Novel Approach Included in U.S. EPA’s Recent Secondary Lead Smelting Proposal

On October 1, 2025, the U.S. Environmental Protection Agency (U.S. EPA) proposed amendments to the National Emission Standards for Hazardous Air Pollutants (NESHAP) for the Secondary Lead Smelting source category (40 CFR Part 63, Subpart X). The proposal addresses multiple items: the required technology review, unregulated hazardous air pollutants (HAP), and outstanding petition issues. U.S. EPA is also proposing revisions related to startup, shutdown, and malfunction (SSM), electronic reporting, monitoring requirements, and other minor technical issues. The novel approach hinted at in the title of this article relates to their analysis of unregulated HAP.

Technology Review

U.S. EPA is required to perform a one-time risk review of each NESHAP eight years after promulgation and an ongoing technology review every eight years after promulgation. The risk review is performed to determine if any changes are needed to address remaining risk associated with HAP emissions from the source category. The technology review is performed to assess whether there have been cost-effective developments in practices, processes, or control technologies that should be incorporated into the rule. These deadlines are often missed; U.S. EPA originally promulgated the NESHAP in 1996 and finalized its risk and technology review of Subpart X in 2012.

This proposal represents the second technology review. U.S. EPA has determined that wet electrostatic precipitators (WESPs) and fenceline monitoring were the most relevant potential developments for consideration in its review. The review determined that requiring a WESP at the outlet of a facility’s existing particulate control (e.g., baghouse) was not cost effective and WESPs are therefore not proposed as a requirement. U.S. EPA also determined that the 2012 Subpart X amendments reduced fugitive HAP metal emissions and ambient concentrations at facility fencelines. Because of these reductions, the additional monitoring programs already required by the rule, and the fact that there has been no fenceline monitoring method proposed or promulgated for HAP metals, U.S. EPA is not proposing to require fenceline monitoring as part of the technology review.

Unregulated HAP

As a result of the D.C. Circuit Court’s 2020 decision in the LEAN case, U.S. EPA has been reviewing NESHAPs to determine if they adequately address all sources of HAP in the source category and all HAP known to be emitted from affected sources. As a result of that review for Subpart X, emissions of hydrochloric acid (HCl), chlorine (Cl₂), and carbonyl sulfide from all furnace types and emissions of total hydrocarbon (THC) and dioxin/furan (D/F) from rotary furnaces were identified as unregulated. Therefore, emissions testing was required as part of an information collection request and based on the collected emissions data, U.S. EPA proposes to:

• Add THC and D/F emissions limits for co-located rotary and reverberatory furnaces,
• Use THC as a surrogate for carbonyl sulfide (COS) emissions (thermal controls used for THC control also control COS), and
• Not require standards for emissions of hydrochloric acid (HCl) and chlorine (Cl₂) because emissions are so minimal as to be considered de minimis.

This is not the first NESHAP where U.S. EPA has added THC and D/F limits to sources or applied THC as a surrogate for non-D/F organic HAP. However, this is the first time since the LEAN decision that U.S. EPA has articulated a position that HAP emitted in “de minimis” amounts do not require either numeric emissions limits or work practice standards. U.S. EPA estimates that HCl and Cl₂ emissions have been reduced to 1.5 and 0.2 tons per year (tpy), respectively, from the 11 facilities covered by Subpart X because polyvinyl chloride (PVC) has been phased out as a separator material in the batteries processed at secondary lead smelters. Their reasoning in determining that these levels of emissions do not require additional controls is that Congress did not intend to subject trivial levels of emissions to controls under Clean Air Act Section 112. Various court decisions are cited in the proposal preamble that revolve around the principle that the law does not concern itself with trifling matters and that a general statutory requirement does not encompass de minimis concerns unless otherwise indicated. U.S. EPA supports its position by pointing out that Section 112 establishes a major source as one that emits 10 tpy or more of any HAP or 25 tpy of total HAP, and that area sources are only regulated if they present a threat of adverse effects to human health or the environment. Also mentioned is the Supreme Court’s admonition in one case that no statute yet known pursues its purpose at all costs. U.S. EPA contends that because Congress required them to establish priorities for promulgating NESHAP that included the effects of HAP on public health and the environment and the quantity and location of HAP that each source category emitted, thus supporting the treatment of HAP with trivial emissions and low impacts as de minimis.

Specific to the secondary lead smelting category and HCl/Cl₂ emissions, U.S. EPA points out that the emissions levels per source are nowhere near the thresholds that trigger major source regulation and do not amount to a level that presents an adverse effect to human health or the environment (the 2012 risk review found that the NESHAP provides an ample margin of safety to protect public health). In addition, neither of these two HAP persist in the environment for long periods of time, transform into other HAP, or bioaccumulate and they are not non-threshold carcinogens. U.S. EPA states that this analysis differs from its prior decision to reject de minimis treatment for these two pollutants under the cement kilns source category because HCl was emitted by each kiln at major source levels.

Finally, it is noted by U.S. EPA that the decision to apply this treatment to these pollutants for this NESHAP is highly fact specific. While U.S. EPA could use the de minimis criteria in future NESHAP reviews, it remains uncertain exactly what emissions level constitutes de minimis. However, in this proposal, de minimis seems to be an amount less than the major source threshold. U.S. EPA’s future use of the de minimis principle is most likely to occur in other gap-filling exercises where emissions are small, risks are low, and there is no indication that emissions of the HAP(s) from the source category are likely to increase over time. While the use of the de minimis criteria by U.S. EPA is discretionary, it will not likely occur for a HAP such as mercury because of its associated risks but could occur for certain trace HAP associated with combustion systems or HAP that pose minimal health risk.

Although U.S. EPA considered a health-based standard for HCl as part of its review of the Lime Manufacturing NESHAP in 2024, they estimated reductions of 884 tpy from application of the numeric emissions limit that was ultimately finalized, which is much higher than HCl emissions levels in the secondary lead smelting source category. The Lime Manufacturing NESHAP is one of several NESHAP where the previous administration added HAP emissions limits that are currently being reconsidered, so it will be interesting to see for which rules and pollutants U.S. EPA applies a novel approach to avoiding or removing standards.

Other Proposed Changes

Other proposed changes to Subpart X include:

• Revisions to building differential pressure monitoring requirements,
• Automatic approval of 24-month performance test extensions under certain circumstances,
• Changes to SSM provisions,
• Updates to the General Provisions applicability table,
• Expansion of electronic reporting requirements, and
• Removal of the affirmative defense provisions.

Comments on the proposal are due November 17, 2025.

Summary

ALL4 is tracking proposed and final revisions to several U.S. EPA air regulations and the approaches the agency is using to justify those revisions. We’re following the reviews and reconsiderations of NESHAP, so stay tuned for future articles. We have extensive experience preparing technical comments on regulations, evaluating final regulations, preparing compliance strategies, and helping facilities implement and sustain compliance approaches. If you need help developing comments or understanding how changes to regulations affect you, please reach out to your ALL4 Project Manager.