What is Hydrogen Sulfide or H2S? – [Updated for 2023]

Poisonous H2S Sign

Hydrogen sulfide, also known as H2S gas, sewer gas, stink damp, and other names, is an important gas to be familiar with when considering chemical safety at home and in the workplace.

Below you will learn more about how individuals can be exposed to hydrogen sulfide gas, differentiating between different concentration levels, and what to do when exposed. Information and proper training will help you and your employees stay safe around this substance.  

Our H2S Training Video can help in this regard.  For now, let’s examine hydrogen sulfide, its uses, and its dangers below.

Why Understanding Hydrogen Sulfide Gas is Important

Hydrogen sulfide is a colorless gas known for its pungent smell that resembles rotten eggs at lower concentrations. The characteristic odor is its most well-known feature, but this colorless gas is also highly flammable and toxic, making it extremely dangerous to humans.

This is why the Occupational Safety and Health Administration (OSHA) has implemented mandatory guidelines for employers and employees to follow. These guidelines help employers and their employees stay safe on the job. They also teach individuals with a low risk of exposure to hydrogen sulfide gas where they can come in contact with it. This can include rare occasions such as when they come into contact with volcanic gases or when visiting hot springs.

Environmental health standards also ensure people are aware of the dangers hydrogen sulfide presents to humans. Environmental health standards regulate job processes, including crude petroleum,  kraft paper mills, and even oil refining. Chemical safety rules come into play to instruct individuals on handling hydrogen sulfide and chemicals that may be used when putting out a gas source or helping in an emergency.

There are a variety of ways that individuals can be exposed to hydrogen sulfide gas and some of its alternate forms can be just as dangerous.  If you want to share an H2S Toolbox Talk with your crew, we have one of those as well.

What Does Hydrogen Sulfide Do to Humans?Gas Detector

Some of the most common sources of H2S emissions include oil and natural gas extraction and processing and natural emissions from geothermal fields.

The gas can also be created during the bacterial breakdown of human and animal wastes. These emissions are created from sewage treatment facilities and landfills, literally sewer gas.

So what does being exposed to hydrogen sulfide gas do? This public health and environmental health question has been strongly regulated by governing bodies to keep people who come into contact with hydrogen sulfide safe.

While being exposed to hydrogen sulfide gas at low levels may not be deadly, it can still cause issues that will affect your health. High concentrations of this sewer gas can result in chronic issues or even death.

This fact makes it vital that you know if you’re in the vicinity of hydrogen sulfide, how much and for how long, and what to do if you come in contact with it.

Some of the effects of being exposed to hydrogen sulfide gas include:

  • Eye irritation
  • Respiratory paralysis
  • Apnea
  • Coma
  • Convulsions
  • Dizziness
  • Headaches
  • Weakness
  • Irritability
  • Insomnia
  • Sore throat
  • Cough
  • Upset stomach
  • In liquid form, frostbite

Hydrogen sulfide affects the mucous membranes of the body. Mucous membranes can be found lining the respiratory tract and other body parts vital for basic functioning. Workers can be harmed if exposed to any concentration of hydrogen sulfide, but the symptoms and effects worsen with higher concentrations or increased exposure time. This is why sharing h2s safety tips with your workers is so important.

Effects of Exposure to Hydrogen Sulfide Gas

There are a variety of health effects that people have experienced after their exposure to hydrogen sulfide. Even the lowest concentrations, such as 2PPM for about 20 minutes, have had health effects, specifically for asthmatics.

Low concentrations of 20-50 ppm typically can cause health effects, including eye irritation. Higher concentrations can cause upper respiratory irritation or sore throat.

Even extended exposure to low levels can have harmful health effects. This level of exposure can irritate the eyes and painful skin rashes. However, repeated exposure over an extended period can cause convulsions, brain and heart damage, coma, and even death.

