4 The record articles

Redoing Your (MACT) Floors

Posted: November 7th, 2013

Author: All4 Staff 

More often than not, people will redo the floors in their homes for aesthic reasons, not because of an underlying problem with how they were constructed. In the air quality regulatory world, we routinely deal with a floor that is not the type you walk on, but rather the type that establishes an emission limit and a host of testing, monitoring, recordkeeping, and reporting requirements for a certain set of regulated air pollutants. So where am I going with this?

The Clean Air Act Amendments of 1990 (1990 CAAA) required the U.S. Environmental Protection Agency (U.S. EPA) to regulate emissions of hazardous air pollutants (HAPs) using technology-based standards. The Section 112 standards are known as the National Emissions Standards for Hazardous Air Pollutants (NESHAPs), and are commonly referred to as Maximum Achievable Control Technology, or MACT, standards. When developing a MACT standard for a particular source category, U.S. EPA evaluates the level of emissions currently being achieved by the best-performing similar sources through use of HAP-compliant materials, clean processes, control devices, work practices, or other methods. These emissions levels set a baseline (referred to as the “MACT floor”) for the new standard. At a minimum, a MACT standard must achieve, throughout the industry, a level of emissions control that is at least equivalent to the MACT floor. The MACT floor is established differently for existing sources and new sources (which are defined based on the date when a NESHAP is proposed):

  • For existing sources, the MACT floor must equal the average emissions limitations currently achieved by the best-performing 12% of sources in that source category, if there are 30 or more existing sources. If there are fewer than 30 existing sources, then the MACT floor must equal the average emissions limitation achieved by the best-performing five sources in the category.
  • For new sources, the MACT floor must equal the level of emissions control currently achieved by the best-controlled similar source.

This approach has been followed in the past without too much fanfare. However, several of the more recent Section 112 determinations (e.g., the Cement and Boiler MACT) that U.S. EPA has finalized have incoportated a different approach for calculating the floor, which has not always been consistent with the procedures followed in the initial Section 112 determinations. Specifically, an August 20, 2013 ruling by the U.S. Court of Appeals for the District of Columbia Circuit requires U.S. EPA to explain how its use of the upper prediction limit (UPL), rather than the statistical averaging as described above to establish the MACT floor for several standards, satisfies the 1990 CAAA. The Court’s ruling was related specificially to MACT standards for sewage sludge incinerators (SSIs) under Section 129 of the Clean Air Act (CAA); however, the collateral damage of potentially requiring U.S. EPA to redo the MACT floors for this rule could “carry major implications” according to a former U.S. EPA official, impacting standards such as the Utility MACT, Boiler MACT, Portland Cement MACT, and rules for medical waste incinerators. Those challenging the use of the UPL, which is based on a statistical confidence limit such as the 90th or 99th percentile, rather than an average, do not believe the approach is in the public’s interest since UPL limits are less stringent than those that would be established using a statistical average or even an upper confidence limit (UCL).  Click here for more detailed information related to the the background of MACT and the statistical tools.

A former U.S. EPA official states that under the Utility MACT standard, the mercury limit would be lowered by two-thirds without the UPL methodology. U.S. EPA contends that the use of UPLs is appropriate to account for variability in the performance of emission sources and when a sufficient sample size of emsisions data is not available. However, if U.S. EPA’s explanation of the validity of the UPL does not satisfy the Court, it’s possible that many Section 112 and Section 129 MACT standards might need to be overhauled (i.e., resulting in lower emission limits), which would completely change the course of MACT compliance for the rest of this decade and beyond.


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