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 / 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
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
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 / Review of other studies
Scent chemicals released by perfumes and scented products can react with ozone in indoor air to form new pollutants, including formaldehyde and fine particles.
Strong evidence.
Caveats
Formation is well established (Fiedler 2005: 40 ppb O3 + a limonene/alpha-pinene-containing VOC mix -> HCHO 13 -> 40 µg/m³, UFP 2,500 -> 46,000 cm-3; Singer 2006; Rossignol 2013; Destaillats 2006 gives the underlying per-ozone yield). Amounts depend on dose and ozone; the harm at real indoor levels is not shown. A far more rigorous, independent toxicologist review (Wolkoff & Nielsen 2017) reaches the same qualitative chemistry conclusion via its own literature synthesis and adds a quantitative caution: in the two largest human tests to date (Fiedler 2005, n=130; Fadeyi 2015, n=71), the resulting reaction mixture caused no significant airway/eye effects, and its own threshold estimate is that >200 µg/m³ ozone at high limonene would be needed before sensory effects are expected. Cite primaries for numbers.
Source: Alblooshi 2025, Frontiers in Toxicology Link to this fact
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
Air fresheners, deodorizers and moth crystals were identified as the household source of p-dichlorobenzene, which reached about 300 µg/m³ indoors while being nearly absent outdoors.
Strong evidence: P-DCB is a deodorizing active, not a fragrance ingredient.
Source: Weisel 2002, Environmental Health Perspectives Link to this fact
Indoor air / Measured in people or real products
Formaldehyde in New Jersey homes averaged about seven times the outdoor level (55 vs 13 parts per billion).
Strong evidence: Regional sample; building materials are a major source alongside products.
Source: Weisel 2002, Environmental Health Perspectives Link to this fact
Indoor air / Measured in people or real products
For p-dichlorobenzene, an often unlabeled deodorizer in air fresheners and moth repellents, 97% of people's exposure came from sources inside the home, not outdoor air.
Strong evidence: Deodorizer/pesticide active, not a fragrance ingredient.
Source: Wallace 1991, Environmental Health Perspectives 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
Limonene was 13x higher indoors than outdoors in European offices and classrooms, one of the two highest indoor/outdoor ratios of 23 chemicals measured.
Strong evidence: Ratio of medians.
Source: Geiss 2011, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
Most European homes had low limonene (median 9.5 ug/m3 over a week), but the heaviest-use homes reached weekly averages up to 493 ug/m3, the range used in lab studies of indoor ozone chemistry.
Strong evidence: Upper tail only; median home 20-60x below chamber levels.
Source: Geiss 2011, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
In a real test house, using a limonene-scented foam cleaner exactly as the label directed produced a burst of 45,000-150,000 tiny new particles per cubic centimeter within minutes, but only when ozone was in the air.
Strong evidence.
Caveats
One product, one house; particle number not mass. Independently replicated in a second real building (a working primary school; Morawska 2009), with a different detergent and country, at a similar order of magnitude (5.89-6.85×10^4 cm-3 in controlled tests) and the same water-only-no-burst, ozone-dependent pattern.
Source: Rossignol 2013, 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 / Lab study (cells or chemistry)
What you breathe after using a scented cleaner is not only what was in the bottle: the scent chemicals keep reacting in the air to make new compounds.
Strong evidence: Well-established chemistry; magnitude indoors depends on ozone levels.
Indoor air / Review of other studies
Scented cleaners react with ozone indoors and release bursts of tiny new particles; every study able to separate them found this.
Strong evidence.
Caveats
Particle number, not mass; magnitude depends on instrument size cutoff and dose. Now confirmed across chamber, real test-house, real-office and real-classroom settings (Rossignol 2013, Rosales 2022, Wu 2024, Morawska 2009).
Source: Salonen 2024, Environment International 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
Four people in a clean, well-ventilated test room removed about two-thirds of the ozone coming in, confirmed in a second campaign that included women as well as men.
Strong evidence: Same DTU chamber as Wang 2024; ~65% is evidence base arithmetic from a ~100 ppb target inlet; furnished homes also lose ozone to surfaces.
Source: Zannoni 2021, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
When scent chemicals from a pine-oil cleaner, an orange-oil degreaser and a plug-in air freshener met ozone in a lab chamber, about one formaldehyde molecule formed for every 3-5 ozone molecules they used up (yields of 20-30% in most tests).
