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Approximately 40 million Americans suffer from hay fever, while 16 million adults endure asthma.

Although genetics play an important role in these conditions, recent research is finding that higher temperatures and more carbon dioxide are making allergy seasons worse, stimulating plants to produce more pollen and increasing fungi growth.

There have been significant increases in allergies and asthma in recent decades, which obviously cannot be explained by any change in genetics, said Christine Rogers, a research associate in Environmental Science and Engineering at Harvard University.

Live Science, 22 Nov 2005