Every fact on this site, with its evidence grade, its caveats, and a link to the source.
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
In the two largest, most realistic human tests to date, breathing a fragrance-and-ozone reaction mixture (the kind formed when a citrus-scented product meets indoor ozone) produced no measurable increase in eye, nose or throat irritation, in either healthy people, or asthmatics.
Moderate evidence.
Caveats
Both are single, short (140 min / 3 h) acute exposures using self-reported symptom scales, not objective blink rate; neither tests repeated exposure or more severe asthma, and the doses are still above many typical indoor levels. Fadeyi 2015 had no clean-air control arm (its null is 'ratings stayed below slight irritation', not a test against no exposure); Laumbach 2005 did have a masked-air control. Neither refutes the single-eye blink-rate findings (different route and endpoint).
Source: Wolkoff & Nielsen 2017, Environment International Link to this fact
Indoor air / Animal study
A Danish government toxicology lab derived a specific reference value for 4-oxopentanal (4-OPA), a common byproduct when ozone reacts with skin oils and fragrance terpenes indoors: 30 ppb (123 ug/m3), from mouse bronchoconstriction data with standard safety margins applied. Measured office and aircraft-cabin 4-OPA levels (2-10 ppb) sit below this value, though ventilation-filter and occupied-room measurements (up to 10-16 ppb) come closer.
Moderate evidence.
Caveats
Single-lab animal bioassay (60-min, ppm-range mouse exposures) extrapolated to humans via assessment factors; no direct human 4-OPA exposure or epidemiology data exist anywhere in our evidence base. A 2014 corrigendum corrected two unrelated numbers in the same results table; the 30 ppb 4-OPA airflow-limitation value itself is unaffected.
Source: Wolkoff et al. 2012/2013, Toxicology Letters 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
In a study of 25 UK homes using real whole-air canister sampling (not a lab chamber), the scent chemicals D-limonene and alpha-pinene were the single most abundant VOC measured in 94% of homes, regardless of building age, size, or occupancy.
Moderate evidence.
Caveats
N=25 homes (19 London + 6 York), one country; dominance among 8 quantified VOCs, not an absolute-concentration health claim A larger same-lab study (Heeley-Hill 2021, 60 UK homes, daily diaries; Givaudan-funded fieldwork) found cumulative product-use frequency had almost no predictive power for measured VOC levels (R²<0.001), so treat the product-use link as suggestive.
Source: Wang 2017, Environmental Science: Processes & Impacts 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
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
In a study of 60 UK homes with real air sampling (not a lab chamber), the VOC found in the highest concentration indoors was n-butane, an aerosol-spray propellant, not a fragrance chemical; it reached a wintertime maximum of 4,630 ug/m3, and aerosol antiperspirant/deodorant was the single most universally used VOC-releasing product, used in all 60 homes.
Moderate evidence: One UK town's panel-recruited cohort (Ashford, Kent); Givaudan-funded (fieldwork), analysis independent at University of York.
Source: Heeley-Hill 2021, Environmental Science: Processes & Impacts Industry-funded Link to this fact
Indoor air / Measured in people or real products
Indoor VOC levels exceeded outdoor levels in the large majority of homes studied — 84% in summer and 100% in winter — confirming that for this cohort, indoor air, not outdoor air, was the dominant source of VOC exposure.
Moderate evidence: Outdoor samples were not individually paired with every indoor home/period; mean seasonal outdoor values used for comparison.
Source: Heeley-Hill 2021, Environmental Science: Processes & Impacts Industry-funded 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 / Animal study
In a mouse study, when scientists physically separated the invisible ultrafine particles from the gas fumes made by citrus-scent limonene reacting with ozone, the airway-irritating effect disappeared entirely, showing it's the gas products, led by formaldehyde, that irritate, not the particles themselves.
Moderate evidence.
Caveats
Single lab, not independently replicated. Mouse ppm-range reactant concentrations are several orders of magnitude above real indoor limonene/ozone levels; this is a causation/mechanism study, not an exposure-response study at realistic doses.
