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HaggartyD
Daniel Haggarty

Feature chemical – hydrogen sulfide

13/12/2013

The latest NCEC call to be featured in the national news was a chemical incident on Oxford’s Port Meadow during the weekend of 7/8 December when a student was found dead inside a tent. It is thought the incident involved hydrogen sulfide gas.

Hydrogen sulfide (H2S) is a toxic and flammable gas, produced naturally and by human activity. It is sometimes known as sewer gas or slurry gas because it can be produced by bacterial breakdown of organic materials such as sewage. Extracting and refining petroleum and natural gas can also release H2S.

H2S is no stranger to high-profile and deadly incidents. In 2003, in Chongqing, China  a blowout at a gas well released a high concentration of natural gas and H2S, resulting in the deaths of over 200 people and the hospitalisation of 10,000. Seemingly caused by a catalogue of errors, the human cost was high. The scale of the incident response reflected the severity of the incident, with emergency crews implementing a 5km evacuation zone and igniting the gas residues to reduce the concentration of gas before entering the affected areas.

Being heavier than air, H2S can collect in low-lying places near to an incident. This means it is possible for flammable or toxic accumulations to gather nearby – an important problem when considering an exclusion zone, but also giving the option for large-scale incidents to be dealt with as in Chongqing.

Despite the severity of the incident in Chongqing, H2S incidents are often on a much smaller scale and frequently happen closer to home. One of the really dangerous times is when workers enter uncleaned pits, silos and tanks where H2S has formed. Farmers seeing dead birds or livestock close to enclosed organic matter should be especially cautious. One tragic example in the agricultural world was in Northern Ireland in 2012, where three members of the same family became overcome by slurry gas on a farm. Because of the fast effects of the gas, rescuers can become casualties themselves.

Members of the emergency services need to have good training in how to respond to incidents involving H2S and prevent themselves becoming casualties. The distinctive rotten-egg smell of H2S is a good indicator of its presence, but the gas itself reduces a person’s sense of smell. This means you may be least aware of its presence at the time you need to be most aware. If you smell rotten eggs, get away quickly! A 1,000ppm concentration of hydrogen sulfide will quickly be fatal, but a reduced sense of smell occurs at just 100 – 150ppm.

Following tragically fatal incidents, hazards still remain. Even if the gas has dispersed, exposure can still happen through ‘gassing off’ of the deceased. Look out for an unresponsive person in an enclosed space, warning signs, illness amongst bystanders and react quickly to the distinctive smell. Ventilate – quickly, and if safe to do so* – stand back and bring protective equipment to the scene.

*Don’t forget the flammable properties of hydrogen sulfide. If in an enclosed area (e.g. a sealed car), opening the door to ventilate could cause a spark. Breaking a window may be a safer option.

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