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Leaves of three, let them be.” No doubt you’ve heard this warning about poison ivy, a weedy plant that each year causes more than 350,000 reported cases of human contact dermatitis, and probably many thousands more unreported cases.
Anecdotes from doctor’s offices indicate that this year is shaping up as a particularly nasty one for poison ivy, or Toxicodendron radicans, and evidence suggests that increased concentrations of carbon dioxide in the air have contributed to bumper crops with a more potent toxin.

But the rising risk of developing an extremely itchy, blistering rash from poison ivy is only one of the recent changes in human exposures to toxic or harmful plants.

Many homes and gardens play host to an increasing number of hazardous plants, and children are most often at risk. In 2003, according to an authoritative new book, poison control centers nationwide received more than 57,000 calls relating to exposure to potentially harmful plants, and 85 percent of them involved children under age 6. Most, however, were considered simply exposures; either no toxin was ingested or the amount consumed was too small to be harmful.

The book, “Handbook of Poisonous and Injurious Plants” by Dr. Lewis S. Nelson, Dr. Richard D. Shih and Michael J. Balick, was produced under the auspices of the New York Botanical Garden, where Dr. Balick is director of the Institute of Economic Botany. While its primary mission is to help health care professionals identify and treat plant-caused injuries, this lavishly illustrated book can be a helpful guide to ordinary people. It highlights hundreds of troublesome plants, providing photographs and written descriptions, common names, geographic distributions, toxic parts and toxins, effects on the body and information on medical management.

I was stunned to realize just how many of these potentially dangerous plants were in my own home and garden, including aloe, elephant’s ear, jade, peace lily (Spathiphyllum), philodendron and dumbcane (Dieffenbacchia), as well as foxglove (Digitalis purpurea), hellebore, vinca, rhododendron and chrysanthemum. I count my blessings that none of my children or grandchildren tried to chomp on one of them.

Of course, plant-based poisons have an important role to play, especially in discouraging predators. And through the ages and into modern times, many have served important medicinal roles. Vinca, for example, was the original source of the anticancer drug vincristine, and foxglove gave us the valuable heart stimulant digitalis.

Deer, which have become a horrific horticultural nuisance in the Northeast, somehow know to avoid dining on several of the toxic plants, like vinca and foxglove, enabling gardeners to plant them in unfenced areas. If only our children were equally knowledgeable.

Common Risks

Dr. Nelson, of New York University School of Medicine and the New York City Poison Control Center, said the problem often began with the fact that many toxic plants are beautiful and colorful, prompting people to pick them to adorn their homes and gardens. But their very attractiveness is what creates a hazard for small children, who may be tempted to put toxic berries, flowers or foliage in their mouths.

A second risk involves adults, who pick what they think are edible or medicinal plants but mistakenly choose a toxic look-alike. In a recent incident cited by Dr. Nelson, a group of people picked what they thought were wild leeks, or ramps, cooked and ate them. What they really consumed was the cardiac toxin from young false hellebore. Fortunately, they survived the resulting heart rhythm disturbance.

Other cases have involved people who picked foxglove before it flowered, thinking it was a helpful herb that could be made into a medicinal tea. And sometimes herbal teas that should be safe are not because they were accidentally contaminated by a toxic plant. Thus, it is best to stick to well-known commercial brands packaged in the United States.

While ingested plant poisons are the most common hazard for small children, for adults and older children the usual sources of misery are plants that create problems on physical contact, like poison ivy. I asked Dr. Nelson what people do wrong after coming into contact with poison ivy, and the answer was simple: “They don’t wash their hands quickly and thoroughly enough. If you wash off the toxin with soap and water within 10 or 15 minutes, it’s unlikely to cause a reaction.”

This can be a particular problem for outdoor sports enthusiasts, landscapers and other outdoor workers who may not notice their contact with the plant or may not have a means of quickly washing away the toxin, called urushiol. Even those who do wash may fail to scrub off the urushiol that gets under fingernails and then spread it to other parts of the body, Dr. Nelson said.

Over the course of hours or days, urushiol causes a slowly developing rash characterized by pain, itchiness, redness, swelling and blisters. Contrary to what many people think, the rash itself does not spread. Rather, people spread the toxin around their bodies through scratching and contact with contaminated clothing.

Other Problem Plants

Poison ivy is hardly the only source of urushiol, a class of toxins with varying potencies. It is also found in the skin of mangoes, as I sadly learned after eating a mango off the rind. It was still winter when I called my dermatologist and said, “If I didn’t know better, I’d say I had poison ivy of the mouth.” His immediate response: “You’ve been eating mangoes.”

Why, I wondered, had this not happened years ago? The answer was that after repeated exposures to urushiol that caused no reaction, I had become sensitized to the allergen and thereafter any contact with it could cause the same miserable reaction. Dr. Nelson said 85 percent of the population has the potential to develop sensitivity to urushiol. So if you think you can safely traipse through poison ivy, think again. Sooner or later you are likely to suffer as I did.

Treatment of a poison ivy rash typically involves relieving the itch with calamine lotion and taking an oral antihistamine or, in more serious cases, a corticosteroid.

Another common source of contact dermatitis involves the stinging nettle, a weedy plant that also seems to be thriving in our carbon dioxide-enriched environment, Dr. Balick said. These plants are a source of mechanical irritants. They have fragile hypodermic-like tubules containing a mixture of irritant chemicals that are injected when bare skin brushes against the plant and stinging hairs from the stems and leaves break the skin. Unlike poison ivy, the burning, itchy rash caused by stinging nettles is short-lived.

Still other problem plants contain chemical irritants, like capsaicin from chili peppers. This chemical is a mucous membrane irritant that causes the release of a substance that stimulates pain fibers and inflammation. This is especially painful when contaminated fingers transfer the chemical to the eyes or genitalia. To relieve the discomfort, it takes thorough and repeated washing, an analgesic to relieve the pain and, in some cases, anti-inflammatory medication.

Some plants, including agave, snow-on-the-mountain, crown-of-thorns, marsh marigold and buttercup, contain an irritant sap or latex, which can cause a chemical burn on the skin.

Finally, there are plants that contain phototoxins — substances that increase the sensitivity of the skin to ultraviolet light and can result in a blistering sunburn. Among these are yarrow, rue and Queen Anne’s lace.

Source: The New York Times