A 26-year-old woman with asthma is reviewed after three hours of emergency department treatment with inhaled salbutamol and ipratropium, systemic corticosteroid and oxygen. She is now comfortable at rest, has no accessory muscle use and speaks normally; oxygen saturation is 97% on room air. Her peak expiratory flow has risen but remains 48% of her predicted value, measured correctly on two occasions 30 minutes apart. What is the most appropriate disposition?