6560-50-P

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[OAR-2003-0189; FRL-]

RIN: 2060-AK73 

List of Hazardous Air Pollutants, Petition Process, Lesser 

Quantity Designations, Source Category List

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

SUMMARY: The EPA is proposing to amend the list of 

categories of sources that was developed pursuant to section

112(c)(1) of the Clean Air Act (CAA) by deleting four 

subcategories from the Stationary Combustion Turbines source 

category. Final maximum achievable control technology 

(MACT) standards creating the following subcategories were 

published on March 4, 2004: lean premix gas-fired 

stationary combustion turbines, diffusion flame gas-fired 

stationary combustion turbines, emergency stationary 

combustion turbines, and stationary combustion turbines 

located on the North Slope of Alaska. This action is being 

taken in part to respond to a petition submitted by the Gas 

Turbine Association (GTA) and in part upon the EPA 

Administrator’s own motion. Petitions to remove a source 

category from the source category list are permitted under 
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section 112(c)(9) of the CAA. The proposed rule is based on

EPA’s evaluation of available information concerning the 

potential hazards from exposure to hazardous air pollutants 

(HAP) emitted from the four subcategories and includes a 

detailed rationale for removing the subcategories from the 

source category list. We request comment on the proposed 

rule.

Although the proposed rule would delete certain 

subcategories from the Stationary Combustion Turbines source 

category, the MACT standards for the subcategories will take 

effect upon publication of the standards. Because the MACT 

standards require immediate compliance by new sources, some 

sources in the subcategories which we are proposing to 

delist may need to make immediate expenditures on emission 

controls which will not be required if we adopt a final rule

to delete the subcategories. In view of our initial 

determination that the statutory criteria for delisting have

been met for the subcategories, we consider it inappropriate 

and contrary to statutory intent to mandate such 

expenditures until after a final determination has been made

whether or not the subcategories should be delisted. 

Accordingly, we are publishing elsewhere in this 

Federal Register a proposal to stay the enforcement of the 

MACT standards for new sources in the subcategories during 
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the pendency of the rule to delete the subcategories.

DATES: Comments. Written comments on the proposed rule 

must be received by [INSERT DATE 60 DAYS FROM PUBLICATION OF

THE PROPOSED RULE IN THE FEDERAL REGISTER].

Public Hearing. A public hearing regarding the proposed 

rule will be held if requests to speak are received by the 

EPA on or before [INSERT DATE 15 DAYS FROM PUBLICATION OF 

THE PROPOSED RULE IN THE FEDERAL REGISTER]. If requested, a 

public hearing will be held on [INSERT DATE 28 DAYS FROM 

PUBLICATION OF THE PROPOSED RULE IN THE FEDERAL REGISTER]. 

ADDRESSES: Comments. Comments may be submitted 

electronically, by mail, or through hand delivery/courier. 

Electronic comments may be submitted on-line at 

http://www.epa.gov/edocket/. Written comments sent by U.S. 

mail should be submitted (in duplicate if possible) to: Air

and Radiation Docket and Information Center (Mail Code 

6102T), Attention Docket ID Number OAR-2003-0189, Room B108,

U.S. EPA, 1200 Pennsylvania Avenue, NW., Washington, DC 

20460. Written comments delivered in person or by courier 

should be submitted (in duplicate if possible) to: Air and 

Radiation Docket and Information Center (Mail Code 6102T), 

Attention Docket ID Number OAR-2003-0189, Room B102, U.S. 

EPA, 1301 Constitution Avenue, NW., Washington, DC 20460. 

The EPA requests a separate copy also be sent to the contact
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person listed below (see FOR FURTHER INFORMATION CONTACT).

Public Hearing. If a public hearing is requested by [INSERT

DATE 15 DAYS FROM PUBLICATION OF THE PROPOSED RULE IN THE 

FEDERAL REGISTER] the public hearing will be held at the EPA

facility complex, T.W. Alexander Drive, Research Triangle 

Park, NC [INSERT DATE 28 DAYS FROM PUBLICATION OF THE

PROPOSED RULE IN THE FEDERAL REGISTER]. Persons interested 

in presenting oral testimony should contact Ms. Kelly A. 

Rimer, Risk and Exposure Assessment Group, Emission 

Standards Division (C404-01), U.S. EPA, Research Triangle 

Park, North Carolina 27711, telephone number (919) 541-2962.

Persons interested in attending the public hearing should 

also contact Ms. Rimer to verify the time of the hearing. 

FOR FURTHER INFORMATION CONTACT: Ms. Kelly A. Rimer, Risk 

and Exposure Assessment Group, Emission Standards Division 

(C404-01), U.S. EPA, Research Triangle Park, NC 27711, 

telephone number (919) 541-2962, electronic mail address 

rimer.kelly@epa.gov.

SUPPLEMENTARY INFORMATION:

Regulated Entities. Categories and entities potentially regulated by this 

action include:

------------------------------------------------------------

 Category SIC NAICS Examples of

 regulated entities

------------------------------------------------------------
5

Any industry 4911 2211 Electric power generation, 

using a transmission, or stationary combustion 

distribution

turbine as 4922 486210 Natural gas transmission

defined 1311 211111 Crude petroleum and natural

in the gas production

regulation. 1321 211112 Natural gas liquids producers

4931 221 Electric and other services 

combined

This table is not intended to be exhaustive, but rather provides a guide for 

readers regarding entities likely to be affected by this action. If you have any 

questions regarding the applicability of this action to a particular entity, consult 

the person listed in the preceding FOR FURTHER INFORMATION CONTACT 

section.

Docket. The EPA has established an official public docket 

for this action under Docket ID Number OAR-2003-0189. The 

official public docket is the collection of materials that 

is available for public viewing at the EPA Docket Center 

(Air Docket), EPA West, Room B-108, 1301 Constitution 

Avenue, NW, Washington, DC 20004. The Docket Center is open

from 8:30 a.m. to 4:30 p.m., Monday through Friday, 

excluding legal holidays. The telephone number for the 

Reading Room is (202) 566-1744, and the telephone number for

the Air Docket is (202) 566-1742.

Electronic Access. An electronic version of the public 

docket is available through EPA’s electronic public docket 

and comment system, EPA Dockets. You may use EPA Dockets at
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http://www.epa.gov/edocket/ to submit or view public 

comments, access the index of the contents of the official 

public docket, and access those documents in the public 

docket that are available electronically. Once in the 

system, select "search" and key in the appropriate docket 

identification number.

Certain types of information will not be placed in the 

EPA dockets. Information claimed as confidential business 

information (CBI) and other information whose disclosure is 

restricted by statute, which is not included in the official

public docket, will not be available for public viewing in 

EPA’s electronic public docket. The EPA’s policy is that 

copyrighted material will not be placed in EPA’s electronic 

public docket but will be available only in printed, paper 

form in the official public docket. Although not all docket

materials may be available electronically, you may still 

access any of the publicly available docket materials 

through the EPA Docket Center.

For public commenters, it is important to note that 

EPA’s policy is that public comments, whether submitted 

electronically or in paper, will be made available for 

public viewing in EPA’s electronic public docket as EPA 

receives them and without change unless the comment contains

copyrighted material, CBI, or other information whose 
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disclosure is restricted by statute. When EPA identifies a 

comment containing copyrighted material, EPA will provide a 

reference to that material in the version of the comment 

that is placed in EPA’s electronic public docket. The

entire printed comment, including the copyrighted material, 

will be available in the public docket.

Public comments submitted on computer disks that are 

mailed or delivered to the docket will be transferred to 

EPA’s electronic public docket. Public comments that are 

mailed or delivered to the docket will be scanned and placed

in EPA’s electronic public docket. Where practical, 

physical objects will be photographed, and the photograph 

will be placed in EPA’s electronic public docket along with 

a brief description written by the docket staff.

Comments. You may submit comments electronically, by mail, 

by facsimile, or through hand delivery/courier. To ensure 

proper receipt by EPA, identify the appropriate docket 

identification number in the subject line on the first page 

of your comment. Please ensure that your comments are 

submitted within the specified comment period. Comments 

submitted after the close of the comment period will be 

marked “late.” The EPA is not required to consider these 

late comments.

