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.
Health effects / Other evidence
Daily sunscreen use produced significantly fewer squamous-cell carcinoma tumors on the treated sites (28 vs 46; rate ratio 0.61, 95% CI 0.46-0.81), but the number of people with squamous-cell carcinoma did not differ significantly (22 vs 25; rate ratio 0.88, 0.50-1.56).
Moderate evidence.
Caveats
The significant result is a secondary tumor-count analysis on small numbers; the authors say chance cannot be ruled out and person-based numbers were insufficient for conclusive statements; control arm used sunscreen at its own discretion.
Source: Green 1999, The Lancet Industry-funded Link to this fact
Health effects / Other evidence
Daily betacarotene supplements (30 mg) for 4.5 years neither prevented nor significantly increased basal-cell or squamous-cell skin cancer in a randomized trial of 1,621 adults.
Moderate evidence: Squamous-cell numbers were small (40 vs 28 people); the authors judged the nonsignificant SCC excess a chance finding; 4.5-year adult trial only; betacarotene supplied by Roche.
Source: Green 1999, The Lancet Industry-funded Link to this fact
Health effects / Other evidence
Contaminated cosmetics still reach the market: of 56 US-recalled products 45% carried Pseudomonas aeruginosa and 33% Burkholderia cepacia, and EU recalls from 2005 to May 2008 (24 products) were at least 42% P. aeruginosa.
Moderate evidence: Recalls are selected for detected problems; the denominator is recalled products, not products sold.
Health effects / Government agency
The EPA work plan relays a disputed literature on quat disinfectants and asthma (a French cohort with an adjusted odds ratio of 7.56, 95% CI 1.84-31.05, for physician-diagnosed asthma in exposed health-care workers, versus a hospital-clinic review calling such effects exceedingly rare) and takes no position, saying it will consider all epidemiology in its risk assessment.
Moderate evidence: EPA reports others' studies and the exchange between Weber and Pechter and Rosenman; it states no hazard classification. The Weber review is a single clinic's records.
Source: US EPA 2017 Link to this fact
Health effects / Other evidence
The more types of personal care product a man had used in the previous 48 hours (cologne, aftershave, hair gel, deodorant, lotion), the higher his urinary MEP, rising about 33% per additional product type.
Moderate evidence: Five product categories; very few men used four or five types (22 and 3); no health outcome measured.
Source: Duty 2005, Environmental Health Perspectives Link to this fact
Health effects / Review of other studies
Both too little and too much iodine in pregnancy are linked to subclinical hypothyroidism: urinary iodine of 250 ug/L or more carried odds ratios of 1.72 to 2.17, while low iodine in the first trimester carried OR 2.47, and severe deficiency causes endemic cretinism.
Moderate evidence: U-shaped relationship rests on single urinary iodine samples, a weak proxy for individual status; iodine is not a fragrance chemical.
Health effects / Other evidence
In a random survey of 1,582 adults in the Atlanta, Georgia metro area (1999-2000), 12.6% said they were allergic or unusually sensitive to everyday chemicals like cleaning products, paints, perfumes, detergents and insect spray, and 3.1% said a doctor had diagnosed multiple chemical sensitivity or environmental illness.
Moderate evidence: Self-report only, one metro area; sampling frame, interview mode and response rate are not reported in this brief; no clinical verification.
Source: Caress & Steinemann 2004 (AJPH), American Journal of Public Health Link to this fact
Health effects / Other evidence
About 7 in 10 Atlanta adults who called themselves chemically sensitive were women (71.7%), but about 3 in 10 were men (28.3%), and they spanned all ages and education levels.
Moderate evidence: Women were over-represented in the sample itself (60.7%), so the authors say the sample exaggerates the gap; no sex-specific prevalence is printed.
Source: Caress & Steinemann 2004 (AJPH), American Journal of Public Health Link to this fact
Health effects / Other evidence
In a 2005-06 national phone survey of 1,058 US adults, about 3 in 10 adults with asthma (31.4%) also said they are unusually sensitive to everyday chemicals, and about 1 in 3 of those chemically sensitive (34.9%) said they have asthma.
