As Company 4 Members began this evening’s drill, one of the duty crews was dispatched to an EMS call. This is a routine occurrence during drill and the Medic unit responded for a male having trouble breathing. Upon arrival of EMS it was noted that there was a heavy chlorine odor as the crew approached the residence and a noticeable cloud as they entered where the patient was located. The crew immediately snatched the patient and retreated to the exterior, requesting a Fire response to the scene. Engine 41 and Truck 4 responded and found the patient had set up an inflatable hot tub in his basement and unintentionally mixed a moderate amount of granular chlorine and a poly-styrene gel causing a reaction. The issue was minimized through copious dilution, the home ventilated and personnel decontaminated. Company 4 remained on scene for approximately 2 hours.