1

MEMORANDUM

SUBJECT: Non-Hg Case Study Chronic Inhalation Risk Assessment for the Utility MACT 

Appropriate and Necessary Analysis

FROM: Madeleine Strum, Emissions Inventory and Analysis Group (C339-02)

James Thurman, Air Quality Modeling Group (C439-01)

Mark Morris, Air Toxics Assessment Group (C539-02)

TO: Docket EPA-HQ-OAR-2009-0234

DATE: March 16, 2011

To estimate the potential for human health impacts from current emissions of HAPs other 

than mercury from coal- and oil-fired electric utility steam generating units (EGUs), several 

facilities were selected as case studies for a chronic inhalation risk assessment, and this memo 

documents the methods and results of the assessment for these case studies. The assessment was 

performed in support of the appropriate and necessary analysis for coal- and oil-fired EGUs.

1. Case Study Selection

An initial set of eight case study facilities was selected based on several factors. First, we 

considered facilities with the highest estimated cancer and non-cancer risks using the 2005 

National Emissions Inventory (NEI) data and the Human Exposure Model (HEM-3)

1

. The 2005 

NEI data were used because the initial set of case study facilities was selected before we received 

the bulk of the emissions data from the “Information Collection Effort for New and Existing 

Coal- and Oil-fired Electric Utility Steam Generating Units” (EPA ICR No. 2362.01; OMB 

Control Number 2060-0631). The criteria considered in the selection of the initial set of case 

study facilities are given below:

o Focused on the highest risk facilities based upon previous studies. We identified the top

facilities in terms of total inhalation cancer and non-cancer risk based on the 2005 NEI and 

the use of HEM-3, following the same approach used for risk assessments performed for 

section 112(f) of the Clean Air Act. 

o Screened the top facilities to exclude those that (a) were not power generating facilities that 

contribute to the grid, (b) for which ICR facility data were not available, and (c) installed

emissions control equipment, suggested that the 2005 NEI may not reflect current conditions 

at those facilities (and, therefore, may have significantly lower risks). 

o Provided coverage, to the extent possible, for the different cancer and non-cancer risk drivers 

identified in risk modeling for the top EGUs in each category.

o Provided coverage for different regions of the country, including those believed to have 

higher US EGU mercury impacts (e.g., eastern portion of the country, primarily NE and SE). 

o Visually inspected aerial images of each EGU location to verify that there are potential 

residential locations located nearby and, consequently, possible near-field inhalation impacts. 

 1 http://www.epa.gov/ttn/fera/hem_download.html
2

Based on application of these criteria, the initial set of case study facilities included: Xcel 

Bayfront, SC&E Canadys, Dominion Chesapeake Energy Center, Exelon Cromby Generating 

Station, Spruance Genco, PSI Energy – Wabash River, Heco Waiau, and Dominion – Yorktown. 

After the receipt of more data through the ICR, additional case study facilities were selected, 

based on the magnitude of emissions, heat input values (throughput), and level of emission 

control. For the initial set of case study facilities, emissions were estimated for all HAPs 

included in the available ICR data. Because dispersion modeling and risk estimates were 

performed for the initial set of case study facilities before the selection of the additional facilities, 

the results of the initial set were used to focus the assessment of the additional facilities. The 

results for the initial set indicated that nickel, hexavalent chromium, and arsenic were the cancer 

risk drivers, and that non-cancer risks were not significant. Although the non-cancer risks were 

low (the maximum chronic hazard index was 0.4), they were driven by emissions of nickel, 

arsenic and hydrogen chloride. For these reasons, the selection of additional case study facilities 

focused on nickel, hexavalent chromium, and arsenic. The following criteria were used to select 

the additional case study facilities:

o Considered the potential emissions of EGUs based on generating capacity (> 300 MW).

o Focused on coal-fired EGUs with the highest emission rates of chromium and arsenic from 

test data. We computed an approximate emission rate using 2008 heat input data from the 

Clean Air Markets data and maps2

o Focused on untested EGUs with the highest throughput and minimal emission controls. For 

throughput, we used 2008 heat input data.

, a unit-specific emission factor estimated from the ICR 

data, and average capacity utilization based on 2007-2009 data provided in the ICR data.

