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 / 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.