In plumbing a smoke test forces non-toxic, artificially created smoke through waste and drain pipes under a slight pressure to find leaks. Plumes of smoke form where there are defects. This test can be performed when the plumbing is brand new, but more often it is used to find sewer gas leaks that may plague a building or an area. Any sign of smoke escaping can be considered a possible site for sewer gas to escape. Sewer gas typically has a rotten egg smell and can contain methane gas, which is explosive, or hydrogen sulfide gas, which is deadly.