An additional eight case study facilities were selected using these criteria: Cambria Cogen, 

Conesville, TVA Gallatin, City Utilities of Springfield -James River, Amerenue-Labadie, 

PSHNH –Merrimack, Monticello Steam Electric Plant, and OG&E –Muskogee. Considering 

both sets of case study facilities, there were a total of sixteen facilities, fifteen that use coal as 

fuel, and one that uses oil. The locations of the case study facilities are shown in Figure 1.

It is important to note that the case study facilities likely do not represent the highest-emitting 

(and highest-risk) sources. Although facility selection criteria included high estimated cancer 

and non-cancer risks using the 2005 NEI data, high throughput, and minimal emission control, 

another necessary criterion was the availability of ICR data for the EGUs at those facilities (or 

for similar EGUs at other facilities). Because the ICR data were collected for the purpose of 

developing the MACT standards, the ICR was targeted towards better performing sources, with a 

smaller set of random recipients. Therefore, facilities for which ICR data were available may not 

represent the highest-emitting sources.

 2 http://camddataandmaps.epa.gov/gdm/
3

Figure 1. Location of Case Study Sites.

2. Methods

This section describes the methods used to estimate emissions for the case study facilities, 

and the dispersion modeling and risk estimation methods used.

2.1 Emissions Estimation

For the initial set of case study facilities, emissions were estimated using ICR data for metals, 

HCl, and several organic compounds, such as ethylene dibromide, polycyclic aromatic 

hydrocarbons, and naphthalene. However, because the ICR emission rate estimates of organic 

HAP emissions were based on minimum detectable levels, they were not used in the assessment 

of any of the case study facilities. As discussed above, only emissions of nickel, hexavalent 

chromium, and arsenic were estimated for the additional set of case study facilities.

For all case study facilities, emissions were developed using the data from Part III of the 

ICR. Emission factors, in lbs of pollutant per million BTU, were developed directly from the 

test data, where available. Where test data were not available for a specific unit, emission factors

were derived from similarly configured units at the facility (site-average across tested units) or 

from similarly configured units from other facilities tested (average across tested units with 
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similar configuration). Only EGUs were considered in the assessment; other emitting processes

located at the facilities were not considered.

Speciated chromium emissions data were not available from the ICR. Speciated chromium 

emissions data are available from existing test data on four coal-fired boilers and seven oil-fired 

boilers. These test data indicate that hexavalent chromium is on average twelve percent and 

eighteen percent of the emissions from coal- and oil-fired boilers, respectively, and we applied 

these values to the total chromium emissions estimates to estimate hexavalent chromium 

emissions.

“Actual” annual emissions were computed with the following equation:

In the above equation, the annual emission rate was computed in tons per year using an 

emission factor in lb/MMBtu and the maximum heat input in MMBtu/hr. The capacity factor 

was taken as the boiler-specific 3-year average capacity factor provided by ICR respondents of 

Part I of the ICR. If a respondent did not provide a 3-year average capacity factor for a boiler, a 

default value of 100 percent was used. For the set of additional case study facilities, potential 

emissions were computed using a 100 percent average capacity factor.

Hourly emissions for each hour in years 2005-2009 were generated from the actual annual 

emissions by using hourly heat input as a temporalization factor. That is, the hourly heat input 

divided by the annual heat input was multiplied by the annual emissions to get hourly emissions 

for each pollutant. The same annual emissions value was used for all 5 years. Hourly heat input

values were obtained from Clean Air Markets continuous emission monitoring (CEM) data. 

Where data were missing for several months for a specific year, the data for the same period of a 

different year were used. In situations where hourly heat inputs were unavailable for the entire 

year, a State average temporalization for the year 2005 was used for all years. The State average

data were readily available from national scale modeling efforts. For the Heco Waiau case 

study, neither unit nor State average data were available, so we used facilities with CEM data

from Broward and Miami-Dade Counties as a surrogate for Hawaii CEM heat input for each of 

the five years. Hourly emissions derived from the potential emissions were created assuming the 

same emissions for each hour. 

Table 1 provides facility and unit location, and stack parameter data for all units in the case 

studies. Table 2 provides estimates of actual and potential emissions.
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Table 1. Unit Locations and Stack Parameters.