Source: Wolkoff et al. 2008, Toxicology Letters Link to this fact
Indoor air / Animal study
In the same mouse study, formaldehyde formed by the limonene-ozone reaction, together with the leftover limonene itself, accounted for roughly three-quarters of the airway-irritating effect measured; formaldehyde was the dominant contributor.
Moderate evidence.
Caveats
Single lab, single animal study; the estimate for formaldehyde and limonene's individual contributions is model-based (from the group's own earlier single-compound dose-response studies), not a direct blocking experiment. The remaining ~25% of the effect, and the entire conducting-airway (airflow-limitation) effect's causal chemical, are unidentified.
Source: Wolkoff et al. 2008, Toxicology Letters 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 / Measured in people or real products
Switching to fragrance-free laundry detergent also reduced D-limonene in the laundry room's own indoor air, not just at the dryer vent, by up to 72.7% (mean 45.8%) within one month.
Moderate evidence: 4 households, single background-air sample per round, no statistical testing.
Source: Goodman 2019, Air Quality, Atmosphere & Health Link to this fact
Indoor air / Measured in people or real products
Every one of 25 fragranced laundry products tested (US and Australian detergents and dryer sheets) emitted a terpene fragrance chemical (such as limonene, alpha-pinene, or eucalyptol), while none of 10 fragrance-free laundry products emitted any terpene at all.
Moderate evidence.
Caveats
This paper restates chemical-panel data from Steinemann 2015 and Nematollahi 2018/2019, which are not yet independently checked in this evidence base; treat the 100%-vs-0% split as a secondary report until those primaries are read directly.
Source: Goodman, Nematollahi & Steinemann 2020/2021, Air Quality, Atmosphere & Health Link to this fact
Indoor air / Other evidence
The 25 mg/m3 total-VOC chamber dose that produced a measurable lung-function decline in asthmatics is, in the authors' own framing, a maximal concentration seen in new houses but only a moderate one in the work environment — not a typical existing-home level.
Moderate evidence: This is the paper's own characterization of the dose, not a broad survey of home VOC levels for our cross-paper comparison.
Source: Harving et al. 1991, American Review of Respiratory Disease Link to this fact
Indoor air / Other evidence
The 25 milligram per cubic meter total-VOC chamber dose used in this and related studies to demonstrate airway inflammation is representative of a new home or office (i.e., freshly built or renovated), not of typical existing homes, which run far lower.
Moderate evidence.
Caveats
The paper itself only states the mixture is representative of new construction; the comparison to typical existing-home levels being far lower is a evidence base cross-reference, not this paper's own measurement.
Source: Koren et al. 1992, Archives of Environmental Health: An International Journal 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 / Animal study
One fabric-softener pad left for 24 hours in a small, nearly sealed room (about 5 cubic meters, essentially no forced ventilation) raised bulk air TVOC to only 130 ppm — far below the same pad's raw emissions in a closed test chamber (190-750 ppm) — yet still caused significant sensory irritation in exposed mice (61% of breaths).
Moderate evidence.
Caveats
The room had essentially no forced air exchange for 24 hours, an extreme low-ventilation scenario rather than a normally ventilated home; TVOC is an unspeciated bulk flame-ionization reading, not a compound-specific, µg/m3-scale concentration comparable to real household measurements.
Source: Anderson & Anderson 2000, Journal of Toxicology and Environmental Health, Part A Link to this fact
Indoor air / Test-chamber measurement
A 21-chemical VOC mixture modeled on indoor air caused dose-related increases in respiratory, eye, and other symptoms in 15 healthy adults, worsening as the chamber concentration rose from 25 to 50 mg/m3 over 4-hour exposures.
Moderate evidence: Both tested concentrations (25 and 50 mg/m3) are roughly 100-1,000x higher than typical home indoor-air VOC levels, so this does not show ordinary indoor air causes these symptoms.
Source: Pappas et al. 2000, Int J Occup Environ Health 6:1-8 Link to this fact
Indoor air / Test-chamber measurement
In a chamber test at a realistic indoor ozone level (~40 ppb), adding ozone to an indoor-representative mixture containing the fragrance terpenes d-limonene and alpha-pinene generated formaldehyde (13 to 40 micrograms per cubic meter), several other irritant aldehydes, organic acids, and fine secondary particle pollution (140 micrograms per cubic meter, versus under 5 without ozone).
