Supplement to the Non-Hg Case Study Chronic 

Inhalation Risk Assessment In Support of the 

Appropriate and Necessary Finding for Coaland Oil-Fired Electric Generating Units

EPA-452/R-11-013

November 2011

Supplement to the Non-Hg Case Study Chronic Inhalation Risk Assessment In Support of 

the Appropriate and Necessary Finding for Coal- and Oil-Fired Electric Generating Units

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Research Triangle Park, NC

1

Introduction

A previous document1discussed the methods and results of the chronic inhalation risk 

assessment of hazardous air pollutants (HAPs) other than mercury from coal- and oil-fired 

electric utility steam generating units (EGUs) at sixteen case study facilities, which was 

performed in support of the “appropriate and necessary” finding for coal- and oil-fired EGUs. 

Several changes were made to the emissions estimates, dispersion modeling, and risk 

characterization of these facilities in response to public comments on the proposed rule, and this 

report documents those changes and their impact on the estimated risks from the case study 

facilities.

1. Emissions

In response to comments on the proposed rule, the emissions data used in the case studies 

were updated for several facilities in two ways. First, in response to comments that EPA’s 

methodology was not sufficiently refined, EPA used year-specific heat input for the modeled 

years 2005 through 2009, rather than the average 2007-2009 heat input for all years. Second, in 

response to comments that EPA’s approach to emission factor development should use outlier 

tests, EPA revised its calculations for emission factors to apply to those units that had not been 

tested in the ICR. Only the arsenic, chromium, and nickel emissions were recomputed for use in 

modeling because these pollutants were the key risk drivers for the case studies. Both of these 

updates were made only to the case study facilities that had estimated cancer risks near 1 in a 

million at proposal. The subsections below provide more information on these updates. In 

addition, the detailed calculations for the case study emissions at the unit level are provided in 

the spreadsheet “Case_Study_Emis_MATS_Final.xlsx,” which is included in the docket (EPAHQ-OAR-2009-0234-2939).

1.1 Unit-Level Annual Heat Input

The preferred source of unit-level annual heat input data was EPA’s Clean Air Markets 

continuous emission monitoring (CEM) program. Unit-level annual heat input data for calendar 

years 2002-2010 were obtained for all units that report these data to EPA. Heat input data are

important because emissions are proportional to heat input. The only facility without CEM data 

was the HECO Waiau facility (ORIS 766). This facility was contacted directly to obtain actual 

unit-specific annual heat input data. The year-specific heat input data used for the final rule are 

an improvement over the approach used at proposal, which used a multi-year average heat input 

for each modeled year.

Table 1 provides the total facility-level heat input for each year, as well as the average 

value. While the average value was not used for any calculations, it is provided for comparison 

to the value used at proposal. For three of the case study facilities remodeled for the final rule, 

the revised heat input is higher (Amerenue-Labadie, 0.3%; James River, 2.5%; and Conesville, 

24%). For the rest of the facilities, the heat input is lower (Dominion – Yorktown, 12%; 

Chesapeake Energy Center, 8.8%, Heco Waiau, 2.5%; OG &E – Muskogee, 8.6%; PSHNH –

 

1 Non-Hg Case Study Chronic Inhalation Risk Assessment for the Utility MACT “Appropriate and Necessary”

Analysis, March 16, 2011. Docket ID No. EPA-HQ-OAR-2009-0234-2939. Available at www.regulations.gov.
2

Merrimack, 16%; and TVA Gallatin, 3.2%). While these facility-level changes give some 

indication of the impact on risk, because each unit at a facility can contribute differently to the 

risk, the unit-level heat input changes can give a somewhat different view of the changes. These 

unit-level changes are available in the case study emissions spreadsheet discussed above.

Table 1: Final case study heat input (MMBtu) compared to heat input used at proposal.

