Q: We have been pulling weeds in our garden. Several of the people we hired, my husband and I have had to go to the emergency room because of a blistering rash. What kind of plant could do this? We have a mix of common perennials and weeds, nothing out of the ordinary.

A: What you might be experiencing is called phytophotodermatitis (PPD). That means it is a plant- and sunlight-induced rash. There are plenty of plants that can cause this rash. The reaction will occur when your skin is exposed to ultraviolet light and the plant's oils. Both the ultraviolet light and the oils are necessary to cause the problem. Washing off the oil before too much exposure to sunlight will prevent the problem. Sometimes, the reaction is minor and nothing more than a simple rash, similar to poison ivy, will occur. Depending on the plant and the person, the rash can be similar to dipping your skin in acid. In the worst cases, large blisters, chunks of loose skin and lots of itching will occur. Afterward, the healing skin may maintain a bright red coloring for many months, which is known as hyperpigmentation.

Let's look at some possible culprits. Queen Anne's lace or the wild carrot (Daucus carota) is a serious weed in all of the cultivated areas of the world. The young small root is edible like a store-bought carrot, but the leaves are a possible source of PPD.

The wild parsnip (Pastinaca sativa) is in the same family and has a much stronger toxicity. It is a European native found in much of the United States where it invades natural areas. I have been poisoned by this plant while pulling it out of a restored prairie. The blisters on my arms lasted several weeks, but the red coloring lasted from July into the winter. But the other guy working with me had no reaction at all.

The cow parsnip (Heracleum maximum) is a North American native found from the sub-arctic south to all but the Gulf Coast. Giant hogweed (Heracleum mantegazzianum) is a huge perennial plant that can reach 15 feet to 20 feet tall. It has huge leaves to 4 feet across. It has become established in the northeast and northwestern parts of the United States where it is often found in wet areas. It is an extremely phototoxic plant that can cause blindness.

All of the above plants are in the umbel family of plants. They have flower clusters that are formed at the top of a stem. Each cluster is at the top of smaller stems. The overall appearance is similar to an umbrella and its arms radiate out from the center. Other plants in this family that have caused reactions in farm workers include parsley, celery and carrots. Landscape workers have developed PPD when string trimming susceptible plants.

Other plants with phytophototoxic compounds include the citrus, fig and bean families. Limes have caused PPD on bartenders and people who spread lime juice on their skin for the fragrance or as a mosquito repellent when sunbathing. Sucking on lemons, limes and oranges in sunlit areas can cause PPD on the lips and face. Other perennials in the citrus family and known sources of PPD are the burning bush or gas plant (Dictamnus albus) — not to be confused with the burning bush shrub — the garden rue (Ruta graveolens) and the coast spicebush (Cneoridium dumosum) in California.

Phytophototoxic chemicals are found in the fig leaves but not the edible fruit. In the bean family plant, it is the scurf pea (Psoralea corylifolia), which is found in India.

Other plant families like the daisy, buttercup, cabbage, St. John's wort, morning glory and sumac families have all been implicated in having members that can cause PPD, but they have not been proven to cause it. They may just be responsible for causing a contact dermatitis like poison ivy that doesn't need to be exposed to ultraviolet light.

Lastly, there are some plants that have caused PPD in livestock when they ate the plants and then were exposed to the sun.

E-mail questions to Jeff Rugg, University of Illinois Extension at [email protected]. To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.