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
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
Health effects / Measured in people or real products
The 17 fragrances together contained 24 chemicals EWG classed as skin sensitizers, about 10 per product (19 in Armani Acqua Di Gio); limonene was in 16 products, linalool in 14, and geraniol in 12.
Moderate evidence: Presence is lab- or label-based; sensitizer status is EWG's literature classification, not a potency or dose assessment; the sensitizer table omits one of the 17 products.
Source: Sarantis et al. 2010 (Not So Sexy, Canadian edition) Link to this fact
Hidden ingredients / Review of other studies
Sanitary pads, panty liners and tampons can contain undisclosed fragrance allergens, chiefly alpha-isomethyl ionone (in every scented tampon tested), hexyl cinnamal and benzyl salicylate, plus trace limonene, linalool and geraniol, and in the EU they do not have to declare any of them.
Moderate evidence.
Caveats
Primary source (Desmedt 2020, validated HS-GC-MS/FID, 10 scented Belgian AHPs) now in our evidence base. The compounds actually found above the EU 10 ug/g labeling threshold were alpha-isomethyl ionone (all 4 tampons, 12.6-19.1 ug/g), hexyl cinnamal, and benzyl salicylate (one panty liner); limonene, linalool and geraniol were all trace (<10 ug/g) — the original claim named the wrong allergens as the notable ones. Graded moderate: rests on one product-testing study relayed through a review.
Source: Pastor-Nieto 2021 (Desmedt 2020), Current Treatment Options in Allergy Link to this fact
Hidden ingredients / Animal study
The air freshener's exact formulation is proprietary; GC/MS of its emissions identified toluene, limonene, alpha- and beta-pinene, terpinolene, sabinene, and about a dozen other volatiles the consumer has no way to learn about from the label.
Moderate evidence: Qualitative GC/MS of one product, one test atmosphere, recoveries unknown; only peaks with 90%+ library-match certainty reported; brand unnamed.
Source: Anderson & Anderson 1997, Archives of Environmental Health Link to this fact
Hidden ingredients / Measured in people or real products
The chemical composition of all five fabric-softener dryer-sheet products tested was undisclosed (a trade secret); gas chromatography/mass spectrometry of one scented brand's emissions found only petroleum-derived aromatic compounds (toluene, styrene, xylenes, trimethylbenzene, isopropylbenzene) plus phenol and thymol — no fragrance terpenes, esters, or alcohols were detected, even though prior published analyses had found such compounds (limonene, linalool, benzyl acetate, and others) in fabric-softener emissions generally.
Moderate evidence.
Caveats
Only one of the five brands was chemically analyzed, and only qualitatively (recovery percentages unknown); about half of the detected GC/MS peaks could not be identified at all, so unidentified compounds — possibly including fragrance materials — cannot be ruled out.
Source: Anderson & Anderson 2000, Journal of Toxicology and Environmental Health, Part A Link to this fact
Hidden ingredients / Measured in people or real products
In a database analysis of Korean household chemical products, the most prevalent chemicals flagged as endocrine disruptors were compounds added as fragrance ingredients or preservatives (e.g. linalool, D-limonene, hexyl cinnamaldehyde), not the product's main functional ingredient.
Moderate evidence.
Caveats
Ingredient-list matching against EPA CompTox/ToxCast hazard flags, not measured concentrations or confirmed toxicity in the products as sold; single-country (South Korea) database of household chemical products (air fresheners, cleaners, detergents, fabric softeners, deodorants), not personal-care/cosmetic products specifically.
Source: Lee & Ji 2022, Ecotoxicology and Environmental Safety Link to this fact
Hidden ingredients / Review of other studies
d-Limonene, a common fragrance/scent chemical, was detected in 100% of personal and area air samples in 7 nail salons despite not appearing as an ingredient on any safety data sheet the authors reviewed for the nail products used there.
Moderate evidence.