Facility 2005 2006 2007 2008 2009 Average

Proposed 

Rule

AmerenueLabadie 168,514,660 170,157,435 182,401,603 165,591,139 166,226,274 170,578,222 171,126,775

City Utilities of 

Springfield -

James River

16,452,496 15,943,500 13,610,535 15,181,310 11,968,020 14,631,172 14,997,208

Conesville 96,719,546 92,193,174 108,230,769 100,770,581 66,087,859 92,800,386 115,092,253

Dominion -

Yorktown 40,366,974 21,700,400 29,638,959 22,409,846 19,286,905 26,680,617 23,447,200

Dominion 

Chesapeake 

Energy Center

41,070,023 37,711,371 40,777,823 36,222,677 35,255,050 38,207,389 34,853,246

Heco Waiau 14,673,660 14,397,785 14,935,405 13,320,107 14,884,983 14,442,388 14,080,684

OG&E -

Muskogee 106,454,236 104,916,428 87,737,665 104,566,459 92,434,936 99,221,945 90,739,465

PSHNH -

Merrimack 34,045,565 34,410,607 36,317,955 30,332,534 25,319,652 32,085,263 26,821,184

TVA Gallatin 73,113,053 74,908,225 77,474,644 79,699,781 64,123,359 73,863,812 71,515,151

1.2 Development of Emission Factors

For several of the case study facilities, emission factors were used as the best available 

alternative source of emissions information because appropriate facility-specific stack test data 

were not available to compute emissions. The case study facilities using emission factors that 

were not based on site-specific stack test data are Conesville (3% of the risk driver emissions), 

PSHNH – Merrimack (35%), and all risk driver emissions at Chesapeake Bay Energy Center, 

Dominion – Yorktown, OG&E – Muskogee, and Amerenue-Labadie. The risk driver emissions 

are those emissions that contributed the most to the total facility risk. The emission factors used 

in these cases were recalculated based on revised methods that had not been used at the time of

proposal, including the use of well-established, robust outlier checks (Dixon or Rosner tests), 

depending on the number of values and when more than three values were evaluated. 

The complete documentation for the arsenic, chromium, and nickel emission factors is available

in the docket in separate documents: Coal_Fired_Utility_Boiler_Arsenic.pdf, 

Coal_Fired_Utility_Boiler_Chromium.pdf, and Coal_Fired_Utility_Boiler_Nickel.pdf. The 

same hexavalent chromium percentages of total chromium used at proposal were used for the 

final rule analysis (12% for coal units and 18% for oil units).
3

1.3 Emissions Estimates

Tables 2 through 4 give the estimated emissions for the facilities remodeled. These 

emissions estimates reflect the changes in heat inputs and emission factors described above. 

While the average values shown were not used in the modeling (the year-specific values were 

used), they are provided for comparison to the emissions values used at proposal. The detailed 

calculations and unit-level emissions are available in the case study emissions spreadsheet. 

Table 2: Final case study arsenic emissions (tons/year) compared to emissions used at 

proposal.

Facility 2005 2006 2007 2008 2009 Average

Proposed 

Rule

Amerenue-Labadie 1.05E+00 1.06E+00 1.14E+00 1.03E+00 1.04E+00 1.07E+00 1.72E+00

City Utilities of Springfield 

-James River 1.96E-02 1.90E-02 1.63E-02 1.82E-02 1.43E-02 1.75E-02 1.78E-02

Conesville 1.65E-01 1.46E-01 1.76E-01 1.76E-01 1.13E-01 1.55E-01 3.26E-01

Dominion - Yorktown 2.29E-01 1.32E-01 1.73E-01 1.35E-01 1.17E-01 1.57E-01 2.09E-01

Dominion Chesapeake 

Energy Center 2.57E-01 2.36E-01 2.55E-01 2.26E-01 2.20E-01 2.39E-01 3.46E-01

Heco Waiau 2.53E-02 2.50E-02 2.56E-02 2.30E-02 2.57E-02 2.49E-02 2.46E-02

OG&E -Muskogee 6.65E-01 6.56E-01 5.48E-01 6.54E-01 5.78E-01 6.20E-01 9.11E-01

PSHNH -Merrimack 2.76E-01 2.79E-01 2.94E-01 2.46E-01 2.05E-01 2.60E-01 2.33E-01

TVA Gallatin 1.45E-02 1.49E-02 1.54E-02 1.58E-02 1.27E-02 1.47E-02 1.42E-02

Table 3: Final case study hexavalent chromium emissions (tons/year) compared to 

emissions used at proposal.

