The report of the appearance of photodermatitis in individuals picking parsnips (PJ, October 30, p782) reminds me of an unpleasant experience I underwent many years ago in connection with the same vegetable.
While out walking my dog in midsummer in that magical corner of the land where Hampshire meets Sussex, I had occasion to climb a steep down overgrown waist-high with a yellow-flowered umbellifer. It was an uncommon plant in those parts, but I diagnosed it as being wild parsnip. I waded through the thicket in the heat of the early afternoon sun, and thought nothing of it. During the evening of the same day, however, I became aware of a pricking sensation on the backs of my hands and wrists and noticed a speckly red rash. I decided that the contact with parsnip juice had conferred photosensitivity on my skin. I thanked my lucky stars that I had not been wearing shorts at the time. I have treated parsnip plants with suspicion ever since.
Another photosensitiser with which I used to make contact in that vicinity was the giant hogweed introduced by Doctor Mantegazzi from the Caucasus, and often straying from gardens where it was a prize exhibit. Like parsnip, the giant hogweed contains furocoumarins which sensitise tissues to ultraviolet radiations. There were several thickets of it, perhaps 10ft high, in a wooded valley where I often walked with my dog, and I knew its reputation and handled it gingerly. I was particularly intrigued to see long columns of wood ants ascending and descending the main stems, and this was one reason why I avoided handling the plant. I imagine that the insects were collecting nectar from the fleshy flower heads.
Despite my close attention to such phenomena, I was never to suffer from irritation or photodermatitis from this notorious immigrant, and I think that the periodical outcries about its hazard which the local papers carried every summer were a sheer panic reaction which shamed a civilised country.