Electronically. If you submit an electronic comment as 
8

prescribed below, EPA recommends that you include your name,

mailing address, and an e-mail address or other contact 

information in the body of your comment. Also include this 

contact information on the outside of any disk or CD ROM you

submit and in any cover letter accompanying the disk or CD 

ROM. This ensures that you can be identified as the 

submitter of the comment and allows EPA to contact you in 

case EPA cannot read your comment due to technical 

difficulties or needs further information on the substance 

of your comment. The EPA’s policy is that EPA will not edit

your comment and any identifying or contact information 

provided in the body of a comment will be included as part 

of the comment that is placed in the official public docket 

and made available in EPA’s electronic public docket. If 

EPA cannot read your comment due to technical difficulties 

and cannot contact you for clarification, EPA may not be 

able to consider your comment.

Your use of EPA’s electronic public docket to submit 

comments to EPA electronically is EPA’s preferred method for

receiving comments. Go directly to EPA Dockets at 

http://www.epa.gov/edocket, and follow the online 

instructions for submitting comments. Once in the system, 

select “search” and key in Docket ID No. OAR-2003-0189. The

system is an “anonymous access” system, which means EPA will
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not know your identity, e-mail address, or other contact 

information unless you provide it in the body of your 

comment.

Comments may be sent by electronic mail (e-mail) to 

a-and-r-docket@epa.gov, Attention Docket ID No. OAR-2003-

0189. In contrast to EPA’s electronic public docket, EPA’s 

e-mail system is not an “anonymous access” system. If you 

send an e-mail comment directly to the docket without going 

through EPA’s electronic public docket, EPA’s e-mail system 

automatically captures your e-mail address. E-mail 

addresses that are automatically captured by EPA’s e-mail 

system are included as part of the comment that is placed in

the official public docket and made available in EPA’s 

electronic public docket.

You may submit comments on a disk or CD ROM that you 

mail to the mailing address identified in this document. 

These electronic submissions will be accepted in WordPerfect

or ASCII file format. Avoid the use of special characters 

and any form of encryption.

By Mail. Send your comments (in duplicate, if possible) to:

EPA Docket Center (Air Docket), U.S. EPA West, (MD-6102T), 

Room B-108, 1200 Pennsylvania Avenue, NW, Washington, DC 

20460, Attention Docket ID No. OAR-2003-0189.

By Hand Delivery or Courier. Deliver your comments (in 
10

duplicate, if possible) to: EPA Docket Center, Room B-108, 

U.S. EPA West, 1301 Constitution Avenue, NW, Washington, DC 

20004, Attention Docket ID No. OAR-2003-0189. Such 

deliveries are only accepted during the Docket Center’s 

normal hours of operation.

By Facsimile. Fax your comments to: (202) 566-1741, Docket 

ID No. OAR-2003-0189.

CBI. Do not submit information that you consider to be CBI 

through EPA’s electronic public docket or by e-mail. Send 

or deliver information identified as CBI only to the 

following address: Kelly Rimer, c/o Roberto Morales, OAQPS 

Document Control Officer (C404-02), U.S. EPA, Research 

Triangle Park, NC 27709, Attention Docket ID No. OAR-2003-

0189. You may claim information that you submit to EPA as 

CBI by marking any part or all of that information as CBI 

(if you submit CBI on disk or CD ROM, mark the outside of 

the disk or CD ROM as CBI and then identify electronically 

within the disk or CD ROM the specific information that is 

CBI). Information so marked will not be disclosed except in

accordance with procedures set forth in 40 CFR part 2.

In addition to one complete version of the comment that

includes any information claimed as CBI, a copy of the 

comment that does not contain the information claimed as CBI

must be submitted for inclusion in the public docket and 
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EPA’s electronic public docket. If you submit the copy that

does not contain CBI on disk or CD-ROM, mark the outside of 

the disk or CD-ROM clearly that it does not contain CBI. 

Information not marked as CBI will be included in the public

docket and EPA’s electronic public docket without prior 

notice. If you have any questions about CBI or the 

procedures for claiming CBI, please consult the person 

identified in the FOR FURTHER INFORMATION CONTACT section.

Worldwide Web (WWW). In addition to being available in the 

docket, an electronic copy of today’s proposed rule will 

also be available on the WWW through the Technology Transfer

Network (TTN). Following the Administrator’s signature, a 

copy of the proposed rule will be placed on the TTN’s policy

and guidance page for newly proposed or promulgated rules at

http://www.epa.gov/ttn/oarpg. The TTN provides information 

and technology exchange in various areas of air pollution 

control. If more information regarding the TTN is needed, 

call the TTN HELP line at (919) 541-5384.

Outline. This preamble is organized as follows:

I. Background and Criteria for Delisting

II. Summary of Petitioner’s Request and EPA’s Initial 

Delisting Determination

III. Description of the Four Stationary Combustion Turbine Subcategories

IV. Analysis of Gas-Fired Subcategories

A. Analytical Approach

B. Planning and Scoping

C. Source Characterization

D. Emissions Characterization
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E. Air Dispersion Modeling

F. Human Health Effects of Emitted HAP

G. Human Health Values Used 

H. Human Health Risk Results-Inhalation Pathway

I. Multipathway Considerations

J. Effects Due to Acute Exposure

K. Environmental Effects Evaluation

V. Analysis of the Emergency Turbine Subcategory 

VI. Analysis of the North Slope Turbine Subcategory

VII. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

B. Paperwork Reduction Act

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act 

E. Executive Order 13132: Federalism

F. Executive Order 13175: Consultation and Coordination 

with Indian Tribal Governments

G. Executive Order 13045: Protection of Children from 

Environmental Health Risks and Safety Risks

H. Executive Order 13211: Actions Concerning Regulations 

that Significantly Affect Energy supply, Distribution, or 

Use

I. National Technology Transfer and Advancement Act

I. Background and Criteria for Delisting

Section 112 of the CAA contains a mandate for EPA to 

evaluate and control emissions of HAP from industry sectors 

called source categories. Section 112(b)(1) includes a list 

of 188 specific chemical compounds and classes of compounds 

identified as HAP. Section 112(c) requires the EPA to 

publish a list of all categories and subcategories of 

sources of HAP which will be subject to regulation. Each 

category or subcategory which includes major sources of HAP 

must be listed for regulation. Under section 112(d), the 

CAA requires EPA to establish national emission standards 
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for major source categories based on MACT for each category 

or subcategory which is included in the list.

The EPA published the initial source category list in 

the Federal Register on July 16, 1992 (57 FR 31576); you can

find the most recent update to the source category list in 

the February 12, 2002 Federal Register (67 FR 6521).

Section 112(c)(9) of the CAA provides for the deletion 

of a source category from the list of source categories. A 

source category may be deleted from the list under section 

112(c)(9)(A) if the category no longer satisfies the 

criteria for inclusion on the list because of the deletion 

of one or more HAP from the HAP list pursuant to section 

112(b)(3) or a source category may be deleted from the list 

under section 112(c)(9)(B) if certain substantive criteria 

are satisfied. The EPA construes these provisions to apply 

to each listed subcategory as well. This construction is 

logical in the context of the general regulatory scheme 

established by the statute and is the most reasonable one 

because section 112(c)(9)(B)(ii) expressly refers to 

subcategories. If EPA takes final action to delete a listed

source category or subcategory, this eliminates any 

requirement that MACT standards be promulgated for the 

category or subcategory in question. If MACT standards have

already been promulgated, EPA will amend or rescind the 
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standards in question.

A proceeding to delete a listed category or subcategory

under section 112(c)(9)(B) of the CAA may be commenced 

either in response to a petition or on the initiative of the

EPA Administrator. A source category delist petition is a 

formal request to the EPA from an individual or group to 

remove a specific source category or subcategory from the 

source category list. The Administrator must either grant 

or deny a petition within 1 year after receiving a complete 

petition. To grant such a petition, or to commence a 

proceeding to delete a category or subcategory on the 

Administrator’s own motion, the Administrator must make an 

initial determination that: 

(1) In the case of HAP emitted by sources in the 

category or subcategory that may result in cancer in humans,

a determination that no source in the category or 

subcategory emits such HAP in quantities that may cause a 

lifetime risk of cancer greater than 1 in 1 million to the 

individual in the population who is most exposed to 

emissions of such HAP from the source;

(2) In the case of HAP that may result in adverse 

health effect in humans other than cancer, a determination 

that emissions from no source in the category or subcategory

exceed a level which is adequate to protect public health 
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with an ample margin of safety; and 

(3) In the case of HAP that may result in adverse 

environmental effect, a determination that no adverse

environmental effect will result from emissions from any 

source in the category or subcategory.

