Why “natural” fragrance and essential oils can irritate skin too

A lotion scented with lavender oil feels like a kinder choice than one scented with something made in a factory. It smells like a garden, the label says “natural”, and there’s no mysterious “parfum” on the list. But your skin doesn’t read labels. It responds to molecules, and the molecules in essential oils are the very same fragrance chemicals that cause most fragrance allergies.

What is an essential oil, really?

An essential oil is a concentrated liquid pressed or distilled from a plant: the peel of an orange, the flowers of lavender, the leaves of a tea tree. It carries the plant’s scent because it’s packed with volatile aromatic compounds, the small molecules that evaporate easily and reach your nose.

The word “oil” is a bit misleading. These aren’t moisturizing oils like the ones in a cream. DermNet notes that a single essential oil can contain over 100 different constituents, and many of them are known skin sensitizers. A sensitizer is a substance that can teach your immune system to react to it, producing allergic contact dermatitis (an itchy, red, scaly rash that returns every time the skin meets that substance again).

So when a product is scented with essential oils, it is scented with fragrance. “Natural fragrance” is still fragrance.

Limonene and linalool: the two names to know

Two molecules do a lot of the work in both natural and synthetic scents. According to DermNet, limonene is extracted from the peel of citrus fruits, and linalool comes from herbs, flowers and woods. They show up in shampoos, conditioners, moisturizers, deodorants and household cleaning products, whether the scent is described as natural or not.

Here’s the twist. On their own, both are weak sensitizers. But DermNet explains that on exposure to oxygen in the air, both oxidize into hydroperoxides, which are much more potent sensitizers. Oxidation is simply a chemical change caused by contact with air, the same process that makes cooking oil go rancid.

This matters in your bathroom. A freshly opened product may be fine, but as DermNet puts it, after a period of time once the pack has been opened, the parent fragrance oxidizes and can cause an allergic dermatitis. That half-used bottle of lotion from last year is chemically different from the day you bought it.

How common is this? DermNet reports that 10% of people investigated for a suspected contact dermatitis were found to be allergic to these hydroperoxides, more often women and people over 40. A 2024 review in Acta Dermato-Venereologica describes limonene and linalool as prehaptens, meaning substances that are not allergens themselves but become allergens after air exposure, and describes the hydroperoxides they form as clinically relevant sensitizers.

Citral, geraniol and the other plant chemicals

Limonene and linalool aren’t alone. The same 2024 review documents contact allergy to citral, the lemony molecule behind lemongrass and lemon-scented oils, and to geraniol, a rose-scented compound that is both a prehapten (activated by air) and a prohapten (activated by enzymes in the skin). In other words, several of the plant molecules that smell the nicest are also the ones your skin is most likely to learn to hate.

Which oils cause the most trouble? DermNet lists tea tree oil, ylang-ylang oil, lemongrass oil, sandalwood oil, clove oil, jasmine absolute and narcissus absolute as essential oils that commonly cause allergic contact dermatitis. The people most at risk are those who handle them all day, such as aromatherapists, massage therapists, hairdressers and beauticians, along with people who have atopic dermatitis (eczema), older adults and women.

Tea tree oil: the most reported offender

Tea tree oil is popular for blemishes, dandruff and minor skin problems, and it’s often applied straight from the bottle. That’s exactly where the trouble starts.

A 2016 review in the journal Contact Dermatitis found that of all essential oils, tea tree oil has caused the most published allergic reactions since the first cases were reported in 1991. In routine patch testing, between 0.1% and 3.5% of people react to it. Fresh tea tree oil is a weak to moderate sensitizer, but oxidation increases its allergenic potency. As it ages, the oil changes and forms new compounds, including one called ascaridole, which is among its main sensitizers.

The review also notes something practical: most reactions are caused by applying the pure oil, and cosmetics containing it are the culprits in a minority of cases. Undiluted, aged tea tree oil dabbed on skin is a much bigger exposure than a small amount blended into a wash.

Lavender oil: gentle reputation, real reactions

Lavender is the essential oil most associated with calm and gentleness, and it appears in countless baby and “sensitive” products. Yet its main components are linalool and linalyl acetate, and a 2016 Swedish study in Acta Dermato-Venereologica found that both autoxidize when exposed to air, generating sensitizing hydroperoxides.

In that study, 2.8% of consecutively patch-tested patients reacted to oxidized lavender oil. Just over half of them also reacted to oxidized linalool or linalyl acetate, and about half reacted to the standard fragrance markers, which means a standard patch test would have missed some of them. The authors concluded that an oxidized lavender oil preparation could be a useful diagnostic tool, which tells you how real the problem is.

What Health Canada says about “natural”

Canada’s regulator doesn’t treat plant origin as a safety guarantee. Health Canada’s page on essential oils and botanical extracts states that natural doesn’t always mean safe, and explains that many plants evolved to produce substances that protect them from insects and animals, some of which can harm people.

Health Canada also warns that do-it-yourself uses, such as homemade massage oils, bath products and moisturizers, can lead to high levels of exposure when applied to skin, and it does not recommend or approve DIY uses of essential oils. It flags extra caution for infants, toddlers and pregnant people, and its list of oils with specific restrictions includes cedarwood, jasmine and myrrh for skin contact, along with clove oil, which it says shouldn’t be used in perfume for children under 9.

How to use this when you shop

You don’t have to fear essential oils. Many people use them without any problem. But if your skin is reactive, a few habits help:

• Read the ingredient list, not the front label. Essential oils appear by plant name, and limonene, linalool, citral and geraniol may be listed individually.
• Treat “natural fragrance”, “botanical scent” and “essential oil blend” as what they are: fragrance.
• Be wary of old, half-used scented products, especially citrus and lavender ones. Air exposure is what turns mild molecules into strong allergens.
• Never apply undiluted essential oil to skin, and keep it away from babies and young children.
• For anything that stays on your skin all day, fragrance-free is the simplest choice. Our fragrance-free products filter can help.

If you get a rash that keeps coming back and you can’t work out why, ask your doctor about patch testing, which can identify the exact molecule. And if a rash isn’t better after two weeks of avoiding scented products, see your doctor or pharmacist.

The thing to remember is that your immune system responds to chemistry, not to marketing. Limonene from an orange peel and limonene from a lab are the same molecule, and both become stronger allergens once the bottle is open. Choose products for what’s in them, not for how natural they sound.