In this system, a single duct is provided and is normally operated by a blowing fan. After blasting, airflow is reversed and the fumes and dust are exhausted. Ventilation is again reversed to blowing, when the work at the face is resumed. The usual arrangement is to use two fans, one for forcing, one for exhausting, at the mouth of the heading. This arrangement is particularly suited to underground use as it allows clean air to be drawn from, and contaminated air to be discharged to, separate points in the main airways.