Hydrogen Sulfide (H2S) is a highly toxic gas, with a pungent “rotten eggs” odor at low concentrations but no visible color. At higher concentrations, the gas acts as a nerve agent to de-sensitize human smell, so that it seems odorless. Its paralytic effect on smell extends to more important bodily functions such as breathing, causing rapid loss of consciousness and asphyxiation. A photograph of a safety chart (taken at a wastewater treatment facility) shows just how toxic hydrogen sulfide gas is:
Note how concentrations in the parts per million range are hazardous, and how little H2S concentration is required to paralyze one’s sense of smell. Hydrogen sulfide also happens to be flammable, its LEL value in the air is 4.3%. However, the toxic properties of the gas are generally the more pressing concern when released into the atmosphere. Another hazardous property of hydrogen sulfide is its density: 1.18 times that of air. This means H2S gas will tend to collect in low areas such as pits, electrical vaults, and empty underground storage vessels.
The principal source of hydrogen sulfide gas is an anaerobic (oxygen-less) decomposition of organic matter. Sewage treatment facilities, pulp mills, and oil refineries generate H2S gas in significant quantity, and so employees at such facilities must be continually aware of the associated hazards.
One of the most popular analytical sensing technologies for H2S gas appropriate for portable monitoring includes an electro-chemical reaction cell similar in principle to the micro fuel cells used to detect oxygen concentrations. Hydrogen sulfide gas entering such a cell engages in a specific chemical reaction, creating a small electrical current proportional to the gas concentration. Like oxygen-sensing fuel cells, these chemical cells also have limited lives and must be routinely replaced.
Instrumentation Basics 