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 53.34 414.26 17.68 3.96

Unit 2 36.7706 -76.3011 53.34 412.59 18.25 3.96

Unit 3 36.7709 -76.3009 60.96 413.15 21.29 3.66

Unit 4 36.7712 -76.3008 60.96 382.04 21.45 4.27

Conesville Conesville, OH 4 coal

3 40.1648 -81.9044 137.16 416.48 12.19 5.33

4 40.1862 -81.8787 243.84 416.48 25.36 7.92

5 40.1856 -81.8798 243.84 324.82 23.92 7.92

6 40.1856 -81.8798 243.84 324.82 23.92 7.92

Exelon Cromby Generating 

Station Phoenixville, PA coal (unit 1) 

oil (unit 2)

Unit 1 40.1524 -75.5303 91.44 388.71 17.92 4.27

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 152.86 408.15 21.76 7.62

2 36.3156 -86.4005 152.86 408.15 21.76 7.62

3 36.3151 -86.4009 152.86 408.15 24.57 7.62

4 36.3151 -86.4009 152.86 408.15 24.57 7.62

City Utilities of Springfield -

James River Springfield, MO 3 coal

3 37.1084 -93.2602 60.96 422.04 4.57 3.66

4 37.1084 -93.2598 60.96 422.04 5.70 3.66

5 37.1084 -93.2605 106.68 422.04 14.72 2.53

Amerenue-Labadie Labadie, MO 4 coal

1 38.5626 -90.8381 213.36 444.26 28.04 6.25

2 38.5621 -90.8377 213.36 444.26 28.04 6.25

3 38.5614 -90.8371 213.36 444.26 28.04 8.84

4 38.5614 -90.8371 213.36 444.26 28.04 8.84

PSHNH -Merrimack Bow, NH 2 coal 1 43.142 -71.4685 69.19 433.15 45.42 2.59
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Facility Location Unit Type Unit ID Lat Long

Stack

Height

(m)

Stack

Temperature 

(K)

Stack

Velocity 

(m/s)

Stack

Diameter 

(m)

2 43.1418 -71.4682 97.23 449.82 31.09 4.42

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 17.13 7.32

5 35.7619 -95.288 106.68 402.04 17.13 7.32

6 35.7621 -95.2872 152.40 402.04 24.84 6.55

Spruance Genco Richmond, VA 8 coal

GEN1 37.4552 -77.4312 76.20 355.37 17.03 2.62128

GEN2 37.4555 -77.4309 76.20 355.37 17.03 2.62128

GEN3 37.4557 -77.4307 76.20 355.37 17.03 2.62128

GEN4 37.4559 -77.4304 76.20 355.37 17.03 2.62128

PSI Energy – Wabash River West Terre 

Haute, IN

1 coal-gas 

(unit 1)

2 coal (units 

4&6)

PG7221FA 39.5303 -87.4256 68.58 452.59 19.17 5.49

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 3.2

W8 21.3884 -157.9603 41.91 392.04 16.12 3.2

Dominion - Yorktown Yorktown, VA

2 coal (units 

1&2)

1 oil (unit 3)

Units 1&2 37.2154 -76.4622 98.76 402.04 32.77 5.18

Unit 3 37.2152 -76.4612 149.05 415.93 33.52 6.86
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Table 2. Unit Emissions Estimates.

Facility Location Unit Type Unit ID

As

(TPY) 

actual

As

(TPY) 

potential

Cr+6

(TPY)* 

actual

Cr+6

(TPY)* 

potential

Nickel 

(TPY)

actual

Nickel 

(TPY) 

potential

HCl (TPY) 

actual

Basis of Emissions 

Factor 

(lbs/MMBtu) **

Xcel 

Bayfront Ashland, WI 1 coal 5 n/a n/a n/a 40 similar config

Cambria 

Cogen Ebensburg, PA 2 coal B1 used 

potential

1.0E-03 used 

potential

3.6E-02 used 

potential

2.2E-01 not 

modeled

test

B2 1.1E-03 7.8E-03 7.2E-02 test

SC&E 

Canadys Canadys, SC 3 coal

CAN001 5.1E-02

not 

modeled

1.9E-02

not 

modeled

1.3E-01

not 

modeled

114 similar config

CAN002 4.9E-02 1.8E-02 1.3E-01 111 similar config

CAN003 2.2E-02 2.4E-03 9.6E-02 4.8 test (except HCl)

