Limonene is a terpene and is found naturally in the skin of citrus fruits, including orange, lemon, lime and grapefruit.  

Limonene is a colorless, liquid hydrocarbon, and is classified as a cyclic monoterpene with the molecular formula C10H16.

Limonene has 3 isomers:

1. D-limonene, also known as (R)-limonene, (+)-limonene, is found abundantly in citrus fruits.

2. L-limonene, also known as (S)-limonene, (−)-limonene, is found in pine needles and cones.

3. DL-limonene, also called dipentene, is a mixture of D-limonene and L-limonene.

These limonene 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.

Limonene is a terpene found naturally in citrus peel.

Limonene is considered to have low toxicity and is recognized as a safe ingredient for skincare under the current regulatory guidelines for cosmetics.

In the USA, the FDA considers limonene as GRAS (generally recognized as safe) when used 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

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.

There are many benefits of limonene for skin.

Limonene is widely used in skincare products for its fresh citrus aroma.

Limonene is a gentle, yet powerful cleaner that is reputed to effectively remove grease, tar, oil, soot, grime and soils.

Limonene has antioxidant activity and may protect your skin by neutralizing unstable free radicals.

Antioxidants such as limonene can also increase the shelf life of skincare products.

Limonene has cleansing activity, antioxidant activity and may boost the absorption of active compounds into your skin.

The delivery of active ingredients into the skin is a challenge.

Permeation enhancers help the skin absorb active ingredients.

This can be challenging, as the goal is to increase absorption, without causing any damage to the skin barrier. 

Terpenes such as limonene are permeation enhancers with low skin irritation.

Limonene may boost absorption of nutrients and active ingredients, including vitamins into your skin barrier.