If the Administrator decides to deny a petition, the 

Agency publishes a written explanation of the basis for 

denial in the Federal Register. A decision to deny a 

petition is final Agency action subject to review. If the 

Administrator decides to grant a petition, the Agency 

publishes a written explanation of the Administrator’s 

decision, along with a proposed rule to delete the affected 

source category or subcategory. After affording an 

opportunity for notice and comment, the Administrator will 

issue a final rule determining whether or not the affected 

category or subcategory will be delisted. If the final rule

delists any affected source category or subcategory, the 

Administrator will also take all necessary actions to revise

the source category list and to amend or to rescind affected

MACT standards.

We do not interpret section 112(c)(9)(B) of the CAA to require absolute 

certainty that a source category or subcategory will not cause adverse effects on 

human health or the environment before it may be deleted from the source 

category list. The use of the words “may” and “adequate” indicate that the 
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Agency must weigh the potential uncertainties and their likely significance. 

Uncertainties concerning risks of adverse health or environmental effects may be 

mitigated if we can determine that projected exposures are sufficiently low to 

provide reasonable assurance that such adverse effects will not occur. Similarly, 

uncertainties concerning the magnitude of projected exposures may be mitigated 

if we can determine that the levels which might cause adverse health or 

environmental effects are sufficiently high to provide reasonable assurance that 

exposures will not reach harmful levels. 

II. Summary of Petitioner’s Request and EPA’s Initial

Delisting Determination

On August 28, 2002, the GTA submitted a petition 

requesting EPA to create and then delete two subcategories 

from the Stationary Combustion Turbines source category:

lean premix stationary combustion turbines firing natural 

gas as a primary fuel with limited oil backup capability, 

and a low-risk subcategory of stationary combustion 

turbines.

Upon receiving a source category or subcategory 

deletion petition, EPA must first determine whether there is

a match between the source category or subcategory to which 

the petition applies and a listed category or subcategory. 

When MACT standards have been promulgated for the category 

in question, EPA will consult the definitions in those 
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standards to determine whether or not a petition refers to a 

listed category or subcategory. 

In this case, neither of the two subcategories to which

the petition refers existed at the time the petition was 

received, nor do they coincide with the subcategories which 

we have recently adopted in the final MACT standards for 

stationary combustion turbines. However, based on the 

information and the arguments presented in the petition, we 

decided to conduct our own analysis on the subcategories as 

they were defined in the final MACT standards to determine 

whether any of the subcategories meet the criteria of 

section 112(c)(9)(B) of the CAA. In the analysis on which 

our initial determinations are based, we used the data and 

analysis presented in the petition in those instances where 

we felt it was relevant and technically appropriate to do 

so, and we collected additional data and performed further 

analysis where those in the petition were considered 

inadequate.

We construe the issuance of the proposed rule to 

constitute a partial grant and a partial denial of the GTA 

petition. The lean premix gas-fired turbines subcategory in

the final MACT standards is similar to one of the 

subcategories that the petitioner proposed: namely, the 

lean premix stationary combustion turbine firing natural gas
18

as a primary fuel with limited oil use. We have made an 

initial determination that the substantive criteria for 

delisting are satisfied for this subcategory. However, in 

the final MACT standards, we did not create any subcategory 

coinciding with the low-risk subcategory proposed by the 

petitioner. Therefore, we must deny that portion of the 

petition. Also, we have made an initial determination that 

several additional subcategories included in the final MACT 

standards satisfy the substantive criteria for delisting. 

These additional subcategories are: diffusion flame gasfired stationary turbines, emergency stationary combustion 

turbines, and stationary combustion turbines located on the 

North Slope of Alaska.

III. Description of the Four Stationary Combustion Turbines

Subcategories

The final MACT standards (40 CFR 63.6175) define stationary 

combustion turbines as:

all equipment including, but not limited to, the turbine, the 

fuel, air, lubrication and exhaust gas systems, control 

systems (except emissions control equipment), and any 

ancillary components and sub-components comprising any 

simple cycle stationary combustion turbine, any 

regenerative/recuperative cycle stationary combustion 

turbine, or the combustion turbine portion of any stationary 

combined cycle steam/electric generating system. 

Stationary means that the combustion turbine is not selfpropelled or intended to be propelled while performing its 

function. A stationary combustion turbine may, however, be 

mounted on a vehicle for portability or transportability.
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Currently, there are approximately 8,000 stationary combustion turbines 

operating in the United States.

For the purposes of the MACT standards, stationary combustion turbines 

have been divided into eight subcategories. Four of the subcategories are the 

subject of the proposed delisting rule: (1) stationary lean premix combustion 

turbines when firing gas and when firing oil at sites where all turbines fire oil no 

more than 1,000 hours annually (also referred to as “lean premix gas-fired 

turbines”); (2) stationary diffusion flame combustion turbines when firing gas and 

when firing oil at sites where all turbines fire oil no more than 1,000 hours 

annually (also referred to herein as “diffusion flame gas-fired turbines”); (3) 

emergency stationary combustion turbines; and (4) stationary combustion 

turbines operated on the North Slope of Alaska (defined as the area north of the 

Arctic Circle (latitude 66.5 North)).

The stationary combustion turbines MACT standards also define the

subcategories. The lean premix gas-fired turbines subcategory includes those 

stationary combustion turbines that use lean premix technology which was 

introduced in the 1990's and was developed to reduce nitrogen oxide (NOx) 

emissions without the use of add-on controls. In a lean premix combustor, the 

air and fuel are thoroughly mixed to form a lean mixture for combustion. Mixing 

may occur before or in the combustion chamber. Lean premix combustors emit 

lower levels of NOx, carbon monoxide (CO), formaldehyde and other HAP than 

diffusion flame combustion turbines.
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Diffusion flame gas-fired turbines operate in a different manner than lean 

premix units. In a diffusion flame combustor, the fuel and air are injected at the 

combustor and are mixed only by diffusion prior to ignition. Emergency 

stationary combustion turbines are stationary combustion turbines that operate in 

an emergency situation. Examples include stationary combustion turbines used 

to produce power for critical networks or equipment (including power supplied to 

portions of a facility) when electric power from the local utility is interrupted, or 

stationary combustion turbines used to pump water in the case of fire or flood, 

etc. Emergency stationary combustion turbines do not include stationary 

combustion turbines used as peaking units at electric utilities or stationary 

combustion turbines at industrial facilities that typically operate at low capacity 

factors. Emergency stationary combustion turbines may be operated for the 

purpose of maintenance checks and readiness testing, provided that the tests 

are required by the manufacturer, the vendor, or the insurance company 

associated with the turbine. 

The subcategory stationary combustion turbines located on the North 

Slope of Alaska refers to all stationary combustion turbines that are located north 

of the Arctic Circle. They have been identified as a subcategory due to operating 

limitations and uncertainties regarding the application of controls to these units.

IV. Analysis of Gas-Fired Subcategories

A. Analytical Approach

In conducting the risk assessment for the four source 
21

subcategories, EPA uses a tiered, iterative process 

recommended by the National Research Council (NRC) of the 

National Academy of Sciences. This process begins with the 

use of relatively inexpensive screening techniques and moves

to more resource-intensive levels of data-gathering, model 

construction, and model application, as the particular 

situation warrants (NRC, 1994). In applying this approach, 

EPA typically conducts the first (and in some cases the 

only) iteration of the risk assessment using limited amounts

of data and simple, health-protective assumptions. This 

results in risk estimates that we expect will over-predict 

the actual risk. If the initial estimates of risk exceed a 

level of concern, then successive refinements with regard to

data and models may be useful to more accurately 

characterize the actual risk. If the initial estimates are

below a level of concern, then a more sophisticated analysis

may not be necessary for decision-making purposes. 

The analysis discussed here represents an initial 

assessment based on simple, conservative assumptions. This 

screening approach has not sought to modify the assumptions 

in a way that would yield high-end estimates (e.g., 95th

percentile) within the distribution of expected exposures. 

Instead, through the compounding of conservative 

assumptions, we feel this approach yields exposure estimates
22

that exceed exposures to the most exposed individuals in the 

population.

B. Planning and Scoping

The first step in conducting a tiered, iterative risk 

assessment is to plan and scope the assessment. The EPA 

provides guidance for this step in the Risk Characterization

Handbook (EPA, 2000) and in the Framework for Cumulative 

Risk Assessment (EPA, 2003). The general process of 

planning and scoping includes defining the elements that 

will or will not be included in the risk assessment and 

explaining the purposes for which the risk assessment 

information will be used (EPA, 2000).

