What is the difference between an occlusive and semi-occlusive dressing?

An occlusive dressing is a non-permeable dressing, which means that no air or moisture can penetrate in or out. A semi-occlusive (semi-permeable, transparent) dressing allows the wound to “breathe” (air can penetrate in and out) but at the same time, protects the wound from outside liquids. Commonly used dressings in vascular access are semi-occlusive, transparent dressings.