Strong evidence.
Caveats
Same three products as Singer 2006, same research group/CARB contract, not independent replication; the highest single value (90%) occurred at air-freshener vapor loads 30-45x a real room's plug-in levels.
Source: Destaillats 2006, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
The moment ozone was added to pine-oil, citrus and air-freshener scent vapors in a lab chamber, a burst of new ultrafine particles formed immediately and grew within minutes into fine particles, reaching hundreds of micrograms per cubic meter.
Strong evidence.
Caveats
Bench chamber at cleaning-pulse-scale vapor loads; ultrafine particles were only 1-4% of the mass at steady state, not the 4-16% the paper itself mislabels as an 'ultrafine' yield (that figure is total particle mass, all sizes).
Source: Destaillats 2006, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
In a controlled test room, people removed roughly 60–69% of the indoor ozone in every one of five very different groups tested — teenagers, young adults and people in their 70s alike — showing this is a general property of occupied rooms with ozone present, not a quirk of one age group.
Strong evidence: Chamber study (22.5 m³, DTU, mixing fans push ozone deposition to the top of the literature range), fragrance-free hygiene products only, no health outcome measured.
Indoor air / Self-reported survey
19.4% of Australians reported health problems from being near someone wearing fragrance and 16.4% from air fresheners.
Moderate evidence: Self-report; no exposure measurement; counts 213/1,098 and 180/1,098 confirmed in Steinemann 2018 (MCS, AU) Table 2, same dataset.
Source: Steinemann 2017, Preventive Medicine Reports Link to this fact
Indoor air / Self-reported survey
About 1 in 6 adults (17%) in four countries said air fresheners or deodorizers make them unwell; of those, half reported breathing problems.
Moderate evidence: Self-report, no objective measure.
Source: Steinemann 2019, Air Quality, Atmosphere & Health Link to this fact
Indoor air / Self-reported survey
41% of US adults with asthma reported health problems from air fresheners, and 36.7% said they can't use a public restroom that has one.
Moderate evidence: Self-report.
Source: Steinemann 2018, Air Quality, Atmosphere & Health Link to this fact
Indoor air / Self-reported survey
23.5% of Swedes reported health problems from being near someone wearing a fragranced product.
Moderate evidence: Self-report.
Source: Steinemann 2018 (Sweden), Air Quality, Atmosphere & Health Link to this fact
Indoor air / Self-reported survey
About 1 in 5 US adults (20.4%) reported health problems from air fresheners, and 23.6% from being near someone wearing a fragranced product.
Moderate evidence: Self-report; no exposure measurement.
Source: Steinemann 2016, Air Quality, Atmosphere & Health Link to this fact
Indoor air / Self-reported survey
About 1 in 5 Australians with asthma (21.4%) said they can't or won't use public toilets that have air fresheners.
Moderate evidence: Self-report.
Source: Steinemann 2018, Air Quality, Atmosphere & Health Link to this fact
Indoor air / Measured in people or real products
Scented laundry products put fragrance chemicals into outdoor air: in two homes, limonene (a citrus scent chemical) came out of the dryer vent only when fragranced detergent or dryer sheets were used (up to 14.7 ug/m3), not in the no-product runs.
Moderate evidence.
Caveats
2 homes (Steinemann 2013), single grab samples; undiluted vent air, not a breathing-zone dose. Replicated at much higher magnitude and with a dose-response signal by Goodman et al. 2019 (6 Melbourne homes, before/after switch design): fragranced-detergent vent D-limonene 2.35-118 ug/m3 vs 0.24-0.26 ug/m3 in fragrance-free-only homes, and switching from fragranced to fragrance-free detergent cut vent D-limonene by up to 99.7% (mean 79.1%) within a month. The two studies share one author (Anne Steinemann) and neither ran statistical significance tests, so the combined evidence (8 homes total) stays below strong, but the qualitative and dose-response finding is now cross-country replicated within one research program rather than resting on a single 2-home study.
Source: Steinemann 2013, Air Quality, Atmosphere & Health Link to this fact
Indoor air / Measured in people or real products
In a room-sized test chamber, mopping with a pine-oil cleaner or spraying an orange-oil degreaser, with ozone at typical summer-smog levels, produced 130-280 µg/m³ of fine particles, versus 2-5 µg/m³ when the same products were used without ozone.
