tainment areas”—those classified as not meeting the NAAQS2 (CAA § 172(b)(5), 42 USC § 7502 (b)(5)). An overview of both NSR programs is provided later in this chapter.

NSR covers modifications of existing sources as well as construction of new sources. The CAA defines a modification as “any physical change, or change in the method of operation of a stationary source” that significantly increases the emissions of air pollution from the source (CAA §111(a)(4), 42 USC § 7411(a)(4)).3 This definition explicitly applies to both the PSD (CAA §169(2)(C), 42 USC § 7479(2)(C)) and Part D NSR (CAA § 171(4), 42 USC § 7501(4)) programs.

EPA regulations governing NSR (40 CFR 51.166 and 40 CFR 52.21)4 elaborate on the statutory definition of a modification. In 2002 and 2003, EPA altered those regulations. The first of those revisions, published December 31, 2002 (67 Fed. Reg. 80186 [2002]), altered the rules in five ways:

  • Nonutility sources may now calculate annual emissions before the physical change by averaging annual emissions in any 24-month period within the 10 years before the change. This average must be reduced to reflect any tightening of the source’s emission limit since that 24-month period. The previous rules had required those sources to average their annual emissions over the past 2 years


The NAAQS specify the maximum allowable concentrations of criteria pollutants in ambient air that are protective of public health and welfare. The six criteria pollutants are carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. Pollutants for which there are NAAQS are known as criteria pollutants because EPA prepares “criteria documents” describing the sources and effect of these pollutants.


As discussed below, EPA, with court approval, has exempted nonsignificant increases from NSR. EPA requires that, in judging whether a significant increase would occur, all other contemporaneous increases and decreases be considered; hence, the determination of whether there is a significant increase is done on a “net basis.”


Section 51.166 sets out the requirements that states must meet to have PSD programs approved as part of their state implementation plan. Section 52.21 specifies the elements of an EPA-run program in a state without an approved PSD program in its state implementation plan. The two sections are nearly identical, and so, for convenience, citations are to 51.166 only.

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