dissenting.
The majority concludes that the Environmental Protection Agency (“EPA”) acted arbitrarily and capriciously in failing to adequately explain its basis or theory in denying Kane and Du Page Counties’ request to be reclassified from a status of nonattainment to one of attainment. To support its position, the majority posits several theories (i.e. urban v. county designation) that may explain the EPA’s denial of Kane and Du Page Counties’ redesignation request; and the majority argues that the EPA has failed to consistently apply either of these alleged theories in defining what constitutes an appropriate nonattainment area. Because the majority believes that the EPA failed to consistently apply the same theory in assessing the requests for redesignation, it remands this case to the EPA for further explanation. After reviewing the previous statements issued by the EPA concerning the appropriate size for nonattainment areas and the reason proffered by the EPA for denying Kane and Du Page Counties’ request for redesignation — namely, that Du Page and Kane Counties are within the Chicago urban ozone area and contribute to the urban area’s ozone problem — it is clear that the EPA has applied the same theory jn assessing a request for redesignation from nonattainment to attainment. Specif-icaiiy( the theory applied by the EPA is that the entire Chicago urban area is con-gjdered to be one nonattainment zone and that Kane and Du Page Counties are with-jn that nonattainment zone for purposes of assessing their redesignation requests, since the EPA has consistently applied the theory that the entire Chicago urban area js considered one nonattainment area, there js n0 reason to remand this case to the EPA to clarify its ruling. Thus, I respectfully dissent.
This court’s standard of review of a federal agency decision was most recently stated in Motor Vehicle Mfrs. Assn. v. state Farm Mut., 463 U.S. 29, 103 S.Ct. 2856, 77 L.Ed.2d 443 (1983).
“The scope of review under the ‘arbitrary and capricious’ standard is narrow an¿ a court jg not to substitute its judgment for that of the agency. Nevertheless, the agency must examine the relevant data and articulate a satisfactory explanation for its action including a ‘rational connection between the facts found and the choice made.’ ... In reviewing that explanation, we must ‘consider whether the decision was based on a consideration of relevant factors and whether there has been a clear error of judgment.’ ... Normally, an agency rule would be arbitrary and capricious if the agency has relied on factors which Congress has not intended it to consider, entirely failed to consider an important aspect of the problem, offered an explanation for its decision that runs counter to the evidence before the agency, or is so implausible that it could not be ascribed to a difference in view or the product of agency expertise. The re*1152viewing court should not attempt itself to make up for such deficiencies; we may not supply a reasoned basis for the agency’s action that the agency itself has not given____ We will, however, ‘uphold, a decision of less than ideal clarity if the agency’s path may reasonably be discerned.’”
Id. at 43, 103 S.Ct. at 2866 (citations omitted) (emphasis added). The theory the EPA employed in denying Kane and Du Page Counties’ request for redesignation— that Kane and Du Page Counties are part of the Chicago urban area and will be considered to be in nonattainment until the Chicago urban area achieves attainment— can certainly be “reasonably discerned” from the record.
I
Congress passed the Clean Air Act in 1970 and established a combined state and federal program to control air pollution. See Pub.L. No. 91-604, 84 Stat. 1676 et seq. 1970. As part of this program Congress required the EPA to establish a National Ambient Air Quality Standard (“NAAQS”), see sections 108 and 109, and required that each state adopt and submit a “state implementation plan” (“SIP”) to the EPA designed to attain the NAAQS in the designated Air Quality Regions (“AQCRs”), section 107(b)-(c), 42 U.S.C. § 7407(b)-(c). Attainment of the NAAQS was to be achieved by 1977. Currie, Air Pollution, § 6-1 (1981). It became apparent, however, that many of the AQCRs would not meet the deadline for compliance with the Clean Air Act. Id. at 62. Thus, in 1977 Congress added subsection (d) and (e) to Section 107 of the Act. See 42 U.S.C. § 7407(d) and (e). Subsection (d) provides that states “submit to the Administrator a list, together with a summary of the available information, identifying those air quality control regions, or portions thereof,” that either satisfy the NAAQS (“attainment”), failed to satisfy the NAAQS (“nonattainment”) or could not be classified because of a lack of information. Nonattainment was defined as “an area which is shown by monitored data or which is calculated by air quality modeling ... to exceed any national ambient air quality standard____” Section 171, 42 U.S.C. § 7501.1 Each state was required to submit to the EPA a list of those air quality control regions or AQCRs that failed to comply with the NAAQS; the EPA was then to promulgate the list within sixty days of receipt “with such modifications as he [the Administrator] deems necessary.” 42 U.S.C. § 7407(d)(2). For those areas listed as nonattainment, Congress provided that the SIPs were to provide for incremental emission reductions from existing sources in the nonattainment area. 42 U.S.C. § 7502(b)(3). The reductions in pollutants could be achieved through “reasonably available control technology,” id., and through permits restricting the construction and operation of new sources of pollutants in the nonattainment area. 42 U.S.C. §§ 7502(b)(6) and 7503.
