link to this fact
Fragrance is everywhere / Animal study
The vapor from a single cologne contained at least twenty identifiable chemicals – limonene, linalool, linalyl acetate, alpha- and beta-pinene, terpinolene, citronellol, anethole, coumarin, diethyl phthalate among them – none of which appear on the label.
Moderate evidence: Qualitative GC/MS of one test atmosphere, recoveries unknown, isomers possibly unresolved; one unnamed product.
Source: Anderson & Anderson 1998, Archives of Environmental Health Link to this fact
Fragrance is everywhere / Measured in people or real products
All 31 UK shampoos, shower gels, conditioners, moisturizers, and foundations with reported emission rates gave off fragrance terpenes (measured together as limonene); shampoos and shower gels released the most.
Moderate evidence.
Caveats
Lab headspace of products bought at one UK supermarket; SIFT-MS cannot separate monoterpenes, so 'limonene' is the sum of all of them; brands not named. Same lab's second, separate set of 16 UK day moisturizers (Yeoman 2022) also all emitted monoterpenes (0.08–0.41 µg/s/g), green and regular alike.
Fragrance is everywhere / Review of other studies
A 1991 US EPA survey of 31 fragrance products already found limonene, linalool, alpha-pinene, benzyl acetate, benzyl alcohol, camphor, and acetone among the most common chemicals, the same compounds found in fragranced products tested in the 2010s.
Moderate evidence: Known only second-hand (Kendall 1995 list; Steinemann 2011 quotes its prevalences); the EPA report itself is not in our evidence base.
Source: DONTSPRAYCALIFORNIA 2008 (DOJ ADA docket) Link to this fact
Fragrance is everywhere / Measured in people or real products
Plug-in and hanging car air fresheners mainly release fragrance terpenes (limonene, pinene, dihydromyrcenol) and fruity-smelling esters, the same terpenes found across larger product surveys.
Moderate evidence.
Caveats
7 products; presence only. Lamorena & Lee 2008 (KAIST chamber, GC-MS/FT-IR) independently confirms terpene-dominant emissions (α-pinene, β-pinene, p-cymene, limonene) from a car air freshener, but that product is an oil-based liquid type, not plug-in or hanging, so it corroborates the general 'car fresheners emit terpenes' pattern without directly extending evidence for the plug-in/hanging sub-types this claim specifies.
Source: Pino-Delgado 2021; Steinemann 2011, Journal of Student Research Link to this fact
Fragrance is everywhere / Measured in people or real products
In winter Manhattan air the dominant terpene was limonene, the citrus scent chemical used in cleaners and personal care, while nearby forests gave off pine-type terpenes: scented products are a measurable source of city air pollution in New York.
Moderate evidence.
Caveats
One city (NYC 2018); isomer pattern by GC-MS; fragranced products explain ~220 +/- 170 of 520-860 mg/person/day anthropogenic monoterpenes, the rest possibly cooking/building materials.
Source: Coggon 2021 Link to this fact
Fragrance is everywhere / Measured in people or real products
A 2015 survey of 817 unique detergent products from 131 German households found limonene and linalool — the two fragrance chemicals most often linked to skin allergy when oxidized — named on 23.1% and 20.1% of product labels, more than any of the other 24 EU-declarable fragrance allergens.
Moderate evidence: Label-declaration frequency above the EU's 0.01% threshold, not a chemical measurement; Northern Germany, 2015, convenience sample, not representative.
Source: Wieck 2018, Regulatory Toxicology and Pharmacology Link to this fact
Fragrance is everywhere / Advocacy group testing
The UV filter ethylhexyl methoxycinnamate, found in several of the tested perfumes, and limonene, one of the most common fragrance ingredients, are both flagged by this report as environmentally problematic; the EU classifies limonene as very toxic to aquatic life.
Moderate evidence.
Caveats
The water-contamination claim for ethylhexyl methoxycinnamate specifically is supported in the report only by a citation to a review of a different, though related, UV filter (oxybenzone); however, an independent peer-reviewed marine-ecotoxicology review (Hodge et al. 2025) now corroborates the general pattern that organic UV filters including EHMC are detected in marine water, sediment, and biota worldwide (ng/L-mg/L), including remote regions with no direct use (Antarctica, the Arctic), and lists EHMC as the third-most-studied organic UV filter in that literature.
