TUESDAY, May 30 (HealthDay News) -- An annoying consequence of global warming could be a dramatic increase in the amount of poison ivy and its ability to cause allergic reactions, researchers report.

About 80 percent of people develop an allergic reaction to poison ivy that includes an itchy rash and blisters, caused by skin contact with the oily sap -- or resin -- of the plant. As the virulence of the plant increases due to global warming from increased atmospheric carbon dioxide, even more people will be susceptible, the researchers suggest.

"Under atmospheric carbon dioxide (CO2) concentrations that the whole planet will reach by the middle of this century, poison ivy grows not only faster and bigger, but also more poisonous," said study lead author Jacqueline E. Mohan, a post-doctoral scientist at the Ecosystems Center, Marine Biological Laboratory, at Woods Hole, Mass.

Only humans and some apes are allergic to poison ivy, Mohan noted. But with carbon dioxide levels increasing, Mohan suspects the plant's growth rate and virulence are already on the rise.

"It makes the forest an even scarier place," Mohan said. "But not just forest -- backyards too. Poison ivy is a remarkable plant. It can grow just about anywhere."

The study findings appear in this week's edition of the Proceedings of the National Academy of Sciences.

In their six-year experiment, Mohan and her colleagues showed that elevated atmospheric carbon dioxide in an intact forest ecosystem increases photosynthesis, water use efficiency, growth and biomass of poison ivy. "This was out in the real world," Mohan said.

Poison ivy plants exposed to elevated CO2 levels averaged 149 percent faster growth compared with control plants, Mohan said. "Something we did not expect to happen, but indeed did -- the form of the poison they make was more poisonous," she added.

"This is kind of sad news, not only for humans but for forests," Mohan said. "Increased vine abundance inhibits tree regeneration by killing young trees," she added.

One ecological expert thinks the findings are the first to link increased growth and toxicity with rising levels of CO2.

"This is a very interesting paper," said Kevin L. Griffin, an associate professor at the Lamont-Doherty Earth Observatory of Columbia University. "The increase in the growth rate with elevated CO2 is very large. Similar rates have been reported for potted plants in short-term experiments, but for these to be maintained in the field with natural environmental variation is really quite surprising."

"I also like the linkage between the growth data, the physiology and the toxicity, which I believe is unique in the elevated CO2 literature," he added.

Another expert predicts future medical problems from more toxic plants.

"This important research is yet another reason to be concerned about mankind's steadily increasing production of carbon dioxide through the burning of fossil fuels," said Dr. Daniel W. Shaw, an associate clinical professor of dermatology at the University of California, San Diego.

These experiments predict that poison ivy will likely become more abundant, resulting in more exposures to these dangerous plants, and at the same degree of exposure it is likely to cause worse skin reactions than currently seen, Shaw said. "This is worrisome because of the intense skin reactions that these plants already commonly produce in the majority of the exposed population," he said.

And another expert sees the study findings carrying far-reaching consequences.

"The most worrisome message here is less about this particular plant and more about the whole forest," said Dr. David L. Katz, an associate professor of public health and director of the Prevention Research Center at Yale University School of Medicine.

"We are upsetting a balance in ecosystems and that will have far-reaching effects, many of which we are first now beginning to guess," Katz said.

More information

The National Institutes of Health can tell you more about poison ivy.

SOURCES: Jacqueline E. Mohan, Ph.D., post-doctoral scientist, The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Mass.; Kevin L. Griffin, Ph.D., associate professor, Lamont-Doherty Earth Observatory of Columbia University, Palisades, N.Y.; Daniel W. Shaw, M.D., associate clinical professor of dermatology, University of California, San Diego; David L. Katz, M.D., M.P.H., associate professor of public health, director, Prevention Research Center, Yale University School of Medicine, New Haven, Conn.; May 29-June 2, 2006, Proceedings of the National Academy of Sciences

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