Caveats
Authors could not identify the actual source (possible fragrance classification, or an unreviewed product category such as lotions, scrubs, waxes or cleaning products); small pilot (7 salons).
Hidden ingredients / Measured in people or real products
Di(2-ethylhexyl) phthalate (DEHP), a plasticizer used to make nail polish more flexible and durable, was the most abundant of six phthalates measured on silicone wristbands in all 10 nail salons tested (median 391.80 ng/g-wristband), with diethyl phthalate (DEP) second highest (median 154.4 ng/g-wristband) — but the paper never discusses what produces its measured DEP, unlike its explicit attribution of DEHP to nail polish and d-limonene to cleaning/fragrance products.
Moderate evidence.
Caveats
Small sample (n=10 for phthalates); wristband measurement is a sorbed dose, not a direct air concentration; the paper's own figures and Discussion text mislabel this unit as ppb instead of the Methods/footnote's ng/g-wristband, an internal inconsistency in the published article.
Source: Han et al. 2022 Link to this fact
Hidden ingredients / Animal study
In a Dutch government study, isoeugenol was the only one of five skin-allergy fragrance chemicals to stimulate the lymph nodes of mice that breathed it (about 7-fold more cell growth after 6 hours a day for 3 days), while limonene and linalool did not.
Moderate evidence.
Caveats
Mice (6 per group), one lab, nose-only at 75 mg/m3, an unvalidated assay tested mostly on strong sensitizers; a hazard signal, not human disease. Government-funded (NVWA), no industry money declared.
Source: ter Burg 2014, Inhalation Toxicology Link to this fact
Hidden ingredients / Other evidence
Many chemicals on the founding 1965 FEMA flavor list are also common fragrance ingredients or labeled contact allergens, among them eugenol, isoeugenol, cinnamaldehyde, citral, linalool, geraniol, hydroxycitronellal, and d-limonene; the list covers eating them at the reported food levels, not breathing or skin contact.
Moderate evidence: FEMA numbers read from scanned OCR and checked on the page image only for eugenol and methyl eugenol; the paper does not discuss fragrance use.
Source: Hall & Oser 1965 (FEMA GRAS 3), Food Technology Industry-funded Link to this fact
Hidden ingredients / Other evidence
Wet cleaning rests on water, but the detergents and spotting agents hold alcohol ethoxylates, glycol ethers, D-limonene, monoethanolamine, quaternary ammonia and trade secrets; many safety data sheets lacked ingredient data, some noted skin and eye irritation, and ethoxylated alcohols can carry 1,4-dioxane (county study pending).
Moderate evidence: From the county's review of a sample of safety data sheets, not testing of products or garments; the report still rated wet cleaning health-green.
Source: SCDHS 2018 Link to this fact
Hidden ingredients / Other evidence
The hydroperoxides that cause this allergy are not listed on labels, so patients can only look for limonene and linalool, two of the 26 EU-declarable fragrance allergens.
Moderate evidence: Authors' statement about labeling in the EU; limonene and linalool on a label show the parent chemical, not whether any hydroperoxide is present.
Source: Dittmar & Schuttelaar 2019, Contact Dermatitis Link to this fact
Hidden ingredients / Other evidence
Limonene, linalool and Lyral are fragrance ingredients commonly found in personal care products, processed foods and beverages, and perfumes, and the society judges them relevant for many patch-tested patients.
Moderate evidence: Committee judgment based on cited reviews; no product-survey data in this paper.
Hidden ingredients / Other evidence
The fragrance terpenes in the restroom's urinal screens (alpha-terpinene, linalool, limonene, linalyl acetate and others, about 20 ppb total in one test) were the main chemicals that reacted with the lamp-made ozone.
Moderate evidence: Single room; deodorizer screens, not household fragrance; alpha-terpinene dominance is specific to this product.