While smelling hydrogen sulfide gas doesn’t mean it will harm you, it can cause anxiety or worry. However, repeated exposure to the odor can lead to headaches, nausea, and fatigue. Although these are not technically considered direct health effects, they can cause discomfort to those who experience them.

When exposed to hydrogen sulfide gas, serious health effects are not likely to be met until air levels reach a minimum of 2 PPM for at least 30 minutes, the same rate that can cause individuals with asthma to experience symptoms.

Additional health effects have been reported with exposures greater than 50 PPM, known to cause eye irritation, which is higher than the average odor threshold.

Exposure to slightly higher hydrogen sulfide gas levels, such as over 300 PPM, can cause serious health effects. However, this level of exposure beyond the odor threshold is only common for those who encounter occupational or industrial accidents.

50-100 PPM can cause eye damage, whereas 100-150 PPM can paralyze the olfactory nerve with just a few inhalations of hydrogen sulfide gas.

0.00047 PPM or 0.47 PPB is considered the odor threshold, which is the point where 50% of individuals can detect the presence of an odor without being able to identify it.


Measuring for H2S

There are permissible levels of sewer gas that employees can encounter as well as existing concentrations that can cause severe health issues or death. Some effects that individuals may experience can be as insignificant as a sore throat or they can be fatal after just a few breaths of hydrogen sulfide have been taken.

  • 0.00011-0.00033: Acceptable hydrogen sulfide levels.
  • 0.01-1.5: This is where the odor threshold is, meaning you can detect the characteristic odor of rotten eggs. This odor becomes stronger between 3-5 PPM. Anything above 30 PPM, the odor is described to be incredibly sweet.
  • 2-5: This is the level where exposure to hydrogen sulfide begins to cause nausea, headaches, and the inability to sleep. Airways problems can occur in people who have asthma.
  • 20– Health effects, including exhaustion, lack of desire to eat, headache, agitation, memory loss, and dizziness.
  • 50-100: Individuals can experience slight conjunctivitis and respiratory irritation after about an hour. This level of exposure can also cause digestive issues and no desire to eat.
  • 100: Many individuals experience coughing, eye irritation, loss of smell within a few minutes, breathing problems, sleepiness after up to about half an hour or less, an irritated throat after an hour, and a slow increase in the severity of symptoms within several hours post-exposure. Death can occur after two days of the last exposure.
  • 100-150: Loss of smell.
  • 200-300: Conjunctivitis and respiratory tract irritation after one hour. Pulmonary edema can occur in cases of extended exposure.
  • 500-700: Individuals may stagger or collapse within five minutes. Serious damage to the eyes can occur after 30 minutes. Death can occur after about half an hour to an hour.
  • 700-1000: Individuals can experience rapid unconsciousness or immediate collapse after only a few breaths. Breathing stops and death occurs within a few minutes.
  • 1000-2000: Almost instant death.

Adverse Health Effects

While hydrogen sulfide gas doesn’t accumulate in the body, repeated and/or repeated exposure to moderate levels can lead to symptoms including headaches, loss of appetite, weight loss, and low blood pressure.

Prolonged exposure is when individuals begin to experience more adverse health effects. This can include painful skin rashes and eye irritation.

Repeated exposure to high hydrogen sulfide gas concentrations can cause convulsions, coma, brain and heart damage, and even death. The Centers for Disease Control strongly advises individuals to adhere to all guidelines and to go to the hospital if they believe they have been exposed to hydrogen sulfide gas before they begin to experience adverse health effects.

If someone inhales too much hydrogen sulfide or inhales it for too long, they can become unconscious. Long-term health effects such as these can result after waking up:

  • Headaches
  • Poor attention span
  • Poor memory
  • Decreased motor function
  • There have also been reports of problems within the cardiovascular system after exceeding exposure limits

Not everyone who is exposed to hydrogen sulfide gas works in petroleum refineries or kraft paper mills. Some individuals may work in other, more common industries or may have visited hot springs where organic matter is known to decay and create this stinky and dangerous gas.

