Regulation Database – Non-GHG Emission Standards

In 2015, based on EPA’s review of the air quality criteria for ozone (O3) and related photochemical oxidants and for O3, EPA revised the levels of both standards. EPA revised the primary and secondary ozone standard levels to 0.070 parts per million (ppm), and retained their indicators (O3), forms (fourth-highest daily maximum, averaged across three consecutive years) and averaging times (eight hours).

While EPA did not quantify the greenhouse gas reduction co-benefits of this rule, it did recognize in an implementation memo that sources of ozone pollution may generate GHG emissions (e.g., motor vehicles) and thus actions to reduce ozone pollution will also have GHG co-benefits.

Following a review, in December 2020 the EPA decided not to update the 2015 standards.

In May 2011, EPA proposed National Emission Standards for Hazardous Air Pollutants (NESHAP) for coal and oil-fired electric utility steam generating units, commonly referred to as the Mercury and Air Toxics Standards (MATS) rule, which would require facilities to achieve an emissions rate for mercury and air toxics consistent with the implementation of the maximum available control technology (MACT). As part of the same rulemaking, EPA also proposed New Source Performance Standards (NSPS) for criteria pollutants (PM, NOx, and SO2) from fossil fuel-fired electric utility, industrial, commercial, and institutional steam generating units.

The final MATS / NSPS Rule was published in February 2012. Responding to an industry challenge, EPA revised certain aspects of the rule in April 2013 to soften standards for new power plants. In November 2014, EPA revised certain aspects of the rule pertaining to startup and shutdown standards. In April 2016, EPA issued technical corrections to the rule.


In Michigan v. EPA, 135 S.Ct. 2699 (2015), the U.S. Supreme Court ruled that EPA had improperly failed to consider compliance costs at the outset of developing the MATS Rule. The rule remained in effect pending remand to EPA, and in April 2016, EPA issued a supplemental finding considering compliance costs and concluding that the MATS Rule was appropriate and necessary.

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Deregulatory Action:

On May 22, 2020, EPA published in the Federal Register a reconsideration of the April 2016 supplemental cost finding while ensuring that power plants will emit no more mercury to the air than before.  EPA concluded that it is not "appropriate and necessary" to regulate mercury from coal-fired power plants. The decision relies on EPA's rejection the value of co-benefits — reductions in other pollutants not directly covered by MATS and a key part of the original cost-benefit calculus. Although EPA did not rescind the MATS rule, advocates worry that its refusal to consider co-benefits will make it more difficult to justify future rulemakings that are protective of public health.


On June 19, 2020, a coalition of environmental and civil rights groups challenged the "appropriate and necessary" finding in court.

Reregulatory Action:

On January 31, 2022, the Environmental Protection Agency (EPA) proposed revoking the 2020 rule that stated it was not appropriate and necessary to regulate mercury and air toxics (MATS) from coal- and oil-fired electric utility steam generating units (EGUs) under Clean Air Act (CAA) Section 112. EPA’s January 2021 proposed rule would also reaffirm the pre-2020 finding that it is appropriate and necessary to regulate hazardous air pollutant (HAP) emissions from EGUs after considering cost.

In August 2011, EPA published the final Cross-State Air Pollution Rule (CSAPR). The rule replaced EPA’s 2005 Clean Air Interstate Rule (CAIR). The rule establishes a regulatory framework for reducing interstate sulfur dioxide (SO2) and nitrogen oxide (NOx) emissions from power plants in “upwind” states that contribute to non-attainment with, or impaired maintenance of, National Ambient Air Quality Standards (NAAQS) for ozone and fine particulate matter (PM2.5) in “downwind” states. In December 2011, EPA supplemented the final rule to cover additional states for certain pollution. EPA has also issued minor revisions to the rule’s compliance deadlines since it has been finalized. As of January 2017, the CSAPR requires 28 states in the eastern United States to reduce power plant emissions of SO2, annual NOX, and ozone season NOX affecting downwind states.

The U.S. Supreme Court upheld the CSAPR in EPA v. EME Homer City Generation, L.P., 134 S. Ct. 1584 (2014), reversing the D.C. Circuit Court of Appeals’ 2012 overturn of the rule. With the litigation ended, EPA began implementing Phase I of the CSAPR in 2015. EPA is scheduled to begin implementing Phase II in 2017, which would require certain states to make additional reductions in SO2 emissions.

In September 2016, EPA finalized Federal Implementation Plans (FIPs) in the CSAPR Update to address air quality impacts of the interstate transport of ozone air pollution in the eastern United States.

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Deregulatory Action:

On December 6, 2018, EPA published final rule announcing its determination that the 2016 CSAPR Update fully satisfies “good neighbor” obligations for the 2008 ground-level ozone NAAQS. The Rule is called the “Close-Out” Rule, because it will not require further emissions reductions from upwind states.


On January 30, 2019, a coalition of states asked the DC Circuit to review the CSAPR Close-Out Rule. New York v. EPA, No. 19-1019 (D.C. Cir.). A coalition of environmental groups also filed a petition for review. On September 13, 2019, in Wisconsin v. EPA, the D.C. Circuit largely upheld the 2016 CSAPR Update, except for a provision that allows upwind states to continue to pollute downwind states past the statutory deadline for downwind states to comply with the NAAQS. This portion of the rule is sent back to EPA for review. On October 1, 2019, the DC Circuit rejected EPA’s CSAPR Close-Out Rule, finding that EPA has not implemented stringent enough rules to deal with cross-state air pollution affecting downwind states. On October 30, 2020, EPA issued a proposed rule in response to the D.C. Circuit’s remand in Wisconsin v. EPA. EPA finalized the rule on March 15, 2021.

Regulation on Use of Science at EPA

Deregulatory Action: In April 2018, EPA proposed the Strengthening Transparency in Regulatory Science Rule, which would restrict EPA to relying only on scientific research for which the underlying data has been made available to the general public. Commenters warned that researchers are often unable to share the confidential information of participants in public health studies. In the revised proposal, EPA offers two alternatives: 1) prohibiting EPA from relying on studies for which the underlying data is not available for validation; or (2) allowing EPA to use such studies, but giving them lesser weight than those for which the underlying data is available. EPA is seeking comment on which approach should be adopted.

On January 5, 2021, EPA finalized a rule to limit the use of scientific studies for which underlying data is not publicly available. The final rule applies to scientific data that the agency uses in crafting regulations as well as all "influential scientific information" at the agency, a broad term that could apply to informal guidance or other agency actions.

Litigation: On January 19, 2021, a coalition of states challenged the rule in the District Court for the Southern District of New York. (S.D.N.Y. 1:21-cv-00462). A parallel challenge was filed in the District Court for the District of Montana, which on February 1 vacated the rule at the request of the EPA under the administration.

Revised Clean Air Act Cost-Benefit Analysis

Deregulatory action: On December 9, 2020, the EPA finalized a rule to create standard practices for analyzing new regulations' costs and benefits under the Clean Air Act. The rule could be used to limit the agency's consideration of co-benefits when developing and justifying new regulations. This could constrain the agency's ability to consider climate regulations' ancillary benefits, such as reductions in conventional pollutants like soot.

Litigation: On January 19, 2021, a coalition of states challenged the rule in the D.C. Circuit.

Reregulatory action: On May 13, 2021, EPA rescinded the rule and requested public comments.