FACT SHEET

Final Amendments to Air Toxics Standards for
Mercury Cell Chlor-Alkali Plants

ACTION

•	On April 29, 2022, the US. Environmental Protection Agency (EPA) finalized
amendments to the 2003 National Emission Standards for Hazardous Air
Pollutants (NESHAP) Mercury Cell Chlor-Alkali Plants.

•	The amendments will provide clarifications, corrections and improve compliance
and reduce emissions of hazardous air pollutants.

•	Mercury cell chlor-alkali plants produce chlorine and caustic using mercury cells.

Caustic is used to neutralize acidic compounds.

•	Following a residual risk and technology review (RTR) conducted under the Clean
Air Act (CAA), EPA is finalizing amendments to:

o Prohibit mercury emissions from existing mercury cell chlor-alkali plants within three years;
o Amend the requirements for cell room fugitive mercury emissions to require work practice
standards for the cell rooms and instrumental monitoring of cell room fugitive mercury
emissions during the period of time before they eliminate the emissions of mercury;
o Add standards for fugitive chlorine emissions from mercury cell chlor-alkali plants, which

are not currently covered in the 2003 NESHAP;
o Correct and clarify regulatory provisions related to emissions during periods
of startup, shutdown and malfunction (SSM), including proposing to eliminate
exemptions during periods of SSM; and
o Require facilities to submit electronic copies of notification of compliance
status reports and electronic copies of performance test results and reports.

RESIDUAL RISK ASSESSMENT

•	The CAA requires EPA to assess the risk remaining after application of the final air
toxics standards. This is known as a residual risk assessment.

•	The maximum individual cancer risk for inhalation is estimated to be less than 1-in-l
million for the Mercury Cell Chlor-Alkali Plants source category.

•	Based on the completed risk assessment, available health information and associated
uncertainties, EPA determined risks from mercury cell chlor-alkali plants to be
acceptable and provide an ample margin of safety to protect public health.

TECHNOLOGY REVIEW

•	The CAA also requires EPA to assess, review and revise the air toxics standards as
necessary, taking into account developments in practices, processes and control
technologies since the standards were first issued.

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•	The conclusions of our technology review and our beyond-the-floor MACT determination
resulted in a prohibition of mercury emissions from existing mercury cell chlor-alkali
plants within 3 years.

•	The technology review for mercury cell chlor-alkali plants also identified cost-effective
developments that include implementing a combination of continuous cell room
mercury monitoring and work practices for mercury for the period of time before the
prohibition of mercury deadline comes into effect.

•	In addition, pursuant to a recent court decision regarding the need to fill regulatory
gaps as part of the required technology review, we are finalizing work practice
standards to address fugitive chlorine emissions.

BACKGROUND

•	The CAA requires EPA to regulate hazardous air pollutants, also known as air toxics, from
categories of industrial facilities in two phases.

•	The first phase is "technology-based," where EPA develops standards for controlling the
emissions of air toxics from sources in an industry group (or "source category"). These
maximum achievable control technology (MACT) standards are based on emissions levels
that are already being achieved by the best-controlled and lower-emitting sources in an
industry.

•	Within 8 years of setting MACT standards, the CAA directs EPA to assess the remaining
health risks from each source category to determine whether the MACT standards protect
public health with an ample margin of safety and protect against adverse environmental
effects. This second phase is a "risk-based" approach called residual risk. Here, EPA must
determine whether more health-protective standards are necessary.

•	Also, every 8 years after setting MACT standards, the CAA requires that EPA review and
revise the standards, if necessary, to account for improvements in air pollution controls
and/or prevention.

FOR MORE INFORMATION

•	To download a copy of the final rule notice, go to EPA's website at
https://www.epa.gov/stationarv-sources-air-pollution/mercury-cell-chloralkali-plants-
national-emissions-standards.

•	Today's action and other background information are available electronically at
https://www.regulations.gov/. EPA's electronic public docket and comment system.

o Materials for this final action can be accessed using Docket ID No. EPA-HQ-OAR-2020-
0560.

•	For further technical information about the rule, contact Phil Mulrine, EPA's Office of Air
Quality Planning and Standards, Sector Policies and Programs Division, at (919) 541-5289 or
mulrine.phil@epa.gov.

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