Moderate evidence.
Caveats
Single chamber study, one research group; the total VOC and terpene concentrations used (6.6 ppm total VOC, 125-160 ppb terpenes) were deliberately set above typical nonindustrial indoor air, by the authors' own statement, to match a specific human-exposure protocol.
Source: Fan et al. 2003, Environmental Science & Technology Link to this fact
Indoor air / Test-chamber measurement
Removing just the two fragrance terpenes (d-limonene and alpha-pinene) from the 23-chemical mixture, while keeping ozone and the other 21 chemicals, collapsed the secondary particle pollution to background levels (2-7 micrograms per cubic meter) and eliminated most of the irritant aldehydes — identifying these two terpenes as the drivers of essentially all the ozone-initiated chemistry in this mixture.
Moderate evidence.
Caveats
Single chamber study; the 21 non-terpene VOC in the mixture are mostly saturated compounds that react very slowly with ozone, so this result may not generalize to mixtures containing other reactive unsaturated compounds.
Source: Fan et al. 2003, Environmental Science & Technology Link to this fact
Indoor air / Test-chamber measurement
A meaningful share of the formaldehyde (56-70%) and fine-particle mass (19-29%) generated when ozone reacts with fragrance terpenes comes from hydroxyl radicals that the ozone-terpene reaction itself produces as a byproduct, not from ozone reacting directly with the terpene or other chemicals.
Moderate evidence.
Caveats
The hydroxyl radical concentration itself was not measured directly but estimated from a mass-balance model using rate constants published in other studies; one of the model's own predictions (formaldehyde in the terpene-free mixture) did not match the measured value and the paper says the reason is unclear.
Source: Fan et al. 2003, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
Among 170 UK homes monitored for a year with passive VOC samplers, daily or near-daily use of air fresheners or of other household aerosols/sprays during pregnancy was statistically associated with the home's indoor total volatile organic compound (TVOC) concentration being in the top quartile measured across the study.
Moderate evidence.
Caveats
Association based on chi-square/Fisher's Exact tests in 146-148 homes, not a controlled dose comparison; measured home TVOC levels were, per the authors, well below levels established as producing acute effects in occupational settings.
Source: Farrow 2003, Archives of Environmental Health: An International Journal 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
A single person in a typical room is estimated to remove about 10-25% of the room's ozone just through skin-surface chemistry, before any ventilation, wall, or furnishing losses are counted.
Moderate evidence.
Caveats
Derived from two occupants in one 28.5 m3 office chamber (deposition velocities 0.4-0.5 cm/s), then extrapolated to a generic 30 m3 room with typical total removal rates from the literature; not a direct measurement in a real, furnished home.
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 / Journalism
An independent indoor-air chemist (Charles Weschler) cautioned that the reported dryer-vent acetaldehyde concentrations are high enough to raise sensory-irritation concern if dryers are vented indoors, whether intentionally via indoor dryer-vent kits or unintentionally through equipment malfunction.
Moderate evidence: An expert's stated concern about a hypothetical indoor-venting scenario, not a measurement of indoor air; the underlying study measured only outdoor vent emissions.
Source: Kessler 2011 (EHP Forum), Environmental Health Perspectives Link to this fact
Indoor air / Review of other studies
Regulatory limits for airborne irritants like formaldehyde and ammonia are typically derived from a mouse reflex-breathing test (the Alarie/RD50 bioassay): the human occupational limit is estimated as about 3% of the mouse concentration that cuts breathing rate in half, a relationship checked across 89 chemical data pairs spanning six orders of magnitude.
Moderate evidence.
Caveats
This is a narrative review restating a decades-old correlation (Alarie 1981; Schaper 1993), not new data; the California EPA's independent check (Kuwabara 2007) adds some replication, but the underlying data pairs are not systematically re-verified here.
Source: Nielsen & Wolkoff 2017, Regulatory Toxicology and Pharmacology Industry-funded Link to this fact
Indoor air / Review of other studies
Approved nasal-spray and oral-inhalation pharmaceutical products deliver estimated benzalkonium chloride epithelial doses of roughly 0.1-5 micrograms per square centimeter (nasal) or under 1 nanogram per square centimeter (oral inhalation) — doses far below the concentrations used in most in vitro cytotoxicity and ciliotoxicity assays (typically 0.001-0.02% BKC, i.e., 1-200 micrograms/mL).
