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.
Fragrance is everywhere / Hypothesis
Immigrant women in the US most often work as house cleaners and make up 71% of manicurists; an expert job-exposure tool rates both jobs as probably exposed to fragrance chemicals, and house cleaning also to cleaning-product ingredients such as alkylphenols and antimicrobials.
Weak evidence: Exposure ratings from job descriptions (WORC), not measurements.
Source: Knox 2026, Journal of Exposure Science & Environmental Epidemiology Link to this fact
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.
Health effects / Other evidence
Nail salon workers have reported elevated rates of headaches, respiratory irritation, skin problems, and nasal/airway symptoms in prior cross-sectional surveys, consistent with (but not directly tested by) this study's indoor air quality measurements.
Weak evidence: This paper measured air chemistry only, not health outcomes; the symptom literature it cites is cross-sectional survey data from other studies.
Source: Pavilonis, Roelofs & Blair 2018, citing prior nail-salon worker surveys, Journal of Occupational and Environmental Hygiene Link to this fact
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
The industry / Other evidence
This review of BAK's harms to the trabecular meshwork was partly funded by Merck, Inc., and its senior author, Paul Kaufman, discloses paid consultant, honoraria, and financial-support relationships with Alcon and Allergan, both manufacturers of BAK-free alternative eye-drop preservatives (SofZia and Purite respectively) that the review discusses favorably.
Weak evidence.
Caveats
This is a conflict-of-interest disclosure, not an evidentiary finding. The funding pattern runs opposite to a typical 'industry defends its own product' case: the paper argues against a preservative (BAK) while its senior author has financial ties to companies selling competing, BAK-free alternatives.
Source: Rasmussen et al. 2014, Journal of Ocular Pharmacology and Therapeutics Industry-funded Link to this fact
What you can do / Self-reported survey
People affected by fragrance say policies work only with purchasing rules for fragrance-free, low-emission products, staff education, better ventilation, and consequences for repeat violations, enforced like smoke-free rules.
Weak evidence: Participants' and authors' proposals; not tested as an intervention.
Source: Roy 2026 Link to this fact
What you can do / Review of other studies
Fragrance-free policies are easier to make work when they start with a needs assessment and an inventory of the cleaning and personal products in use, involve employees in drafting, and agree how violations are handled before launch.
Weak evidence: Lessons from unnamed organizations; no outcome data.
What you can do / Review of other studies
Because people with MCS describe both a fear of asking for accommodations (risking hostility) and a fear of staying silent (risking worsening symptoms), workplaces and services that proactively offer fragrance-free accommodation without requiring the person to ask remove one source of this dilemma.
Weak evidence.
Caveats
Qualitative theme ('suppressing vs expressing needs', 9 findings/5 studies) describing participants' experience, not a tested intervention; no study in this review evaluated an accommodation policy's effect. Downgraded moderate→weak 2026-09-27 (Jev audit): the accommodation recommendation is an inference, not a finding.
Source: Driesen 2020, Journal of Psychosomatic Research Link to this fact
What you can do / Animal study
People who work for hours daily near burning scented candles under high job stress (for example, spa, hospitality, or restaurant staff who tend candle displays), rather than casual home users, are the population most directly addressed by this animal model, and may want to reduce burn time or improve ventilation in that specific occupational setting.
Weak evidence.
Caveats
Based on a single small, unreplicated mouse study using an occupational-intensity exposure chamber; no human occupational data were collected, and the specific candle's composition was not disclosed.
Source: Chandrasekaran 2021, Current Research in Toxicology Link to this fact
Information, not medical advice. See also: myths we won’t tell you.