EPA Revises National Ambient Air Quality Standard

EPA finalized a rule on February 7, 2024, to tighten the annual national ambient air quality standard for fine particulate matter (PM_2.5) by 25%*. EPA says the decision is based on available science and comments on its 2023 proposed rule. This week’s rule did not change the 24-hour standard for fine particulate matter or any other NAAQSs.

*The standard is lowering from 12 micrograms per cubic meter to 9 micrograms per cubic meter.

Back in December 2020, EPA finalized a rule to retain the National Ambient Air Quality Standard (NAAQS) for particulate matter—meaning, the Agency decided not to revise it.

See EPA's news release covering the rule.

Why make a rule without changes?

The Clean Air Act requires EPA to reevaluate the NAAQS standards every five years. In 2020, EPA determined that the rule did not need to change.

In 2021, the Agency said that it reconsidered the 2020 decision to retain the old standards because “the available scientific evidence and technical information indicated that the standards may not be adequate.” This led to the 2023 proposed rule and the subsequent final rule that we are seeing now.

What is fine particulate matter?

Fine particulate matter includes dust, dirt, soot, or smoke, and other particles that are “so small they can only be detected using an electron microscope.” For reference, the average human hair is 50–70 micrometers thick. These particles are 2.5 micrometers wide or smaller—significantly smaller, even, than the width of a human hair.

Some of these particles are a result of chemical reactions in the air, and others are emitted directly from construction sites, unpaved roads, fires, and other sources.

Exposure to particulate matter can negatively impact human health and the environment by affecting the lungs and heart, making lakes and streams acidic, affecting the diversity of ecosystems, damaging sensitive forests and farm crops, and more.

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