The double cylinder design raises a safety issue. In the event of a fire, occupants will be prevented from escaping through double-cylinder locked doors unless the correct key is used. This is often an avoidable cause of death in house fires.  The risk can be mitigated by locking the deadlock only when there are no occupants inside the building, or leaving the key near the keyhole. Some fire departments suggest putting the key on a small nail or screw near the door at floor level, since the cleanest air is at floor level and you may be crawling to get to the exit, thus placing the key where it is easiest to find.