NEW YORK (Reuters Health) - If predictions about current trends in global warming hold true, come the year 2050, allergy sufferers may have a lot more to sneeze about.

Researchers at Harvard University are reporting that ever-rising levels of carbon dioxide (CO2) may up the amount of allergy-causing ragweed pollen in the air by as much as 61% as early as mid-century.

In the study, lead author Dr. Peter Wayne of Harvard University in Cambridge, Massachusetts and his team measured pollen counts of ragweed grown in greenhouses at levels of atmospheric CO2 that are predicted for the years 2050 to 2100.

Concentrations of CO2 have increased by 29% since pre-industrial times and are expected to double again during the latter half of this century, according to the report published in the March issue of the Annals of Allergy, Asthma, and Immunology.

Rising atmospheric concentrations of CO2 and other "greenhouse gases" produced by burning fossil fuels are believed by many scientists to be contributing to a gradual warming of the earth's climate.

In the study, a doubling of the atmospheric CO2 concentration in the greenhouses stimulated ragweed pollen production by 61%, the investigators found.

"Our observation that a doubling of the atmospheric CO2 concentration markedly stimulates ragweed pollen production suggests that the incidence of hay fever and related respiratory diseases may increase in the future," the authors write.

"Additional research is warranted to more accurately evaluate the future impact of allergenic pollen on public health and to help develop effective ecologic, public health, and policy strategies for mitigating these threats," Wayne and colleagues conclude.

About one in six Americans suffer from allergies, according to the American Academy of Allergy, Asthma, and Immunology. Common sources of allergy-causing pollens include ragweed, grass and some trees.

SOURCE: Annals of Allergy, Asthma, and Immunology 2002;88:279-282.

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