## Facts

- EPA revised the primary annual National Ambient Air Quality Standard (NAAQS) for fine particulate matter (PM₂.₅) under Clean Air Act (CAA) §109, lowering it from 15.0 µg/m³ to 12.0 µg/m³ while retaining the 24-hour standard.
- EPA relied largely on epidemiological evidence associating long-term PM₂.₅ exposure with adverse health outcomes at concentrations below the prior standard and set the standard with an “adequate margin of safety.”
- The notice of proposed rulemaking solicited comment on “all issues” related to lowering the PM₂.₅ standard; industry groups and states submitted comments opposing tightening and citing contrary studies.
- The rule addressed compliance measurement and monitoring, including (i) declining to permit certain uses of “spatial averaging” across monitors because it could mask localized higher-concentration areas and (ii) requiring near-roadway monitors in large metropolitan areas to better capture exposures near major roads.
- Petitioners sought review under CAA §307(b)(1), challenging the tightened standard, notice-and-comment adequacy, EPA’s scientific reasoning and responses to comments, monitoring choices, and the absence of contemporaneous implementation guidance.

## Issues

1. Whether EPA provided adequate notice and opportunity to comment on the question of whether to revise the PM₂.₅ NAAQS.
2. Whether EPA acted arbitrarily or capriciously in tightening the annual PM₂.₅ standard based on the scientific record.
3. Whether EPA adequately responded to significant comments and studies submitted by petitioners.
4. Whether EPA reasonably explained its decision not to allow spatial averaging for compliance determinations.
5. Whether EPA had authority and a reasonable basis to require near-roadway monitoring in large metropolitan areas.
6. Whether the CAA required EPA to issue implementation guidance, or delay effectiveness, before promulgating or requiring compliance with the revised NAAQS.

## Decision

- The D.C. Circuit denied the petitions for review and upheld the rule in full.
- The court held the proposal’s request for comment on “all issues” concerning lowering the standard provided adequate notice, including for arguments that EPA should not revise the NAAQS at all.
- The court concluded EPA reasonably weighed the scientific evidence and explained why the record supported tightening the standard to protect public health with an adequate margin of safety.
- The court held EPA adequately addressed significant comments without needing to respond separately to every cited study or data point.
- The court upheld EPA’s rejection of spatial averaging as reasonably explained to avoid masking localized higher exposures.
- The court upheld the near-roadway monitoring requirements as within EPA’s monitoring discretion and reasonably justified by exposure concerns near major roads.
- The court rejected the claim that EPA had to issue implementation guidance or delay the standard, finding no such statutory requirement.

## Legal Principles

- In setting NAAQS under CAA §109, EPA has substantial discretion to evaluate complex scientific evidence and choose standards “requisite to protect the public health” with an adequate margin of safety; judicial review is deferential under arbitrary-and-capricious standards.
- Notice-and-comment is satisfied when the proposal fairly alerts interested parties to the subjects at issue; a request for comment on “all issues” related to a proposed tightening can encompass whether tightening should occur.
- Under the APA and CAA §307(d), EPA must respond to significant comments but need not provide a line-by-line rebuttal to every comment or individually discuss each cited study if the agency addresses the main points and explains its reasoning.
- EPA has broad discretion over air-quality monitoring design, including monitor siting and data treatment choices, so long as the agency provides a reasonable explanation.
- The CAA does not require EPA to issue implementation guidance as a precondition to promulgating or enforcing a revised NAAQS.

## Conclusion

The D.C. Circuit sustained EPA’s 2013 tightening of the annual PM₂.₅ NAAQS and related monitoring decisions, holding that EPA gave adequate notice, reasonably evaluated the scientific record, sufficiently answered significant comments, acted within its monitoring authority, and was not required by statute to issue implementation guidance before the revised standard took effect.


Nat'l Ass'n of Mfrs. v. EPA, 750 F.3d 921 (D.C. Cir. 2014)