40 C.F.R. § 81.51
The Portland Interstate Air Quality Control Region (Oregon-Washington) has been revised to consist of the territorial area encompassed by the boundaries of the following jurisdictions or described area (including the territorial area of all municipalities (as defined in section 302(f) of the Clean Air Act, 42 U.S.C. 1857h(f) ) geographically located within the outermost boundaries of the area so delimited):
In the State of Oregon: Benton County, Clackamas County, Columbia County, Lane County, Linn County, Marion County, Multnomah County, Polk County, Washington County, Yamhill County.
In the State of Washington: Clark County, Cowlitz County, Lewis County, Skamania County, Wahkiakum County.
40 C.F.R. §81.51
For purposes of identification, the Portland Interstate Air Quality Control Region (Oregon-Washington) will be referred to by Washington authorities as the Portland (Oregon)-Southwest Washington Interstate Air Quality Control Region.