Moderate evidence.
Caveats
Chamber, 1 product per type, 120 ppb ozone supply; real homes measured lower SOA mass. Corroborated in a second, smaller chamber with the same products (Destaillats 2006 bench-scale: steady-state SOA mass 11-229 µg/m³, same order of magnitude), but that study shares the same research group, products and contract, so it does not count as independent replication.
Source: Singer 2006, Atmospheric 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 / Self-reported survey
Air fresheners were the most common secondhand trigger: 15.5% of UK adults reported health problems from them, and 12.1% avoid or are reluctant to use public restrooms that have them.
Moderate evidence: Self-report.
Source: Steinemann 2018, Air Quality, Atmosphere & Health Link to this fact
Indoor air / Measured in people or real products
Keeping a gel air freshener in a car raised the fragrance chemical limonene in the air the driver breathed 3.6-4.7 times, to about 19-28 ug/m3, during real commutes (10 cars tested).
Moderate evidence: N=5 per group, non-randomized; mostly drivers' own products.
Indoor air / Measured in people or real products
In lab tests, gel air fresheners released limonene at up to 1.5 mg per hour, far more than any other chemical measured.
Moderate evidence.
Caveats
Chamber emission rates; products chosen for high limonene (upper end). Singer 2006 (Indoor Air) found the opposite pattern in a different air-freshener type (plug-in scented oil): limonene was a minor emission (39 mg/day) versus non-reactive esters (bornyl acetate 460, benzyl acetate 320 mg/day). 'Limonene dominates' holds for the limonene-selected gels Jo 2008 tested, not for air fresheners generally.
Indoor air / Measured in people or real products
Indoors, vapors from these products run about 7 times higher than outdoors, and include fragrance chemicals such as limonene.
Moderate evidence: Box model checked against indoor measurements.
Indoor air / Measured in people or real products
Mopping a room with a terpene-based (pine- or citrus-type) cleaner caused bursts of tiny new particles, about 100 times the particle count of outdoor air, as the scent chemicals reacted with ozone.
Moderate evidence.
Caveats
One cleaner, one room, one volunteer; particle number, not mass. The ~100x figure is specific to sub-3 nm nanocluster aerosol; a second real classroom mopping event (Morawska 2009, a school, >=5 nm instrument) reached only ~3-7x the outdoor baseline mopping the same class of product, showing the fold-increase depends heavily on the particle-size cutoff of the instrument used.
Indoor air / Measured in people or real products
During mopping with a terpene cleaner, scent chemicals such as limonene and pinene peaked at 280-380 ppb in the room air.
Moderate evidence.
Caveats
Brief peaks; typical home averages 3-7 ppb (EXPOLIS/Gokhale/Sarigiannis) — but Wang 2017 found one heavily fragranced/cleaned UK home's 5-day AVERAGE reached ~258 ppb, within the same order of magnitude as this mopping peak, showing the 'brief peak vs low average' distinction does not hold for the heaviest product-use homes.
Indoor air / Measured in people or real products
Homes painted, refurnished or re-floored in the previous 6 months had higher levels of indoor chemicals released into the air (including toluene and xylenes) than other homes.
Moderate evidence: Not a fragrance finding.
Source: Maisey 2013, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
For the third of homes using p-dichlorobenzene products, the upper-bound cancer risk was about 3x the population average (~270 vs 90 in a million).
Moderate evidence: Upper-bound 1991 potencies, 7-10x above best estimates.
Source: Wallace 1991, Environmental Health Perspectives Link to this fact
Indoor air / Measured in people or real products
A limonene-scented foam cleaner released formaldehyde, an irritant and carcinogen, both directly and through reactions with ozone after use.
Moderate evidence.
Caveats
Primary formaldehyde source ingredient unidentified (Rossignol's own ISO chamber measured 25.6 ug/m2/h at 0-30 min without ozone). Destaillats 2006 tested three different terpene-containing products (pine-oil cleaner, orange-oil degreaser, plug-in air freshener) under controlled bench conditions and found NO primary formaldehyde in any of them (negligible before ozone was added). This does not contradict Rossignol's own direct measurement of a different, specific product, but it shows primary formaldehyde is not a general feature of terpene/fragrance cleaners, supporting the reading that Rossignol's primary source is a formulation additive (e.g. a preservative or surfactant impurity) rather than the fragrance/terpene component itself.
