Why should I run hot water faucets before using the dishwasher if it hasn't been used in a while?

If the hot water system has not been used for two weeks or more, hydrogen gas may be produced. Before using the dishwasher, turn on all hot water faucets and let water flow from each for several minutes to release any accumulated hydrogen gas. As the gas is flammable, do not smoke or use an open flame during this time.