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Crowcon #5: Hydrogen Sulphide: toxic and deadly

Hydrogen Sulphide: toxic and deadly – Chris explains more about this dangerous gas

Many of you will have come across hydrogen sulphide (H2S). If you have ever cracked a rotten egg the distinctive smell is H2S.

H2S is a hazardous gas that is found in many work environments, and even at low concentrations it is toxic. It can be a product of man-made process or a by-product of natural decomposition. From offshore oil production to sewerage works, petrochemical plants to farms and fishing vessels, H2S presents a real hazard to workers.

Man-made by-product

Hydrogen sulphide is a by-product of processes, such as petroleum refining, mining, paper mills, and iron smelting. It is commonly associated with the refining of crude oil and processing of “soured” natural gas. With increasing demand for oil and gas, wells that previously may have not been developed because they were too sour, are being brought on line. This, combined with a trend for increasingly tough regulation on sulphur levels in the end product, has led to significant money being invested into improved methods of neutralising this dangerous gas from hydrocarbon energy sources.

Incidental by-product

Hydrogen sulphide is also a common by-product of the biodegradation of organic matter, as a result of the action of sulphur-reducing bacteria. In environments with low or no oxygen, these bacteria use sulphur instead of oxygen to create energy. They “oxidise” organic matter, producing H2S. Pockets can collect in enclosed spaces or build up in the material itself, and be released when it is disturbed.

Workers in sewerage and waste water plants and pipework can be overcome by H2S, with fatal consequences. Farmers are often unaware of the risks when entering to inspect an empty slurry tank. The slurry may have been removed, but the gas could still be present.

Mariners are also at risk. Incorrectly stored and refrigerated fish potentially endanger the lives of fisherman on board. There is also a hazard from H2S in ship ballast and sewage tanks; one incident occurred when crew members on a luxury liner succumbed to H2S while undertaking repairs on a pipe in the ship’s propeller room.

How H2S is dangerous

Hydrogen sulphide is flammable in high concentrations. However, the main danger is that, like carbon monoxide, it is toxic. It inhibits cellular respiration and uptake of oxygen. Prolonged exposure to 2-5 parts per million (ppm) H2S can cause nausea and headaches, and bring tears to the eyes. At 20ppm, symptoms include fatigue, headaches, irritability, dizziness and impaired memory. Severity of symptoms increase with concentration through coughing, conjunctivitis, olfactory paralysis (loss of sense of smell), collapse and rapid unconsciousness. Exposure at higher levels can result in death almost instantaneously.

As a toxic gas, H2S can have instantaneous effects, and prolonged exposure may cause chronic illness or even death. Because of this, many monitors will have both instantaneous and TWA (Time-Weighted Average) alerts.

Heavier than air, H2S sinks to the floor and may accumulate in shafts, trenches or floor-level ducting. Smelling of rotten eggs, the pungent odour of H2S is very noticeable, at first. However, the gas quickly deadens the sense of smell, giving the false impression that the gas has dispersed. Unaware of the persisting danger, someone may continue to work, failing to take adequate precautions against the toxic risk.

Gas detector types

Both portable and fixed gas detectors can be used for monitoring H2S. Fixed systems typically comprise one or more detector heads connected to a separate control panel. If a detector reads a dangerous gas level, the panel raises the alarm by triggering external sirens and beacons.

Often a compact portable unit for monitoring exposure over time is more suitable. Combining one (or more, if other gas hazards may be present) sensor with powerful audible and visual signals to warn when pre-set gas levels are reached, portable detectors can be carried or worn wherever they are needed. In addition, a compact instrument is easily carried in a confined space, ensuring that pockets of high gas concentrations are not missed.

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