The agency made a number of regulatory revisions to the NSR programs in December 2002 (the 2002 NSR rules). These included revisions in methods for determining what particular physical or operational changes in a facility might result in significant emission increases and thus invoke NSR requirements. In support of these revisions, EPA concluded that the 2002 changes would be likely to result either in some degree of emission reduction or in no significant changes in emissions. However, EPA indicated that it did not have sufficient data to quantify the emission changes that might result and also said it could not reliably determine the locations of any potential emission changes. Thus the agency could not estimate the rule’s impacts on public health.

In October 2003, EPA made additional NSR revisions, referred to as the equipment replacement provision, that allow facilities to make certain equipment replacements without an NSR permit even if pollutant emissions increase significantly, as long as the facility does not exceed its maximum level of allowable emissions.² This expanded the scope of the exemption from NSR for “routine” maintenance. After conducting a computer model analysis of the electric power industry and six case studies of other industrial sectors, EPA concluded that the equipment replacement provision would have little impact on future emissions.

The rule changes have provoked much dispute. EPA and other supporters of the revisions say the NSR changes will provide industry with greater flexibility in operating its facilities, increase energy efficiency, and help to modernize plants—all without causing substantial emission increases. Opponents say that the NSR revisions will slow progress in cleaning the nation’s air, and thus damage human health, and that the changes are not necessary to provide operating flexibility in industry.

Congress directed EPA to arrange for an independent study by the National Research Council (NRC) to estimate the effects of the 2002 NSR rule changes and the 2003 equipment replacement provision. Congress called for an assessment of changes in emissions of pollutants regulated under the NSR programs, the effects on human health, and changes in operating efficiency (including energy efficiency), pollution prevention, and pollution-control activities at facilities subject to the revised NSR programs. The study task statement specified that the study should consider the data and methods necessary to assess specific effects of the NSR rules expected to occur in the coming years.

Several factors made it difficult to assess the effects of the NSR rule changes. As of mid-2006, the 2002 NSR rules have gone into effect in only a few states, and few permits have been issued under the 2002 rules. In ad-