Hydrogen sulfide (H₂S) is a gas you don’t want to underestimate. Colourless, flammable, and toxic even at low concentrations, H₂S has earned a notorious reputation across high-risk industries. Known by nicknames like sour gas, sewer gas, or stink damp, it’s instantly recognisable by its “rotten egg” odour—though relying on your nose could be a fatal mistake.

Despite its hazards, H₂S is also vital in industrial processes and scientific research. This dual identity makes understanding H₂S essential—it supports workplace safety, environmental compliance, and responsible handling wherever H₂S is present..

In this guide, we’ll try to summarize everything you need to know about hydrogen sulfide: how it forms, why it’s dangerous, where it’s used, and—critically—how to detect and protect against it.

What is Hydrogen Sulfide (H₂S)?

Hydrogen sulfide (H₂S) is a naturally occurring gas that’s as dangerous as it is common in certain industries. It’s colourless, flammable, and highly toxic—even small amounts can present serious risks. One of its most recognisable features is its odour: a pungent smell often compared to rotten eggs. But that smell can’t be relied on for safety, as exposure can quickly impair the sense of smell, making it undetectable without proper monitoring.

Known in industrial circles as sour gas or stink damp, H₂S is produced through the breakdown of organic matter in oxygen-deprived environments. That includes swamps, sewers, volcanic vents, and even some natural gas and petroleum reserves. For this reason, workers in wastewater treatment, oil and gas, mining, and agriculture are frequently exposed to it—sometimes without realising it.

Its presence isn’t always obvious, but the consequences of ignoring it can be immediate and severe. That’s why awareness, detection, and preparation are central to keeping people safe in H₂S-prone environments.

Physical and Chemical Properties

Understanding the physical and chemical characteristics of hydrogen sulfide (H₂S) is key to identifying the risks it poses and selecting the right safety measures. These properties directly influence how the gas behaves in the workplace and the environment.

Physical Properties

• Molecular weight: 34.08 g/mol.
• Appearance: Colourless gas.
• Odour threshold: Detectable at concentrations as low as 0.13 ppm, although this detection is unreliable at higher exposures due to olfactory fatigue.
• Density: Heavier than air (vapor density 1.19), which causes it to accumulate in low-lying or enclosed areas—especially dangerous in confined spaces.
• Solubility: Slightly soluble in water, forming a weak acid (hydrosulfuric acid).

Chemical Properties

• Flammability: Highly flammable, with an ignition temperature around 260°C (500°F). It can form explosive mixtures with air at concentrations between 4.3% and 46%.
• Combustion by-products: Burning H₂S produces sulphur dioxide (SO₂), another toxic gas that can irritate the respiratory system.
• Corrosiveness: Reacts with certain metals to form iron sulfide, which can spontaneously ignite when exposed to air—a lesser-known but serious hazard.
• Reactivity: Capable of causing stress corrosion cracking in metals, making it a concern not just for health, but for structural integrity in industrial systems.

These traits make hydrogen sulfide a complex threat—one that behaves differently depending on its environment. Safe handling requires a thorough understanding of how and where it can accumulate, how it reacts under pressure or heat, and what it leaves behind when it burns.

Sources of Hydrogen Sulfide

Hydrogen sulfide doesn’t need to be manufactured to become a problem—it shows up naturally and through human activity in a range of environments. Knowing where it originates helps identify risk areas and guides monitoring strategies.

Natural Sources

• Anaerobic decomposition: The most common natural source of H₂S is microbial activity in oxygen-deprived environments. As organic matter breaks down in places like swamps, marshes, and stagnant ponds, bacteria release H₂S as a by-product.
• Volcanic activity and geothermal features: Sulphur springs, volcanoes, an geysers can emit H₂S naturally. These areas often have elevated concentrations of the gas in both air and water.
• Natural gas and crude oil deposits: H₂S often occurs in underground fossil fuel reserves. In these cases, it’s typically referred to as sour gas and requires processing before the fuels can be safely used or transported.

Industrial Sources

• Oil and gas operations: Hydrogen sulfide is a well-known hazard in drilling, refining, and natural gas processing. Workers may encounter it in extraction zones, wellheads, or storage tanks.
• Wastewater and sewage treatment: Anaerobic digestion in sewers, septic tanks, and sludge pits produces H₂S. Confined spaces in these facilities are particularly hazardous.
• Pulp and paper mills: Sulphide-based pulping processes can release hydrogen sulfide during breakdown and recovery stages.
• Mining and metal refining: Some ores contain sulphur compounds that, when processed, generate H₂S gas.

The variety of sources means that hydrogen sulfide can be present in industries that may not immediately associate with gas hazards. This makes it essential for employers and workers to assess environments carefully—even when exposure seems unlikely.

