Limonene

Limonene is what citrus smells like. It is in orange peel, in lemon oil, in a great many cleaning products and cosmetics, and it is one of the most commonly encountered fragrance materials there is.

It is on this site because the thing people react to is not quite the thing on the label.

The molecule is not the allergen

Fresh limonene is a weak sensitiser. What sensitises is what it turns into.

Exposed to air and light, limonene oxidises and forms hydroperoxides. Those are potent contact allergens, and they are what patch test panels use when testing for this.

The European Society of Contact Dermatitis guideline records the trend: “limonene and linalool hydroperoxides have recently been found to be more common in the younger age groups, which underscores the burden of fragrance sensitization”.

So the substance named on the label is the precursor. The allergen forms afterwards, in the bottle, over time, and no label records that.

What follows, and it is genuinely useful

An old product is more allergenic than a new one. Nothing was added and nothing went off in the usual sense. The chemistry moved. A bottle opened two years ago contains more of the thing people react to than the same bottle did on day one.

Buy smaller. A large bottle of something scented, used slowly, is exactly the configuration that maximises oxidation. This is one of the few places where package size is a safety-relevant decision.

Keep the lid on and keep it out of the light. Air and light are the reaction conditions.

Natural is the wrong axis here

Limonene is the reason a citrus essential oil smells of citrus. Cold-pressed orange oil is largely limonene.

So a product using natural essential oils instead of synthetic fragrance is not avoiding this allergen. It is often delivering more of it, and essential oils, being less stable and less controlled than synthetic aroma chemicals, oxidise readily.

The intuition that natural fragrance is gentler runs directly against the mechanism here. That does not make essential oils bad. It makes “natural” useless as a guide to this particular question, which is the wider argument in the guide on what natural and clean legally mean.

Why you see it on some labels and not others

European rules require certain fragrance substances to be named separately above a threshold, which is why limonene appears at the end of European ingredient lists.

US rules permit fragrance to be declared simply as fragrance. So the same product can name limonene in one market and hide it inside one word in another.

If you suspect fragrance and can find a European label for a product you use, it will tell you more than the American one. That is a real and underused technique, and it is set out on the fragrance allergy page.

In food

Limonene is used as a flavouring and is present in citrus foods generally. Eating it is a different exposure from wearing it, and contact allergy is a skin phenomenon rather than a food one.

Someone with a contact allergy to oxidised limonene does not generally need to avoid oranges, and that distinction is worth stating because the same ingredient name appearing in two aisles causes a good deal of unnecessary worry.