Facility 2005 2006 2007 2008 2009 Average

Proposed 

Rule

Amerenue-Labadie 4.94E-01 4.99E-01 5.35E-01 4.86E-01 4.88E-01 5.00E-01 5.77E-01

City Utilities of Springfield 

-James River 5.55E-01 5.42E-01 4.66E-01 5.24E-01 4.13E-01 5.00E-01 4.99E-01

Conesville 4.06E-01 2.85E-01 3.77E-01 4.53E-01 2.74E-01 3.59E-01 6.74E-01

Dominion - Yorktown 1.51E+00 2.84E-01 7.78E-01 3.67E-01 2.51E-01 6.37E-01 4.25E-01

Dominion Chesapeake 

Energy Center 1.20E-01 1.11E-01 1.20E-01 1.06E-01 1.03E-01 1.12E-01 1.27E-01

Heco Waiau 2.86E-03 2.89E-03 2.86E-03 2.60E-03 2.91E-03 2.82E-03 2.87E-03

OG&E -Muskogee 3.12E-01 3.08E-01 2.57E-01 3.07E-01 2.71E-01 2.91E-01 3.06E-01

PSHNH -Merrimack 3.69E-02 3.73E-02 3.94E-02 3.29E-02 2.75E-02 3.48E-02 4.77E-02

TVA Gallatin 1.16E+00 1.19E+00 1.23E+00 1.27E+00 1.02E+00 1.17E+00 1.14E+00
4

Table 4: Final case study nickel emissions (tons/year) compared to emissions used at 

proposal.

Facility 2005 2006 2007 2008 2009 Average

Proposed 

Rule

Amerenue-Labadie 2.31E+00 2.33E+00 2.50E+00 2.27E+00 2.28E+00 2.34E+00 4.21E+00

City Utilities of Springfield 

-James River 3.36E+00 3.27E+00 2.80E+00 3.15E+00 2.48E+00 3.01E+00 3.03E+00

Conesville 3.26E+00 2.59E+00 3.25E+00 3.55E+00 2.21E+00 2.97E+00 3.95E+00

Dominion - Yorktown 7.26E+01 1.18E+01 3.63E+01 1.59E+01 1.04E+01 2.94E+01 1.84E+01

Dominion Chesapeake 

Energy Center 5.63E-01 5.17E-01 5.59E-01 4.96E-01 4.83E-01 5.23E-01 8.89E-01

Heco Waiau 3.96E+00 3.93E+00 4.01E+00 3.60E+00 4.02E+00 3.90E+00 3.87E+00

OG&E -Muskogee 1.46E+00 1.44E+00 1.20E+00 1.43E+00 1.27E+00 1.36E+00 2.23E+00

PSHNH -Merrimack 1.55E-01 1.56E-01 1.65E-01 1.38E-01 1.15E-01 1.46E-01 2.85E-01

TVA Gallatin 5.48E+00 5.61E+00 5.80E+00 5.97E+00 4.80E+00 5.53E+00 5.35E+00

Annual emissions estimates for all case study facilities are given in Table 5, including estimates 

for facilities that have been revised as discussed above, and the same estimates used at the time 

of proposal for facilities without revised emissions estimates.
5

Table 5. Case study average annual emissions (2005-2009).

Facility Location Unit Type Unit ID As (TPY) Cr+6 (TPY) Ni (TPY)

Xcel Bayfront Ashland, WI 1 coal 5

Cambria Cogen Ebensburg, PA 2 coal B1 1.00E-03 3.60E-02 2.20E-01

B2 1.10E-03 7.80E-03 7.20E-02

SC&E Canadys Canadys, SC 3 coal

CAN001 5.10E-02 1.90E-02 1.30E-01

CAN002 4.90E-02 1.80E-02 1.30E-01

CAN003 2.20E-02 2.40E-03 9.60E-02

Dominion Chesapeake Energy Center* Chesapeake, VA 4 coal

Unit 1 4.1E-02 1.9E-02 8.9E-02

Unit 2 4.4E-02 2.1E-02 9.6E-02

Unit 3 6.5E-02 3.0E-02 1.4E-01

Unit 4 9.0E-02 4.2E-02 2.0E-01

Conesville* Conesville, OH 4 coal

3 4.6E-02 3.3E-01 1.8E+00

4 4.7E-02 1.1E-02 5.1E-01

5 3.2E-02 7.2E-03 3.5E-01

6 3.0E-02 6.9E-03 3.3E-01

Exelon Cromby Generating Station Phoenixville, PA 1 coal Unit 1 6.60E-02 3.00E-03 1.80E-02