We have already established the motivation for 

conducting the risk assessment. Prompted by industry, we 

conducted the assessment under section 112(c)(9)(B) of the 

CAA to determine whether regulatory relief for the industry 

was warranted. The assessment needed to show whether or not 

any source in each of the four subcategories exceeds the 

human health and ecological criteria described in the 

statute. In designing the assessment, we considered the 

statutory requirements, the amount and type of available 

information on the subcategories to include in the 

assessment, and the available methods and models.

Based on the criteria, we designed an assessment to 
23

estimate cancer risks and noncancer hazards from a worstcase exposure scenario which would likely exceed the 

exposure to the person most exposed. We began by conducting

a human health risk analysis on stationary lean premix 

combustion turbines when firing gas and when firing oil at 

sites where all turbines fire oil no more than 1,000 hours 

annually, and stationary diffusion flame combustion turbines

when firing gas and when firing oil at sites where all 

turbines fire oil no more than 1,000 hours annually. To 

evaluate the risks, hazards and potential for adverse 

environmental effects from the emergency turbines and north 

slope turbines subcategories, we used available information 

on the subcategories and the results of the assessment on 

the lean premix and diffusion flame subcategories. 

We designed the assessment to address cancer risks and

noncancer hazards to humans from the air and ingestion 

pathways and also evaluated the potential for adverse 

environmental effects. As we describe above, we used a 

tiered, iterative approach to the assessment. Given that 

there are thousands of facilities in the four subcategories

and that current information on the facilities is limited, 

it was not feasible to identify all turbines and their 

operating characteristics on a site-specific basis. 

Therefore, we used several health-protective assumptions 
24

where we lacked data. This is an appropriate approach to 

evaluating whether to remove a source category or 

subcategory from regulation as the CAA specifies that in 

order to be delisted, “no source in the category” may exceed

the cancer, noncancer or environmental criteria. 

We created a worst-case exposure scenario by using a combination of 

actual data and health-protective assumptions. For the air pathway, our 

approach was to:

(1) Determine which type of turbine would result in

the highest modeled air concentration of HAP.

(2) Hypothetically “place” eleven of the turbines at

an actual facility to create our model plant. (An actual facility is permitted for 

eleven turbines, but seven turbines are currently operated there.)

(3) Calculate cancer risks, noncancer hazards and the

potential for adverse environmental effects based on the highest ambient air 

concentrations of HAP calculated by the model.

For the multipathway analysis, we developed and evaluated an exposure 

scenario for our model plant using meteorologic data from locations around the 

country: Allentown, PA; Baton Rouge, LA; Indianapolis, IN; Kansas City, KS; 

Los Angeles, CA; Minneapolis, MN; Seattle, WA; and Tampa, FL. Our goal here 

was to account for the effect of meteorologic variability on the risks and hazards.

We feel the health-protective assumptions we used, 

which when compounded in the assessment, lead to very 
25

health-protective risk estimates. Given the combination of 

data and assumptions used, we have conducted an assessment 

consisting of a single iteration that adequately addresses 

the questions posed, that is responsive to the requirements 

in section 112(c)(9)(B)of the CAA, that overestimates actual 

risks, and shows that the statutory criteria for deletion 

are met. See the technical memo located in the docket for 

the a more detailed description of the analysis (Combustion 

Turbines Source Category Risk Characterization, January 

2004).

C. Source Characterization

Stationary combustion turbines can be operated in two 

basic cycles: simple cycle and combined cycle. The simple 

cycle mode consists of the combustion turbine-generator 

combination operating and producing electricity with the 

turbine exhaust vented through a stack directly to the 

atmosphere. In the combined cycle mode, the exhaust from 

the turbine is passed through a heat recovery steam 

generator to generate steam that is then used to produce 

additional electricity. The heat extraction at this step 

cools the exhaust gas stream resulting in a lower exhaust 

temperature (reduced plume buoyancy). Thus, emissions from 

a turbine operating in the combined cycle mode will often 

produce higher ground level pollutant concentrations. As a 
26

conservative assumption, our analysis only examined the 

combined cycle units.

To conduct our analysis, we used information on the 

physical characteristics of these turbines that was 

submitted by the petitioner after we determined the data 

were of sufficient quality to do so. The GTA provided data 

on a set of typical turbines ranging in power output from 5

to 253 megawatts (MW) each. These characteristics include 

turbine type (i.e., make and model), heat input, stack 

parameters (height, diameter, exit velocity, temperature), 

and building dimensions. 

D. Emissions Characterization

With regard to emissions, we agree with the petitioner 

that the following HAP are emitted from turbines when 

natural gas is used as the fuel: 1,3-butadiene, 

acetaldehyde, acrolein, benzene, ethylbenzene, formaldehyde,

naphthalene, polycyclic aromatic hydrocarbons (PAH, which 

the EPA classifies as a subset of a larger group of HAP, 

polycyclic organic matter (POM)), propylene oxide, toluene, 

and xylenes (mixed). We also agree with the petitioner that

the following non-metallic HAP are emitted from turbines 

when distillate oil is used as the fuel: 1,3-butadiene, 

benzene, formaldehyde, naphthalene, and PAH. However, the 

petitioner claimed that metallic HAP are not detectable in 
27

distillate oil and are, thus, not present in turbine 

emissions; they subsequently amended this claim to state 

that only chromium and lead are emitted. We disagree with 

these claims and have collected additional data showing the 

following HAP metals can be emitted when turbines burn 

distillate oil, although the levels can vary by oil type: 

arsenic, beryllium, cadmium, chromium VI, lead, manganese, 

mercury, nickel and selenium. We used emission factors for 

the emitted HAP that are based on the most recent available 

data. Also, we developed separate emission factors for 

large and small turbines based on the burner design-type 

(lean premix or diffusion flame) and based on the 

differences in heat input between small versus large 

turbines. To develop conservative, yet still realistic 

emission values, we calculated emission factors by selecting

the upper 95 percent confidence interval around the mean of 

each dataset. We then developed turbine-specific emission

estimates by multiplying the pollutant-specific emission 

factors with the heat input of each unit. 

E. Air Dispersion Modeling

The goal of our air dispersion modeling approach was to determine the 

maximum annual ambient average concentration for all emitted HAP that a 

person living in the vicinity of a turbine would experience. We used these 
28

maximum annual ambient average concentrations, without regard to whether a 

person actually lives at that location, as surrogates for 

exposure. This is a health protective, or conservative, 

approach to assessing exposure. 

We used the SCREEN3 model (Version 96043) to estimate 

the maximum annual ambient average concentration from all 

emitted pollutants. SCREEN3 consists of algorithms that 

tend to overestimate HAP concentrations in air, along with 

worst-case meteorologic conditions, to estimate ambient 

concentrations of HAP in air. This results in estimates of 

HAP concentrations in air that are likely to be an 

overestimate of what we expect people to actually breathe. 

We used this health-protective modeling approach to evaluate

the four subcategories of stationary combustion turbines 

because it is not feasible to identify all turbines and 

their operating characteristics due to the large number of 

facilities. Also, we want to ensure that our assessment is 

not underestimating potential exposures and risks. This is 

an important consideration when we are evaluating whether to

grant a petition to remove a source category from regulation

as the CAA specifies that in order to be delisted, “no 

source in the category” may exceed the cancer, noncancer or 

environmental criteria.

Our approach to modeling was to first determine which 
29

type of turbine (of the ten turbine types identified by the 

petitioner) produces the highest maximum annual ambient 

average concentrations using SCREEN3. We then built a 

simulated facility and ran SCREEN3 for all HAP emitted from 

lean premix gas-fired turbines and also for diffusion flame 

gas-fired turbines, using regulatory default mode, full 

meteorology, building downwash, flat nearby terrain, rural 

dispersion, automated receptor arrangement (50-2000 meter),

and a conversion factor of 0.08 to obtain annual average 

concentrations from maximum 1-hour concentrations. As 

stated above, we used turbine characteristics submitted by 

the petitioner and developed updated emission factors 

ourselves. We used these data as inputs into the SCREEN3 

model in order to obtain the maximum annual average air 

concentrations from a worst-case type of turbine. Our 

dispersion modeling showed that the W501F turbine resulted 

in the highest air concentrations.

