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 / Review of other studies
A chemical released indoors is roughly 1,000 times more likely to be breathed in than the same amount released outdoors (exact when comparing the low end of each range; only ~100x at the high end, since the ranges overlap at 10⁻³), so cleaning products and air fresheners (about 1% of California's smog-forming emissions) may deliver an inhaled dose on the same scale as all outdoor sources combined.
Strong evidence.
Caveats
Modeled population intake fractions (not measurements), population-average estimate. Traced to the primary source, Lai, Thatcher & Nazaroff 2000 (JAWMA 50:1688-1699), which derives outdoor PITF 10⁻⁶-10⁻³ vs indoor/in-vehicle PITF 10⁻³-10⁻¹ from Gaussian-plume and mass-balance modeling (Nazaroff co-author of both papers, so this is not independent replication). The ranges overlap at 10⁻³, so '1000x' is a floor-to-floor comparison, not a fixed multiplier under all conditions.
Source: Nazaroff & Weschler 2004, Atmospheric Environment Link to this fact
Indoor air / Review of other studies
Scent chemicals such as limonene and pine oil react with ozone indoors to make new pollutants, including formaldehyde and fine particles, that keep forming for hours after cleaning ends.
Strong evidence: Size of the effect depends on dose and indoor ozone; confirmed by later chamber and field studies.
Source: Nazaroff & Weschler 2004, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
Questionnaires about habits and routines correctly sorted only 20% of people by their measured exposure to cleaning and fragrance chemicals, vs 61% for traffic fumes; exposure depends on individual behavior and product choice.
Strong evidence.
Caveats
'Product choice' is the authors' explanation, not directly tested in this paper; independently corroborated by a larger, more direct 2021 UK study (Heeley-Hill 2021, 60 homes, daily diary-logged — not recalled — product-use frequency vs measured concentrations) that found cumulative product-use frequency has essentially no predictive power for measured indoor VOC concentrations (R²<0.001 for total VOC; no individual VOC significant).
Source: Edwards 2006, Atmospheric Environment Industry-funded Link to this fact
Indoor air / Measured in people or real products
In week-long air measurements across 11 European cities, the scent chemicals limonene and alpha-pinene were the compounds most concentrated in homes compared with workplaces (about 4x), attributed to perfumes, cleaning and fragranced products.
Strong evidence: Source attribution by the authors, not measured per product.
Source: Geiss 2011, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
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
Scent chemicals in cleaners and air fresheners react with ozone that drifts in from outdoors to make formaldehyde: cleaning raised formaldehyde by 9-16 ppb for 4 hours, and a plug-in air freshener raised it by about 6 ppb for as long as it ran.
Moderate evidence.
Caveats
Secondary formation only, depends on indoor ozone. The underlying mechanism is now directly quantified at bench scale (Destaillats 2006: formaldehyde 16-38% per ozone consumed for the same three products), but that bench study is the same research group/products/contract as this one, not an independent replication, so the grade is not raised on that basis alone.
Source: Singer 2006, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
After 2-7 minutes of cleaning with a terpene cleaner, indoor air chemistry stayed altered for 10-12 hours because the scent chemicals soaked into walls and floors and came back out.
Moderate evidence.
Caveats
Empty chamber; furnished rooms may differ. Singer 2006 (Indoor Air primary-emissions companion, same 50 m3 room without ozone) corroborates the sorption/desorption mechanism directly: alpha-terpineol stays at 260-700 ug/m3 for hours and ~90 ug/m3 at 4-24 h via surface water films and wallboard sorption, and towels left in the room raise 24-h emissions 35-100%. Not independent (same lab/room/products as the ozone paper).
Source: Singer 2006, Atmospheric Environment 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 / 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 / Review of other studies
The standard week-long air tests used across Europe miss the short spikes of chemicals released after cleaning, which could change the risk picture for eye and airway irritation.
Moderate evidence: Authors' methodological caveat, not a measured result.
Source: Sarigiannis 2011, Environment International Link to this fact
Indoor air / Measured in people or real products
Indoor ozone reacts with the natural oils on our skin. In a test room with four people and 35 ppb ozone, this alone kept highly reactive hydroxyl (OH) radicals at about 7×10⁵ per cm³, similar to levels measured in empty rooms after cleaning with pine or citrus products.
