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Rising carbon dioxide levels are bad news for allergy sufferers because they
increase pollen production by common ragweed, the plant responsible for most
autumn hay fever cases in North America. Researchers for the US Department of
Agriculture found that when they grew ragweed at pre-industrial CO₂
levels, the plants produced 5 grams of pollen on average, half as much as today.
“We get a nice correlation with the increase in asthma rates” over the past few
decades, Lewis Ziska told last week’s Global Warming International Conference in
Boston.