National Ambient Air Quality Standards (NAAQS)

The Clean Air Act is the primary federal law governing air pollution in the U.S. The Clean Air Act gives the U.S. Environmental Protection Agency (EPA) authority to set and enforce limits on atmospheric levels of harmful air pollutants, including particulate matter like coal dust.

The EPA has identified six "criteria pollutants" that must not exceed concentration thresholds known as the National Ambient Air Quality Standards (NAAQS), pronounced as "the nacks". The criteria pollutants are fine particulate matter (PM_2.5) and coarse particulate matter (PM₁₀), also known as dust, as well as ozone (O₃), lead (Pb), carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂).

To determine NAAQS compliance, atmospheric concentrations of criteria pollutants must be measured using designated regulatory air monitors by an agency with the authority to do so, which in Virginia is the Virginia Department of Environmental Quality (VA DEQ). When locations exceed the NAAQS for any of the criteria pollutants, regulatory action is triggered, including stricter permitting requirements, emission reductions plans, enforcement measures, and public health advisories.

Particulate Matter

• The PM₁₀ NAAQS is a 24-hour average mass concentration of 150 μg/m³, not to be exceeded more than three times over three years.
• There are two PM_2.5 NAAQS: a 24-hour average mass concentration of 35 μg/m³ and a recently revised annual average mass concentration of 9 μg/m³.

The NAAQS are set to protect public health, including the health of more sensitive individuals such those with asthma as well as children and elders. That said, there is no known safe level of exposure to PM_2.5, with documented health impacts below the NAAQS.