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