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Hydrogen Sulfide

A Dangerous Gas

Hydrogen sulfide, or H2S, is one of the greatest hazards faced by workers on the drill site. Both highly toxic and flammable, H2S  is most often encountered in association with natural gas. Natural gas with high concentrations of H2S is referred to as sour gas.

Hydrogen sulfide is most easily recognizable by its pungent “rotten eggs” smell. However, smell should never be relied on to assess the risk posed by H2S. This is because the olfactory nerve, which is responsible for the sense of smell, is quickly incapacitated by the gas, causing the smell to disappear as concentrations increase.1Iowa State University Extension, May 2004, The Science of Smell Part 1: Odor Perception and physiological response, (accessed May 5, 2014).

Exposure to H2S at low concentrations causes itchy eyes, noses, and throats. At higher concentrations, exposure to H2S can cause rapid death by asphyxiation or neurotoxicity. Because H2S is more dense than air, it flows downhill and settles in low places. One breath of air sufficiently rich in hydrogen sulfide can cause unconsciousness, and a few breaths, death.

Although H2S will always be a hazard on drilling and production sites, by educating ourselves about H2S and following established safety protocols, the risks posed by the gas can be mitigated.

Settings for Encountering H2S during Oil and Gas Operations

Hydrogen sulfide is produced by sulfur-reducing bacteria. Oil and gas form as a result of the anoxic decomposition and chemical alteration of ancient living things; thus, these compounds are very often associated with H2S left over from the original anoxic decomposition of plant matter.

Hydrogen sulfide can be found anywhere natural gas is present. The H2S may come to the surface during drilling and completion when gas escapes from the well unexpectedly (e.g., unscrewing the drill string at the surface), or during regular gas production activities (e.g., pipe corrosion). Specific situations from these settings are discussed further in our course on petroleum engineering and technology.

Health Hazards of Hydrogen Sulfide

The health hazards associated with H2S are wide-ranging and well documented. As the concentration of H2S in the air increases, the severity of the risks increases. Studies in humans suggest that the respiratory tract and nervous system are the most sensitive targets of H2S toxicity.2Agency for Toxic Substances and Disease Registry (U.S. Department of Health & Human Services), 2017, January 12, ToxFAQ for Hydrogen Sulfide, (accessed January 6, 2023).

Hydrogen Sulfide Health Hazards based on Concentration3U.S. Department of Health and Human Services, 2014, Toxicology Profile for Hydrogen Sulfide and Carbonyl Sulfide, (accessed June 29, 2015).

ppm Health Hazard
0.005 ppm The odor threshold, the point at which 50% of a human panel can detect the presence of the compound
10⁠–⁠20 ppm The borderline concentration for eye irritation
50⁠–⁠100 ppm Leads to eye damage
100⁠–⁠150 ppm The olfactory nerve is paralyzed after a few inhalations, and the sense of smell disappears, often together with awareness of danger
320⁠–⁠530 ppm Leads to pulmonary edema with the possibility of death
530⁠–⁠1,000 ppm Causes strong stimulation of the central nervous system and rapid breathing, leading to loss of breathing
800 ppm Is the lethal concentration for 50% of humans for 5 minutes exposure
1,000 ppm Causes immediate collapse with loss of breathing, even after inhalation of a single breath

Regulatory agencies usually require that workplaces meet certain standards, including a maximum concentration at any one point in time and requirements that average concentrations over a period of time are less than a certain value. The Occupational Safety and Health Administration (OSHA) set an acceptable ceiling limit of 20 ppm for H2S in workplace air. The ceiling limit is a 15-minute time-weighted average that cannot be exceeded at any time during the working day. The National Institute for Occupational Safety and Health (NIOSH) recommends a 10-minute ceiling limit of 10 ppm. NIOSH also determined that 100 ppm is immediately dangerous to life or health of workers. People usually can smell H2S at low concentrations in air ranging from 0.0005 to 0.3 ppm.4Agency for Toxic Substances and Disease Registry (U.S. Department of Health & Human Services), 2017, January 12, ToxFAQ for Hydrogen Sulfide, (accessed January 6, 2023).

Workplace Safety and Preventative Measures

Fortunately, there are many preventative measures that can be taken to reduce the hazards posed by H2S. And some of the risks associated with H2S can be reduced through the use of safety technology.

H2S monitors are the first line of defense. There are numerous gas meters on the market that are carried with an individual while on site that measure the amount of a given gas in an area. Gas metering devices do require regular maintenance and calibration to be effective.

Windsocks are often installed on a site so that workers can assess wind speed and direction. If the air is still, gas is more likely to build up to dangerous concentrations. In addition, in the event of a release of H2S, a wind sock allows workers to quickly determine the upwind direction and move to a safe upwind location.

Blowers can be installed in low lying areas where gas is likely to be present. The blowers serve to disperse any gases that are released and can help lower concentrations. However, the presence of blowers does not guarantee the absence of high concentrations of H2S.

Most jurisdictions require the installation of H2S sensors on rigs where H2S is likely to be encountered. These sensors are usually connected to an alarm system, which can alert rig workers to the presence of gas and allow them to take appropriate safety measures. We’ll talk about the kinds of sensors available in the next section.

Self Contained Breathing Apparatuses (SCBAs) allow workers to enter areas of high H2S concentrations in order to assist victims or prevent further leakage. In addition, “escape” SCBAs provide enough oxygen to allow workers to escape from a site. These escape units are not meant for long term use. Enough escape SCBAs for each worker should be present in a given work area.

