Why an old bottle of essential oil is a different substance

Last updated: 13 September 2026

An old bottle is not a weaker bottle

The intuition is that essential oils fade, like a spice losing its smell. Something else happens as well, and it runs in the opposite direction.

Several of the molecules that make up an essential oil react with oxygen in the air. The European Commission’s scientific committee has a name for substances that behave this way: prehaptens. They are not allergens when you buy them. They become allergens through air oxidation, outside the body, in the bottle.

The named examples are the ones you own. Limonene, the main component of citrus oils. Linalool, which lavender is largely made of. Linalyl acetate, lavender’s other main component. On contact with air they form hydroperoxides, and then secondary products — aldehydes and epoxides. The committee’s conclusion is the sentence that matters: those oxidation products are more potent sensitisers than the parent substance.

So an old bottle is not a diminished version of a new one. Part of its contents has turned into something the new bottle did not contain.

How much of a lavender bottle is made of this

It is easy to treat this as a trace-level worry. The numbers say otherwise.

ISO 3515 is the international standard for lavender oil, written by ISO Technical Committee 54, which does nothing but essential oils. It defines what a lavender oil has to look like: relative density, refractive index, optical rotation, and the proportions of sixteen characteristic components.

For linalool it gives a range of roughly 20–45% depending on origin; for linalyl acetate, roughly 25–47%. Spontaneous French lavender, for instance, is specified at 25–38% linalool and 25–45% linalyl acetate.

Add those together. Between half and four fifths of a lavender oil is made of the two substances the Commission lists as prehaptens. This is not a trace contaminant. It is most of the bottle.

Which oils go first

Oxidation rate is not the same for every oil, and the ranking follows the chemistry rather than the price.

• Citrus oils — lemon, wild orange, bergamot, grapefruit — are the fastest. They are largely limonene, and limonene is the textbook prehapten.
• Pine, fir and spruce oils are rich in monoterpenes and behave similarly.
• Tea tree oxidises in a way that is well documented in the dermatology literature and is a common cause of contact allergy to that oil specifically.
• Lavender is slower than citrus but is, as above, largely made of two prehaptens.
• Heavier oils — sandalwood, vetiver, patchouli — are the most stable, and some genuinely improve with age.

What actually slows it down

The Commission’s own statement is the useful one: air oxidation of prehaptens can be partly prevented by protective storage and antioxidants. Partly. Not stopped.

• Keep the cap on tightly. Every minute the bottle is open is oxygen contact. This is the single largest thing you control.
• Headspace is the enemy. A half-empty 15 ml bottle contains more air than a full one. Decanting a nearly finished oil into a smaller bottle genuinely helps.
• Cold and dark. Amber glass is not decoration. Heat and light both accelerate the reaction. A cupboard is fine; a windowsill is not; the fridge is better for citrus.
• Write the opening date on the bottle. The PAO symbol — the open jar with a number in it — counts from opening, not from purchase, and nobody remembers when they opened anything.
• Never decant into plastic for storage. Essential oils dissolve some plastics, and what dissolves ends up in the oil.

Why this is about to appear on your labels

This is not a separate subject from the allergen rules. Limonene and linalool are among the substances that Regulation (EU) 2023/1545 requires to be declared on cosmetic labels, with a deadline of 31 July 2026 for new products. The reason those two are on the list at all is precisely the oxidation chemistry described above.

So the ingredient list on a new bottle and the age of an old bottle are two halves of the same question.

Common questions

How can I tell if an oil has oxidised?

Often you cannot, reliably, by smell alone — though a citrus oil that has gone sharp, turpentine-like or flat has usually turned. Thickening, a change in colour, or a bottle that no longer smells like the one you bought are all signs. If in doubt, stop putting it on skin.

Can I still use an oxidised oil for cleaning?

For cleaning surfaces, the sensitisation risk is lower because the contact is brief and mostly not on your skin. Wear gloves. It should not go into anything that stays on skin.

Do essential oils have an expiry date?

Not in the way food does. What they have is a period after opening, and it depends on the oil and on how you stored it. A citrus oil opened two years ago and kept on a shelf above a radiator is a different proposition from an unopened one.

Does refrigeration really help?

For citrus and conifer oils, yes. Let the bottle come to room temperature before opening it, so that condensation does not form inside — water in the bottle brings its own problems.

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The oils mentioned on this page

• Lemon
• Wild Orange
• Lavender

Each product page lists the botanical name, the size, the directions and the cautions, with a buy button for your country.

Read next

• The EU allergen list just tripled
• How to read an essential oil label in Europe
• Essential oil safety: dilution, sunlight and food use