There’s a warming trend in cities across American, and that’s extending allergy season for millions of Americans.

A new study by Climate Central found five cities in Minnesota have increased their pollen season — in some cases by nearly a month. The Twin Cities allergy season has expanded by 27 days, Duluth is seeing an average of 24 extra days of allergy season, while Mankato is experiencing 11 and Rochester is averaging 10 extra days, said Kristoffer Tigue, a reporter for the Minnesota Star Tribune.

“It’s not just the extended pollen season that the studies have have shown are are making allergies worse for people,” said Tigue. “The plants themselves are growing larger, and that’s because there’s more carbon dioxide in the air. There’s even research that suggests that the pollen itself is becoming more allergenic.”

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