Source: Tegengif 2022 Link to this fact
Fragrance is everywhere / Other evidence
The 2005 flavor-industry paper names, as components of the essential oils it covers, chemicals that are also common fragrance allergens: linalool (coriander oil), limonene, citral (lemongrass), eugenol (clove bud oil) and cinnamaldehyde.
Moderate evidence: The paper evaluates oral intake only and never mentions allergy or sensitization; it supports the shared chemistry, not any skin or airway effect. Industry-authored (FEMA).
Source: Smith 2005, Food and Chemical Toxicology Industry-funded Link to this fact
Fragrance is everywhere / Other evidence
Retailers diffuse a scent blend of about 20 listed materials, including limonene, coumarin, lilial and hexyl cinnamal, through a shop from a ceiling-mounted unit running from one hour before opening, with the door open so the scent also drifts outside.
Moderate evidence: Describes one store; ingredient names only, no concentrations; the paper does not say how common this is and makes no allergen or health statement.
Green, natural and unscented / Lab study (cells or chemistry)
There is no known 'safe terpene'. Eucalyptol is inactive on hormone receptors but acts weakly on a hormone-making enzyme; alpha-terpineol is the most active on receptors and only middling on the enzyme; limonene is near-inert on receptors and a comparable enzyme inhibitor. The two rankings disagree in both directions.
Moderate evidence: Both rankings are in vitro and at concentrations far above real exposure. The point is that one screen does not clear a molecule, not that any of these molecules is proven harmful.
Source: Sharma 2024 + Ramsey 2019, Biomolecules Link to this fact
Green, natural and unscented / Measured in people or real products
In a check of 100 'natural' personal-care products from 10 European natural-cosmetics brands, 82 listed at least one of the EU's labeled fragrance allergens (up to 11 in one product); limonene was on 71 labels and linalool on 70.
Moderate evidence: Label declarations only, no chemical analysis; 10 products from each of 10 anonymous brands, not representative; 2013-15, when the EU list had 26 allergens.
Source: Klaschka 2016, Environmental Sciences Europe Link to this fact
Green, natural and unscented / Measured in people or real products
All 16 UK day face moisturizers tested gave off fragrance terpenes (measured together as 'limonene'), and the 8 'green'-marketed ones released the same amount as the 8 regular ones (0.25 vs 0.26 µg per second per gram), as well as the same ethanol.
Moderate evidence.
Caveats
Lab headspace (SIFT-MS) of 16 products from UK shops; 'limonene' is the sum of all monoterpenes; no statistical test; green products did have less synthetic-only solvents (t-butyl alcohol, 1,3-butanediol).
Green, natural and unscented / Measured in people or real products
A headspace test of the fragranced and fragrance-free versions of the same detergent brand found 34 volatile chemicals in the fragranced version but only 7 in the fragrance-free version, and no fragrance terpenes (such as D-limonene or linalool) at all in the fragrance-free version.
Moderate evidence: Single product pair tested (one Australian brand); presence/absence only, not a full quantitative comparison.
Source: Goodman 2019, Air Quality, Atmosphere & Health Link to this fact
Green, natural and unscented / Measured in people or real products
Ethanol, linalool, and D-limonene, common fragrance and solvent terpenes found in both synthetic and "natural" scented products, were the single most-listed chemicals overall (about 27%, 16%, and 12% of products respectively) in a database of over 7,000 Korean household chemical products, ahead of synthetic fragrance aldehydes like hexyl cinnamaldehyde (about 8%).
Moderate evidence: Ingredient-list prevalence, not measured air or dermal exposure; does not distinguish products marketed as "natural"/"green" from conventional ones.
Source: Lee & Ji 2022, Ecotoxicology and Environmental Safety Link to this fact
Health effects / Measured in people or real products
Nitrate radicals alone, generated from ozone and nitrogen dioxide with no terpene present at all, raised human eye-blink frequency 21% — comparable in size to the effect of isoprene ozonolysis products — suggesting that some of the eye-irritating power of 'scented product plus ozone' indoor chemistry may come from reactive radicals in general, not from limonene specifically.