Source: Link 2024, Environmental Science: Processes & Impacts Link to this fact
Hidden ingredients / Other evidence
Limonene and beta-pinene scent terpenes were measurable in every perfume, deodorant and cleaning liquid tested (about 0.1-0.3% and 0.04-0.65% of product weight), even though labels only say 'fragrance' or, for the cleaners, name limonene.
Moderate evidence.
Caveats
Six products; GC-MS of three terpenes only, so other scent chemicals are unmeasured; alpha-pinene was below the detection range. The abstract's beta-pinene range (0.4-0.7%) differs from Table 1 (0.04-0.65%).
Indoor air / Measured in people or real products
Scented laundry products put fragrance chemicals into outdoor air: in two homes, limonene (a citrus scent chemical) came out of the dryer vent only when fragranced detergent or dryer sheets were used (up to 14.7 ug/m3), not in the no-product runs.
Moderate evidence.
Caveats
2 homes (Steinemann 2013), single grab samples; undiluted vent air, not a breathing-zone dose. Replicated at much higher magnitude and with a dose-response signal by Goodman et al. 2019 (6 Melbourne homes, before/after switch design): fragranced-detergent vent D-limonene 2.35-118 ug/m3 vs 0.24-0.26 ug/m3 in fragrance-free-only homes, and switching from fragranced to fragrance-free detergent cut vent D-limonene by up to 99.7% (mean 79.1%) within a month. The two studies share one author (Anne Steinemann) and neither ran statistical significance tests, so the combined evidence (8 homes total) stays below strong, but the qualitative and dose-response finding is now cross-country replicated within one research program rather than resting on a single 2-home study.
Source: Steinemann 2013, Air Quality, Atmosphere & Health Link to this fact
Indoor air / Measured in people or real products
Keeping a gel air freshener in a car raised the fragrance chemical limonene in the air the driver breathed 3.6-4.7 times, to about 19-28 ug/m3, during real commutes (10 cars tested).
Moderate evidence: N=5 per group, non-randomized; mostly drivers' own products.
Indoor air / Measured in people or real products
In lab tests, gel air fresheners released limonene at up to 1.5 mg per hour, far more than any other chemical measured.
Moderate evidence.
Caveats
Chamber emission rates; products chosen for high limonene (upper end). Singer 2006 (Indoor Air) found the opposite pattern in a different air-freshener type (plug-in scented oil): limonene was a minor emission (39 mg/day) versus non-reactive esters (bornyl acetate 460, benzyl acetate 320 mg/day). 'Limonene dominates' holds for the limonene-selected gels Jo 2008 tested, not for air fresheners generally.
Indoor air / Measured in people or real products
Indoors, vapors from these products run about 7 times higher than outdoors, and include fragrance chemicals such as limonene.
Moderate evidence: Box model checked against indoor measurements.
Indoor air / Measured in people or real products
During mopping with a terpene cleaner, scent chemicals such as limonene and pinene peaked at 280-380 ppb in the room air.
Moderate evidence.
Caveats
Brief peaks; typical home averages 3-7 ppb (EXPOLIS/Gokhale/Sarigiannis) — but Wang 2017 found one heavily fragranced/cleaned UK home's 5-day AVERAGE reached ~258 ppb, within the same order of magnitude as this mopping peak, showing the 'brief peak vs low average' distinction does not hold for the heaviest product-use homes.
Indoor air / Measured in people or real products
A limonene-scented foam cleaner released formaldehyde, an irritant and carcinogen, both directly and through reactions with ozone after use.
Moderate evidence.
Caveats
Primary formaldehyde source ingredient unidentified (Rossignol's own ISO chamber measured 25.6 ug/m2/h at 0-30 min without ozone). Destaillats 2006 tested three different terpene-containing products (pine-oil cleaner, orange-oil degreaser, plug-in air freshener) under controlled bench conditions and found NO primary formaldehyde in any of them (negligible before ozone was added). This does not contradict Rossignol's own direct measurement of a different, specific product, but it shows primary formaldehyde is not a general feature of terpene/fragrance cleaners, supporting the reading that Rossignol's primary source is a formulation additive (e.g. a preservative or surfactant impurity) rather than the fragrance/terpene component itself.
