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Effluent Guidelines

Toxic and Priority Pollutants Under the Clean Water Act

""The Toxic Pollutant List was developed in 1976 and subsequently added to the Clean Water Act by Congress in 1977. The list was intended to be used by EPA and states as a starting point to ensure that Effluent Guidelines regulations, water quality criteria and standards, and NPDES permit requirements addressed the problems of toxics in waterways. However, this list consisted of broad categories of pollutants rather than specific, individual pollutants. Therefore, EPA developed the Priority Pollutant List in 1977 to make implementation of the Toxic Pollutant List more practical for water testing and regulatory purposes.

Portions of both lists are outdated. For example, they contain some pesticides that have not been manufactured in the United States for many years and are therefore unlikely to be discharged into surface waters. As such, the pollutants on these lists are not the only ones regulated under the CWA programs. EPA considers a wider range of pollutants—toxic, conventional and nonconventional—when developing Effluent Guidelines. (See Learn about Effluent Guidelines.) Neither Congress nor EPA have updated either list, but EPA is providing them here as a reference because EPA, states, and authorized tribes continue to consult the lists when developing ambient water quality criteria pursuant to CWA sections 303(c)(2)(B) and 304(a). (See Water Quality Standards: Regulations and Resources)

On this page:

Toxic Pollutant List

Key Features

  • The Clean Water Act references the Toxic Pollutant List at section 307(a)(1); 33 U.S.C. 1317(a)(1).
  • The list appears in the Code of Federal Regulations at 40 CFR 401.15.
  • The list contains 65 entries. Many of the entries, such as "haloethers," are for groups of pollutants.

Relationship between CWA Section 307(a)(1) and the Toxic Pollutant List

  • Section 307(a)(1) says:
    ...the list of toxic pollutants or combination of pollutants subject to this Act shall consist of those toxic pollutants listed in table 1 of Committee Print Numbered 95-30 of the Committee on Public Works of the House of Representatives…
  • Committee Print 95-30 (November 1977) is titled "Data Relating to H.R. 3199 (Clean Water Act of 1977)."
  • Table 1 is titled "Section 307—Toxic Pollutants." EPA codified Table 1 at 40 CFR 401.15.

History of the Toxic Pollutant List

  • Source of the list: The list was negotiated among parties to a settlement agreement (NRDC et al. vs Train, 6 ELR 20588 Exit, D.D.C. June 9, 1976). This agreement is sometimes referred to as the "Toxics Consent Decree" or the "Flannery Decision" (for presiding U.S. District Court Judge Thomas A. Flannery).
  • Congress subsequently ratified the Settlement Agreement and the Toxic Pollutant List when it amended the CWA in 1977 (Pub. L. 95-217, December 27, 1977 (PDF)).(46 pp, 7.7 MB, About PDF)
  • Background document:
    Copeland, Claudia. Congressional Research Service (1993). "Toxic Pollutants and the Clean Water Act: Current Issues." Report No. 93-849. Exit
  • The list was first published on January 31, 1978 in the Federal Register (43 FR 4108).
  • On July 31, 1979 (44 FR 44501), EPA published the list again and added the list to the CFR at 40 CFR 401.15.
  • On November 8, 1983 (48 FR 51405), EPA added regulations at 40 CFR 131.11 that require states and authorized tribes to adopt water quality criteria, sufficient enough to protect the designated use, for section 307(a) toxic pollutants.
  • On February 4, 1987, Congress amended CWA section 303(c)(2) by adding subparagraph (B) which requires that a state, whenever reviewing, revising, or adopting new water quality standards, must adopt numeric criteria for all toxic pollutants listed pursuant to section 307(a)(1) for which criteria have been published under section 304(a).
  • For the 14 states that did not timely adopt such numeric criteria, EPA promulgated the National Toxics Rule (NTR) on December 22, 1992 (57 FR 60848). The NTR uses the criteria listed in section 304(a) to promulgate numeric criteria for states that had not yet adopted numeric criteria for those toxic pollutants.


  • EPA removed three pollutants from the list in 1981, after determining that their chemical properties did not justify their inclusion:
    • Dichlorodifluoromethane and trichlorofluoromethane were de-listed on January 8, 1981 (46 FR 2266) at the request of E.I. duPont de Nemours and Co. because of low solubility in water and high volatility combined with low human and mammalian toxicity. Bis(chloromethyl) ether was de-listed on February 4, 1981 (46 FR 10723) based on data that indicated a half-life in water of 38 seconds at 20°C.
  • De-listing the three pollutants did not change the 65 entries because the three de-listed pollutants were specific compounds within entries for the groups Halomethanes (list entry 38) and Haloethers (list entry 37).

Priority Pollutant List


Key features of the Priority Pollutant List and its relationship to the Toxic Pollutant List:
  • The Priority Pollutants are a set of chemical pollutants EPA regulates, and for which EPA has published analytical test methods.
  • The Priority Pollutant List makes the list of toxic pollutants more usable, in a practical way, for the purposes assigned to EPA by the Clean Water Act. For example, the Priority Pollutant list is more practical for testing and for regulation in that chemicals are described by their individual chemical names. The list of toxic pollutants, in contrast, contains open-ended groups of pollutants, such as "chlorinated benzenes." That group contains hundreds of compounds; there is no test for the group as a whole, nor is it practical to regulate or test for all of these compounds.


Starting with the list of toxic pollutants, EPA used four criteria to select and prioritize specific pollutants:
  • We included all pollutants specifically named on the list of toxic pollutants;
  • There had to be a chemical standard available for the pollutant, so that testing for the pollutant could be performed;
  • The pollutant had to have been reported as found in water with a frequency of occurrence of at least 2.5 percent, and
  • The pollutant had to have been produced in significant quantities, as reported in Stanford Research Institute's "1976 Directory of Chemical Producers, USA."

Number of Entries

Originally, there were 129. When three pollutants were removed from the list of toxic pollutants in 1981 (see "Modifications" above), they were also removed from the Priority Pollutant List.
  • Entry numbers 17, 49, and 50 were removed.
  • The last number on the list is still 129, although there are 126 entries.


Q: Why is the Priority Pollutant List published at 40 CFR Part 423, Appendix A, rather than at Part 401, or some other, more general section?
  • One of the first industrial categories for which EPA developed Effluent Guidelines was the Steam Electric Power Generating Category (40 CFR Part 423). The Priority Pollutant List was included to support regulations for that category.
  • Although the other sections within Part 423 apply only to the Steam Electric Power Generating Category, the Priority Pollutant List in Appendix A is not limited in terms of its relevance to that one industrial category.
  • Some users find it helpful to think of Appendix A to Part 423 as a convenient storage place for the list, or as a matter of convenience for citation.

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