MEMORANDUM

SUBJECT: Documents developed in connection with external peer review of the EPA's report 

titled “Methods to Develop Inhalation Cancer Risk Estimates for Chromium and 

Nickel Compounds”

FROM: Ines Pagan

Health and Environmental Impacts Division

Air Toxics Assessments Group

TO: Mercury and Air Toxics Standards Rule docket EPA-HQ-OAR-2009-0234

The documents included in the attached pdf file were developed in connection with the external 

peer review of the EPA's report titled “Methods to Develop Inhalation Cancer Risk Estimates for 

Chromium and Nickel Compounds” in support of EPA’s recently proposed (March 2011) Mercury and 

Air Toxics Standards (MATS) Rule. These materials include the following documents: 1) an external 

peer review report of chromium and nickel methods (May 2011), and associated attachments (A and B), 

2) charge questions for the peer reviewers, 3) charge responses by three external peer reviewers, and 4) a 

summary of the peer review including reviewers recommendations and EPA’s conclusions based on 

reviewers’ recommendations.
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

Compounds – EXTERNAL PEER REVIEW REPORT.

1

Methods to Develop Inhalation Cancer Risk Estimates for 

Chromium and Nickel Compounds

1 Purpose

The purpose of this paper is to discuss the methods used to develop inhalation cancer risk 

estimates associated with emissions of chromium and nickel compounds from coal and oil-fired electric 

utility steam generating units (EGUs) in support of EPA’s recently proposed Air Toxics Rule.

1 The 

derivation of cancer risk estimates is based on the speciation data available from selected source 

categories, and on the available unit risk estimates (UREs) reflecting the dose that corresponds to a 

specific level of cancer risk. This document includes discussions for both emissions of chromium

(Section 2) and nickel (Section 3) compounds with regards to (1) the methods and rationale used in 

previous analyses (where applicable), (2) the methods used in the recent analysis for the Air Toxics Rule 

(considering previous methods and currently available data), and (3) a discussion of the uncertainties 

and/or limitations of the methods used. In this document, we consider the emissions of chromium 

compounds from both coal- and oil-fired EGUs and the emissions of nickel compounds from oil-fired 

EGUs, since these are major contributors to inhalation cancer risk estimates from each of these source 

types.

2 Chromium and Compounds

2.1 Background

Chromium compounds occur in nature and are found primarily in the earth’s crust. The largest 

source of chromium is the ore mineral chromite, FeCr2O4 or MgCr2O4, where magnesium can substitute 

for iron.

2

 1 US EPA, 2011. National Emission Standards for Hazardous Air Pollutants from Coal- and Oil-fired 

Electric Utility Steam Generating Units and Standards of Performance for Fossil-Fuel-Fired Electric 

Utility, Industrial-commercial-Institutional, and Small Industrial-Commercial-Institutional Steam 

Generating Units Rule. Available online at 

 Chromium can also be found in small concentrations in certain types of igneous rocks, coal, tar, 

asphalt, and crude oil. Chromium oxidation valences vary from –2 to +6, but only the 0, +2, +3 and +6 

http://www.epa.gov/ttn/atw/utility/utilitypg.html. 2 Guertin, Jacques, James A. Jacobs and Cynthia P. Avakian. Chromium (VI) Handbook. Independent 

Environmental Technical Evaluation Group (IETEG). CRC Press, 2005. ISBN 9781566706087
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

Compounds – EXTERNAL PEER REVIEW REPORT.

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valence states are common.3

 However, the hexavalent state (Cr+6), also denoted as Cr (VI), rarely occurs 

naturally, and is usually produced from anthropogenic sources (US EPA 1984).4

 The trivalent state 

(Cr+3), also denoted as Cr (III), and the Cr(VI) state are the first and second most stable, respectively, and 

thus the forms considered more important with regards to human health.5

Chromium compounds are used in a variety of industrial applications and operations. They are 

used in: alloys, such as stainless steel; paint pigments; refractory bricks that line furnaces and kilns; wood 

preservatives; production and processing of insoluble salts; leather tanning; catalysts for halogenation, 

alkylation, and catalytic cracking of hydrocarbons; fuel and propellant additives; and more.

 

6

 Many of 

these applications use substances containing Cr(VI) compounds, including various chromates, 

dichromates, and chromic acid.7

 Disposal of chemicals containing chromium or burning fossil fuels can 

also release chromium to the air, soil and water. In addition, some industrial processes may produce 

chromium emissions, wherein the hexavalent state is favored in an oxidizing alkaline environment, and 

the trivalent state is favored in a reducing acidic environment.8

In its 1998 the Integrated Risk Information System (IRIS) Toxicological Review, the U.S. EPA 

classified Cr(VI), including its compounds, as “a known human carcinogen by the inhalation route of 

exposure” based on evidence from occupational epidemiological studies consistently indicating dosedependent associations between chromium inhalation exposure and lung cancer, and on supporting 

evidence from animal inhalation studies (EPA, 1998)

 However the exact distribution between 

the hexavalent and trivalent chromium states is unknown.

9

 3 Dayan AD and Paine AJ. Mechanisms of chromium toxicity, carcinogenicity and allergenicity: review 

of the literature from 1985 to 2000. Hum and Experiment Toxicol 2001, 20:439-451.

. With regards to trivalent chromium Cr(III), the 

1998 IRIS Toxicological Review notes that while the evidence from the occupational database “could be 

used to suggest that total chromium is carcinogenic by inhalation, animal data support the human 

carcinogenicity data only on hexavalent chromium”. Thus, based on these considerations the EPA 

concluded that only Cr(VI) should be classified as a human carcinogen via the inhalation route of 

4 US EPA, 1984. Health assessment document for chromium. Research Triangle Park, NC: Environmental 

Assessment and Criteria Office, U.S. Environmental Protection Agency. EPA600883014F. 5 Dayan AD and Paine AJ. Mechanisms of chromium toxicity, carcinogenicity and allergenicity: review 

of the literature from 1985 to 2000. Hum and Experiment Toxicol 2001, 20:439-451. 6 Guertin, Jacques, James A. Jacobs and Cynthia P. Avakian, 2005. Chromium (VI) Handbook. 

Independent Environmental Technical Evaluation Group (IETEG). CRC Press. ISBN 9781566706087 7 Agency for Toxic Substances and Disease Registry (ATSDR), 1993. Toxicological Fact Sheet for 

chromium. Available online at www.nativeknowledge.org/db/files/tfacts7.htm 8 Guertin, Jacques, James A. Jacobs and Cynthia P. Avakian. Chromium (VI) Handbook. Independent 

Environmental Technical Evaluation Group (IETEG). CRC Press, 2005. ISBN 9781566706087 9 US EPA, 1998. Integrated Risk Information System (IRIS) assessment for hexavalent chromium. 

Available at: http://www.epa.gov/ncea/iris/subst/0144.htm.
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

Compounds – EXTERNAL PEER REVIEW REPORT.

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exposure.

10 EPA is currently reevaluating the carcinogenicity of hexavalent chromium via the oral route 

of exposure in light of a 2-year chronic bioassay conducted by the National Toxicology Program (NTP) 

concluding that Cr(VI) is carcinogenic when ingested in drinking water. The International Agency for 

Research on Carcinogenicity, or IARC, concluded that there is sufficient evidence for the carcinogenicity 

of Cr(VI) compounds in humans based on the combined evidence from the epidemiological, animal and 

mechanistic data available. In agreement with the EPA, the IARC stated that compounds of chromium III 

are not classifiable as to their carcinogenicity.

11 Scientific views similar to those of EPA and IARC are 

reflected in the NTP’s 11th Report of the Carcinogens.

12

The inhalation unit risk estimate (URE) for Cr (VI) on IRIS is 0.012 per μg/m3, which means that 

inhaling Cr(VI) in air at an average concentration of 1 μg/m3 daily for a lifetime poses an estimated 

increased risk of cancer of 12,000 in a million.13 This inhalation cancer potency value is among the 

highest values of any of the hazardous air pollutants (HAP) listed in the Clean Air Act. 

2.2 Approach for Estimating Inhalation Cancer Risks from Chromium 

Compounds

Based on the information in the previous section, we estimate cancer risks due to the inhalation of 

all chromium compounds based on the exposure concentration of Cr(VI) alone. Thus:

Risk = ECCr(VI) * 0.012 (ug/m3

)

-1

Where

ECCr(VI) is the chronic exposure concentration estimate for hexavalent chromium and

0.012 (ug/m3

)

-1 is the cancer unit risk estimate for hexavalent chromium. In terms of chronic inhalation 

exposure to total chromium compounds, we need to know the mass percentage of the total chromium 

mixture which is hexavalent in order to estimate chronic cancer risks. Thus:

 10 US EPA, 1998 Integrated Risk Information System (IRIS) assessment for hexavalent chromium. Available online

at http://www.epa.gov/iris/subst/0144.htm. 11 International Agency for Research on Cancer (IARC), 1990. IARC monographs on the evaluation of carcinogenic 

risks to humans. Chromium, nickel and welding. Vol. 49. Lyons, France: International Agency for Research on 

Cancer, World Health Organization 49-256. 12 National Toxicology Program (NTP), 2005. Report on carcinogens. 11th ed. Research Triangle Park, NC: US

Department of Health and Human Services (DHHS), Public Health Service. Available at http:// 

http://ntp.niehs.nih.gov/ntp/roc/eleventh/profiles/s045chro.pdf. 13 US EPA, 1998. Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units – Final 

Report to Congress. US EPA #453/R-98-004. Available at http://www.epa.gov/ttn/caaa/t3rc.html.
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

Compounds – EXTERNAL PEER REVIEW REPORT.

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Risk = ECCr * MPCr(VI) * 100 * 0.012 (ug/m3

)

-1

Where

ECCr is the chronic exposure concentration for the mixture of all chromium compounds and 

MPCr(VI) is the mass percent of the inhaled mixture which is hexavalent chromium.

2.3 Approach for Developing Speciation Profiles for Selected Source 

Categories

As discussed above, the toxicity of chromium compounds is largely dependent on its oxidation 

state (i.e., Cr(VI) vs. Cr(III), primarily) which is an important factor in evaluating the health effects from 

exposure to chromium compounds. Where the specific compound or oxidation state of chromium is 

available, the appropriate dose response value may be applied for the purposes of risk assessment. 

However, for generically reported chromium emissions where there is no information provided regarding 

the oxidation state of the chromium, we generate default “speciation profiles” that reflect the proportions 

of emissions that would likely be in the Cr(VI) and Cr(III) oxidation states. Appendix A contains a paper

by several EPA staff wherein available chromium speciation data for different industrial categories of 

emission sources have been analyzed to develop default chromium speciation profiles. The following 

sections describe the data, analyses, limitations and conclusions regarding chromium speciation for the 

two selected source categories of coal-fired and oil-fired utility boilers.

2.4 Chromium Speciation for Specific Source Categories

2.4.1 Coal-Fired Boilers

Based on source test data from 7 units, including 4 utility boilers and 3 industrial boilers (which 

are similar in process to utility boilers but often smaller), an average of 12 percent hexavalent chromium 

was derived to be used as the speciation default for coal-fired boilers without available chromium 

speciation information. The average hexavalent chromium for the 4 utility boilers was 11 percent and the 

range for the 4 tests was 0.4 percent to 23 percent.

14

 14 US EPA, 1998. Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units – Final 

Report to Congress. U.S. EPA #453/R-98-004. Available at http://www.epa.gov/ttn/caaa/t3rc.html.

Although the range of values was not reported, the 

average hexavalent chromium for the 3 industrial boilers was 12 percent. (Emissions database compiled 

November 30, 2000, in support of National Emission Standards for Hazardous Air Pollutants for 

Industrial/Commercial/Institutional Boilers and process Heaters, Final Rule, 69FR55217 September 13, 
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

Compounds – EXTERNAL PEER REVIEW REPORT.

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2004). Because of the limited number of units tested, we chose the value of 12 percent (the highest 

percent average of hexavalent chromium when considering both utility and industrial boilers) to be used 

as the default for our analysis of potential chromium risks from coal-fired utility boilers.

2.4.2 Oil-Fired Boilers

Based on source test data from 7 units, an average of 18 percent hexavalent chromium was derived 

to be used as the speciation default for oil-fired boilers without available chromium speciation 

information. The average hexavalent chromium for the 7 utility boilers was 18 percent, and the range for 

the 7 tests was 5 to 34 percent.

15

2.5 Uncertainties and Limitations 

The uncertainties and limitations associated with this analysis are related primarily to the speciation 

profile data availability, and the assumptions made to generate the default speciation profiles. 

Assumptions were made in this analysis to: 1) select the compound species of chromium driving cancer 

risks based on health effects information, and 2) select the appropriate percentage of the chromium

species driving cancer risk for derivation of the inhalation cancer risk estimates for chromium emitting 

facilities. This section briefly describes the assumptions made and associated uncertainties and 

limitations, and a qualitative characterization of the relative confidence (assigned as low, moderate or

high) on those assumptions. 

A major limitation of this analysis is the small number of test data (i.e., speciation profile from 7 

units) available to derive chromium default speciation profile values for both coal- and oil- fired boilers. 

These data represent a very small portion of the units that exist within the respective source categories

(approximately 1,200 combined coal- and oil-fired EGUs), which introduces uncertainties when making 

generalizations on how accurately the default profiles derived in this analysis reflect actual speciation 

profiles for coal- and oil- fired boilers emissions. With regards to selecting the appropriate percent 

chromium to derive default speciation profiles based on the available speciation data, we chose to 

calculate the percent average based on pooling similar test data from the available units for coal- and oilfired boilers. Although this seems a reasonable quantitative approach given the limited data set, an 

alternative, and more conservative, approach would have been to use the maximum measured percentage 

of hexavalent chromium rather than the average of the range. Further uncertainty exists as to the percent 

hexavalent chromium given the wide range present in the test data for either coal- or oil-fired units. 

