Tar can be removed with Polyoxyethylene sorbitan. Polysorbates are found in Sudocream and Flamazine which are available from most pharmacy departments. Soil, that may have become ingrained in the wound, also has a severely damaging effect on tissues. Damage to blood vessels results in the classic bluish-purple skin discolouration. The red pigment haemoglobin turns blue as it loses its oxygen and is later broken down into green and yellow bile pigments.
Stagnant blood is an ideal environment for bacteria to multiply in.
Any slight breach in the surface of the skin can result in infection. Contusions, while classically not breaching the skin's integrity are generally accompanied by other injuries which do. A force applied to the tissues literally squashes them resulting in a crush injury. The location of the injury determines the mortality or morbidity. For example, crush injuries to the chest, can result in fractures to the sternum and the ribs, and involve internal organs such as the lungs, heart, liver or spleen.