EPA’s Final Rule for particulate matter (PM) NAAQS

On February 7, 2024, the U.S. Environmental Protection Agency (EPA) published a Final Rule on its reconsideration of the National Ambient Air Quality Standards (NAAQS) for Particulate Matter (PM), to lower the primary annual PM2.5 standard from 12.0 micrograms per cubic meter to 9.0 micrograms per cubic meter. The EPA retained the current standards for primary 24-hour PM2.5, primary and secondary 24-hour PM2.5, and primary and secondary 24-hour PM10.  Whew, that is a mouthful.

What does this all mean? Let’s break it down. 

Under the Clean Air Act, the EPA sets certain standards for air emissions to protect against particle pollution, one of the six primary pollutants regulated by the Clean Air Act. Particle pollution includes fine particles, such as emissions from vehicles, smokestacks, fires, or particles formed when gases emitted by processes such as power plants or gasoline engines react in the atmosphere. These fine particles are 2.5 micrometers in diameter and smaller – referred to as PM2.5. There are also coarse particles, such as dust kicked up by road traffic, certain industrial processes, and construction operations. These particles have diameters between 2.5 and 10 micrometers – referred to as PM10. 

The EPA sets two types of NAAQS – primary standards, which are those that protect public health – and secondary standards, which are those that protect public welfare. The primary standards must be “requisite to protect public health with an adequate margin of safety,” including the health of more sensitive populations such as those with heart or lung disease, children or older adults, communities of color, or low-income communities.  Secondary standards are intended to address adverse PM-related visibility, climate, and materials impacts – for example haze, or the erosion of buildings caused by acid rain. 

In order to protect against both the short-term and long-term effects from particulate matter, the EPA looks at levels at both 24-hour averaging time and annual averaging time. The various standards, as revised by the February 7 Final Rule, are as follows: 



Averaging Time 







Annual arithmetic mean, averaged over 3 years  




Primary and Secondary 



98th percentile, averaged over 3 years 


Primary and Secondary 

Annual and 24-hours 


Not to be exceeded more than once per year on average over 3 years 

What does this mean for Iowa?  

The revisions to primary annual PM2.5 trigger the state’s review to reassess and determine areas of the state that do not meet the revised standard. Areas that do not meet the revised standard, known as nonattainment areas, will need to develop a plan to work towards meeting the standard. Whether a certain area is designated attainment or nonattainment also determines the requirements for preconstruction review and permitting in the event a new major source decides to build in that area or an existing major source desires to expand.  Major sources are those “that emit or have the potential to emit 10 tons per year or more of a hazardous air pollutant or 25 tons per year or more of a combination of hazardous air pollutants.” Construction and operating-related permitting may become more stringent for new areas of the state designated as nonattainment, in an effort to lower exposure to particulate matter by Iowa citizens. 

If you have any questions about the ruling or its applicability to you, please contact Ellen Hames or your BrownWinick attorney