One of the most dangerous things about H2S is that it deadens your sense of smell at low levels. If you begin to smell H2S, you should immediately vacate the area. The toxicity comes from the affinity to bind to iron and prevent cellular respiration in such places like in the mitochondria of any cells or on hemoglobin on red blood cells. In short its toxic because it prevents our cells from getting oxygen properly. As the H2S levels increase, an untrained individual may take this as a sign that they are now out of harm’s way, but unfortunately they may be exposed to lethal levels of the chemical. If you are in a plant that processes H2S, ask for a H2S badge or monitor. At least 1 in 5 people working in an area should have a monitor on them. It also have wide explosive limit and spark is enough to initiate ignition