If you Google "is d-Limonene safe," you'll find a confusing mix of answers. Some sites call it a miracle ingredient. Others mention cancer studies in rats. A few suggest it irritates skin. Most don't explain the actual science behind any of it.
We use d-Limonene as one of only three ingredients in Carbon Cleanse, so we have a direct interest in being honest about this. Not promotional. Not evasive. Just straightforward about what the research says — including the parts that require nuance.
Here's the full picture.
The Short Answer
D-Limonene has been extensively studied and is classified as safe for human use by the FDA, the EPA, and multiple independent scientific review panels. It occurs naturally in citrus fruits, is present in thousands of food and consumer products, and has been consumed by humans for as long as people have been eating oranges.
That said, the full answer has more layers — and you deserve to see all of them.
What the FDA Says
The U.S. Food and Drug Administration classifies d-Limonene as Generally Recognized as Safe (GRAS) under 21 CFR 182.60. This means it has been approved as a flavoring agent and food additive. You'll find it in fruit juices, soft drinks, baked goods, ice cream, pudding, candy, and chewing gum.
GRAS status isn't handed out casually. It requires a long history of safe use or sufficient scientific evidence demonstrating that the substance is safe under its intended conditions of use. D-Limonene meets both criteria — it's been part of the human diet through citrus fruit consumption for centuries, and it's been studied in formal toxicology settings for decades.
What the EPA Says
The U.S. Environmental Protection Agency has reviewed d-Limonene multiple times. Their most recent human health risk assessment found no data deficiencies in the toxicology database and concluded that quantitative risk assessments weren't even necessary — the existing evidence was sufficient to determine that currently registered uses of d-Limonene don't pose risks of concern.
The EPA also classifies d-Limonene as eligible for 25(b) minimum risk exemption under FIFRA (the Federal Insecticide, Fungicide, and Rodenticide Act). This is a category reserved for pesticide active ingredients that the EPA considers so low-risk they don't require formal registration. Products qualifying for 25(b) exemption must use only ingredients classified as minimal risk by the EPA or recognized as safe by the FDA.
What 25(b) means in practice: The EPA has enough confidence in d-Limonene's safety profile that products containing it as an active ingredient face fewer regulatory hurdles than conventional cleaning chemicals. Every ingredient must be disclosed on the label, and all components — active and inert — must come from approved low-risk substance lists.
The Cancer Question — and Why It Won't Go Away
This is the part most brands skip over. We're not going to.
In 1990, the National Toxicology Program (NTP) published a study that found kidney tumors in male rats exposed to high doses of d-Limonene. That finding entered the scientific record, and it still shows up in search results and ingredient databases today. If you've seen a warning flag next to d-Limonene somewhere online, this study is almost certainly why.
Here's what happened next — and why the finding doesn't apply to humans.
The Myth: "D-Limonene causes cancer."
The Reality: The kidney tumors observed in the NTP study occurred exclusively in male rats — not in female rats, not in mice of either sex, and not in any other species tested. Subsequent research identified the specific mechanism: a protein called α2u-globulin that is produced only by adult male rats. When d-Limonene metabolites bind to this protein, it accumulates in the kidney and causes cell damage. No other species — including humans — produces this protein or experiences this effect.
This isn't a matter of scientific debate. The mechanism has been thoroughly established across multiple peer-reviewed studies. The International Agency for Research on Cancer (IARC) evaluated d-Limonene and determined that the male rat kidney tumors result from a species-specific mechanism that has no relevance to human cancer risk. Both d-Limonene and its primary oxide metabolite tested negative in mutagenicity screens, meaning they don't cause the kind of DNA damage that drives cancer in humans.
A comprehensive risk assessment published in the Journal of Toxicology and Environmental Health calculated the margin of exposure (MOE) at 169 and the hazard index (HI) at 0.592 — both well within safe limits. The researchers' conclusion: d-Limonene poses no serious risk for human exposure and may be regarded as a safe ingredient.
More interesting still: subsequent research has found that d-Limonene actually demonstrates chemopreventive properties — meaning it may help prevent cancer rather than cause it. Studies have shown it inhibited lung carcinogenesis in mice, preneoplastic stages of colon carcinogenesis in rats, and pancreatic carcinogenesis in hamsters when used alongside known carcinogens.
What About Skin Irritation?
This is where d-Limonene safety does require some nuance, and we want to be upfront about it.
D-Limonene itself is generally well-tolerated on skin. However, when d-Limonene is exposed to air over time, it can oxidize and form byproducts — primarily limonene hydroperoxides — that have been shown to cause skin sensitization in some individuals. This is why some people report irritation from old or improperly stored citrus-based products.
This is a real concern, but it's a formulation and storage concern, not an inherent toxicity concern. There are important differences:
Fresh d-Limonene in a properly formulated product has low irritation potential. The EPA's own data classifies it as having low acute dermal toxicity.
Oxidized d-Limonene — from products left open, stored in heat, or sitting on shelves for years — can develop sensitizing compounds. This is why proper packaging, storage, and shelf-life management matter.
Concentration matters. At the 5% concentration used in Carbon Cleanse, d-Limonene is well within the range considered safe for regular use. Many industrial degreasers use concentrations of 80-95%, which is an entirely different exposure scenario.