Moderate evidence: Self-reported answers, one cross-sectional survey, no clinical confirmation or confidence intervals for the overlap; shows co-occurrence, not cause or order.
Source: Caress & Steinemann 2009 (Toxicol Ind Health), Toxicology and Industrial Health Link to this fact
Health effects / Other evidence
In the same survey, adults with asthma more often said air fresheners cause them breathing problems (37.2%) and that scented products irritate them (38%) than the whole sample (20.5% and 29.9%).
Moderate evidence: Self-report; subgroup of 137; no exposure measured. Same dataset as the 2005-06 arm of the J Environ Health paper, not an independent replication.
Source: Caress & Steinemann 2009 (Toxicol Ind Health), Toxicology and Industrial Health Link to this fact
Health effects / Other evidence
A 2023 narrative review of paraben studies found that human TSH and thyroid-hormone results run in different directions depending on the paraben, the stage of pregnancy and the study; it calls the evidence scarce and conflicting.
Moderate evidence: Narrative review with no search protocol, study count or grading; the review gives few sample sizes; association only. Parabens are not fragrance ingredients.
Source: Azeredo 2023, International Journal of Molecular Sciences Link to this fact
Health effects / Review of other studies
A 2013 narrative review by Cardiff University psychiatric geneticists concluded that no clear-cut causal environmental risk factor for ADHD had been identified, apart from rare extreme early deprivation; prenatal smoking, alcohol, maternal stress, lead, pesticides and PCBs were graded as risks that are not proven causal.
Moderate evidence: Narrative review, no systematic search or risk-of-bias grading; literature to about 2012; the causal grades are the authors' judgment.
Source: Thapar 2013, Journal of Child Psychology and Psychiatry Link to this fact
Health effects / Other evidence
A 2025 systematic review of European children and adolescents with ADHD on methylphenidate found 7 eligible studies (2007-2016) and none of them examined genetic variants linked to sensory differences.
Moderate evidence: Absence of studies within a narrow filter (European only, methylphenidate, under 18, English, 2000-2024); not evidence that no genetic link exists.
Source: Recarey-Rama 2025, Journal of Psychiatric Research Link to this fact
Hidden ingredients / Measured in people or real products
When researchers tested 25 best-selling US laundry, cleaning, personal-care, and air-freshener products, they found 133 different chemicals released into the air, but only 1 of them was listed on any product label.
Moderate evidence: Single headspace sample per product; VOC counts depend on detection threshold.
Source: Steinemann 2011, Environmental Impact Assessment Review Link to this fact
Hidden ingredients / Self-reported survey
About two-thirds (64.6%) of US adults did not know fragrance ingredients don't have to be listed on the label, and more than half would prefer fragrance-free workplaces (53.2% vs 19.7% against) and health care (54.8% vs 22.4%).
Moderate evidence: Opinion survey.
Source: Steinemann 2016, Air Quality, Atmosphere & Health Link to this fact
Hidden ingredients / Self-reported survey
About a third of the personal care products people actually used were not in EWG's Skin Deep database at all, including 39% of perfumes and nearly all intimate-care products, so their hazards could not be rated.
Moderate evidence: Database coverage as of July 2022; EWG has since added products.
Source: Barrett 2025, Journal of Exposure Science & Environmental Epidemiology Link to this fact
Hidden ingredients / Review of other studies
Plastic particles are added to cosmetics on purpose for texture, a silky or powdery feel, opacity, shimmer, and as carriers for other ingredients, not only as scrub beads; and the EU chemicals agency's definition does not count liquid, semi-solid, or water-soluble polymers as microplastics.
Moderate evidence: Definition statement is the review's reading of ECHA; check against Regulation (EU) 2023/2055.
Hidden ingredients / Measured in people or real products
In 2018, testing commissioned by Breast Cancer Prevention Partners of 25 personal care and 7 cleaning products found chemicals flagged for cancer, hormone disruption, respiratory, or developmental hazards that were not on the label; 99 of the 124 were on the fragrance industry's own ingredient list.
