Hydrogen sulfide or H2S is a colorless, flammable, extremely hazardous gas with a "rotten egg" smell. Exposure to H2S can result in varying responses depending on the length of the exposure and the concentration - effects can range from eye, nose and throat irritation to death via asphyxiation. Developing a thorough, well planned out Job Safety Analysis (JSA) or Activity Hazard Analysis (AHA) is a key strategy for ensuring the on-the-job health and safety of employees who may be exposed to H2S.
As a chemical asphyxiant, H2S affects both oxygen utilization and the central nervous system. In addition, although effects may have been tolerated previously at low levels, repeated exposure can result in dangerous health effects. To develop a job safety analysis that will help minimize potential health threats, the HSE Manager should 1) identify where H2S hazards may be found on the job site, 2) identify the characteristics of H2S and the affiliated health risks, 3) determine the possible means of exposure while working around known or possible sources of H2S, and 4) develop a plan to control and mitigate the risks posed by H2S.
First, where does H2S occur, and where could you possibly be exposed to it? Hydrogen sulfide in the air typically comes from natural sources. It is produced when bacteria break down plant and animal material. H2S occurs naturally in crude petroleum, natural gas, hot springs, volcanoes, and underwater thermal vents.
(Image source: Kathy created from a video clip of the lava flows in Hawaii (in 2018)
Industrial sources of hydrogen sulfide include petroleum and natural gas extraction and refining, sewer and wastewater treatment, agricultural silos and pits, pulp and paper manufacturing, food processing, rayon textile production, hot asphalt paving, mining, coke ovens, tanneries, chemical manufacturing, and waste disposal. Source
Here's a closer look at some common sources:
- Organic Decomposition. H2S is present in swamp and sewer environments, which is why these areas emit the odors that we often smell.
- Geomorphology and Chemical Reactions. H2S is present in areas of active natural geomorphology such as volcanoes, hot springs and underwater sub-sea vents.
- Man-made environments and industries. It is often found in the farming, coal mining, pulp and paper, petroleum, tanning and smelting industries. The largest industrial cause of H2S is through petroleum refineries as the hydrodesulfurization process separates sulfur from petroleum. Small amounts of H2S occur in crude petroleum and natural gas can contain up to 90% H2S. Source
In general, working in the following areas and conditions increases a worker's risk of overexposure to hydrogen sulfide:
- Confined spaces (for example pits, manholes, tunnels, wells) where H2S can build up to dangerous levels.
- Windless or low-lying areas that increase the potential for pockets of H2S to form.
- Marshy landscapes where bacteria break down organic matter to form H2S.
- Hot weather that speeds up rotting of manure and other organic materials and increases the H2S vapor pressure.
Many workers are at risk for exposure to hydrogen sulfide, and those who work in confined spaces are particularly at risk. Some examples of the types of jobs that may expose workers to unsafe levels of H2S include:
Sanitation workers when cleaning or maintaining municipal sewers and
- Farm workers when cleaning manure storage tanks or working in manure pits.
- Workers in oil and natural gas drilling and refining including workers performing tank gauging (tanks at production, pipeline and refining operations) or routine field maintenance involving hydrocarbons, tank batteries and/or wells.
H2S may be generated at construction sites, specifically within piles of construction and demolition debris. For example, a major component of gypsum wallboard is calcium sulfate which can be degraded by bacteria and generate H2S. If construction and demolition debris contain large quantities of wallboard, large amounts of H2S may be formed. Production is greatest when the wallboard is finely crushed and when there is little oxygen, such as when the debris is buried and soaked with water.
Because H2S is heavier than air, it may travel along the ground, and collect in low-lying and enclosed, poorly-ventilated areas such as basements, manholes, sewer lines, under-ground telephone vaults and manure pits. Hydrogen sulfide can also exist as a liquid compressed gas.
- In its pure state is colorless.
- It is heavier than air.
- It has the odor of rotten eggs at low concentrations; In higher concentrations, it can deaden the sense of smell (olfactory nerve).
- H2S is soluble in water, is flammable and poses a definite threat of explosion.
- It is the second most toxic gas known to man, ranking behind hydrogen cyanide and ahead of carbon monoxide.
- Toxic By-Products: H2S can create sulfur dioxide which can ignite without warning.
