University of Vermont Extension 
Department of Plant and Soil Science

Summer News Article
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SOME PLANTS AND SUN DON’T MIX

 
Dr. Leonard Perry, Extension Professor
University of Vermont
 
If you’re a gardener or merely enjoy being outdoors, you may have heard if not learned first hand of the skin reactions caused by some plants—dermatitis.  Common examples are rashes from the oil of poison ivy or brushing the leaves of stinging nettles.  Also common, but often overlooked and misdiagnosed, are the rashes caused by exposure to certain plants in sunlight.  Some common culprits are garden plants such as gas plant, wildflowers such as Queen Anne’s Lace, vegetables such as celery, and one of the most common—limes.

These “phytophotodermititis” skin rashes (dermatitis) are caused by the reaction of the plant (phyto) toxic chemicals (“furocoumarins”), combined with the UV rays from the sun (photo).  While not all people get these reactions, they are possible in anyone if high enough concentrations under the right conditions.   Agricultural workers and gardeners, bartenders (on beaches and outside), and children (from running through wild growth areas) are some of the populations in which this problem is often seen. 

If going to the beach on vacation, or merely partying on a sunny patio, beware of popular drinks with citrus wedges and juice, particularly limes (the problem is more the juice from the rinds than from the pulp).  A beer commercial that appears humorous on the screen may cause dire problems in reality.  In the scene, a woman squirts lime juice on her boyfriend for looking at a gorgeous girl. Does she know she may be causing phytophotodermititis (sometimes called “margarita dermatitis” or “lime disease”), resulting in a rash on his face?  Others have reactions from using lime juice remedies as insect repellents.

Rashes and lesions begin about a day after exposure, often burning and may blister.  While they peak in 2 to 3 days, the pigmentation may last for weeks or months.  The more fortunate only get the pigmentation without the burning and blistering.  Exposure is more common in mid to late summer when the amount of chemical is highest in offending plants, skin exposure to sunlight is greatest, skin is wet from swimming or sweating, and lime drinks are more common outdoors.

Symptoms of photodermititis are different from those of poison ivy and other plants in that they cause burning not itching, are only in areas exposed to sunlight, are often in random linear streaks or patterns (such as from lime juice running down skin or brushing against plants), and develop pigmentation.  Such pigmented areas may be sensitive to UV light for several years.  Getting the toxic substance on hands and then touching skin (as parents grabbing children), can lead to fingerprint patterns.  Offending weed parts thrown against skin, as with weed trimmers, can cause a buckshot appearance to the rash. 
 
There are three main plant families that cause this disease.  The carrot family, Umbelliferae or Apiaceae, is the most common in gardens and natural areas.  The false Queen Anne’s lace or bishop’s flower (Ammi majus) has a flower similar to Queen Anne’s lace (Daucus carota), only is annual not biennial as the latter.  The former was used as early as 2000 BCE for the treatment of certain diseases, but the cause of this plant and sun reaction was not discovered until the 1930’s.  Queen Anne’s lace is the common roadside wildflower, is useful to attract beneficial insects and pollinators, is the relative of our cultivated carrot, and yet can cause such reactions. The false Queen Anne’s lace is often used in flower arranging.

Other members of the carrot family growing wild that can cause problems are the closely related cow parsley, cow parsnip, and the invasive giant hogweed.  These have white flowers compared to the wild parsnip with yellow flowers, the latter escaped from cultivation and also becoming invasive in many regions.  Herbs in this family to use care with include angelica (wild angelica too), parsley, and fennel.  Handling the edible crops celery and carrot in this family, especially repeatedly by agricultural workers, can cause problems.  An interesting point found through research is that some plants, such as celery, may develop much higher levels of these toxic (to humans) furocoumarins in response to a disease attack.  Such substances, termed “phytoalexins,” help plants to resist disease organisms.

The rue family, Rutaceae, includes the tropical citrus culprits of this disease, as well as the temperate garden plants garden rue (Ruta graveolens) and gas plant (Dictamnus albus).  The latter exudes a “gas” in the air on still summer evenings that can be ignited briefly.  Some conjecture that this may have been the burning bush that Moses saw on Mount Sinai.

The last main family of significance with this disease is the mulberry, Moraceae, the main culprit being the fig tree (Ficus carica).  Similar to the citrus, symptoms arise from getting the juice on the skin (leaving it on lips can be a problem) rather than from eating the fruit.  Pruning figs, harvesting, even using homemade fig decoctions as tanning lotions can cause problems.

The first step to treatment is to avoid such plants, or treat them with caution until you learn your sensitivity.  Wear gloves and cover skin when working around such plants.  If you do end up with lesions, apply cool wet compresses.  If lesions are extensive and quite painful, consult a physician who may prescribe topical steroids. 

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