EPA standards for emissions from heavy-duty diesel trucks have been lowered dramatically since 2001. The EPA guidelines stipulated in 2001 that models manufactured from January 2010 must reduce PM emissions to 0.01 per brake horsepower hour (g/bhp-hr) and NO_xemissions to 0.20 g/bhp-hr. A 'phase-in' period allowed NO_x emissions of 1.2 g/bhp-hr by 2007. The EPA implemented a bank and trade system for NO_x, which meant manufacturers who exceeded the requirements before 2010 (such as Cummins and Navistar) could accrue NO_xcredits which could be used to sell models producing emissions above the standard after the deadline. There was no banking and trading of PM emissions.