What is H2S?

Hydrogen sulfide, notated as H₂S, is a toxic gas that can be recognized by its rotten egg smell. H₂S is a byproduct of microbial decomposition of organic materials, this process is often referred to as anaerobic digestion due to the lack of oxygen during decomposition.

The first thing to know is the lower explosive limit and the upper explosive limit (LEL and UEL) of H₂S gas. The ratio of H₂S saturation in the air between those limits is known as the flammable range, for hydrogen sulfide that range is 4%-44%. Serious injuries and even death can result from these types of toxic and flammable gas explosions in areas like confined spaces since the gas has no way of dispersing itself. It often collects in areas that are not well ventilated and with the gas being heavier than air, it pools in areas such as basements and sewers.

Where is H₂S Found and What is it Used for?

Hydrogen sulfide can be found in places such as sewers and swamps. Because of that, the hazardous chemical acquired the name “sewer gas.” However, hydrogen sulfide can also be found in natural gas, volcanic gases, water sources, and even the human body (in very minute amounts) as a signaling molecule. 

Aside from natural occurrences of H₂S, the hazardous gas is often found in the following industrial environments:

• The Petroleum Industry – Hydrogen sulfide forms when sulfur is removed from petroleum products. It's also present during the drilling process as sour gas.
• Mining Operations – Coal mining is a culprit of producing hydrogen sulfide contaminated groundwater. This is primarily due to sulfate-reducing bacteria.
• Wastewater Treatment Plants – Hydrogen sulfide gas is especially common within wastewater treatment plants. The organic material, if turned septic, naturally produces a biological reduction of sulfates through decomposition.
• Agricultural Sites – High levels of hydrogen sulfide can be present at agricultural sites via manure decomposition in large vats or what is called manure lagoons.
• Paper Processing Plants – Something as simple as the paper is quite energy intensive and dangerous to make. Through the process of creating pulp, the slurry has to be treated with several chemicals to create a white paper. Unfortunately, one of the byproducts of this process is hydrogen sulfide gas.
• Rayon Textile Manufacturing – Rayon is a synthetic fiber created from regenerated cellulose that can mimic wool, cotton, and other natural textiles. During the acidification process of this textile, hydrogen sulfide is created.

At the frequency of hydrogen sulfide's presence in certain occupations, employees need to be aware of the risks that the toxic gas poses to their health. Occupational safety measures for this chemical must include PPE and preventative maintenance, as well as minimization of exposure through reasonable means.

The Necessary Precautions and PPE to Prevent H₂S Exposure

Never rely on your sense of smell! To determine the risk of being exposed to hydrogen sulfide, workers must use special instruments to measure the saturation levels within the air. This precaution is taken because the gas can deaden your sense of smell after mere seconds if the concentration of H₂S is high enough. Going into areas where H₂S is present incredibly dangerous since the LEL is so low at 4%.

On top of being flammable, hydrogen sulfide is also an irritant and chemical asphyxiant that causes problems with the body's nervous system and oxygen utilization. At low exposures, the gas causes the eyes to burn and irritates the throat and lungs. At higher levels of exposure, the victim may experience convulsions, shock, the inability to breathe, rapid unconsciousness, coma, and even death. These symptoms can occur in just a few breaths saturated with the hazardous gas. 

To gather more of an understanding of when these symptoms start to appear, OSHA has defined the level of ppm exposure that correlates with health decline. The information is as follows: 

• 0.00011-0.00033ppm is the typical background concentration of this gas. Therefore, it does not pose a significant risk to those in the area.
• 0.01-1.5ppm is the level at which a rotten egg smell be noticed by those with a sensitive nose.
• 2-5ppm The rotten egg smell is much more noticeable at this level. Not only that, but symptoms of nausea, watering eyes, headaches, and loss of sleep become apparent. Bronchial constriction has been noted in victims with asthma.
• 20ppm brings on fatigue, appetite loss, headaches, irritability, memory problems, and dizziness.
• 50-100ppm after one hour of exposure this level of saturation often results in what's called a gas eye (conjunctivitis), digestive issues, and appetite loss.
• 100ppm is the threshold of saturation that causes coughing, eye irritation, olfactory fatigue after 2-15 minutes, altered breathing, drowsiness after 15-30 minutes, throat irritation after an hour, and an over gradual increase in severity after several hours of exposure. In serious cases, death may even occur after 48 hours.
• 100-150ppm often results in the victim's loss of smell.
• 200-300ppm exposure causes pulmonary edema, a condition resulting in excess fluid residing in the lungs. This can occur as quickly as an hour with this level of hydrogen sulfide exposure.
• 500-700ppm reaches a critical point in which the exposed victim will collapse around 5 minutes, serious eye damage will occur after 30 minutes, and death follows closely after 30-60 minutes.
• 700-1000ppm exposure usually causes an immediate collapse after 1-2 breaths, the victim's breathing will stop, and their death will follow a few short minutes later.
• 1000-2000ppm causes death almost instantly. 