Moderate evidence.
Caveats
Dose estimates rely on assumed nasal/lung surface areas and deposition fractions from separate reference studies, not direct measurement in the same subjects; applies only to pharmaceutical delivery devices, not to disinfectant sprays or other consumer BAC exposure routes.
Source: Johnson 2018, Journal of Aerosol Medicine and Pulmonary Drug Delivery Link to this fact
Indoor air / Measured in people or real products
A flickering (flame-disturbed) candle can emit up to 1,700 times more soot than the same candle burning steadily and undisturbed, based on chamber measurements of one candle type under both conditions.
Moderate evidence.
Caveats
Comparison is for a single candle type (candle 1) with one steady-burn control experiment versus two replicate stressed-burn experiments; the 60-1700x range reflects the size of the effect across the measurement, not a single precise multiplier.
Indoor air / Measured in people or real products
Candle wax and wick choice makes up to a 27-fold difference in soot, fine-particle, and particle-bound PAH emissions between candle types, but essentially no difference in NOx, formaldehyde, or gas-phase PAH emissions, in a controlled chamber study of five unscented pillar candles.
Moderate evidence.
Caveats
Five candle types, two replicate experiments each, single chamber, and single research group; not verified across other candle shapes (e.g. tapered, container candles) or other labs.
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
Burning a scented candle releases far more organic combustion by-products than burning an unscented candle of the same wax: total VOC emission rates reached up to 14,388 micrograms per candle per hour for scented candles versus 70-204 for the four unscented candles tested (24 fuel/fragrance combinations, chamber study).
Moderate evidence.
Caveats
Single lab (Fraunhofer WKI), industry/candle-association and fragrance-house funded (European Candle Association, National Candle Association, Latin American Candle Manufacturers Association; advisory committee incl. Arylessence, Belmay, Firmenich, Givaudan, IFF, Symrise, Takasago); n=4 candles per combination, purpose-built research candles not verified as representative of specific retail products.
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
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
The fragrance added to a candle, not the wax it is made from, is the main driver of extra combustion emissions: unscented candles made from four different waxes (palm, paraffin, soy, stearin) formed the lowest-emission cluster in the study's statistical analysis, while scented versions of the same waxes released far more VOCs, PAHs, and formaldehyde.
Moderate evidence.
Caveats
Based on a hierarchical cluster analysis of 24 experiments in one chamber study; individual scented fragrance families varied in how much they raised emissions (floral, oriental, and spice & edibles raised emissions less than fresh and fruit).
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
Indoor air / Other evidence
For rodent inhalation studies of irritating chemicals, extending a subchronic study to predict chronic effects needs roughly a 2-fold safety margin, and extending a shorter subacute study to chronic needs about 4-fold — both smaller than the values regulators previously used (2-fold and 6-fold).
Moderate evidence: Single reanalysis by one research team of one database; not independently replicated.
Source: Mangelsdorf et al. 2021, Int J Hyg Environ Health, International Journal of Hygiene and Environmental Health Link to this fact
Indoor air / Other evidence
A rat inhalation study's no-effect concentration for eye/airway irritation predicts the human no-effect concentration reasonably well, but rats are on average about 3 times less sensitive than humans, so a safety factor of about 3 is needed when extrapolating from rats to people.
Moderate evidence: Small dataset (10-25 chemicals); many human studies only tested one concentration so the true no-effect level may be higher than reported.
Source: Mangelsdorf et al. 2021, Int J Hyg Environ Health, International Journal of Hygiene and Environmental Health Link to this fact
Indoor air / Other evidence
German government toxicologists reviewing the data behind indoor air guide values concluded there is not enough evidence to lower the standard safety margin (10 to 20 times) currently used to protect sensitive groups such as people with chemical intolerance from irritating chemicals.
Moderate evidence: This is a decision to keep an existing regulatory default under uncertainty, not new evidence that the value of 10-20 is itself correct.
