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Toxic plants: A growing danger

Plucking a wild fennel frond on a summer walk nearly left Barbara Hall with scars for life. Few are aware, she explains, that sunshine can turn some plants toxic

With the long summer days comes the vision of walks through long dappled grass, of plucking wild herbs fragrant from the sun, and of cool drinks infused with limes. But it isn't widely known that the sun, certain plants and our skin make a fiery and toxic combination.

Three summers ago I went for a walk in idyllic countryside, just north of Paris. A clump of tall wild fennel plants was too delicious to leave alone, so I plucked a handful of seeds, squeezed them to release the aromatic oil and passed my hand under the appreciative noses of my companions. It was a cloudy day and the walk was cut short by the rain.

A few days later we repeated the expedition, and again the magic of the squeezed fennel seeds was highly praised. We walked on a sandy footpath, this time under blazing sun. A gust of wind blew dust in my face. I wiped it off with my fingers and we walked on. Towards the evening I felt a strange sensation on my face, as if my skin had been invaded by running ants, passing just under the skin with light, barely touching feet. There was nothing to see, but the feeling continued as my fingers brushed my skin and I was conscious something was happening.

The next day I had to fly abroad on a business trip. I was getting ready to meet my colleagues when I stopped in front of a mirror. My face was like Munch's The Scream.

Across each of my cheeks, clearly delineated over the cheek bones, ran two grotesque, parallel streaks of what looked like dark red war paint. The skin wasn't raised and there was no blistering, but it felt as if a fire was raging just under the surface.

I ran my fingers over the marks and instantly remembered having done the same thing the day before when I had wiped the dust off my face with fingers smeared with fennel oil. Furious at my own carelessness, I realised that I had just managed to vividly reproduce on my own face the phototoxic reaction which, until then, I had seen only in dermatological publications. A classic case of failing to heed the old dictum medice, cura te ipsum – healer, heal thyself.

As I was away from home it was a week before I could be seen by a dermatologist and the slow process of healing the skin could begin. It took about a year of check-ups and treatment with powerful creams before the signs more or less disappeared. Today all visible traces have gone, leaving only a memory of my close encounter.

My skin reaction was characteristic of phytophotodermatitis, a skin inflammation caused by simultaneous exposure to a phototoxic plant and sunlight (UVA radiation). It is a toxic dermal eruption that typically produces a strong "sunburn-like" response which may be accompanied by blisters followed by darkening of the skin (hyperpigmentation). There are many phototoxic plants, both domestic and wild. The most common are celery, parsnips, fennel, dill, carrots (Umbelliferae family), limes, bitter oranges (Rutaceae) and figs (Moraceae). In the wild they include cow parsnip (hogweed), cow parsley, angelica and other Umbelliferae. What they have in common are chemical substances called furocoumarins (psoralens), which the plants produce to defend themselves from micro-organisms.

Both furocoumarins and sunlight must be present for the skin reaction to occur. First, the furocoumarins activated by the UVA bind to the DNA of skin cells to form DNA-psoralen adducts. These then target the lipid-rich membranes of skin cells, killing the cells and damaging the skin. The burning analogy is entirely fitting as heat and sweat intensify the phototoxic skin response. The healing process consists in replacing the damaged tissue.

The most commonly affected body areas are those that come into contact with plants: the back of the hands, the forearms and the lower legs. Typical manifestations of a phototoxic reaction are a burning or painful sensation on the skin, intense redness, and curious marks, often shaped like fingerprints or drips. There may also be blistering. Medical help should be sought to prevent permanent damage to the skin.

The incidence of phytophotodermatitis is highest in spring and summer, when psoralen concentration in plants and people's exposure to sunlight reach a peak.

So which groups of people are most at risk? Here are a few examples. An airline flight attendant, serving drinks on the way to the Caribbean, spilled lime juice on her forearm. This incident, followed by sunbathing under the intense tropical sun, resulted in extensive blistering of the skin. The scarring was still visible two months later. In fact, hers was a case of "margarita photodermatitis", a little-known but real hazard of bartending and cocktail drinking in the sun.

Also at risk are grocers and farmers who handle celery, fennel bulbs and other phototoxic vegetables ("harvester's photodermatitis") and gardeners and park-keepers ("strimmer photodermatitis").

Other reported cases have been linked to fig leaf concoctions used to speed up suntanning; to limes rubbed into the skin to prevent insect bites or squeezed onto hair to bleach it in the sun; and, more generally, to a combination of sun exposure and herbal "folk" remedies.

More bizarre cases involve people eating large quantities of parsnips or celery root (celeriac) before using a UV bed.

Friends have often remarked that more publicity should be given to this summer hazard – hence this story. But my purpose is not to create a Hitchcock-style nature scare. Rather, it is to add a note of caution to today's widely held belief in the benevolence of nature. Encouraged by the media and advertising, we have forgotten that the plants we encounter today have survived millions of years of ruthless natural selection that has honed their chemical armouries to a sometimes frightening degree. For this, the plants deserve our fullest respect.

For my part, I will never again pluck wild fennel on a sunny day with my bare hands. It's worth knowing that this beautiful plant is a prolific wild herb which can be often found in seaside locations in the UK. By all means use it in summer salads and fish barbecues during this long hot summer, while you sip cool margaritas – but wait until the sun's gone down.

Barbara Hall is a Eurotox-registered toxicologist