Most of the information on human health effects from hydrogen sulfide exposure comes from accidental and industrial exposures to high levels. Exposure to high levels can cause muscle cramps, low blood pressure, slow respiration, and loss of consciousness. Short-term exposure to moderate amounts of hydrogen sulfide in the workplace produces eye, nose and throat irritation, nausea, dizziness, breathing difficulties, headaches and loss of appetite and sleep. Continued exposure can irritate the respiratory passages and can lead to a buildup of fluid in the lungs. H2S affects nerve centers of the brain which control breathing.
The Permissible Exposure Level of H2S is 10 ppm for an 8-hour, time weighted average. A level of H2S gas at or above 100 ppm is Immediately Dangerous to Life and Health (IDLH).
People working in industries where H2S exposure is common, but is usually below the OSHA 8-hour standard (10 ppm), may have decreased lung function and increased risk of spontaneous abortion and impaired neurological functions (including reaction time, balance, color discrimination, short-term memory and mood) compared to unexposed workers. People living near industries that emit hydrogen sulfide have an increased risk of eye irritation, cough, headache, nasal blockage and impaired neurological function (same measures as above) compared to unexposed residents.
Exposure Routes. The primary route of exposure is inhalation and the gas is rapidly absorbed by the lungs. Absorption through the skin is minimal. People can smell the "rotten egg" odor of hydrogen sulfide at low concentrations in air. However, with continuous low-level exposure, or at high concentrations, a person loses his/her ability to smell the gas even though it is still present (olfactory fatigue). This can happen very rapidly and at high concentrations, the ability to smell the gas can be lost instantaneously. Therefore, DO NOT rely on your sense of smell to indicate the continuing presence of hydrogen sulfide or to warn of hazardous concentrations.
Protection against H2S exposure. The best way to prevent possible exposure and serious injury or possibly death from H2S is to fully understand the risks that may be present at the work site and during the work task. Conducting a thorough job safety analysis, or activity hazard analysis will ensure that the people in charge of Health & Safety fully evaluate the risks.
Before entering areas where H2S may be present:
The air must be tested for the presence and concentration of hydrogen sulfide by a qualified person using air monitoring equipment, such as hydrogen sulfide detector tubes or a multi-gas meter that detects the gas.
Testing should also determine if fire/ explosion precautions are necessary.
- If the gas is present, the space/area must be ventilated continually to remove the gas.
If the gas cannot be removed, the person entering the space/area
must use appropriate respiratory protection and any other
necessary personal protective equipment, rescue and communication
equipment. If air concentrations are elevated, eye irritation may become
a serious issue. If a half-mask respirator is used, tight fitting goggles
must also be used.
Entry into IDLH atmospheres can only be made using: 1) a full facepiece pressure demand self-contained breathing apparatus (SCBA) with a minimum service life of thirty minutes, or 2) a combination full facepiece pressure demand supplied-air respirator with an auxiliary self-contained air supply, with a 5-minute escape pack.
- At least one person (per two workers), equipped with a SCBA, should act as a stand-by person; this person should not participate in the work being performed until the atmosphere has been tested and found to have no H2S present in quantities over 10 ppm. The stand-by person should be stationed up wind, within 100 feet and in clear view of the workers.
- Workers in areas containing hydrogen sulfide must be monitored for signs of overexposure. NEVER attempt a rescue in an area that may contain hydrogen sulfide without using appropriate respiratory protection and with-out being trained to perform such a rescue.
- Employees should be trained in the dangers associated with working in around H2S, have completed a recent respiratory fit test and be cleared by a medical authority to wear respiratory protection.
- Employees also need to be trained in the proper use and calibration of H2S monitors and gas detection instruments.
- Instrumentation must be appropriate for the application then checked and calibrated for proper operation. Employers must ensure that fixed or portable monitors will alarm at 10 ppm and personal or area monitors will alarm when at 10 ppm.
Any time employees are asked to perform a task that may put them in harm's way, development of a thorough Job Safety Analysis is just the first step toward identifying the associated risks, then coming up with a strategy to either eliminate or reduce the risks associated with the task. Done properly, a Hazard Analysis will require that the H&S manager and/or affected employee identify each step that will be required, the possible hazards that may be encountered, identify controls that will be put into place to eliminate or reduce the hazards, and finally the tools and PPE necessary to perform the work.
Using JSABuilder is a great way to develop your H2S Job Safety Analysis. JSABuilder lists dozens of the types of hazards that may be encountered at a job site, and also includes a listing of over 300 chemicals and their associated health risks, including hydrogen sulfide. JSABuilder leads the user through the formation of the JSA or AHA that will help keep employees safe.