A 27-year-old tall, thin man presents with sudden onset right-sided pleuritic chest pain and dyspnoea. He is a non-smoker and otherwise healthy. On examination he is comfortable at rest, respiratory rate 18 breaths/min, oxygen saturation 98% on room air and there is reduced breath sounds over the right apex. Chest X-ray shows a right primary spontaneous pneumothorax with a 3 cm rim of air between lung margin and chest wall at the hilum. What is the most appropriate initial management?