Limonene and linalool: why old fragrance can bite harder

Limonene smells like orange peel and linalool smells like lavender. Between them, they turn up in a large share of scented products, and on their own they rarely cause trouble. The catch is what happens after the bottle is opened: air slowly changes them into something your skin is far more likely to react to.

Two of the most common scent molecules

Limonene is the citrus note; linalool is the floral, lavender-like one. A 2024 review in Acta Dermato-Venereologica cites an analysis of 2,044 cosmetic products in which limonene and linalool were major fragrance substances in around 30% of them. They were also among the three most common fragrances labelled on products for babies and children, alongside benzyl alcohol.

DermNet lists where you meet them: shampoos, conditioners, moisturizers, deodorants and cosmetics, but also dishwashing liquids, laundry detergents and industrial products. Limonene is even used as a solvent and degreaser. You can’t really avoid encountering them, which is why how they behave over time matters.

What air does to them

Both molecules are what allergists call prehaptens. The European Union’s Scientific Committee on Consumer Safety (SCCS) defines a prehapten as a chemical that is non- or low-sensitizing by itself but is changed outside the skin, by air oxidation or light, into a hapten, the small molecule that actually triggers an allergic response.

The change is called autoxidation. The SCCS describes it as a free radical chain reaction: oxygen from the air attaches to the molecule step by step, and the products include hydroperoxides, which the committee identifies as the main allergens formed when linalool oxidizes. For limonene, the Acta review names two main hydroperoxides that are known to be potent sensitizers.

The SCCS sums up the practical point: the non-oxidized compounds rarely cause allergic reactions, but oxidized limonene, linalool and linalyl acetate give frequent allergic reactions when tested in people with dermatitis. Linalyl acetate is a close chemical cousin of linalool.

How common is allergy to the oxidized forms?

It depends a lot on the clinic and the test concentration, so it helps to look at several numbers rather than one.

• A large Danish registry. A 2026 study of 6,719 consecutive patients patch tested at a Danish hospital between 2014 and 2025 found positive reactions in 1.6% for oxidized limonene and 3.1% for oxidized linalool. Positive linalool reactions were more likely in people over 40.
• The upper end. The Acta review reports that the highest prevalences published so far reach 20% for oxidized linalool and 9.4% for oxidized limonene.
• DermNet’s summary. About 10% of people investigated for suspected contact dermatitis were allergic to these hydroperoxides, more often women and people over 40.

These are people referred for patch testing because they already had a rash, not the general public. Still, the Danish authors concluded that the evidence supports adding both hydroperoxides to the standard European screening series.

The honest debate: how much forms in real products?

Here’s where the science gets less tidy. The Acta review notes that it’s hard to know whether a prehapten has changed before it reaches the skin, and that market surveillance studies have struggled to pin down how much hydroperoxide real products contain. In one study it cites, a cream with realistic levels of oxidized linalool, from 44 to 440 parts per million, applied for four weeks, did not cause dermatitis on normal skin in 31 people already allergic to it.

A 2026 study pushed further. Twelve allergic volunteers applied a simulated perfume containing oxidized linalool twice a day for 21 days, at three strengths. Reactions appeared in a third of them at the highest strength (1,630 parts per million), in two at the middle one, and in one at the lowest (179 parts per million). None of the 20 non-allergic volunteers reacted. The reaction depended on the dose, and it was real, but not everyone reacted at everyday-sounding levels.

So the direction is clear, and the exact risk from a given bottle is not. That’s an honest place for the research to be.

Freshness, storage and antioxidants

The SCCS says air oxidation can be prevented to a certain extent by avoiding air exposure during handling and storage of ingredients and finished products, and by adding suitable antioxidants. It adds a caution: some antioxidants can themselves be activated into sensitizers once oxidized.

DermNet makes the everyday version of the point: people who are allergic may tolerate a newly opened product, but after the pack has been open for a while, the fragrance oxidizes and can cause dermatitis.

A few low-effort habits follow from that:

• Close caps and lids after each use rather than leaving jars open.
• Don’t keep a half-used scented lotion or perfume for years.
• Keep products out of direct sunlight, since the SCCS notes light can also activate prehaptens.
• If a scented product that never bothered you starts to sting or itch after months of use, its age is worth considering.

How to find them on a label

Canada now requires both to be named. Limonene and linalool are on Health Canada’s first list of 24 fragrance allergens, which must appear on labels above 0.001% in leave-on products and 0.01% in rinse-off ones. Linalyl acetate joins the expanded list for new products from August 2026. Look for the exact words “limonene”, “linalool” and “linalyl acetate”; they are often right before “parfum”.

The label tells you the fresh ingredient is present. It can’t tell you how much has oxidized, because that depends on age, storage and how often the container has been opened. If you prefer to skip the question, our fragrance-free products filter lists products with no added scent.

If you get a rash that flares with scented products and a standard patch test didn’t explain it, ask your doctor whether testing with the oxidized forms of limonene and linalool is available. And if a rash isn’t improving after two weeks of avoiding the product you suspect, see your doctor or pharmacist.

The thing to remember: limonene and linalool are mild when fresh and stronger allergens once air gets at them. The research on exactly how much that matters in real products is still being worked out, but closing lids, using products up and reading for these two names are simple ways to stay ahead of it.