A 22-year-old man with known asthma presents to the emergency department with a 2-day history of worsening wheeze and shortness of breath after a viral illness. He is speaking in full sentences, respiratory rate is 22 breaths/min, oxygen saturation is 94 percent on room air, and peak expiratory flow is 70 percent of his personal best. There is diffuse expiratory wheeze but no accessory muscle use. What is the most appropriate initial treatment?