The rising carbon dioxide (CO2) levels that are leading to global warming may also worsen hay fever, according to a new study by the U.S. Department of Agriculture to be published in the journal World Resource Review. Researchers say higher CO2 concentrations in the atmosphere seem to be causing ragweed to produce twice as much pollen as it did 100 years ago, and pollen production will likely rise still further this century with continued increases in CO2 levels, possibly aggravating allergy problems. Ragweed, a common plant in the U.S., is a major cause of hay fever in the fall.