Limonene is a terpene found naturally in citrus peel.
Pure limonene is safe for most people to use, when it is properly diluted.
However, oxidized limonene (old limonene) may cause irritation in some people with dermatitis.
Limonene is a weak skin sensitizer, and labelled by some as a "worst ingredient" for skin.
But is limonene actually bad for your skin? Should you avoid limonene in skincare?
This article will cover:
- What is limonene?
- Why is limonene used in skincare?
- Is limonene safe?
- What is oxidized limonene?
- Sensitizing potential of pure limonene
- Sensitizing potential of oxidized limonene
- How to store limonene to prevent oxidation
- How to formulate skincare safely with limonene
- Who should avoid limonene?
What is limonene?
Limonene is a terpene found naturally in citrus peel.
Limonene is a naturally occurring compound found mainly in the skin of citrus fruits, including orange, lemon, lime and grapefruit.
Limonene is a cyclic monoterpene with the molecular formula C10H16.
Limonene has 3 isomers:
- D-limonene (also known as (R)-limonene, (+)-limonene) is found abundantly in citrus fruits.
- L-limonene (also known as (S)-limonene, (−)-limonene) is found in pine needles and cones.
- DL-limonene (also called dipentene) is a mixture of D-limonene and L-limonene.
These isomers are found in different amounts and ratios in various plants. The predominant form of limonene is D-limonene, which is present in nearly 98% of all citrus oils (Bråred Christensson et al., 2009).
D-limonene is the main constituent of cold-pressed essential peel oils from citrus fruits, including tangerine, sweet orange, bitter orange, grapefruit, mandarin, lemon, lime and bergamot.
Sweet orange essential oil contains ~ 95% limonene.
Why is limonene used in skincare?
Limonene is widely used in skincare products for:
Limonene is used for its fresh, pleasant citrus aroma. However, the purer the limonene is, the less odor it has (Chiralt et al, 2002; Sell et al, 2004; Kvittingen et al, 2021).
Commercially, D-limonene is isolated from orange peel. The cruder the D-limonene is, the more impurities it contains, and the more it will smell like orange (Kvittingen et al, 2021).
2. Cleansing activity
Limonene is used as a cleanser and a degreaser. Limonene is used as a solvent in water-free hand-cleansers at a concentration of about 10% (Karlberg et al, 1997).
3. Antioxidant activity
Limonene has antioxidant activity (Anandakumar et al, 2021) and may increase product shelf life. For instance, limonene has been shown to inhibit lipid oxidation, and is being investigated to extend shelf-life in food products (reviewed by Bora et al, 2020).
Learn more: Why is Limonene used in Skincare?
4. Permeation enhancer
Limonene is a permeation enhancer (Mendanha et al, 2017) and can boost absorption of nutrients and active ingredients (Valgimigli et al, 2012).
Is limonene safe?
Limonene is considered to have low toxicity (reviewed by Sun, 2007) and is recognized as a safe ingredient for skincare under the current regulatory guidelines for cosmetics (reviewed by Kim et al, 2013).
In the USA, the FDA considers limonene as GRAS (generally recognized as safe) when used as a food additive or flavoring, and as a fragrance additive.
In Europe, limonene is classified as a potential skin sensitizer. This means that it has the potential to cause a temporary skin reaction (such as red, bumpy, or itchy skin) in some individuals. The pure compound limonene can also be broken down to oxidized limonene, e.g. limonene hydroperoxides, which are potential allergens in sensitive individuals (Topham et al, 2003).
There is no max dermal limit for limonene, but the EU Cosmetics Regulations require companies to declare if limonene is present at more than 0.001% in a leave-on or 0.01% in a rinse-off product.
What is oxidized limonene?
Typically, limonene has a shelf-life of 1 year.
Limonene becomes oxidized limonene when it is exposed to oxygen in the air. This process happens naturally over time, but occurs faster when limonene is improperly processed or stored.
When limonene is exposed to oxygen, it breaks down into hydroperoxides called limonene-1-hydroperoxide and limonene-2-hydroperoxide, which are both strong contact allergens (de Groot, 2019).
Limonene-1-hydroperoxide is a strong sensitizer (Bråred Christensson et al, 2008) and a clinically important allergen (Bråred Christensson et al, 2014).
There is potential for skin sensitization to occur when skin is repeatedly exposed to limonene break down products.
To induce oxidized limonene in a lab setting, limonene preparations are exposed to air in an open flask and stirred multiple times a day for 2 months (Karlberg et al, 1991; Karlberg et al, 1992). This procedure was developed in order to mimic industrial handling of limonene when used as solvent (e.g. in paints or as degreasing agent for metal workers), but later it was also applied to various terpenes as a routine test for oxidation (Kern et al, 2014).
While this 'worst-case scenario' is useful to evaluate and compare the potential of different terpenes and essential oils to spontaneously oxidize, it is not directly relevant to how limonene is stored or used in consumer products or skincare products (Kern et al, 2014).
For instance, during storage, limonene can be protected from oxidation by several factors, including protection from light, heat and oxygen exposure.