Moderate evidence.
Caveats
High-ozone condition (35 ppb, 3.2 air changes/h, mixing fans that push ozone uptake to the top of the literature range); at the typical ~5 ppb indoor ozone the authors' model gives ~3×10⁴, 25–30× less. The companion ES&T paper (Zannoni 2021) replicates the OH-reactivity side (8–11 → 25–35 s⁻¹, ozone 85% of variance) but tested ozone only on/off, not a dose-response, and reports a geranyl acetone → 6-MHO inlet artifact that may overstate 6-MHO's share. No fragrance or health outcome studied. The ICHEAR protocol paper (Bekö 2020) shows the underlying skin-oil chemistry (6-MHO, geranyl acetone, 4-OPA) is reproducible across five groups spanning teenagers to seniors, but it reports no OH concentration data itself, so the 7×10⁵ cm⁻³ figure still rests on only the original three adult groups; it also found isoprene — the basis of this paper's alternative "precursor-product" OH cross-check — has poor day-to-day repeatability independent of the clothing explanation offered for the two methods' 1.7× disagreement.
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 / Measured in people or real products
Leaving used paper towels in the room after cleaning increased the solvent and scent chemicals released by 35-100%.
Moderate evidence: Measured over 24 h in an empty chamber.
Source: Singer 2006, Indoor Air, Indoor Air 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
The UK home with the heaviest, most frequent use of cleaning and fragranced products (9 different products, some used more than 10 times in a week, plus a scented candle) had a 5-day average D-limonene concentration of up to 1,439 ug/m3, around 80 times the study's median home and possibly the highest domestic value reported in the literature at the time.
Moderate evidence: Single home out of 25 driving the extreme; 5-day average, not a short-term peak, which the authors state was likely higher.
Source: Wang 2017, Environmental Science: Processes & Impacts Link to this fact
Indoor air / Measured in people or real products
Even with daily diary records of exactly how often 60 households used 13 different types of scented and cleaning products over three days, how often people used these products did not predict how much VOC was in their home's air (R-squared less than 0.001 for total VOC; no individual VOC, including limonene, showed a significant relationship).
Moderate evidence: Frequency of use, not dose per use or product formulation, was recorded; a narrow, weak exception existed for limonene with insecticide/plug-in-air-freshener use specifically.
Source: Heeley-Hill 2021, Environmental Science: Processes & Impacts Industry-funded Link to this fact
Indoor air / Other evidence
Methacrolein, a reaction product of ozone and isoprene (a natural component of human breath), can form indoors or in the airway itself whenever an ozone source is present, with no fragrance or cleaning product needed.
Moderate evidence.
Caveats
The formation reaction itself is not measured in this paper; it is cited as the motivation for the study. Later chamber work (Zannoni 2022) directly measured methacrolein forming from breath isoprene + indoor OH radicals in an occupied, product-free room.
Source: Larsen & Nielsen 2000, Toxicology Letters Industry-funded Link to this fact
Indoor air / Other evidence
The one real-world indoor measurement of gas-phase methacrolein in the wiki (309 ng/m3, during ozone-terpene cleaning-product chemistry) sits about 2,800 times below this paper's own proposed occupational exposure limit for methacrolein, and about 96,000 times below the mouse RD50.
Moderate evidence.
Caveats
Compares an acute, occupational-limit-oriented mouse endpoint to a short measured cleaning-event concentration in one test house; not a chronic or population exposure estimate. See contradictions register C63.
Source: Larsen & Nielsen 2000, Toxicology Letters Industry-funded 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
After adjusting for outdoor PM2.5, season, cooking, cleaning, and smoking, incense-burning events were the largest contributor to indoor PM2.5 among all monitored indoor activities in 32 Hanoi homes, raising the geometric mean of total indoor PM2.5 by 120.3% (versus +71.6% for cooking, +52.2% for smoking, +40.5% for cleaning).
Moderate evidence: Single-city, single-cohort study with no independent replication; percentages are specific to this sample's burn frequency, ventilation, and background air quality.