1 oil Unit 2 2.40E-03 6.00E-04 3.40E-01

TVA Gallatin* Gallatin, TN 4 coal

1 3.5E-03 2.8E-01 1.3E+00

2 3.4E-03 2.8E-01 1.3E+00

3 3.9E-03 3.2E-01 1.5E+00

4 3.8E-03 3.1E-01 1.4E+00

City Utilities of Springfield -James River* Springfield, MO 3 coal

3 3.9E-03 1.0E-01 6.3E-01

4 4.2E-03 3.8E-02 4.1E-01

5 9.4E-03 3.6E-01 2.0E+00

Amerenue-Labadie* Labadie, MO 4 coal

1 2.4E-01 1.1E-01 5.4E-01

2 2.7E-01 1.3E-01 5.9E-01

3 2.7E-01 1.3E-01 6.0E-01

4 2.8E-01 1.3E-01 6.1E-01

PSHNH –Merrimack* Bow, NH 2 coal 1 7.8E-02 1.0E-02 4.4E-02

2 1.8E-01 2.4E-02 1.0E-01

Monticello Steam Electric Plant Mount Pleasant, TX 3 coal 1 6.10E-02 1.16E-01 4.10E-01

2 6.20E-02 1.17E-01 4.10E-01
6

Facility Location Unit Type Unit ID As (TPY) Cr+6 (TPY) Ni (TPY)

3 1.10E-01 1.98E-02 1.70E-01

OG&E –Muskogee* Fort Gibson, OK 3 coal

4 2.0E-01 9.5E-02 4.4E-01

5 2.0E-01 9.3E-02 4.4E-01

6 2.2E-01 1.0E-01 4.8E-01

Spruance Genco Richmond, VA 8 coal

GEN1 8.40E-04 3.10E-04 2.40E-03

GEN2 9.00E-04 3.20E-04 8.30E-03

GEN3 3.50E-03 5.60E-04 6.20E-03

GEN4 2.40E-03 3.60E-04 4.10E-03

PSI Energy – Wabash River West Terre Haute, IN

1 coal-gas PG7221FA 1.30E-03 7.50E-04 5.00E-03

2 coal

4 1.10E-01 4.50E-03 4.10E-02

6 3.30E-01 1.40E-02 1.20E-01

Heco Waiau* Waiau, HI 6 oil

W3 2.0E-03 2.1E-04 3.2E-01

W4 2.0E-03 2.1E-04 3.0E-01

W5 2.8E-03 2.9E-04 4.3E-01

W6 2.1E-03 1.2E-04 3.2E-01

W7 8.5E-03 1.2E-03 1.4E+00

W8 7.5E-03 7.9E-04 1.2E+00

Dominion – Yorktown* Yorktown, VA 2 coal Units 1&2 1.2E-01 5.6E-02 2.6E-01

1 oil Unit 3 3.9E-02 5.8E-01 2.9E+01

* Facility was remodeled for the final rule. Facilities not remodeled for the final rule have the same annual emissions estimate for each year.
7

2. Dispersion Modeling

The methodology used for dispersion modeling of the case study facilities was discussed in 

more detail in the previous document.2 The modeling was done using AERMOD, EPA’s 

preferred model for near-field dispersion. Table 6 provides the model scenario information used 

for the case study facilities. This information is unchanged from the proposed rule. Table 7 

provides the stack parameters used for the final rule modeling.

Table 6. Model scenario information.

Facility Downwash

Urban/rural 

(population) Surface station Upper air station

Xcel Bayfront No Rural

Ashland Kennedy 

Memorial Airport, WI Minneapolis, MN

Cambria Cogen Yes Rural

Johnstown Cambria 

County Airport, PA Pittsburgh, PA

SC&E Canadys No Rural

Charleston Intl. 

Airport, SC

Charleston Intl. 

Airport, SC

Dominion Chesapeake 

Energy Center* No Urban (200,000)

Norfolk Intl. Airport, 

VA

Washington Dulles, 

VA

Conesville* No Rural

Zanesville Municipal 

Airport, OH Wilmington, OH

Exelon Cromby 

Generating Station No Rural

Philadelphia Intl. 

Airport, PA

Washington Dulles, 

VA

TVA Gallatin* No Rural

Nashville Intl. Airport, 

TN

Nashville Intl. 

Airport, TN

City Utilities of 

Springfield -James 

River* No Rural

Springfield Regional 

Airport, MO

Springfield Regional 

Airport, MO

Amerenue-Labadie* No Rural

St. Louis Lambert Intl. 

