Limonene is a compound naturally found in citrus fruits that is used as a fragrance in cosmetics and personal care products, and is also added to foods to enhance flavor.
Limonene is a clear, colorless liquid that is the major component in the oil of citrus fruit peels, including lemon, orange, mandarin, lime, and grapefruit. The name “limonene” was derived from the name of lemon, Citrus limon. Limonene is classified as a cyclic monoterpene. It is one of the most common terpenes in nature.
Limonene is a chiral molecule. A chiral molecule is non-superimposable on its mirror image, which is known as an enantiomer or isomer. Biological sources (citrus fruit) produce D-limonene, which is the (R)-isomer. D-limonene is obtained commercially from citrus fruits through two primary methods: centrifugal separation or steam distillation. The less common L-isomer is found in mint oils and has a piny, turpentine-like odor. For the purposes of this article, the term “limonene” will refer to the D-limonene isomer.
Limonene is used in perfumes, soaps, and various other personal care products because of its pleasant fragrance, penetration enhancing capabilities, and anti-inflammatory properties. Limonene has a light, fresh, and sweet citrus odor.
According to an article published in Prime, the International Journal of Aesthetic and Anti-Ageing Medicine, limonene is known for its capacity for high penetration particularly appreciable in the skin, which is why it is referred to as an ‘enhancer’. The enhancing power of limonene enables it to be used as a carrier for other molecules that are unable to cross epithelial barriers alone. In fact, limonene and other terpenes are used to improve transdermal drug delivery since they reduce skin barrier resistance with low irritancy potential.
Essential oils that contain limonene, such as orange peel oil, are also used to enhance immunity, according to Dr. Josh Axe. Orange oil even has cancer-fighting abilities, since monoterpenes have been shown to be very effective chemopreventive agents against tumor growth.
How Limonene benefits the skin
Studies on limonene have demonstrated that this ingredient has the ability to reduce inflammation through numerous mechanisms. One way limonene reduces inflammation is through the inhibition of angiogenesis. Angiogenesis is the formation of new blood vessels, which is an inherent component of inflammation. Thus, by inhibiting this process, limonene can help to reduce inflammation and also improve wound healing. It is also known that terpenes from essential oils, such as limonene, are able to repress the production of tumor necrosis alpha (TNF) and interleukin-6 (IL-6), both being major pro-inflammatory cytokines.
Like most volatile fragrance components, limonene has strong antioxidant benefits. Antioxidants are also known as reducing agents, which means they can donate an electron to another atom or molecule. By donating electrons to free radicals, antioxidants are able to limit oxidative damage caused by free radicals. The International Dermal Institute explains, “Antioxidants help prevent such damage by giving free radicals what they want before they attack your cells.” This is important because when oxidative damage accumulates in the skin the result is inflammation, uneven skin tone, and premature signs of aging. Therefore, it’s important to reinforce your skin by eating a diet rich in antioxidant foods and also by applying antioxidant-rich skincare products.
However, some skin care experts believe that the antioxidant effects of limonene may be negligible because once these highly volatile antioxidant compounds are exposed to air, they oxidize and become capable of sensitizing skin.
Limonene is listed in the Code of Federal Regulation as GRAS (Generally Recognized As Safe) and precise concentrations are indicated for its usage as a flavoring agent.
According to a 2013 safety evaluation published in the Journal of Toxicology and Environmental Health, skin irritation or sensitizing potential was reported following widespread use of this agent in various consumer products. In experimental animals and humans, oxidation products or metabolites of d-limonene were shown to act as skin irritants. Other than the adverse dermal effects noted in humans, other notable toxic effects of d-limonene have not been reported. Based on this information, it is often recommended to those with sensitive skin or skin conditions like eczema, psoriasis, or rosacea to avoid products containing limonene.
References: Wikipedia, “Limonene”, Altern Med Rev. 2007 Sep;12(3):259-64, PRIME Journal, “Anti-inflammatory and skin repair treatments with d-limonene”, 2015, Cosmetics Info, “Limonene”, AAPS J. 2008 Mar; 10(1): 120, Dr. Axe, “Orange Oil”, Indian J Exp Biol. 2002 Mar;40(3):245-51, Wikipedia, “Antioxidant”, Dermalogica, “Better Skin Through Antioxidants”, Paula’s Choice, “Limonene”, J Toxicol Environ Health B Crit Rev. 2013;16(1):17-38.