 15 US EPA, 1998. Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units – Final 

Report to Congress. U.S. EPA #453/R-98-004. Available at http://www.epa.gov/ttn/caaa/t3rc.html.
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

Compounds – EXTERNAL PEER REVIEW REPORT.

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Based on this information, we have low confidence in the results based on the overall selected 

assumptions.

With regards to the cancer risk from chromium and chromium compounds, we selected hexavalent 

chromium as the species likely to be driving cancer risks based on solid evidence from the health effects 

database for chromium and compounds. Major scientific bodies that conduct thorough reviews on 

carcinogenic compounds (e.g., IARC, NTP) agree that there is sufficient evidence for the carcinogenicity 

of Cr(VI) compounds in humans based on the combined evidence from the epidemiological, animal and 

mechanistic data available. These reports also state that compounds of chromium III are not classifiable 

as to their carcinogenicity. Based on this information, we have high confidence on the assumption that 

hexavalent chromium is the carcinogenic species and that the carcinogenic risk of chromium-emitting 

facilities is proportional to the mass of hexavalent chromium contained in the emissions from those 

facilities.

2.6 Conclusions

Based on the information above, we have high confidence that hexavalent chromium is the species 

driving the carcinogenic risk of chromium-emitting facilities and, thus, we consider it reasonable to derive 

default speciation profiles based on the mass of hexavalent chromium contained in the emissions from 

chromium-emitting facilities. We have less confidence in the default speciation profiles derived based on 

a limited data set with a wide range of percentages of hexavalent chromium. An alternative and more 

conservative approach would be to use the maximum measured percentage of hexavalent chromium 

rather than the average of the range.
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

Compounds – EXTERNAL PEER REVIEW REPORT.

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3 Nickel and Compounds

3.1 Background

Nickel (Ni) is a lustrous white, hard, ferromagnetic metal found in transition group VIII of the 

Periodic Table. It has high ductility, good thermal conductivity, high strength, and fair electrical 

conductivity. It constitutes approximately 0.009 percent of the earth's crust, making it the 24th most 

abundant element. Nickel can achieve several oxidation states including -1, O, +1, +2, +3, and +4, and 

can be combined with many other elements to form different nickel compounds; however, the majority of 

nickel compounds are nickel +2 species. Indirect sources, primarily coal and oil combustion, are 

estimated to release from 85 to 94 percent of the total anthropogenic nickel emissions to the air. Nickel 

compounds are used for nickel alloys, electroplating, batteries, coins, industrial plumbing, spark plugs, 

machinery parts, stainless-steel, nickel-chrome resistance wires, and as catalysts.

16

Nickel and nickel compounds have been classified as human carcinogens. National and 

international scientific bodies including the IARC (1990)

17, the World Health Organization (WHO, 

1991)18, and the European Union’s Scientific Committee on Health and Environmental Risks (SCHER, 

2006)19

 16 US EPA, 1986. Health Assessment Document for Nickel. EPA/600/8-83/012F. National Center for 

Environmental Assessment, Office of Research and Development, Washington, DC.

, agree that nickel compounds (including nickel sulfate), in general, are carcinogenic. In their 11th 

Report of the Carcinogens, the NTP has classified nickel compounds as known to be human carcinogens 

based on sufficient evidence of carcinogenicity from studies in humans, including epidemiological and 

mechanistic information, which indicates a causal relationship between exposure to nickel compounds 

and human cancer. This classification is based on consistent findings of increased risk of cancer in 

exposed workers, and support by evidence from experimental animals that shows that exposure to an 

assortment of nickel compounds by multiple routes causes malignant tumors at various organ sites and in 

multiple species. The 11th Report of the Carcinogens states that the “combined results of 

epidemiological studies, mechanistic studies, and carcinogenesis studies in rodents support the concept 

17 International Agency for Research on Cancer (IARC), 1990. IARC monographs on the evaluation of carcinogenic 

risks to humans. Chromium, nickel and welding. Vol. 49. Lyons, France: International Agency for Research on 

Cancer, World Health Organization Vol. 49:256. 18 International Labour Organization/United Nations Environment Programme, World Health Organization (WHO), 

1991. Nickel. In Environmental Health Criteria No 108 Geneva. 19European Commission, Scientific Committee on Health and Environmental Risks (SCHER), 2006. Opinion on: 

Reports on Nickel, Human Health part. SCHER, 11th plenary meeting of 04 May 2006 

[http://ec.europa.eu/health/ph_risk/committees/04_scher/docs/scher_o_034.pdf].CHER 2006
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

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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”. Although the precise nickel 

compound (or compounds) responsible for the carcinogenic effects in humans is not always clear, studies 

indicate that nickel sulfate and the combinations of nickel sulfides and oxides encountered in the nickel 

refining industries cause cancer in humans. In agreement with the above mentioned scientific bodies, the 

US Department of Health and Human Services (DHHS, 1994)

20 has determined that nickel compounds 

are known human carcinogens, and that another nickel compound, nickel metal, may reasonably be 

anticipated to be a carcinogen. The EPA has only evaluated two nickel compounds for their carcinogenic 

potential -- nickel subsulfide and nickel refinery dusts -- and has classified them both as group A, human 

carcinogens. Another assessment for the carcinogenic potential of nickel and nickel compounds has been 

developed by the California Department of Health Services (CDHS, 1991)21

The major scientific bodies mentioned above have recognized that there are potential differences 

in toxicity and/or carcinogenic potential across the different nickel compounds. These differences are 

believed to be due, in part, to differences in solubility properties and/or different speciation of nickel 

compounds that in turn make nickel ions more or less available at target sites. The view of the IARC’s 

nickel review group (IARC’s Working group Special Report 2009)

. With regards to speciation, 

it is noted in this assessment that the available epidemiological data is inadequate to develop separate unit 

risk factors for different nickel compounds, nevertheless the view of CDHS is consistent with 

consideration of both insoluble and soluble nickel compounds as carcinogenic based on the available 

epidemiological evidence.

22

 20US Department of Health and Human Services (DHHS), 1994. Seventh annual report on carcinogens: Summary 

1994. Research Triangle Park, NC: National Institute of Environmental Health Sciences (NIEHS), 262-269.

is that, in addition to the release of 

nickel ions at target sites, there may be other less understood factors that promote accumulation of nickel 

ions at critical target sites and, given the available scientific evidence, it is difficult to predict the relative 

carcinogenic hazard of nickel compounds. There have been differences of opinions on the consideration 

of all nickel compounds as carcinogenic to humans. Some authors believe that water soluble nickel, such 

as nickel sulfate, should not be considered a human carcinogen, based primarily on a negative nickel 

sulfate NTP 2-year rodent bioassay (which is different than the positive nickel subsulfide 2-year NTP 

21 California Department of Health Services (CDHS), 1991. Health Risk Assessment for Nickel. Initial Statement 

for Rulemaking, Proposed Identification of Nickel as a Toxic Air Contaminant – Technical Support Document, part 

B. Available online at http://www.arb.ca.gov/toxics/id/summary/nickel_tech_b.pdf 22 International Agency for Research on Cancer (IARC) Working Group: Special Report. Policy; A review of 

human carcinogens; Part C: metals, arsenic, dusts, and fibres. Lancet Oncol 2009, 10:453-454.
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bioassay).

23,24,25 Although these authors agree that the epidemiological data clearly supports an 

association between nickel and increased cancer risk, they sustain that the data are weakest regarding

water soluble nickel. A recent review by Grimsrud et al., (2010)26

As mentioned above, the EPA has only derived URE values for nickel subsulfide and for nickel 

refinery dusts. Nickel subsulfide has a URE of 0.00048 per μg/m3

, which means that inhaling nickel 

subsulfide in air at an average concentration of 1 μg/m3 daily for a lifetime poses an estimated increased 

risk of cancer of 480 in a million. The nickel refinery dust has a value of 0.00024 μg/m3

, which suggests 

that this compound may have half of the carcinogenic potential of nickel subsulfide. There are two other 

available UREs for nickel compounds derived by the CDHS (URE derived for nickel compounds as a 

group) and the Texas Commission on Environmental Quality (TCEQ, Draft assessment, 2009)

highlights the robustness and 

consistency of the epidemiological evidence across several decades showing associations between nickel 

and nickel compounds exposure (including nickel sulfate) and cancer.

27, with 

values of 0.00026 per μg/m3 and a range from 0.00039-0.0009 μg/m3 (this range corresponds to different 

adjustments in the modeling), respectively.

3.2 Approach for Estimating Inhalation Cancer Risks from Nickel Compounds

Based on past technical and scientific considerations, the determination of the percent of nickel 

subsulfide (which has an IRIS URE value) versus nickel sulfate (with no available URE value) was 

considered a major factor for estimating the extent and magnitude of the risks of cancer due to nickelcontaining emissions. Thus, in previous analyses, we estimated cancer risks due to the inhalation of all 

nickel compounds based on the estimated exposure concentration of nickel subsulfide alone. Thus:

Risk = ECNi subsulfide * 0.00048 (ug/m3

)

-1

Where

 23 Oller A. Respiratory carcinogenicity assessment of soluble nickel compounds. Environ Health Perspect 2002, 

110:841-844. 24 Heller JG, Thornhill PG, Conard BR: New views on the hypothesis of respiratory cancer risk from soluble nickel 

exposure; and reconsideration of this risk's historical sources in nickel refineries. J Occup Med Toxicol 2009, 4:23. 25 Goodman JE, Prueitt RL, Thakali S, and Oller AR. The nickel iron bioavailability model of the carcinogenic 

potential of nickel-containing substances in the lung. Crit Rev Toxicol 2011, 41:142-174. 26 Grimsrud TK and Andersen A. Evidence of carcinogenicity in humans of water-soluble nickel salts. J Occup Med 

Toxicol 2010, 5:1-7. Available online at http://www.ossup-med.com/content/5/1/7. 27 Texas Commission on Environmental Quality (TCEQ), 2009. Development support document draft for 

arsenic/inorganic arsenic compounds, and nickel compounds. Available online at 

http://www.tceq.texas.gov/toxicology/dsd/reviews.html#arsenic
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ECNi subsulfide is the chronic exposure concentration estimate for Ni subsulfide and 0.00048 (ug/m3

)

-1

is the cancer unit risk estimate for nickel subsulfide. In terms of chronic inhalation exposure to total 

nickel compounds, we need to know the mass percentage of the total nickel mixture which is subsulfide in 

order to estimate chronic cancer risks. Thus:

Risk = ECNi * MP Ni subsulfide * 100 * 0.00048 (ug/m3

)

-1

Where

ECNi is the chronic exposure concentration for the mixture of all Ni compounds and MPNi sunsulfide is 

the mass percent of the inhaled mixture which is nickel subsulfide.

3.3 Approach for Developing Speciation Profiles for Selected Source 

Categories

Similar to chromium, the speciation data for nickel emissions from different source categories is 

limited. Several approaches have been considered by the EPA, over time, in the determination of the 

cancer risks posed by the inhalation of nickel and nickel compounds emitted by large combustion sources 

(including oil-combustion). The following subsections describe the data, analyses and assumptions 

regarding nickel speciation for risk analysis derived in the past (A and B), and potential approaches (C) 

based on recent nickel speciation data from oil-fired power plants.

3.3.1 A. Approach to Derive Unit Risk Estimates: Electric Utility Report to Congress 1998

A speciation analysis performed by the Electric Power Research Institute28

 28 US EPA, 1998. Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units – Final 

Report to Congress. US EPA #453/R-98-004. Available at http://www.epa.gov/ttn/caaa/t3rc.html.

using data from 1992 

to 1994 on 5 sites with oil-fired utilities showed the presence of numerous nickel compounds and a wide 

range of the percentages of different nickel species measured. This report indicated the following general 

speciation profile: soluble nickel compounds 25 to 60 percent, sulfidic nickel compounds 4 to 26 percent, 

metallic nickel compounds 0 to 4 percent, and oxidic nickel compounds 27 to 70 percent. Based on these

data, there were two proposed approaches to calculating nickel inhalation risk estimates from these 

sources. The first approach used the assumption that the mix of nickel compounds emitted by oil-fired

utilities was 50 percent as carcinogenic as nickel subsulfide (the nickel compound with the higher cancer 

potency based on the existing UREs). The second approach was also based on the application of the IRIS 

URE for nickel subsulfide; however it provided a suite of risk estimates depending on the percentage of 

nickel subsulfide (ranging from 100 to 0 percent) believed to be present in the emissions from oil-fired 
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

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utilities. The assumption used for both approaches was that only the nickel subsulfide fraction 

contributes to the cancer risk. The cancer risk due to the other nickel compounds was considered as 

unknown, and thus not considered as contributing to cancer. 

3.3.2 B. Approach to Derive Unit Risk Estimates: NATA 2000

Based on previous analysis of some of the largest nickel-emitting combustion sources indicating

that at least 35 percent of total nickel emissions may be soluble compounds29, the EPA assumed that as 

much as 65 percent of emitted nickel could be in the insoluble form and that all insoluble nickel could be 

crystalline. Because the URE listed in IRIS for nickel subsulfide represents a form which is pure 

insoluble crystalline nickel, this assessment assumed that 65 percent of the total mass of emitted nickel 

might be of this form, and thus might be carcinogenic. Thus, for the purposes of the 2000 NATA 

assessment, the EPA assumed that 65 percent of emitted nickel was insoluble, that all insoluble nickel 

was crystalline, and that the IRIS URE for nickel subsulfide could be applied to this portion. On this 

basis, the URE for nickel subsulfide (representing pure insoluble crystalline nickel) was multiplied by 

0.65 and applied to the mass emissions of all nickel compounds.