Airport, MO Lincoln, IL

PSHNH –Merrimack* No Rural

Concord Municipal 

Airport, NH Albany, NY

Monticello Steam 

Electric Plant No Rural Tyler Pounds Field, TX Shreveport, LA

OG&E –Muskogee* No Rural

Muskogee Davis Field, 

OK Norman, OK

Spruance Genco No Urban (200,000)

Richmond Intl. Airport, 

VA

Washington Dulles, 

VA

PSI Energy – Wabash 

River No Rural

Terre Haute Hulman 

Regional Airport ,IN Wilmington, OH

Heco Waiau* Yes Urban (300,000)

Honolulu Intl Airport, 

HI Lihue, HI

Dominion –

Yorktown* Yes Rural

Newport News Intl. 

Airport, VA

Washington Dulles, 

VA

* Facility was remodeled for the final rule.

 

2 Non-Hg Case Study Chronic Inhalation Risk Assessment for the Utility MACT “Appropriate and Necessary”

Analysis, March 16, 2011. Docket ID No. EPA-HQ-OAR-2009-0234-2939. Available at www.regulations.gov.
8

Table 7. Stack parameter data.

Facility Location Unit Type Unit ID Lat Long

Stack Height 

(m)

Stack 

Temperature 

(K)

Stack 

Velocity 

(m/s)

Stack 

Diameter 

(m)

Xcel Bayfront Ashland, WI 1 coal 5 46.5872 -90.9018 59.44 371.48 13.05 1.86

Cambria 

Cogen Ebensburg, PA 2 coal B1 40.4748 -78.703 70.10 466.48 27.83 2.29

B2 40.4748 -78.703 70.10 466.48 27.83 2.29

SC&E Canadys Canadys, SC 3 coal

CAN001 33.0646 -80.6235 60.96 415.37 11.00 4.88

CAN002 33.0653 -80.6232 60.96 412.04 12.62 4.88

CAN003 33.065 -80.6218 60.96 413.71 19.93 4.88

Dominion 

Chesapeake 

Energy 

Center*

Chesapeake, VA 4 coal

Unit 1 36.7705 -76.3012 53.34 430.35 17.37 3.97

Unit 2 36.7706 -76.3011 53.34 429.25 17.37 3.97

Unit 3 36.7709 -76.3009 60.96 407.05 17.98 3.97

Unit 4 36.7712 -76.3008 60.96 427.05 21.34 4.27

Conesville* Conesville, OH 4 coal

3 40.1648 -81.9044 137.16 416.48 10.31 5.33

4 40.1862 -81.8787 243.84 416.48 25.30 7.93

5 40.1856 -81.8798 243.84 324.82 23.90 7.93

6 40.1856 -81.8798 243.84 324.82 23.90 7.93

Exelon Cromby 

Generating 

Station

Phoenixville, PA

1 coal Unit 1 40.1524 -75.5303 91.44 388.71 17.92 4.27

1 oil Unit 2 40.152 -75.5304 91.44 388.71 17.11 4.27

TVA Gallatin* Gallatin, TN 4 coal

1 36.3156 -86.4005 152.70 406.48 17.07 7.62

2 36.3156 -86.4005 152.70 406.48 17.07 7.62

3 36.3151 -86.4009 153.01 395.37 18.90 7.62

4 36.3151 -86.4009 153.01 395.37 18.90 7.62

City Utilities of 

Springfield -

James River*

Springfield, MO 3 coal

3 37.1084 -93.2602 60.96 427.59 9.45 3.65

4 37.1084 -93.2598 60.96 427.59 10.06 3.65

5 37.1084 -93.2605 106.68 433.15 27.70 3.12

AmerenueLabadie* Labadie, MO 4 coal

1 38.5626 -90.8381 213.36 436.21 32.66 6.25

2 38.5621 -90.8377 213.36 424.21 30.32 6.25

3 38.5614 -90.8371 213.36 413.54 31.96 6.25

4 38.5614 -90.8371 213.40 446.93 34.83 6.25

PSHNH –

Merrimack* Bow, NH 2 coal 1 43.142 -71.4685 68.58 391.48 41.98 2.62

2 43.1418 -71.4682 96.62 422.04 36.92 4.42
9

Facility Location Unit Type Unit ID Lat Long

Stack Height 

(m)

Stack 

Temperature 

(K)

