The pollen count in Chicago peaks in April–September, though allergy sufferers can experience symptoms from as early as early spring. The Midwest has clearly defined seasons: tree pollen in spring, grass in early summer, and ragweed dominating fall.
The primary driver is Mixed Trees — a mix of oak, maple, birch, and elm pollen peaks throughout spring. On warm, dry, and windy days the pollen count in Chicago can spike dramatically, while rain temporarily brings it down.
Tree pollen starts in early spring and runs through spring. Grass pollen peaks during late spring through early summer. Ragweed dominates from August through October.