Poisonous H2S Sign

Long-Term Health Effects

There have been reports that individuals who breathed in high enough levels of hydrogen sulfide gas to become unconscious continue to experience symptoms including headache, poor attention span, lack of memory, and decreased motor skills after waking up.

There have also been reports of cardiovascular problems in people who were exposed to a concentration exceeding the exposure limit.

It’s important to note that individuals with asthma or other chronic health issues that are characterized by difficult breathing can experience more severe symptoms when they are exposed to lower concentrations of the gas than those who do not have breathing issues.

Who is at Risk?

Any exposure to hydrogen sulfide can be potentially dangerous or even deadly, causing an individual to experience adverse health effects. Although hydrogen sulfide occurs naturally in the body and in nature as a result of waste and decay, it is extremely toxic and dangerous to breathe.

Many workers in the industries we discussed are at risk of being exposed to hydrogen sulfide gas while working. Those most at risk are:

  • Sanitation workers cleaning or maintaining sewers and septic tanks
  • Farm workers cleaning manure storage tanks or working in manure pits
  • Workers in oil refineries and natural gas extraction and refining when working in gas deposits
  • People working around coke ovens
  • Petroleum refineries’ employees
  • Individuals who have chronic conditions that affect their respiratory system are at even greater risk than those who do not have a chronic condition such as asthma.

How You Can Be Exposed to Hydrogen Sulfide Gas

You can be exposed to hydrogen sulfide gas in a few ways, but the most common way is through breathing it in. However, you can also be exposed through contact with the eyes and skin. Exposure to the gas can happen at work or in public places such as hot springs, marshes, or other places where organic matter can decay.

In some places, entire communities can be exposed to hydrogen sulfide generated by the breakdown of organic compounds. For example, coastal communities in Western Australia have been affected by the breakdown of seaweed that has amassed on the shore. Another natural way that individuals not in specific industries can be exposed is through certain chemical processes. The hydrogen sulfide generated can be converted to sulfur.

Some of these places can also include farms or areas close to landfills or open sewers where human and animal wastes are in close proximity to where people live. Exposure to a toxic waste dump can also cause adverse health effects on people. Exposure to hydrogen sulfide this way puts the general population at risk because it stems from natural sources that can not be as easily avoided.

H2S Chemical Compound

What Areas and Conditions Increase the Risk of Exposure to Hydrogen Sulfide?

Confined spaces that include pits, tunnels, manholes, and wells are some of the most dangerous places to be when in contact with hydrogen sulfide. Hydrogen sulfide can build up to dangerous concentrations in tight, ill-ventilated spaces. Not only does the lack of ventilation create higher concentrations of hydrogen sulfide gas, but it can also make it impossible to get a breath of fresh air.

This lack of oxygen both facilities an increased concentration of hydrogen sulfide and decreases your chances of breathing in any regular air if you are in a tight space with hydrogen sulfide without the PPE that could otherwise keep you safe, depending on the concentration level.

Places to avoid where hydrogen sulfide can include:

  • Low-lying areas with no airflow. These spaces increase the potential for pockets of hydrogen sulfide to form.
  • Marshy or swampy landscapes. An increased amount of bacterial breakdown occurs naturally to form hydrogen sulfide.
  • Hot weather. This speeds up the breakdown process for manure and other organic compounds, increasing the vapor pressure of hydrogen sulfide.
  • Hydrogen sulfide occurs naturally in some sulfur springs and is associated with the decomposition of organic matter.

The Agency for Toxic substances and Disease Registry (ATSDR) is one of the governing bodies that regulate how employers and employees must handle working in these types of environments to protect the safety of all workers, including those with increased risk.

What is Hydrogen Sulfide Used For?

