“Phyto” derives from the Greek word for plant, “photo” from light and “derma” from skin, so “phytophotodermatitis” is a skin reaction initiated both by exposure to light and to some plant material. Both exposure to a photosensitizing agent and to ultraviolet light are necessary for the reaction to occur, although not necessarily concurrently. If exposure to the photosensitizing substance takes place in the absence of ultraviolet light, such as at night, a reaction can still occur on subsequent exposure to the sun if the substance has not been completely washed off the skin.
The photosensitizing chemical culprits belong to a class of compounds known as furocoumarins, which certain plants produce to fight off disease. Parsnips, celery, fennel, dill, lime, lemon, mustard, fig, chrysanthemums, cow parsley, bergamot orange and the giant hogweed are just some of the plants that produce furocoumarins and can cause phytophotodermatitis. The classic inflammatory reaction, characterized by burning pain, generally occurs about twenty-four hours after exposure and lasts several days, but may be followed by a darkening of the skin in the affected area that can last months or even years. Cool, wet dressings help with the pain as does the application of hydrocortisone cream.
Heracleum mantegazzianum, better known as giant hogweed, has received a great deal of publicity because of its impressive size, its clusters of beautiful white flowers, its ready proliferation and its ability to cause severe phytophotodermatitis. This plant, which can reach heights of fifteen to twenty feet, was originally introduced into Europe and the Americas from its native Asia as a garden curiosity, but now has spread broadly enough to have become a pest. Children are particularly captivated by the towering hogweed plants and have been known to make peashooters and “telescopes” from its stalk. Contact with furocoumarins in the eye region is particularly dangerous and in theory can even cause blindness. But let’s get one thing straight. Hogweed will not go out of its way to attack innocent people. Some newspaper accounts would have us believe that these are killer plants, massing to launch a chemical attack on humanity. Certainly all contact with the sap that oozes from the leaves and stalk should be avoided, and removal of hogweed from areas where such contact is a possibility should be considered. Removal, though, presents its own risks. Best time to attempt this is at night, while clad in protective clothing from head to toe. Herbicides such as 2,4-D and glyphosate will kill the plant, as well as everything around it, but will not destroy the roots, which need to be dug out. Although municipalities have launched efforts to eradicate hogweed, it is unlikely they will be successful. The flowers produce thousands of seeds that are spread by the wind and can be viable for as long as fifteen years. As far as the giant hogweed, and the numerous other potentially phototoxic plants go, it is best to adopt a “look but don’t touch” philosophy. “Natural” chemicals aren’t always so desirable, are they?