Stack 

Velocity 

(m/s)

Stack 

Diameter 

(m)

Monticello 

Steam Electric 

Plant

Mount Pleasant, 

TX 3 coal

1 33.0907 -95.0375 121.92 453.15 24.69 6.55

2 33.0914 -95.038 121.92 453.15 24.69 6.55

3 33.0923 -95.0378 140.21 354.26 26.52 2.44

OG&E –

Muskogee* Fort Gibson, OK 3 coal

4 35.7618 -95.2886 106.68 402.04 14.02 7.31

5 35.7619 -95.288 106.68 402.04 14.02 7.31

6 35.7621 -95.2872 152.40 402.04 25.13 6.55

Spruance 

Genco Richmond, VA 8 coal

GEN1 37.4552 -77.4312 76.20 355.37 17.03 2.62

GEN2 37.4555 -77.4309 76.20 355.37 17.03 2.62

GEN3 37.4557 -77.4307 76.20 355.37 17.03 2.62

GEN4 37.4559 -77.4304 76.20 355.37 17.03 2.62

PSI Energy –

Wabash River

West Terre 

Haute, IN

1 coal-gas PG7221FA 39.5303 -87.4256 68.58 452.59 19.17 5.49

2 coal

4 39.5274 -87.4232 137.16 410.93 34.26 7.62

6 39.5274 -87.4232 137.16 410.93 34.26 7.62

Heco Waiau* Waiau, HI 6 oil

W3 21.3891 -157.9615 42.09 469.26 12.25 3.05

W4 21.389 -157.9613 42.09 469.26 12.25 3.05

W5 21.3888 -157.9612 41.91 414.26 12.25 2.74

W6 21.3887 -157.961 41.91 414.26 12.25 2.74

W7 21.3885 -157.9606 41.91 392.04 16.12 3.20

W8 21.3884 -157.9603 41.91 392.04 16.12 3.20

Dominion –

Yorktown* Yorktown, VA 2 coal Units 1&2 37.2154 -76.4622 98.80 417.35 22.60 4.90

1 oil Unit 3 37.2152 -76.4612 149.05 415.93 33.52 6.86

* Facility was remodeled for the final rule.
10

The list below discusses the inputs used in the modeling of the case study facilities for the 

final rule, and notes where there were differences compared to the proposed rule.

1. Each boiler or combination of boilers was modeled as an individual emission point in 

AERMOD, unchanged from the proposed rule analysis.

2. Hourly emissions for 2005-2009 were modeled explicitly for each emission point for 

arsenic, chromium (VI), and nickel. Five-year average concentrations were calculated 

within AERMOD. For the proposed rule analysis, a unit emission rate (1 gram per 

second ) was used, and the resulting hourly concentrations were scaled using hourly heat 

input values to derive pollutant-specific 5-year average concentrations calculated outside 

of AERMOD. This methodology difference does not change the concentration estimates.

3. Building parameterization and surface characteristics were unchanged from the proposed 

rule analysis.

4. Some stack parameters changed because of new data received during the public comment 

period.

5. Current versions of AERMINUTE (11059) and AERMET (11059) were used, and the 

meteorological data used for the proposed rule analysis were reprocessed using these 

versions. Beta versions of AERMINUTE and AERMET were used for the proposed rule 

analysis.

6. Receptor locations (Census blocks within 20 km of the source) were unchanged from the 

proposed rule analysis. The current version of AERMAP (11103) was used to calculate 

source and receptor elevations, whereas version 09040 was used for the proposed rule 

analysis.

7. The current version of AERMOD (11103) was used. A beta version of AERMOD was 

used for the proposed rule analysis.

As noted above, updated versions of AERMAP, AERMINUTE, AERMET, and AERMOD 

were used in the modeling of the case study facilities for the final rule. The changes between 

versions 09040 and 11103 of AERMAP resulted in no differences in elevations of sources and 

receptors. The meteorological data used for the proposed rule were reprocessed using versions 

11059 of AERMINUTE and AERMET, and processed along with the proposal emission inputs 

in version 11103 of AERMOD to compare differences due to AERMET and AERMOD changes. 

The differences in the modeled concentrations were insignificant between the beta and current 

versions of AERMET and AERMOD.

