Regulation Database – Endangerment

Before regulating a new pollutant under certain Clean Air Act programs, EPA must make a finding that the pollutant endangers public health or welfare, and that emissions targeted by the regulations cause or contribute to such pollution.  In Massachusetts v. EPA, 549 U.S. 497 (2007), the Supreme Court concluded that greenhouse gases are air pollutants under the Clean Air Act, and that EPA must determine under section 202(a) of the Clean Air Act whether or not greenhouse gas emissions from new motor vehicles cause or contribute to air pollution that would endanger public health or welfare. On December 7, 2009, EPA promulgated an “endangerment finding” for six greenhouse gases, finding them to threaten public health and welfare: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). At the same time, EPA promulgated a “cause or contribute finding” for new motor vehicles, concluding that vehicular emissions of these greenhouse gases contribute to the endangerment created by greenhouse gas pollution.

EPA used the December 2009 endangerment and contribution findings as a basis for subsequent motor vehicle emission standards. EPA also extensively referenced the endangerment finding in proposing and finalizing subsequent emission standards for existing power plantsnew and modified power plants, and the oil and gas sector.


Twenty-seven cases challenging the Endangerment Finding were consolidated in Utility Air Regulatory Group v. Environmental Protection Agency (D.C. Cir. 2012). In June 2012 the Court of Appeals for the DC Circuit Court upheld the Endangerment Finding, in addition to the Tailpipe Rule, holding that the EPA’s interpretation of the governing Clean Air Act provisions are unambiguously correct.

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In July 2015, EPA proposed a rule under section 231(a) of the Clean Air Act finding that greenhouse gas emissions from certain aircraft engines contribute to air pollution that endangers public health and welfare. EPA referenced the rational and scientific backing for the 2009 endangerment finding as support for the proposed rule. In August 2016, EPA published the final rule. Aircraft covered by the rule include small jet aircraft such as the Cessna Citation CJ2+ and the Embraer E170, up to and including the largest commercial jet aircrafts—the Airbus A380 and the Boeing 747.

The July 2015 proposed rule also included an advanced notice of proposed rulemaking for greenhouse gas emissions standards for aircraft, which would be developed in coordination with international standards under the International Civil Aviation Organization.  In making the endangerment and contribution findings, the August 2016 rule anticipated future action to propose domestic greenhouse gas emission standards for aircraft under a separate notice-and-comment process.

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