After establishing that maximum annual ambient average 

concentration are the highest from the W501F turbine, we 

built another simulated facility. We placed 11 W501F

turbines at our simulated facility because the highest 

number of large turbines permitted to operated at an actual 

facility is 11. After accounting for source separation (see

technical memo for details), we ran SCREEN3 on our simulated
30

facility for four scenarios: (1) assuming the 11 turbines 

are lean premix gas-fired turbines collectively using 1,000 

hours of oil per year; (2) assuming the 11 turbines are diffusion flame gas-fired 

turbines collectively using 1,000 hours of oil per year; (3) assuming the 11 

turbines are lean premix and burn only natural gas; and (4)assuming the 11 

turbines are diffusion flame turbines and burn only natural gas. We conducted 

the analyses assuming the turbines burn only natural gas, and assuming the 

turbines burn natural gas plus 1,000 hours of oil per year because not all 

facilities use oil, and because emissions are different when only natural gas is 

used as fuel (no metals are emitted but formaldehyde emissions are higher). 

The maximum annual ambient average concentrations for each emitted pollutant 

for natural gas plus 1,000 hours of oil per year and for natural gas only for the 11 

W501F turbines can be found in Table 4 of the technical memo (see docket).

We consider the maximum annual average concentrations resulting from 

our dispersion modeling analysis to be conservative. That is, we feel that the 

resulting air concentrations over- rather than under-estimate actual exposures to 

people. This is because our analysis used conservative source parameters and 

atmospheric dispersion modeling methodology; relied on conservative emission 

factors for all HAP; used the maximum annual ambient average concentrations 

of the emitted HAP as a surrogate for exposure; and assumed 70 years, 24 

hours a day, 365 days a year of continuous exposure. Even though actual 

emission rates, and thus ambient concentrations, of HAP may 
31

increase above annual average levels during certain shortduration transient operations such as unit startup, the 

conservative analysis approach accounts for such transient 

increases in the conservative estimates of annual average 

exposures. Thus, the analyses, even though they do not 

explicitly incorporate these short term events, reasonably 

account for these events and result in health-protective 

estimates of risk.

F. Human Health Effects of Emitted HAP

Although numerous HAP may be emitted from combustion 

turbines, a few account for essentially all the mass of HAP 

emissions from stationary combustion turbines. These HAP 

are formaldehyde, toluene, benzene, and acetaldehyde. Other

emitted HAP are of potential concern not so much because of 

the emitted amounts, but due to their high potency via the 

inhalation route. These include arsenic and PAH. Four of 

the emitted HAP are of potential concern from the ingestion 

route: PAH, which are of concern for cancer; and cadmium, 

lead and mercury which are of concern for noncarcinogenic 

effects.

The HAP emitted in the largest quantity is 

formaldehyde. Formaldehyde is a probable human carcinogen 

and can cause irritation of the eyes and respiratory tract, 

coughing, dry throat, tightening of the chest, headache, and
32

heart palpitations. Acute (short-term) inhalation has 

caused bronchitis, pulmonary edema, pneumonitis, pneumonia, 

and death due to respiratory failure. Chronic (long-term) 

exposure can cause dermatitis and sensitization of the skin 

and respiratory tract. 

Other HAP emitted in significant quantities from 

stationary combustion turbines include toluene, benzene, and

acetaldehyde. The health effect of primary concern for 

toluene is dysfunction of the central nervous system (CNS). 

Toluene vapor also causes narcosis. Controlled exposure of

human subjects produced mild fatigue, weakness, confusion, 

lacrimation, and paresthesia; at higher exposure levels 

there were also euphoria, headache, dizziness, dilated 

pupils, and nausea. After-effects included nervousness, 

muscular fatigue, and insomnia persisting for several days. 

Acute exposure may cause irritation of the eyes, 

respiratory tract, and skin. It may also cause fatigue, 

weakness, confusion, headache, and drowsiness. Very high 

concentrations may cause unconsciousness and death.

Benzene is a known human carcinogen. The health 

effects of benzene include nerve inflammation, CNS 

depression, and cardiac sensitization. Acute exposure can 

cause dizziness, euphoria, giddiness, headache, nausea, 

staggering gait, weakness, drowsiness, respiratory 
33

irritation, pulmonary edema, pneumonia, gastrointestinal 

irritation, convulsions, and paralysis. Benzene can also 

cause irritation to the skin, eyes, and mucous membranes. 

Chronic exposure to benzene can cause fatigue, nervousness, 

irritability, blurred vision, and labored breathing and has 

produced anorexia and irreversible injury to the bloodforming organs; effects include aplastic anemia and 

leukemia. 

Acetaldehyde is a probable human carcinogen. 

Inhalation exposures to acetaldehyde can cause irritation of

the eyes, mucous membranes, skin, and upper respiratory 

tract, and CNS depression in humans. Acute exposure can 

cause conjunctivitis, coughing, difficult breathing, and 

dermatitis. Chronic exposure may cause heart and kidney 

damage, embryotoxicity, and teratogenic effects. 

Arsenic, a naturally occurring element, is found throughout the 

environment. For most people, food is the major source of exposure to arsenic. 

The EPA has classified inorganic arsenic as a human carcinogen. Acute highlevel inhalation exposure to arsenic dust or fumes has resulted in gastrointestinal 

effects (nausea, diarrhea, abdominal pain); central and peripheral nervous 

system disorders have occurred in workers acutely exposed to inorganic arsenic.

Chronic inhalation exposure to inorganic arsenic in humans is associated with 

irritation of the skin and mucous membranes. Chronic oral exposure has 
34

resulted in gastrointestinal effects, anemia, peripheral neuropathy, skin lesions, 

hyperpigmentation, and liver or kidney damage in humans. Inorganic arsenic 

exposure in humans, by the inhalation route, has been shown to be strongly 

associated with lung cancer, while ingestion of inorganic arsenic in humans has 

been linked to a form of skin cancer and also to bladder, liver, and lung cancer.

Polycyclic aromatic hydrocarbons are a group of 

compounds that fit within the POM HAP category. Dermal 

exposures to mixtures of PAH cause skin disorders in humans 

and animals. No information is available on the 

reproductive or developmental effects of PAH mixtures in 

humans, but animal studies have reported that oral exposure 

to benzo(a)pyrene (BaP, a PAH compound) causes reproductive 

and developmental effects. Human studies have reported an 

increase in lung cancer in humans exposed to PAH-bearing 

mixtures including coke oven emissions, roofing tar 

emissions, and cigarette smoke. Animal studies have 

reported respiratory tract tumors from inhalation exposure 

to BaP and forestomach tumors, leukemia, and lung tumors 

from oral exposure to BaP. The EPA has classified seven PAH

compounds: (BaP, benz(a)anthracene, chrysene, 

benzo(b)fluoranthene, benzo(k)fluoranthene, 

dibenz(a,h)anthracene, and indeno(1,2,3-cd)pyrene) as Group 

B2, probable human carcinogens.
35

The EPA reports in the Integrated Risk and Exposure 

Assessment (IRIS) that cadmium has been shown to cause 

kidney damage via the oral route. IRIS also reports that

there are no positive cancer studies of orally ingested 

cadmium suitable for quantification. Consequently, we 

evaluated noncancer hazards only for cadmium ingestion. The

major effect from chronic oral exposure to inorganic mercury is also kidney 

damage. Animal studies have reported effects such as alterations in testicular 

tissue, increased resorption rates, and abnormalities of development from oral 

exposure to inorganic mercury. Mercuric chloride (an inorganic mercury 

compound) exposure has been shown to result in forestomach, thyroid, and 

renal tumors in experimental animals. For lead, oral exposures can 

lead to central nervous system effects, as well as effects 

on the blood, blood pressure kidneys and Vitamin D 

metabolism. Children are especially sensitive to the 

chronic effects of lead, and can exhibit slowed cognitive 

development and reduced growth.

G. Human Health Values Used

With the exception of formaldehyde, we used the human 

health values currently used by EPA’s air toxics program and

available at: 

http://www.epa.gov/ttn/atw/toxsource/summary.html. These 

dose response values come from several sources including 
36

EPA’s IRIS, the United States department of Health and Human

Service’s Agency for Toxic Substances Disease Registry, and 

California EPA. See Table 5 in our technical memo for a 

summary of the human health values we used in our 

assessment.