Health Hazards of H₂S Exposure

Hydrogen sulfide is acutely toxic and fast-acting. Even brief exposure to elevated levels can lead to serious health consequences or death. What makes it particularly dangerous is how symptoms escalate quickly, often without warning—and in some cases, without smell.

Acute Effects

• Respiratory distress: Inhalation can cause irritation of the nose, throat, and lungs, leading to coughing, shortness of breath, and tightness in the chest.
• Neurological impact: Headaches, dizziness, nausea, and disorientation can occur at relatively low concentrations.
• Eye and mucous membrane irritation: Burning sensations in the eyes and nose often accompany early exposure.
• Loss of consciousness and death: At higher concentrations (typically above 500 ppm), H₂S can cause rapid unconsciousness—sometimes within a single breath. This is often referred to as the knockdown effect, and fatalities can occur without warning.

Chronic Exposure Risks

Prolonged or repeated exposure, even at low levels, can have long-term health implications:

• Olfactory damage: The sense of smell may be permanently impaired.
• Cognitive effects: Memory issues, reduced reaction time, and mood disturbances have been observed.
• Respiratory issues: Chronic bronchitis and ongoing respiratory inflammation can result from ongoing low-level exposure.

Concentration and Effects

Here’s a breakdown of concentration levels and their typical effects:

High-Risk Groups

The following workers are particularly vulnerable to exposure:

• Oil and gas field personnel.
• Sewer and wastewater workers.
• Agricultural and food processing staff.
• Pulp and paper mill employees.
• Those working in or near confined spaces.

Lone workers—especially those in isolated or confined environments—require dedicated monitoring solutions to ensure their safety in the event of H₂S exposure.

Early recognition of symptoms, combined with monitoring and protective equipment, is crucial to protecting health and saving lives.

Industrial and Scientific Applications

Despite its toxicity, hydrogen sulfide plays a practical role in several industries. Its controlled use in manufacturing, processing, and research makes it an important—though tightly managed—chemical.

Industrial Uses

• Sulphur and sulphuric acid production: One of the primary uses of H₂S is as a raw material for manufacturing elemental sulphur and sulphuric acid. These substances are fundamental to the production of fertilisers, detergents, and other industrial chemicals.
• Metal processing: Hydrogen sulfide is used to purify certain metals by removing impurities such as iron oxides or other contaminants during refining.
• Petrochemical industry: In sour gas treatment, H₂S is removed from crude oil and natural gas through processes such as amine gas treating and Claus sulphur recovery.

Scientific and Medical Research

• Biological signalling: H₂S is being studied for its role as a signalling molecule in the human body, similar to nitric oxide. It may be involved in processes such as inflammation, blood pressure regulation, and cellular defence.
• Therapeutic potential: Early research suggests H₂S could play a part in developing treatments for cardiovascular diseases and neurodegenerative conditions—though this research remains in its early stages due to safety challenges.

Analytical Applications

• Water quality testing: In laboratory settings, hydrogen sulfide is used to test for the presence of metal ions in water samples.
• Chemical analysis: It’s a reagent in qualitative analysis, helping identify and precipitate specific metal ions by forming distinctive sulphide compounds.

Even with these benefits, strict protocols are always required. The risks associated with handling H₂S mean that any operational use must include rigorous safety systems, real-time monitoring, and staff training.

Environmental Impact of H₂S

Hydrogen sulfide doesn’t only pose risks to human health—it also has serious environmental consequences. When released in significant quantities, either accidentally or through industrial operations, it can damage ecosystems, lower air and water quality, and threaten wildlife.

Air Quality and Atmospheric Effects

When H₂S is burned—either intentionally in flares or during accidental ignition—it produces sulphur dioxide (SO₂), a toxic gas that contributes to:

• Acid rain: SO₂ reacts with water vapour in the atmosphere, forming sulphuric acid. This leads to acid rain, which can damage crops, corrode buildings, and alter soil chemistry.
• Smog and respiratory pollution: In industrial regions with poor emission controls, airborne H₂S and SO₂ add to the overall burden of air pollution, raising public health concerns and lowering regional air quality indices.

Water Contamination

Hydrogen sulfide is moderately soluble in water, and once dissolved, it forms a weak acid (hydrosulfuric acid). In natural water bodies, this can:

• Lower oxygen levels: Decomposition of H₂S consumes oxygen, threatening fish and aquatic life.
• Disrupt ecosystems: Even small concentrations can be toxic to aquatic organisms, leading to biodiversity loss in contaminated lakes, ponds, or rivers.
• Affect drinking water supplies: In groundwater or well water, the presence of H₂S is often indicated by a rotten egg smell and can corrode plumbing systems and impact taste and safety.

Odour and Community Impact

Even at very low concentrations, H₂S’s strong odour can be a source of complaints in nearby communities. Persistent smells near industrial sites or sewage plants often lead to public concerns, regulatory scrutiny, and reputational damage for operators.

