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.
Information, not medical advice. See also: myths we won’t tell you.