Dominion 

Chesapeake 

Energy 

Center

Chesapeake, 

VA 4 coal

Unit 1 5.1E-02

not 

modeled

1.9E-02

not 

modeled

1.3E-01

not 

modeled

113 similar config

Unit 2 5.6E-02 2.1E-02 1.4E-01 125 similar config

Unit 3 1.0E-01 3.8E-02 2.7E-01 232 similar config

Unit 4 1.4E-01 5.0E-02 3.5E-01 304 similar config

Conesville Conesville, OH 4 coal

3 9.1E-02 1.2E-01 6.6E-01 8.5E-01 3.5E+00 4.5E+00

not 

modeled

test

4 1.0E-01 1.6E-01 8.1E-03 1.3E-02 1.9E-01 3.0E-01 similar config

5 6.8E-02 8.2E-02 5.3E-03 6.5E-03 1.2E-01 1.5E-01 similar config

6 6.5E-02 8.2E-02 5.1E-03 6.5E-03 1.2E-01 1.5E-01 similar config

Exelon 

Cromby 

Generating 

Station

Phoenixville, 

PA

coal (unit 1) 

oil (unit 2)

Unit 1 6.6E-02

not 

modeled

3.0E-03

not 

modeled

1.8E-02

not 

modeled

5.7 similar config

Unit 2 2.4E-03 6.0E-04 3.4E-01 0.97 test for arsenic 

only

TVA Gallatin Gallatin, TN 4 coal

1 3.3E-03 4.8E-03 2.6E-01 3.8E-01 1.2E+00 1.8E+00

not 

modeled

site average

2 3.3E-03 4.8E-03 2.7E-01 3.8E-01 1.2E+00 1.8E+00 test

3 3.8E-03 5.3E-03 3.0E-01 4.2E-01 1.4E+00 2.0E+00 site average

4 3.8E-03 5.3E-03 3.1E-01 4.2E-01 1.4E+00 2.0E+00 site average

City Utilities 

of 

Springfield -

James River

Springfield, 

MO 3 coal

3 4.1E-03 5.1E-03 1.1E-01 1.3E-01 6.6E-01 8.1E-01

not 

modeled

site average

4 4.6E-03 5.4E-03 4.1E-02 4.9E-02 4.5E-01 5.4E-01 test

5 9.1E-03 1.1E-02 3.5E-01 4.4E-01 1.9E+00 2.4E+00 test

AmerenueLabadie Labadie, MO 4 coal

1 3.9E-01 5.4E-01 1.3E-01 1.8E-01 9.5E-01 1.3E+00

not 

modeled

similar config

2 4.3E-01 5.4E-01 1.5E-01 1.8E-01 1.1E+00 1.3E+00 similar config

3 4.4E-01 5.4E-01 1.5E-01 1.8E-01 1.1E+00 1.3E+00 similar config

4 4.6E-01 5.4E-01 1.5E-01 1.8E-01 1.1E+00 1.3E+00 similar config

PSHNH - Bow, NH 2 coal 1 8.2E-02 9.4E-02 2.7E-02 3.2E-02 2.0E-01 2.3E-01 not similar config
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Facility Location Unit Type Unit ID

As

(TPY) 

actual

As

(TPY) 

potential

Cr+6

(TPY)* 

actual

Cr+6

(TPY)* 

potential

Nickel 

(TPY)

actual

Nickel 

(TPY) 

potential

HCl (TPY) 

actual

Basis of Emissions 

Factor 

(lbs/MMBtu) **

Merrimack 2 1.5E-01 2.1E-01 2.0E-02 2.9E-02 8.5E-02 1.2E-01 modeled test

Monticello 

Steam 

Electric 

Plant

Mount 

Pleasant, TX 3 coal

1 6.1E-02 6.7E-02 1.16E-01 1.26E-01 4.1E-01 4.5E-01

not 

modeled

site average

2 6.2E-02 6.7E-02 1.17E-01 1.26E-01 4.1E-01 4.5E-01 test

3 1.1E-01 1.2E-01 1.98E-02 2.09E-02 1.7E-01 1.8E-01 similar config

OG&E -

Muskogee

Fort Gibson, 

OK 3 coal

4 3.1E-01 4.7E-01 1.0E-01 1.6E-01 7.5E-01 1.2E+00

not 

modeled

similar config

5 2.8E-01 4.7E-01 9.5E-02 1.6E-01 6.9E-01 1.2E+00 similar config

6 3.2E-01 4.6E-01 1.1E-01 1.5E-01 7.9E-01 1.1E+00 similar config

Spruance 

Genco Richmond, VA 8 coal

GEN1 8.4E-04

not 

modeled

3.1E-04

not 

modeled

2.4E-03 4.5 test

GEN2 9.0E-04 3.2E-04 8.3E-03 0.0 test

GEN3 3.5E-03 5.6E-04 6.2E-03 0.0 test

GEN4 2.4E-03 3.6E-04 4.1E-03 0.1 test

PSI Energy –

Wabash 

River

West Terre 

Haute, IN

1 coal-gas 

(unit 1) 2 

coal (units 

4&6)