In line with these regulations, Illinois in 1978 submitted to the EPA, and the EPA promulgated, a list of nonattainment areas. This list included, among others, Cook, Lake, Du Page, Kane, Will, and McHenry Counties. Indiana also submitted its list to the EPA of nonattainment areas that included Porter and Lake Counties in Indiana. Porter and Lake County, Indiana are located along the southern short of Lake Michigan adjoining the Illinois border. Cook and Lake Counties, Illinois are located on the western shore of Lake Michigan just northwest of Lake and Porter Counties, Indiana, while Du Page and Kane Counties are located southwest of Lake County and immediately west of Cook County, Illinois. The County of Will is located south of Cook County while McHenry County is located north of Kane and Du Page Counties.
On January 27, 1983, the State of Illinois requested that the EPA redesignate Kane *1153and Du Page Counties from nonattainment to attainment status. To support its request, Illinois noted that the ozone monitors located within those counties had registered no violations of the NAAQS for a three-year period. In its proposed ruling, the EPA denied the redesignation request reasoning that Kane and Du Page Counties were part of the Chicago urban area and contributed to the ozone problem in the Chicago urban area because of the prevailing southwest winds during the summer months.
“However, U.S. EPA has determined that the high population densities in Du-Page and eastern Kane Counties are associated with relatively high densities of area and mobile source volatile organic compound emissions. Because the prevailing winds during the ozone season are from south through west, ozone precursors emissions from DuPage and Kane Counties can contribute significantly to ozone NAAQS exceedances which continue to be observed in the Chicago area. U.S. EPA, therefore, proposes to reject the Illinois proposal to redesignate Kane and DuPage Counties to attainment for ozone. These counties will continue to be designated nonattainment until sufficient data is available to warrant a change. These counties must continue to be considered as part of the Chicago urbanized ozone nonattainment area for the purpose of the 1982 SIP.”
48 Fed.Reg. 46082, 46084 (1983). After Illinois submitted comments on the proposed ruling, the EPA, on June 12, 1984, issued its final ruling and denied the state’s request that Kane and Du Page Counties be redesignated to an ozone attainment status.
II
The majority holds that the EPA’s action in denying Kane and Du Page Counties’ request for redesignation from nonattainment to attainment is arbitrary and capricious as the EPA has failed to set forth a consistent theory in approving the boundaries for the nonattainment areas. Essentially, the majority argues that the EPA has been less than consistent in its decision of whether the nonattainment boundaries should be determined on a county or on an urban wide area basis. Specifically, the majority observes that in 1978 the EPA approved a list submitted by Illinois designating the counties in the Chicago area as nonattainment. The majority next notes that the reason Du Page and Kane Counties were denied redesignation to attainment status was that monitors downwind from these counties in the Chicago urban area have been registering ozone violations. The majority concludes that if the EPA considers the county to be the area unit for defining attainment/nonattainment status and these counties are to be held responsible for ozone measured downwind and outside the county boundaries, the EPA should clarify the standards as to when the county is to be held accountable for ozone outside its borders.2 On the other hand, the majority observes that the EPA may have considered the entire urban Chicago area as one nonattainment area (as all the counties within the urban area were listed as nonattainment), but that this designation is inconsistent with the EPA’s approval of Illinois’ list of nonattainment areas defined on a county basis. Thus the majority concludes that if the EPA has changed its policy as to how it defines the attainment/nonattainment area “it must explain how and why; if it has not it must articulate an explanation that will account for both the earlier and the most recent actions it has taken.”