3. Chronic Inhalation Risk Assessment

For chronic inhalation exposures, we used the 5-year average ambient concentrations of

HAP estimated from the refined dispersion modeling. The estimated ambient concentration at 

each nearby census block centroid was used as a surrogate for the chronic inhalation exposure 

concentration for all the people who reside in that census block. We assessed non-cancer health 

effects from chronic exposures by comparing the chronic inhalation exposure concentration to 

the Reference Concentration values. We calculated the maximum individual risk, or MIR, for 

each facility as the cancer risk associated with a continuous lifetime (24 hours per day, 7 days 

per week, and 52 weeks per year for a 70-year period) exposure to the maximum concentration at 

the centroid of an inhabited census block. Individual cancer risks were calculated by multiplying 
11

the estimated lifetime exposure to the ambient concentration of each HAP (in micrograms per 

cubic meter) by its cancer unit risk estimate (URE), which is an upper bound estimate of an 

individual's probability of contracting cancer over a lifetime of exposure to a concentration of 1 

microgram of the pollutant per cubic meter of air. We used URE values for arsenic and 

hexavalent chromium from EPA’s Integrated Risk Information System (IRIS), which is a human 

health assessment program that evaluates quantitative and qualitative risk information on effects 

that may result from exposure to environmental contaminants. Unit risk estimates in IRIS have 

undergone both internal and external peer review. 

In the preamble to the proposed rule, we discussed the reasons for using 65% of the IRIS 

URE for nickel subsulfide for all nickel compounds. In July 2011, we completed an external 

peer review (using three independent expert reviewers) of the methods used to evaluate the risks 

from nickel compounds emitted by EGUs in a report titled, “Methods to Develop Inhalation 

Cancer Risk Estimates for Chromium and Nickel Compounds.”

3 Based on the views of major 

scientific bodies such as the International Agency for Research on Cancer (IARC), the World 

Health Organization (WHO), and the European Union’s Scientific Committee on Health and 

Environmental Risks (SCHER), and those of the expert peer reviewers who commented on our

approaches to risk characterization of nickel compounds, we consider all nickel compounds to be 

carcinogenic as a group and do not consider nickel speciation or nickel solubility to be strong 

determinants of nickel carcinogenicity. 

 

Based on this review, we decided to use 100 percent of the current IRIS URE for nickel 

subsulfide, rather than assuming that 65 percent of the total mass of emitted nickel might be 

nickel subsulfide, as used in previous analyses. We used the IRIS URE value because IRIS 

values are typically preferred for use in HAP risk assessments performed in support of air toxics 

regulations under the Clean Air Act.

4

The IRIS values are preferred because they are developed 

in accordance with EPA risk assessment guidelines and because of the level of peer review IRIS

values receive. We used 100 percent of the IRIS value because of the concerns about the 

potential carcinogenicity of all forms of nickel raised by the major national and international 

scientific bodies. Nevertheless, taking into account that there are potential differences in toxicity 

and/or carcinogenic potential across the different nickel compounds, and given that there have 

been two URE values derived for exposure to mixtures of nickel compounds that are 2-3 fold 

lower than the IRIS URE for nickel subsulfide 5, the EPA also considers it reasonable to use a 

value that is 50 percent of the IRIS URE for nickel subsulfide for providing an estimate of the 

lower end of a plausible range of cancer potency values for different mixtures of nickel 

compounds. 

The health reference values used in the assessment are given in Table 8.

 

3 Mercury and Air Toxics Standards Rule Docket, ID No. EPA-HQ-OAR-2009-0234. Available at 

www.regulations.gov.

4

http://www.epa.gov/ttn/atw/toxsource/chronicpriority.html

5 Two UREs (other than the current IRIS values) have been derived for nickel compounds: one developed by the 

California Department of Health Services (http://www.arb.ca.gov/toxics/id/summary/nickel_tech_b.pdf) and the 

other by the Texas Commission on Environmental Quality 

(http://www.epa.gov/ttn/atw/nata1999/99pdfs/healtheffectsinfo.pdf).
12

Table 8. Health reference values used in the assessment.

Pollutant CAS Number

URE

(1/μg/m3) Source

RfC

(mg/m3) Source

Arsenic 7440382 4.3E-03 IRIS 0.000015 CalEPA

Chromium (VI) 18540299 0.012 IRIS 0.0001 IRIS

Nickel 7440020 0.00048 IRIS 0.00009 CalEPA

HCl 7647010 0.02 IRIS

3. Results.

The results of the assessment are given in Table 9. Based on estimated actual emissions, 

the highest estimated lifetime cancer risk from any of the sixteen case study facilities was 20 in a 

million, driven by nickel emissions from the one case study facility with only oil-fired EGUs. 