Source: Rossignol 2013, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
Scent was only part of what a scented cleaner released: its glycol-ether solvent reached about 1,100 ug/m3 in room air against 71 ug/m3 for limonene.
Moderate evidence.
Caveats
One product; Singer 2006 (Indoor Air, 5 more US cleaners, 50 m3 room) found the opposite pattern in a cleaner containing both a glycol ether and limonene (limonene 3-5x the 2-BE), so 'solvent exceeds scent' is product-specific, not general.
Source: Rossignol 2013, Atmospheric Environment Link to this fact
Indoor air / Review of other studies
Average indoor formaldehyde in homes exceeded California's long-term health limit (9 µg/m³) in every European country the EU's own researchers assessed.
Moderate evidence: Secondary data; formaldehyde mainly from building materials, not shown to be from fragrance.
Source: Sarigiannis 2011, Environment International 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 / Lab study (cells or chemistry)
In lab chamber tests set up to mimic a room, the citrus scent chemical limonene reacted with ozone in the air to form fine particles carrying reactive oxygen species.
Moderate evidence.
Caveats
Chamber concentrations exceed typical measured indoor exposure by 5-10x. Particle formation is confirmed in a human chamber (Fiedler 2005), but ROS were not measured there and 130 women showed no acute effect; don't pair this claim with 'linked to irritation'. Wolkoff et al. 2008 (now read directly, the true primary behind this line of evidence) is a mouse gas-vs-particle denuder study showing denuded limonene-ozone-reaction ultrafine particles (~9.5-12.0 mg/m3, gas phase >95% removed) cause NO sensory irritation or airflow-limitation effect at all, statistically indistinguishable from clean air — reinforcing, from a different technique (whole-body mouse bioassay vs in-vitro ROS assay), that particle-bound ROS formation should not be equated with a demonstrated health effect.
Indoor air / Lab study (cells or chemistry)
Limonene produced 2 to 11 times more of these reactive particles than alpha-pinene, the pine scent chemical.
Moderate evidence: Chamber comparison only.
Indoor air / Government agency
EPA notes that people using products can expose themselves and others to very high pollutant levels, and that elevated levels can persist in the air long after the activity ends.
Moderate evidence: About product use in general; the peak example given is paint stripping (up to 1,000x outdoor), not fragrance.
Source: US EPA 2026 Link to this fact
Indoor air / Measured in people or real products
In an office-sized test room with some ozone in the air, a single ordinary spray of perfume, body spray or hair spray, or a dab of hand lotion, set off a burst of new ultrafine particles within minutes, reaching 34,000–200,000 particles per cubic centimeter, more than in many city streets.
Moderate evidence: One chamber, 5 products, one use each; ozone held at 35–40 ppb (high end for indoors); masked volunteer; no toxicity measured.
Source: Wu 2024, Environmental Science & Technology Letters Link to this fact
Indoor air / Measured in people or real products
Almost everything in a perfume ends up in the air: one perfume released 964 mg of vapor per gram used, mostly alcohol but also about 32 mg per gram of terpene fragrance chemicals.
Moderate evidence: One perfume; emission factors are lower bounds (PTR-MS blind to some compounds).
Source: Wu 2024, Environmental Science & Technology Letters Link to this fact
Indoor air / Measured in people or real products
Skipping showers for one or two days raised skin-derived acetic acid emissions by 84% and 172%, and increased the products of ozone reacting with skin.
Moderate evidence: 4 men; bacterial origin of the acids is inferred, not measured; fragrance-free soap/shampoo used throughout.
Source: Wang 2024, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
Pure limonene only irritates human eyes at about 445 mg/m³, far above indoor levels. A limonene-ozone mixture reacted for just 10 minutes raises eye-blink rates at roughly 1,000-2,000x less limonene: Klenø & Wolkoff (2004, n=8, primary) found residual 0.42 mg/m³ limonene + 0.08 mg/m³ ozone raised blink frequency 42% (P<0.0001); Nøjgaard (2005, n=10, primary) found a lower dose, 0.23 mg/m³ limonene + 0.08 mg/m³ ozone, gave a smaller but still significant +17% (p=0.003).
Moderate evidence.