3.3.3 C. Approach to Derive Unit Risk Estimates: Alternatives Based on Speciation of

Residual Oil Fly Ashes from Power Plants.

Recently, a nickel speciation analysis was conducted by the Electric Power Research Institute30

 29 US EPA, 2001. National-Scale Air Toxics Assessment for 1996 - Appendix-G: Health Effects Information Used 

in Cancer and Noncancer Risk Characterization for the NATA 1996 National-Scale Assessment, EPA-453/R-01-

003. Available online at http://www.epa.gov/ttn/atw/sab/appendix-g.pdf.

on residual oil fly ashes (ROFA) obtained from stacks of utility boilers from two of the largest oil-fired 

power plants in the country, which are located in Hawaii (Honolulu and Waiau, respectively). The ROFA 

samples were analyzed using x-ray absorption fine structure (XAFS) spectroscopy and x-ray diffraction 

(XRD). Speciation analysis results indicated that 80 to 95 percent of the total Ni present in the ROFA 

samples was nickel sulfate hexahydrate (NiSO4.6H2O), and that approximately 20 percent of the total Ni 

was present as a Ni-containing spinel compound, similar in composition to nickel iron oxide (NiFe2O4). 

Sulfidic Ni compounds (such as nickel subsulfide) were not detected in this analysis. Given that the 

ROFA samples tested did not contain nickel subsulfide, using an approach to derive unit risks estimates 

similar to that used in past analysis (e.g., applying the URE for nickel subsulfide to the mass emissions of 

all nickel compound) would result in a unit risk estimate equal to zero. Based on what we know 

30 See appendix B. Report of Analysis on the “ Determination of nickel species in particulate matter collected on 

quartz thimble filters from flue gas emissions of power plants burning residual No. 6 fule oil”. Prepared by Frank 

Huggins, University of Kentucky, Lexington, July 2011.
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

Compounds – EXTERNAL PEER REVIEW REPORT.

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regarding the health effects of nickel compounds as a group, making an assumption of no risk associated 

with exposure to nickel-containing emissions would clearly not be health-protective. For this reason, we 

considered the following alternative approaches.

1- Using the same approach as 2000 NATA. That is, using the IRIS URE for nickel subsulfide 

and applying a factor of 0.65 to the mass emissions of all nickel compounds. While it does 

not take the new speciation data into account, this approach was developed for NATA and 

RTR assessments years ago, and has been peer reviewed by the Science Advisory Board 

(SAB), each time receiving favorable reviews. It would also help to provide continuity and 

comparability between current and past assessments. This is the approach used in 

assessments performed in support of the EPA’s Toxics Rule proposed in April 2011.

2- Considering a more health-protective approach. Based on the consistent views of major 

scientific bodies (i.e., NTP in their 11th ROC, IARC, and other international agencies) that 

consider nickel compounds to be carcinogenic as a group, we propose considering all nickel 

compounds as carcinogenic as nickel subsulfide. This approach would include the 

application of the IRIS URE without a factor, assuming 100 percent of nickel compounds as 

carcinogenic as nickel subsulfide.

3- Considering alternative UREs. This approach would involve the direct application of URE 

values developed for nickel compounds as a group by governmental agencies other than the 

EPA rather than deriving a value based on the IRIS URE for nickel subsulfide. The 

application of the CDHS URE would be yield a unit risk estimate roughly twofold lower than 

using 100 percent of the IRIS URE for nickel subsulfide. The application of the TCEQ URE 

range would yield unit risk estimates that would range from about 20 percent lower than to 

about twofold higher than that based on using 100 percent of the IRIS URE for nickel 

subsulfide.

3.4 Uncertainties and limitations 

The uncertainties and limitations associated with these analyses are related primarily to the 

speciation profile data availability, and the assumptions made to generate the default speciation profiles. 

Assumptions were made in this analysis to: 1) select the compound species of nickel driving cancer risks 

based on health effects information, and 2) select the appropriate percentage of the nickel species driving 

cancer risk for derivation of the inhalation cancer risk estimates for nickel-emitting facilities. This section 
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

Compounds – EXTERNAL PEER REVIEW REPORT.

13

briefly describes the assumptions made and associated uncertainties and limitations, and a qualitative 

characterization of the relative confidence (assigned as low, moderate or high) on those assumptions. 

Similar to chromium, a major limitation of these analyses is the small number of test data (i.e., 3 

individual nickel speciation analysis of few units) available to derive nickel default speciation profile 

values for and oil-fired units. Another source of uncertainty is the lack of a speciation profile patterns 

across the different nickel speciation analysis available, and the wide range of percentages for the 

individual nickel species. Based on this information, we have low confidence in the derivation default 

nickel speciation profiles based on the available speciation data.

With regards to estimating the cancer risk from inhaling nickel and its compounds, we have in the 

past selected nickel subsulfide as the species likely to be driving cancer risks based on health effects 

evidence indicating that nickel subsulfide is a carcinogenic in both humans and animals, and on the 

potentially higher cancer potency of nickel subsulfide when compared to that of nickel refinery dusts 

(which has the only other IRIS URE value). Although this approach was considered reasonable at the 

time (i.e., , and consistent with approaches used for derivation of speciation defaults for other compounds 

(e.g., chromium), an alternative and more conservative approach would be based on consideration of all 

nickel compounds to be as carcinogenic as nickel subsulfide.

It is important to note that the selection of an approach using the two discussed UREs (one from 

California and one from Texas) rather than an approach based on use of the IRIS URE for nickel 

subsulfide would yield risk estimates roughly similar to those using the IRIS URE.
Chromium Emissions Speciation for Selected Source Categories March, 2011

1

TO: Ted Palma, HEID

FROM: Chuck French and Anne Pope, SPPD

DATE: March 11, 2011

SUBJECT: Chromium Emissions Speciation for Selected Source Categories 

I. Purpose

This document provides a summary of the available chromium speciation data for 

selected source categories addressed in 2005 NATA and the rationale for the chromium 

speciation values used in the preliminary inhalation risk analyses.

II. Background

Chromium compounds occur in nature and are found primarily in the earth’s crust. 

The largest source of chromium is the ore mineral chromite, FeCr2O4 or MgCr2O4, where 

magnesium can substitute for iron (Guertin, 2005). Chromium can also be found in small

concentrations in certain types of igneous rocks, coal, tar, asphalt, and crude oil (Guertin, 

2005). Chromium exists in several different oxidation states, but the most stable and 

most commonly found are hexavalent chromium (+6 valence state) (or Cr VI) and 

trivalent chromium (+3 valence state) (Cr III) (ATSDR, 2008). 

Chromium compounds are used in a variety of industrial applications and 

operations. They are used in alloys, such as stainless steel; paint pigments; refractory 

bricks that line furnaces and kilns; wood preservatives; production and processing of 

insoluble salts; in leather tanning; as catalysts for halogenation, alkylation, and catalytic 

cracking of hydrocarbons; as fuel and propellant additives; and more (Guertin, 2005). 

Many of these applications use substances containing CrVI compounds, including 

various chromates, dichromates, and chromic acid. In addition, some industrial processes 

may produce chromium emissions, wherein the hexavalent state is favored in an 

oxidizing alkaline environment, and the trivalent state is favored in a reducing acidic 

environment (Guertin, 2005).

Cr VI, including its compounds, is classified by U.S. EPA as “a known human 

carcinogen by the inhalation route of exposure” because there is sufficient scientific 

evidence that inhalation exposure to this form of chromium increases the risks of cancer 

in humans (EPA, 1998). Because carcinogenicity by the oral route of exposure cannot be 

determined based on the information available, it is not classified as to the human 

carcinogenicity from this route of exposure (EPA, 1998). Cr VI has an inhalation cancer 

potency value of 0.012 per microgram per cubic meter (μg/m3

), which means that 

inhaling Cr VI in air at an average concentration of 1 μg/m3 daily for a lifetime poses an 

estimated increased risk of cancer of 12,000 in a million (EPA, 1998). This inhalation 

cancer potency value is among the highest values of the hazardous air pollutants (HAP) 
Chromium Emissions Speciation for Selected Source Categories March, 2011

2

listed in the Clean Air Act. On the other hand, trivalent chromium (Cr III) is “not 

classified as to its human carcinogenicity,” as no data are available on the carcinogenic 

potential of chromium (III) compounds alone (EPA, 1998). Therefore, it has no cancer 

potency value for inhalation or ingestion exposures and is not evaluated for cancer effects 

in risk assessments.

For non-carcinogenic effects, Cr VI has a Reference Concentration for Chronic 

Inhalation Exposure (RfC) inhalation value of 0.0001 per milligram per cubic meter 

(mg/m3

), which means that a person inhaling Cr VI in air at an average concentration of 

0.0001 mg/m3 or less daily is likely to be without an appreciable risk of deleterious 

effects during a lifetime (EPA, 1998). On the other hand, data are considered to be 

inadequate for development of an RfC for Cr III due to the lack of a relevant toxicity 

study addressing respiratory effects of Cr III (EPA, 1998). Therefore, it has no noncancer dose-response value for inhalation and is not evaluated for non-cancer effects in 

risk assessments. Thus, when estimating the increased risk of cancer due to chromium 

emissions, the determination of the percent Cr VI versus Cr III, is a major factor for 

estimating the extent and magnitude of the risks. 

A preliminary risk assessment was conducted based on data extracted from EPA’s 

2005 National-Scale Air Toxics Assessment (NATA) National Emissions Inventory 

(NEI). While chromium emissions are sometimes reported in the NEI as the specific 

compound emitted (e.g., potassium dichromate, strontium chromate, etc.) they can also 

be reported to the NEI as a general category of emissions such as “chromium & 

compounds,” or “chromium.” As the toxicity of these compounds is largely dependent 

on the oxidation state, it is an important factor in evaluating the health effects from 

exposure to chromium compounds. Where the specific compound or oxidation state of 

chromium is reported, the appropriate dose response value may be applied for the 

purposes of risk assessment. However, for generically reported chromium emissions, 

there is no information provided regarding the oxidation state of the chromium. For the 

source categories with generically reported chromium emissions, available data were 

examined to determine the proportions of emissions that would likely be in the 

Cr VI and Cr III oxidation states. Preliminary “speciation profiles” were developed from 

this effort and applied in the preliminary risk analyses. 

III. General Approach for Developing Speciation Profiles for Source Categories

The available chromium speciation data were examined for source categories. 

The available chromium speciation data ranged from no data available at all for a few 

source categories, to speciated data for individual emission point types for source 

categories. For several source categories, speciation data were available at a facility-wide 

level for one or more facilities in the source category. 

In general, the data were examined and evaluated, and the speciation profile 

determined to be most appropriate for each source category was chosen according to a 

hierarchy in the quality of the data. Where available, emission point-specific speciation 
Chromium Emissions Speciation for Selected Source Categories March, 2011

3

data from that source category was given the highest weighting. Next in the hierarchy,

overall emissions speciation data that represented the profile for an entire facility (as 

opposed to individual emission points at the facility) were considered. If data for a 

particular source category were unavailable or very limited, then we reviewed available 

data and information about chromium emissions from other similar industrial source 

categories or similar processes. In some cases, we also considered the NEI default 

chromium speciation values for a source category or emission point type when source 

category data were not available or limited. 

Since the available chromium speciation data are sparse for most source 

categories, a conservative approach was applied in determining an appropriate profile. 

For example, if available data indicated a range of percentages for Cr VI, the higher end 

of the range was generally selected. Also, in general, an overall maximum value (or 

upper end value) was selected to be applied source category-wide to all emission points

and at all facilities with generically reported chromium emissions. 

While we believe the conservative approach applied in determining chromium 

speciation profiles to use for the preliminary risk analyses is appropriate, the chromium 

speciation profiles will be re-evaluated considering data received from information 

collection requests (ICR) under CAA Section 114, public comments received in response 

to proposed rule-making, and any relevant emissions test data that EPA may obtain from 

other efforts. Depending upon the amount and quality of data available during this reevaluation, a less conservative approach may be applied. In addition, where available, 

emission point type-specific chromium speciation profiles may be applied. Any revised 

chromium speciation values will be applied in the risk analyses performed for the 

purposes of developing residual risk standards. 

The following sections describe the data, analyses, assumptions, and conclusions 

regarding chromium speciation for selected source categories included in the 2005 

NATA with generically reported chromium emissions. 

IV. Chromium Speciation for Specific Source Categories

1. Coal Boilers

Chromium is naturally present as a trace element in coal. When coal is burned, 

chromium is released to the atmosphere. The quantity of chromium emitted is dependent 

on many factors including the concentration of chromium in the coal, control device 

efficiency and type of boiler. Based on source test data from 7 units, an assumption of 

12% hexavalent chromium is used for coal boilers in 2005 NATA modeling. The 7 units 

included 4 utility boilers and 3 industrial boilers. The average hexavalent chromium for 

the 4 utility boilers was 11 % and the range for the 4 tests was 0.4 % to 23%. (1998 EPA 

“Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating 

Units – Final Report to Congress. U.S. EPA #453/R-98-004. February 1998). As part of 

the development of industrial boiler rule in 2000, hexavalent chromium was measured at 

3 industrial boiler units. The average hexavalent chromium for the 3 industrial boilers 
Chromium Emissions Speciation for Selected Source Categories March, 2011

4

was 12% (Emissions database compiled November 30, 2000, in support of National 

Emission Standards for Hazardous Air Pollutants for Industrial/Commercial/Institutional 

Boilers and process Heaters, Final Rule, 69FR55217 September 13, 2004) Because of the 

limited number of units tested, the utility and industrial boiler data sets were combined 

and the more conservative value of 12% was used as a default in the 2005 NATA.

