Pacemakers are used to stimulate a regular heartbeat when the body's natural electrical pacing system is irregular or not transmitting properly. Over the years, various power sources have been used for pacemakers, including thermoelectric batteries containing 2 to 4 curies of plutonium-238 (88 year half-life). As the term "thermoelectric" implies, the heat from the decaying plutonium is used to generate the electricity that stimulates the heart. At present (2003), there are between 50 and 100 people in the US who have nuclear powered pacemakers. When one of these individuals dies, the pacemaker is supposed to be removed and shipped to Los Alamos where the plutonium will be recovered. The pacemaker shown in these photos, produced by Medtronic, has had its plutonium removed.