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
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
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 / Measured in people or real products
Limonene was the most prevalent and highest-concentration of 88 volatile organic compounds measured in a national survey of Canadian homes: detected in 99% of homes, with an arithmetic mean of about 45 ppb (roughly 250 ug/m3) and a geometric mean of about 25 ppb (roughly 140 ug/m3), levels well above limonene concentrations reported in Australian, European, and Californian indoor-air studies.
Strong evidence.
Caveats
The ppb-to-ug/m3 conversion is evidence base arithmetic (limonene MW 136.23, 25C/1atm), not stated by the authors, who reported concentrations only in ppb. Comparator studies used different sampling durations, locations, and methods, so the size of the gap should be read cautiously even though the direction (Canada higher) is clear.
Source: Dales & Cakmak 2019, Environmental Pollution Industry-funded Link to this fact
Indoor air / Other evidence
Of 109 air fresheners bought in the Netherlands in 2009, 20 of the 24 regulated fragrance allergens turned up; linalool was in 87% of products and limonene in 69%, and one product was 15% linalool by weight.
Strong evidence: One survey of one national market in January 2009; weight fractions only, not what is emitted into air.
Source: ter Burg 2014, Inhalation Toxicology Link to this fact
Indoor air / Government agency
When limonene and linalool sit in air they oxidize into allergens: the EU committee said oxidized limonene and oxidized linalool rank among the most common contact allergens in patch testing (about 2-3% of consecutive patients for limonene at 3-5%, and 0.8-7.2% for linalool depending on test strength).
Strong evidence.
Caveats
Clinic patients, not the general population; test concentrations differ between studies (limonene 3-5%, linalool 2-11%); no published data on peroxide content of finished cosmetic products existed when written; transformation chain supported by multiple independent clinics but patch-test groups overlap (several Swedish centers).
Indoor air / Other evidence
The 2018 list printed d-limonene as Group 3 (Vols 56, 73, 1999), acetaldehyde as 2B, and styrene as 2A (Vol 121, listed 'In prep.'), alongside household combustion of biomass fuel (2A) and coal (1).
Strong evidence: 2018 snapshot; Group 3 means not classifiable, not safe; hazard not risk.
Source: IARC 2018, Agents Classified by the IARC Monographs, Vols 1-123 Link to this fact
Indoor air / Other evidence
Terpene ingredients in scented cleaners, air fresheners, perfumes, colognes and essential oils (limonene, pinene, linalool, linalyl acetate) are listed as ozone-reactive indoor sources that make formaldehyde and tiny particles when ozone is present.
Strong evidence.
Caveats
Table 1 is a literature summary; perfumes and essential oils rest on three chamber citations (Chao 2005, Karamalegos 2005, Shu 1997). No product was measured in this paper and it gives no product-specific numbers.
Source: Weschler 2006, Environmental Health Perspectives Link to this fact
Information, not medical advice. See also: myths we won’t tell you.