PG7221FA 1.3E-03

not 

modeled

7.5E-04

not 

modeled

5.0E-03

not 

modeled

1.1 test

4 1.1E-01 4.5E-03 4.1E-02 43.6 test

6 3.3E-01 1.4E-02 1.2E-01 131.0 test

Heco Waiau Waiau, HI 6 oil

W3 1.4E-03

not 

modeled

7.7E-05

not 

modeled

2.1E-01

not 

modeled

0.2 site average

W4 1.6E-03 8.7E-05 2.4E-01 0.2 site average

W5 2.0E-03 1.1E-04 3.0E-01 0.2 site average

W6 2.0E-03 1.1E-04 3.1E-01 0.2 site average

W7 8.3E-03 1.2E-03 1.3E+00 0.3 tested

W8 9.3E-03 1.3E-03 1.5E+00 0.4 tested

Dominion -

Yorktown Yorktown, VA

2 coal (units 

1&2)

 1 oil (unit 

3)

Units 1&2 1.9E-01

not 

modeled

6.8E-02

not 

modeled

4.8E-01

not 

modeled

415.5 similar config

Unit 3 2.4E-02 3.6E-01 1.8E+01 1.6 tested

* hexavalent chromium was assumed to be 12 percent of total chromium for coal fired units (same value used in the EPRI report) and 18 percent for oil fired units. Coal gas speciation was assigned the 

coal speciation (12 percent).

** Emission factors were based on unit-specific test data ("test"), data from the average of tested similar units at the same facility ("site average"), or units that had similar boiler / control configurations 

("similar config")
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2.2 Dispersion Modeling

Dispersion modeling for the case study facilities was done using AERMOD3, EPA’s 

preferred model for near-field dispersion. The AERMOD modeling system has two input data 

processors that are regulatory components of the system: AERMET, a meteorological data 

preprocessor that incorporates air dispersion based on planetary boundary layer turbulence 

structure and scaling concepts, and AERMAP, a terrain data preprocessor that incorporates 

complex terrain using USGS digital elevation data. Other nonregulatory components of the 

AERMOD system include AERSURFACE, a surface characteristics preprocessor, and the

Building Profile Input Program for PRIME (BPIPPRM), a processor that calculates building 

parameters (e.g., height and projected building width) for use in building downwash algorithms 

within AERMOD.

For each facility listed in Table 1, each boiler or combination of boilers (e.g., Yorktown 

Units 1 and 2) was modeled as an individual emission point. Sources were modeled with a unit 

emission rate of 1 g/s. Annual and hourly emissions were used in post-processing to calculate 

concentrations for all modeled HAPs. The EGU location coordinates and stack parameters used 

in the modeling are given in Table 1. When stack parameters such as stack height and diameter 

differed among the boilers, plane view and aerial photos were used to assign the correct source 

identifier to the stacks. Source locations (in UTM coordinates) were entered into AERMAP to 

calculate source elevations. Census block centroids within 20 km of each facility were used as 

model receptors. Receptor coordinates (in UTM coordinates) were entered into AERMAP to 

calculate receptor elevations. 

Because data were not readily available for building parameters, building downwash was 

ignored except for Waiau, Cambria, and Yorktown. Photos of these facilities revealed that 

downwash may play a role in dispersion. Building parameters were generated using aerial and 

plane view photos. Later analysis revealed that downwash may not play a role in Cambria and 

Yorktown. Table 3 summarizes the use of downwash for each facility.

The urban/rural classification for each facility was based on methodologies listed in Section 

7.2.3 of the Guideline on Air Quality Models4

. These methodologies are a landuse method and a 

population density based method. With both methods, a circle with 3 km radius is centered on 

the source. If 50 percent of the area within the circle is commercial, industrial, or high intensity 

residential landuse, the source is considered urban. If the population density within the circle is 

750 people per sq. km or more, the source is considered urban. Also, if a source is near an urban 

area but not considered urban based on the above methods, it was still modeled as urban. Based 

on these methods, three facilities, Dominion Chesapeake, Spruance Genco, and Heco Waiau 

were considered urban. Urban populations were based on nearby city populations. All others 

were modeled as rural sources.

