Our latest study finds that the U.S. will face up to a 200 percent increase in total pollen this century if the world continues producing carbon-dioxide emissions from vehicles, power plants, and other sources at a high rate. Under that scenario, the spring pollen season will generally start up to 40 days earlier and last up to 19 days longer than it does today.

https://www.theatlantic.com/science/archive/2022/03/climate-change-allergies-pollen/627103/