During the handling of the oil, i.e. during loading and unloading in particular, large quantities of the light components of the oil evaporate. These oil vapours are normally called VOC, short for Volatile Organic Compounds. Evaporation also occurs during the voyage when the oil splashes around in the tanks. To prevent the oil vapours exploding, the tanks are filled with inert gas, which normally consists of cleaned combustion gas with an oxygen content below 8%.