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.
Health effects / Measured in people or real products
Two German gas-works plumbers who repeatedly hand-poured the gas odorant tetrahydrothiophene in an unventilated room developed chronic rhinitis and lasting airway obstruction; neither had allergies on skin testing, and the accident insurer accepted both as occupational disease.
Weak evidence: Two case reports; exposure estimated, not measured; one worker smoked ~16 pack-years; industrial odorant, not fragrance.
Source: Baur & Bittner 2009, American Journal of Industrial Medicine Link to this fact
Health effects / Review of other studies
A cohort of laborers acutely exposed to gasoline vapors above allowed limits during an underground tunnel-excavation job later developed symptoms similar to those reported in multiple chemical sensitivity, and the study authors argued the workers' sociocultural profile made simple psychosomatic suggestibility an unlikely sole explanation.
Weak evidence.
Caveats
Cited secondhand from Davidoff et al. (primary study not yet in our evidence base); occupational cohort with no independent control for pre-existing chemical sensitivity, and "unlikely to be suggestible" is the original authors' qualitative judgment, not a tested exclusion of a psychological contribution.
Source: Rossi 2018, Journal of Occupational & Environmental Medicine Link to this fact
Health effects / Other evidence
In cultured human trabecular meshwork cells, BAK concentrations as low as 0.00002% — well below the 0.005-0.02% range found in commercial glaucoma drops — significantly reduced cell density and cell growth.
Weak evidence.
Caveats
Single in vitro study; bovine trabecular meshwork cells in a different study required about 50x that concentration (0.001%) to show the same effect, so results vary by species and cell state.
Source: Rasmussen et al. 2014, citing Samples et al. 1989, Journal of Ocular Pharmacology and Therapeutics Industry-funded Link to this fact
Health effects / Lab study (cells or chemistry)
BKC aerosol exposure altered secretion of 166 proteins in bronchial epithelial cells (84 up, 82 down); a gene-disease network of the upregulated secreted proteins linked GDF15, MMP1, and VEGFA (each increased two- to three-fold) to atherosclerosis and cardiovascular disease, suggesting inhaled BKC could have a direct, not just stress-mediated, systemic cardiovascular effect.
Weak evidence.
Caveats
This is a novel hypothesis from one cell-culture secretome analysis cross-referenced against disease-association databases (DisGeNET, CTD), not a measured blood biomarker change in an exposed animal or human; the authors describe it as warranting further investigation.
Source: Kim et al. 2025, Ecotoxicology and Environmental Safety Link to this fact
Health effects / Other evidence
A screening-level model of a heavy occupational scented-candle exposure (aromatherapy workers, 8 hours a day for 40 years) found a 95th-percentile lifetime lung cancer risk from PAHs alone of 1.20 in a million, marginally exceeding the commonly used 1-in-a-million risk threshold, under the single worst combination of low ventilation and intermediate humidity tested.
Weak evidence.
Caveats
PAH-only estimate (excludes co-emitted benzene, formaldehyde, and particulate matter that would add to total risk); occupational 8h/day/40-year exposure scenario, not typical home candle use; single candle formulation and small chamber; only the 95th-percentile Monte Carlo estimate under the single worst condition exceeded the threshold, not the mean case.
Information, not medical advice. See also: myths we won’t tell you.