225 ILCS 705/31.31
In a gassy mine, air that has passed through an abandoned panel which is inaccessible for inspection, or air that has passed through a similar abandoned area which is inaccessible for inspection, or air which has been used to ventilate a pillar line, or air which has been used to ventilate an area from which the pillars have been removed, shall not be used to ventilate any active face area in such mine; but if this Section cannot be complied with in such mine on the effective date of this Act, such mine may continue to be operated after such date as it was operated immediately prior to such date, for a reasonable time until future mine development and ventilation of such mine can be changed to comply with this Section. In no event shall such air be used to ventilate any area in such mine in which men work or travel if such air contains more than 1% of methane.
225 ILCS 705/31.31