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 / Other evidence
The estimated mass of new particles in the 0.1-0.3 um range was about 27-40 ug/m3 with injected limonene and about 3-24 ug/m3 with a lemon-scented solid air freshener.
Weak evidence: Estimates from counts at 0.1 um and above by assumption, not weighed; chamber at 60-100 ppb ozone, well above typical indoor levels outside summer peaks.
Source: Wainman 2000, Environmental Health Perspectives Link to this fact
Indoor air / Test-chamber measurement
A 15-second spray of lemon-scented furniture polish on one coffee table in a 25 m3 chamber raised limonene to 175 ppb (about 975 ug/m3), and it took about 3 hours to clear.
Weak evidence: One run, one product; the run had no added ozone and no particle measurement, so the particle risk from polish is the authors' inference.
Source: Wainman 2000, Environmental Health Perspectives Link to this fact
Indoor air / Other evidence
In the model, limonene is the largest single VOC loss for each of the three oxidants (ozone 68%, NO3 59%, OH 24% of each oxidant's VOC loss), but OH also attacks ethanol, formaldehyde and other VOCs, so limonene is only about a quarter of OH conversion; this is a model result, not a measurement.
Weak evidence: Model with median VOC concentrations; limonene is not specific to fragrance (also wood and cleaning products); products formed were not computed.
Source: Waring & Wells 2015, Atmospheric Environment Link to this fact
Indoor air / Other evidence
Plants kept in water gave off far more limonene in a lab chamber (about 810 vs 74 micrograms an hour) than plants in soil, but in real rooms the extra limonene was only about 1,000 vs 530 micrograms over 3 hours, about 14% of what the lab rate predicted.
Weak evidence.
Caveats
Lab rate assumes all monoterpene signal is limonene; number of plants in the chamber not stated; reasons for the gap (ozone reactions, air exchange, sorption) were named but not measured.
Indoor air / Other evidence
Limonene alone made only a small share of the aerosol from these products: the authors estimate about 0.4-2.8% of the seeded and 1.9-26.5% of the unseeded aerosol, so other product ingredients mattered as much or more.
Weak evidence: Calculated from limonene content and pure-limonene yield; the other precursors are not identified; chamber at 6.5 ppm ozone.
Indoor air / Other evidence
Adding seed particles raised aerosol yields 1.5 to 14 times, but pure limonene made about 52-53% aerosol with or without seed, so how much aerosol a scented product makes depends on the particles already in the air.
Weak evidence: Single lab, chamber at 6.5 ppm ozone; ammonium sulfate seed only; mechanisms not tested (no product speciation).
Skin allergy / Measured in people or real products
People without skin disease reacted to oxidized limonene and linalool as strongly as dermatitis patients.
Weak evidence: Small groups (9 controls per comparison after exclusions).
Source: Bråred Christensson 2009, Contact Dermatitis Industry-funded Link to this fact
Skin allergy / Lab study (cells or chemistry)
In lab tests on human skin cells, the oxidized forms of limonene and linalool reshaped the cells' fats in the same way as a classic potent skin allergen (DNFB), a pattern consistent with oxidative stress.
Weak evidence: One cell line, one dose.
Skin allergy / Lab study (cells or chemistry)
In lab tests on human skin cells, the air-oxidized forms of limonene and linalool (the forms that cause fragrance allergy) damaged the fats in cell membranes at concentrations from 100 µM, with oxidized limonene strongest; a possible second route to skin inflammation not yet shown in people.
Weak evidence: Cell line and liposomes only; added iron; real skin doses unknown.
Skin allergy / Measured in people or real products
Once someone is sensitized, patch tests can trigger reactions at low ppm concentrations of oxidized terpene, but the specific 'as low as 1 ppm' figure is for Δ³-carene in older, narratively-reviewed occupational turpentine literature; for the two terpenes that dominate consumer exposure, modern peer-reviewed dose-response studies put the threshold one to two orders of magnitude higher (limonene hydroperoxides: ~24 ppm single patch application, ~140 ppm repeated realistic exposure, ED₁₀ ≈85 ppm; linalool hydroperoxides: ~560 ppm repeated realistic exposure).
Weak evidence.
Caveats
The 1 ppm figure applies to Δ³-carene in historical occupational patch testing, not to limonene or linalool, and its own study design (occlusion, participant numbers) is not given in the citing review. No limonene- or linalool-specific study supports reactions at ~1 ppm.
Source: Karlberg & Lepoittevin 2021, Contact Dermatitis Link to this fact
Skin allergy / Review of other studies
A 2007 clinical review reports that oxidized limonene caused positive patch-test reactions in about 2.6% of dermatitis patients in a European study, leading to a call to add oxidized limonene to routine allergy screening.