Five years of meteorological data were used as input for the modeling. The most recent five 

complete years (2005 through 2009) were chosen. Integrated Surface Hourly (ISH) 

 3 http://www.epa.gov/ttn/scram/dispersion_prefrec.htm#aermod

4 U.S. EPA, 2005. Guideline on Air Quality Models. 40 CFR Part 51 Appendix W. 

http://www.epa.gov/ttn/scram/guidance/guide/appw_05.pdf.
10

meteorological data and upper air data were processed through a beta version of AERMET 

which includes the input of hourly averaged winds from a beta preprocessor (AERMINUTE) that 

processes two-minute wind speeds for Automated Surface Observing Stations (ASOS). Surface 

characteristics were processed using AERSURFACE. For each station, the most representative 

National Weather Service surface and upper air stations were chosen. This was often the closest 

station or station with the least amount of missing data when multiple stations were considered. 

A list of selected stations for each facility is shown in Table 3, along with urban/rural 

classification and whether downwash effects were modeled.

Table 3. 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
11

AERSURFACE was used to process 1992 land cover data to calculate the following 

surface characteristics at the surface meteorological stations: albedo, Bowen ratio and surface 

roughness. All variables were allowed to vary seasonally. Bowen ratio was also allowed to vary 

seasonally and by year, as it is affected by the moisture conditions at the station (average, dry, or

wet) based on precipitation amounts. Precipitation was calculated for each season for each year 

from 2005 to 2009. For stations that do not use default month-season assignments (e.g., March 

may not be considered spring at some locations), AERSURFACE output was varied monthly but 

precipitation used to define whether the location experienced dry, wet, or average moisture 

conditions was based on seasonal totals. A season was considered average if the seasonal 

precipitation was within 25 percent of the seasonal normal. A season was dry if the seasonal 

precipitation was less than 75 percent of the seasonal normal and a season was wet if the season 

exceeded 125 percent of the seasonal normal. Surface roughness also varied spatially at a 

location, based on aerial photos and analysis of the 1992 land cover data. Surface roughness 

may vary spatially around a station because of the proximity of vegetation and structures around 

the meteorological tower.

A potential concern related to the use of NWS meteorological data for dispersion 

modeling is the often high incidence of calms and variable wind conditions reported for the

ASOS stations in use at most NWS stations since the mid-1990’s. In the METAR5 coding used 

to report surface observations beginning July 1996, a calm wind is defined as a wind speed less 

than 3 knots and is assigned a value of 0 knots. The METAR code also introduced the variable 

wind observation that may include wind speeds up to 6 knots, but the wind direction is reported 

as missing if the wind direction varies more than 60 degrees during the 2-minute averaging 

period for the observation. The AERMOD model currently cannot simulate dispersion under 

calm or missing wind conditions. To reduce the number of calms and missing winds in the 

surface data, archived 1-minute winds for the ASOS stations can be used to calculate hourly 

average wind speed and directions, which are used to supplement the standard archive of hourly 

observed winds processed in AERMET. 

Recently, the National Climatic Data Center (NCDC) began archiving the 2-minute 

average wind speeds for each minute of the hour for most ASOS stations for public access. 

These 2-minute values have not been subjected to the METAR coding for calm and variable 

winds, but have also been subjected to only limited quality control measures. The 1-minute 

ASOS wind data consists of running 2-minute average winds, reported every minute, for 

commissioned ASOS stations. The 1-minute ASOS wind data can be obtained without cost 

through the National Climatic Data Center (NCDC) website 

(ftp://ftp.ncdc.noaa.gov/pub/data/asos-onemin/). AERMINUTE reads the one minute data files 

and calculates hourly average winds for each hour from available minutes. In AERMET 

processing, the hourly average winds supersede the winds from the standard observations also 

processed by AERMET.

For actual emissions, AERMOD hourly concentrations were output at each receptor for 

each boiler at a facility for the period 2005-2009. For example, if a facility had three boilers, 

each receptor would have an hourly concentration contribution from each boiler. A unit 

 5 METAR literally translates to Météorologique Aviation Régulière. The Federal Aviation Administration defines 

the report as the aviation routine weather report.
12

emission rate of 1 g/s for each boiler was modeled in AERMOD. The resulting modeled 

concentration at each receptor was scaled by the hourly emissions of each HAP to calculate an 

hourly concentration for each HAP. A 5-year concentration for each HAP was calculated by 

averaging the hourly values (that were not calm or missing hours) for the 2005-2009 period.