Moderate evidence.
Caveats
Single small study; the specific irritant chemical species were not identified or measured, only the percent of ozone chemically consumed. Not yet tested by chemical fractionation in any study in this evidence base.
Health effects / Measured in people or real products
In a 3-hour chamber test at realistic ozone and citrus-terpene (limonene) levels, people with mild asthma rated the air as less irritating to their eyes, nose, and throat than healthy people did, but reported more chest tightness, headache, and flu-like symptoms.
Moderate evidence.
Caveats
Single lab, n=33 mild asthmatics only (moderate/severe excluded by design), no unexposed/clean-air control arm, so this is a between-group comparison at one dose, not a test of the chemistry against no exposure.
Health effects / Measured in people or real products
People with mild asthma performed a concentration task significantly less accurately than healthy people during a 3-hour ozone-limonene chamber exposure, despite working faster.
Moderate evidence: Single study, no unexposed control arm, so it is not established that the chemistry (rather than general chamber occupancy or task load) caused the accuracy difference.
Health effects / Measured in people or real products
In the nasal-lavage arm of the same 130-woman chamber study, breathing a VOC-ozone mixture containing citrus and pine scent chemicals (limonene, alpha-pinene) caused no significant increase in nasal irritation symptoms or in objective inflammation markers (immune cells, IL-6, IL-8, protein) in nasal fluid.
Moderate evidence.
Caveats
Single 2-hour exposure in healthy, non-asthmatic, non-chemically-sensitive women; a different, smaller earlier study using a different lavage technique found a positive result at a similar VOC dose.
Source: Laumbach 2005, Journal of Occupational and Environmental Medicine Link to this fact
Health effects / Measured in people or real products
In a population-representative sample of about 2,900 Canadians (Canadian Health Measures Survey, 2012-2013), a 100% increase in measured residential indoor-air limonene was associated with a 17-18% relative increase in the adjusted odds of both wheezing (OR 1.17, 95% CI 1.16-1.18) and doctor-diagnosed asthma (OR 1.18, 95% CI 1.17-1.19), plus a 1.55% (95% CI 1.28-1.89) increase in exhaled nitric oxide (FeNO), an objective marker of airway inflammation.
Moderate evidence.
Caveats
Single cross-sectional study; cannot establish temporal sequence or fully rule out reverse causation, though the authors argue reverse causation would plausibly run the opposite way (airway-hypersensitive people avoiding scented products). Lung function (FEV1, FVC) showed no significant association with limonene.
Source: Dales & Cakmak 2019, Environmental Pollution Industry-funded Link to this fact
Health effects / Other evidence
The flavor industry's panel judged the food flavor d-limonene (also the most common fragrance terpene) still GRAS after the US National Toxicology Program found clear evidence of kidney tumors in male rats, reasoning the finding is irrelevant to human eating; this is a swallowed-route conclusion by the industry panel.
Moderate evidence: Industry panel's own reading; the male-rat alpha-2u-globulin mechanism is accepted by US EPA, but the call covers food intake, not skin or inhalation exposure to limonene.
Source: Hallagan 1995, Regulatory Toxicology and Pharmacology Industry-funded Link to this fact
Health effects / Other evidence
For 3 of the 18 flavorings (vanilla extract, cumin oil, apricot kernel oil) the panel found a child's lower body weight pushed intake above the generic safety threshold, and cleared each with a single animal no-effect level (ethyl vanillin 500 mg/kg/day in rats, D-limonene 215 mg/kg/day, apricot kernel oil 10 mg/kg/day in a one-dose 13-week feed study).
Moderate evidence: Each rescue relies on one animal study, in one case a surrogate chemical (ethyl vanillin for the vanillin group); industry panel's own judgment; no human child data.
Source: Guengerich 2026 (FEMA Expert Panel), Food and Chemical Toxicology Industry-funded Link to this fact
Information, not medical advice. See also: myths we won’t tell you.