This action finalizes the residual risk and technology review (RTR) conducted for the Mercury Cell Chlor-Alkali Plants source category regulated under national emission standards for hazardous air pollutants (NESHAP). In addition, this action finalizes the beyond-the- floor determination that EPA performed in response to a petition for reconsideration of the 2003 NESHAP. These final amendments prohibit mercury emissions from existing mercury cell chlor-alkali plants based on the results of our technology review and our beyond-the-floor maximum achievable control technology (MACT) determination. The compliance date for this requirement is three years. Since mercury emissions will be eliminated as a result of the final rule standards, any adverse health or environmental effects from mercury emissions from the source category will also be eliminated in that three-year time frame. Furthermore, the EPA is finalizing work practice standards and instrumental monitoring of mercury to minimize fugitive mercury emissions from the cell rooms during the period of time before emissions are eventually eliminated. In addition, the EPA is finalizing work practice standards to minimize fugitive chlorine emissions from mercury cell chlor-alkali plants, which were not previously regulated under the NESHAP. The EPA is also finalizing revisions related to emissions during periods of startup, shutdown, and malfunction (SSM) and amendments to correct a few minor errors in compliance provisions in the 2003 rule.