Phytophotodermatitis

For the plant most often responsible in Europe, see Giant hogweed.

Phytophotodermatitis (from Greek phyton, “plant”, phōs, “light”, and derma, “skin”) is a burn-like reaction of the skin caused by contact with certain plant saps followed by exposure to ultraviolet light.¹ The sap is harmless in the dark. Contact is painless and leaves no mark: the injury appears only after the skin has been exposed to light, becoming visible after about a day and reaching its height two to three days later.² The affected skin often stays dark for months, and may remain sensitive to sunlight for years.³

Because of this delay, many of those affected do not connect their injuries with their cause.the light does not cause the harm; it shows it The streaks and splashes of the burn follow the pattern of contact — a brush of a leaf, a drip of sap, the shape of a hand — and have been mistaken for other injuries, including, in children, for signs of abuse.⁴

Cause

The reaction is caused by furanocoumarins, compounds produced by several plant families as a defence against insects and fungi. In the carrot family, Apiaceae, they are found in giant hogweed and its relatives, wild parsnip and angelica; in the citrus family they occur in limes, where they cause the “margarita burn” of summer bartenders.¹ The giant hogweeds are the most dangerous source in Europe because of the quantity of sap they carry and the size of the plants, whose leaves and stems easily brush bare skin.

The compounds present in hogweed include psoralen, bergapten, xanthotoxin and angelicin, which is often the most abundant of them. They are found in every part of the plant, and in the highest concentration in the fruit.⁵ See furanocoumarin § In hogweed.

Mechanism

Furanocoumarins penetrate the skin within minutes of contact. When they are struck by long-wave ultraviolet light (UVA, roughly 320–400 nm), they bind to the DNA of the skin cells and generate reactive oxygen species, killing the cells and triggering inflammation.¹ The skin is at its most sensitive from about half an hour to two hours after contact, but it remains sensitive for much longer.

The same reaction stimulates the pigment cells, which is why a darkening of the skin — hyperpigmentation — follows the burn and outlasts it.

Signs and symptoms

  contact      light         ~24 h         ~48 h         ~72 h        weeks - years
     |           |             |             |             |               |
  ---o-----------o-------------o-------------o-------------o---------------o------>
     |           |             |             |             |               |
  painless    UVA activates  redness,      blisters      peak of the    dark marks;
  and          the sap within burning                    reaction       skin stays
  invisible    minutes                                                  sensitive to
                                                                        light

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First aid

If sap touches the skin:²⁶

1. Wash it off at once with soap and plenty of cold water.
2. Cover the area and keep it out of sunlight for at least 48 hours — including on dull, overcast days, when ultraviolet light is still present.
3. Seek medical advice, especially if blisters appear. Early treatment with topical corticosteroids may reduce the reaction; large burns are treated as burns.
4. If sap gets into the eyes, rinse them with water at once, wear sunglasses, and seek emergency care.

After healing, the affected skin should be protected with sunscreen for several months.⁶

Prevention

Contact is avoided by not touching the plant, and by working among it only in full protective clothing: waterproof synthetic garments, long gloves and eye protection (see Control of giant hogweed § Safety). Strimmers and brush-cutters, which spray sap over the operator, should not be used.⁷ Children should be taught to recognise the plant and warned not to use its hollow stems as pea-shooters or telescopes.³

Prognosis

Burns heal over days to weeks. The hyperpigmentation that follows usually lasts months and can persist for years, and some patients remain unusually sensitive to sunlight on the affected skin for several years after the injury.³ In rare cases burns may be extensive enough to need hospital treatment.

See also

• Giant hogweed
• Furanocoumarin
• Control of giant hogweed