How to store perfume oil, and can it go off?

Yes, perfume oil can change over time, like any perfume, though usually slowly. How you store it matters more than almost anything else.

The short version

• Keep it cool and dark, in its box in a drawer: heat and sunlight age a fragrance.
• Cap it straight after use: oxygen slowly changes scent molecules.
• If it smells flat, sour or rancid, don't put it on your skin: common scent molecules become stronger allergens as they oxidise.

What ages a fragrance

Research on essential oils, which share many perfume ingredients, points to three things: light, heat and oxygen. In one study, lavender and winter savory oils only stayed unchanged when kept dark with no oxygen.

All three feed one process. Oxygen reacts with scent molecules and changes them. Light and heat start and speed up the reaction.

Light

UV light carries enough energy to break chemical bonds, and violet-blue light can drive oxidation too. A scent molecule that absorbs that energy can split into free radicals, reactive fragments that start the chain reaction described below. That's photo-oxidation.

Light can also bend a molecule into a new shape. Under UV, citral (what makes lemon smell like lemon) flips shape and breaks down.

Amber glass made to pharmacy standards lets through no more than a tenth of UV and blue light (290 to 450 nanometres). A box or drawer blocks the rest.

Heat

Warmth speeds these reactions up, because more molecules meet with enough energy to react. A rule of thumb says many reaction rates roughly double with every 10°C rise. On that rule, a shelf at 30°C ages a scent about twice as fast as one at 20°C.

In a small university study, commercial perfumes kept at 40°C began to oxidise and smell rancid. Above 50°C, they lost their chemical stability.

A room won't get that hot. But a sunny windowsill or a car in summer can get far warmer.

Air

The slow, everyday version is a chain reaction called autoxidation:

• Many scent molecules have weak spots where a hydrogen atom can be pulled off. What's left is a free radical.
• The radical grabs oxygen from the air.
• The result pulls a hydrogen off the next molecule, and so on down the line.

Terpenes, a big family of plant compounds, are especially prone. They include limonene (from citrus peel) and linalool (a main part of lavender oil).

The first products, hydroperoxides, are unstable. They break down into aldehydes, epoxides and other compounds nobody added, changing the smell.

In one lab test, linalool left in air for ten weeks fell to about 80% of its starting amount. New compounds formed in its place.

Fresh limonene and linalool rarely cause allergies, but their oxidised forms are much stronger skin sensitisers. Patch tests on 1,511 people at six European clinics found oxidised linalool a common contact allergen.

That's why there are warnings and limits:

• The EU's consumer safety committee calls oxidised limonene and linalool allergens of high concern.
• IFRA, the fragrance industry's standards body, says both should have a peroxide value (a measure of oxidation) under 20 millimoles per litre.
• UK and EU cosmetics law sets the same limit for d-limonene as an ingredient.

More air above the liquid means more oxygen to react. In a fish oil test using dark glass bottles, those with more empty space at the top oxidised faster.

So a nearly empty bottle can age a little faster. A small bottle spends less of its life half-empty. Cap it straight away, and use up the last few millimetres rather than saving them.

Steam and swings in temperature

Bathrooms combine heat, steam and daily temperature swings. Every hot shower warms the room, then it cools. So a bottle by the basin gets a daily dose of heat.

Steam also settles as water on anything cooler than the air.

What about the fridge?

Cold slows the reactions and a fridge is dark, so in principle it's a good place. But a cold bottle in a warm room collects condensation. And daily trips in and out are exactly the swing to avoid.

If you use one, leave the bottle there and let it warm up before opening.

What antioxidants do

Some fragrance materials and scented products contain added antioxidants, such as tocopherol (a form of vitamin E) or BHT. They react with free radicals before the scent molecules can, breaking the chain.

IFRA's limonene and linalool standards cite 0.1% BHT or tocopherol, added during production, as working well. But antioxidants only slow oxidation, and they get used up. So storage still matters.

Where to keep it, and where not to

The best place is a boring one: somewhere dry and dark that stays cool.

• Good: a bedroom drawer, a wardrobe, a cupboard away from the oven and radiators.
• Not good: a sunny windowsill, a shelf above a radiator, the bathroom, a car in summer.

Oils and alcohol sprays age in different ways

Scent molecules react with light, heat and air whatever they're dissolved in. What differs is the base.

Alcohol-based sprays

Alcohol doesn't go rancid, but it evaporates easily. At 20°C, ethanol's vapour pressure is about two and a half times water's.

In a sealed bottle, the vapour has nowhere to go. With the cap off or a loose seal, the alcohol slowly escapes and the level drops.

Perfume oils

If a perfume oil has a plant-oil base, that base can go off on its own. Plant oils are fats, and fats go rancid through the same radical chain. The ingredients on the box or product page show what yours contains.

Each extra double bond in a fatty acid at least doubles how fast it oxidises. So polyunsaturated oils turn sooner than more saturated ones.

Jojoba is the odd one out. It's technically a liquid wax with its own natural tocopherols. In one lab test, it held up better than olive oil.

We couldn't find a study ageing one scent as both an oil and a spray. So be wary of claims that one format always keeps longer. Our guide to the chemistry of perfume oils covers how they differ on skin.

Has my perfume gone off? The signs

• It smells different: the bright opening has gone flat, or there's a sour or rancid edge.
• It looks cloudy, or the liquid has thickened.
• A spray's level has dropped, though you've barely used it.

Colour is a weaker clue: a perfume can darken with age without having turned. But if it smells off, don't put it on your skin.

How long does perfume last?

There's no fixed number, and we found no independent study that gives one.

But light, citrus-led scents tend to change sooner than richer ones. That's because citrus oils are mostly limonene (around 60 to 80% of lemon and mandarin oil), which oxidises readily in air. In one storage test, orange oil mainly lost its citrus character over three months.

What the open-jar symbol means

On the box or bottle, a small open pot marked 12M, 24M and so on is the period-after-opening symbol. It shows how many months the maker says a product can be used without harm once opened.

UK cosmetics law is based on EU Regulation 1223/2009. Products that last 30 months or less need a "best used before the end of" date. Ones that last longer show the open-jar symbol instead.

EU guidance says the symbol isn't needed where ageing poses no risk of harm. So a perfume may carry no date at all. Either way, the symbol is about safe use, not when the scent smells its best.

Travelling and flying with perfume oil

Perfumes and oils count as liquids at UK airport security. At most airports, that means:

• Containers of 100 ml or less.
• All in one clear, resealable plastic bag of up to a litre, about 20 cm by 20 cm.
• One bag per person.

Some airports with newer CT scanners allow up to 2 litres. But not all do, and the list keeps changing. Check every airport on your route before you pack.

Small roll-ons sit well inside the limit. Keep them upright.

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