Hydrogen Sulphide – Hazardous Gas Management

Hydrogen sulphide (H₂S) is a colourless, flammable and highly toxic gas. It can occur naturally in oil and gas deposits and from the breakdown of organic material. It is also commonly produced as an unwanted by-product during petroleum refining, sewage treatment, pulp manufacture and other industrial processes.

This distinction matters: hydrogen sulphide is not normally added to crude oil to remove sulphur. During refinery processing, sulphur compounds can be converted into H₂S, which is then removed from process streams and recovered or converted into products such as elemental sulphur or sulphuric acid. The UK Health Security Agency describes hydrogen sulphide as an undesirable by-product of refining sulphur-containing crude oil.

Why Hydrogen Sulphide Is Dangerous

Hydrogen sulphide can irritate the eyes and respiratory tract. As concentration rises, exposure can cause headache, dizziness, confusion, collapse, respiratory failure and death. NIOSH's hydrogen sulphide guidance explains the danger from high concentrations, which can overwhelm a person rapidly and create a serious risk to anyone who attempts an unplanned rescue.

The familiar rotten-egg smell is not a control measure. The sense of smell becomes fatigued and can stop providing warning while the gas is still present. A calibrated gas detector is needed wherever H₂S could accumulate.

Hydrogen sulphide is also flammable. Controls must address both toxic exposure and ignition, using the classification, handling and emergency information in the SDS for the specific product or process.

Where Exposure Can Occur

• Oil and gas extraction, refining and natural-gas processing
• Sewers, wastewater treatment, sludge handling and manure systems
• Pulp and paper manufacture
• Tanks, pits, vessels and other confined spaces where organic material decomposes
• Processes where sulphur-containing materials react or are heated

Hydrogen sulphide is heavier than air and can collect in low points and poorly ventilated spaces. Confined-space work needs a task-specific safe system, atmospheric testing, suitable rescue arrangements and competent people. Ventilation alone should not be assumed to make an unknown atmosphere safe.

UK Workplace Exposure Limits

The current HSE publication EH40/2005 Workplace Exposure Limits lists hydrogen sulphide at 5 ppm (7 mg/m³) as an eight-hour time-weighted average and 10 ppm (14 mg/m³) over 15 minutes. These are exposure limits, not dividing lines between safe and dangerous work. Employers must still prevent exposure where reasonably practicable or otherwise control it adequately under COSHH.

Controlling the Risk

• Identify every task and location where H₂S can be released or accumulate.
• Eliminate the release where possible, or use enclosed systems and effective extraction.
• Test the atmosphere with calibrated direct-reading equipment before entry and continuously where conditions can change.
• Use alarms, restricted access and clear evacuation arrangements.
• Select respiratory protective equipment from the measured or foreseeable concentration and the SDS. Unknown or immediately dangerous atmospheres require specialist breathing apparatus, not a disposable mask.
• Train workers and practise rescue arrangements without relying on an unprotected person entering the contaminated area.

Why Rescue Attempts Create More Casualties

Most H₂S incidents that involve more than one casualty follow the same pattern. Someone goes down in a confined space, a colleague climbs in after them without pausing to think, and within seconds there are two people on the floor instead of one. At high concentrations, hydrogen sulphide can overwhelm a person within one or two breaths. There is rarely time to shout a warning, let alone climb back out.

Rescues should be planned around that fact. The atmosphere that put the first casualty down has probably not improved just because a few minutes have passed, so it should be treated as hazardous until testing proves otherwise. Where the task allows it, retrieval happens without anyone entering: a harness and line rigged before work starts, or a tripod and winch positioned at the access point. If entry is unavoidable, the person going in wears specialist breathing apparatus and works alongside a trained standby person stationed outside, watching and ready to raise the alarm.

Emergency Response

Raise the alarm and keep unprotected people out of the affected area. Do not attempt a rescue unless the atmosphere has been assessed and the rescuer has the required training, equipment and backup. Move the casualty to fresh air only when it is safe, call emergency services and follow the SDS. If breathing has stopped, trained responders should begin appropriate resuscitation. Oxygen should be administered only by properly trained personnel.

Because symptoms can be severe or delayed, anyone with suspected significant exposure needs prompt medical assessment.