For formaldehyde, we do not use the dose-response value reported in 

IRIS. The dose-response value in IRIS is based on a 1987 study, and no longer 

represents the best available science in the peer-reviewed literature. Since that 

time, significant new data and analysis have become available. We based the 

dose-response value we used for formaldehyde on work conducted by the CIIT 

Centers for Health Research (CIIT) (formerly, the Chemical Industry Institute of 

Toxicology). In 1999, the CIIT published a risk assessment which incorporated 

mechanistic and dosimetric information on formaldehyde that had been 

accumulated over the past decade (see the docket for the CIIT report). The risk 

assessment analyzed carcinogenic risk from inhaled formaldehyde using 

approaches that are consistent with EPA’s draft guidelines for carcinogenic risk 

assessment. The CIIT model is based on computational fluid dynamics (CFD) 

models of airflow and formaldehyde delivery to the relevant parts of the rat and 

human respiratory tract, which are then coupled to a biologically-motivated, twostaged clonal growth model that allows for incorporation of different biological 

effects. These biological effects, such as interaction with DNA and cell 

proliferation, are processes by which formaldehyde may contribute to 
37

development of cancer at sites exposed at the portal of entry (e.g., respiratory 

tract). The two-staged model is a much more advanced approach for examining 

the relevance of tumors seen in animal models for human populations.

We believe that the CIIT modeling effort represents the best available 

application of the available mechanistic and dosimetric science on the doseresponse for portal of entry cancers due to formaldehyde exposures. We note 

here that other organizations, including Health Canada, have adopted this 

approach. Accordingly, we have used risk estimates based on the CIIT airflow 

model coupled to a two-staged clonal growth model as the basis for the doseresponse values for this analysis. The formaldehyde risk value obtained by 

extrapolating with the CIIT model that we used in our analysis differs slightly from 

the values used by the petitioner. The CIIT model incorporates state-of-the-art 

analyses for species-specific dosimetry, and encompasses more of the available 

biological data than any other currently available model. As with any model, 

uncertainties exist, and the CIIT model is sensitive to the inputs, but we believe it 

represents the best available approach for assessing the risk of portal-of-entry 

cancers due to formaldehyde exposures.

The Agency is aware of recently completed epidemiologic analyses that 

update existing cohorts of workers exposed to formaldehyde. Not all these 

results have been published in peer-reviewed journals at this point, and the new 

data are not fully available. These studies examine the potential for 

formaldehyde to cause cancer in other organs. These new results, in 
38

combination with the previously available literature regarding the potential for 

systemic effects, indicate the pollutant may be taken into the body and 

distributed to extra respiratory organs, potentially causing adverse effects that 

are not picked up by the CIIT model. As these new studies are published in 

peer-reviewed journals and datasets are made available for further analysis, we 

will evaluate them in the context of the full body of literature to determine the 

relevance to our conclusions with regard to risks from exposure to 

formaldehyde. Based on our review of the currently 

available data, the formaldehyde human health value used in 

our assessment is based on the CIIT CFD airflow model 

coupled with a two-stage clonal growth model.

H. Human Health Risk Results-Inhalation Pathway

We calculated the maximum excess lifetime cancer risk 

for the inhalation pathway that results from the reasonable 

worst-case exposure scenario described above. We estimated 

risks for both the primary firing of natural gas with 1,000 

hours of oil firing per year, per facility, and for the 

continuous firing of natural gas. Diffusion flame gas-fired

turbines produced the highest risk. When firing natural gas

plus 1,000 hours of oil per year, the total excess lifetime 

cancer risk from the all emitted pollutants from the 

diffusion flame turbines in our analysis is 7.7 x 10-7. The 

total excess lifetime cancer risk from the continuos burning
39

of natural gas for our modeled scenario is 3.9 x 10-7. 

In addition to estimating cancer risks, we evaluated 

noncancer hazards for each pollutant where there is a 

noncancer human health value. To do this, we used a hazard 

quotient (HQ) approach and calculated the ratio of the 

exposure concentration to the noncancer human health value 

(e.g., inhalation reference concentration (RfC) for each 

emitted HAP. This is represented by the formula HQ= 

(exposure concentration)/(RfC). The RfC is a peer-reviewed 

value defined as an estimate (with uncertainty spanning 

perhaps an order of magnitude) of a daily inhalation 

exposure to the human population (including sensitive 

subgroups) that is likely to be without appreciable risk of 

deleterious noncancer effects during a lifetime.

We then generated hazard indices (HI) by summing HQ 

across HAP. We can generate two types of hazard indices. 

The first type is generated by adding HQ for all emitted HAP

regardless of their target organ. This results in an HI 

that is considered health-protective since the HQ for all 

pollutants are added even though some pollutants cause 

distinctly different effects. For our modeled scenario, the

total HI for all noncarcinogens for the natural gas plus 

1,000 hours of oil scenario is 0.6. The HI for the natural 

gas burning scenario is 0.4.
40

We can also calculate HI by summing HQ from HAP that 

affect the same target organ. In this assessment, 

pollutants that affect the same target organ are acrolein 

and formaldehyde; they affect the respiratory system. These

also are the two HAP with the highest individual hazard 

quotients. When accounting for the fact that acrolein and 

formaldehyde affect the same target organ, we calculate a HI

of 0.4. None of the other HAP affect the same target organ,

thus, we calculated a HI for the respiratory system only. 

The other HAP had HQ ranging from 10-6 (nickel) to 0.1 

(manganese). 

I. Multipathway Considerations

In order to fully characterize risks and hazards to 

humans from the subcategories, we considered exposures from 

ingestion as well as inhalation for four of the emitted HAP:

cadmium, lead, mercury and PAH. We chose these HAP because

of all the HAP emitted, only these four appear on lists of 

chemicals that EPA considers to be persistent, 

bioaccumulative, and toxic (PBT) substances under the 

Pollution Prevention Program, the Great Waters Program, or 

the Toxics Release Inventory. (See the multipathway HAP 

memo in the docket for more information.) Therefore, in 

addressing the potential for the subcategories to be of 

concern due to multipathway routes of exposure, we need to 
41

consider emissions of cadmium, lead, mercury and PAH.

Several of the emitted PAH are carcinogenic via the 

ingestion pathway and, thus, we evaluated these pollutants 

in the multipathway analysis: benzo(a)anthracene, 

benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene, 

chrysene, dibenz(a,h)anthracene, and indeno(1,2,3-cd)pyrene.

We evaluated noncarcinogenic effects for cadmium, lead, 

mercury and the following noncarcinogenic PAH: 

acenaphthene, fluoranthene, fluorene,and pyrene. 

To evaluate the potential for these HAP to cause cancer

risk or noncancer hazard to humans due to ingestion, we 

conducted a screening level multipathway analysis. As with 

the inhalation assessment, we did not have enough data to 

evaluate actual exposures across the entire source category.

We did not structure this assessment to reflect actual 

exposures, rather we developed a worst-case exposure model 

scenario based on limited data and assumptions which, when 

considered in total, provide for a health-protective 

analysis. This approach ensures that we are not 

underestimating actual risks and hazards from emissions from

the four subcategories. 

We structured this analysis to estimate maximum risks 

to an individual exposed via routes other than inhalation 

(e.g., ingestion of contaminated food) for HAP emitted from 
42

combustion turbines. We used our modeled facility and 

evaluated human ingestion of contaminated food, water and 

soil. The Human Health Risk Assessment Protocol for Hazardous Waste 

Combustion Facilities (HHRAP) (U.S. EPA, 1998) provided the primary source of 

chemical-specific parameter values and default environmental parameters. We

started with the HHRAP and replaced specific inputs as necessary, either due to 

updated science or due to policy choices that we made in order to be consistent 

with the mandate to assess risks to the individual most exposed.

To evaluate a worst-case potential exposure from our 

modeled facility, we used a subsistence farmer scenario. 

This scenario reflects an adult living on a farm that we 

hypothetically assumed to be located close to our modeled 

facility. We assumed the farmer consumes meat (pork and 

beef), dairy, fruit, and vegetables that the farm produces

as a portion of his/her diet. The animals raised on the 

farm subsist primarily on feed grown on the farm. We also 

assumed that the farmer is a recreational fisher and eats 

the fish he/she catches. Finally, we assumed that the 

farmer drinks treated, local surface water (water which has 

gone through minimal municipal treatment). 

For several reasons, we consider this approach to 

multipathway assessment scenario to be health-protective. 

We used the ambient air concentrations from our modeled 
43

facility which, as we have stated above, produces higher 

ambient air concentrations than we expect to actually occur 

anywhere in the U.S. Also, we used a water body size, flow 

rate, watershed size and other parameters that were 

developed for the health protective analysis scenario 

analyzed in the Mercury Study Report to Congress. Further, 

we applied maximum pollutant deposition rates to the entire 

watershed. Thus, we feel our modeled scenario will overpredict actual risks and hazards from ingestion and is, 

therefore, health-protective.

