{"data":{"id":"us/40-cfr-51.301","jurisdiction":"us","citation":"40 CFR 51.301","heading":"Definitions.","body":"For purposes of this subpart:\nAdverse impact on visibility means, for purposes of section 307, visibility impairment which interferes with the management, protection, preservation, or enjoyment of the visitor's visual experience of the Federal Class I area. This determination must be made on a case-by-case basis taking into account the geographic extent, intensity, duration, frequency and time of visibility impairments, and how these factors correlate with (1) times of visitor use of the Federal Class I area, and (2) the frequency and timing of natural conditions that reduce visibility. This term does not include effects on integral vistas.\nAgency means the U.S. Environmental Protection Agency.\nBART-eligible source means an existing stationary facility as defined in this section.\nBaseline visibility condition means the average of the five annual averages of the individual values of daily visibility for the period 2000-2004 unique to each Class I area for either the most impaired days or the clearest days.\nBest Available Retrofit Technology (BART) means an emission limitation based on the degree of reduction achievable through the application of the best system of continuous emission reduction for each pollutant which is emitted by an existing stationary facility. The emission limitation must be established, on a case-by-case basis, taking into consideration the technology available, the costs of compliance, the energy and nonair quality environmental impacts of compliance, any pollution control equipment in use or in existence at the source, the remaining useful life of the source, and the degree of improvement in visibility which may reasonably be anticipated to result from the use of such technology.\nBuilding, structure, or facility means all of the pollutant-emitting activities which belong to the same industrial grouping, are located on one or more contiguous or adjacent properties, and are under the control of the same person (or persons under common control). Pollutant-emitting activities must be considered as part of the same industrial grouping if they belong to the same Major Group (i.e., which have the same two-digit code) as described in the Standard Industrial Classification Manual, 1972 as amended by the 1977 Supplement (U.S. Government Printing Office stock numbers 4101-0066 and 003-005-00176-0 respectively).\nClearest days means the twenty percent of monitored days in a calendar year with the lowest values of the deciview index.\nCurrent visibility condition means the average of the five annual averages of individual values of daily visibility for the most recent period for which data are available unique to each Class I area for either the most impaired days or the clearest days.\nDeciview is the unit of measurement on the deciview index scale for quantifying in a standard manner human perceptions of visibility.\nDeciview index means a value for a day that is derived from calculated or measured light extinction, such that uniform increments of the index correspond to uniform incremental changes in perception across the entire range of conditions, from pristine to very obscured. The deciview index is calculated based on the following equation (for the purposes of calculating deciview using IMPROVE data, the atmospheric light extinction coefficient must be calculated from aerosol measurements and an estimate of Rayleigh scattering):\nDeciview index = 10 ln (bext/10 Mm−1).\nbext = the atmospheric light extinction coefficient, expressed in inverse megameters (Mm−1).\nEnd of the applicable implementation period means December 31 of the year in which the next periodic comprehensive implementation plan revision is due under § 51.308(f).\nExisting stationary facility means any of the following stationary sources of air pollutants, including any reconstructed source, which was not in operation prior to August 7, 1962, and was in existence on August 7, 1977, and has the potential to emit 250 tons per year or more of any air pollutant. In determining potential to emit, fugitive emissions, to the extent quantifiable, must be counted.\nFossil-fuel fired steam electric plants of more than 250 million British thermal units per hour heat input,\nCoal cleaning plants (thermal dryers),\nKraft pulp mills,\nPortland cement plants,\nPrimary zinc smelters,\nIron and steel mill plants,\nPrimary aluminum ore reduction plants,\nPrimary copper smelters,\nMunicipal incinerators capable of charging more than 250 tons of refuse per day,\nHydrofluoric, sulfuric, and nitric acid plants,\nPetroleum refineries,\nLime plants,\nPhosphate rock processing plants,\nCoke oven batteries,\nSulfur recovery plants,\nCarbon black plants (furnace process),\nPrimary lead smelters,\nFuel conversion plants,\nSintering plants,\nSecondary metal production facilities,\nChemical process plants,\nFossil-fuel boilers of more than 250 million British thermal units per hour heat input,\nPetroleum storage and transfer facilities with a capacity exceeding 300,000 barrels,\nTaconite ore processing facilities,\nGlass fiber processing plants, and\nCharcoal production facilities.\nFederal Class I area means any Federal land that is classified or reclassified Class I.\nFederal Land Manager means the Secretary of the department with authority over the Federal Class I area (or the Secretary's designee) or, with respect to Roosevelt-Campobello International Park, the Chairman of the Roosevelt-Campobello International Park Commission.\nFederally enforceable means all limitations and conditions which are enforceable by the Administrator under the Clean Air Act including those requirements developed pursuant to parts 60 and 61 of this title, requirements within any applicable State Implementation Plan, and any permit requirements established pursuant to § 52.21 of this chapter or under regulations approved pursuant to part 51, 52, or 60 of this title.\nFixed capital cost means the capital needed to provide all of the depreciable components.\nFugitive Emissions means those emissions which could not reasonably pass through a stack, chimney, vent, or other functionally equivalent opening.\nGeographic enhancement for the purpose of § 51.308 means a method, procedure, or process to allow a broad regional strategy, such as an emissions trading program designed to achieve greater reasonable progress than BART for regional haze, to accommodate BART for reasonably attributable impairment.\nImplementation plan means, for the purposes of this part, any State Implementation Plan, Federal Implementation Plan, or Tribal Implementation Plan.\nIndian tribe or tribe means any Indian tribe, band, nation, or other organized group or community, including any Alaska Native village, which is federally recognized as eligible for the special programs and services provided by the United States to Indians because of their status as Indians.