Caveats
Fadeyi et al. 2015 (now read directly; n=71 — 38 non-asthmatic, 33 mild asthmatic, corrects an earlier undifferentiated count — 3 h, whole-body) found no significant eye/nose/throat irritation difference between asthma-status groups, at doses comparable-to-LOWER than Kleno/Nojgaard's residual concentrations, not higher as previously stated: limonene 35-36 ppb vs. Kleno's 75 ppb / Nojgaard's 41 ppb, and ozone 20-37 ppb (rising) vs. their steady ~41 ppb. This is a different endpoint (self-reported symptom rating vs. objective blink count), and the study has no unexposed/clean-air control arm, so 'no significant' is a between-group comparison with ratings staying below the scale's 'slight irritation' anchor, not a formal null against no exposure. Not a refutation of the blink-frequency result, but it tempers real-world relevance at doses at or below the LOEL pair, over a much longer (3 h vs 20 min), whole-body exposure. Laumbach et al. 2005 (n=130, same chamber cohort as Fiedler 2005) adds a whole-body objective-biomarker null at a higher combined terpene dose (limonene 125 ppb + alpha-pinene 160 ppb, with ozone, 135 min): no nasal symptom or nasal-lavage PMN/IL-6/IL-8/protein change. It's a different route and endpoint from the single-eye blink studies, so it doesn't refute them, but it weighs against generalizing them.
Source: Wolkoff 2013, International Journal of Hygiene and Environmental Health Link to this fact
Indoor air / Other evidence
In mouse tests, about three-fourths of the airway irritation from limonene reacting with ozone came from formaldehyde and residual limonene together (formaldehyde the dominant contributor, at roughly 17-22 percentage points; residual limonene a small but real contributor, under 5 percentage points), not from the particles.
Moderate evidence.
Caveats
Animal data at ppm-range reactants; the author's own lab; not independently replicated by another group. This is now grounded directly in the true primary, Wolkoff et al. 2008 (*Toxicol Lett* 181:171-176), read in full: its own Abstract and Conclusion state formaldehyde and residual limonene COMBINED explain 'up to three-fourth' (~75%), correcting this page's earlier paraphrase ('two-thirds, formaldehyde only, excluding limonene') which had both the wrong number and the wrong attribution. The entire conducting-airway (VD/VT, airflow-limitation) effect's causal chemical remains unidentified in the primary.
Source: Wolkoff 2013, International Journal of Hygiene and Environmental Health Link to this fact
Indoor air / Review of other studies
An indoor-air toxicologist recommends measuring limonene in offices because this common fragrance 'easily undergoes ozonolysis producing oxidation products that includes formaldehyde'.
Moderate evidence.
Caveats
Wolkoff & Nielsen 2017 (same author) reinforces the same recommendation-level framing and adds a quantitative synthesis ('>200 ug/m3 ozone at high limonene concentrations would be necessary to expect sensory airway and eye effects'), consistent with human nulls in Fiedler 2005 and Fadeyi 2015. Laumbach et al. 2005 (n=130, now read directly, the Fiedler cohort's nasal-lavage companion) adds a null on the one endpoint most directly relevant to a monitoring recommendation for a chemical that 'easily undergoes ozonolysis': no nasal symptom or objective nasal-inflammation-marker change despite the same ozonolysis chemistry occurring (formaldehyde and SOA both confirmed to rise during exposure). This keeps the claim at the level of a monitoring/indicator recommendation, not a finding of harm at current office levels.
Source: Wolkoff 2013, International Journal of Hygiene and Environmental Health Link to this fact
Indoor air / Measured in people or real products
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
OH radicals attack almost every organic chemical in indoor air, not only the ~10% that ozone reacts with, and these reactions happen in the air closest to our bodies; the researchers note that many of the resulting products have never been tested for toxicity.
Moderate evidence: No toxicity or health endpoint measured; the 'untested' statement is the authors' assessment.
Indoor air / Instrument measurement
The 'air quality index' on home air monitors is driven by particles; it did not move when only solvent-type products (nail polish remover, hair dye) were used, even though the VOC reading rose.
Moderate evidence: One unnamed device; its AQI does not follow the US EPA scale.
Indoor air / Journalism
In 2003 Southern California air regulators estimated that household and small-business products, including cleaners, cosmetics, perfume and room fresheners, released about 108 tons of smog-forming chemicals a day in the Los Angeles region, second only to vehicle tailpipes.
