Chlorine is an irritant gas that is widely used in water purification. Several previous reports had reported accidents of inhalation injuries at swimming pools. However, there have been limited data on the detection of on-site chlorine concentration. This study aims to report a chlorine leakage accident at a swimming pool caused by improper disinfection operations. Calculation using the gas diffusion simulation software showed that the on-site chlorine concentration was 221.45 ppm. When the accident occurred, there were 92 individuals at the swimming pool and the gym, among which 61 were referred to the emergency department of five different hospitals for feeling ill. Among them, 22 patients underwent chest high-resolution computed tomography scans in our hospital. According to the findings, 4 (18.2%) patients had peribronchitis, 3 (13.6%) had tracheobronchitis, 4 (18.2%) had pneumonia, 4 (18.2%) had interstitial pulmonary edema, and 3 (13.6%) had alveolar pulmonary edema. The symptoms of 22 patients who visited our hospital significantly improved after comprehensive treatment. Three months after the accident, 8 of 17 patients presented obstructive ventilation defects or small airway dysfunction. The accidental exposure to chlorine may induce acute poisoning with various respiratory injuries and prolonged lung dysfunction.