Allergy seasons have been getting longer and more problematic. The U.S. Centers for Disease Control and Prevention estimate that approximately one in four adults and one in five children in the U.S. suffer from seasonal allergies. The Asthma and Allergy Foundation of America anticipates that 2025 will be a severe year for seasonal allergies across the country, with the worst-afflicted cities located in the southern region. Notably, at the end of March, Atlanta and Houston set pollen count records.

Pollen is dispersed by trees in the spring, grasses in the summer and ragweed during autumn, but these patterns have shifted due to climate change. Elevated atmospheric carbon dioxide levels induce many plants to produce more pollen, increase their size and growth rate, and yield more flowers. Warmer temperatures result in warmer, shorter winters and earlier springs, leading to longer growing seasons and overlapping pollen seasons. A 2021 study in Scientific Reports also noted that climate change is augmenting the number and duration of allergenic mold spores in some areas.