2. Ferroalloys Production

Ferroalloys are alloys of iron and one or more other elements, such as chromium, 

manganese, and/or silicon. Ferroalloy production occurs when an electric arc furnace 

(EAF) is charged with raw materials to begin smelting the ores. The molten product is 

“tapped” or poured from the furnace. Raw material and product handling (e.g., crushing 

and screening operations) also occur as part of the ferroalloy production process. 

Emissions of chromium compounds may be caused by chromium present in the EAF 

feedstocks (coal and ore) and/or the refractory linings of the furnaces. 

The ANPRM data set for the ferroalloys production source category includes one 

facility with chromium emissions (out of a total of three facilities), which are reported

generically as “chromium & compounds” and “chromium” emissions. 

During the development of the MACT for ferroalloys, there was a ferrochromium 

(FeCr) plant (Macalloy) located at Charleston, South Carolina. This facility was a 

potential major source of HAP, but closed down before the MACT standard was 

promulgated. A letter addressed to Conrad Chin (Environmental Engineer, U.S. EPA) 

from the South Carolina Department of Health and Environmental Control (DHEC) dated 

December 19, 1996, contains a table that provides Cr VI emissions data from electric arc 

furnaces (EAF) no. 15 and no. 14 of the Macalloy facility (South Carolina DHEC, 1996). 

Based on these data, Cr VI comprises approximately 2 percent of the total chromium 

emissions from the EAFs. 

We do not have speciated chromium emissions data for other Ferroalloys 

production processes (e.g., for ferromanganese or silicomanganese). However, we do 

have some data for EAFs used for steel production. We believe that steel production 

EAFs, particularly EAF melting processes, are somewhat similar to ferroalloy processes, 

in that similar feedstocks are used and the EAF would include similar refractory linings. 

Therefore, we also reviewed the chromium data for the melting process from steel 

production EAFs for possible application to the ferroalloy category. We have Cr VI data 

for the baghouse dust from EAF melting for one steelmaking facility. These data include 

9 samples of EAF melting baghouse dust that ranged from 0.8 percent to 2.5 percent Cr 

VI; the average of these data was 1.5 percent; and, the median was 1.4 percent

(Consumers Energy, Laboratory Services, 2004). These data, all from a single plant, are 

not from emissions stack test data but rather are from analyses of EAF baghouse dust (i.e., 

we assumed that captured baghouse dust concentrations are the same as the dust 

concentrations emitted from the baghouse). We believe the EAF melting data from the 

baghouse dust is appropriate for ferroalloy production because the steel production EAF 

melting is similar to ferroalloys processing.
Chromium Emissions Speciation for Selected Source Categories March, 2011

5

Based on these data, and assuming that captured baghouse dust concentrations are 

the same as the dust concentrations emitted from the baghouse, the estimated Cr VI 

emissions from one closed ferroalloy facility were 2 percent of the total chromium

emissions generated by the EAF, and emissions of Cr VI are a maximum of 2.5 percent

of EAF total chromium emissions at one steel production facility. Applying a 

conservative approach, we have assumed that 3 percent of the total chromium emissions 

from ferroalloys production facilities are Cr VI, and that 97 percent are Cr III, and this 

speciation profile was used in the preliminary risk analysis for generically reported 

chromium emissions.

3. Oil Boilers

Chromium is naturally present as a trace element in oil. When oil is burned, 

chromium is released to the atmosphere. The quantity of chromium emitted is dependent 

on many factors including the concentration of chromium in the oil, control device 

efficiency and type of boiler. Based on source test data from 7 units, an assumption of 

18% hexavalent chromium is used for coal boilers in 2005 NATA modeling. The 

average hexavalent chromium for the 7 utility boilers was 18 %, and the range for the 7 

tests was 5 % to 34%. (1998 EPA “Study of Hazardous Air Pollutant Emissions from 

Electric Utility Steam Generating Units – Final Report to Congress. U.S. EPA #453/R98-004. February 1998). 

4. Pesticide Active Ingredient Production

Pesticide Active Ingredient (PAI) Production facilities manufacture active 

ingredients in insecticides, herbicides, fungicides, and related products. Typically, the 

active ingredients are subsequently formulated with inert ingredients to create endproduct pesticides for application. 

Emissions of chromium compounds are generally not expected from facilities in 

this source category. However, chromium emissions are included in the ANPRM data set 

for one facility (out of a total of 18 facilities), and are reported generically as “chromium 

& compounds” and “chromium” emissions. 

We have no data for the PAI facility reporting chromium emissions that would 

provide a chromium speciation profile or information to develop such a profile. 

Consequently, we considered using chromium speciation data from source categories 

with similar processes. However, as chromium emissions appear to be unique to one PAI 

facility and are not necessarily emitted by a specific process used within this industry or 

other industries, a process-type comparison between facilities or industries was not 

appropriate in this case. We next considered the NEI default chromium speciation 

profiles. The NEI does not have a default chromium speciation profile specific to the 

MACT code for PAI production (0911), but there is a NEI default chromium speciation 

profile specific to the SCC for these emissions (30190013) of 4 percent Cr VI and 96
Chromium Emissions Speciation for Selected Source Categories March, 2011

6

percent Cr III. Considering that all the chromium emissions for this source category are 

from one facility and one emissions point classified by this SCC, we applied the NEI 

default chromium speciation profile to these generically reported chromium emissions. 

Therefore the speciation profile used in the preliminary risk analysis was 4 percent Cr VI 

and 96 percent Cr III. 

5. Phosphoric Acid and Phosphate Fertilizer Production

Phosphoric acid production facilities produce phosphoric acid by reacting 

phosphate rock with sulfuric acid. Phosphate fertilizer production facilities react 

phosphoric acid or phosphate rock with other materials to create a fertilizer product. 

These operations are generally co-located, as phosphoric acid manufacturing facilities 

provide feedstock for phosphate fertilizer production facilities, and much of the 

phosphoric acid produced in the U.S. is consumed in the manufacture of fertilizers. 

Chromium emissions from these source categories originate from native chromium that is 

present in the phosphate rock ore.

The ANPRM data set for these facilities includes emissions for both source 

categories together. Of the 15 facilities in the ANPRM data set for the phosphoric acid 

and phosphate fertilizer production source categories, 2 facilities report “chromium” 

emissions. 

We have no emissions data specific the facilities reporting these chromium 

emissions or to the phosphoric acid or phosphate fertilizer production industries that 

would provide a chromium speciation profile or information to develop such a profile for

these processes. As such, we considered using chromium speciation data from source 

categories with similar processes. Another facility type that produces inorganic 

chemicals and that also processes ore containing chromium compounds are chromium 

chemicals production facilities. We have emissions tests from 1996 and 2000 for two 

facilities that manufacture chromium chemicals, which include chromium speciation 

measurements. The estimates of chromium speciation were 20 percent Cr VI and 80

percent Cr III for one facility and 21 percent Cr VI and 79 percent Cr III for the other 

facility (METCO 1996 and 2000). We expect that ores used for chromium chemicals 

manufacturing would have much higher levels of total chromium than phosphate rock 

ores. However, the speciation of the chromium that is in the ores used by these industries 

and that is emitted through the ore processing and production of the inorganic chemicals 

could be similar.

In the absence of chromium speciation data specific to phosphoric acid and 

phosphate fertilizer production facilities, we applied a conservative approach and

assumed that the proportion of Cr VI emissions from phosphate fertilizer and phosphoric 

acid production facilities is similar to the upper end of the proportion of Cr VI emissions 

from chromium chemical production facilities. As the proportion of Cr VI measured at 

two of these facilities was 20 percent and 21 percent, and considering the possible 

differences in the ores used in by the chromium chemical production facilities and the 

ores used by phosphoric acid and phosphate fertilizer production facilities, we 
Chromium Emissions Speciation for Selected Source Categories March, 2011

7

conservatively assumed a value of 25 percent of the total chromium emissions from 

phosphoric acid and phosphate fertilizer production facilities are Cr VI, and that 75 

percent are Cr III. This speciation profile was used in the preliminary risk analysis for 

generically reported chromium emissions for these source categories.

6. Pulp and Paper Production

The pulp and paper production source category includes any facility engaged in 

the production of pulp and/or paper, including integrated mills (where pulp alone or pulp 

and paper or paperboard are manufactured on-site), non-integrated mills (where paper or 

paperboard are manufactured, but no pulp is manufactured on-site), and secondary fiber 

mills (where waste paper is used as the primary raw material). The pulp and paper 

production operations (e.g., pulping, bleaching, and chemical recovery) are addressed in 

three MACT standards as follows:

• MACT I (40 CFR 63, subpart S) - regulates hazardous air pollutant (HAP) 

emissions from the pulp production areas and bleaching operations at chemical 

pulp mills (Kraft, sulfite, semi-chemical, and soda wood pulping processes)

• MACT II (40 CFR 63, subpart MM) - regulates HAP emissions from the 

chemical recovery combustion areas of chemical pulp mills (kraft, sulfite, semichemical, and soda pulping processes)

• MACT III (40 CFR 63, subpart S) - regulates HAP emissions from pulp and 

paper production areas of pulp mills using mechanical, secondary fiber, and nonwood pulping, and papermaking systems at all mills

Emissions of chromium and other metallic HAP are not expected from MACT I 

and III sources (other than for supplemental or in-process fuel use), but are expected from 

many of the MACT II chemical recovery combustion sources (e.g., direct contact 

evaporator (DCE) and nondirect contact evaporator (NDCE) recovery furnaces) at kraft, 

soda, sulfite, and stand-alone semichemical pulp mills; and lime kilns and smelt 

dissolving tanks at kraft and soda pulp mills. 

Three ANPRM data sets were created for pulp and paper facilities. The contents 

of the data sets are based on the process MACT codes assigned to NEI records: 

(1) emissions regulated by MACT I and MACT III, (2) emissions regulated by MACT II, 

and (3) emissions from other pulp and paper sources at pulp and paper mills that are not 

specifically identified as being regulated by MACTs I, II, or III or other MACTs. Of the 

168 facilities in the MACT I and III pulp and paper ANPRM data set, six facilities report 

“chromium & compounds” emissions, six report emissions of “chromium,” and one 

reports emissions of Cr VI; of the 141 facilities in the MACT II pulp and paper ANPRM 

data set, 22 facilities report “chromium & compounds” emissions, 23 report emissions of 

“chromium,” 30 report emissions of Cr VI, 21 report emissions of Cr III, two report 

emissions of chromic acid (VI), one reports emissions of calcium chromate, one reports 

emissions of sodium chromate, and one reports emissions of sodium dichromate; and of 

the 107 facilities in the “other sources” pulp and paper ANPRM data set, one facility 

reports “chromium & compounds” emissions, 3 report emissions of “chromium,” one 
Chromium Emissions Speciation for Selected Source Categories March, 2011

8

reports emissions of Cr VI, and one reports emissions of Cr III. The generically reported 

“chromium” and “chromium & compounds” emissions comprise over 99 percent of all 

the reported chromium emissions in the MACT I and III data set; approximately 38

percent of all the reported chromium emissions from the MACT II data set; and over 99 

percent of all the reported chromium emissions from the “other sources” data set.

Some data were available from the industry to characterize emissions from 

MACT II emissions sources at pulp and paper facilities. The National Council for Air 

and Stream Improvement (NCASI) has produced a technical bulletin, Technical Bulletin 

(TB) 858, that contains emissions factors and summaries of emission test results for total 

chromium and Cr VI for some MACT II emissions sources (NCASI, 2003). For 

emissions sources in which several total chromium and Cr VI emission test results were

presented, we selected the median value for each, based upon assuming all non-detects 

were half of the detection limits, which is a common approach used to estimate emissions 

when samples are below detection limit. We then divided the Cr VI emissions by the 

total chromium emissions to determine the approximate percentage of Cr VI. In other 

instances, only total chromium emissions data were presented, and we could not 

determine chromium speciation from this information. For these emissions sources, the 

general NEI default chromium speciation was assigned (34 percent Cr VI and 66 percent

Cr III). We have no emissions data specific to the MACT I and III or “other” sources at 

pulp and paper facilities that could be used to develop a chromium speciation profile for 

the generically reported “chromium” or “chromium & compounds” emissions. However, 

the NEI has default chromium speciation profiles specific to many of the SCCs for these 

emissions sources, and we applied these NEI default chromium speciation profiles to 

these emissions. Table 1 shows the chromium speciation applied to each type of 

emission point at pulp and paper facilities and the source of the speciation data. These 

speciation profiles were used in the preliminary risk analysis for generically reported 

chromium emissions. 

