This is one of the important toxic gases found in piggeries and it is potentially lethal. The gas is produced by anaerobic bacterial decomposition of organic matter, particularly that found in faeces and slurry. The greatest source of H2S and indeed the greatest potential for disasters comes from slurry that is held in pits beneath slatted finishing houses. Pockets of gas may become trapped and when the slurry is agitated and removed from the house there is a risk of acute and fatal poisoning not only to pigs but also to people. There is a low concentration of H2S in most pig houses, usually less than 100 ppm. H2S concentrations can be detected by the human nose within the range of 0.05 to 200 ppm. When levels get above the upper limit the sensitivity of the nose to detect the gas decreases significantly and the situation becomes potentially dangerous.

Clinical signs

These are dependent on the levels in the air and the following sequence of events can be considered in both pigs and humans:

• Less than 50 ppm - little effect but detected by people.
• 200 to 250 ppm - slight breathing difficulties, distress and irritation of the eyes, nose and back of the throat. The environment is becoming dangerous.
• 250 to 400 ppm - breathing starts to become very difficult with muscular spasms and disorientation.
• 400 to 600 ppm - pigs become totally disorientated and some start to relapse into a coma.
• 600 to 1000 ppm - the pig develops fits and convulsions and becomes extremely short of oxygen Its skin goes blue (cyanosed) and death ensues.