Source: Tran 2021, Building and Environment 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 / Measured in people or real products
Hotel housekeepers' real occupational personal exposure to fragrance terpenes (mainly limonene and alpha-pinene) from direct, repeated contact with cleaning and laundry products averaged just 7.7 µg/m3, even though every cleaning product tested in the study contained terpenes (up to 64 µg/mL in one smoke-removal spray).
Moderate evidence: Occupational, not general-population exposure; short sampling window (6-9h); small sample.
Indoor air / Measured in people or real products
In this hotel study, airborne formaldehyde — the single biggest driver of estimated health risk — did not track terpene-fragrance levels or season (terpene-ozone secondary chemistry would be expected to rise with summer ozone), leading the authors to attribute it instead to building materials, furnishings, and the large volume of bedding and towels used in hotels, not to the terpene-containing cleaning and laundry products.
Moderate evidence: Attribution is inferential, not source-tested directly; the authors call for further study to verify formaldehyde sources in hotels.
Indoor air / Measured in people or real products
In a 2021 pilot study of 20 real nail salons in the NYC area, toluene (median 7.03 ppb) and d-limonene (median 4.21 ppb) were the two most abundant of 31 volatile organic compounds measured in the air, and the authors attribute the d-limonene to cleaning and disinfection products rather than nail products, since d-limonene is not a nail-product ingredient.
Moderate evidence.
Caveats
Small convenience sample (n=20), area/micro-environmental air sampling only (not personal exposure), sampled during COVID-19 when cleaning frequency was likely elevated; source attribution to cleaning products is the authors' inference, not measured (no product-ingredient data collected).
Source: Han et al. 2022 Link to this fact
Indoor air / Measured in people or real products
Nail salons serving 30 or more customers per day had significantly higher air d-limonene (median 6.18 ppb) than salons serving fewer than 30 customers (median 2.30 ppb, p=0.044), consistent with more frequent cleaning of tables between customers.
Moderate evidence: Single small pilot study; cleaning-product use itself was not directly measured or recorded, only inferred from customer volume and observed cleaning behavior.
Source: Han et al. 2022 Link to this fact
Indoor air / Other evidence
Ordinary spray-and-wipe cleaning with quats, bleach, hydrogen peroxide, or a thymol-based botanical disinfectant raised indoor fine-particle (PM2.5) levels by 0.7-14.5 micrograms per cubic meter and total-VOC sensor readings by 10-104 ppbv, measured across a residential bathroom, a university office, two chemistry labs, and a hotel in Canada.
Moderate evidence: Single study, small replicate counts per cleaner/location, and the TVOC sensor is explicitly non-quantitative.
Source: Souza 2025, ACS Earth Space Chem, ACS Earth and Space Chemistry Link to this fact
Indoor air / Other evidence
The lowest-ventilation room tested, a residential bathroom with an air change rate under 1 per hour, showed 146-346% higher cleaning-related PM2.5 increases and about 7-fold higher TVOC increases than a university office and lab with 5.9-9.1 air changes per hour, for the same cleaning products.
Moderate evidence: One bathroom, one office, two labs; not a dose-response series across many ACR values.
Source: Souza 2025, ACS Earth Space Chem, ACS Earth and Space Chemistry Link to this fact
Indoor air / Other evidence
The particulate matter released during cleaning in this study looked like direct, mechanical spray-bottle emission rather than new particles forming from chemical reactions in the air, consistent across all four very different cleaning chemistries tested.
Moderate evidence.
Caveats
No ozone was deliberately added or measured in these experiments, and none of the four products was a terpene-rich scented cleaner, so this does not rule out secondary particle formation under different (ozone-rich, terpene-rich) conditions documented elsewhere.
Source: Souza 2025, ACS Earth Space Chem, ACS Earth and Space Chemistry Link to this fact
Indoor air / Other evidence
Bleach cleaning produced a two-humped (bimodal) rise in total VOC readings, a pattern consistent with bleach's chlorine-based oxidants reacting with ordinary indoor VOCs to form new, oxidized compounds, rather than only releasing VOCs directly.
Moderate evidence: The oxidized compounds were not chemically identified in this study; the PID gives only a total, non-specific VOC signal, so this is an inferred mechanism, not a confirmed one.
