Abstract: At 3:00 p.m. on December 10, 2018, Employee #1 had been administering aquatic physical therapy to a patient and was filling out paperwork related to the patient while sitting at a poolside table. Employees #2 and #3 were working in the office portion of the facility. Employee #1 said he did felt unwell (light-headed, lethargic) and entered Dressing Room #1. The patient (non-employee) activated an emergency switch located inside of Dressing Room #2, which alerted staff in the reception area to an emergency. Employees #2 and #3 responded to the alarm. Employees #2 and #3 gave the patient aid and took the patient's blood pressure readings and administered a pulse oximeter to her finger. Employee #2 alerted the receptionist to call for emergency services. Employee #1 was found in Dressing Room #1 conscious and ambulatory. Employee #3 started noticing symptoms of light-headedness and had the building evacuated. Emergency services took Employees #1 and #2 to Western Maryland Regional Medical Center in Cumberland, Maryland; Employee #3 and the patient were taken to Potomac Valley Hospital in Keyser, WV. The pool heater, a RayPak model C-R66A-EP-X (SN: 1304356686), had a malfunctioning heat exchanger, the component responsible for separating the byproducts of combustion (carbon monoxide) from the ambient breathing air. Employees #1-3 were hospitalized with carbon monoxide poisoning. The employees suffered light-headedness, headaches, and nausea.