Understanding Hydrogen Sulfide (H2S)

Hydrogen Sulfide (H2S) is a colorless gas. It is heavier than air, flammable, explosive, corrosive, and very poisonous. Its foul odor has characteristics of rotten eggs, which is perceptible at very low concentrations.  However, in higher concentrations, H2S affects a person’s ability to smell the toxic gas the longer the person is exposed. In severe cases, a person may eventually not realize they are inhaling the toxic gas until it is too late. Hydrogen Sulfide often results from the bacterial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers. It also naturally occurs in manure pits, well water, oil and gas wells, and volcanoes.

The primary route of exposure is inhalation and the hazardous gas is rapidly absorbed by the lungs. In addition, hydrogen sulfide is highly flammable and gas/air mixtures can be explosive. It can potentially travel to sources of ignition and flash back. If ignited, the gas burns to produce toxic vapors and gases, such as Sulfur Dioxide.

Because it is heavier than air, hydrogen sulfide can travel and collect in low-lying and enclosed, poorly-ventilated areas such as manholes, sewers, and underground telecommunication site vaults. For work within enclosed spaces, appropriate procedures for identifying hazards, monitoring, and entering confined spaces must be defined. Hydrogen Sulfide presence makes work in enclosed spaces potentially very dangerous.

Health Hazards

When present in high concentrations, Hydrogen Sulfide may produce these symptoms:

  • Headache
  • Nausea
  • Eye, throat, respiratory irritation
  • Fatigue
  • Olfactory fatigue (loss of smell)
  • Pulmonary edema (excess fluid in lung tissue)
  • Unconsciousness
  • Death

Exposure Limits

When researching H2S gas detection solutions, facility managers should be aware of the following H2S exposure limits:

Exposure Limits Agency
20 ppm (ceiling) OSHA PEL (General Industry)
10 ppm TWA OSHA PEL (Construction Industry)
10 ppm TWA OSHA PEL (Maritime)
10 ppm TWA, 15 ppm STEL ACGIH TLV
10 ppm ceiling (10 minutes) NIOSH REL
100 ppm NIOSH IDLH


Technologies for H2S Gas Detection

Electrochemical Sensors

Electrochemical sensors are fuel cell-like devices consisting of an anode, cathode, and electrolyte. The components of the cell are selected so that a subject gas, allowed to diffuse into the cell, will cause a chemical reaction and generate a current. The cells are diffusion-limited, meaning that the rate of the gas entering the cell is solely dependent on the gas concentration. The current generated is proportional to the rate of consumption of the subject gas in the cell.

Learn more about the advantages of using Sierra Monitor's electrochemical gas sensor modules »

Solid State (Semiconductor) Sensors

Solid state (semiconductor) sensors have a resistance that is affected by oxygen adsorbed on the surface of the sensor. Oxygen atoms capture electrons on the semiconductor surface, thereby increasing its resistance. The sensors can be impregnated with dopants such that the sensor's resistance changes when specific gases displace the adsorbed oxygen.

Learn more about the advantages of using Sierra Monitor's solid state gas sensor modules »