Weak evidence.
Caveats
Secondary relay of Matura et al. 2006 (not independently read in this evidence base); this is an early statement of a position later found by other groups (e.g. the IVDK) to over-call weak/doubtful reactions as true allergy.
Source: Brasch 2007, Current Opinion in Allergy and Clinical Immunology Link to this fact
Skin allergy / Measured in people or real products
Reading a patch test a second time on day 7 revealed extra hydroperoxide allergies: 8% of limonene-hydroperoxide and 4% of linalool-hydroperoxide positives were negative or doubtful on day 3.
Weak evidence: One-clinic count (6 of 77 and 4 of 96); a day-7 reading is a clinic practice, not something consumers control.
Source: Dittmar & Schuttelaar 2019, Contact Dermatitis Link to this fact
What you can do / Measured in people or real products
Keep scented products closed and avoid old, long-opened ones: limonene and linalool became skin irritants after weeks of air exposure.
Weak evidence.
Caveats
Extrapolated from lab-oxidized neat material (10-20 weeks air exposure); product storage not tested by this paper. Two independent industry market surveys have now directly tested the specific mechanism this advice assumes (in-bottle oxidation with age) and both found it absent: Kern 2014 (39 aged fine fragrances + 29 antiperspirants, 9 months accelerated storage) and Natsch 2019 (104 products spanning fine fragrances, antiperspirants, creams, and lotions, including two fine fragrances tracked 3-6 years at room temperature) both found no progressive hydroperoxide accumulation with age, and consumer-product doses 2-4 orders of magnitude below elicitation/sensitization thresholds. Both studies are industry-funded (IDEA consortium: Givaudan, Firmenich, IFF, IFRA) and not independently replicated by a non-industry group, and neither tests essential oils or raw materials (separately known to accumulate hydroperoxide) or every product category. Moved to claims-to-avoid because the specific mechanistic basis for this advice (in-container autoxidation) is now negative in the only two real-product tests that exist, even though the underlying autoxidation chemistry itself, in neat/essential oils, is not disputed. Kern 2014 and Natsch 2019 are both industry studies (Givaudan / IDEA consortium) with Natsch as a co-author on both, so they are not independent replications of each other.
Source: Bråred Christensson 2009, Contact Dermatitis Industry-funded Link to this fact
What you can do / Measured in people or real products
Using less shower product and running the bathroom fan both lower the fragrance-terpene peak: emissions scaled with product amount, and halving ventilation raised the modeled limonene peak from about 375 to 495 ppb.
Weak evidence: Amount-scaling from one person's shower; ventilation effect is modeled; no health outcome measured.
What you can do / Review of other studies
If households switch from fragranced to fragrance-free laundry detergent, modeling suggests dryer-vent limonene emissions across a whole city or state could drop substantially — an estimated 1.58 tons per year for metropolitan Melbourne, or 8.66 tons per year for the state of California.
Weak evidence.
Caveats
This is a modeled extrapolation, not a measurement: it multiplies a mean vent concentration from just 2-6 households (one grab sample each) by census household counts and usage-rate assumptions, with no confidence interval or sensitivity analysis anywhere in the calculation; the Melbourne input is dominated by a single outlier household roughly 11 times the other three.
Source: Goodman, Nematollahi & Steinemann 2020/2021, Air Quality, Atmosphere & Health Link to this fact
What you can do / Review of other studies
Close deodorant containers after use and replace them about a year after opening: two of the most common deodorant fragrances (limonene, linalool) are rated weak allergens fresh but oxidize in air into much stronger sensitizers.
Weak evidence.
Caveats
Klaschka relays this oxidation chemistry from prior studies (Karlberg 1994, Matura 2005, Skold 2008), already in our evidence base; she adds no new dose data of her own. Downgraded to weak 2026-09-28 (Jev audit): the action goes beyond what this paper itself tested.
Source: Klaschka 2012, International Journal of Hygiene and Environmental Health Link to this fact
What you can do / Hypothesis
Unlike outdoor air pollutants such as vehicle exhaust, people can directly reduce their exposure to limonene at home by limiting use of strongly limonene-scented personal-care products and household cleaners.
Weak evidence: This is the authors' own inference from a cross-sectional association, not a tested intervention; no randomized avoidance trial has been done, though the authors call for one.
Source: Dales & Cakmak 2019, Environmental Pollution Industry-funded Link to this fact
Information, not medical advice. See also: myths we won’t tell you.