Table 1. Pulp and Paper Chromium Speciation 

Pulp and 

Paper Process 

Units Emissions Source(s) SCC

Estimated

% Cr VI Basis

MACT I and III 

- Pulp and 

bleach systems 

at kraft, soda, 

sulfite, and 

semichemical 

pulping mills

unknown operations, fuel use –

unknown fuel, and pulping not 

classified operations 

30700X99 100 NEI 

default

fuel use - unknown fuel 3079XXXX 34

General

NEI

default

Fuel Fired Equipment: Natural 

Gas: Process Heaters, Fuel Fired 

Equipment: Natural Gas: 

Incinerators, In-Process Fuel 

Use: Natural Gas, General

30790003

30790013

39000699 4 NEI 

default

wastewater 30700121 19 NEI 
Chromium Emissions Speciation for Selected Source Categories March, 2011

9

default

causticizing 30700122 25 NEI 

default

MACT II -

Chemical 

recovery 

combustion 

sources at kraft, 

soda, sulfite, 

and stand-alone 

semichemical 

pulping mills 

smelt dissolving tank 30700105 100 NCASI

unknown operations, neutral 

sulfite semichemical pulping, 

and pulping not classified

30700X99 100 NEI 

default

recovery furnace/indirect 

contact evaporator 30700110 75 NCASI

lime kiln (with ESP), MgO 

recovery system, NH3 recovery 

system, Na recovery system,

and fluid bed reactor 

30700106,3

0700221, 

30700222, 

30700223, 

30700303

34

General 

NEI 

default

recovery furnace/direct contact 

evaporator 30700104 25 NCASI

incineration of sludge 50300506 19 NEI 

default

process heater – oil, incinerator 

– oil, lime kiln burning oil

30790001, 

30790002, 

30790011, 

30790012, 

39000403, 

39000503

18 NEI 

default

lime kiln (with wet scrubber) 30700106 10 NCASI

process heater – natural gas, 

incinerator - natural gas, 

incinerator - process gas, lime 

kiln burning natural gas, in 

process fuel use – natural gas

30790003,

30790013, 

30790014, 

39000603, 

39000699

4 NEI 

default

unknown operations 39999999 100 NEI 

default

Other, nonMACT 

processes

incineration and incineration of 

sludge

50300107, 

50300506 19

fuel use – natural gas and in 

process fuel use – natural gas 39000699 4

7. Secondary Aluminum Production 

Secondary aluminum production facilities recover aluminum from scrap materials 

such as beverage cans, foundry returns, dross, and other aluminum scrap. These facilities 

conduct the following operations: (1) preprocessing of scrap aluminum, including size

reduction and removal of oils, coatings, and other contaminants, (2) furnace operations 

including melting, in-furnace refining, fluxing, and tapping, (3) additional refining, by 

means of in-line fluxing, and (4) cooling of dross. 
Chromium Emissions Speciation for Selected Source Categories March, 2011

10

Emissions of chromium compounds may be caused by chromium present in the 

furnace feedstock ((i.e., the scrap aluminum) and/or the refractory linings of the furnaces. 

Secondary aluminum feedstock may contain chromium from impurities in the scrap metal 

(such as auto parts) and from contamination of oils, grease, paints and lubricants on the 

aluminum scrap being fed into the furnace. Additionally, refractory linings of various 

parts of the furnace and its appendages contain chromium compounds or chromium ore,

which is emitted from the furnace along with the other metal HAP emissions. 

Of the 180 facilities in the ANPRM data set for the secondary aluminum 

production source category, 70 facilities report “chromium” emissions, 9 report emissions 

of “chromium & compounds,” 5 report emissions of Cr VI, and 2 report emissions of Cr 

III. The generically reported “chromium” and “chromium & compounds” emissions 

comprise over 99 percent of all the reported chromium emissions from the source 

category. 

We have no emissions data specific to the secondary aluminum industry that 

could be used to develop a chromium speciation profile for the generically reported 

“chromium” or “chromium & compounds” emissions from secondary aluminum 

production processes. We have reports from tests conducted at secondary aluminum 

facilities for which total chromium emissions were measured, but these tests did not 

measure the Cr VI fraction of the total chromium emissions. Because the types of metal 

being recycled vary significantly from one facility to another, and because of lack of data 

for these facilities, it is not possible, at this time, to use industry-specific data to estimate 

chromium speciation for the overall industry. We considered using chromium speciation 

data from facilities in source categories that have similar processes or equipment, such as 

wool fiberglass facilities. Facilities in both the wool fiberglass and the secondary 

aluminum source categories use furnaces that have linings made of refractory material 

containing chromium. Due to these refractory materials, melt furnaces have been 

identified as one of the primary sources of chromium emissions from wool fiberglass 

production. While chromium emissions for these furnaces may be similar for both source 

categories, there is uncertainty about the amount of Cr VI that is in the scrap aluminum 

feed, and this speciation could vary depending on the specific characteristics of the scrap 

material. Considering the Cr VI speciation profile for wool fiberglass melt furnaces 

ranges from 5 percent to 95 percent (see the wool fiberglass section for more information 

about this speciation range), the uncertainty of the chromium speciation of the scrap feed 

material, and in applying a conservative approach, we have assumed that 100 percent of 

the total chromium emissions from secondary aluminum production facilities are Cr VI, 

and that 0 percent are Cr III. This speciation profile was used in the preliminary risk 

analysis for generically reported chromium emissions.

8. Secondary Lead Smelting

Secondary lead smelting facilities use blast, rotary, reverberatory, and/or electric 

furnaces to recover lead metal from lead-bearing scrap materials, primarily lead-acid 

batteries. Processes at these facilities include: (1) breaking lead acid batteries and 
Chromium Emissions Speciation for Selected Source Categories March, 2011

11

separating the lead-bearing materials from the other materials, including the plastic case 

material and acid electrolyte, (2) melting lead metal and reducing lead compounds to lead 

metal in the smelting furnace, and (3) refining and alloying the lead to customer 

specifications. While it has been established that emissions of chromium from these 

facilities occur, the source of these emissions is not known.

The ANPRM data set for the secondary lead smelting source category includes 15 

facilities with chromium emissions (out of a total of 15 facilities). The dataset includes 

records for “chromium” emissions for these facilities. 

The data available on speciated chromium emissions from secondary lead 

smelters is limited to test data from one facility in California (Exide). However, as 

operations at this facility are considered typical for the source category, chromium 

emissions are expected to be similar at other smelters. In October 1991 and February 

1992, Exide sampled baghouse dusts from multiple emissions points (EC/R, 2006). In 

the four emissions source types examined at the facility - a blast furnace, material storage 

unit, reverberatory furnace, and rotary dryer, total chromium and Cr VI were measured. 

The concentration of total chromium in the sampled dust ranged from 20 to 120 mg/kg. 

The concentration of Cr VI was below the detection limit of 0.25 mg/kg in each of the 

samples. If we assume Cr VI is one-half the detection limit (0.125 mg/kg), which is a 

common approach used to estimate emissions when samples are below detection limit, 

the range of the percent of Cr VI would be 0.1 percent (0.125 mg/kg ÷ 120 = 0.001) to 

0.6 percent (0.125 mg/kg ÷ 20 mg/kg = 0.00625), depending on the amount of total 

chromium assumed (EC/R, 2006).

Based on these data, and assuming that captured baghouse dust concentrations 

are the same as the dust concentrations emitted from the baghouse, the estimated range of 

Cr VI emissions for secondary lead smelters are from 0.1 percent to 0.6 percent of the 

total chromium emissions generated. Applying a conservative approach, we have 

assumed that 1 percent of the total chromium emissions from secondary lead smelters are

Cr VI, and that 99 percent are Cr III. This speciation profile was used in the preliminary 

risk analysis for generically reported chromium emissions.

9. Wood Furniture (Surface Coating)

The Wood Furniture Manufacturing industry encompasses the manufacture of a 

range of wood and wood-based products, including kitchen cabinets, residential furniture, 

office furniture and fixtures, partitions, shelving, lockers, and other wood furniture. The 

MACT standard for this source category covers the finishing, gluing, cleaning, and wash 

off operations at wood furniture manufacturing facilities. Emissions of chromium 

compounds from these facilities may be caused by the use of varnishes, glazes, paints, 

and other coatings that contain chromium. 

Of the 644 facilities in the ANPRM data set for the wood furniture surface coating 

source category, 48 facilities report “chromium & compounds” emissions, 13 report 

emissions of “chromium,” and 10 report emissions of Cr III. The generically reported 
Chromium Emissions Speciation for Selected Source Categories March, 2011

12

“chromium” and “chromium & compounds” emissions comprise approximately 22 

percent of all the reported chromium emissions from the source category.

To determine whether there could be Cr VI emissions from these facilities, EPA 

contacted representatives from the industry trade association (American Furniture 

Manufacturers Association) and two of the primary coating suppliers (Kazoo Nobel and 

Valspar). Based on these oral communications, we conclude that no product 

formulations for coatings used in the wood furniture industry contain Cr VI compounds. 

Therefore, we have assumed that 0 percent of the total chromium emissions from wood 

furniture manufacturing (surface coating) facilities are Cr VI, and that 100 percent are Cr 

III. This speciation profile was used in the preliminary risk analysis for generically 

reported chromium emissions.

10. Wool Fiberglass Production

Wool fiberglass is manufactured in a process that forms thin fibers from molten 

glass using raw materials such as sand, feldspar, sodium sulfate, anhydrous borax, boric 

acid, or other materials. A typical wool fiberglass manufacturing line consists of the

following processes: (1) preparation of molten glass; (2) formation of fibers into a wool 

fiberglass mat; (3) curing the binder-coated fiberglass mat; (4) cooling the mat; and 

(5) backing, cutting, and packaging the finished product. According to the North 

American Insulation Manufacturers Association (NAIMA), Cr VI emissions are 

generated primarily from high temperature erosion and corrosion of refractory materials 

that contain chromium and some chromium is also generated from tramp chromium 

materials in the batch (Crane, 2008). Melt furnaces and all 

riser/channel/conditioner/forehearth systems that are constructed from chromium-bearing 

refractory materials emit Cr VI (Crane, 2008). 

Of the 35 facilities in the ANPRM data set for the wool fiberglass production 

source category, 24 facilities report “chromium & compounds” emissions, 5 report 

emissions of “chromium,” 3 report emissions of Cr VI, and 1 reports emissions of Cr III. 

The generically reported “chromium” and “chromium & compounds” emissions 

comprise over 95 percent of all the reported chromium emissions from the source 

category.

Information provided by NAIMA reports estimated percentages of Cr VI relative 

to total chromium emissions for the equipment that generates chromium emissions at 

wool fiberglass production facilities. This information is based on stack sampling

conducted over the past several years and compiled by NAIMA (Crane, 2008). Table 2

summarizes this information.

Table 2. Cr VI as a Percent of Total Chromium Emissions for Chromium 

Emissions Sources at Wool Fiberglass Production Facilities

Chromium Emissions Source Type Cr VI Percent of Total 

Chromium Emissions

Melt furnace - OxyFuel unit with a precipitator 95%
Chromium Emissions Speciation for Selected Source Categories March, 2011

13

Melt furnace - OxyFuel unit with electric-boost and 

a precipitator 85%

Melt furnace - electric (TECO, Intech, Western 

Fiberglass Hot Spot, etc.) 5%

Melt furnace - hot top other than an OxyFuel unit 75%

Channel/forehearth - sealed, un-vented electric 40%

Channel/forehearth - gas fired and vented 60%

Channel/forehearth - hybrid using burner and 

electric resistance heating 75%

Channel/forehearth - cooled without added heating 30%

In addition to this data, information provided to EPA by the Kansas Department 

of Health and Environment, indicates that one facility, Owens Corning in Kansas City, 

KS estimates Cr VI makes up 76 to 77 percent of total chromium emissions from melt 

furnaces and conditioning processes (Edmiston-Bennett, 2008). 

Based on these data, the estimated Cr VI emissions range from 5 percent to 95

percent of the total chromium emissions from melt furnaces and range from 30 percent to 

75 percent of the total chromium emissions from channel/forehearths. As our data do not 

indicate the specific type of equipment used by facilities, and given the wide variation in 

the estimated chromium speciation, we have applied a conservative approach and 

assumed that 100 percent of the total chromium emissions from all emission points 

emitting chromium at wool fiberglass production facilities are Cr VI, and that 0 percent

are Cr III. This speciation profile was used in the preliminary risk analysis for 

generically reported chromium emissions. 

D. Summary of Chromium Speciation Profiles for Selected 2005 NATA Categories

The following table presents a summary of the estimated percent Cr VI and Cr III 

used in the 2005 NATA preliminary inhalation risk analyses for each source category, 

along with the data used to support these estimates. 

Table 3. Summary of Chromium Emissions Speciation for Selected Source 

Categories in the 2005 NATA

Source Category % Cr VI % Cr III Basis for speciation

Coal Boilers 12 88 Source test data

Ferroalloys Production 3 97 Source category test 

data, surrogate 

source category test 

data

Pesticide Active 

Ingredient Production

4 96 NEI SCC default

Phosphate Fertilizer and 

Phosphoric Acid 

25 75 Surrogate source 

category test data
Chromium Emissions Speciation for Selected Source Categories March, 2011

14

Source Category % Cr VI % Cr III Basis for speciation

Production

Pulp and Paper 

Production 

4 to 100 

depending on 

type of process 

unit

0 to 96 depending 

on type of process 

unit

Industry data, NEI 

SCC defaults

Secondary Aluminum 

Production

100 0 Surrogate source 

category test data

Secondary Lead Smelting 1 99 Source category test 

data

Wood Furniture (Surface 

Coating)

0 100 Discussion with 

trade association and 

coatings suppliers

Wool Fiberglass 

Production

100 0 Trade association 

reported estimates

E. References:

Agency for Toxic Substances and Disease Registry (ATSDR), 2008. Toxicological 

profile for Chromium (Draft for Public Comment). Atlanta, GA: U.S. Department of 

Health and Human Services, Public Health Service. Available 

at: http://www.atsdr.cdc.gov/toxprofiles/tp7.html#bookmark08.

Consumers Energy, Laboratory Services, 2004. Analytical Test Results On Baghouse 

Dust – at the Quanex Corporation Macsteel facility. February 27, 2004.

Crane, 2008. Email from Angus Crane (Vice President of General Council for NAIMA) 

to Mr. Jeff Telander (Environmental Engineer, U.S. EPA) which presents a summary of 

chromium emissions speciation data. February 8, 2008.

EC/R, 2006. Secondary Lead Smelter Industry - Source Characterization for 

Residual Risk Assessment. Prepared for EPA/OAR/OAQPS/SPPD/MMG by 

EC/R Incorporated. November 2006.

Guertin, Jacques, James Alan Jacobs, Cynthia P. Avakian, 2005. Chromium (VI) 

Handbook. CRC Press. ISBN 9781566706087.