Source: Souza 2025, ACS Earth Space Chem, ACS Earth and Space Chemistry Link to this fact
Indoor air / Other evidence
During real COVID-19 disinfection at a hotel, repeated use of a quats-based disinfectant raised fine-particle levels by 5.5 to 14.2 micrograms per cubic meter in the lobby, elevator, and a public bathroom, exposing cleaning staff to repeated particle bumps throughout a work shift.
Moderate evidence.
Caveats
One day of field measurement, one hotel; cleaning protocol there also deviated from the product label (solution left on surfaces rather than wiped off) to reduce staff vapor exposure.
Source: Souza 2025, ACS Earth Space Chem, ACS Earth and Space Chemistry Link to this fact
Indoor air / Other evidence
A 2011 Georgetown study that sent test fabrics through seven Northern Virginia dry cleaners found perchloroethylene (PCE) solvent still in polyester, wool, and cotton after one cleaning at all five shops that use PCE, about 10-57 nmol/cm2 depending on fabric and shop.
Moderate evidence.
Caveats
One jacket and one cleaning per shop, unpressed swatches sewn into the lining, seven shops in one region; residue measured, not what anyone breathes or absorbs. No industry funding stated.
Source: Sherlach 2011, Environmental Toxicology and Chemistry Link to this fact
Indoor air / Other evidence
In a lab chamber with a very high ozone level (about 6.5 ppm), all six ordinary perfumes, deodorants and cleaning liquids tested formed new particles; about 0.2% to 17% of the product's evaporated mass became aerosol, with an average near 4%.
Moderate evidence.
Caveats
One lab, six products, a closed 0.68 m3 chamber at roughly 100 times typical indoor ozone, very dry air, no vapor wall-loss correction; a yield, not an indoor concentration. NSF and Sloan funded; no industry funding.
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
The authors judged that the PCE from dry-cleaned clothing is not itself a major health concern in Hong Kong because most households dry-clean rarely and ventilate, but high levels can occur in poorly ventilated homes with frequent dry cleaning.
Weak evidence.
Caveats
Authors' judgment from 3 rooms and a 500-household phone survey (64.6% dry-clean; 5% every 2-3 days, 34% every few months, 39% yearly); their own tables show some days above the CARB benchmark at all three sites.
Source: Chao 1998, Building and Environment Link to this fact
Indoor air / Other evidence
Wool dry cleaned repeatedly at the same shop kept absorbing more PCE with each cycle (about 47 nmol/cm2 after one cleaning to about 195 after six, read from Fig. 5), while cotton and polyester leveled off.
Weak evidence: One shop, one jacket, 1-6 cycles, replicate extractions not independent garments; values read from a chart.
Source: Sherlach 2011, Environmental Toxicology and Chemistry Link to this fact
Indoor air / Other evidence
OSHA says concentrations of many VOCs are consistently higher indoors, 'up to ten times higher', than outdoors, and lists cleaning supplies and pesticides among the thousands of product sources.
Weak evidence: A blanket multiplier with no data in the booklet; EPA pages say 2 to 5 times; primary data show compound-specific ratios from about 1 to about 60.
Source: OSHA 3430-04, 2011 Link to this fact
Indoor air / Other evidence
Someone doing their own house cleaning in a region with moderate outdoor ozone was estimated to inhale about 20 micrograms a day of formaldehyde and 35 micrograms a day of new particles from ozone reacting with cleaning-product and air-freshener ingredients.
Weak evidence.
Caveats
A modeled estimate borrowed from another review, not a measurement, for cleaning products and air fresheners together, not for perfume. It adds to formaldehyde and particles from other sources.
Source: Weschler 2006, Environmental Health Perspectives Link to this fact
Indoor air / Other evidence
Two products in the same category can behave very differently: with seed particles present, one deodorant turned about 17% of its evaporated mass into aerosol and the other about 6% (about, read from Fig. 2), and the two cleaning liquids ran about 5% and 12%.
Weak evidence: Approximate values read from Fig. 2; two products per category; the authors say the products are not representative of the market.
Information, not medical advice. See also: myths we won’t tell you.