National Wildlife Federation
Unchecked global warming will worsen respiratory allergies for approximately 25 million Americans. Ragweed—–the primary allergen trigger of fall hay fever—–grows faster, produces more pollen per plant, and has higher allergenic content under increased carbon dioxide levels. Longer growing seasons under a warmer climate allow for bigger ragweed plants that produce more pollen later into the fall. Springtime allergies to tree pollens also could get worse. Warmer temperatures could allow significant expansion of the habitat suitable for oaks and hickories, which are two highly allergenic tree species. Changing climate conditions may even affect the amount of fungal allergens in the air.