Polyurethane is a very durable, organic and easy to clean material. It has good buoyancy, grip adhesion and resistance to water, oil, grease and abrasion.
For cleaning regularly, wipe with a soft cloth dampened with cool water. Refer to manufacturer’s care instructions for a machine washable option. Never use hot water. Do not dry clean. Allow to air-dry naturally away from direct heat.
Birkenstocks are designed to be roomy this is needed to perform the natural rolling motion of the foot without restrictions - just as if you were barefoot.
Your toes and heel should not touch the rim of your shoes while standing in them. Most people are fine with a regular fitting, but if your foot moves side to side when you walk and adjusting any straps does not help, a narrow fitting may be more suitable.