The methodology for estimating concentrations due to potential emissions was similar, except 

the potential emissions were uniformly distributed throughout a year (i.e., no hourly variability).

2.3 Chronic Inhalation Risk Assessment

In the 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 

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 the estimated lifetime exposure to 

the ambient concentration of each carcinogenic 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.

The IRIS URE for nickel subsulfide was derived from evidence of the carcinogenic effects of 

insoluble nickel compounds in crystalline form. Soluble nickel species such as nickel sulfate, and 

insoluble species in amorphous form, do not appear to produce genotoxic effects by the same 

toxic mode of action as insoluble crystalline nickel, although other agencies, including the 

International Agency on Research in Cancer6 and the Danish EPA7 for example, have 

determined that nickel sulfate, specifically, and nickel compounds, in general, are carcinogenic. 

Although there is some discussion of how risk varies with different combinations of nickel 

compounds, the collection of epidemiology studies provides strong evidence of carcinogenicity 

specifically with respect to soluble nickel (nickel sulfate). In 2009, the National Toxicology 

Program (NTP)8

 6 World Health Organization International Agency for Research on Cancer, 1997. IARC Monographs on the 

Evaluation of Carcinogenic Risks to Humans, Volume 49, Chromium, Nickel and Welding. 

http://monographs.iarc.fr/ENG/Monographs/vol49/volume49.pdf

noted that "The combined results of epidemiological studies, mechanistic 

studies, and carcinogenesis studies in rodents support the concept that nickel compounds 

generate nickel ions in target cells at sites critical for carcinogenesis, thus allowing consideration 

and evaluation of these compounds as a single group." Nickel speciation information for large 

combustion sources (including oil combustion, coal combustion, and others) suggests that at least 

7 Danish Environmental Protection Agency, 2008. European Union Risk Assessment Report, 

Nickel and Nickel Compounds.

8 Report on Carcinogens, Eleventh Edition; U.S. Department of Health and Human Services, Public Health Service, 

National Toxicology Program.
13

35% of total nickel emissions may be soluble compounds, while the remaining insoluble nickel 

emissions are not well-characterized. More recent data specifically from oil combustion suggest 

that the nickel emissions can be 85-100% nickel sulfate. EPA does not have an inhalation unit 

risk estimate for nickel sulfate and is not aware of any other organization that has developed one. 

However, the EPA does have a unit risk for one of the insoluble nickel compounds, nickel 

subsulfide. This unit risk is 4.8 X 10-4 per ug/m3. A draft assessment from Texas Commission 

on Environmental Quality (May 2009) derives a unit risk for total nickel which is between 3.9 X 

10-4 and 9.0 X 10-4 per ug/m3 (these are upper bound estimates, where the range corresponds to 

different adjustment procedures in the modeling). This estimate is roughly similar to the IRIS 

unit risk for nickel subsulfide. For the purposes of this assessment, we have assumed that 65% 

of emitted nickel is insoluble, that all insoluble nickel is crystalline, and that the URE for nickel 

subsulfide can be applied to this fraction. On this basis, the URE for nickel subsulfide 

(representing pure insoluble crystalline nickel) was multiplied by 0.65 and applied to all nickel 

compounds.9

 We note that this approach, while attempting to account for potential differences 

in toxicity between the soluble and insoluble forms of nickel, is not completely consistent with 

the NTP suggestion that all forms of nickel can be considered as a single group, and, therefore, 

this approach may result in an underestimation of the health risks from nickel emissions from 

EGUs.

To assess the risk of non-cancer health effects from chronic exposures, following the 

approach recommended in EPA’s Mixtures Guidelines10,11, we summed the hazard quotients 

(HQ) for all HAPs that affect a common target organ system to obtain the HI for that target organ 

system (target-organ-specific HI, or TOSHI). The HQ for chronic exposures is the estimated 

chronic exposure (again, based on the estimated 5-year average ambient concentration at each 

nearby census block centroid) divided by the chronic non-cancer reference level, which is 

usually the EPA Reference Concentration (RfC), defined as ``an estimate (with uncertainty 

spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human 

population (including sensitive subgroups) that is likely to be without an appreciable risk of 

deleterious effects during a lifetime.'' In cases where an IRIS RfC is not available, EPA utilizes 

the following prioritized sources for chronic dose-response values: (1) The Agency for Toxic 

