Hydrogen sulfide is a potentially lethal gas produced by anaerobic decomposition of organic matter. It can be found in sewers, swamps & in oil fields where drilling is taking place.
In the image, note the two strips to the left. The left one was wetted before exposure to H2S while the middle strip was exposed dry. By wetting, the lead acetate infused strip can detect as low as 5ppm of H2S which is well below the danger level permitting time to don gasmasks.