Don’t take our word for it. Here’s every fact on this site, with its evidence grade, its caveats, and a link to the source, so you can check us.
We start you on the strong ones: findings that have been replicated or come from one large, well-controlled study. Moderate and weak facts are here too, labeled for what they are. Use “Link to this fact” to share a single card.
Indoor air / Review of other studies
A chemical released indoors is roughly 1,000 times more likely to be breathed in than the same amount released outdoors (exact when comparing the low end of each range; only ~100x at the high end, since the ranges overlap at 10⁻³), so cleaning products and air fresheners (about 1% of California's smog-forming emissions) may deliver an inhaled dose on the same scale as all outdoor sources combined.
Strong evidence.
Caveats
Modeled population intake fractions (not measurements), population-average estimate. Traced to the primary source, Lai, Thatcher & Nazaroff 2000 (JAWMA 50:1688-1699), which derives outdoor PITF 10⁻⁶-10⁻³ vs indoor/in-vehicle PITF 10⁻³-10⁻¹ from Gaussian-plume and mass-balance modeling (Nazaroff co-author of both papers, so this is not independent replication). The ranges overlap at 10⁻³, so '1000x' is a floor-to-floor comparison, not a fixed multiplier under all conditions.
Source: Nazaroff & Weschler 2004, Atmospheric Environment Link to this fact
Indoor air / Review of other studies
Scent chemicals such as limonene and pine oil react with ozone indoors to make new pollutants, including formaldehyde and fine particles, that keep forming for hours after cleaning ends.
Strong evidence: Size of the effect depends on dose and indoor ozone; confirmed by later chamber and field studies.
Source: Nazaroff & Weschler 2004, Atmospheric Environment Link to this fact
Indoor air / Measured in people or real products
Questionnaires about habits and routines correctly sorted only 20% of people by their measured exposure to cleaning and fragrance chemicals, vs 61% for traffic fumes; exposure depends on individual behavior and product choice.
Strong evidence.
Caveats
'Product choice' is the authors' explanation, not directly tested in this paper; independently corroborated by a larger, more direct 2021 UK study (Heeley-Hill 2021, 60 homes, daily diary-logged — not recalled — product-use frequency vs measured concentrations) that found cumulative product-use frequency has essentially no predictive power for measured indoor VOC concentrations (R²<0.001 for total VOC; no individual VOC significant).
Source: Edwards 2006, Atmospheric Environment Industry-funded Link to this fact
Indoor air / 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
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
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 / 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
Information, not medical advice. See also: myths we won’t tell you.