Helicopter pilots refer to certain operations as hot or cold. A hot operation is one where the engines are kept running during the procedure and cold is with everything shut down. The arguments for and against hot operations center around safety vs. time savings.
For EMS operators, one of the more common hot operations is loading the helicopter. Many times at an accident scene the pilot will keep the helicopter running while the medical crew gets out to retrieve the patient. Since the idea of helicopter EMS, especially with trauma patients, is to save time, the hot loading of patients is performed routinely. However, there have been studies that have shown very little time difference between hot loading, shutting down the helicopter, and then restarting it to depart. The argument for shutting down is that maneuvering around a running helicopter can be hazardous. For example, people have walked into tail rotors and objects have come in contact with the main rotor system. On the other hand, helicopters are mechanical machines and there have been cases where the helicopter failed to start. On an accident scene, this could shut down a highway for a much longer period of time and delay getting the patient to a trauma center.
Another hot operation that is performed is refueling. Pilots trying to save time or an engine start (turbine engines have start cycle TBOs) will ask to be hot refueled. For trained personnel this can be preformed safely on most helicopters especially when the fueling point is low and below the engine. A fueling port high up on the fuselage and above the engine increases the possibility of a fire if fuel spills. Also, climbing on a ladder or other object to reach the fuel port can place personnel dangerously close to a spinning rotor system.
The case for proper training was apparent several years ago when I was watching a Bell JetRanger giving rides at an air show. When the pilot needed fuel, I watched someone drive a pickup truck, with a fuel tank in the bed, up close to the helicopter. The driver climbed out, ran around to the back, and jumped up into the bed. He stood completely up and then quickly ducked. He obviously felted how close his head came to the spinning rotor system. I turned away because I thought he was going to get hit. I remember thinking, wow, he was lucky!