Hydrogen sulphide is a flammable, colourless gas with a characteristic odour and has an offensive smell of rotten eggs. This has to be handled very carefully because it is very poisonous and corrosive. The smell indicates the presence of hydrogen sulphide. Hydrogen sulphide gas also can result from industrial activities, such as food processing, coke ovens, kraft paper mills, tanneries and petroleum refineries. Hydrogen sulphide gas is also known as “sewer gas” because it is often produced by the breakdown of waste material.