How long essential oils last, by note family

The number on the bottle, and what it is worth

Most bottlers state a shelf life of three years. That is their figure for their product, and it is the one on the label in your hand. Everything below explains why one three-year-old bottle is still excellent and another is finished after eighteen months.

An essential oil does not spoil the way food does — nothing grows in it. It oxidizes. Oxygen, warmth and light break the aromatic molecules into other molecules, and those other molecules smell wrong and behave differently on skin. Every time you open the bottle you let in a fresh charge of oxygen, so a half-empty bottle ages faster than a full one, which is the least intuitive thing on this page.

The order in which notes turn

Grouped by what the chemistry does, not by what the label says. This ordering is standard practice in the trade rather than a regulator's table — treat it as a guide to which bottle to smell first, not as a set of dates.

Turn first — the citrus and light monoterpene end

Lemon, Sweet Orange, Tangerine, Grapefruit, Bergamot, Lemongrass, Citronella.

These are dominated by limonene and its relatives, which are noted as sensitive to heat and light. Left open, warm or sunny, they oxidize readily — and oxidized citrus oil is not merely a smell problem. Oxidized R-limonene is a recognized, frequent skin sensitizer, common enough in Europe to be tracked in patch-test data. That is a reason to retire an old citrus bottle rather than use it up on your wrists. In a diffuser it is a smaller issue, since nothing lands on you.

The middle — herbs, needles and greens

Basil, Rosemary, Thyme, Peppermint, Eucalyptus, Cajeput, Tea Tree, Pine Needles, Cypress, Juniper, Fennel, Clary Sage, Camphor.

Steady for a few years if capped and cool. Tea tree is the one to watch in this group; it goes sharp and a little medicinal-plasticky as it ages.

Slow — flowers and softer notes

Lavender, Chamomile, Geranium, Ylang Ylang, Rose, Jasmine, Black Pepper, Ginger, Cinnamon, Clove.

Years, not months. Cinnamon and clove darken with time, which is normal and not the same as turning.

Last, and often improve — woods, resins, roots

Sandalwood, Cedarwood, Vetiver, Patchouli, Frankincense, Myrrh, Vanilla.

These are the exception to everything above. Patchouli and sandalwood are traditionally considered better after a few years, and a resin will comfortably outlive the bottle it came in. If you buy one bottle of anything to keep, buy it here.

How to tell, in thirty seconds, without a lab

1. Smell it against the memory of new. The specific failure is not "weaker" — it is sharper, thinner, a bit like a solvent or fresh paint. Citrus goes acrid rather than sweet.
2. Look at the liquid. Cloudiness, visible thickening, or a stickiness on the rim of the neck.
3. Watch how it behaves in water. Old oil beads and films oddly on the surface of the diffuser tank instead of dispersing.
4. Check the fill level and the age together. A bottle that has been three-quarters air for a year is older than its date.

If in doubt: do not put it on skin, and it is fine to run it out in a diffuser in a ventilated room — or bin it. A 10 ml bottle is not worth an argument with yourself.

Storing them so this takes longer

• Cap tight, every time. The single biggest factor, because it controls oxygen.
• Cool and dark. A drawer or a cupboard, not the windowsill and not the bathroom shelf above a hot shower. Heat and light are named directly in the research on limonene stability.
• Keep the amber glass. It is not decoration; it is a UV filter. Never decant into clear glass, and never into plastic — oils dissolve it over time.
• Buy small for citrus. 10 ml of lemon used steadily beats 30 ml going slowly stale.
• Upright, always — a cap lined with oil degrades and then leaks.

What this means for buying

Buy 10 ml, one note per bottle, in amber glass — the size and the glass are both doing the work described above. The honest advice is that nobody needs a shelf of them: keep three or four in rotation, and let citrus be the ones you replace.

An oil that has gone flat is not a diffuser fault. If the mist itself is the problem rather than the smell, that is a cleaning question — can a diffuser cause mold covers the tank, and are diffusers bad for you covers what the water carries into the air.

Read next

• Does aromatherapy work, and where does the evidence stop? — the pillar this sits under.