A passive autocatalytic recombiner (PAR) is a device that removes hydrogen from air. Its purpose is to prevent hydrogen explosions. They are passive devices that work automatically as soon as the hydrogen concentration increases. nside a recombiner there are plates or pellets that are coated with platinum or palladium catalyst. On the surface of the catalyst, hydrogen and oxygen molecules react chemically at low temperature and low hydrogen concentration. The reaction generates steam. The reaction starts spontaneously when the hydrogen concentration reaches 1–2 percent. Burning of hydrogen in air requires at least 4 percent hydrogen concentration, and even higher for an explosion. Therefore, a recombiner is able to remove hydrogen from the containment before a flammable concentration is reached