Now, how exactly are employees supposed to prevent this kind of exposure? Remember the Hierarchy of Hazard Controls! Elimination, substitution, engineering controls, administrative controls, and lastly PPE, in that order. First attempt to remove the hydrogen sulfide gas entirely or swap the hazardous chemical with something less dangerous. Change the way the process works as well as how people work next. And only after all of those options have been gone over should the employer need to provide them with personal protective equipment if exposure risk is still significant.

OSHA also provides other protection measure requirements against hydrogen sulfide exposure. The organization suggests:

1. Testing the air with a multi-gas meter or hydrogen sulfide detector tubes before a person enters an environment. This must be done by a qualified person and must also take into account the possibility of fire or an explosion.
2. Ventilate the area if it has been proven that hydrogen sulfide gas is present.
3. If the gas cannot be removed entirely, then the employer must give the employees performing a task in that environment PPE.

Personal protective equipment used specifically for hydrogen sulfide gas include:

• Chemical safety goggles or a face shield
• Gloves
• Aprons
• Boots
• A full body hazmat suit including an SCBA may be necessary for some environments
• Full-face piece air purifying respirator, powered or not, with cartridges that specifically protect against H₂S

NIOSH has several respirator recommendations that go into detail regarding the assigned protection factor, or APF. The APF highly depends on the type of respirator used as well as the environment those respirators will be used in.

First Aid Procedures if Exposed

Because hydrogen sulfide is such a dangerous chemical; first aid must be administered quickly to prevent severe and unrepairable damage as well as death. According to the CDC, three different areas of the body will need medical attention depending on exposure level and the lengthh of time the person was exposed. Those are the eyes, the skin, and the respiratory system.

Hydrogen sulfite gas poses a unique risk in that it is oftentimes in the form of compressed gas, becoming liquid. This is particularly dangerous because liquid hydrogen sulfide can cause severe frostbite. If this happens to the eyes the victim must be taken for immediate medical attention in the event the eye tissue has been completely frozen. If it is not, then it's recommended that the eyes be flushed with water for at least 15 minutes. NIOSH notes that if there is any lingering pain, swelling, lacrimation, or light sensitivity, then the victim seek further medical attention. 

Logically, if frostbite can happen to the eyes, then the skin is also at risk in the presence of compressed hydrogen sulfide gas. If frostbite has occurred after skin exposure make sure to not flush the area with water, rub the area at all, or remove clothing from the frostbitten area, this will damage the skin tissue further. In this case, the victim must seek medical attention swiftly. However, if frostbite has not occurred, the victim must rinse the area with soap and water thoroughly. 

Aside from the risk of being exposed to the liquid chemical, by far the most common exposure route is through inhalation. If a person is exposed to a high concentration of hydrogen sulfide take them outside to get fresh air immediately. Keep that person at rest and warm, if they stop breathing perform artificial respiration. While this is going on, seek medical attention for them immediately. Once the medical team arrives, they may administer an oxygen mask or aerosolized bronchodilators if the situation is serious.

Exposure to hydrogen sulfide gas can be life threatening. Use the correct precautions to ultimately avoid stressful situations like the ones mentioned above. 

Labeling H₂S Containers

Labeling hydrogen sulfide gas containers is an important part of occupational safety efforts. Health administrations such as OSHA have standards and regulations that need to be followed to maximize the level of safety that workers need to be able to avoid injuries. Two of those that immediately come to mind include the Globally Harmonized System of Classification and Labeling of Chemicals, or GHS, and OSHA's Hazard Communication standard.   

Utilizing GHS essentially gives employees and visitors an easy-to-understand crash course on the chemical that has been labeled. The label will always include hazard pictograms, a hazard statement, the precautionary statement, and a signal word. All of these provide the reader with crucial information that actively helps protect others as well as themselves.

The Hazard Communication Standard is particularly important because it has been merged with GHS.  Both work together to provide more clear guidelines on how chemical labeling and hazardous substances, such as hydrogen sulfide, should be handled.