Hydrogen sulfide is used for a variety of industries, including and is a by-product of several industrial processes, including:

  • Oil refining
  • Wood pulp and paper processing
  • Petroleum refineries
  • Rayon manufacturing
  • Kraft paper mills
  • Sewer and wastewater treatment
  • Hot asphalt paving
  • Food processing
  • Textile manufacturing
  • Agricultural silos and pits
  • Coke ovens
  • Industrial Chemistry

Although this isn’t a use, hydrogen sulfide gas is commonly found in animal wastes. Animal wastes such as manure are frequently used on farms, which can potentially create concentrations of hydrogen sulfide gas which can become dangerous if not properly handled.

The strong odor of animal wastes can help individuals identify if there is a problem before the gas concentration builds up to a concerning level.

Natural Gas Hazards

Understanding and identifying the hazards of this sewer gas or stink damp helps employees identify chemical hazards. While the smell of rotten eggs can easily deter someone from being around the source of the gas, not being able to smell it after a certain period or beyond a certain concentration makes it difficult to know when it’s there.

Hydrogen sulfide gas is noticeable by its strong odor before reaching the odor threshold. However, it’s a problem when you can no longer smell it and, therefore, may not be aware of its presence because it has a toxicity level similar to that of carbon monoxide. The health effects that result because of this toxicity prevent cellular respiration.

Being aware of any industrial emissions, industrial materials, decay, lack of ventilation, and other hazards in the area can cause hydrogen sulfide gas emissions. It can make a difference between life and death for you and the people around you.

H2S Danger Sign

Other Ways Hydrogen Sulfide Can Harm You

Since hydrogen sulfide is a highly explosive and flammable gas, so it can cause life-threatening situations if it is not handled properly. In addition to it being flammable, when hydrogen sulfide gas burns, it creates toxic vapors and gases, such as sulfur dioxide.

An individual can be exposed to hydrogen sulfide in the air, and someone can come in contact with it in the form of a liquid. In this form, hydrogen sulfide can cause frostbite or “blue skin.” If clothes become wet after contact, make sure to avoid anything that can cause ignition. The clothes should be removed immediately and left in a safe and isolated place for the hydrogen sulfide to evaporate.

Hydrogen sulfide gas is commonly found when drilling or producing crude oil and natural gas. This toxic gas is the result of microbial breakdown of organic material when there is a lack of oxygen.

The lack of oxygen that causes the rise of H2S gas causes the liberation of the sulfur dioxide-producing amino acids and bacteria, which also causes the odor of rotten eggs, similar to the pungent odor of flatulence.

Organic material decays in these spaces that are deprived of oxygen, which is often common in swamps and marshes. Crude petroleum, natural gas, volcanic gases, liquid manure, sewage, landfills, sulfur springs, and human and animal wastes can also release it. Although you may not come into contact with many of these items or locations, it’s important to know where hydrogen sulfide gas can be found, just in case.

How is Hydrogen Sulfide Harmful As a Colorless Gas?

Since hydrogen sulfide gas is colorless, so it can’t be seen. However, it can be recognized by its distinguishable odor of rotten eggs. This odor of rotten eggs is only detectable at low-level exposure because the olfactory system becomes paralyzed once high concentrations are reached.

These detectable levels are around 0.0005 parts per million to 0.3 ppm. The smell of rotten eggs itself does not produce any harmful effects. However, it can be unpleasant.

Also commonly known as hydrosulfuric acid, stink damp, among other names, this flammable gas with a strong odor can be found in many places, although it’s often difficult. Workers often require the use of testing equipment to be able to locate where the hydrogen sulfide gas is located and at what concentrations to determine whether or not it is safe to be in.

The impact on short-term safety is significant since hydrogen sulfide gas is presented as a silent and invisible threat. Inhalation is the primary route of exposure and can’t be avoided if it’s nearby.

Even though it can be easily smelled in low concentrations, exposure to lower concentrations is enough to weaken the senses, including smell. In addition, even lower concentrations can cause serious health effects with prolonged or repeated exposure.

