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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules the-floor standard under CAA section 112d2. However, we are soliciting comments, data, and other information regarding this proposed decision, including data and information regarding the capital and annual costs, cost effectiveness, non-air impacts, and other relevant information that would be relevant for the remaining facility regarding whether the NESHAP should include a zero-mercury standard as a beyond-the-floor MACT standard. We intend to consider any such submitted data and information, in addition to the data and information contained in the records for the 2008 and 2011 proposals and in this proposal, in reaching final conclusions under CAA section 112d2 regarding a zero-mercury standard beyond-the-floor.
B. What are the results of the risk assessment and analyses?
As described above, for the Mercury Cell Chlor-Alkali Plant source category, we conducted an inhalation risk assessment for all HAP emitted, a multipathway screening assessment for the PBHAP emitted, and an environmental risk screening
assessment for the PBHAP emitted from the source category. When we initiated this RTR and developed the risk input files, there were two facilities operating in the source category Ashta in Ohio and Westlake in West Virginia;
however, as noted above, Ashta has since permanently shut down the mercury cell process. We also conducted an environmental screening for HCl, because we initially had some HCl emissions in our data set, but as described above, after further review, we conclude those HCl emissions are due to non-category sources. We present results of the risk assessment briefly below and in more detail in the Residual Risk Assessment for the Mercury Cell Chlor-Alkali Plant Source Category in Support of the Risk and Technology Review 2020 Proposed Rule, which is available in the docket for this action.
1. Chronic Inhalation Risk Assessment Results The EPA estimated inhalation risk is based on actual and allowable emissions. The estimated baseline MIR
posed by the source category is less than
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1-in-1 million based on actual emissions and MACT-allowable emissions. The total estimated cancer incidence based on actual or allowable emission levels is 0.0000003 excess cancer cases per year, or one case every 3 million years.
Emissions of 1,3-dichloropropene from the mercury cell building at Ashta accounted for 100 percent of the cancer incidence. No one is exposed to cancer risk greater than or equal to 1-in-1
million based upon actual and allowable emissions see Table 1 of this preamble. However, based on the available data, the 1,3-dichloropropene was only emitted from Ashta, which is no longer operating as a mercury cell facility, as discussed above.
Furthermore, we have no indication or data suggesting that this pollutant is emitted from the one remaining facility.
The maximum chronic noncancer TOSHI values for the source category were estimated to be less than 1 0.05
based on actual and allowable emissions. For both actual and allowable emissions, respiratory risks were driven by chlorine emissions from the mercury cell building.

TABLE 1INHALATION RISK ASSESSMENT SUMMARY FOR MERCURY CELL CHLOR-ALKALI PLANT 1 SOURCE CATEGORY
Maximum individual cancer risk 1-in-1 million 3

Number of facilities 2

Risk assessment
Estimated population at increased risk of cancer 1-in-1 million
Estimated annual cancer incidence cases per year
Maximum chronic noncancer TOSHI 4

Maximum screening acute noncancer HQ 5

Baseline Actual Emissions Source Category

2

0.004

0

0.0000003

0.05 respiratory
Facility-Wide

2

0.3

0

0.0001

0.05 respiratory
0.0000003

0.05 respiratory
2 REL, 7E4
AEGL2.

Baseline Allowable Emissions Source Category

2

0.004

0

1 Based
on actual and allowable emissions.
of facilities in the risk assessment includes two facilities subject to 40 CFR part 63, subpart IIIII.
individual excess lifetime cancer risk due to HAP emissions from the source category.
4 Maximum TOSHI. The target organ with the highest TOSHI for the source category is the respiratory system.
5 The maximum estimated acute exposure concentration was divided by available short-term threshold values to develop an array of HQ values. The acute HQ shown was based upon the lowest acute 1-hour dose-response value, the REL for mercury elemental. When an HQ exceeds 1, we also show the HQ using the next lowest available acute dose-response value.
2 Number
3 Maximum
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2. Screening Level Acute Risk Assessment Results Based on our refined screening analysis of reasonable worst-case acute exposure to actual emissions from the category, both facilities exceeded an HQ
of 1 the HQ was 2 when compared to the 1-hour REL for mercury elemental.
As discussed in section III.C.3.c of this preamble, we used an acute hourly multiplier of 10 for all emission processes. For this HAP, there are no AEGL1 or ERPG1 values for
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comparison, but AEGL2 or ERPG2
values are available. For elemental mercury, when the maximum off-site concentration is compared with the AEGL2 and ERPG2, the maximum acute noncancer HQ is well below 1
0.0007.
3. Multipathway Risk Screening Results PBHAP emissions based on estimates of actual emissions were reported from both facilities in the source category with both exceeding the Tier 1 non-cancer screening threshold
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emission rate for mercury. A Tier 2
screening analysis was conducted with no facilities having an SV greater than 1 for any scenario the fisher and farmer had the highest SV at 0.4. There are no carcinogenic PBHAP emitted from the source category. So, there are no cancer SVs to report. Further details on the Tier 2 screening analysis can be found in the Residual Risk Assessment for the Mercury Cell Chlor-Alkali Plant Source Category in Support of the Risk and Technology Review 2020 Proposed Rule, and Appendix 10 of this report.

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