Source: Rossignol 2013, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
Scent was only part of what a scented cleaner released: its glycol-ether solvent reached about 1,100 ug/m3 in room air against 71 ug/m3 for limonene.
Moderate evidence.
Caveats
One product; Singer 2006 (Indoor Air, 5 more US cleaners, 50 m3 room) found the opposite pattern in a cleaner containing both a glycol ether and limonene (limonene 3-5x the 2-BE), so 'solvent exceeds scent' is product-specific, not general.
Source: Rossignol 2013, Atmospheric Environment Link to this fact
Indoor air / Review of other studies
EU researchers attributed large swings in home limonene levels within the same city to whether people had used a fragrance, air freshener, or cleaning product before the air was sampled.
Moderate evidence.
Caveats
Authors' interpretation of variance; a larger, more rigorous 60-home UK diary study (Heeley-Hill 2021) found cumulative product-use frequency has little predictive power for measured concentrations (R²<0.001), with only weak covariance between limonene and insecticide/plug-in-air-freshener use specifically, not general fragrance/cleaning-product use Note the exposure measures differ: this claim concerns product use shortly before sampling, whereas Heeley-Hill 2021 tested cumulative use frequency over the sampling period, so it does not directly test the claim. Evidence is mixed: Wang 2017 (25 UK homes) found the highest-limonene homes were heavy product users; Heeley-Hill 2021 (60 UK homes; fieldwork funded by Givaudan, a fragrance manufacturer; analysis by the University of York) found frequency predicted little.
Source: Sarigiannis 2011, Environment International Link to this fact
Indoor air / Lab study (cells or chemistry)
In lab chamber tests set up to mimic a room, the citrus scent chemical limonene reacted with ozone in the air to form fine particles carrying reactive oxygen species.
Moderate evidence.
Caveats
Chamber concentrations exceed typical measured indoor exposure by 5-10x. Particle formation is confirmed in a human chamber (Fiedler 2005), but ROS were not measured there and 130 women showed no acute effect; don't pair this claim with 'linked to irritation'. Wolkoff et al. 2008 (now read directly, the true primary behind this line of evidence) is a mouse gas-vs-particle denuder study showing denuded limonene-ozone-reaction ultrafine particles (~9.5-12.0 mg/m3, gas phase >95% removed) cause NO sensory irritation or airflow-limitation effect at all, statistically indistinguishable from clean air — reinforcing, from a different technique (whole-body mouse bioassay vs in-vitro ROS assay), that particle-bound ROS formation should not be equated with a demonstrated health effect.
Indoor air / Lab study (cells or chemistry)
Limonene produced 2 to 11 times more of these reactive particles than alpha-pinene, the pine scent chemical.
Moderate evidence: Chamber comparison only.
Indoor air / Measured in people or real products
Pure limonene only irritates human eyes at about 445 mg/m³, far above indoor levels. A limonene-ozone mixture reacted for just 10 minutes raises eye-blink rates at roughly 1,000-2,000x less limonene: Klenø & Wolkoff (2004, n=8, primary) found residual 0.42 mg/m³ limonene + 0.08 mg/m³ ozone raised blink frequency 42% (P<0.0001); Nøjgaard (2005, n=10, primary) found a lower dose, 0.23 mg/m³ limonene + 0.08 mg/m³ ozone, gave a smaller but still significant +17% (p=0.003).
Moderate evidence.
