Final answer:
When the airway radius decreases, the % FEV1 decreases, indicating less air can be expelled in the first second of forced expiration.
Step-by-step explanation:
When the airway radius decreases, the % FEV1 (the percentage of forced expiratory volume in 1 second) decreases. This means that less air can be expelled from the lungs in the first second of forced expiration. The airway radius plays a crucial role in determining the ease with which air flows through the respiratory system. As the radius narrows, there is increased resistance to airflow, leading to a decrease in % FEV1.