Source: Mangelsdorf et al. 2021, Int J Hyg Environ Health, International Journal of Hygiene and Environmental Health Link to this fact
Indoor air / Measured in people or real products
In a sealed-room test, a reed diffuser's top three released chemicals were linalool acetate, linalool, and alpha-pinene, with linalool acetate alone making up 31-44% of everything measured.
Moderate evidence: Single product tested; sealed airtight chamber built to reach a maximum equilibrium, not a lived-in room.
Source: Zhang 2024, Cell Reports Physical Science Link to this fact
Indoor air / Measured in people or real products
Scented wax melts, even though they involve no flame or smoke, released enough fragrance terpenes to react with ordinary indoor ozone and trigger bursts of new nanoparticle formation in a full-scale test house, reaching concentrations above 1 million particles per cubic centimeter.
Moderate evidence: One test house, 3 scented wax melt products; a real-world field measurement, but a single research group's single building, not yet replicated elsewhere.
Source: Patra et al. 2025, Environmental Science & Technology Letters Link to this fact
Indoor air / Measured in people or real products
The estimated respiratory tract deposited dose of these new nanoparticles from scented wax melt use (median 2.9 x 10^10 particles per minute) was comparable to the dose rates reported for burning a scented candle, running a gas stove, or diesel and natural gas engine exhaust, and about 1,000 times higher than the same test house's background air.
Moderate evidence.
Caveats
Modeled dose rate from measured particle size distributions using a published respiratory deposition method, not a direct measurement of dose in a human airway; combustion-source comparison values come from separate studies with different instruments and settings.
Source: Patra et al. 2025, Environmental Science & Technology Letters Link to this fact
Indoor air / Measured in people or real products
In a real-home field study of 32 occupied Hanoi houses monitored continuously for a week each (not a chamber), an incense-burning event raised PM2.5 in the burning room to a mean of 201.3 (SD 132.2) ug/m3, with a 1-minute peak of 825.5 ug/m3.
Moderate evidence.
Caveats
Single-city, single-cohort field study (32 homes); incense is direct combustion of solid material, not evaporation from a finished fragranced product — do not generalize to candles, perfume, or air fresheners.
Source: Tran 2021, Building and Environment 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 / Measured in people or real products
Habitually burning incense frequently (4+ times/week) raised whole-week geometric-mean indoor PM2.5 by 61.6% versus non-use homes, after adjusting for outdoor air quality, season, house type, road proximity, and other indoor activities; occasional use (up to 3 times/week) raised it by 18.5%.
Moderate evidence: Habit groups were self-reported by occupants, not independently verified; effect size may not generalize outside this Hanoi sample.
Source: Tran 2021, Building and Environment Link to this fact
Indoor air / Measured in people or real products
Across all 32 Hanoi homes, the 24-h average indoor PM2.5 concentration (52.1 ug/m3) exceeded the WHO 24-h guideline in 97% of homes and Vietnam's own 50 ug/m3 ambient standard in 47% of homes, even before separating out incense burning.
Moderate evidence: Reflects Hanoi's generally poor ambient air quality as much as any single indoor source; outdoor PM2.5 (54.4 ug/m3) was itself higher than indoor.
Source: Tran 2021, Building and Environment Link to this fact
Indoor air / Measured in people or real products
In a lab chamber test, three of six candles — a strawberry candle, a clean-cotton candle, and a plain (unscented) reference candle — produced formaldehyde levels above both the ACGIH occupational guideline (325 ppb) and OSHA's legal limit (730 ppb) while lit, up to 2,098 ppb for the strawberry candle.
Moderate evidence.
Caveats
These are concentrations from a small impinger/chamber system fed by 2 L/min of pure air pulled directly past the flame, not measured or modeled room air; single lab, six specific retail products, no replication.
Source: Ahn et al. 2015, Journal of Hazardous Materials 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
Using a mass-balance model of a realistic room scenario (one person present 8 h, two candles lit for 4 h, 20 cubic-meter room, 0.5 air changes per hour), Kim et al. 2016 calculate an 8-hour time-weighted-average formaldehyde concentration exceeding 100 ppb in the worst case, more than 6 times NIOSH's 16 ppb 8-hour recommended exposure limit.
