Contact dermatitis and plant allergies

Allergic contact dermatitis is a delayed hypersensitivity reaction to something touching the skin. It is the commonest skin problem caused by plant preparations, and the mechanism explains its characteristic pattern: sensitisation happens first with no reaction at all, and the allergy appears on a later exposure. That is why someone can use a neat essential oil happily for months and then react to it permanently.

The plant sensitisers to know.

• Tea tree and other Melaleuca oils — one of the most frequently reported causes of allergic contact dermatitis from a "natural" product. The oxidation products of the oil, more than the oil itself, are the allergens, which means an old bottle is more sensitising than a fresh one.
• Lavender — linalool and linalyl acetate oxidise to allergens with the same effect; an old bottle is worse.
• The cinnamon, clove and citrus materials — cinnamaldehyde, eugenol, and the citrus terpenes are potent sensitisers.
• Compositae family plants — chamomile, arnica, feverfew, yarrow, tansy, and others. A whole-family allergy is common and catches people who react to chamomile cream.
• Balsam of Peru and fragrance mixes are the standard patch-test allergens for this reason.

Phototoxic versus photoallergic reactions. Phototoxicity is not an allergy: a furanocoumarin in cold-pressed citrus peel oil, or a psoralen in celery or fig sap, absorbs UV and damages the skin directly. It can burn anyone who applies enough and goes into the sun — bergamot being the classic. Photoallergy is a genuine immune reaction. The practical difference is that phototoxicity happens on the first exposure if the dose and the sun are enough, while photoallergy requires prior sensitisation. Both look like a burn or a rash in sun-exposed skin only.

Patch testing. If a plant preparation is suspected, the way to know is patch testing by a dermatologist, which uses a standard series plus the suspected product. Doing your own patch test on a small area of forearm for 48-72 hours is a reasonable preliminary and it is different from a proper diagnosis.

The rule. Dilute to 2-3% for skin. Use fresh oils and store them dark and cool, because oxidation creates allergens. Never neat on a large area, never neat repeatedly, never on broken skin. If a rash develops where a product is applied, stop it — re-exposure makes the allergy worse and permanent.