What we do about it: Carbon Cleanse is formulated at a 5% d-Limonene concentration, packaged in opaque bottles to minimize oxidation from light exposure, and designed for use as a spray-and-wipe cleaner — not a product that sits on your skin for extended periods. If you have known citrus sensitivities, we recommend testing on a small area first, just as you would with any cleaning product.
What Independent Safety Organizations Say
Beyond the FDA and EPA, several independent organizations have evaluated d-Limonene:
The Environmental Working Group (EWG) includes d-Limonene in both their Skin Deep cosmetics database and their Guide to Healthy Cleaning. Their assessment notes that it is allowed in EWG VERIFIED products (their strictest tier) with adequate substantiation. The primary concerns they flag — skin irritation and sensitization — are rated as low and relate primarily to the oxidation issue discussed above.
The International Agency for Research on Cancer (IARC) evaluated d-Limonene and determined the male rat tumor mechanism is species-specific and not relevant to human cancer risk.
The Code of Federal Regulations lists d-Limonene as GRAS under 21 CFR 182.60, and it appears in the FEMA (Flavor and Extract Manufacturers Association) GRAS list as well — an independent industry evaluation separate from the FDA's own assessment.
The Joint Expert Committee on Food Additives (JECFA), administered by the FAO and WHO, has also evaluated d-Limonene for use in food products internationally.
Clinical Use in Humans
D-Limonene isn't just passively tolerated by the human body — it has been used in clinical settings. Published research documents its use as a gallstone solubilizer (it dissolves cholesterol-containing gallstones), and as a treatment for heartburn and gastroesophageal reflux (GERD) due to its gastric acid neutralizing properties. In these applications, patients consumed d-Limonene orally at therapeutic doses for extended periods with low reported toxicity.
These clinical applications don't directly relate to its use as a cleaning ingredient, but they do demonstrate something important: d-Limonene has been studied in human subjects, at doses well above what you'd encounter using a spray cleaner, and the safety profile holds up.
The Full Picture: What's Real, What's Not
✓ TRUE: D-Limonene is FDA GRAS, EPA-reviewed, and classified as low toxicity across every major regulatory body that has evaluated it.
✓ TRUE: Kidney tumors were observed in male rats in a 1990 NTP study — but the mechanism (α2u-globulin accumulation) is specific to male rats and has been conclusively determined to be irrelevant to humans.
✓ TRUE: Oxidized d-Limonene can cause skin sensitization in some people. This is a product formulation and storage issue, not an inherent toxicity issue.
✓ TRUE: At the concentrations used in consumer cleaning products, d-Limonene poses no serious risk for human exposure.
✗ FALSE: D-Limonene causes cancer in humans. There is no evidence supporting this claim. The compound is non-mutagenic and non-genotoxic.
✗ FALSE: D-Limonene is a harsh chemical. It's a naturally occurring monoterpene found in every orange, lemon, lime, and grapefruit you've ever eaten.
Why Carbon Cleanse Uses d-Limonene
We use d-Limonene at 5% concentration because it's one of the most effective natural degreasers available — capable of dissolving grease, oil, adhesives, and grime that water alone can't touch. Combined with decyl glucoside (our plant-derived surfactant) and purified water, it creates a cleaning formula that's three ingredients, fully disclosed, and backed by decades of safety research.
We don't use d-Limonene despite the research. We use it because of the research.
If you want to learn more about the other ingredients in Carbon Cleanse, read our companion posts:
→ What Is d-Limonene? The Natural Citrus Solvent Behind Carbon Cleanse
→ What Is Decyl Glucoside? The Plant-Based Surfactant Behind Carbon Cleanse
Three Ingredients. Full Transparency.
No hidden formulas. No fine print. Just the cleanest clean you've ever had.
Shop Carbon Cleanse →Sources & Further Reading
Yoon, K. et al. — Safety Evaluation and Risk Assessment of d-Limonene. Journal of Toxicology and Environmental Health, Part B, 16:1, 17-38 (2013)
Sun, J. — D-Limonene: Safety and Clinical Applications. Alternative Medicine Review, 12(3), 259-264 (2007)
IARC Monographs on the Evaluation of Carcinogenic Risks to Humans — d-Limonene, Volume 73 (1999)
Review of Toxicological Assessment of d-Limonene, a Food and Cosmetics Additive. Food and Chemical Toxicology (2018)
EPA — d-Limonene: Revised Human Health Risk Assessment (2020)
EPA — Limonene: Reregistration Eligibility Decision (RED) Fact Sheet
Lehman-McKeeman, L.D. — The Human Relevance of the Renal Tumor-Inducing Potential of d-Limonene in Male Rats (1991)
Dietrich, D.R., Swenberg, J.A. — The Presence of α2u-Globulin Is Necessary for d-Limonene Promotion of Male Rat Kidney Tumors. Cancer Research, 51(13), 3512-3521 (1991)
EWG Skin Deep — d-Limonene Ingredient Profile
FDA — d-Limonene: Food Substance Listing (GRAS, 21 CFR 182.60)
0 comments