Moderate evidence: Not peer reviewed; labs, detection limits, and concentrations not reported; hazard = database flag; IFRA list includes multifunctional solvents/preservatives.
Source: BCPP 2018 Link to this fact
Hidden ingredients / Advocacy group testing
In a 2010 survey of more than 11,000 Canadian personal-care products with ingredient lists, 'fragrance' or 'parfum' was the most common ingredient of concern, listed on 56% of products and on about two-thirds of shampoos, soaps, and deodorants.
Moderate evidence: Self-selected volunteers read labels; product entries not de-duplicated; 2010 formulations; label count is a lower bound on fragranced products.
Hidden ingredients / Review of other studies
FDA's own 1998 registry data, reproduced in full by the industry's cosmetic safety panel, show benzyl alcohol's biggest single cosmetic use was hair dye (130 of 1,572 formulations), with smaller counts in baby shampoo (5 of 21), baby lotion (3 of 53), moisturizers (8 of 769) and body/hand skin care (16 of 796).
Moderate evidence: Voluntary registry counts (Jan 1998); concentration-of-use reporting to FDA had already stopped by 1992 per the same report, so these are presence counts, not amounts used.
Source: Nair 2001 (CIR Expert Panel), International Journal of Toxicology Industry-funded Link to this fact
Hidden ingredients / Measured in people or real products
A 2024 lab study of six cosmetics found that ingredient labels can misdescribe the PFAS actually present: one product's named ingredient (a silane) appeared to be a different molecular isomer than the standard, and another's named phosphate-ester ingredient did not disclose that it contained shorter fluorocarbon chain lengths than its name implies.
Moderate evidence: Small sample (6 products from one country, 2 with naming issues); does not establish how common INCI-naming errors are across the wider market.
Source: Namazkar 2024, Environmental Science: Processes & Impacts Link to this fact
Hidden ingredients / Measured in people or real products
Benzyl alcohol, a fragrance allergen dubbed the "covert fragrance," was found in 32 of 533 US baby and children's personal care products (6%) with no "fragrance," "perfume," or "parfum" wording anywhere on the label.
Moderate evidence: Doesn't establish whether these products are marketed "fragrance-free"; measures label wording, not chemical content verification.
Hidden ingredients / Measured in people or real products
A common personal-care-product ingredient can appear on ingredient labels under several different chemical names (the silicone D5 may be listed as 'cyclopentasiloxane' or 'cyclomethicone'), which may not read as the same substance to a consumer trying to identify or avoid it.
Moderate evidence: Based on this study's own tested product labels and general INCI naming conventions for volatile methyl siloxanes, not a systematic label-disclosure survey.
Source: Coggon 2018, Environmental Science & Technology Link to this fact
Hidden ingredients / Other evidence
California state scientists estimate that the mass of PFAS in cosmetics is concentrated not in makeup but in shaving creams and gels, hair care products, facial cleansers, sun care products, and lotions and moisturizers, which together make up more than 90% of the total estimated PFAS mass in California cosmetics sold each year, while all makeup categories combined make up less than 3%.
Moderate evidence.
Caveats
Estimated from use-concentration ranges compiled from industry websites and patents that the authors say 'differed by orders of magnitude'; the model assumes 100% of PFAS reaches wastewater and exactly one PFAS ingredient per product; total mass estimates span roughly two orders of magnitude (650-56,000 kg/year).
Source: Balan 2024, Environmental Science & Technology Link to this fact
Hidden ingredients / Measured in people or real products
In a French study of 543 healthcare workers who used disinfectants, 99 (18.2%) could not be classified by exposure status from self-report alone, and researchers had to review Material Safety Data Sheets with 3 experts to determine which products contained quaternary ammonium compounds.
Moderate evidence.
Caveats
This describes a French occupational healthcare-worker population in 2006-2007, not consumers; it does not measure how well consumers can identify QACs in household cleaning products from labels today.
