Some substances released from plants only have an irritant effect on the skin after being irradiated by UV light. In Western Europe hogweed (Heracleum sphondylium), giant hogweed (Heracleum mantegazzianum) and rue (Ruta graveolens) are best known for this. In tropical and subtropical regions, irritation is seen after skin contact with fig trees, mango trees and many other plants. Phytophotodermatitis (syn. dermatitis pratensis) is identified clinically by strange red stripes at the site of contact (forearms, hands, legs, sometimes the face) that is exposed to the sun. The skin lesions are similar to burns. There is a delay between the skin contact and the first signs of irritation. Berloque dermatitis, triggered by perfumes containing for example bergamot oil, is a known condition in dermatology. There is generally residual pigmentation. Attempts have been made to use this in the treatment of vitiligo. Plant substances such as 8-methoxypsoralen and similar furocoumarins (coumarins bound to a furan ring) are obtained from Ammi majus (bullwort). These so-called psoralens are photodynamic substances. They absorb UV light, become activated and then cause cell damage by inhibiting DNA synthesis. They are used in PUVA therapy of psoriasis [psoralen + Ultraviolet A]. Animals that eat such plants develop vesicles, eye lesions and erythema of the udder after exposure to sunlight. Phytophotodermatitis should not be confused with contact allergy or with photoallergic reactions such as polymorphous light eruption, persistent light reaction or solar urticaria. The treatment consists of thorough cleansing of the skin and application of a steroid cream. Foods containing large amounts of psoralens (celery, parsnips, figs, and parsley) should be avoided by people with photodermatitis, e.g. in SLE (systemic lupus).