After reviewing the various statements and rulings issued by the EPA since 1978— the year that the states were to submit a list of those areas it considered to be in *1154nonattainment — the record is clear that the EPA has consistently followed the same policy in assessing a redesignation request from counties located in the Chicago urban area: that any county in the Chicago urban area contributing to the Chicago urban area’s ozone problem will remain in nonat-tainment status until the Chicago urban area problem of ozone is adequately controlled and thus reaches attainment status. In denying Kane and Du Page Counties’ request for redesignation, the EPA stated:
“Kane and DuPage counties are part of the Chicago urbanized area and are therefore to be considered nonattainment until the greater Chicago area and its downwind peak impact areas are designated as attainment. This is consistent with EPA’s designation policy for ozone as outlined in the January 3, 1978 memorandum from David G. Hawkins, Assistant Administrator for Air and Waste Management entitled ‘Attainment/Nonattainment Status Designations’____”
49 Fed.Reg. 24128, 24130 (1984) (emphasis added). The Hawkins’ memo discusses the appropriate size of the attainment/nonat-tainment areas.
“Priority should be given to those 105 urban areas with populations greater than 200,000. These areas are to be considered nonattainment for oxidants unless monitored data supports attain-ment____ If a state fails to designate any of these areas or submits unacceptable designations, the Regional Office should make the required designations. The Regional Office requires that the designated areas be of sufficient size to include most of the significant hydrocarbon sources. ”
See Memorandum from David Hawkins to Regional Administrators of USEPA, January 3, 1978 (reprinted in Appellee’s Appendix at A-39) (emphasis added). Thus, for the past seven years, as far back as 1978, it has been the EPA’s position that the entire urban area should be designated as nonat-tainment, including in this area the significant sources of hydrocarbon precursors. This position is also reflected in the comments that the EPA made when it promulgated the list of nonattainment areas that the states submitted to the EPA: “There are 105 urban areas in the United States with populations greater than 200,000. These major urban areas ... are where the oxidant problem is most severe. ” 43 Fed.Reg. 8962-63 (1978) (emphasis added). Further, in 1978, Porter County, Indiana, a part of the Chicago urban area, requested that it be classified as in attainment with the NAAQS; the EPA, however, denied this request noting “[t]he nonattainment designation of Porter County was based on the fact that portions of Porter County are part of the Chicago-northwest Indiana urbanized area.” 43 Fed.Reg. 45997 (1978) (emphasis added). Finally, in its proposed ruling, denying redesignation of Kane and Du Page Counties, the EPA observes that:
“DuPage and eastern Kane Counties are associated with relatively high densities of area and mobile source volatile organic compound emissions. Because the prevailing winds during the ozone season are from south through west, ozone precursors emissions from DuPage and Kane Counties can contribute significantly to ozone NAAQS exceedances which continue to.be observed in the Chicago area.... These counties must continue to be considered as part of the Chicago urbanized ozone nonattainment area____”
48 Fed.Reg. 46084 (1984) (emphasis added). Thus, since 1978 the EPA has been concerned that the entire Chicago urban area be considered one nonattainment area for purposes of determining whether the NAAQS has been satisfied. This areawide designation is consistent with the EPA’s attempt to control the ozone problem. Ozone is an area wide, multi-chemical phenomenon that depends upon the transformations of VOC or hydrocarbon precursors under varying meteorological conditions. State of Texas v. Environmental Protection Agency, 499 F.2d 289, 293 n. 1 (5th Cir.1974), cert. denied, 427 U.S. 905, 96 *1155S.Ct. 3191, 49 L.Ed.2d 1199 (1976).3 Since science to date has been unable to make exact predictions concerning the transportation of ozone, the designation of an area sufficient in size to control the ozone problem is well within the discretion and expertise of the EPA. See Citizens Against Refinery’s Effects v. Environmental Protection Agency, 643 F.2d 183, 186 (4th Cir.1981).
The majority notes that the EPA may have been inconsistent in the theory it applies to determine the appropriate nonat-tainment area as the State originally submitted, and the EPA approved, a list of nonattainment areas based upon the counties boundaries. As the majority concedes, at the time the state submitted its original list to the EPA all counties in the Chicago urban area were listed as nonattainment; and thus the EPA approval of this list does not demonstrate in and of itself that the EPA approved the nonattainment status of this area on a county basis. I would agree with the majority that remand for clarification of the grounds upon which the EPA evaluates a redesignation request would be proper if the approved state list was the only evidence in the administrative record in this case as to the previous action taken by the EPA in approving nonattainment areas. However, as discussed earlier in this dissent, the reports and comments issued by the EPA since 1978 demonstrate that the EPA considered the urban area, sufficient in size to contain the major sources of the VOC precursors, to be the proper size for determining the boundaries of the nonattainment/attainment zone. See 43 Fed.Reg. 8962 (1978); 43 Fed.Reg. 45997 (1978); Memo from David Hawkins to Regional Administrators of USEPA, (January 3, 1978) (Appellee’s Appendix at A-39). Thus, it is clear that the EPA never intended to consider the attainment/nonat-tainment status of the Chicago urban area on a county basis, but rather the EPA considered the entire Chicago urban area as one nonattainment zone.
