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.
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
Across decades of US monitoring where people carried personal air samplers, the air they breathed held more of almost every hazardous chemical than outdoor air did: exposure happens indoors, not at the outdoor monitoring station.
Strong evidence: Older studies include smokers' homes; compound-specific.
Source: Weisel 2002, Environmental Health Perspectives 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
In a controlled chamber, the people themselves were the main thing reacting with indoor ozone: four seated adults removed about 60–65% of it, in two separate campaigns with men and with women, and their skin oils turned it into carbonyls such as 4-oxopentanal and 6-MHO. The same DTU chamber's protocol paper independently reproduced the ~60–69% removal in three more groups, teenagers and seniors as well as adults, across 26 experiments, and showed the sink is dermal (a breath-only control lost almost no ozone).
Strong evidence.
Caveats
Same DTU 22.5 m³ chamber and fan setup in all campaigns (Wang 2024: 4 men, ~37 ppb; Zannoni 2021: 2 groups of 2 women + 2 men, ~35 ppb; Bekö 2020: 5 groups — 3 adult, teenagers, seniors — same chamber/fans, ~35 ppb); mixing fans put deposition velocities at the top of the literature range, and a furnished home also loses ozone to surfaces, so the occupant share there will be lower. Chemistry only, no health endpoint; ICHEAR 6-MHO values may be inflated by an inlet artifact.
Source: Wang 2024, Environmental Science & Technology 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
Indoor air / Measured in people or real products
In unscented candles, formaldehyde and other carbonyl emissions stayed flat and low (39-54 micrograms per hour) regardless of wax type, wick type, or whether the flame was steady or flickering, which the study's authors attribute to the absence of added fragrance or dye, since those additives (not the wax or wick) are understood to be the main source of a candle's aldehyde emissions.
Moderate evidence.
Caveats
The attribution to fragrance/dye is the authors' interpretation, citing prior literature (Derudi et al. 2014), not a within-study comparison against scented versions of the same candles; this study tested no scented candles.
Indoor air / Measured in people or real products
Benzo[a]pyrene, a combustion-linked carcinogen, was measurable in 5 of 24 candle-burning experiments (up to 12 nanograms per candle per hour) and only in scented candles; none of the 4 unscented candles produced a detectable amount.
Moderate evidence: Small n (5/24 detections); detection near the analytical limit of quantification, which the authors themselves note adds uncertainty to the calculated exposure comparisons.
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
Indoor air / Measured in people or real products
One kiwi-melon scented candle emitted more total measured VOC while sitting unlit (12,742 ppbC) than while it was actually burning (2,766 ppbC) — more than 4 times as much — and the unlit figure was comparable to a published emission rate for a carpet cleaner.
Moderate evidence.
Caveats
TVOC is a summed, toluene-equivalent proxy, not a hazard-weighted measure; the carpet-cleaner comparison is to a different study's figures for a different product, not a side-by-side measurement.
Source: Ahn et al. 2015, Journal of Hazardous Materials Link to this fact
Indoor air / Measured in people or real products
Of every ingredient/route combination in this deodorant risk assessment, the tightest safety margin was for inhaling didecyldimethylammonium chloride (DDAC), a quaternary-ammonium biocide, from spray deodorants used at home or in cars: its calculated margin of exposure was 30, only about 1.2 times the regulatory target of 25, driven by an unusually low reference concentration for DDAC rather than by high measured exposure.
Moderate evidence.
Caveats
Still above the regulatory safety threshold in this study's own framework (no risk was formally identified); a single Korean government assessment, modeled exposure rather than measured personal or biomonitoring exposure.
Source: Kim et al. 2018, Science of The Total Environment Link to this fact
Indoor air / Other evidence
Burning scented candles releases VOCs, SVOCs, and particulate matter, including polycyclic aromatic hydrocarbons (benzo[a]pyrene, naphthalene, anthracene, pyrene) classified as carcinogenic.
Moderate evidence.
Caveats
This is a secondhand restatement in a two-page opinion piece, not a new measurement; our own chamber primaries (Salthammer 2021, Andersen 2021) independently confirm PAH/VOC output but find it strongly formulation-dependent rather than universal to all scented candles.
Source: Singh 2023, Environmental Science & Technology Link to this fact
Indoor air / Other evidence
In a small case series, removing mothballs and reducing scented cleaning/personal-care products, candles, and stored chemicals cut targeted indoor-air VOC categories by roughly a quarter to over 95% (e.g., naphthalene from mothballs down 96%, p-dichlorobenzene from moth crystals down 56-80%, odorants/fragrances down 26-80%) across three homes, measured by GC/MS air sampling before and after a structured home intervention.
Moderate evidence.
Caveats
Small, uncontrolled, non-blinded case series with multiple co-occurring intervention elements per home (product substitution, education, counseling, and in one case new chemical storage); cannot isolate the VOC-reduction effect from other changes.
Source: Rincón et al. 2024, Primary Health Care Research & Development Link to this fact
Indoor air / Animal study
Air brought from a federal office building under litigation for poor indoor air quality (carried to the lab in sealed bags) caused the same objective sensory-irritation signature in mice as the consumer-product tests, dropping their respiratory rate to about 46% of baseline before partial recovery; air from buildings people considered acceptable did not cause this effect.
Moderate evidence: Single building, single lab, one comparison air sample; the underlying human complaints were not independently verified against objective health measures in this paper.
Source: Anderson & Anderson 1999, Toxicology and Industrial Health Link to this fact
Indoor air / Other evidence
Spraying a trigger-type car air freshener for about a minute made a burst of fine particles in a parked SUV: particle counts at the driver seat rose from about 14,100 to as high as about 23,600 per cubic centimeter, and most of the particles were smaller than 100 nanometers.
