d-Limonene

Aroma and sensory profile

Fragrance references describe d-limonene at full strength as citrus, orange, fresh, and sweet, and its taste at 30 parts per million as sweet, orange, citrus, and terpene-like [3]. It is the smell of freshly zested orange peel more than of orange juice, since peel oil is about 95 percent d-limonene [4].

Limonene is chiral. The (R)-(+) form, d-limonene, makes up 98 to 100 percent of the limonene in most citrus oils, while the (S)-(-) form predominates in oils such as citronella and lemongrass [4]. Textbooks have long assigned orange to the (R) form and lemon to the (S) form. That pairing does not hold up. In two odor tests with analytical-grade samples, few participants matched (R)-(+)-limonene to oranges or (S)-(-)-limonene to lemons, and the claim was traced to a single 1971 article repeated without checking [1]. A 2025 follow-up treats the orange-versus-lemon story as a myth and questions whether the odors attributed to each enantiomer come from limonene itself or from derivatives formed in the nose [2]. The two forms are reported to differ in character and intensity; a fruit-for-fruit assignment is not supported.

Effects and research

The strongest human evidence concerns anxiety. In a double-blind crossover trial, 20 healthy adults who used cannabis intermittently inhaled vaporized THC alone at 15 or 30 mg, d-limonene alone, both together, or placebo across nine sessions. Giving 30 mg THC with 15 mg d-limonene significantly reduced ratings of feeling anxious or nervous and paranoid compared with 30 mg THC alone, while d-limonene by itself produced no notable effects [5]. This is a single small study and it measured limonene's ability to blunt a THC effect, not a standalone benefit.

Everything else is preclinical. A 2025 pharmacology review reports that limonene suppresses Escherichia coli and Staphylococcus aureus in vitro, that oral d-limonene lowered plasma glucose and glycosylated hemoglobin in an animal diabetes model, and that it inhibited lymphoma cell proliferation in mice and altered p53 and Bcl-2 expression in leukemia cells in vitro [6]. None of these findings has been confirmed in people, and IARC's earlier assessment of the animal cancer data found only limited evidence [4]. Claims that limonene lifts mood or relieves depression in humans have no trial behind them in the sources reviewed here.

Natural sources

Limonene is, with the possible exception of alpha-pinene, the most frequently occurring monoterpene in nature. It is the major constituent of citrus peel oils and appears at lower levels in many other fruits and vegetables [4]. IARC records it in more than 300 essential oils [4]. Orange peel oil and the essence oil recovered from juice processing run about 95 percent d-limonene, which is why commercial d-limonene is a by-product of the citrus juice industry rather than something synthesized [4]. Outside citrus, the (S)-(-) form dominates in citronella and lemongrass oils [4]. Cannabis flower also produces limonene, but at fractions of a percent of plant mass, far below the concentration in a single orange peel.

Boiling point and vaporization

PubChem gathers several published boiling points for (+)-limonene: 175 to 177 °C from JECFA, 348 to 349 °F from CAMEO Chemicals, 177.6 °C from the Hazardous Substances Data Bank, and 178 °C from the ILO-WHO chemical safety card [7]. This profile uses 176 °C (349 °F), inside the JECFA range. In practice that means limonene is among the first constituents to leave a citrus oil during distillation and is lost quickly from an open pan or a hot vaporizer chamber. Heating it far beyond that point is where the hazard begins: in a study that heated pure limonene, myrcene, and linalool under conditions simulating dabbing, methacrolein rose from 131 parts per billion per dose at 403 °C to 185 at 526 °C, and benzene appeared only at the highest temperature tested [8]. The terpene evaporates intact at less than half those temperatures.

Safety and regulation

Limonene in its d-, l-, and dl- forms is listed by the US Food and Drug Administration among synthetic flavoring substances generally recognized as safe for their intended use in food, at 21 CFR 182.60 [9]. IARC evaluated d-limonene in 1993 and placed it in Group 3, not classifiable as to its carcinogenicity to humans, citing limited evidence in animals and no human data [4].

The documented hazard is on the skin, and it comes from oxidation rather than from limonene itself. Fresh limonene has low sensitizing potential, but on exposure to air it forms hydroperoxides that are potent sensitizers [10]. In consecutive patch testing of dermatitis patients, 9.4 percent reacted to hydroperoxides of limonene [10]. An opened bottle of citrus oil or a limonene-containing product that has sat in air is therefore a different material from a fresh one. No inhalation-specific human safety study was found among the sources reviewed.

Industries and uses

Fragrance and cosmetics: d-limonene and orange oil have been used for roughly fifty years as fragrance additives in perfume and soap [4], where it supplies the opening citrus note. Culinary: the same materials are flavor additives in food and beverages, and limonene holds FDA flavoring status [4][9]. Cleaning: d-limonene serves as a solvent, cleaner, and odorant, including in the petroleum industry, which is why citrus-scented degreasers exist [4]. Aromatherapy: citrus peel oils are staple materials in the practice; no source in this review evaluates aromatherapy use specifically.