The majority also notes that the EPA’s theory that the entire Chicago urban area is to be considered as one nonattainment zone is inconsistent with several EPA statements that sources of the ozone pollution must be included within the nonattainment area. See Majority opinion at 12 citing 48 Fed.Reg. 46084 and 49 Fed.Reg. 24130. To support this position the majority argues that peak ozone concentrations may be miles downwind from the source, yet under the urbanized area theory these areas would not be included within the nonattainment zone since these areas may not be considered, under the urban area theory, to be part of the nonattainment area because of their rural setting. For example, the majority argues that for the EPA to be truly consistent in its theory that sources of the ozone are to be included within the nonattainment zone, the counties in southeast Wisconsin must be considered as part of the Chicago urban area for non-attainment purposes since the Chicago area contributes or is a source of ozone in southeast Wisconsin; yet, the EPA does not consider southeastern Wisconsin a part of the Chicago urban area nonattainment zone.4 The majority’s analysis completely misses the point. The EPA, as demonstrated in David Hawkins’ memo in 1978 that suggests the appropriate size for the nonattainment area, was primarily concerned with controlling ozone in urban areas.5 See also 43 Fed.Reg. 8962 (1978); 43 Fed.Reg. 45997 (1978). In order to reach its goal of attaining compliance with the NAAQS, the EPA suggested that the urban area nonattainment zone include the sources of the ozone (i.e. VOC precursors). *1156The EPA clearly did not intend that rural areas (those with less than a population of 200,000) be considered as part of one massive nonattainment designation simply because ozone from large metropolitan areas may be blown into or effect the rural areas;6 rather, the EPA was concerned with controlling ozone in the urban area and the best method to reach this goal was to include the sources of the ozone affecting the urban area’s air quality within a single nonattainment zone. See Western Oil & Gas Assoc. v. EPA, 767 F.2d 603 (9th Cir.1985).7
Further, the fact that the EPA previously approved of the redesignation request of Will and McHenry Counties Illinois from nonattainment to attainment status fails to reach that quantum of legal reasoning to convince me that the EPA has acted arbitrarily and capriciously in denying Kane and Du Page Counties’ request for redesig-nation. In rejecting Illinois argument that Kane and Du Page Counties should be approved for redesignation simply because Will and McHenry Counties were previously approved for redesignation on the basis that ozone monitored within those counties were within the NAAQS, the EPA noted:
“The main reason EPA concurred with Illinois’ redesignation request for McHenry and Will Counties was that these areas contained essentially none of the Chicago urbanized area or adjacent fringed areas of development. EPA was aware of the significant YOC emissions for Will County. It was determined that these emissions were dominated by stationary source emissions. These emissions have been and will continue to be significantly reduced as a result of Illinois RACT Regulations.” 49 Fed.Reg. 24128, 24130 (1980).
The population of both Will and McHen-ry Counties was less than 200,000 during the period the EPA approved their redesig-nation to attainment status.8 Thus, the EPA decision to approve the redesignation of Will and McHenry Counties was consistent with its policy that counties with a population of less than 200,000 people were not considered to be part of the urban nonattainment area as defined in the 1970 census.9 In contrast, both Kane and Du *1157Page Counties’ populations are greater than 200,000 people, and both are sources of ozone precursors that contribute to the ozone in the Chicago urban area. Thus, the fact that the EPA had previously approved the redesignation of Will and Kane Counties does not mean that the EPA has not acted consistently in applying its policy that those counties within the urban area of Chicago would be considered as part of one nonattainment area.