Edmiston-Bennett, 2008. Email from Robin Bennett-Edmiston (ISB Environmental), to 

Andy Hawkins (Kansas Department of Health and Environment). Subject: National 

Emissions Inventory Kansas City – Chrome. January 16, 2008. 

METCO Environmental, Dallas, TX, 1996. Source Emissions Survey of American 

Chrome & Chemicals, L.P., located in Corpus Christi, TX on August 14 - 16, 1996. 

METCO Environmental, Dallas, TX, 2000. Source Emissions Survey of Elementis 

Chromium, located in Corpus Christi, TX on March 29 and 30, 2000. 
Chromium Emissions Speciation for Selected Source Categories March, 2011

15

NCASI, 2003. Technical Bulletin 858, Compilation of Air Toxic and Total Hydrocarbon 

Emissions Data for Sources at Kraft, Sulfite, and Non-Chemical Pulp Mills – An Update. 

February 2003.

South Carolina, DHEC, 1996. Letter from the South Carolina Department of Health and 

Environmental Control (DHEC) to Conrad Chin (of U.S. EPA). December 19, 1996. 

U.S. EPA, 1998. Integrated Risk Information Service (IRIS) assessment for hexavalent 

chromium. Available at: http://www.epa.gov/ncea/iris/subst/0144.htm. 

U.S. EPA, 1998. Study of Hazardous Air Pollutant Emissions from Electric Utility 

Steam Generating Units – Final report to Congress. EPA#453/R-98-004. February 1998. 

http://www.epa.gov/ttn/atw/combust/utiltox/utoxpg.html

Copyrighted page(s) removed. Copyrighted 

material is not available in Regulations.gov since 

it may not be reproduced without consent of the 

copyright holder. 

Removed pages can be viewed at the USEPA 

Docket Center, Public Reading Room. 

Address: 1301 Constitution Ave, NW 

Room 3334 

Washington, DC 20004 

Telephone: 202-566-1744 

Email: docket-customerservice@epa.gov
Purpose of the Review of the Nickel and Chromium Methods Report and Charge to the External 

Peer Reviewers.

Purpose

The purpose of this review is to seek expert advice (written report that can be shared 

electronically) on the methods used to develop inhalation cancer risk estimates associated with emissions 

of chromium and nickel compounds from coal and oil-fired electric utility steam generating units (EGUs) 

in support of EPA’s recently proposed Air Toxics Rule.1

 The derivation of cancer risk estimates is based 

on the speciation data available from selected source categories, and on the available unit risk estimates 

(UREs) reflecting the dose that corresponds to a specific level of cancer risk. 

Description of the document to be reviewed

This 13-page document includes a brief description of its purpose, followed by discussions of

emissions for both chromium (Section 2) and nickel (Section 3) compounds with regards to (1) the 

methods and rationale used in previous EPA analyses (where applicable), (2) the methods used in the 

recent analysis for the EPA’s Air Toxics Rule (considering previous methods and currently available 

data), and (3) a discussion of the uncertainties and/or limitations of the methods used. In this document, 

we consider the emissions of chromium compounds from both coal- and oil-fired EGUs and the emissions 

of nickel compounds from oil-fired EGUs, since these are major contributors to inhalation cancer risk 

estimates from each these source types.

In addition to these discussions, we have included attachments A and B which contain 

information on chromium speciation profiles for different industrial sources, and nickel species in 

particulate matter for power plants, respectively.

Charge questions

1- Do EPA’s judgments related to speciated chromium emissions adequately take into 

account the available chromium speciation data?

Chromium and Compounds

2- Has the EPA selected the species of chromium (i.e., hexavalent chromium) that 

accurately represents the toxicity of chromium and compounds?

 1 US EPA, 2011. National Emission Standards for Hazardous Air Pollutants from Coal- and Oil-fired 

Electric Utility Steam Generating Units and Standards of Performance for Fossil-Fuel-Fired Electric 

Utility, Industrial-commercial-Institutional, and Small Industrial-Commercial-Institutional Steam 

Generating Units Rule. Available online at http://www.epa.gov/ttn/atw/utility/utilitypg.html.
3- Are the assumptions used in past analysis scientifically defensible, and are there 

alternatives that the EPA should consider for future analysis?

1- Do EPA's judgments related to speciated nickel emissions adequately take into account 

available speciation data, including recent industry spectrometry studies?

Nickel and Compounds

2- Based on the speciation information available and on what we know about the health 

effects of nickel and compounds, and taking into account the existing URE values (i.e., 

values derived for IRIS, Cal EPA and Texas), the EPA has provided several 

approaches to derive unit risk estimates that may be more scientifically defensible than 

those used in past analyses. Which of the options presented would result in more 

accurate and defensible characterization of risks from exposure to nickel and 

compounds? Are there alternative approaches that the EPA should consider?

Timeline for Submittal of Review

The comments from the reviewers must be submitted to the EPA no later than July 29, 2011.
David A. Eastmond

1

Responses to Charge Questions and Comments on the Draft “Methods to Develop 

Inhalation Cancer Risk Estimates for Chromium and Nickel Compounds” 

Document

Chromium and Compounds

1. Do EPA’s judgments related to speciated chromium emissions adequately take into 

account the available chromium speciation data?

The data on chromium speciation has been derived from test data from 7 coal-fired 

boilers and 7 oil-fired boilers. As indicated in the document, this is a small sample size. 

Based on the citations listed, the values used appear to date from before 1998 and do not 

appear to have included more recent studies that may be directly (or indirectly) relevant. 

Determining which articles are specifically relevant is difficult given the limited focus of 

the document and the differences in terminology used in the articles. Some examples of 

more recent articles that would appear to be relevant are listed below. I will attach copies 

as available. 

Huggins FE, Najih M, Huffman GP (1999) Direct speciation of chromium in coal 

combustion by-products by X-ray absorption fine structure spectroscopy, Fuel 78:233-

242. 

Goodarzi F, Huggins FE. (2001) Monitoring the species of arsenic, chromium and nickel 

in milled coal, bottom ash and fly ash from a pulverized coal-fired power plant in western 

Canada. J Environ Monit. 3(1):1-6.

Galbreath KC, Zygarlicke CJ (2004) Formation and chemical speciation of arsenic-, 

chromium-, and nickel-bearing coal combustion PM2.5, Fuel Processing Technology 

85:701-726.

Narukawa T, Riley KW, French DH, Chiba K. Speciation of chromium in Australian fly 

ash. Talanta (2007) 73(1):178-84.

Stam AF, Meij R, Te Winkel H, Eijk RJ, Huggins FE, Brem G. (2011) Chromium 

speciation in coal and biomass co-combustion products. Environ Sci Technol. 

45(6):2450-6.

As appropriate, inclusion of the data in these articles could substantially increase sample 

size for determining chromium speciation in the document and may change the calculated 

value. 

In addition, the Center for Air Toxic Metals has conducted a number of EPA funded 

studies that may also provide useful information. Some examples from the EPA website 

can be found at the following URLs:
David A. Eastmond

2

http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/7

859/report/F

http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/1

809/report/0

2. Has the EPA selected the species of chromium (i.e., hexavalent chromium) that 

accurately represents the toxicity of chromium and compounds?

Yes. As indicated in the document, multiple authoritative bodies including the EPA, 

IARC and the National Toxicology Program have concluded that inhalation exposure to 

chromium VI compounds is carcinogenic to humans. The document is correct in stating 

that the EPA and IARC have concluded that Cr III compounds are not classifiable as to 

their carcinogenicity to humans. However, this does not convey the full story as Cr III 

compounds are widely accepted as essential trace minerals and are commonly included in 

multiple vitamin supplements. Also, in a recent NTP 2-year rodent bioassay report, 

positive results for the induction of cancer were not seen after dietary administration of 

high doses of chromium picolinate, a Cr III compound, to the treated mice and rats. I 

leave it to the EPA to decide whether this information should be included. 

3. Are the assumptions used in past analysis scientifically defensible, and are there 

alternatives that the EPA should consider for future analysis?

I will discuss the current analysis since I am not sure to which past analysis the question 

is referring. The current analysis is based on the assumption that the risk of cancer from 

inhalation exposure to chromium is due to the presence of Cr VI compounds and that this 

risk decreases linearly with dose into the low and very low dose range. Linear 

extrapolation is commonly used for agents that act through a mutagenic mode of action 

and Cr VI compounds are widely recognized as exerting their carcinogenic effects 

through a mutagenic mechanism (Straif et al., 2009; McCarroll et al., 2010). As a result, 

I would consider the use of linear extrapolation for estimating chromium risks to be 

scientifically defensible. As indicated in #2 above, I believe that there is strong scientific 

justification for basing the chromium risks on the dose of Cr VI rather than the total Cr 

dose. 

In the current assessment, as a default assumption to calculate risks, Cr VI compounds 

are estimated to comprise 12% of the total Cr emitted from the coal- and oil-fired electric 

generating units (EGUs). This number is the higher of the averages from the coal- and 

oil fired boilers. From the information presented in the document, this would seem to be 

a reasonable estimate, particularly for the calculation of population risks. It should be 

noted that individual risks for exposed individuals could be higher, particularly if above 

average Cr VI percentages are consistently seen at specific EGUs. 

Specific details on the sampling locations are not presented in the document but it

appears that the speciation values were determined from samples collected at the EGU 
David A. Eastmond

3

itself and not at sites where exposure to the general population would occur. It is 

assumed that the proportion of Cr VI compounds to which general population is exposed

will be that same as that at the collection site at the EGU. This seems to be a reasonable 

assumption but this may not be the case if significant atmospheric transformation of the 

Cr compounds occurs. There is some theoretical and laboratory-based evidence in the 

literature that Cr compounds can be transformed in the environment with a portion of the 

Cr VI compounds being converted into Cr III compounds (Seigneur and Constantinou, 

1995; Werner et al., 2006). 

Other assumptions that have been used are fairly typical and are based on an inhalation 

rate of 20 m3

/day, a body weight of 70 kg, and a standard 70-year lifespan. These default 

values are generally higher than average for the inhalation rate, and lower than average 

for the body weight and lifespan. These do not reflect the ranges of values that are 

commonly seen. If desired, more accurate values could be obtained from the EPA 

Exposure Factors Handbook for this or future analyses. 

Additional Comments

The risk estimate calculated from the unit risk estimate is an upper-bound estimate of the 

excess cancer risk resulting from a lifetime of continuous exposure to Cr VI at a 

concentration of 1 μg/m3 in air. It should be noted on page 3 that the 12,000 in a million 

risk is an upper-bound estimate. Similar changes should be made elsewhere in the 

document such as the risk estimate for nickel described on page 9. 

The formula for calculation of risk on page 4 appears to be incorrect. This can easily be 

seen if one assumes that mass percent of Cr VI in the mixture is 100% and compares the 

results to that of the formula on page 3. The MPCr(VI) should be divided by 100 rather 

than multiplied by 100. Alternatively, the formula could be changed to ECCr * Mass 

FractionCr(VI) * 0.012 (μg/m3)

-1

. 

On page 5 (beginning of 3rd complete paragraph), the speciation data was derived from 

14 units (7+7) rather than 7 as listed. 

The 12th Report on Carcinogens has recently been released by the NTP. The references 

to the 11th Report on Carcinogens mentioned throughout the document could be updated. 

Please note that the report title is incorrect on pages 3 and 7 and possibly elsewhere. 

Nickel and Compounds

1. Do EPA’s judgments related to speciated nickel emissions adequately take into 

account available speciation data, including recent industry spectrometry studies?

Similar to what was described above for the chromium section, it appears that much of 

the nickel speciation information, with the exception of the recent 2011 Huggins study in 

Appendix B, comes from studies that were conducted prior to 1998. There are a number 

of more recent studies that may be directly or indirectly relevant. As indicated above, 
David A. Eastmond

4

determining which articles are directly relevant is difficult given the limited focus of the 

document and the differences in terminology used in the articles. Some examples of 

more recent articles that would appear to be relevant are listed below.

Goodarzi F, Huggins FE. (2001) Monitoring the species of arsenic, chromium and nickel 

in milled coal, bottom ash and fly ash from a pulverized coal-fired power plant in western 

Canada. J Environ Monit. 3(1):1-6.

Galbreath KC, Zygarlicke CJ (2004) Formation and chemical speciation of arsenic-, 

chromium-, and nickel-bearing coal combustion PM2.5, Fuel Processing Technology 

85:701-726.

Goodarzi F, Huggins F. (2004) Speciation of nickel in Canadian subbituminous and 

bituminous feed coals, and their ash by-products. J Environ Monit. 6(10):787-91.

Galbreath KC, Schulz RL, Toman DL, Nyberg CM, Huggins FE, Huffman GP, Zillioux 

EJ. (2005) Nickel and sulfur speciation of residual oil fly ashes from two electric utility 

steam-generating units. J Air Waste Manag Assoc. 55(3):309-18.

Pattanaik S, Huggins FE, Huffman GP, Linak WP, Miller CA. (2007) XAFS studies of 

nickel and sulfur speciation in residual oil fly-ash particulate matters (ROFA PM). 

Environ Sci Technol. 41(4):1104-10.

Huggins FE, Galbreath KC, Eylands KE, Van Loon LL, Olson JA, Zillioux EJ, Ward SG, 

Lynch PA, Chu P. (2011) Determination of nickel species in stack emissions from eight 

residual oil-fired utility steam-generating units. Environ Sci Technol. 45(14):6188-95.

In addition, the Center for Air Toxic Metals has conducted a number of EPA-funded 

studies that may also provide useful information. Some examples from the EPA website 

can be found at the following URL:

http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/7

859/report/F

http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/1

809/report/0

It should be noted that the Huggins article listed above contains speciation information 

for nickel for EGUs in addition to the Hawaiian sites listed in Appendix B. From this and 

the other studies, it appears that the sulfidic nickel compounds are present at quite low 

levels, if present at all, in the emissions from the EGUs. 

