On October 15, 1999, at 0700 hours, the Apparatus Operator on Engine 4 was conducting a routine equipment check when he experienced problems with the Medtronic Physio-Control LIFEPAK 500, which is an automated external defibrillator (AED). He brought the malfunctioning AED to the attention of the Assistant Chief and a Captain, explaining that when he turned on the AED, it shut down instead of self-testing. The Assistant Chief and a Captain had the AED brought into one of the fire department offices, where they placed it on the desk to check it out. When the AED was turned on again, the units message system indicated the battery should be replaced. They removed and checked the lithium battery and noted it had an expiration date of 2004. They replaced the battery in the unit and it was turned on. An explosion or battery rupture occurred, which blew out the back of the AED (see photos). It was reported by the Assistant Chief and the Captain that the concussion from the explosion or battery rupture blew the Assistant Chief out the office door onto the floor and blew the Captain against the office wall. The office was filled with white acrid smoke which made breathing difficult. The Assistant Chief reported he received flash burns to the left arm, a sprained left wrist, and an injured back. The Captain reported he suffered upper respiratory inflammation from the acrid smoke. NIOSH investigators concluded that to reduce the risk of similar occurrences fire departments, emergency medical services, and other users of AEDs should: Follow the manufacturers instructions to replace battery packs immediately when the unit indicates a low battery or replace battery message. Additionally, NIOSH recommends the Food and Drug Administration, which is responsible for approval of medical devices, should continue follow up with Medtronic Physio-Control, manufacturer of the LIFEPAK AED, to reduce the potential hazards with battery packs in these types of devices.