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

Electrochemical Sensors

Electrochemical sensors, used to detect H2S gas, 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.

Advantages to using an electrochemical sensor include allowing sensors to be specific to a particular gas in the parts-per-million range, low power requirements, high accuracy, and lower cost than other gas detection technologies. However, electrochemical sensors are sensitive to temperature, subject to interference from other gases, and have a shorter lifespan the greater the exposure is to the targeted gas.

Sierra Monitor electrochemical sensors provide improved reliability by allowing the gas to diffuse into the sensor through a capillary port, rather than diffusing through membranes. The result is an extremely stable sensor with very low temperature and pressure coefficients, and the capability to monitor gas as a percent by volume (oxygen) and ppm (toxics).

Solid State (Semiconductor) Sensors

Solid state (semiconductor) sensors, also used to detect H2S gas, 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.

As solid state sensors are among the most versatile of all sensors, they are simple and robust and can be used in many different applications. Solid state sensors have the ability to detect both low ppm levels of gases, as well as high combustible levels. However, they are not very specific to a particular gas as often required for industrial applications and could potentially provide false alarms. Solid state sensors may be affected by fugitive volatile organic compounds (VOCs), and must be taken care of to locate them in areas without these gases present.