8.3.4. The employer shall ensure that the fresh air supply for the ventilation system is without impurities, and for this purpose:(a) the ventilation ducts shall be placed so as to prevent the return of foul air into the air supply system through the shaft or tunnel entrance; and
(b) vehicles not directly involved in the underground work site shall be kept at a distance of at least 15 m from the access or opening connected to the underground work site.