Limonene

What it is

Limonene is the molecule that gives orange and lemon peel their smell, and it makes up the bulk of most cold-pressed citrus oils. It is also produced synthetically and used as a fragrance component and a mild solvent. Because it appears on the EU fragrance-allergen list, you will see it declared on a large share of scented face masks, often because a citrus or herbal essential oil elsewhere in the formula contains it, not because pure limonene was added.

What it does well

• Provides a fresh citrus note without a separate perfume oil

  It is one of the most widely used aroma chemicals in the world and does its job reliably at very low levels. This is a formulation benefit rather than a skin benefit.

  Strong evidence

• Acts as a natural solvent that helps disperse other fragrance materials

  Limonene dissolves oils and resins well, which is why it appears both in fragrance blends and in cleaning products. In cosmetics this role is minor compared with its scent contribution.

  Moderate evidence

Worth knowing

• Oxidised limonene is a common contact allergen

  Fresh limonene is a weak sensitiser. On contact with air it forms hydroperoxides, chiefly limonene-2-hydroperoxide, which are far more potent. In consecutive European dermatitis patients, 5.1 to 5.2% test positive to oxidised limonene, and exposure was judged clinically relevant in about a third of those cases.

  Matters most toPeople with eczema, hand dermatitis or a known fragrance allergy.

• Its allergenicity depends on how old the product is, which you cannot see

  Limonene autoxidises during storage and, more quickly, once a jar has been opened and repeatedly exposed to air. A well-preserved, recently made product carries less risk than an old tub of mask, but nothing on the label tells you which you have.

  Matters most toAnyone who keeps opened jars of mask for many months.

• Skin patch testing to fresh limonene can miss the allergy

  Because the sensitiser is the oxidation product rather than the parent molecule, dermatologists test with deliberately oxidised limonene. If you were tested only with pure limonene, a negative result does not fully rule the allergy out.

  Matters most toPeople who have been patch tested and got an unexplained negative.

Where the EU stands

Sources (3)

Every link goes to the original: a journal, a PubMed record or an EU document.

1. [1]
   Positive patch test reactions to oxidized limonene: exposure and relevance

   Contact Dermatitis · 2014 · doi:10.1111/cod.12285

   5.2% of 2900 consecutive dermatitis patients across eight international centres reacted to oxidised R-limonene, and the exposure was judged probably relevant to their dermatitis in 36% of those patients.

2. [2]
   Contact allergy to oxidized linalool and oxidized limonene: Patch testing in consecutive patients with dermatitis

   Contact Dermatitis · 2022 · doi:10.1111/cod.13980

   In 5773 consecutive Swedish patients, 5.1% reacted to oxidised limonene and 7.0% to oxidised linalool, confirming oxidised terpenes as frequent contact allergens.

3. [3]
   Contact Allergy to Hydroperoxides of Linalool and D-Limonene in a US Population

   Dermatitis · 2017 · doi:10.1097/DER.0000000000000318

   Hydroperoxides of limonene and linalool also produced meaningful rates of positive patch tests in a US cohort, showing the problem is not confined to Europe.