Caveats
Fadeyi et al. 2015 (now read directly; n=71 — 38 non-asthmatic, 33 mild asthmatic, corrects an earlier undifferentiated count — 3 h, whole-body) found no significant eye/nose/throat irritation difference between asthma-status groups, at doses comparable-to-LOWER than Kleno/Nojgaard's residual concentrations, not higher as previously stated: limonene 35-36 ppb vs. Kleno's 75 ppb / Nojgaard's 41 ppb, and ozone 20-37 ppb (rising) vs. their steady ~41 ppb. This is a different endpoint (self-reported symptom rating vs. objective blink count), and the study has no unexposed/clean-air control arm, so 'no significant' is a between-group comparison with ratings staying below the scale's 'slight irritation' anchor, not a formal null against no exposure. Not a refutation of the blink-frequency result, but it tempers real-world relevance at doses at or below the LOEL pair, over a much longer (3 h vs 20 min), whole-body exposure. Laumbach et al. 2005 (n=130, same chamber cohort as Fiedler 2005) adds a whole-body objective-biomarker null at a higher combined terpene dose (limonene 125 ppb + alpha-pinene 160 ppb, with ozone, 135 min): no nasal symptom or nasal-lavage PMN/IL-6/IL-8/protein change. It's a different route and endpoint from the single-eye blink studies, so it doesn't refute them, but it weighs against generalizing them.
Source: Wolkoff 2013, International Journal of Hygiene and Environmental Health Link to this fact
Indoor air / Other evidence
In mouse tests, about three-fourths of the airway irritation from limonene reacting with ozone came from formaldehyde and residual limonene together (formaldehyde the dominant contributor, at roughly 17-22 percentage points; residual limonene a small but real contributor, under 5 percentage points), not from the particles.
Moderate evidence.
Caveats
Animal data at ppm-range reactants; the author's own lab; not independently replicated by another group. This is now grounded directly in the true primary, Wolkoff et al. 2008 (*Toxicol Lett* 181:171-176), read in full: its own Abstract and Conclusion state formaldehyde and residual limonene COMBINED explain 'up to three-fourth' (~75%), correcting this page's earlier paraphrase ('two-thirds, formaldehyde only, excluding limonene') which had both the wrong number and the wrong attribution. The entire conducting-airway (VD/VT, airflow-limitation) effect's causal chemical remains unidentified in the primary.
Source: Wolkoff 2013, International Journal of Hygiene and Environmental Health Link to this fact
Indoor air / Review of other studies
An indoor-air toxicologist recommends measuring limonene in offices because this common fragrance 'easily undergoes ozonolysis producing oxidation products that includes formaldehyde'.
Moderate evidence.
Caveats
Wolkoff & Nielsen 2017 (same author) reinforces the same recommendation-level framing and adds a quantitative synthesis ('>200 ug/m3 ozone at high limonene concentrations would be necessary to expect sensory airway and eye effects'), consistent with human nulls in Fiedler 2005 and Fadeyi 2015. Laumbach et al. 2005 (n=130, now read directly, the Fiedler cohort's nasal-lavage companion) adds a null on the one endpoint most directly relevant to a monitoring recommendation for a chemical that 'easily undergoes ozonolysis': no nasal symptom or objective nasal-inflammation-marker change despite the same ozonolysis chemistry occurring (formaldehyde and SOA both confirmed to rise during exposure). This keeps the claim at the level of a monitoring/indicator recommendation, not a finding of harm at current office levels.
Source: Wolkoff 2013, International Journal of Hygiene and Environmental Health Link to this fact
Indoor air / Measured in people or real products
A new Little Tree car air freshener releases most of its limonene in the first few days: in a closed car the limonene signal halved roughly every day and was nearly gone within 4-5 days.
Moderate evidence: Relative peak areas only, no concentrations; one car and one scent; baseline not shown.
Source: Pino-Delgado 2021 (J Student Res), Journal of Student Research Link to this fact
Indoor air / Measured in people or real products
When 40 ppb of ozone was added to a room holding a mix of 23 common indoor chemicals, including the citrus and pine scent chemicals limonene and alpha-pinene, formaldehyde tripled (13 to 40 µg/m³), ultrafine particles rose about 18-fold (2,500 to 46,000 per cm³) and particle mass reached 140 µg/m³.
