Hydrogen Sulfide (H2S) is really prevalent here in the south/southwest, but also is becoming more prevalent in other areas where oil and gas exploration is taking hold in light of the U.S. trying to break our dependence on foreign oil reserves. Examples are Pennsylvania, North Dakota, etc. Those fields which were previously "sweet"....having very little sour gas, if any, have turned around and are becoming much more sour. This process is expected in our industry, but it only goes one way.....from marginally sour to very sour. Never the reverse.
H2S is known as the "silent killer". It's an insidious and invisible gas which smells a bit like rotten eggs at low concentrations. It quickly paralyzes the olfactory nerves (sense of smell) at about 100 part per million (ppm). Many times, upon investigation of major incidents/fatalities, the concentration didn't go from zero to toxic, but simply hit the victim with a deadly concentration immediately .