For determining the length of time between foam dressing changes, you must consider the absorption rate of the particular type of foam dressing you are using. Most foam wound dressings have polyurethane structures available in various kinds: film-backed, polyurethane membranes, and gels. Each type has its independent levels of absorbency and permeability. These differences determine how these foam wound-dressing handle exudate. For example, polyurethane foam gels are exceptionally absorbent and highly permeable to moisture, allowing them to be effective for up to five days. But, for the most part, you should change foam dressings when you see exudate within 1 cm of the edge of the dressing or two to five days.