Sulfur - Hydrogen sulfide gas may have several sources.  Its offensive odor is akin to rotten eggs and in extreme cases is corrosive to plumbing.  Several external factors may also effect its formation.  Because of its volatility, quantitative results can only come from on site testing.  Presences is expressed as positive or negative based on subjective sample analysis or from provided information.  Noting its presence on hot or cold sources is equally important.