Limonene

Limonene is a naturally occurring monoterpene with the molecular formula C10H16. Being a chiral molecule, it exists as two mirror-image forms: D-limonene, the right-handed R-enantiomer that smells of sweet oranges and is by far the more common in nature, and L-limonene, the left-handed S-enantiomer with a sharper, turpentine-like odor. It is a clear, oily liquid that does not mix with water but dissolves readily in oils and organic solvents ^[3].

The compound is the principal component of the essential oil in citrus fruit peel, and it is produced commercially in large quantities as a byproduct of orange-juice manufacturing, recovered from the peels by pressing and steam distillation. It is also found in many other plants, including conifers such as pine and spruce and herbs such as caraway and dill, and it is one of the characteristic terpenes of cannabis ^[3].

Limonene has an unusually broad range of uses. In the food and fragrance industries it serves as a flavoring and a scent, lending citrus notes to foods, drinks, perfumes, and household products, and it can mask bitter tastes in medicines ^[3]. Its ability to dissolve oils makes it a popular ingredient in cleaning products and hand cleaners and an industrial solvent for degreasing and adhesive removal; in laboratories it is used as a safer substitute for xylene when preparing tissue samples, and it can dissolve certain plastics used as supports in 3D printing.

Because it is so widely consumed and inhaled, limonene has been studied for possible biological effects. Laboratory and animal research points to antioxidant, anti-inflammatory, and gastroprotective activity, and reviews have explored its potential in areas ranging from neuroprotection to cancer research ^[2]. Much of this evidence is preliminary, however, and there is no consistent proof that taking limonene, whether as a supplement or by eating citrus, prevents or treats cancer in people. It is sometimes sold as a dietary supplement and has been investigated for symptoms such as heartburn.

Pure limonene is regarded as having low toxicity when swallowed and is treated as generally safe for use in food and cosmetics under current guidelines ^[1]. Its main drawback is on the skin: limonene itself is a mild irritant, and when it is exposed to air it slowly oxidizes into compounds that are more potent allergens and can cause contact dermatitis in sensitized people ^[1]. High-dose studies in male rats produced kidney tumors, but this effect depends on a protein that humans do not make and is not considered relevant to human risk ^[1]. The liquid is flammable and is classed as harmful to aquatic life.

Generally recognized as safe as a food component and well tolerated. High doses can cause mild GI upset, and concentrated citrus oils can irritate skin. It may interact with drug-metabolizing enzymes at high intakes.