Most crude oils are in varying degrees acidic, with sulphur compounds occurring in widely varying amounts, sometimes even in crudes from different fields in the same production area. In general the Middle East crudes have a high sulphur content, some over 6%, while some of the Far East, North African and North Sea crudes may have less than 0.2%. An obnoxious smell may indicate a high hydrogen sulphide content in a crude oil. Exposure to this may dull the senses so that there is the danger of the olfactory warning being ignored. The appropriate handling of each crude type will depend upon its characteristics. The majority of Middle East crude oils, for example, require no special preparation of tanks before loading and, unless very low temperatures will be encountered on the voyage, will not require any heating. The tank cleaning between such cargoes will only be that necessary for ballast purposes and for good housekeeping, i.e. to prevent the build up of sediments in the tanks. Some solids, semi solids, waxy and oily deposits will be left in the cargo tanks after every voyage. An accumulation of these deposits would affect discharging, by hindering the flow of oil across the tank bottoms, so the need for routine cleaning is obvious even when questions of cleanliness for cargo quality reasons, or in order to drydock for repairs, do not arise. Provided the tanks have been well drained after discharging the previous crude (or black oil) cargo, most crudes, whether heavy, medium or light, may be loaded without any further tank preparation. There are exceptions and before loading wax free naphthenic crudes such as Tia Juana Pedado, Cabimas or Lagunilas, the tanks should be hot washed and all wax deposits lifted unless the previous cargo had been a wax free crude or a naphthenic distillate. These particular crudes are not moved in very large parcels. They require a fairly high degree of heating and also with the very large crude carriers, the lifting of deposits is not really a practical proposition.