Proposed Amendments to Air Toxics Standards for Mercury Cell Chlor-
Alkali Plants: Fact Sheet

ACTION

•	On January 4, 2021, the US. Environmental Protection Agency (EPA)
proposed amendments to the 2003 National Emission Standards for
Hazardous Air Pollutants (NESHAP) Mercury Cell Chlor-Alkali Plants Residual
Risk and Technology Review.

•	The proposed amendments will provide clarifications, corrections and
improved 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 proposing to:

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;
o Amend the requirements for cell room fugitive mercury emissions to require work
practice standards for the cell rooms and to require instrumental monitoring of cell
room fugitive mercury emissions;
o Add standards for fugitive chlorine emissions from mercury cell chlor-alkali plants,

which are not currently covered in the 2003 NESHAP; and
o Require facilities to submit electronic copies of notification of

compliance status reports and submit electronic copies of performance
test results and reports.

•	EPA will accept comment on the proposed amendments for 45 days after
publication in the Federal Register.

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.

•	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.

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

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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.

•	The technology assessment for mercury cell chlor-alkali plants identified cost-
effective developments that included implementing a combination of continuous
cell room mercury monitoring and work practices for mercury.

REGULATORY GAPS

•	In addition to the RTR analyses, pursuant to a recent court decision regarding the
need to fill regulatory gaps, EPA is proposing work practice standards to address
fugitive chlorine emissions, which is the one known regulatory gap for this source
category.

BACKGROUND

•	The CAA requires EPA to regulate toxic 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 eight 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 eight 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.

HOW TO COMMENT

•	EPA will accept comment on the proposal for 45 days after publication in the Federal
Register.

•	Comments, identified by Docket ID No. EPA-HQ-OAR-2020-0560, may be submitted by one
of the following methods:

o Go to https://www.regulations.gov/ and follow the online instructions for submitting
comments.

o Send comments by email to a-and-r-docket@epa.gov. Attention Docket ID No. EPA-
HQ-OAR-2020-0560.

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•	Out of an abundance of caution for members of the public and our staff, the EPA Docket
Center and Reading Room are closed to the public, with limited exceptions, to reduce the
risk of transmitting COVID-19. Our Docket Center staff will continue to provide remote
customer service via email, phone and webform. We encourage the public to submit
comments via https://www.regulations.gov/ or email, as there will be a delay in
processing mail and faxes. For further information on EPA Docket Center services, please
visit us online at https://www.epa.gov/dockets.

FOR MORE INFORMATION

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

•	This proposed 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 proposed action can be accessed using Docket ID No. EPA-HQ-
OAR-2020-0560.

•	For further technical information about the proposed 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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