Hydrogen Sulfide Danger Warning Sign

How to Evaluate Your Exposure to Hydrogen Sulfide Gas

The Occupational Safety and Health Administration has created guidelines on how to stay safe when coming in contact with this natural gas.

The best ways for workers to stay protected at work are:

  • Evaluate exposure before working to know whether there is hydrogen sulfide gas in the vicinity and if so, at what level?
  • Attempt to eliminate the source of hydrogen sulfide if possible. This natural gas can be extremely harmful, regardless of its source.
  • However, if the source can’t be eliminated, employees must use engineering or administration tools. These work safety guidelines are based on the Occupational Safety and Health Administration regulations to reduce exposure levels.
  • Always use personal protective equipment (PPE) if these measures don’t meet exposure limits.

Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency have come up with these guidelines to keep people safe when exposed to hydrogen sulfide, specifically for at-risk employees who may come into contact with hydrogen sulfide on a regular or semi-regular basis to do their jobs.

Guidance From The Agency for Toxic Substances and Disease Registry (ATSDR)

The Agency for Toxic Substances and Disease Registry (ATSDR) reports that workers in certain industries are at a much greater risk of hydrogen sulfide exposure or of being exposed to higher concentrations that can cause more serious side effects in comparison to the general population.

Workers who are required to work in areas that contain industrial emissions, poorly ventilated spaces, and other environments that cause hydrogen sulfide gas to be present need to wear personal protective equipment (PPE), including respiratory protection, to stay protected while they work.

OSHA Standards

The Occupational Safety and Health Administration (OSHA) has created regulations for low-level exposure to high-level exposure to hydrogen sulfide or hydrosulfuric acid in order to keep individuals who may come in contact with this dangerous gas at any level.

The regulations work together with those put together by the Agency for Toxic Substances and Disease Registry (ATSDR) to guarantee that workers and individuals who may be exposed to hydrogen sulfide gas can identify the gas, know what to do when they come in contact with the gas, and what to do in an emergency involving hydrogen sulfide.

There are criteria that help workers immediately determine if exposure to hydrogen sulfide gas meets dangerous to life or health concentrations.

50 PPM PEL-Peak, according to The Occupational Safety and Health Administration (OSHA,) is the peak permissible exposure to hydrogen sulfide, meaning this is the level that should never be exceeded.

The regulations set up by employers and the Occupational Safety and Health Administration (OSHA) should always be followed. Not adhering to these standards can lead to harmful physical effects and death in the worst cases.

The Occupational Safety and Health Administration (OSHA) considers the wide variety of industries and jobs that put individuals at risk of coming in contact with hydrogen sulfide gas.

From industrial emissions to natural bacterial breakdown, their guidelines cover every situation individuals, especially employees, can expect to be exposed to hydrogen sulfide or sulfur dioxide while working and how to protect themselves.

The Environmental Protection Agency

The U.S Environmental Protection Agency (EPS) helps protect public health and environmental health. The Environmental Protection Agency also provides guidelines on how to handle hydrogen sulfide gas and what to do in an emergency situation, including hydrogen sulfide gas.

For those handling this gas, it’s important to comply with the EPA regulations to avoid problems for their business.


While low-level exposure to hydrogen sulfide gas or hydrosulfuric acid won’t necessarily harm you, repeated and extended exposure can cause serious effects and even lead to death.

Most people don’t have to worry about potential exposure to hydrogen sulfide gas. Still, there are a variety of jobs within many industries where workers need to be prepared to be exposed to varying hydrogen sulfide levels. What concentrations workers are exposed to depends on their job.

Thankfully, the Occupational Safety and Health Administration (OSHA) has created regulations, guidelines, and additional resources for employers and employees to keep everyone safe at job sites where individuals may be at a high risk of harm. Ensure these guidelines are followed to create a safe working environment.