Source: Gonzalez 2014, Clinical & Experimental Allergy Link to this fact
Hidden ingredients / Measured in people or real products
Besides PFOA, gamma-irradiated PTFE also released smaller amounts of related short-chain perfluorocarboxylic acids: about 46 ppb perfluorobutanoic acid (PFBA), 26 ppb perfluoropentanoic acid, 20 ppb perfluorohexanoic acid, and 15 ppb perfluoroheptanoic acid, and, when irradiated in an air-filled sealed bag, a wider range of these acids from 4 to 13 carbons.
Moderate evidence.
Caveats
Measured in laboratory-irradiated PTFE samples, not a finished product; short-chain PFAS are generally considered less bioaccumulative than PFOA but some (e.g. PFHxA) remain under regulatory scrutiny — this evidence base has not separately assessed their toxicology.
Source: Oshima et al. 2020, Scientific Reports Industry-funded Link to this fact
Hidden ingredients / Review of other studies
Body odor intensity has a genetic component: people carrying the ABCC11 gene variant linked to dry earwax (common in East Asian populations) had less body odor and lower levels of the odor-carrier protein apolipoprotein D than people without it, in the study this review cites.
Moderate evidence.
Caveats
Single cited primary study relayed by this review, not independently verified here; ABCC11's role in foot odor and isovaleric acid formation is separately noted but not from the same study.
Source: Kanlayavattanakul & Lourith 2011, International Journal of Cosmetic Science Link to this fact
Hidden ingredients / Animal study
GC-MS of the scented candle's emissions detected methylene chloride (2.71% of peak area, a carcinogen with two known toxic-metabolite pathways) and the phthalate DEHP (0.1%), neither of which would appear on a candle's fragrance or wax label.
Moderate evidence.
Caveats
Detected in headspace/combustion emissions, not necessarily added as ingredients; percentages are of total VOC peak area, not of candle mass, and the specific candle's ingredient list is not published.
Source: Mohamed et al. 2025, Frontiers in Public Health Link to this fact
Hidden ingredients / Measured in people or real products
FDA's 2010 follow-up testing of 84 consumer cosmetic products found the fragrance solvent diethyl phthalate (DEP) in all 11 fragrance products tested (up to 36,006 ug/g, about 3.6% by weight) and in 20 of 60 adult-use products overall, while dibutyl phthalate (DBP) was found only in nail products (11 of 24, up to 62,607 ug/g), even though phthalates carried in as part of 'fragrance' are not required to be named on the label.
Moderate evidence.
Caveats
Single FDA laboratory follow-up survey, 84 products bought in or near Washington DC or online before August 2010 (exact purchase date not stated); HPLC-UV product concentration, not absorbed dose.
Source: Hubinger 2010, Journal of Cosmetic Science Link to this fact
Hidden ingredients / Government agency
DEA and its condensates (cocamide DEA, lauramide DEA) are not banned from cosmetics, but US Cosmetic Ingredient Review guidance says they should not be combined with nitrite or other nitrosating agents in a formulation, because that reaction can form N-nitrosodiethanolamine (NDELA), an animal carcinogen that is not itself listed on any ingredient label.
Moderate evidence.
Caveats
This paper restates the regulatory rationale (citing 1983/1986 CIR positions) but does not itself measure NDELA formation in any tested product; the formation-measurement gap is still open in our evidence base.
Source: Kraeling 2004, Food and Chemical Toxicology Link to this fact
Hidden ingredients / Government agency
EPA lists moth repellents and air fresheners among household-product sources of organic-gas (VOC) indoor air pollution, and notes that paradichlorobenzene, a chemical carrying an 'avoid breathing vapors' warning on moth-repellent labels, is also the key active ingredient in many air fresheners.
Moderate evidence: EPA names the shared ingredient and the moth-repellent warning; it does not separately assess air-freshener-specific health risk.