2. Based on the speciation information available and on what we know about the health 

effects of nickel and compounds, and taking into account the existing URE values 

(i.e., values derived for IRIS, Cal EPA and Texas), the EPA has provided several 

approaches to derive unit risk estimates that may be more scientifically defensible 

than those used in past analyses. Which of the options presented would result in 
David A. Eastmond

5

more accurate and defensible characterization of risks from exposure to nickel and 

compounds? Are there alternative approaches that the EPA should consider?

Most authoritative bodies, including the EPA, have concluded based on human, animal 

and mechanistic data that nickel compounds as a class, are carcinogenic to humans. The 

sulfidic nickel compounds appear to be the most potent but there is also evidence from 

human studies for the carcinogenicity of soluble nickel compounds, either alone or in 

combination with other nickel species. However, the results of animal studies have 

indicated that some specific nickel compounds such as nickel sulfate hexahydrate and 

nickel metal powder are not carcinogenic in rodents by inhalation whereas other nickel 

compounds showed clear evidence (nickel subsulfide) or limited evidence (nickel oxide) 

of carcinogenicity by the inhalation route. 

Based on the recent studies such as those of Galbreath et al. (2005) and Huggins et al. 

(2011), it appears that very low levels of the most potent sulfidic nickel compounds are 

emitted from EGUs. As a result, neither option 1, the 2000 NATA approach, nor option 

2, treating all nickel compounds as Ni subsulfide, would seem to be appropriate. From 

my perspective, Option 3 would be preferable, and of the alternative UREs, I would 

recommend using the TCEQ URE of 1.7 x 10-4

m3/μg. The risk assessment leading to 

the derivation of this number was performed recently, included an updated and critical 

review of the literature, and appears to be comprehensive with an emphasis on healthprotection. One additional strength of the TCEQ evaluation is its inclusion of the 

Grimsrud et al., (2003) study where soluble nickel compounds, one of the major types 

found in the EGU emissions, were associated with elevated cancer incidence. It should 

also be noted that the TCEQ URE is quite similar to that derived by the CDHS (2.6 x 10-4

m3

/μg) and the EPA for nickel refinery dust (2.4 x 10-4m3

/μg). The TCEQ, CDHS and 

EPA nickel refinery dust UREs could also be combined to derive a composite URE. 

Recently a nickel nuclear bioavailability model has been proposed to explain the 

biological effectiveness and potency of the various nickel compounds (Goodman et al., 

2011). This mechanistically based model looks promising and could allow the URE to be 

adjusted (with appropriate health-protective assumptions) by the relative bioavailability 

of the actual nickel compounds present or likely to be present in the EGU emissions. The 

paper describing the nickel nuclear bioavailability model was favorably reviewed by an 

expert panel prior to its publication (TERA, 2010). 

Additional Comments

The DHHS (1994) reference mentioned on page 8 is an earlier version of the NTP Report 

on Carcinogens. The reference should be updated to use the more recent version and the 

text integrated with that on the previous page (page 7). 

The Texas Commission on Environmental Quality’s final unit risk estimate is 1.7 x 10-4 

μg/m3 and not the range presented on page 9. The range presented appears to be the 

upper 95th percentile unit risk estimates derived based on Texas rates (rather than the 

United States) for one of the two studies evaluated. 
David A. Eastmond

6

As explained above for the chromium risk formula, the formula for nickel on page 10 

also appears to be incorrect. 

On page 12, the section on point 1- the first sentence is an incomplete sentence and needs 

a verb. 

The expert panel convened by TERA (2010) stated that “While the mechanism of 

action for nickel carcinogenesis is not known and there are a number of potential 

MOAs, the panel reached unanimous agreement that nickel does not act by direct 

DNA reactivity. Instead, the available data suggest that nickel-induced tumors result 

from protein-level effects.” If this conclusion represents the general consensus of 

those in the field, use of approaches other than linear extrapolation might be 

warranted in estimating low dose risks from nickel inhalation. It should also be 

noted that there were a number of panelists who did not believe that “ there is 

sufficient evidence to prove that a threshold exists for nickel compound 

carcinogenesis.” 

References

Grimsrud TK, Berge SR, Martinsen JI, Andersen A. (2003) Lung cancer incidence 

among Norwegian nickel-refinery workers 1953-2000. J Environ Monit. 5(2):190-7.

McCarroll N, Keshava N, Chen J, Akerman G, Kligerman A, Rinde E. (2010) An 

evaluation of the mode of action framework for mutagenic carcinogens case study II: 

chromium (VI). Environ Mol Mutagen. 51(2):89-111.

Seigneur C and Constantinou E (1995) Chemical kinetic mechanism for atmospheric 

chromium, Environ. Sci Technol. 29:222-231.

Straif K, Benbrahim-Tallaa L, Baan R, Grosse Y, Secretan B, El Ghissassi F, Bouvard V, 

Guha N, Freeman C, Galichet L, Cogliano V; WHO International Agency for Research 

on Cancer Monograph Working Group. (2009) A review of human carcinogens--part C: 

metals, arsenic, dusts, and fibres. Lancet Oncol. 10(5):453-4.

TERA (2010) Report on the Nickel Bioavailability Workshop, Vol. 1, Workshop 

organized by Toxicology Excellence in Risk Assessment. The report is available online 

at: http://www.tera.org/Peer/NiBioavailability/

Werner M, Nico P, Guo B, Kennedy I, Anastasio C (2006) Laboratory study of simulated 

atmospheric transformations of chromium in ultrafine combustion aerosol particles, 

Aerosol Science and Technology 40:545-556.
Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

Compounds

Peer Review

by

Herman Gibb

Tetra Tech Sciences

2200 Arlington Boulevard, Suite 400

Arlington, VA 22201

Email: hgibb@sciences.com

July 26, 2011
Herman Gibb

1

1. Do EPA’s judgments related to speciated chromium emissions adequately take into 

account the available chromium speciation data?

Chromium and Compounds

Answer: Yes. EPA has focused on the species that are the most stable and for which the 

most information is available. 

2. Has the EPA selected the species of chromium (i.e., hexavalent chromium) that 

accurately represents the toxicity of chromium and compounds?

Answer: Yes. At this point in time, the data support a conclusion of a causal association 

between hexavalent chromium and lung cancer in humans; they do not support a conclusion 

of a carcinogenic risk for trivalent chromium or metallic chromium. The document states:

“With regards to trivalent chromium Cr (III), the 1998 IRIS Toxicological Review notes that 

while the evidence from the occupational database ‘could be used to suggest that total 

chromium is carcinogenic by inhalation, animal data support the human carcinogenicity data 

only on hexavalent chromium.” The animal data on the carcinogenic risk of inhaled trivalent 

chromium are limited. A stronger argument that hexavalent chromium is the species of 

concern would be Gibb et al. (2000)1

. Gibb et al. (2000) found that in a study of 2357 

chromate production workers, trivalent chromium exposure, when adjusted for hexavalent 

chromium exposure, was not associated with an increased risk of lung cancer. 

3. Are the assumptions used in past analysis scientifically defensible, and are there 

alternatives that the EPA should consider for future analysis? 

Answer: The study on which the IRIS 1998 quantitative risk assessment is based is the study 

by Mancuso (1975). Mancuso (1975) reported cancer risk by total chromium exposure. The 

ratio of trivalent to hexavalent chromium in the Painesville facility studied by Mancuso was 

reported to range from 1:1 to 52:1. Exposure estimates were made from industrial hygiene 

data collected in 1949; the cohort studied began work in 1931-37. No information was 

available on the prevalence of smoking in the workforce. EPA’s quantitative assessment was

based on the risk to total chromium, not hexavalent chromium. The risk was justified as a 

risk for hexavalent chromium by the following assumptions: (1) that the ratio of trivalent to 

hexavalent chromium was 6:1 which would underestimate the risk by seven-fold; and (2) that 

higher exposure in 1931-37 compared to 1949 when the industrial hygiene measurements 

were made and a greater prevalence of smoking among the cohort would overestimate

 1 Gibb HJ, Lees PSJ, Pinsky P, Rooney BC. (2000). Lung cancer among workers in chromium chemical 

production. American Journal of Industrial Medicine 38:115-126.

the 

risk by no more than four-fold. Gibb et al. (2000) was undertaken to address the limitations 
Herman Gibb

2

presented by these assumptions. Gibb et al. was a study of a chromate production facility in 

Baltimore. A comparison of the advantages of Gibb et al. (2000) vs Mancuso (1975) for risk 

assessment can be found in the following table: 

Gibb et al. (2000) Mancuso (1975, 1997

update)

Cohort

1205 white males

848 nonwhite males

304 race unknown

332 white males

Industrial hygiene 

measurements vis-à-vis work 

history

Concurrent with the work 

history

Not concurrent with work 

history (Measurements made 

in 1949 were used to estimate 

exposure to workers who 

began employment in 1931-

37)

Industrial hygiene 

measurements vis-à-vis 

chromium species 

measurement

Ambient measurements of 

Cr+6 (Cr+3 derived from ratios 

of Cr+6 to Cr+3 in settled dust)

Ambient measurements of 

total Cr (Cr+6 and Cr+3

Number of lung cancer cases

estimates from settled dust)

122 42 (1975 study); 66 (1997 

update)

Person-years of observation 70,736 5,853 (1975 study); 12,881 

(1997 update)

Smoking data Yes/No for 91% of the cohort 

at time of employment None

Exposure groups (mg/m3-

years)

0 - 0.00149, 0.0015 – 0.0089, 

0.009 – 0.0769, 0.077 – 5.25

< 0.25, 0.25 - 0.49, 0.50 –

1.00, 1.00 – 1.99, > 2.00

Analysis of cancer risk of Cr 

VI and Cr III Multivariate Univariate

As indicated in the table, the exposure groups in Gibb et al. are closer, by at least two orders of 

magnitude, to environmental Cr+6 concentrations than are the exposure groups of Mancuso 

(1975). The Gibb et al. study also had more person years of observation (70,736 vs. 5,853), a 

larger cohort (2357 vs. 332), and more lung cancer deaths (122 vs. 42) than the Mancuso study. 

Exposure measurements in the Gibb et al. study were taken at the time of exposure; exposure in 

the Mancuso study was estimated from industrial hygiene measurements taken over 10 years 

after the cohort was first exposed. The Gibb et al. study had data on smoking prevalence; the 

Mancuso study had none. Exposure concentrations of hexavalent chromium in the Gibb et al. 

study were the result of actual measurements; hexavalent chromium exposure in the Mancuso 

study was estimated from total chromium measurements. 
Herman Gibb

3

Gibb et al., which was funded by EPA, is the basis of the OSHA Permissible Exposure Limit. It 

also is the basis of the quantitative risk assessment by Park et al. (2004)2 that is used by NIOSH 

for its Recommended Exposure Limit for hexavalent chromium3

. Park et al. (2004) is also the 

basis of the cancer risk estimate in the World Health Organization’s draft Concise International 

Chemical Assessment Document (CICAD) on Hexavalent Chromium. The draft CICAD for 

hexavalent chromium states that the environmental cancer risk (as opposed to the occupational 

cancer risk estimate) for hexavalent chromium is 0.04 per μg/m3 (i.e., > 3X the current EPA risk 

estimate of 0.012 per μg/m3. 

1. Do EPA’s judgments related to speciated nickel emissions adequately take into account 

available speciation data, including recent industry spectrometry studies?

Nickel and Compounds

Answer: All nickel compounds should be regarded as carcinogenic. The 12th Report on 

carcinogens (2011)4, reports that “nickel compounds” (no distinction as to solubility) are 

known human carcinogens. IARC (1990)5 concluded that there was sufficient evidence in 

humans for the carcinogenicity of nickel sulfate and of the combinations of nickel sulfide and 

oxides in the nickel refining industry. IARC’s overall conclusion was that nickel compounds

(no distinction with respect to solubility) are carcinogenic to humans. The International 

Committee on Nickel Carcinogenesis in Man (ICNCM) concluded that there was “strong 

evidence” that soluble nickel was associated with an increased risk of respiratory cancer 

(ICNCM 1990)6

. The report further concluded that the epidemiologic studies provided 

evidence of a nasal cancer risk due to soluble nickel and that soluble nickel exposure may 

enhance the risk of lung and nasal cancer associated with less soluble forms of nickel. The 

Committee’s work was funded and supported by the U.S. EPA; the Commission of European 

Communities; Energy, Mines and Resources Canada; National Health and Welfare Canada; 

Ontario Ministry of Labor; and the Nickel Producers Environmental Research Association. 

It should be noted that the carcinogenicity of hexavalent chromium was also thought to be 

related to solubility and that soluble chromium compounds were not believed to be 

 2 Park RM, Bena JF, Stayner LT, Smith RJ, Gibb HJ, Lees PSJ. (2004) Hexavalent chromium and lung 

cancer in the chromate industry: a quantitative risk assessment. Risk Analysis 24(5): 1099-1108. 3

NIOSH. 2008. Draft Criteria Document Update on Occupational Exposure to Hexavalent Chromium. Available at: 

http://www.cdc.gov/niosh/review/peer/HISA/pdfs/DRAFTCriteriaDocumentUpdateOccupationalExposuretoHexava

lentChromium.pdf

4 National Toxicology Program. 2011. Report on Carcinogens, 12th Edition. Available at: 

http://ntp.niehs.nih.gov/index.cfm?objectid=03C9AF75-E1BF-FF40-DBA9EC0928DF8B15 5 IARC. 1990. Chromium, Nickel and Welding. Volume 49. Available at: 

http://monographs.iarc.fr/ENG/Monographs/vol49/mono49-7.pdf 6 International Committee on Nickel Carcinogenesis in Man. Report of the International Committee on Nickel 

Carciongenesis in Man [Chair: Sir Richard Doll]. Scand J Work Environ Health 16(Special):1-82. 
Herman Gibb

4

carcinogenic7. Subsequent epidemiologic studies disproved that notion, however, as strong 

evidence of respiratory cancer risk was found in workers exposed to sodium dichromate 

(chromate production) and chromic acid (chrome plating), both very soluble hexavalent 

chromium compounds3

. 