Substances and Disease Registry (ATSDR) Minimum Risk Level (MRL), which is defined as 

``an estimate of daily human exposure to a substance that is likely to be without an appreciable 

risk of adverse effects (other than cancer) over a specified duration of exposure''; and (2) the 

California Environmental Protection Agency chronic Reference Exposure Level (REL), which is 

defined as ``the concentration level at or below which no adverse health effects are anticipated 

for a specified exposure duration.'' In this assessment, we used the IRIS RfC values for 

hexavalent chromium and hydrogen chloride, the ATSDR MRL for nickel compounds, and the 

California Environmental Protection Agency REL for arsenic.

 9 Recent data from industry have attempted to further characterize the insoluble nickel. While these data show that 

the insoluble nickel is primarily in a spinel form, they do not attempt to characterize the toxicity of that form, noting 

only that it is not in the insoluble crystalline form. As such, it does not provide us with a better means for 

characterizing the risks than the approach described above. 10 US EPA, 1986, Guidelines for the Health Risk Assessment of Chemical Mixtures, EPA-630-R-98-002. 

http://www.epa.gov/NCEA/raf/pdfs/chem_mix/chemmix_1986.pdf.

11 US EPA, 2000. Supplementary Guidance for Conducting Health Risk Assessment of Chemical Mixtures. EPA630/R-00-002. http://www.epa.gov/ncea/raf/pdfs/chem_mix/chem_mix_08_2001.pdf.
14

The health reference values used in the assessment are given in Table 4.

Table 4. 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 0.000312 IRIS 0.00009 CalEPA

HCl 7647010 0.02 IRIS

3. Results

The results of the assessment are given in Table 5. Based on actual emissions, the 

highest estimated lifetime cancer risk from any of the sixteen case study facilities was 10 in a 

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

facilities with coal-fired EGUs, there were three with maximum cancer risks greater than 1 in a 

million (the highest was 8 in a million), all driven by hexavalent chromium, and there were four 

with cancer risks at 1 in a million. All of the facilities had non-cancer TOSHI values less than 

one, with a maximum TOSHI value of 0.4 (also driven by nickel emissions from the one case 

study facility with 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.
15

Table 5. Chronic Inhalation Risk Assessment Results.

Facility

Actual emissions Potential emissions

Max Risk Risk Driver Max HI HI Driver Max risk Risk Driver Max HI HI Driver

Xcel Bayfront 4.0x10-9 Formaldehyde 0.005 HCl N/A N/A N/A N/A

Cambria Cogen 5.0x10-7 Chromium VI 0.003 Nickel 5.0x10-7 Chromium VI 0.003 Nickel

SC&E Canadys 6.0x10-7 Arsenic 0.009 HCl N/A N/A N/A N/A

Dominion Chesapeake 

Energy Center

3.0x10-6 Chromium VI 0.05 HCl N/A N/A N/A N/A

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

Exelon Cromby 

Generating Station

3.0x10-7 Arsenic 0.008 Nickel N/A N/A N/A N/A

TVA Gallatin 1.0x10-6 Chromium VI 0.006 Nickel 1.0x10-6 Chromium VI 0.005 Nickel

City Utilities of Springfield 

-James River

8.0x10-6 Chromium VI 0.04 Nickel 9.0x10-6 Chromium VI 0.05 Nickel

Amerenue-Labadie 8.0x10-7 Arsenic 0.006 Arsenic 9.0x10-7 Arsenic 0.007 Arsenic

PSHNH -Merrimack 1.0x10-6 Arsenic 0.01 Arsenic 2.0x10-6 Arsenic 0.01 Arsenic

Monticello Steam Electric 

Plant

6.0x10-7 Chromium VI 0.003 Arsenic 7.0x10-7 Chromium VI 0.003 Arsenic

OG&E -Muskogee 1.0x10-6 Arsenic 0.01 Arsenic 2.0x10-6 Arsenic 0.01 Arsenic

Spruance Genco 8.0x10-8 Arsenic 0.007 HCl N/A N/A N/A N/A

PSI Energy – Wabash 

River

1.0x10-7 Arsenic 0.001 Arsenic N/A N/A N/A N/A

Heco Waiau 1.0x10-5 Nickel 0.4 Nickel N/A N/A N/A N/A

Dominion - Yorktown 1.0x10-6 Chromium VI 0.02 Nickel N/A N/A N/A N/A