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EPA Proposes to Retain Particulate Matter Standards of Importance to Industries in Arid West

On April 14, 2020, the Environmental Protection Agency (EPA) proposed to retain the current National Ambient Air Quality Standards (NAAQS) for particulate matter, including both the PM10 and PM2.5 standards.1  The PM10 and PM2.5 standards, which are important compliance drivers for industries in the arid western United States, are subject to a five-year review under the Clean Air Act.  EPA anticipates finalization by the end of 2020, which raises uncertainties regarding the standards’ permanence in the event of a change in administrations.

EPA’s current proposal would retain without revision both the 2012 primary (health-based) and secondary (welfare-based) standards for PM10 at 150 µg/m3 (24-hour average) and for PM2.5 (annual average of 12 µg/m3 and 24-hour average of 35 µg/m3).  The proposal follows the Clean Air Scientific Advisory Committee’s (CASAC) consensus advice to retain the 24-hour primary standard for PM2.5, the primary standard for PM10, and the secondary standards. 

CASAC, however, was split on the question of whether to lower or retain the annual primary standard for PM2.5.  Both CASAC’s recommendation and EPA’s proposal conflicts with a required technical report prepared by EPA staff, which recommended lowering the annual primary standard for PM2.5 to between 8 and 10 µg/m3. In his decision to retain the primary standards for PM2.5, EPA Administrator Wheeler cited “important uncertainties” in the evidence in support of lowering the standard that ultimately “do not call into question the adequacy of the current primary PM2.5 standards.”

The uncertainties in the evidence are particularly important for assessment of health impacts of crustal PM, which predominates in the western United States, and is regulated under both the PM2.5 and the PM10 standards.  More than twenty years ago, EPA issued separate standards for PM2.5 and the PM10 due to the “significantly different physio-chemical properties and origins” between fine and crustal PM.  Since 2007, however, EPA has defined PM2.5 to include “inorganic material (including metals, dust, sea salt, and other trace elements) generally referred to as ‘crustal’ material.” Due to the high percentage of crustal PM that makes up PM2.5 in the arid West, a future reduction in either of the PM standards—as a result of the final rulemaking in this administrative proceeding, litigation, or a future administrative proceeding—would have disproportionate impacts on compliance for industries in the arid West.

Comments on the proposed rule are due 60 days after publication in the Federal Register. The final rule will be challenged, and any proceedings would be restricted to those who filed timely and substantive comments. Moreover, the development of the administrative record through detailed comments will be critical in light of the pending election.  

1. Fine particulate matter (PM2.5) encompasses particles less than 2.5 µm in diameter while larger particles between 2.5 and 10 µm in diameter are considered coarse particulate matter (PM10). The primary standard for PM2.5 includes both an annual and a 24-hour standard.


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