By Joe Peré, Technical Director of Air Quality 

Particulate matter less than 2.5 microns in size – known as PM2.5 or “fine particulate matter” – is classified as one of the six criteria pollutants that is federally regulated under the US Clean Air Act’s (CAA) National Ambient Air Quality Standards (NAAQS).  In recent years, PM2.5 has come under greater scrutiny for its increased contributions to adverse health effects, namely respiratory illnesses and cardiovascular disease.  On February 7, 2024, The US Environmental Protection Agency (EPA) released a pre-publication of federal rulemaking, lowering the annual PM2.5 NAAQS from 12 ug/m³ to 9 ug/m³, while retaining the 24-hour standard of 35 ug/m³.  Lowering the NAAQS for PM2.5 is likely to create a wave of change that will be felt throughout the regulatory landscape. The revised NAAQS standard will take effect within 60 days of the final release date, which has not yet occurred. 


PM2.5 is a leading cause of respiratory illness worldwide.  Based on a United Nations World Health Organization (WHO) study, PM2.5 has been attributable to approximately 42 million deaths globally.  It is classified as fine particulate matter and generally results from anthropogenic activities, such as mining, milling, rock crushing, aggregates handling, and fugitive dust from haul roads. Because of the fine size of the particulates, PM2.5 has historically been difficult to accurately quantify, with emission factors and atmospheric dispersion models typically overquantifying and conservatively predicting the behavior of the pollutant. 


Ultimately, States and Tribes will incorporate the revised PM2.5 standard into their respective Implementation Plans (SIPs and TIPs) and, with the EPA, will begin to assess the air quality in certain areas that could trigger nonattainment designations for PM2.5.  Generally, the initial designations are issued by EPA within two years of issuance of the new standard. For areas designated as nonattainment, plans must be submitted within 18 months of the designation. Based on currently available ambient air data, this could impact several areas that are presently classified as being in attainment with PM2.5 and could lead to higher control standards (BACT, RACT) and lower emissions thresholds (LAER) for PM2.5 sources. While EPA currently expects that 99% of U.S. counties will be able to meet the revised standards by 2032 based on current and proposed federal rulemaking actions, much can change between the effective date and 2032. 

These changes will make it much more challenging for the regulated community to comply with PM2.5 standards, especially if the standard is so low that already high background concentrations in some areas will create an even slimmer or nonexistent margin for modeled concentrations. EPA has also indicated that the PM2.5 Significant Impact Level (SIL, or “De Minimis” level) will be lowered upon the effective date.  

Environmental justice is also a factor in analyzing PM2.5 NAAQS.  Ambient air monitoring near at-risk communities will be enhanced and is intended to improve future NAAQS reviews.  EPA’s goal in this action is to improve data quality in these areas to enhance regulatory decision-making and better characterize air quality in communities of concern. 


Many in the regulated community have proposed ongoing permitting projects that will be affected by these revised standards.  INTERA recommends that those affected take the following into consideration for current and future projects: 

  • Perform an assessment of current and upcoming projects to understand potential impacts of PM2.5 against the revised annual standard.  This includes understanding the modeled concentrations and the design values of the region where the projects are being implemented. 
  • Explore strategies to minimize emissions of PM2.5 if modeled concentrations become a challenge for the project. 
  • If modeling PM2.5 becomes challenging once all operational possibilities have been reviewed, explore all available modeling options based on regulatory guidance and rules; there are a variety of tools that can be used to refine concentrations within the confines of what is regulatorily acceptable. 
  • If a permit is currently under review and is not issued by the time of the effective date, the project will be required to demonstrate compliance with the revised PM2.5 standard of 9 ug/m³. During times of similar past regulatory changes, permit applications had been subject to relief while under certain stages of review, but subsequent court rulings have eliminated such relief. 
  • Consider collecting site-specific ambient PM2.5 monitoring data to support background concentrations. Relying on publicly available monitoring data or design values for assessing background concentrations could include data outliers or unaccounted for exceptional events. 
  • Prepare for enhanced rulemaking and enforcement from EPA and local regulators; foresight now can save time and money on projects. 

INTERA’s air quality experts have been monitoring revisions to the PM2.5 NAAQS for several years.  We bring a successful track record of guiding and counseling clients on strategies related to air quality permitting, compliance, dispersion modeling, and much more.  We welcome the opportunity to support your efforts in navigating the changes that will come from these revisions.  Please contact us today should you have any questions.