Moderate evidence: Agency inventory relayed by the LA Times; for measured/modeled figures use McDonald 2018 (LA) and Coggon 2021 (NYC 2018: VCPs comparable to vehicle fuels for ozone).
Source: DONTSPRAYCALIFORNIA 2008 (DOJ ADA docket) Link to this fact
Indoor air / Measured in people or real products
A new Little Tree car air freshener releases most of its limonene in the first few days: in a closed car the limonene signal halved roughly every day and was nearly gone within 4-5 days.
Moderate evidence: Relative peak areas only, no concentrations; one car and one scent; baseline not shown.
Source: Pino-Delgado 2021 (J Student Res), Journal of Student Research Link to this fact
Indoor air / Self-reported survey
Among Australians with diagnosed multiple chemical sensitivities, about three in four (77.5%) said a fragranced product had kept them from going somewhere, and about two in three (64.8%) can't or won't use public toilets with air fresheners.
Moderate evidence: N = 71 (95% CI roughly +/-10-12 points); self-report.
Source: Steinemann 2018, Preventive Medicine Reports Link to this fact
Indoor air / Measured in people or real products
When 40 ppb of ozone was added to a room holding a mix of 23 common indoor chemicals, including the citrus and pine scent chemicals limonene and alpha-pinene, formaldehyde tripled (13 to 40 µg/m³), ultrafine particles rose about 18-fold (2,500 to 46,000 per cm³) and particle mass reached 140 µg/m³.
Moderate evidence: Chamber with VOC load ~10x typical buildings; 23-compound office mixture, not a fragranced product; other alkenes also react.
Source: Fiedler 2005, Environmental Health Perspectives Link to this fact
Indoor air / Self-reported survey
Among US adults reporting diagnosed multiple chemical sensitivity, 59% said they could not use a public restroom with an air freshener and 70% had been kept from going somewhere by fragranced products.
Moderate evidence: Self-report; small subgroup (n=145) in one online panel.
Source: Steinemann 2018 (JOEM), Journal of Occupational and Environmental Medicine Link to this fact
Indoor air / Measured in people or real products
When moisturizer goes on the face, the wearer breathes a short burst of its vapors (alcohol, fragrance terpenes, benzyl alcohol) that room-air monitoring does not capture; at a mannequin's nose, alcohols and terpenes took about 30 minutes to fade and some solvents up to 2.5 hours.
Moderate evidence: Mannequin head, no people; unheated face (21 °C vs 32–34 °C skin) and a 5-ACH lab; the size of the burst is disputed (tabulated doses do not match the plotted traces).
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 / Measured in people or real products
Wearing shorts and t-shirts instead of long clothes raised the skin-ozone products 6-MHO, 4-oxopentanal and geranyl acetone by 35–51%. Clothes that had been worn for a few hours took up ozone about as fast as bare skin, because skin oil moves into the fabric.
Moderate evidence: One group of 4 young adults, ~35 ppb ozone; chemistry only; 6-MHO values may be inflated by an inlet artifact.
Source: Zannoni 2021, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
A dried film of pine-oil cleaner or orange-oil degreaser, with its scent chemicals already evaporated away, still produced formaldehyde and a burst of new particles when later exposed to ozone.
Moderate evidence: Single glass-plate surface test per product in a lab chamber, not tested on real room surfaces (carpet, fabric, furniture).
Source: Destaillats 2006, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
Wiping a small kitchen table with a few grams of an ordinary spray cleaner put 270-2,300 ug/m3 of the solvent 2-butoxyethanol into the air of a room-sized test chamber for the next hour; four of five cleaners tested contained it.
Moderate evidence: 50 m3 empty chamber, use rate at the ~84th percentile of observed users; real furnished rooms sorb glycol ethers faster, lowering peaks.
Source: Singer 2006, Indoor Air, Indoor Air Link to this fact
Indoor air / Measured in people or real products
Cleaning with a pine-oil or citrus cleaner raised scent chemicals (limonene, terpinolene, alpha-terpineol) to about 1,000-6,000 ug/m3 for the first hour in a room-sized chamber, and some stayed elevated for many hours afterwards.
Moderate evidence: Empty chamber; one product per formulation; alpha-terpineol's persistence depends on surface water films and towels left in the room.