For industries operating near residential areas, proactive environmental monitoring and emission control systems are critical—not just for legal compliance, but for maintaining public trust.

Detection and Monitoring Methods

Hydrogen sulfide is most dangerous when it goes undetected. Because it can numb the sense of smell at higher concentrations, relying on odour alone is a serious safety error. That’s why both fixed and portable detection systems are used across industries—each serving a unique purpose. Learn more about the difference between fixed and portable gas detection systems.

Fixed Detection Systems

• Installed in high-risk areas: These systems are commonly placed in processing facilities, treatment plants, or enclosed workspaces. They provide 24/7 monitoring and are often integrated with ventilation, alarms, or automatic shutdown mechanisms.
• Multi-gas sensors: Modern industrial systems often monitor for H₂S alongside other hazardous gases like methane (CH₄), carbon monoxide (CO), or oxygen (O₂) depletion.

A complete monitoring strategy often requires both fixed and portable detection—each playing a distinct role in ensuring worker safety. Learn more about the importance of using both portable and fixed gas detection systems.

Portable Gas Monitors

• Personal safety tools: Worn on the body or carried into confined spaces, these monitors alert workers to dangerous concentrations before symptoms occur.
• Single- or multi-gas models: Portable units can detect one or more gases depending on the work environment and hazard profile.
• Calibration and maintenance: Routine checks and calibration are required to keep readings accurate and prevent false alarms or missed threats.

Olfactory Fatigue and the Illusion of Safety

While the rotten egg smell may seem like a helpful warning sign, relying on it is not safe. Prolonged or high-level exposure can cause olfactory fatigue, where the sense of smell becomes impaired or shuts down entirely. This can happen quickly, leaving workers unaware of the ongoing danger. Mechanical detection is the only reliable method.

Monitoring Best Practices

• Conduct pre-entry gas checks in confined spaces.
• Use bump tests to verify sensor responsiveness before each shift.
• Maintain detection equipment regularly, with records of inspections and replacements.
• Train staff on what to do when an alarm sounds—including evacuation protocols, equipment use, and best practices for gas detection

Accurate detection shouldn’t be just about meeting standards—giving workers the confidence to do their jobs safely and the ability to act when it counts, is even more valuable.

Emergency Response Procedures

When hydrogen sulfide is detected at hazardous levels, time is critical. A rapid, coordinated response can mean the difference between a safe evacuation and a fatal incident. Because H₂S can cause immediate unconsciousness and respiratory failure, emergency procedures must be rehearsed, well-documented, and specific to each work environment.

Immediate Action

• Evacuate the area: All personnel should leave the affected zone without delay. Movement should be calm but immediate—lingering, hesitation, or attempts to investigate without proper equipment can be fatal.
• Activate alarms and alert response teams: Many facilities are equipped with automated alarm systems that trigger when gas thresholds are exceeded. Manual alarms should be used if needed to alert nearby teams.
• Isolate the source if possible: Only trained personnel with proper respiratory protection (e.g., SCBA) should attempt to shut valves or stop processes contributing to the leak.

Rescue Considerations

• Never attempt a rescue without respiratory protection: Too many fatalities occur during attempted rescues. Self-contained breathing apparatus (SCBA) is mandatory. Standard face masks or escape hoods are not sufficient for sustained exposure.
• Use the buddy system: If a rescue must be carried out, trained teams should work in pairs with one member monitoring and prepared to assist the other.
• Maintain communication: Two-way radios and clear emergency communication protocols are vital during response efforts.

First Aid and Medical Support

• Move victims to fresh air immediately.
• Administer oxygen if available and trained to do so.
• CPR should be started if breathing has stopped, but only if it can be done safely.
• Seek emergency medical care as quickly as possible—delays can result in long-term damage or death.

Post-Incident Procedures

• Conduct incident reviews and identify root causes.
• Inspect and calibrate detection systems.
• Provide counselling or medical follow-up for affected personnel.

Emergency response isn’t just about reacting—it’s about preparation. Drills, clear roles, and equipment readiness must all be part of the workplace safety culture.

Workplace Safety Measures and PPE

Managing the risk of hydrogen sulfide exposure begins long before a leak or alarm. Proactive safety protocols, combined with the right personal protective equipment (PPE), create layers of protection that save lives.

Workplace Safety Protocols

• Permit-to-work (PTW) systems: Before beginning high-risk tasks—especially in confined spaces or processing areas—formal permits must be issued. These include hazard assessments, gas testing, and sign-off from authorised personnel.
• Pre-entry gas testing: Always check for the presence of H₂S before entering confined or enclosed areas. If readings are above safe thresholds, entry is prohibited until levels are controlled.
• Ventilation and gas dispersion: Mechanical ventilation can dilute or remove accumulated gas, particularly in tanks, pits, or sumps. Ventilation systems should be tested and maintained regularly.

