Probably the best-recognized toxic gas, with its rotten-eggs smell and its presence wherever organic decomposition is occurring.  It is toxic in small quantities,  with an allowable level of only 10 ppm.  But it is detected by smell at far lower levels.  The treacherous characteristic is that the sense of smell soon gets degraded to the point where toxic levels no longer are noticed.  H2S is readily detectable by electrochemical cells.  This is one of the gases, along with CO, combustibles and lack of oxygen, that is always required to be monitored in confined spaces.