Global warming prolongs sneezin' season. (Allergies).

The level of atmospheric carbon dioxide (C[O.sub.2]) is predicted to increase throughout this century, largely due to the burning of coal, oil, and natural gas. In addition to contributing to global warming, higher concentrations of this greenhouse gas may also be increasing the incidence of allergies and asthma by raising pollen counts. A study published in the March 2002 issue of the Annals of Allergy, Asthma, and Immunology finds that plants produce more pollen when grown under high levels of C[O.sup.2], which is the main fuel for photosynthesis.

Plant pollens are ubiquitous and irritating allergens, and allergies to pollen exacerbate asthma. "Ragweed is common throughout the United States, and it is one of the most allergenic plants," says Paul Epstein, associate director of the Center for Health and the Global Environment at Harvard Medical School and a study co-author. A rise in ragweed pollen may be an unexpected side effect of climbing C[O.sup.2] concentrations, "which is not good news for allergy sufferers," says Epstein.

In 1998 and 1999, Epstein and colleagues at Harvard University cultivated ragweed from seed gathered in the wild. Half the plants were grown in the then-current atmospheric C[O.sup.2] level of 350 parts per million (ppm) and half at 700 ppm C[O.sup.2], a level estimated to occur in the second half of this century. (Atmospheric C[O.sup.2] levels currently stand at about 370 ppm.) At the peak of the flowering season, pollen grains were collected and counted. The plants grown in the higher C[O.sup.2] atmosphere produced 61% more pollen than those grown at then-current conditions. However, the ragweed plants grown in the higher C[O.sup.2] atmosphere stood only 9% taller, indicating that the plants allocated more energy to pollen production than to boosting their size.

Since the early 1900s, the incidence of allergies and asthma has steadily climbed. Experts have looked at many indoor factors, including dust mites, pets, secondhand tobacco smoke, and cockroaches, to explain the surge. Epstein suggests that additional factors are contributing: In addition to C[O.sup.2] directly fueling greater pollen production, rising concentrations of greenhouse gases cause global warming, which triggers warmer winters and earlier springs and thus longer growing seasons. In recent years, "we're seeing pollen counts of several thousands of grains per cubic meter," he says. According to the American Academy of Allergy, Asthma, and Immunology, pollen counts of just 500-1,500 grains per cubic meter signal severe alert warnings for weeds, grasses, and trees.

Because indoor allergens such as dust mites, mold, and pet dander are most commonly implicated in children's allergies, an increase in outdoor allergens "adds another dimension to discussions about the increased prevalence of allergic diseases," says Richard Weber, an allergist at the National Jewish Medical and Research Center in Denver, Colorado. Children are also more sensitive than adults to allergic triggers of asthma, such as pollen, mold, and pet allergens. So if airborne allergens from plants increase in the future, "children will probably be the main sufferers," says Weber.