Storage and oxidised oils

This is where safety and quality meet, and it is rarely talked about.

What happens over time

Oxygen reacts with molecules that have double bonds, especially monoterpenes. Limonene oxidises into heavier peroxides and alcohols.

The problem: oxidation products sensitise the skin more strongly than the original substance. A citrus oil kept too long does not only lose its freshness; it also becomes more likely to cause a skin reaction than a new bottle.

That is why the shelf life of an essential oil is a safety question, not just an aesthetic one.

The oils that spoil fastest

Cold-pressed citrus oils come first, followed by conifer oils such as pine and fir — both rich in monoterpenes. For these, one to two years after opening is a reasonable limit.

Oils rich in phenols and sesquiterpenes last much longer. Some wood and root oils even round out over a few years.

Details by group are in the article on shelf life.

Five things to do

Dark glass bottles. Not plastic — some compounds attack plastic and draw substances from it into the oil.

Close the bottle straight after use. The total time the cap is off matters more than the number of times you open it.

Keep it somewhere cool and dark. A fridge works for citrus oils, but let the bottle come back to room temperature before opening it, so condensation does not get inside.

Move the oil into a smaller bottle once you have used more than half. The air space above the oil sets the pace of oxidation.

Write the opening date on the label. The date printed on the bottle counts from manufacture while it is unopened; once opened, the clock runs at a different pace.

Signs to stop using it on skin

A smell with a damp-paper or paint note. The liquid has thickened or darkened compared with when it was new. Sediment or cloudiness.

An oil in that state can still be used for cleaning or deodorising, but should no longer go on the skin.