H2s smells of rotten eggs for one breath. After that you don't notice it anymore. It deadens the olfactory nerves so that you can not tell it is present after the first detection. If it is light enough when you first smell it, you will think that it is just you as you can't get another verifying whiff. CO2 will kill you though suffication. H2s will kill you even if you only get one breath of it. (once again this depends on the concentration) What is bad is that the H2S will knock you out. Being as it is heavier than air you usually fall into higher concentrations than what you were exposed to. Someone seeing you down and not knowing of the H2S will wind up joining you so that there two down instead of one.