Moderate evidence.
Caveats
One lab, three repeats, three unnamed sprays and three SUVs, stationary and engine off; particle chemistry, instrument size limits and health effects not reported, and no unsprayed control is described.
Source: Lee 2024, Science of The Total Environment Link to this fact
Indoor air / Other evidence
The same car sprays raised limonene, ethyl acetate, and cyclohexane in the cabin air; limonene at the driver seat in one medium SUV rose from about 10 to about 125 micrograms per cubic meter (about 165 with another spray).
Moderate evidence.
Caveats
Single small lab study; limonene values are read from a chart, so they are approximate; the small SUV had a much higher unexplained limonene baseline (about 280 micrograms per cubic meter before spraying, read from Fig. 5b3).
Source: Lee 2024, Science of The Total Environment Link to this fact
Indoor air / Self-reported survey
About 1 in 9 US adults (10.9%) said the scent of laundry products, fabric softeners, or dryer sheets vented outside irritates them (2005-06 survey; asked in the second survey only).
Moderate evidence: Self-report from a single survey; 'irritation' is not defined further and no exposure was measured.
Source: Caress & Steinemann 2009, Journal of Environmental Health Link to this fact
Indoor air / Other evidence
In a four-cigarette comparison, sidestream smoke carried far more of the nitrosamine NDMA (597-735 ng per cigarette) than the smoke a smoker inhales (4.3-31.1 ng), and about 2 to 4 times more benzo[a]pyrene in filtered cigarettes.
Moderate evidence: Single study of four cigarettes, compiled by industry authors; the catechol row's unit is misprinted so it is not used.
Source: Rodgman & Perfetti 2013 Industry-funded Link to this fact
Indoor air / Other evidence
In a controlled office experiment with 30 young women, more outdoor air (3, 10, then 30 liters per second per person) made the air seem fresher on entering (about 38% then 31% then 15% dissatisfied) and reduced dry mouth and throat, difficulty thinking clearly and feeling bad, while a carpet that gave off chemicals stayed in the room.
Moderate evidence.
Caveats
One 4.6-hour exposure per condition, 30 young female students, self-reported ratings, subjects could smell the difference on entering; percentages approximate (read from Fig. 4); the improvement in air quality during the sit was not significant.
Indoor air / Other evidence
A carpet used as the constant pollution source released about 127 micrograms of volatile chemicals per square meter per hour, including toluene, acetic acid, hexanal and small amounts of the scent terpenes limonene and alpha-pinene.
Moderate evidence: One used carpet, lab emission test at 23 C; terpenes were a small part of the mixture; not a fragrance product.
Indoor air / Other evidence
Soot (elemental carbon) measured inside classrooms was about 94% of the level outdoors, and higher indoor soot went with a 13% smaller one-year gain in working memory (4.2% to 23.1%) between the lowest and highest quarters of schools.
Moderate evidence: Single classroom per school, two 1-week measurements; observational association, not a test of filtration or ventilation.
Source: Sunyer 2015 (BREATHE, Barcelona), PLOS Medicine Link to this fact
Indoor air / Review of other studies
During ordinary house cleaning with bleach, chlorine gas concentrations can peak at up to about 0.5 parts per million, and trichloramine is thought to reach similar concentrations when bleach reacts with ammonia or other nitrogen-containing substances.
Weak evidence: This is the authors citing a separate domestic-cleaner cohort study (their own ref. 15) as background, not a measurement made on the children or homes in this study.
Source: Nickmilder, Carbonnelle & Bernard 2007, Pediatric Allergy and Immunology Link to this fact
Indoor air / Other evidence
In a 1996 Hong Kong pilot, five freshly dry-cleaned garments stored in a poorly ventilated, carpeted bedroom wardrobe pushed bedroom perchloroethylene (PCE) to 494 micrograms per cubic meter on the first day, against outdoor readings below about 1 microgram.
Weak evidence.
Caveats
One bedroom per ventilation condition (3 total), no randomization, 24-h tube samples; ventilation not measured during the tests; government-commissioned (no industry funding); not children or health outcomes.
Source: Chao 1998, Building and Environment Link to this fact
Indoor air / Other evidence
Hot, dry car cabins shift the particles from a spray smaller: at 25 degrees C the driver-seat nanoparticle share reached about 81% (versus about 65% at 10-20 degrees C).
Weak evidence: Single SUV and spray; the humidity explanation is the authors' interpretation; text and abstract disagree on the size of the rise (25.3 +/- 0.4% vs +/- 3.2%).
Source: Lee 2024, Science of The Total Environment Link to this fact
Indoor air / Other evidence
EPA's 2006 assessment found modeled inhalation margins for home mopping, wiping, trigger spray, aerosol deodorizer and carpet spray at 820 to 50,000 against a target of 100.
Weak evidence: Modeled with surrogate exposure data, an oral rabbit study as the inhalation endpoint, and judgment-based amounts handled; EPA itself called for confirmatory exposure data.
Source: EPA RED 2006 Link to this fact
Indoor air / Other evidence
The county quotes dry-cleaned-wool scenarios of perchloroethylene up to 240 milligrams per cubic meter in a sealed 3.4 m3 closet and 620 in a warm car, assuming all residual solvent vaporizes.
Weak evidence.
Caveats
Theoretical ceilings with no ventilation; the figures differ from the ppm values on our Sherlach 2011 page (about 50 and 126 ppm, roughly 340 and 850 mg/m3), so they are not reconciled; not measured exposure.
Source: SCDHS 2018 Link to this fact
Information, not medical advice. See also: myths we won’t tell you.