We estimated both cancer risk and noncancer hazards 

from all the ingestion pathways: water, meats, fruits, 

vegetables, soil, and fish. The results of our multipathway

analysis show that the cancer risks from PAH are 0.16 in 1

million (1.6 x 10-7). This is below the statutory cancer 

risk criterion of 1 in 1 million. When we add these risks 

to the lifetime excess cancer risks of 7.7 x 10-7 from the 

inhalation pathway, we get a total cancer risk of .93 in 1

million, which rounds to 0.9 in 1 million (9 x 10-6). Such 

a summation of risks is appropriate only if it is plausible 

that the person with the maximum risks from the air pathway 

is also the person with the maximum risk from the ingestion 

pathway. Inherent in this assumption is that these two 

maximum concentrations (therefore, the maximum risk and 
44

hazards) occur at the exact same location. While we 

calculated risk and hazards for such a person, we feel it 

very unlikely that one person would be located at the point 

of highest impact from both inhalation and ingestion. If we

had more site-specific data with which to conduct this 

assessment, we would likely have found that the maximum 

impact from inhalation was not in the same location as the 

maximum impact from ingestion, and the risks would be lower.

We consider it inappropriate to use this combined inhalation/ingestion scenario 

because we consider it to be implausible. We feel that the actual combined 

risks, from all pathways, will be lower than 1 in 1 million and, therefore, the 

statutory criteria are met. 

We estimated noncancer hazards for cadmium and mercury,

combining hazards from all ingestion pathways. The highest 

total hazard index for all ingestion pathways is 0.1. 

Noncancer hazards are driven by methyl mercury via ingestion

of fish. The HQ for mercury for this route of exposure is 

also 0.1; it is clearly the driver for multipathway 

noncancer effects. 

The EPA uses a slightly different approach in order to 

assess the hazard from ingestion exposures to lead. In 

general, we use a protocol like that in HHRAP to obtain 

media concentrations. We use an additional model called the
45

Integrated Exposure, Uptake and Biokinetic Model (IEUBK) to 

estimate blood lead levels. We then calculate an HQ. In 

this analysis, the inhalation HQ for lead was so low, 

0.000008, that we found it unnecessary to take the 

additional step of modeling further with the IEUBK. Based 

on previous analyses we have conducted on lead, we do not 

feel that an air concentration that leads to an HQ of 

0.000008 would translate into an HQ of concern from the 

ingestion route of exposure. The ingestion HQ would have to

be four to five orders of magnitude higher than the HQ from 

the air pathway to even approach a level of concern. Given 

the very low inhalation HQ for lead from exposure to the

turbine subcategories, the lead emissions from the four 

subcategories do not exceed a level that is adequate to 

protect the public health with an ample margin of safety. 

Therefore, we conclude that both risks and hazards to humans

due to multipathway exposures from all HAP emitted from the 

four combustion turbine subcategories meet the required 

human health criteria in CAA section 112(c)(9)(B). 

Emissions that result in the maximum modeled lifetime 

excess cancer risk of 0.9 in 1 million are within the 

statutory criteria. With regard to noncancer effects, we 

consider the emissions resulting in a target organ-specific 

HI of 0.4 from the turbine subcategories do not exceed a 
46

level that is adequate to protect the public health with an 

ample margin of safety. We consider the actual risks and 

hazards from the turbines in the four subcategories to be 

lower than what we estimated here due to the healthprotective assumptions we included in this assessment. For 

example, in characterizing the physical and operational 

attributes of the turbines, we assumed all turbines were 

operating in combined cycle, used conservative temperature 

and exit velocities, used worst-case meteorology, and 

included the potential for building downwash. These 

assumptions lead to exposures which we feel are higher than 

what we would find from an actual plant. In addition, we 

assumed that individuals are exposed to the maximum modeled 

concentrations of HAP in the air continuously for their 

entire lives (which we approximated as 70 years), and we 

used the maximum annual average concentration as a surrogate

for exposure. These assumptions are also health-protective.

J. Effects Due to Acute Exposure

We determined that emissions from turbines are of 

concern for long-term (chronic) exposures and not from 

short-term (acute) exposures. Short-term exposures may 

arise when a facility starts up or shuts down equipment, 

which may result in short bursts of high emissions due to 

the fact that the unit is not running at peak efficiency 
47

during the time it takes to start up or shut down. In some 

cases, this can lead to exposures that result in adverse 

health effects. In the case of gas-fired turbines, we have 

determined that upon start up, they reach peak efficiency 

quickly, therefore, limiting any bursts of emissions. Shut 

downs take a short amount of time as well. The adverse 

health effects associated with exposure to the emitted HAP 

from combustion turbines have not been associated with acute

exposures at the concentrations of concern in the analyses. 

While the short-duration emissions may slightly increase 

the overall cancer risks, this effect would be so small as 

to be inconsequential. Therefore, we conclude that the 

acute exposures to HAP emissions from stationary combustion 

turbines are not of concern.

K. Environmental Effects Evaluation

In order to assess whether the emissions from our 

modeled facility could lead to adverse environmental 

effects, we performed a screening-level ecological risk 

assessment. We evaluated the inhalation pathway for 

terrestrial mammals, the ingestion pathway for terrestrial 

wildlife, contact with sediment to benthic species, and 

contact with soil for terrestrial plants. We did not 

evaluate terrestrial plants exposed via direct contact with 

the air due to a lack of toxicity data. 
48

We contend that human toxicity values we used in this 

analysis for the inhalation pathway are protective of 

inhalation exposures that may be experienced by terrestrial 

mammals. The human health values were derived based on 

human studies and also considered studies on small 

laboratory animals, primarily rodents. These values are 

significantly less than the level to which an experimental 

animal was exposed. Because the maximum cancer risk and 

noncancer hazards to humans from inhalation exposure are all

below a level of concern, we expect there to be no 

significant and widespread adverse effects to terrestrial 

mammals from inhalation exposures to HAP emitted from gasfired turbines.

In order to assess whether the continuing emissions 

from our modeled facility could contribute to adverse 

environmental effects from the ingestion pathway, we 

performed a screening-level ecological risk assessment. For

screening purposes, we intentionally designed the assessment 

to be health-protective of ecological receptors. We did not

intend the assessment to be used in predicting specific 

types of effects to individuals, species, populations, or 

communities, or to the structure and function of the 

ecosystem. We used the assessment to identify HAP which may

pose potential risk or hazard to ecological receptors and, 
49

therefore, would need to be evaluated in a more refined 

level of risk assessment.

For screening endpoints, we used the structure and 

function of generic aquatic and terrestrial populations and 

communities, including threatened and endangered species, 

that might be exposed to HAP emissions via soil or water.

The assessment endpoints are relatively generic with respect

to descriptions of the environmental values that are to be 

protected and the characteristics of the ecological entities

and their attributes. We assumed in the assessment that 

these ecological receptors were representative of sensitive 

individuals, populations, and communities present near these

facilities. 

The HAP we included in the quantitative ecological 

assessment, are the same HAP that we evaluated in the 

multipathway human health assessment: cadmium, lead, 

mercury and PAH. We derived estimated media concentrations 

for each of these HAP from the media concentrations 

estimated in the multipathway exposures assessment. We 

chose exposure pathways to reflect the potential routes of 

exposure through sediment, soil, water, and air. We 

selected these environments because they are considered 

representative of locations of generic populations and 

communities most likely to be exposed to the HAP. Within 
50

these environments, the receptors evaluated consisted of two

distinct groups: terrestrial and aquatic (i.e., including 

aquatic, benthic, and soil organisms; terrestrial plants and

wildlife; and herbivorous, piscivorus, and carnivorous 

wildlife).

The chronic ecological toxicity screening values used 

in the assessment were estimates of the maximum 

concentrations that would not be expected to affect 

survival, growth, or reproduction of sensitive species after

long-term (more than 30 days) exposure to HAP. We screened 

HAP, pathways, and receptors using the ecological HQ method,

which simply calculates the ratio of the estimated 

environmental concentrations to the selected ecological 

screening values. 

The results of our ecological assessment show that for all pollutants 

assessed, and for all pathways assessed, the ecological HQ values are less than 

1. Therefore, it is not likely that any of the HAP emitted would pose an 

ecological risk to ecosystems near any of these facilities. 

With regard to endangered species, we assumed that the 

screening values were protective of sensitive species, 

including threatened or endangered species. There are no 

available ecological toxicity test data for threatened and 

endangered species for these HAP. As such, the actual 
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sensitivities of any threatened or endangered species 

located in the vicinity of these facilities is unknown. 