(Guide to safe entry into confined spaces)

Personal Protective Equipment (PPE)

• Respiratory protection: In environments where H₂S exposure is possible, workers must wear appropriate respiratory gear:
  • Escape hoods for emergency evacuation.
  • Self-contained breathing apparatus (SCBA) for rescue operations or high-concentration areas.
• Chemical-resistant clothing: Gloves, coveralls, and boots prevent skin exposure and contamination in wet or chemical-laden environments.
• Eye and face protection: Goggles or full-face shields protect against irritation caused by H₂S and its by-products.

For lone workers, devices like the Blackline G7c offer real-time monitoring and immediate response features, helping teams detect exposure and respond faster in emergencies.

Training and Awareness

• H₂S-specific training: All personnel should complete training that includes gas characteristics, detection tools, PPE use, and emergency response.
• Evacuation drills: Regular simulations ensure teams know how to respond, where to go, and how to assist others under pressure.
• Odour fatigue awareness: Workers must be taught that smell is not a reliable warning—relying on detection equipment is non-negotiable.

(The importance of gas detection training)

Engineering Controls

• Gas detection and alarm systems: Fixed and portable devices should be in place, regularly maintained, and visible/audible throughout the facility.
• Automatic ventilation and shutdown systems: In critical settings, engineering controls can isolate or purge H₂S sources without human intervention.
• Drainage design and equipment maintenance: Preventative maintenance reduces the likelihood of buildup in tanks, piping, or other low-lying areas.

Safety is a system, not a checklist. When policies, equipment, and people work together, the risk of H₂S exposure becomes manageable.

Regulations and Compliance

Hydrogen sulfide is strictly regulated due to its toxicity and widespread industrial presence. Governments and health agencies around the world have established exposure limits, monitoring requirements, and enforcement mechanisms. Compliance with these standards isn’t optional—it’s a legal obligation and a core part of protecting human life.

Exposure Limits

Different agencies set varying limits, but the direction is consistent: keep exposure as low as reasonably possible.

• OSHA (Occupational Safety and Health Administration – US):
  • Permissible Exposure Limit (PEL): 10 ppm (8-hour Time Weighted Average)
  • Ceiling Limit: 20 ppm (never to be exceeded)
• NIOSH (National Institute for Occupational Safety and Health – US):
  • Recommended Short-Term Exposure Limit (STEL): 10 ppm (15-minute exposure)
  • Immediately Dangerous to Life or Health (IDLH): 100 ppm
• HSE (Health and Safety Executive – UK):
  • Workplace Exposure Limit (WEL): 5 ppm (8-hour TWA); 10 ppm (15-minute STEL)
• EU Guidelines and Local Regulations:
  • May vary by country, but most align closely with the above.

These limits apply in all occupational settings where H₂S may be present. Employers are responsible for monitoring levels and taking action if readings approach or exceed thresholds.

Industrial Compliance Requirements

• Gas detection systems: Employers must provide accurate, calibrated equipment to monitor H₂S. This includes fixed systems in facilities and portable monitors for individual workers.
• Emergency preparedness: Written procedures must be in place for evacuation, rescue, and medical response in case of H₂S release.
• PPE and respiratory protection: Employers must supply certified equipment, ensure fit testing, and verify correct usage through training and supervision.
• Training documentation: Workers must be trained regularly, and employers must keep records to show compliance during inspections.

Consequences of Non-Compliance

Failure to meet regulatory requirements can result in:

• Fines and enforcement actions from regulatory bodies.
• Site shutdowns or operating restrictions.
• Legal liability in the event of injury or death.
• Reputational damage that can affect operations and community relations.

Regulations aren’t just about ticking boxes—they exist to prevent tragedies. When applied consistently, they form the foundation of a safe and compliant workplace.

* * *

Hydrogen sulfide is one of the most hazardous gases found in industrial settings. It’s invisible, flammable, and toxic at concentrations that may be hard to detect without the right tools. And while its odour is distinctive, relying on smell is a gamble no one can afford to take.

For organisations operating in oil and gas, wastewater treatment, manufacturing, or confined space environments, controlling H₂S risk is not optional—it’s part of the job. That means investing in reliable detection systems, following proven safety procedures, and training every team member to respond without hesitation.

Understanding hydrogen sulfide is more than technical knowledge—it’s a practical safeguard that protects lives, prevents accidents, and keeps operations running smoothly.

Concerned about H₂S risk in your facility or field operations?

OBW Technologies provides advanced gas detection systems, expert assessments, and hands-on training to help you manage hydrogen sulfide safely and confidently. Whether you’re looking to upgrade your monitoring infrastructure or train your team, our specialists are here to support you.

Contact us today to speak with an expert or schedule a consultation.