For the facilities with coal-fired EGUs, there were five with maximum cancer risks greater than 1 

in a million (the highest was five in a million), four driven by hexavalent chromium, and one 

driven by nickel from an oil-fired EGU. There were also two facilities with coal-fired EGUs with 

cancer risks at 1 in a million. All of the facilities had non-cancer target-organ-specific hazard 

index (HI) values less than one, with a maximum HI value of 0.4 (also driven by nickel 

emissions from the one case study facility with only oil-fired EGUs).

The cancer risk estimates from this assessment indicate that the EGU source category 

would not be eligible for delisting under section 112(c)(9)(B)(i) of the CAA, which specifies that 

a category may be delisted only when the Administrator determines “... that no source in the 

category (or group of sources in the case of area sources) emits such hazardous air pollutants in 

quantities which may cause a lifetime risk of cancer greater than one in one million to the 

individual in the population who is most exposed to emissions of such pollutants from the 

source....” We note that, since these case studies do not cover all facilities in the category, and 

since our assessment does not include the potential for impacts from different EGU facilities to 

overlap one another (i.e., these case studies only look at facilities in isolation), the maximum risk 

estimates from the case studies may underestimate true maximum risks.
13

Table 9. Chronic inhalation risk assessment results.

Facility

Proposed Rule Assessment Final Rule Assessment

Max 

Risk Risk Driver Max HI

HI 

Driver

Max 

risk Risk Driver

Max 

HI

HI 

Driver

Xcel 

Bayfront 4.0x10-9Formaldehyde 0.005 HCl Not Remodeled

Cambria 

Cogen 5.0x10-7Chromium VI 0.003 Nickel Not Remodeled

SC&E 

Canadys 6.0x10-7Arsenic 0.009 HCl Not Remodeled

Dominion 

Chesapeake 

Energy 

Center 3.0x10-6Chromium VI 0.05 HCl 2x10-6 Chromium VI 0.05

a

HCl a

Conesville 3.0x10-6Chromium VI 0.01 Nickel 2x10-6 Chromium VI 0.008 Nickel

Exelon 

Cromby 

Generating 

Station 3.0x10-7Arsenic 0.008 Nickel Not Remodeled

TVA Gallatin 1.0x10-6Chromium VI 0.006 Nickel 2x10-6 Chromium VI 0.007 Nickel

City Utilities 

of 

Springfield -

James River 8.0x10-6Chromium VI 0.04 Nickel 5x10-6 Chromium VI 0.03 Nickel

AmerenueLabadie 8.0x10-7

Arsenic 0.006 Arsenic 7x10-7 Chromium VI 0.004 Arsenic

PSHNH -

Merrimack 1.0x10-6Arsenic 0.01 Arsenic 1x10-6 Arsenic 0.01 Arsenic

Monticello 

Steam 

Electric 

Plant 6.0x10-7Chromium VI 0.003 Arsenic Not Remodeled

OG&E -

Muskogee 1.0x10-6Arsenic 0.01 Arsenic 1x10-6 Chromium VI 0.008 Arsenic

Spruance 

Genco 8.0x10-8Arsenic 0.007 HCl Not Remodeled

PSI Energy –

Wabash 

River 1.0x10-7Arsenic 0.001 Arsenic Not Remodeled

Heco Waiau 1.0x10-5Nickel 0.4 Nickel 2x10-5

b

Nickel 0.4 Nickel

Dominion -

Yorktown 1.0x10-6Chromium VI 0.02 Nickel 2x10-6

b

Nickel 0.03 Nickel

a

Although HCl was not included in the remodeling of this facility, HCl was the HI driver pollutant at proposal, and 

this is carried through for the final rule assessment.

b

Based on considering the emitted nickel to be 100% as potent a carcinogen as pure nickel subsulfide; if we 

consider the emitted nickel to be 50% as potent a carcinogen as nickel subsulfide (see text), estimated risks would be 

1x10-5for Heco Waiau and 1x10-6for Dominion - Yorktown.

United States

Environmental Protection

Agency

Office of Air Quality Planning and Standards

Research Triangle Park, NC

Publication No. EPA-452/R-11-013

November 2011