Phototoxicity: citrus oils and sunlight

This is the risk fewest people know about and the one that leaves the most visible mark: a dark patch or a burn with a sharp edge, in exactly the shape of where the oil was applied, which can last for months.

How it works

Some essential oils cold-pressed from fruit peel contain a group of compounds called furocoumarins, of which bergapten is the most studied. In the dark they do no harm.

But when skin that contains them meets ultraviolet light, a photochemical reaction takes place right there in that layer of skin. The result is a localised injury, much stronger than ordinary sunburn.

Oils to watch

The high-risk group includes cold-pressed bergamot, cold-pressed lemon, cold-pressed lime, cold-pressed bitter orange, and angelica root. Cold-pressed grapefruit carries a lower risk.

Sweet orange is not on this list. Sweet orange oil — Citrus sinensis — contains almost no furocoumarins, and the trade literature classes it as non-phototoxic even when cold-pressed. We spell this out because sweet orange is the most common oil of all, and avoiding it by mistake means giving up something there is no need to give up.

For angelica root the species has to be named, because two sources use two different species. The Vietnamese Pharmacopoeia lists Angelica dahurica; IFRA and the fragrance-industry lists give Angelica archangelica. Both contain furocoumarins, but they are not the same plant — so when you read a label, look for the Latin name rather than stopping at the common one.

The key point with citrus: the extraction method decides. For the same species, the cold-pressed oil contains furocoumarins and the distilled oil contains almost none, because these compounds are too heavy to travel with the steam.

This holds for all three kinds of distillation — in water, water and steam, or direct steam. A copper still in a craft village removes furocoumarins just as industrial equipment does, because this is a matter of molecular weight, not of equipment.

This is one of the places where knowing the extraction method is no longer academic but a matter of safety. A label that does not state the method is not a complete label.

But do not turn that into a general rule. Angelica root is on the list above precisely because it breaks the rule: it is a distilled oil, not a cold-pressed one, and furocoumarins still come through. Egyptian cumin seed does the same. Why a few materials let furocoumarins pass through distillation while citrus peel does not is something we have not yet found a full explanation for.

So the advice "choose the distilled version" is only reliable for citrus. For roots and seeds, read the species name and ask the seller rather than reasoning from the method.

Some products are labelled FCF or bergapten-free, meaning this group of compounds has been removed.

Rules in practice

After applying an oil to skin that will be in the sun, avoid ultraviolet light for at least twelve hours, and eighteen hours for the high-risk oils.

This applies to tanning beds as well.

If you plan to be in the sun and are using a citrus oil: choose a distilled oil, choose one with the furocoumarins removed, or apply it only to skin that stays covered. With angelica root the extraction method does not help — avoid the sun or do not apply it.

Diffusing into the air does not cause photosensitivity — this risk comes only from applying oil to the skin.

A figure for reference if you make products: IFRA caps bergamot oil in leave-on skin products at around 0.4 percent. The exact figure can change with the current edition of the standards, so check the latest one before relying on it.

Keep this separate from the list of allergens that must be declared: that is about labelling, while photosensitivity is about using a product in the sun. A substance can be on the declaration list without being photosensitising, and the other way round.