225 ILCS 705/17.05

In crosscuts connecting main and cross entry inlet and outlet air courses, the permanent stoppings shall be erected of masonry, concrete, or other incombustible material, and shall be erected within 600 feet of the face of main and cross entries at all times. Temporary stoppings of wood or other equally effective material shall be maintained, as nearly air tight as possible, between the last permanent stopping and the cross cut nearest the face in main and cross entries. In room and stub entries the stoppings shall be built of wood or other equally effective material. All stoppings shall be kept in good condition, so as to keep the air up to the working faces.