Source: US EPA, "The Inside Story" Link to this fact
Hidden ingredients / Review of other studies
Danish product checks cited in a 2004 review found parabens in 77% of 215 cosmetics (0.01-0.87%) and 87% of skin creams, with ingredient labels wrong about parabens in about 10% of products.
Moderate evidence: Secondhand from Rastogi 1995 and 2000, and from 1990s Danish and Swedish products; not re-checked in this evidence base; current formulations differ.
Source: [[src-2004-endocrine-disrupters-estrogenic-chemicals-body-care-cosmetics|Harvey & Darbre 2004]], Journal of Applied Toxicology Link to this fact
Hidden ingredients / Other evidence
Under an FDA bulk-labeling rule the flavor industry relies on, a multi-ingredient flavor can carry one sentence ('All flavor ingredients contained in this product are approved for use in a regulation of the Food and Drug Administration') instead of a list of each ingredient, provided every flavor ingredient is approved in an FDA regulation; non-flavor ingredients and flavor ingredients outside such a regulation must be listed separately.
Moderate evidence.
Caveats
1995 description by FEMA's own authors, who add that FDA treated FEMA GRAS lists as reliable industry lists for this purpose. The 2024 CFR text of 21 CFR 101.22(g)(2) names only FDA regulations and says nothing about industry lists.
Source: Hallagan 1995, Regulatory Toxicology and Pharmacology Industry-funded Link to this fact
Hidden ingredients / Other evidence
The FEMA panel does not require toxicology tests on each flavor ingredient: for the 119 ring-structured terpene flavors it judged safety mainly from the metabolism and tests of a few parent compounds (such as menthol, carvone, camphor-type ketones) and from low estimated intake.
Moderate evidence.
Caveats
This is the industry panel's own method statement; the paper argues it is scientifically sound because related substances share metabolic pathways, and does not show case-by-case failure.
Source: Adams 1996, Food and Chemical Toxicology Industry-funded Link to this fact
Hidden ingredients / Other evidence
Several flavor chemicals in this review (menthol, carvone, camphor, borneol, terpineol, ionones, damascones and the musks muscone, and civettone) are also fragrance materials, but the FEMA review assesses them only as swallowed food ingredients; it gives non-oral data 'less weight' and has no inhalation or skin-allergy assessment.
Moderate evidence: The paper says some inhalation data may substitute when flavor exposure is far smaller than inhaled exposure; it does not claim any airborne or skin risk from these substances.
Source: Adams 1996, Food and Chemical Toxicology Industry-funded Link to this fact
Hidden ingredients / Other evidence
In 2008, about one in five (19.5%, 6,463 of 33,212) cosmetics and personal care products in the US FDA's voluntary registry contained a formaldehyde-releasing preservative, including 27.3% of rinse-off products and 16.9% of leave-on products.
Moderate evidence: The registry is voluntary, so it may not represent all US products; 2008 data, and the authors found no comparable European database.
Hidden ingredients / Other evidence
FEMA's 2009 list also affirmed sodium lauryl sulfate (FEMA 4437, 0.001 ppm), 2-phenoxyethanol (FEMA 4620, up to 8 ppm in baked goods) and camphor (FEMA 4513, up to 100 ppm in chewing gum) as flavoring substances, so a food 'flavor' can include chemicals better known from personal care products.
Moderate evidence: Industry's own use-level survey; food only; the usual and maximum levels are averages.
Source: Smith et al. 2009 (FEMA Expert Panel), GRAS Flavoring Substances 24, Food Technology 63(6), Food Technology Industry-funded Link to this fact
Hidden ingredients / Other evidence
A personal care product can be EWG Verified even if some of its ingredients do not score green on Skin Deep; EWG requires the overall product score to be green.
Moderate evidence: EWG's own FAQ; the scoring method is described elsewhere and was not reviewed here.
Source: EWG Verified FAQ, verified-portal.ewg.org, accessed 2026-10-16, EWG Verified portal web page 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
Only 41% (62 of 150) of the "Clean" textured-hair products at this Target had a score in EWG's Skin Deep database, and none carried the EWG Verified mark, so most could not be looked up.
