Is there a safety concern with hot water systems that haven't been used recently?

Under certain conditions, hydrogen gas may be produced in a hot water system that has not been used for two weeks or more. If this occurs, before using the dishwasher turn on all hot water faucets and let the water flow from each for several minutes to release any accumulated hydrogen gas. Do not smoke or use an open flame during this time as the gas is flammable. (Page 2)