Moderate evidence.
Caveats
This is a mass-balance model calculation built on chamber-derived emission factors, not a direct room measurement; the two-candle, 4-hour worst-case assumptions may not reflect typical single-candle household use.
Source: Kim et al. 2016, 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 / Animal study
In a mouse chamber study modeling occupational candle exposure, burning scented candles for 4.5 hours a day, 5 days a week, produced measured chamber levels of 2157.5 ng/m3 PAHs, 181 micrograms per cubic meter VOCs, and 219,000 nanoparticles per cubic centimeter (median size 10.7 nanometers) during active burning.
Moderate evidence.
Caveats
This is an occupational-intensity exposure chamber (hours per day, days per week, for 8 weeks), not a measurement of typical home candle-burning exposure; single lab, single candle type (brand/composition undisclosed).
Source: Chandrasekaran 2021, Current Research in Toxicology Link to this fact
Indoor air / Measured in people or real products
The plasma deodorant device itself generates ozone: about 1.5 ppm measured 3 cm from the outlet (roughly 7.5x the 15-minute occupational STEL of 0.20 ppm the authors cite), falling to about 0.16 ppm at 30 cm in a direct line and 0 ppm when the path to the sensor was blocked, which the authors say is below international occupational ozone limits at that distance.
Moderate evidence.
Caveats
Ozone measured by the device's own gas detector at fixed distances in open air with the device stationary; not measured as real personal breathing-zone exposure during actual underarm use, and occupational limits (TWA/STEL) were not designed for close-range personal-care-device exposure.
Source: Bok et al. 2024, Scientific Reports Link to this fact
Indoor air / Animal study
An "unscented" candle was not toxicologically inert: its headspace was 51.65% toluene by GC-MS peak area, and burning it in rats for 8 weeks still caused duration-dependent serum inflammation and lung injury compared with fresh air, though consistently less severe than the scented candle at every exposure duration tested.
Moderate evidence: One unscented candle product; wax/wick composition not disclosed; toluene here is an emission from the candle itself, not an added fragrance ingredient.
Source: Mohamed et al. 2025, Frontiers in Public Health Link to this fact
Indoor air / Other evidence
Burning a scented candle at low ventilation (0.5 air changes per hour) produced significantly higher total PAH and cancer-risk-weighted PAH emissions than at 1.0 or 2.0 air changes per hour, in a controlled chamber study.
Moderate evidence.
Caveats
Single lab, single scented-candle formulation (paraffin + 6% lavender oil), small chamber; total PAH itself was not monotonic with ACH (0.5 > 2.0 > 1.0), only the toxicity-weighted BaPeq fraction fell monotonically.
Indoor air / Other evidence
Relative humidity had a bell-shaped, non-monotonic effect on a scented candle's PAH emissions, peaking at 75% RH rather than at the lowest or highest humidity tested.
Moderate evidence.
Caveats
Single lab, single candle formulation; mechanism (flame-temperature/water-vapor effects) is inferred from indirect combustion indicators, not directly measured — authors call it a plausible hypothesis, not a demonstrated mechanism.
Indoor air / Other evidence
Low-molecular-weight PAHs, dominated by naphthalene, made up roughly 87-92% of total PAH mass from scented candle combustion across all tested ventilation and humidity conditions, and were mostly in the gas phase rather than bound to particles.
Moderate evidence.
Caveats
Single scented-candle formulation, small chamber study; benzo[a]pyrene itself was non-detect in nearly every condition, so toxicity-weighted risk is driven by other high-potency congeners, not BaP.
Indoor air / Measured in people or real products
Burning four unscented paraffin candles at once drove living-room NOx and HONO to 46.86 ppb and 1.63 ppb, about ten times higher than a single lavender-scented candle (0.02 ppb NOx, 0.23 ppb HONO) burned in the same room.
Moderate evidence.
Caveats
Single home, one scented and one unscented candle product; the unscented scenario burned roughly 22x more total wax mass (4 candles vs. 1), so the room-concentration gap partly reflects source size, not just chemistry.
Source: Deng 2026, Environment International Link to this fact
Information, not medical advice. See also: myths we won’t tell you.