Moderate evidence: One store, scraped 2022 and matched to EWG in 2023; how products were matched is not described, and how scored and unscored products differ is unknown.
Source: Madrid Larrañaga 2026, Journal of Exposure Science & Environmental Epidemiology 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
Almost 9 in 10 (88%, 57 of 65) top-rated Amazon sunscreens carried extra claims such as sensitive skin, preservative-free, natural or organic; the authors call these marketing claims, not performance standards.
Moderate evidence: Claims were recorded as printed; no ingredient lists were checked against them in this paper.
Hidden ingredients / Other evidence
A 2022 study found benzyl alcohol in 15.8% of 1,470 skin care products labeled clean, natural, organic or hypoallergenic.
Moderate evidence: Secondary citation; the figure is for those four label terms, not specifically 'fragrance-free'; product sampling frame not described here.
Source: Zheng et al. 2026, ACDS Allergen of the Year (Cutis), Cutis Link to this fact
Hidden ingredients / Review of other studies
A 2021 consensus essay names diethyl phthalate (DEP) and dibutyl phthalate among phthalates used in nail polish, lotions, fragrances and hair-styling products, and says studies show personal-care-product use tracks with urinary phthalate metabolites.
Moderate evidence: One sentence citing a single exposure study (Berger and colleagues); no concentrations given; fragrance is mentioned only in that list.
Source: Engel 2021, American Journal of Public Health Link to this fact
Indoor air / Measured in people or real products
In a European study that measured the air 182 adults actually breathed over 48 hours, exposure to pine- and citrus-type scent chemicals from cleaning products and fragrances rose with the number of children at home (3-carene median 0.9 to 2.5 µg/m³); the authors attribute this to more cleaning.
Moderate evidence: Late-1990s, small subgroups, attribution to cleaning is the authors' inference; CEFIC-LRI co-funded.
Source: Edwards 2006, Atmospheric Environment Industry-funded 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
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 / 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 / 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
With four people in a test room and indoor ozone at about 35 ppb, the chemical reactivity of the air roughly tripled once ozone was added (from about 8–11 to 25–35 per second). Almost all of the increase came from carbonyls made when ozone hit the natural oils on skin, and across the whole study ozone explained 85% of the variation.
Moderate evidence: One chamber (22.5 m³, 3.2 h⁻¹, mixing fans), one ozone level tested on/off; no health endpoint; fragrance-free products used throughout.
Source: Zannoni 2021, Environmental Science & Technology 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
Nearly all (93%) of the DC childcare facilities studied used chlorine bleach to sanitize surfaces, and chloroform (a bleach byproduct classified by IARC as possibly carcinogenic to humans, Group 2B) was detected in 100% of air samples; the authors attribute it mainly to bleach use.
Moderate evidence: Association, not a measured reaction; bleach usage self-reported, chloroform sources not otherwise investigated.
Source: Quirós-Alcalá 2016, Environmental Research Link to this fact
Indoor air / Animal study
Of five common ozone-terpene reaction products tested in mice (4-AMCH, dihydrocarvone, IPOH, 6-MHO, and 4-OPA), the study's own conclusion was that four do not contribute substantially to airway irritation at indoor or ambient concentrations; only IPOH (sensory irritation) and 4-OPA (airflow limitation) were flagged as compounds of concern warranting precaution against excess formation.
Moderate evidence: Single animal study, one lab (NRCWE Copenhagen); precautionary conclusion, not evidence that current indoor exposure causes harm.
Source: Wolkoff et al. 2012/2013, Toxicology Letters Link to this fact
Indoor air / Measured in people or real products
A personal-care product containing the silicone D5 (such as a deodorant applied hours earlier) can raise D5 levels inside a car far above outdoor background, and this passenger-carried D5 vents out of the car and mixes with the vehicle's exhaust-associated emissions when the cabin fan runs.
Moderate evidence.
Caveats
Small controlled experiment (4 passengers, 1 vehicle, plus one deodorant-in-car test); a car cabin is a distinct, small, enclosed microenvironment, not home or building indoor air, which this paper does not measure.
