Under the Clean Air Act Amendments of 1977, EPA is required to review and if appropriate revise the air quality criteria and national ambient air quality standards (NAAQS) every 5 years. On January 13, 2013, the EPA published a final rule revising the primary (health-based) and secondary (welfare-based) NAAQS for particulate matter (78 FR 3086) to provide increased protection against the health effects of PM. This review includes the preparation of an Integrated Review Plan, an Integrated Science Assessment and, if warranted, a Risk/Exposure Assessment, and also a Policy Assessment by EPA, with opportunities for review by EPA's Clean Air Scientific Advisory Committee and the public. These documents will inform the Administrator's proposed decision as to whether to retain or revise the standards. This proposed decision will be published in the Federal Register with opportunity provided for public comment. The Administrator's final decisions will take into consideration these documents and public comment on the proposed decision. As indicated in the Administrator’s May 2018 Back to Basics memo, the current review of the PM NAAQS is being conducted in a manner intended to ensure that any necessary revisions to the standards are finalized by December 2020, and will apply the principles outlined in that memo to this review as appropriate.