Source: Singer 2006, Indoor Air, Indoor Air Link to this fact
Indoor air / Measured in people or real products
A plug-in scented-oil air freshener evaporated 1.5 g of product a day and kept its scent chemicals at 30-160 ug/m3 in the room over three days; its two largest emissions were fragrance esters (bornyl and benzyl acetate, 280-410 ug/m3), not the ozone-reactive terpenes usually studied.
Moderate evidence: One plug-in product; a different air-freshener type (e.g. limonene-selected gels, Jo 2008) can be limonene-dominant instead.
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
Scented spray products — spray and plug-in air fresheners, dusting sprays — were among the specific product types most strongly associated with childhood wheeze and asthma risk in this Canadian birth cohort.
Moderate evidence.
Caveats
The product-specific breakdown was reported in the paper's online Appendix 1, not reproduced in the main-text tables read for this ingest; treat as directional pending direct review of that appendix.
Source: Parks et al. 2020, CMAJ (Canadian Medical Association Journal) Link to this fact
Indoor air / Measured in people or real products
When the same volunteers breathed through a mask into a separate room, so only their breath (not their skin) was in contact with the air, indoor ozone barely changed at all — showing the ozone-removing chemistry happens on skin and clothing, not from exhaling.
Moderate evidence: Based on a single experiment (Experiment 12) in the ICHEAR chamber; not replicated across groups.
Indoor air / Test-chamber measurement
A car air freshener's own terpenes (alpha-pinene, beta-pinene, p-cymene, limonene) react with in-vehicle ozone to form ultrafine particles and irritant gases (formaldehyde, acetaldehyde, acrolein) that never appear on the product's ingredient list, because they are made in the cabin air, not sold in the bottle.
Moderate evidence.
Caveats
N=1 product (oil-based liquid, citrus); single run per condition, no replicates; the 100 ppb ozone condition exceeds the paper's own cited in-vehicle ozone range (2-86 ppb) and RH/ventilation were excluded, so treat the numbers as chemistry demonstration, not a real-commute dose.
Source: Lamorena & Lee 2008 (J Hazard Mater), Journal of Hazardous Materials Link to this fact
Indoor air / Measured in people or real products
In a real working primary school, mopping a classroom with a limonene-containing detergent produced bursts of tens of thousands of tiny new particles per cubic centimeter within minutes, but only when ozone was present in the air; a plain water-only mop produced no such burst.
Moderate evidence.
Caveats
One school, one detergent; particle number not mass. Classroom mopping peak ~35,000/cm3; matched controlled lecture-theater trials with the same detergent confirmed peaks up to ~68,500/cm3.
Source: Morawska 2009, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
A limonene-cleaning event needs indoor ozone above roughly 5 parts per billion to trigger a detectable burst of new ultrafine particles; below that threshold, in this study's controlled trials, essentially no particle formation occurred.
Moderate evidence.
Caveats
One product, one 140 m3 lecture theater, one air-exchange rate (1.08/h); threshold is specific to this detergent's limonene loading and may not generalize to other products or rooms.
Source: Morawska 2009, Environmental Science & Technology Link to this fact
Indoor air / Measured in people or real products
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 / Measured in people or real products
Among 3,411 Czech households with an infant, the most commonly used chemical products were window cleaner (88%), deodorant (82%) and aerosol/hairspray products (74%) — not air fresheners, which were the second-least-used of 18 chemical product categories (26%).
Moderate evidence: Self-reported product use frequency, one Central European cohort; product prevalence does not equal exposure dose or emission rate.
Source: Mikeš 2019, Science of the Total Environment Link to this fact
Indoor air / Measured in people or real products
A 10-minute-old reacted mixture of limonene and ozone (residual 75 ppb limonene + 40 ppb ozone) raised human eye-blink frequency 42% relative to clean air, while the same men's eyes exposed separately to just limonene, or just ozone, at similar or higher levels, did not blink more.
Moderate evidence.
Caveats
Small (n=8), single-lab, local single-eye exposure (not whole-body). Starting limonene (220 ppb) and residual limonene (75 ppb) are one to two orders of magnitude above typical indoor levels (usually <10 ppb, up to ~70 ppb in cleaning agents/new housing, per the authors' own Introduction). A same-lab follow-up re-test of a nominally comparable mixture gave a 3.5x smaller effect (12% vs 42%), so the magnitude has not replicated within the same group.
