A covering is required for the floor of the hamster cage to provide a soft, comfortable surface for the hamster and also to soak up the urine. The most common type of floor covering available is wood shavings - fine sawdust should be avoided as this can cause irritation to the eyes.
Cedar wood shavings (usually distinguished by a red tint) should not be used as the phenols they contain can cause severe irritations in hamsters. Although Pine is also a softwood Pine wood shavings cause less problems than Cedar and kiln dried Pine can be used without problems.
Any form of scented shavings should be avoided as these can also cause irritation and Corn cob bedding is not generally recommended as it has a tendency to mould and hamsters have been known to eat it and it can then swell when wet inside the stomach.
Wood shavings from hardwoods such as Aspen or small animal litter made from wood pulp (such as Carefresh) are the safest forms of floor covering to use.