Training Measures

Of course, the best technology in the world will never make a drill site completely safe. That’s why it’s critical that workers are well-versed in the risks of H2S and ways of mitigating these risks. Workers should be periodically refreshed on safety rules on sites where H2S may occur.

Important concepts that should be covered in training include the following:

      • Enclosed Spaces: Most H2S-related accidents occur in enclosed spaces, where gases are able to build up. There are many regulations in place related to enclosed spaces. Often, a certification is required to enter them. Workers should test enclosed areas for the presence of H2S before entering them.

      • Self Contained Breathing Apparatus Use: Training in the use of SCBA equipment is essential, because SCBAs are often the only lifeline available to a worker in an emergency situation.

      • Working with a partner: When working in an area where H2S might be present, workers should work in pairs. This decreases the possibility of H2S poisoning because one worker can work to alert others and move the disabled worker to safety.

    Warning signs can serve to remind workers of the dangers specific to certain areas of the drill site. In most jurisdictions, their presence is required by law.

    Detecting Hydrogen Sulfide

    There are several types of H2S sensors. These sensors can be wired to alarm systems, which sound if a dangerous level of H2S is detected.

    Usually, the tiny sensors are contained in small casings with metal contacts. These contacts allow the sensor to communicate with a device that allows humans to read out the H2S measurements. The readout device allows sensors to be replaced as they reach the end of their useful lives, and sometimes allows for sensor types to be changed depending on conditions.

    Each type of sensor has distinct advantages and disadvantages, and it is important to take care when selecting which combination of sensors will work best at a particular site.

    Fixed sensors are mounted in locations deemed to be at high risk for H2S buildup. They can collect data and automatically report the data to a central, safe location. In contrast, portable sensors require the deployment of rig workers to measure gas concentrations. This capability can prove useful if gas builds up in an unexpected place, but it is not optimal because it poses a risk to workers.

    Here’s a brief overview of the sensor types:

    Electro-chemical Sensors

    Electrochemical sensors work by measuring tiny electrical currents generated when gases interact with metal electrodes. In most sensors, a membrane allows a small amount of H2S to enter the housing and interact with an electrode. Because the amount of gas that enters through the membrane is proportional to the amount of gas in the air outside, the total current generated by the electrodes will be proportional to the gas concentration outside the sensor.

        • Electro-chemical sensors work best in moderate conditions.

        • Their accuracy decreases markedly in excessively cold or dry environments.

        • Electro-chemical sensors require little maintenance, so they are a good choice for a low-stress operating environment.

      Metal Oxide Semiconductor Sensors

      Metal oxide semiconductor sensors measure the amount of gas by testing the electrical resistivity of a thin metal oxide strip. The resistivity of the strip is affected by the amount of absorbed H2S, which is related to the concentration of H2S in the air. By measuring the resistivity of the strip, concentrations of H2S in the air can be determined. 

          • Unlike electro-chemical sensors, metal oxide semiconductor sensors perform well in dry and hot conditions.

          • They do not do as well with changes in humidity.

        Optical Sensors

        Optical sensors use changes in light intensity to determine the concentration of gas. In a point type optical sensor, air is passed between a fixed light source and detector. Based on changes to the amount of light received at certain frequencies, the sensor can determine the concentration of H2S.

        In contrast to a small optical sensor, a line of sight sensor consists of a light source and receiver that can be moved relative to each other. The presence of gas is measured along a line of sight that can be up to 100 meters (330 feet) in length. However, the concentration of gas cannot be measured with this type of sensor – it can only detect the total amount of gas present. A two-meter-wide cloud with a concentration of 100 ppm would give the same reading as a 20 meter wide cloud with a concentration of 10 ppm (1/10th the concentration for a cloud 10 times wider).

            • Optical sensors are the most expensive option.

            • They perform well in a much wider range of conditions than electro-chemical and metal oxide semiconductor sensors.

            • An additional advantage is that optical sensors are fail-safe – if any component stops functioning properly, the sensor will register a high H2S concentration. This ensures that false positive alarms are more common than false negatives, which are much more dangerous.

          It is critical that sensors be calibrated frequently according to the guidelines for each specific sensor. An uncalibrated sensor may give frequent false alarms, or worse, fail to detect a dangerous concentration of gas. Most sensors need to be calibrated weekly or daily.

          What to do when gas is detected

          If gas is detected by a fixed location sensor, all personnel should leave the area of the sensor or put on SCBAs. Since gas disperses as it travels from a source, a low but elevated value detected at a fixed sensor does not imply that concentrations in the entire area are low. Lethal concentrations of H2S may exist near the source of the gas.

          Before work can resume, workers need to locate and contain the source of the gas. Detection can be accomplished with portable H2S sensors operated by SCBA-protected staff. Only when gas concentrations in the entire work area have fallen to safe levels should crews go back to work.

          If you smell H2S and suspect a leak or release but don’t have access to a sensor, hold your breath and immediately exit the area or don a SCBA.

          If someone collapses unexpectedly, especially in a confined space, H2S is likely to blame. Do not enter a confined space to help a victim without first testing H2S concentrations and putting on a SCBA if needed. Many accidents have been made worse by unprepared or uneducated rescuers who became victims themselves.


          This section has provided a basic overview of the hazards of H2S and the methods used to manage these hazards. The specifics of H2S risk management vary from company to company and across legal jurisdictions, and it’s important to learn the procedures associated with operators you encounter.

          Images: “Wind Sock” by HHakim via iStock