Section 72.500 - Emission limits for permissible diesel-powered equipment(a) Each piece of permissible diesel-powered equipment introduced into an underground area of an underground coal mine after May 21, 2001 must emit no more than 2.5 grams per hour of diesel particulate matter.(b) As of July 19, 2002, each piece of permissible diesel-powered equipment operated in an underground area of an underground coal mine must emit no more than 2.5 grams per hour of diesel particulate matter. 66 FR 5704 , Jan. 19, 2001, as amended at 66 FR 15033 , Mar. 15, 2001; 66 FR 27866 , May 21, 2001