Indoor air / Review of other studies
The four most common airborne fragrances in homes and offices — pinene, limonene, linalool and eugenol — are usually present at levels 2 to 3 times below the concentration needed to physically irritate the eyes and airways, but at or above the much lower level needed just to smell them.
Moderate evidence.
Caveats
Sensory-irritation thresholds here are derived (mouse RD50 values run through a regression, or human LOAELs divided by an assessment factor of 5), not all directly measured; this restates the same author group's own 2013 conclusion for an updated audience, not independent confirmation.
Source: Wolkoff & Nielsen 2017, Environment International Link to this fact
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 / Measured in people or real products
In a week-long study of 7 working adults in Leipzig, the air at home made up 42-73% of the airborne chemicals they breathed, more than the outdoor air or their workplaces.
Weak evidence: N=7, one week, 25 VOCs; home share includes infiltrated outdoor air.
Source: Gokhale 2008, Science of the Total Environment Link to this fact
Indoor air / Measured in people or real products
In a real shower test, limonene in the bathroom air rose to about 80, 150 and 380 ppb as the amount of shower product went up, roughly in proportion to the amount used.
Weak evidence: One participant, one bathroom, PTR-MS; limonene only; peaks last minutes.
Indoor air / Hypothesis
A computer model of a morning shower found fragrance terpenes from the products react with indoor ozone to make formaldehyde (about +4 ppb), other aldehydes, irritant PAN-type nitrates and a little fine particle matter, even with strong ventilation.
Weak evidence: Modeled (INDCM), not measured; increments are small and last about an hour; water-soluble products may dissolve in shower water, so these are upper limits.
Indoor air / Animal study
A continuously running essential-oil diffuser is now the exposure pattern with an animal result attached to it: continuous ambient lavender exposure advanced puberty in rats, while a larger once-daily nasal dose did not.
Weak evidence: Neither exposure was quantified, so the comparison shows a pattern, not a dose-response; with five animals per group the difference rests partly on one outlier.
Source: Kim & Lim 2022, Journal of Korean Medical Science Link to this fact
Indoor air / Instrument measurement
In a small closed test room, three short bursts of aerosol hairspray pushed fine-particle (PM2.5) readings to about 90 µg/m³, roughly 2.5 times the US 24-hour outdoor standard, though only for minutes.
Weak evidence: Uncalibrated consumer monitor; tiny room; short peak vs 24-h standard; primary spray aerosol, no fragrance characterized.
Indoor air / Measured in people or real products
A fresh, 10-minute-old mixture of limonene and ozone, at concentrations close to the high end measured in real homes and offices, made people's eyes blink significantly more often in a small controlled study, at roughly 2,000 times less limonene than pure limonene alone needs to irritate eyes.
Weak evidence.
Caveats
This is the paper's own LOEL (residual 0.23 mg/m3 limonene + 0.08 mg/m3 ozone, +17%, p=0.003) and was previously mis-cited on our evidence base to Klenø & Wolkoff 2004. Now verified as primary, but the same lab's own comparable re-test of Klenø & Wolkoff 2004 gave a 3.5x smaller effect (12% vs 42%) at nearly identical residual concentrations, and no dose-response held across the paper's own four LOPs mixtures (R2=0.7, ns).
Indoor air / Measured in people or real products
Methacrolein, a known limonene/isoprene ozonolysis product, produced a similarly-sized eye-blink-frequency increase on its own (+18% at 286 ppb) as the freshly reacted limonene-ozone mixture did, supporting the idea that it is reaction products, not parent limonene, that irritate eyes.
Weak evidence.
Caveats
Methacrolein was tested alone, not measured inside the LOPs mixtures, so this is an analogy of magnitude, not a demonstration that methacrolein specifically drives the limonene-ozone effect. Single small study, single lab.
Indoor air / Measured in people or real products
Whether an office air-conditioning filter was new or had been in service for about 10 months made no measurable difference to ozone or particle chemistry, or to occupants' reported symptoms, during ozone-limonene exposure.
Weak evidence.
Caveats
Compares only new vs. moderately used (MERV 13) filters at fixed ventilation/recirculation rates; does not test badly fouled filters or other filter grades, so it should not be read as showing filter replacement is never protective.
Information, not medical advice. See also: myths we won’t tell you.