Moderate evidence: Chamber with VOC load ~10x typical buildings; 23-compound office mixture, not a fragranced product; other alkenes also react.
Source: Fiedler 2005, Environmental Health Perspectives Link to this fact
Indoor air / Measured in people or real products
Cleaning with a pine-oil or citrus cleaner raised scent chemicals (limonene, terpinolene, alpha-terpineol) to about 1,000-6,000 ug/m3 for the first hour in a room-sized chamber, and some stayed elevated for many hours afterwards.
Moderate evidence: Empty chamber; one product per formulation; alpha-terpineol's persistence depends on surface water films and towels left in the room.
Source: Singer 2006, Indoor Air, Indoor Air Link to this fact
Indoor air / Test-chamber measurement
A car air freshener's own terpenes (alpha-pinene, beta-pinene, p-cymene, limonene) react with in-vehicle ozone to form ultrafine particles and irritant gases (formaldehyde, acetaldehyde, acrolein) that never appear on the product's ingredient list, because they are made in the cabin air, not sold in the bottle.
Moderate evidence.
Caveats
N=1 product (oil-based liquid, citrus); single run per condition, no replicates; the 100 ppb ozone condition exceeds the paper's own cited in-vehicle ozone range (2-86 ppb) and RH/ventilation were excluded, so treat the numbers as chemistry demonstration, not a real-commute dose.
Source: Lamorena & Lee 2008 (J Hazard Mater), Journal of Hazardous Materials Link to this fact
Indoor air / Measured in people or real products
In a real working primary school, mopping a classroom with a limonene-containing detergent produced bursts of tens of thousands of tiny new particles per cubic centimeter within minutes, but only when ozone was present in the air; a plain water-only mop produced no such burst.
Moderate evidence.
Caveats
One school, one detergent; particle number not mass. Classroom mopping peak ~35,000/cm3; matched controlled lecture-theater trials with the same detergent confirmed peaks up to ~68,500/cm3.
Source: Morawska 2009, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
A limonene-cleaning event needs indoor ozone above roughly 5 parts per billion to trigger a detectable burst of new ultrafine particles; below that threshold, in this study's controlled trials, essentially no particle formation occurred.
Moderate evidence.
Caveats
One product, one 140 m3 lecture theater, one air-exchange rate (1.08/h); threshold is specific to this detergent's limonene loading and may not generalize to other products or rooms.
Source: Morawska 2009, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
A 10-minute-old reacted mixture of limonene and ozone (residual 75 ppb limonene + 40 ppb ozone) raised human eye-blink frequency 42% relative to clean air, while the same men's eyes exposed separately to just limonene, or just ozone, at similar or higher levels, did not blink more.
Moderate evidence.
Caveats
Small (n=8), single-lab, local single-eye exposure (not whole-body). Starting limonene (220 ppb) and residual limonene (75 ppb) are one to two orders of magnitude above typical indoor levels (usually <10 ppb, up to ~70 ppb in cleaning agents/new housing, per the authors' own Introduction). A same-lab follow-up re-test of a nominally comparable mixture gave a 3.5x smaller effect (12% vs 42%), so the magnitude has not replicated within the same group.
Indoor air / Review of other studies
The four most common airborne fragrances in homes and offices, pinene, limonene, linalool, and eugenol, are usually present at levels 2 to 3 times below the concentration needed to physically irritate the eyes and airways, but at or above the much lower level needed just to smell them.
Moderate evidence.
Caveats
Sensory-irritation thresholds here are derived (mouse RD50 values run through a regression, or human LOAELs divided by an assessment factor of 5), not all directly measured; this restates the same author group's own 2013 conclusion for an updated audience, not independent confirmation.
Source: Wolkoff & Nielsen 2017, Environment International Link to this fact
Information, not medical advice. See also: myths we won’t tell you.