\nIn existence means that the owner or operator has obtained all necessary preconstruction approvals or permits required by Federal, State, or local air pollution emissions and air quality laws or regulations and either has (1) begun, or caused to begin, a continuous program of physical on-site construction of the facility or (2) entered into binding agreements or contractual obligations, which cannot be cancelled or modified without substantial loss to the owner or operator, to undertake a program of construction of the facility to be completed in a reasonable time.\nIn operation means engaged in activity related to the primary design function of the source.\nInstallation means an identifiable piece of process equipment.\nIntegral vista means a view perceived from within the mandatory Class I Federal area of a specific landmark or panorama located outside the boundary of the mandatory Class I Federal area.\nLeast impaired days means the twenty percent of monitored days in a calendar year with the lowest amounts of visibility impairment.\nMajor stationary source and major modification mean major stationary source and major modification, respectively, as defined in § 51.166.\nMandatory Class I Federal Area or Mandatory Federal Class I Area means any area identified in part 81, subpart D of this title.\nMost impaired days means the twenty percent of monitored days in a calendar year with the highest amounts of anthropogenic visibility impairment.\nNatural conditions reflect naturally occurring phenomena that reduce visibility as measured in terms of light extinction, visual range, contrast, or coloration, and may refer to the conditions on a single day or a set of days. These phenomena include, but are not limited to, humidity, fire events, dust storms, volcanic activity, and biogenic emissions from soils and trees. These phenomena may be near or far from a Class I area and may be outside the United States.\nNatural visibility means visibility (contrast, coloration, and texture) on a day or days that would have existed under natural conditions. Natural visibility varies with time and location, is estimated or inferred rather than directly measured, and may have long-term trends due to long-term trends in natural conditions.\nNatural visibility condition means the average of individual values of daily natural visibility unique to each Class I area for either the most impaired days or the clearest days.\nPotential to emit means the maximum capacity of a stationary source to emit a pollutant under its physical and operational design. Any physical or operational limitation on the capacity of the source to emit a pollutant including air pollution control equipment and restrictions on hours of operation or on the type or amount of material combusted, stored, or processed, shall be treated as part of its design if the limitation or the effect it would have on emissions is federally enforceable. Secondary emissions do not count in determining the potential to emit of a stationary source.\nPrescribed fire means any fire intentionally ignited by management actions in accordance with applicable laws, policies, and regulations to meet specific land or resource management objectives.\nReasonably attributable means attributable by visual observation or any other appropriate technique.\nReasonably attributable visibility impairment means visibility impairment that is caused by the emission of air pollutants from one, or a small number of sources.\nReconstruction will be presumed to have taken place where the fixed capital cost of the new component exceeds 50 percent of the fixed capital cost of a comparable entirely new source. Any final decision as to whether reconstruction has occurred must be made in accordance with the provisions of § 60.15 (f) (1) through (3) of this title.\nRegional haze means visibility impairment that is caused by the emission of air pollutants from numerous anthropogenic sources located over a wide geographic area. Such sources include, but are not limited to, major and minor stationary sources, mobile sources, and area sources.\nSecondary emissions means emissions which occur as a result of the construction or operation of an existing stationary facility but do not come from the existing stationary facility. Secondary emissions may include, but are not limited to, emissions from ships or trains coming to or from the existing stationary facility.\nSignificant impairment means, for purposes of § 51.303, visibility impairment which, in the judgment of the Administrator, interferes with the management, protection, preservation, or enjoyment of the visitor's visual experience of the mandatory Class I Federal area. This determination must be made on a case-by-case basis taking into account the geographic extent, intensity, duration, frequency and time of the visibility impairment, and how these factors correlate with (1) times of visitor use of the mandatory Class I Federal area, and (2) the frequency and timing of natural conditions that reduce visibility.\nState means “State” as defined in section 302(d) of the CAA.\nStationary Source means any building, structure, facility, or installation which emits or may emit any air pollutant.\nVisibility means the degree of perceived clarity when viewing objects at a distance. Visibility includes perceived changes in contrast, coloration, and texture elements in a scene.\nVisibility impairment or anthropogenic visibility impairment means any humanly perceptible difference due to air pollution from anthropogenic sources between actual visibility and natural visibility on one or more days. Because natural visibility can only be estimated or inferred, visibility impairment also is estimated or inferred rather than directly measured.\nVisibility in any mandatory Class I Federal area includes any integral vista associated with that area.\nWildfire means any fire started by an unplanned ignition caused by lightning; volcanoes; other acts of nature; unauthorized activity; or accidental, human-caused actions, or a prescribed fire that has developed into a wildfire. A wildfire that predominantly occurs on wildland is a natural event.\nWildland means an area in which human activity and development is essentially non-existent, except for roads, railroads, power lines, and similar transportation facilities. Structures, if any, are widely scattered.","path":["Title 40—Protection of Environment","CHAPTER I—ENVIRONMENTAL PROTECTION AGENCY","SUBCHAPTER C—AIR PROGRAMS","PART 51—REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF IMPLEMENTATION PLANS","Subpart P—Protection of Visibility"],"source_url":"https://www.ecfr.gov/api/versioner/v1/full/2026-08-25/title-40.xml","current_through":"2026-08-25","vintage":"","retrieved_at":"2026-08-27T02:26:04Z","sha256":"4c712ef9f44c4d71bf3844ab040da9bbbbc0c8a6537581aa2c17426d59983830","source_id":"us-cfr","stale":true,"prev":"us/40-cfr-51.300","next":"us/40-cfr-51.302"},"notice":"GroundRules: Original legal text. Not legal advice."}