H2S or Hydrogen Sulphide Gas is a combination of two hydrogen atoms and one sulphur atom.  It is a very simple compound, unfortunately for living creatures it is one of the most deadly gases known to man.

H2S quickly overcomes our body and our senses rendering us unconscious.  First off the gas is invisible, so we cannot see it, only smell, but our sense of smell can be deadened quickly with even a relatively low amount of the gas (100pm/0.001%).  The next step it to paralyze our central nervous system (200ppm/0.002%) we are not longer able to adequately reason.  The third step is unconsciousness, consciousness can even be immediate at 700ppm/0.07%.  Even with a quick rescue and resuscitation, the combination of the gas and our lung fluids can create sulphuric acid leaving us with burnt lungs, some of those exposed die in hospital when they drown on the fluid build up in their lungs.

Prevention of exposure to the gas is the best way to protect yourself.  Safety training with certification in H2S Alive certification is the first step.