H2S or Hydrogen Sulfide -- see the [hydrogen sulfide page](/wiki/hydrogen-sulfide) is, according to OSHA.gov, "Hydrogen sulfide is a colorless, flammable, extremely hazardous gas with a “rotten egg” smell. It occurs naturally in crude petroleum and natural gas, and can be produced by the breakdown of organic matter and human/ animal wastes (e.g., sewage)."...

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