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HOSTILE TAKEOVER

Fueled by a diet growing richer in carbon dioxide, poison ivy is bigger, stronger and itchier than ever. Be on the lookout.

Poison ivy, the scourge of summer campers, hikers and gardeners, appears to be growing faster and producing more potent oil, compared with earlier decades.

The reason? Rising ambient carbon dioxide levels create ideal conditions for the plant, producing larger, hardier leaves, faster growth and oil that's even more irritating.

Although the data on poison ivy come from controlled studies, they suggest the vexing plant is more abundant than ever. And the more potent oil produced by the plants may result in itchier rashes.

"If it's producing a more virulent form of the oil, then even a small or more casual contact will result in a rash," says Lewis Ziska, a plant physiologist for the U.S. Department of Agriculture.

The latest research, led by Ziska, studied poison ivy plants in Maryland under different levels of carbon dioxide exposure. One group of plants was exposed to about 300 parts per million of carbon dioxide - about the same level found in the atmosphere in the 1950s. Another group was exposed to 400 parts per million of CO2 - about the same level as in the atmosphere today.

After about eight months, leaf size, stem length and weight, and oil content of the plants raised at current carbon dioxide levels were, on average, 50 to 75 percent higher than the plants under the 1950s conditions.

Not only did the higher CO2 level double the growth rate, but it made for hardier plants that recovered more quickly from the ravages of grazing animals.

The latest research follows a Duke University report last year that higher carbon dioxide levels create a chemical change in poison ivy that results in a more potent form of urushiol, the oil that triggers an itchy rash in about 70 percent of people exposed to it.

"It is more abundant and allergenic," says Jacqueline E. Mohan, who led the Duke study.

Hikers have long known the adage "leaves of three, let them be." But poison ivy, usually found east of the Rocky Mountains, can sometimes have more leaves and look like a shrub or vine.

The leaves can range from 1 to 6 inches, can be notched or smooth and, depending on the season, can be red or green.

The Web site www.poison-ivy.org offers cards with life-size images to help identify the plant in its various incarnations.

Long pants, long-sleeve shirts and socks can help, but clothes need to be removed and washed to avoid contact with urushiol that may have brushed on clothing.

The oil can even penetrate rubber gloves and boots, so vinyl gloves are recommended if you're trying to remove the plants from the ground.

One treatment, sold as IvyBlock, is rubbed on exposed areas before contact to prevent a rash. A study found this treatment prevented or significantly reduced poison ivy reactions.

Despite protective clothes and washing, Yale nursing professor Patricia Jackson Allen still contracted a rash on her forearms after gardening last month. She said that while her precautions prevented a more severe rash, "I haven't found anything that works 100 percent."

FAST FACTS

What to do

- If you think you've come into contact with poison ivy, wash immediately. But if more than 10 minutes has passed, soap and water removes only about half the oil.

- Tecnu, an over-the-counter poison ivy wash, is about 70 percent effective in eliminating urushiol two to eight hours after exposure.

- Goop, a grease remover, and Dial Ultra dishwashing soap were about 60 percent effective, according to a 2004 article in Pediatric Nursing. Rubbing alcohol also helps.

- If exposure does occur, over-the-counter topical lotions may provide some relief. Doctors can treat severe cases with steroids.

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