CFR > Title 40 - Protection of Environment > CHAPTER I—ENVIRONMENTAL PROTECTION AGENCY > SUBCHAPTER C—AIR PROGRAMS (Parts 50–99) > PART 50—NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY STANDARDS > 50.1—Definitions.

50.1—Definitions.

(a) As used in this part, all terms not defined herein shall have the meaning given them by the Act.
(b) Act means the Clean Air Act, as amended (42 U.S.C. 1857-185 71, as amended by Pub. L. 91-604).
(c) Agency means the Environmental Protection Agency.
(d) Administrator means the Administrator of the Environmental Protection Agency.
(e) Ambient air means that portion of the atmosphere, external to buildings, to which the general public has access.
(f) Reference method means a method of sampling and analyzing the ambient air for an air pollutant that is specified as a reference method in an appendix to this part, or a method that has been designated as a reference method in accordance with part 53 of this chapter; it does not include a method for which a reference method designation has been cancelled in accordance with § 53.11 or § 53.16 of this chapter.
(g) Equivalent method means a method of sampling and analyzing the ambient air for an air pollutant that has been designated as an equivalent method in accordance with part 53 of this chapter; it does not include a method for which an equivalent method designation has been cancelled in accordance with § 53.11 or § 53.16 of this chapter.
(h) Traceable means that a local standard has been compared and certified either directly or via not more than one intermediate standard, to a primary standard such as a National Bureau of Standards Standard Reference Material (NBS SRM), or a USEPA/NBS-approved Certified Reference Material (CRM).
(i) Indian country is as defined in 18 U.S.C. 1151.
(j) Exceptional event means an event that affects air quality, is not reasonably controllable or preventable, is an event caused by human activity that is unlikely to recur at a particular location or a natural event, and is determined by the Administrator in accordance with 40 CFR 50.14 to be an exceptional event. It does not include stagnation of air masses or meteorological inversions, a meteorological event involving high temperatures or lack of precipitation, or air pollution relating to source noncompliance.
(k) Natural event means an event in which human activity plays little or no direct causal role.
(l) Exceedance with respect to a national ambient air quality standard means one occurrence of a measured or modeled concentration that exceeds the specified concentration level of such standard for the averaging period specified by the standard.

Code of Federal Regulations

[36 FR 22384, Nov. 25, 1971, as amended at 41 FR 11253, Mar. 17, 1976; 48 FR 2529, Jan. 20, 1983; 63 FR 7274, Feb. 12, 1998; 72 FR 13580, Mar. 22, 2007]