One of the reasons H2S is so dangerous is because it is lethal at low doses AND it deadens the sense of smell once it exceeds about 15-20 parts per million (ppm) and is immediately dangerous to life and health (IDLH) at 100 ppm. The lethal dose at which 50% (LD50) of rats exposed to it die is 356 ppm for a 4 hour exposure period.
Is Hydrogen Sulphide (H2S) the stuff that is put into propane? I always smell it when the propane tank for my stove is almost empty. I thought the smell added to propane was harmless...