Hydrogen Sulphide (H2S)is a flammable, colourless, toxic gas which has a distinctive odour of rotten eggs. It is usually formed from the bacterial breakdown of organic matter in the absence of oxygen (anaerobic digestion), often occurring in places such as sewers and swamps. It can also be found in volcanic gases, natural gases and some well water. Hydrogen Sulphide is corrosive and can leave some steels brittle, thus causing sulphide stress cracking.
Hydrogen is heavier than air so tend to accumulate at the bottom of poorly ventilated spaces. Although the smell is very noticeable at first it can soon deaden the sense of smell causing potential victims to be unaware of its presence until it is too late. Once Inhaled the hydrogen sulphide forms a complex bond with iron thus preventing oxygen form binding and stopping cellular respiration eventually resulting in death.
However we do have some hydrogen sulphide in the human body and so we have enzymes that detoxify it to the less harmful sulphate and so we can cope with small amounts. Once concentrations increase the body cannot cope with the breakdown and it starts to have a detrimental effect.
Symptoms to small concentrations include eye irritation, a sore throat and cough, nausea, shortness of breath and fluid in the lungs. The threshold where the body is no longer able to cope with H2S is believed to be around 300 â€” 350 ppm but gas detection meters will go into alarm at as little as 5 to 10 ppm and go into high alarm at 15ppm as the first symptoms such as eye irritation can occur as low as 10ppm. A concentration of 800ppm can cause death within 5 minutes.