The scope of our review under the “arbitrary and capricious” standard is limited and we are not to substitute our judgment for that of the agency. Where the agency’s decision is not as precise as it may be, we are obliged to uphold the decision “if the agency’s path may reasonably be discerned.” Motor Vehicle Mfrs. Assn., 463 U.S. at 43, 103 S.Ct. at 2867. The majority argues that the record reveals the EPA has not consistently applied the same theory as to what it believes to be the proper boundaries of a nonattainment area. However, the EPA’s path may reasonably and very easily be discerned in this case as a review of the administrative record reveals that since the attainment/nonattainment designation was introduced in 1978, the EPA considered the entire urban area, including those areas that are sources of the ozone in the urban areas, to be one zone for purposes of establishing that area’s attainment/nonattainment status. Thus, if we are to follow the proper standard of review in assessing the decision of a federal agency, I am unable to agree with the majority’s limited remand to clarify and further explain the theory the EPA followed in assessing the proper boundaries of the non-attainment areas. I respectfully dissent.
. In 1979, the EPA revised the federal chemical oxidant NAAQS and relaxed the standard from 0.08 part per million ("ppm") to 0.12 ppm and changed the chemical designation of the standard from photochemical oxidants to ozone. 40 C.F.R. § 50.9 (1984).
. Presently, the guidelines indicate that the status of whether an area is considered attainment or nonattainment is determined by monitors located within the designated nonattainment area. See Guidelines for the Interpretation of Ozone Air Quality Standards (EPA 450/4-79003).
. VOC or hydrocarbon precursors are those emissions from smokestack industries and automobiles that when mixed with oxygen and sunlight under the right meteorological conditions help to create ozone. State of Texas, 499 F.2d at 293 n. 1.
. It should be noted that we have no knowledge to date of the EPA approving single nonattainment areas across state boundary lines.
. As noted by Mr. Hawkins, "[P]riority should be given to those 105 urban areas with populations greater than 200,000.”
. See Hawkins Memo, supra, noting that the approved designation zone for rural areas could be based upon county boundaries.
. See abo Memo from David Hawkins, Assistant Administrator for Air and Waste Management, January 3, 1978 (Appellee’s Appendix at A-39) ("Priority should be given to those 105 urban areas with populations greater than 200,000. These areas are to be considered nonattainment for oxidants unless monitored data supports attainment ... the Regional Office should require that the designated area be of sufficient size to include most of the significant hydrocarbon sources.”) (oxidant is a technical term for ozone); Memo from Helms, Chief Control Programs Operations Branch, March 5, 1984 (“In general, nonattainment areas should be large enough to include both the areas where the monitored violations occur and the areas where the sources causing these violations are located. The urbanized area should be the minimum size for ozone____ As you know, this reflects EPA’s policy since the first Section 107 designations were made on March 3, 1978 (43 F.R. 8962-9059)”).
. See Chart of Populations of Urbanized Areas: 1970 and 1960 (Appellee’s Appendix at A-485) (listing the populations of Will and McHenry Counties as less than 200,000 people).
. Memo from David Hawkins, Assistant Administrator for Air and Waste Management, January 3, 1978 (Appellee’s Appendix at 39) (Noting that for those areas whose population does not exceed 200,000 the "county should be designated as a minimum [for purposes for designating a nonattainment zone] unless the county is exceptionally large and the extent of the violations is minimal.’’) Letter from Darryl Tyler, Director Control Programs Development Division, March 2, 1984, (Noting that for ozone the area of nonattainment must include the urbanized area "as defined by the U.S. Bureau of Census" and other fringe areas with significant volatile organic compound sources).
In its brief, the EPA admits that if anything it may have made a mistake in redesignating Will County because the businesses within that county do contain a large number of stationary VOC sources. The EPA notes, however, that in redes-ignating Will County it acted in a consistent manner with its policy that those areas not considered to be part of the urban area could have the attainment/nonattainment boundaries defined by their county borders. Moreover, the EPA believed that it had the authority to redes-ignate Will County as nonattainment if Illinois failed to promulgate and enforce the VOC controls. See 43 Fed.Reg. 40412. However, our court has since held that once an area is redesig-nated, the EPA may not unilaterally change the *1157designation. Bethlehem Steel Corp. v. EPA, 723 F.2d 1303 (7th Cir.1983). Finally, the sources of VOCs in Will County are stationary and thus easier to control through implementation of various SIP and RACT programs. However, because of the large population located in Du Page and Kane Counties, a large source of VOCs is vehicular traffic, which makes it more difficult to control the source problem.