In consumer products, limonene is used in a diluted form and mixed with other ingredients, including antioxidants. In addition, consumer products that contain limonene are often sold in closed containers, with limited air contact (Kern et al, 2014).
Indeed, the stability of limonene in fine fragrances was tested, and the data indicate that limonene was stable over a 9-month period, with no accumulation of oxidation products (Kern et al, 2014).
Sensitizing potential of pure limonene
The pure compound limonene is a weak sensitizer (Karlberg et al, 1991; Karlberg et al, 1992), and pure limonene rarely causes positive patch test reactions (Santucci et al, 1987; Matura et al, 2005; Schnuch et al, 2007).
In 10 studies in which pure limonene (non-oxidized) was routinely patch tested at 1%, 2%, 3%, or 10% during 1977–2017, frequencies of sensitization ranged from 0% to 1.6%, but 9 of 10 scored 0.7% or lower (reviewed by Matura et al, 2002; de Groot et al, 2019).
No positive skin reactions were observed when testing 2% pure D-limonene in 1200 patients with contact dermatitis (Santucci et al, 1987).
No positive skin reactions were observed when testing 2% pure D-limonene in 320 patients with eczema, who were suspected of having contact allergy to fragrances (van Oosten et al, 2009).
No positive skin reactions were observed when testing 20% pure D-limonene in 20 dermatitis patients (Brared Christensson et al, 2009).
High concentrations of limonene may be a dermal irritant for some individuals (Falk et al, 1991, York et al, 1995, Topham et al, 2003).
Irritant contact dermatitis among workers in citrus fruit canning industries has been reported (Beerman et al, 1938; Schwartz et al, 1938; Birmingham et al, 1951).
To reduce the risk of irritation, limonene should always be properly diluted before use on skin.
Sensitizing potential of oxidized limonene
Oxidized D-limonene has been tested in several multicenter studies.
At 2 European centers, between 1991 and 1995, there were 2800 consecutive patients with dermatitis that were tested with 2%, 3% or 5% D-limonene that was air-oxidized for 10 or 20 weeks. Altogether, 32 out of 1318 (2.4%) patients reacted to any of the test preparations of D-limonene used at the Stockholm test center, while only 17 out of 1482 (1.1%) patients reacted at the Leuven test center (Karlberg et al, 1997).
Patients were also tested with 0.5% or 1% limonene-hydroperoxides. Altogether, 19 out of 685 (2.8%) patients reacted to the hydroperoxide fraction in the Stockholm population. Only the 0.5% limonene-hydroperoxide fraction was tested in Leuven, with 4 out of 869 (0.9%) testing positive (Karlberg et al, 1997).
In another multicenter study in Europe in 2002, a total of 2273 patients with dermatitis were tested using 3% oxidized limonene, and 2.8% patients showed positive patch test reactions (Matura, 2002).
In 2009, a total of 2900 consecutive dermatitis patients were evaluated in 9 centers in Copenhagen, Malmo, Odense, Barcelona, London, Singapore, Melbourne, Seville and Gothenburg. The overall prevalence of positive reactions to 3% oxidized limonene was 5.2%, ranging from 2.3% to 12.1% in the individual centers (Bråred Christensson et al, 2013).
These studies led to the development of standardized patch test materials of oxidized limonene 3%, with a standardized content of limonene hydroperoxides at 0.3% (hydroperoxides of Limonene 0.3% pet.®) which are manufactured by Chemotechnique Diagnostics AB, Vellinge, Sweden (Ogueta et al, 2021).
In subsequent multicenter and single-center studies, patch testing with limonene hydroperoxides at 0.3% has shown positive patch test reactions in 1.2% to 9.4% of cases (reviewed by Ogueta et al, 2021).
How to store limonene to prevent oxidation
Temperature, light, and oxygen availability are recognized to have a crucial impact on essential oil integrity (reviewed by Turek et al, 2013).
During storage, limonene can be protected from oxidation by several factors, including protection from light, heat and oxygen exposure.
To avoid oxidation of d-limonene, citrus oils should be stored in a dark air-tight container and placed at 4◦C (Karlberg et al, 1994; Tisserand et al, 2014).
Cold and dark storage of d-limonene in closed vessels has been shown to prevent auto-oxidation for 1 year without addition of anti-oxidant (Karlberg et al, 1994).
However, adding antioxidants to limonene is recommended (Karlberg et al, 1994; Tisserand et al, 2014).
How to formulate skincare safely with limonene
To reduce the risk of irritation, old, or expired limonene should be avoided.
Limonene should always be properly diluted when used for skincare, with safety ranges suggested between 0.1 - 3%. Limonene should never form more than 5% of the final cosmetic product, as this increases the risk of potential irritation.
Skincare products should be packaged in closed containers with pump lids to minimize air contact.
Who should avoid limonene?
People who are sensitive or allergic to limonene should avoid limonene and citrus essential oils.
People who have a disturbed or damaged skin barrier should avoid limonene or use limonene with caution, especially with leave-on products.
Pure limonene is safe for most people to use, when it is properly diluted.
Oxidized limonene may cause irritation in people with dermatitis or other skin issues.
Always choose fresh products, and avoid the use of old or expired products that contain limonene.
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