There is some question whether metallic nickel is carcinogenic. IARC (1990)5 indicated that 

metallic is possibly carcinogenic. The 12th Report on Carcinogens4 indicates that metallic 

nickel is reasonably anticipated to be a carcinogen. Considering that metallic nickel is 

reasonably anticipated to be a carcinogen, it would be appropriate to consider nickel and 

nickel compounds as carcinogenic. 

2. Based on the speciation information available and on what we know about the health 

effects of nickel and compounds, and taking into account the existing URE values (i.e., 

values derived for IRIS, Cal EPA and Texas), the EPA has provided several approaches 

to drive unit risk estimates that may be more scientifically defensible than those used in 

past analyses. Which of the options presented would result in more accurate and 

defensible characterization of risks from exposure to nickel and compounds? Are there 

alternative approaches that the EPA should consider?

Answer: Use the TCEQ URE of 2.64-4per μg/m3 (smoking adjusted Relative Risk Estimate

with a U.S. background rate)

8

. This approach: (1) uses human data for the risk estimate, (2)

takes advantage of a nickel-exposed cohort (Grimsrud 20039

) for which there are data on the 

prevalence of smoking. None of the other epidemiologic studies used for quantitative risk 

estimates by TCEQ, CDHS, or EPA had data on smoking prevalence. Furthermore, the study 

has a large cohort (N = 5,297), person-years of observation (153,952.9), and lung cancer 

cases (147). 

SUMMARY OF RECOMMENDATIONS

1. Use Gibb et al. (2000) to justify selecting hexavalent chromium compounds (vs. trivalent 

chromium compounds) as the carcinogenic species of chromium.

2. Use a risk estimate based on Gibb et al. (2000) for hexavalent chromium. One can use the 

risk estimate from Park et al. (2004) adjusted for environmental exposures (24 hours/day; 

 7 Levy LS, Martin PA, Bidstrup PL. 1986. Investigation of the potential carcinogenicity of a range of chromium 

containing materials on rat lung. British Journal of Industrial Medicine 43(4): 243-256. 8 Texas Commission on Environmental Quality (TCEQ), 2009. Development support document draft for 

arsenic/inorganic arsenic compounds, and nickel compounds. Available online at 

http://www.tceq.texas.gov/toxicology/dsd/reviews.html#arsenic 9 Grimsrud TK, Berge SR. Martinsen JI, Andersen A. 2003. Lung cancer incidence among Norwegian nickelrefinery workers 1953-2000. J Environ Monit 5(2):190-7. 
Herman Gibb

5

365 days/year; lifetime exposure) or the cancer risk estimate in the WHO CICAD on 

hexavalent chromium. Both are derived from Gibb et al. (2000). 

3. Regard all nickel species as carcinogenic.

4. Use the TCEQ smoking-adjusted risk estimate from the Grimsrud (2003) study with U.S. 

background data as the cancer risk estimate for nickel (2.64-4 per μg/m3). 
Michael Waalkes

This is a review of the methods used to develop cancer risk estimates associated with emissions of 

chromium and nickel compounds from coal and oil-fired electric utility steam generated units 

(EGU). Specifically the reviewer was charged with several questions mostly with regard to use 

of speciation data. The charge questions and comments (in bold) are as follows:

Chromium and compounds:

General comments: 

Specific compounds of chromium are not discussed at all in the document. Species of 

chromium are discussed. This amounts to an implied assumption that the chromium will 

exist in an ionic form in the emission, which may or may not be the case. The ionic 

hexavalent form would be presumed to be the most carcinogenic but is there clear evidence 

that this is the species that is in emissions? The data are sparse. This is also in sharp 

contrast to the Nickel section where specific compounds are considered.

More testing of speciation is required for truly accurate cancer risk estimates. Only very 

few data were available and these varied greatly (e.g. 0.4% to 23% for utility boilers) for 

hexavalent chromium. 

The speciation my change with fuel source. Default emissions profiles may change as 

industrial practices change.

Charges Questions:

1) Do EPA’s judgments related to speciated chromium emissions adequately take into account the 

available chromium speciation data.

Yes but these are very sparse and limit the confidence in the resulting estimates.

2) Has the EPA selected the species of chromium (i.e. hexavalent chromium) that accurately 

represents the toxicity of chromium and chromium compounds?

Hexavalent chromium ion is likely the carcinogenic species and to the extent this 

“represents the toxicity” of chromium and chromium compounds this is true for cancer but 

perhaps not all toxicity nor is it true for all chromium compounds (see general comments). 

The draft does not address specific compounds or if all compound would be degraded to 

ionic form. In addition the term “chromium” could be interpreted as chromium metal.

3) Are the assumptions used in past analysis scientifically defensible, and are there alternatives 

that the EPA should consider for future?

The EPA recognizes the small number of speciation data sets as a major limitation. This 

needs to be corrected in the future with advanced and consistent methods that look at 

extensive factors that could influence levels. With a data set this small, and the ranges as 

large as they are, it is questionable if using the single maximum level is defensible. Then 
Michael Waalkes

there is no concept of variability, a key factor in defense of this approach, and this is largely 

done blind. Basing any major decisions on data that vary over 57-fold is a poor choice but 

there seems to be no alternative. Additional speciation and compound data is a critical 

need.

Nickel and nickel compounds:

General comments:

The 12 ROC is now available and should be cited. 

Not considering soluble nickel carcinogenic is contrary to the views of major Scientific 

bodies and this reviewer and should not be an option.

More independent testing of emissions would enhance confidence.

The draft talks about selecting appropriate nickel “species” when it is actually trying to 

select appropriate compounds - it is assumed that all the compounds are +2 (page 12 two 

line up from bottom).

1) Do EPA judgments ...including recent industry spectrometry studies?

Yes. 

2) Based on ..?

Option 2 based on clear evidence of the carcinogenic effects of soluble nickel compounds in 

humans. Alternative approaches would require a great deal of additional testing for 

carcinogenic potency and would be very costly and time consuming. 
1

Summary of the Peer Review of the Technical Support Document Titled: Methods to Develop 

Inhalation Cancer Risk Estimates for Chromium and Nickel Compounds.

Three independent external peer reviewers1,2,3 were asked to participate in the review of the 

methods for developing inhalation cancer risk estimates associated with emissions of chromium and 

nickel compounds from coal- and oil-fired electric utility steam generating units (EGUs) estimates in 

support of EPA’s recently proposed Mercury Air Toxics Rule.4

 The approaches and rationale for the 

technical and scientific considerations used to derive inhalation cancer risks were summarized in the 

document titled Methods to Develop Inhalation Cancer Risk Estimates for Chromium and Nickel 

Compounds. A series of charge questions were posed to the peer reviewers in order to determine the 

technical and scientific relevance of the approaches used to develop the inhalation unit risk estimates 

which are based on the speciation data available from selected source categories, and on the available unit 

risk estimates (UREs) reflecting the dose that corresponds to a specific level of cancer risk. This 

document presents the individual charge questions (chromium and compounds presented first, followed 

by nickel compounds), a brief summary of the peer reviewers’ comments and recommendations, and how 

those recommendations can be addressed by the EPA in future risk characterizations.

1-

a. Do EPA’s judgments related to speciated chromium emissions adequately take 

into account the available chromium speciation data? Two of the reviewers were 

in agreement with the EPA in that, although the chromium speciation data available 

are limited, the EPA had both considered appropriately the available data and 

highlighted the high level of uncertainty associated with these data. The third author 

also agreed that the chromium speciation database is limited and recommended 

several additional studies for EPA’s consideration. The EPA reviewed these studies 

and added relevant information from studies of coal-combustion speciation indicating 

the percentage of hexavalent chromium ranging from below detection limits to levels 

two fold higher than those reported in past speciation analyses. Thus, the more 

Chromium and Compounds

 1 Dr. Michael Waalkes, National Toxicology Program, National Institute of Environmental Health 

Sciences- National Institutes of Health, Research Triangle Park, North Carolina. 2

Dr. Herman Gibbs, Sciences International Inc., Alexandria, Virginia.

3

Dr. David Eastmond, Department of Cell Biology and Neuroscience, University of California, Riverside. 4 US EPA, 2011. National Emission Standards for Hazardous Air Pollutants from Coal- and Oil-fired 

Electric Utility Steam Generating Units and Standards of Performance for Fossil-Fuel-Fired Electric 

Utility, Industrial-commercial-Institutional, and Small Industrial-Commercial-Institutional Steam 

Generating Units Rule. Available online at http://www.epa.gov/ttn/atw/utility/utilitypg.html.
2

recent speciation data available is unlikely to reduce the uncertainty of the chromium 

speciation analyses used by EPA as the bases for risk characterization analysis. 

b. Has the EPA selected the species of chromium (i.e., hexavalent chromium) that 

accurately represents the toxicity of chromium and compounds? All three 

authors were in agreement that hexavalent chromium was the best scientifically 

supported carcinogenic species that represents the toxicity of chromium and 

compounds.

c. Are the assumptions used in past analysis scientifically defensible, and are there 

alternatives that the EPA should consider for future analysis? Two reviewers 

considered the assumptions applied (primarily, the assumption that hexavalent 

chromium constitutes 12 % of the total coal-fired utilities, and 18% of oil-fired 

utilities, based on the speciation data available) to be reasonable given the limited 

data available for chromium speciation profiling. Given the limited chromium 

speciation data available, and the view of the majority of the peer reviewers, the EPA 

considers it reasonable to use the same approach in future risk analyses. The third 

reviewer did not comment on the speciation assumptions used in past analysis.

2-

a. Do EPA's judgments related to speciated nickel emissions adequately take into 

account available speciation data, including recent industry spectrometry 

studies? The reviewers, which are in agreement with the views of the EPA, consider 

all nickel compounds as carcinogenic, and thus did not focus on nickel speciation or 

nickel solubility as strong determinants of carcinogenicity. These views are based in 

agreement of major scientific bodies (i.e., NTP, IARC, WHO) in that the integrated 

evidence from epidemiological studies, mechanistic studies and carcinogenesis 

studies in rodents support the concept that nickel compounds should be considered 

carcinogenic, as a group. Further, with exception of IRIS (which has only derived 

unit risk estimates for nickel subsulfide and nickel refinery dusts)

Nickel and Compounds

5

 5 IRIS derived URE values for 2 nickel compounds, nickel subsulfide (0.00048 per μg/m3

) and nickel refinery dust 

(0.00024 μg/m3

). The other available UREs that have both been derived for nickel compounds, as a group, are the 

one developed by the California Department of Health Services (CDHS, 1991) and another by the Texas 

Commission on Environmental Quality (TCEQ, Development Support Document, 2011) , with values of 0.00026 

per μg/m3 and 0.00017 μg/m3

, respectively.

, the other two unit 

risk estimates derived by California Department of Health Services (CDHS, 1991) 
3

and the Texas Commission on Environmental Quality (TCEQ, Development Support 

Document, 2011)6

b. Based on the speciation information available and on what we know about the 

health effects of nickel and compounds, and taking into account the existing 

URE values (i.e., values derived for IRIS, Cal EPA and Texas), the EPA has 

provided several approaches

have been derived for nickel compounds as a group. One

reviewer recommended that the EPA review several manuscripts that might (directly 

or indirectly) be relevant to nickel speciation profiling. However, as mentioned 

above, EPA, in agreement with the peer reviewers, considers nickel compounds to be

carcinogenic as a group.

7

Two of the reviewers suggested the consideration of all nickel compounds as 

carcinogenic, which would lead to using the URE selected without application of a 

factor. The third reviewer pointed at only considering nickel subsulfide as the most 

potent carcinogen among nickel compounds (which was an approach used by the EPA 

in past analyses) with the application of a factor that would account for the fraction of 

nickel subsulfide from total nickel emissions. EPA disagrees with the latter reviewer 

based on the information discussed above (i.e., the available scientific evidence 

supports considering all nickel compounds as carcinogenic). The other aspect of this 

charge question was related to whether the EPA should consider an alternative 

approach. Two of the reviewers suggested using the URE derived by TCEQ rather 

than the one derived by Integrated Risk Information System (IRIS). The third author 

did not comment on alternative approaches. The EPA has decided to continue using 

the current IRIS URE because IRIS derived values are at the top of our hierarchy with 

respect to dose response information used in EPA’s risk characterization.

to derive unit risk estimates that may be more 

scientifically defensible than those used in past analyses. Which of the options 

presented would result in more accurate and defensible characterization of risks 

from exposure to nickel and compounds? Are there alternative approaches that 

the EPA should consider?

8

 6 Texas Commission on Environmental Quality (TCEQ), 2011. Development Support Document for nickel and 

inorganic nickel compounds. Available online at 

http://www.tceq.state.tx.us/assets/public/implementation/tox/dsd/final/june11/nickel_&_compounds.pdf

7

See section 3.3 of the document titled “Methods to Develop Inhalation Cancer Risk Estimates for Chromium and 

Nickel Compounds”. Available online at http://cfpub.epa.gov/si/si_public_record_Report.cfm?dirEntryId=238881. 8 Health Effects Information Used in Cancer and Noncancer Risk Characterization
4

Nevertheless, taking into account that there may be 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 

two to three fold lower than the IRIS URE for nickel subsulfide, 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.

 

for the 1999 National-Scale Assessment. Available online at 

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