Hydrogen sulfide gas is a naturally occuring chemical (chemical formula H2S).
The gas has a characteristic rotten egg odor at low concentrations. About half of the population can smell it at concentrations as low as 8 parts per billion (ppb) in air, and more than 90% can smell it at levels of 50 ppb. At higher concentrations, hydrogen sulfide rapidly deadens the sense of smell. For most people, this occurs at approximately 150 ppm.
Hydrogen sulfide is heavier than air, and it often settles in low-lying areas where it can accumulate in concentrations that can injure or kill livestock, wildlife, and human beings. Additionally, hydrogen sulfide has been found to migrate into surface soils and groundwater.
Sources of H2S
Hydrogen sulfide occurs naturally in the environment (e.g., in volcanic gases, marshes, swamps, sulfur springs, decaying organic matter). It is produced by living organisms, including human beings, through the digestion and metabolism of sulfur-containing materials. Hydrogen sulfide is also a byproduct of many industrial processes, such as paper manufacturing, sewage treatment, landfills, or concentrated animal feed operations (CAFOs).
Hydrogen sulfide gas also is found in petroleum and natural gas. Oil or natural gas is considered sour if it has a high percentage of hydrogen sulfide. Natural gas can contain up to 28 percent hydrogen sulfide gas, consequently, it may be an air pollutant near petroleum refineries and in oil and gas extraction areas. The principal source of anthropogenic hydrogen sulfide is as a by-product in the purification of natural gas and refinement of crude oil. Atmospheric releases of hydrogen sulfide represent the most significant public health concern for the geothermal energy industry.