Source: Coggon 2018, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
Dryer-vent air carried far more of the citrus-scent chemical D-limonene when households used fragranced laundry detergent (2.35-118 ug/m3, mean 33.34) than when they used fragrance-free detergent (0.13-1.50 ug/m3 after switching; 0.24-0.26 ug/m3 in households that never used fragranced detergent).
Moderate evidence.
Caveats
6 Melbourne-area homes, D-limonene only (not the fuller VOC/HAP panel), no statistical significance testing; shares one author (Anne Steinemann) with the earlier 2-home study it builds on, so not a fully independent replication.
Source: Goodman 2019, Air Quality, Atmosphere & Health Link to this fact
Indoor air / Self-reported survey
In a small (n=69) follow-up of Atlanta-area adults with chemical hypersensitivity, the products most commonly reported to trigger symptoms were cleaning agents (88.4%), pesticides (81.2%) and perfume (81.2%), followed by tobacco smoke (82.6%) and car exhaust (72.5%).
Moderate evidence: Small, non-random follow-up sample (n=69, about 35% of the original 199 hypersensitive respondents); self-report of trigger identification, not exposure measurement.
Source: Caress & Steinemann 2003 (EHP), Environmental Health Perspectives Link to this fact
Indoor air / Self-reported survey
The 11 household chemical products in this UK cohort's prenatal composite exposure score were used, in descending order of prevalence, as follows: disinfectant (87%), bleach (85%), aerosols (72%), air fresheners (68%), window cleaner (61%), carpet cleaner (36%), paint or varnish (33%), turpentine/white spirit (23%), pesticides/insect killers (21%), paint stripper (6%) and dry-cleaning fluid (5%).
Moderate evidence.
Caveats
Descriptive prevalence data, not an effect estimate. The 11 products were selected from an initial list of 15 by keeping only those used by at least 5% of the study sample, so rarer products are excluded from the score.
Indoor air / Measured in people or real products
Ordinary indoor ozone reacts with the natural oils on everyone's skin (mainly squalene) to produce a family of carbonyl and dicarbonyl air pollutants — with no fragrance, cleaning product, or air freshener needed. In a simulated two-person office at everyday occupied-room ozone levels (14-18 ppb), two of the compounds (6-MHO and 4-oxopentanal) reached 0.5-2.3 ppb.
Moderate evidence: Single research group; a small 28.5 m3 chamber with only 2 occupants, not a real home; concentrations scale with occupancy and ozone level.
Source: Wisthaler & Weschler 2010, Proceedings of the National Academy of Sciences Link to this fact
Indoor air / Measured in people or real products
Using real-time air analysis (PTR-MS), researchers identified three dicarbonyl compounds and tentatively identified two hydroxy-ketone compounds from ozone reacting with skin oil that had never been reported before, none of which are picked up by the sampling methods routinely used to survey indoor air pollutants.
Moderate evidence: Single-lab chemical identification (bench-scale and simulated-office experiments); not yet independently replicated by another group.
Source: Wisthaler & Weschler 2010, Proceedings of the National Academy of Sciences Link to this fact
Indoor air / Measured in people or real products
In this study, carbonyl compounds coming off human skin were negligible when no ozone was present, and appeared within minutes once ozone was introduced — suggesting these carbonyls are produced by ozone reacting with skin oils rather than being emitted endogenously by the skin itself, as some earlier surveys of human skin volatiles had assumed.
Moderate evidence.
Caveats
Based on a small in vivo skin-enclosure experiment (forehead/cheek/forearm) in this one paper; the earlier 'endogenous' literature it challenges (e.g. Gallagher et al. 2008) is not yet in this evidence base and may not have controlled for ambient ozone during its own sampling, which could reconcile rather than truly contradict the two findings.
Source: Wisthaler & Weschler 2010, Proceedings of the National Academy of Sciences Link to this fact
Information, not medical advice. See also: myths we won’t tell you.