The Dansko Crumble--Read if you Buy/Wear Dansko's

Hydrolysis and polyurethane explained
Many of our shoes have the soles attached to the uppers by a process where 2 materials are brought together in a mould, the resultant chemical reaction forms polyurethane (PU). the use of polyurethane makes a lightweight flexible sole which is not only shock absorbent but also extremely hard wearing, that's why polyurethane is ideal for shoes.

One of the main technical challenges with polyurethane is overcoming an ageing deterioration known as ‘hydrolysis’. Hydrolysis is the chemical breakdown of the PU polymer and the resulting physical breakdown or crumbling of the PU sole by the attack of water (usually in vapour form), occurring over a period of several years (even when the shoes are in store!).

This process is accelerated by warmth and high humidity. It will, therefore, happen more quickly in tropical climates, but also in confined spaces (such as lockers) if the safety shoes are put away damp. In the most advanced state of hydrolysis, the PU sole will lose all its physical strength, thus cracking or crumbling.