Allergy season is getting longer, and the amount of pollen in the air is exploding, causing a growing number of adults in their 30s, 40s and 50s to seek help from doctors.

A large driver of adult-onset seasonal allergies appears to be greenhouse gases and climate change. As CO2 levels go up, plants and trees produce more pollen — about 20% more than 30 years ago, according to the co-director of the Climate Health Institute at George Washington University.