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.
Health effects / Measured in people or real products
Nitrate radicals alone, generated from ozone and nitrogen dioxide with no terpene present at all, raised human eye-blink frequency 21% — comparable in size to the effect of isoprene ozonolysis products — suggesting that some of the eye-irritating power of 'scented product plus ozone' indoor chemistry may come from reactive radicals in general, not from limonene specifically.
Moderate evidence.
Caveats
Single small study; the specific irritant chemical species were not identified or measured, only the percent of ozone chemically consumed. Not yet tested by chemical fractionation in any study in this evidence base.
Health effects / Measured in people or real products
In a 3-hour chamber test at realistic ozone and citrus-terpene (limonene) levels, people with mild asthma rated the air as less irritating to their eyes, nose, and throat than healthy people did, but reported more chest tightness, headache, and flu-like symptoms.
Moderate evidence.
Caveats
Single lab, n=33 mild asthmatics only (moderate/severe excluded by design), no unexposed/clean-air control arm, so this is a between-group comparison at one dose, not a test of the chemistry against no exposure.
Health effects / Measured in people or real products
People with mild asthma performed a concentration task significantly less accurately than healthy people during a 3-hour ozone-limonene chamber exposure, despite working faster.
Moderate evidence: Single study, no unexposed control arm, so it is not established that the chemistry (rather than general chamber occupancy or task load) caused the accuracy difference.
Health effects / Measured in people or real products
In the nasal-lavage arm of the same 130-woman chamber study, breathing a VOC-ozone mixture containing citrus and pine scent chemicals (limonene, alpha-pinene) caused no significant increase in nasal irritation symptoms or in objective inflammation markers (immune cells, IL-6, IL-8, protein) in nasal fluid.
Moderate evidence.
Caveats
Single 2-hour exposure in healthy, non-asthmatic, non-chemically-sensitive women; a different, smaller earlier study using a different lavage technique found a positive result at a similar VOC dose.
Source: Laumbach 2005, Journal of Occupational and Environmental Medicine Link to this fact
Health effects / Measured in people or real products
In a population-representative sample of about 2,900 Canadians (Canadian Health Measures Survey, 2012-2013), a 100% increase in measured residential indoor-air limonene was associated with a 17-18% relative increase in the adjusted odds of both wheezing (OR 1.17, 95% CI 1.16-1.18) and doctor-diagnosed asthma (OR 1.18, 95% CI 1.17-1.19), plus a 1.55% (95% CI 1.28-1.89) increase in exhaled nitric oxide (FeNO), an objective marker of airway inflammation.
Moderate evidence.
Caveats
Single cross-sectional study; cannot establish temporal sequence or fully rule out reverse causation, though the authors argue reverse causation would plausibly run the opposite way (airway-hypersensitive people avoiding scented products). Lung function (FEV1, FVC) showed no significant association with limonene.
Source: Dales & Cakmak 2019, Environmental Pollution Industry-funded Link to this fact
Health effects / Other evidence
The flavor industry's panel judged the food flavor d-limonene (also the most common fragrance terpene) still GRAS after the US National Toxicology Program found clear evidence of kidney tumors in male rats, reasoning the finding is irrelevant to human eating; this is a swallowed-route conclusion by the industry panel.
Moderate evidence: Industry panel's own reading; the male-rat alpha-2u-globulin mechanism is accepted by US EPA, but the call covers food intake, not skin or inhalation exposure to limonene.
Source: Hallagan 1995, Regulatory Toxicology and Pharmacology Industry-funded Link to this fact
Health effects / Other evidence
For 3 of the 18 flavorings (vanilla extract, cumin oil, apricot kernel oil) the panel found a child's lower body weight pushed intake above the generic safety threshold, and cleared each with a single animal no-effect level (ethyl vanillin 500 mg/kg/day in rats, D-limonene 215 mg/kg/day, apricot kernel oil 10 mg/kg/day in a one-dose 13-week feed study).
Moderate evidence: Each rescue relies on one animal study, in one case a surrogate chemical (ethyl vanillin for the vanillin group); industry panel's own judgment; no human child data.
Source: Guengerich 2026 (FEMA Expert Panel), Food and Chemical Toxicology Industry-funded Link to this fact
Health effects / Measured in people or real products
The 17 fragrances together contained 24 chemicals EWG classed as skin sensitizers, about 10 per product (19 in Armani Acqua Di Gio); limonene was in 16 products, linalool in 14, and geraniol in 12.
Moderate evidence: Presence is lab- or label-based; sensitizer status is EWG's literature classification, not a potency or dose assessment; the sensitizer table omits one of the 17 products.
Source: Sarantis et al. 2010 (Not So Sexy, Canadian edition) Link to this fact
Information, not medical advice. See also: myths we won’t tell you.