However, in order to be conservative, we selected ecological

screening values for the most sensitive species available 

for use in the analysis. Also, we are not familiar with any species that 

have become threatened or endangered as a result of emissions of these 

chemicals from stationary combustion turbines. Therefore, we feel it is not likely 

that any threatened and endangered species, if they exist around these facilities, 

would be adversely affected by these HAP emissions.

V. Analysis of the Emergency Turbine Subcategory

Emergency stationary combustion turbines are stationary

combustion turbines that operate in an emergency situation. 

Examples include stationary combustion turbines used to 

produce power for critical networks or equipment (including 

power supplied to portions of a facility) when electric 

power from the local utility is interrupted, or stationary 

combustion turbines used to pump water in the case of fire 

or flood, etc. Emergency stationary combustion turbines do 

not include stationary combustion turbines used as peaking 

units at electric utilities or stationary combustion 

turbines at industrial facilities that typically operate at 

low capacity factors. Emergency stationary combustion 

turbines may be operated for the purpose of maintenance 
52

checks and readiness testing, provided that the tests are 

required by the manufacturer, the vendor, or the insurance 

company associated with the turbine. 

Usually one or two emergency turbines are located at a 

given facility. These units run mostly on oil and operate 

approximately 30 hours per year, per turbine. Regular 

testing of these units (done to ensure they will be 

operational during an emergency) may bring the total 

operating hours for a turbine up toward 200 hours per year,

per turbine, or approximately 400 hours per facility. Given

that these units burn less oil than allowed under the MACT 

standards for lean premix and diffusion flame gas-fired 

turbines (1,000 hours per facility), we expect the maximum 

annual average HAP concentrations in air to be much less for

emergency turbines. Therefore, we expect the risks and 

hazards to be less.

VI. Analysis of the North Slope Turbine Subcategory 

We have identified 120 stationary combustion turbines 

that are located on the North slope of Alaska. Of these, 

112 are diffusion flame gas-fired units, and eight are lean 

premix gas-fired turbines. The total number of oil hours 

used, per year, by any facility we identified on the North 

Slope is much less than 1,000 hours. Because we have 

determined that facilities burning oil for fewer than 1,000 
53

hours per year meet the statutory criteria for delisting, we 

concluded that stationary combustion turbines located on the

North Slope of Alaska also meet the delisting criteria. 

Given the standard EPA risk assessment methods used, 

and the conservative assumptions made in the assessment, we 

have made an initial determination that all sources in the 

four subcategories meet the human health and environmental 

criteria in CAA section 112(c)(9)(B) and should be removed 

from the source category list.

VII. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

Under Executive Order 12866 (58 FR 51735, October 4, 

1993), EPA must determine whether the regulatory action is 

“significant” and, therefore, subject to Office of 

Management and Budget (OMB) review and the requirements of 

the Executive Order. The Executive Order defines

“significant regulatory action” as one that is likely to 

result in a rule that may:

(1) Have an annual effect on the economy of $100 

million or more or adverse affect in a material way the 

economy, a sector to the economy, productivity, competition,

jobs, the environment, public health or safety, or state, 

local or tribal governments or communities;

(2) create a serious inconsistency or otherwise 
54

interfere with an action taken or planned by another agency;

(3) materially alter the budgetary impact of 

entitlements, grants, user fees, or loan programs, or the 

rights and obligation of recipients thereof; or 

(4) raise novel legal or policy issues arising out of 

legal mandates, the President’s priorities, or the 

principles set forth in the Executive Order.

Pursuant to the terms of Executive Order 12866, it has 

been determined that the proposed action does not constitute

a “significant regulatory action” and is, therefore, not 

subject to OMB review.

B. Paperwork Reduction Act

This action does not impose an information collection 

burden under the provisions of the Paperwork Reduction Act, 

44 U.S.C. 3501 et seq. The proposed action will remove two 

subcategories from the combustion turbine source category 

and, therefore, eliminate the need for information 

collection toward regulatory compliance under the CAA. 

Burden means the total time, effort, or financial resources 

expended by persons to generate, maintain, retain, or 

disclose or provide information to or for a Federal agency. 

This includes the time needed to review instructions; 

develop, acquire, install, and utilize technology and 

systems for the purposes of collecting, validating, and 
55

verifying information, processing and maintaining 

information, and disclosing and providing information; 

adjust the existing ways to comply with any previously 

applicable instructions and requirements; train personnel to

be able to respond to a collection of information; search 

data sources; complete and review the collection of 

information; and transmit or otherwise disclose the 

information. An Agency may not conduct or sponsor, and a 

person is not required to respond to a collection of 

information unless it displays a currently valid OMB control

number. The OMB control numbers for EPA’s regulations are 

listed in 40 CFR part 9 and 48 CFR chapter 15.

C. Regulatory Flexibility Act

The Regulatory Flexibility Act (RFA) generally requires

an agency to prepare a regulatory flexibility analysis of 

any rule subject to notice and comment rulemaking 

requirements under the Administrative Procedure Act or any 

other statute unless the agency certifies that the rule will

not have a significant economic impact on a substantial 

number of small entities. Small entities include small 

business, small organizations, and small governmental

jurisdictions. For the purposes of assessing the impacts of

today’s proposed action on small entities, small entity is 

defined as: (1) a small business that meets the definitions
56

for small business based on the Small Business Association 

(SBA) size standards which, for this proposed action, can 

include manufacturing (NAICS 3999-03) and air transportation

(NAICS 4522-98 and 4512-98) operations that employ less 

1,000 people and engineering services (NAICS 8711-98) 

operations that earn less than $20 million annually; (2) a 

small governmental jurisdiction that is a government of a 

city, county, town, school district or special district with

a population of less than 50,000; and (3) a small 

organization that is any not-for-profit enterprise which is 

independently owned and operated and is not dominant in its 

field.

After considering the economic impact of today’s 

proposed action on small entities, I certify that the 

proposed action will not have a significant economic impact 

on a substantial number of small entities. In determining 

whether a rule has significant economic impact on a 

substantial number of small entities, the impact of concern 

is any significant adverse economic impact on small 

entities, since the primary purpose of the regulatory 

flexibility analysis is to identify and address regulatory 

alternatives “which minimize any significant economic impact

of the proposed rule on small entities.” (5 U.S.C. 603 and 

604). Thus, an agency may certify that a rule will not have
57

a significant economic impact on a substantial number of 

small entities if the rule relieves regulatory burden, or 

otherwise has a positive economic effect on all of the small

entities subject to the rule. The proposed rule will 

eliminate the burden of additional controls to be applied to

two subcategories of the combustion turbine source category,

and associated operating, monitoring and reporting 

requirements. We have, therefore, concluded that today’s 

proposed rule will relieve regulatory burden for all small 

entities. We continue to be interested in the potential 

impacts of the proposed rule on small entities and welcome 

comments on issues related to such impacts.

D. Unfunded Mandates Reform Act

Title II of the Unfunded Mandates Reform Act of 1995 

(UMRA), Public Law 1044, establishes requirements for 

Federal agencies to assess the effects of their regulatory 

actions on State, local, and tribal governments and the 

private sector. Under section 202 of the UMRA, EPA 

generally must prepare a written statement, including a 

cost-benefit analysis, for proposed and final rules with 

“Federal mandates” that may result in expenditures to State,

local, and tribal governments, in the aggregate, or to the 

private sector, of $100 million or more in any 1 year. 
58

Before promulgating an EPA rule for which a written 

statement is needed, section 205 of the UMRA generally 

requires EPA to identify and consider a reasonable number of

regulatory alternatives and adopt the least costly, most 

cost-effective or least burdensome alternative that achieves

the objectives of the rule. The provisions of section 205 

do not apply when they are inconsistent with applicable law.

Moreover, section 205 allows EPA to adopt an alternative 

other than the least costly, most cost-effective or least 

burdensome alternative if the Administrator publishes with 

the final rule an explanation why that alternative was not 

adopted. Before EPA establishes any regulatory requirements

that may significantly or uniquely affect small governments,

including tribal governments, it must have developed under 

section 203 of the UMRA a small government agency plan. The

plan must provide for notifying potentially affected small 

governments, enabling officials of affected small 

governments to have meaningful and timely input in the 

development of EPA regulatory proposals with significant 

Federal intergovernmental mandates, and informing, 

educating, and advising small governments on compliance with

the regulatory requirements.

Today’s proposed rule contains no Federal mandates for 

State, local, or tribal governments or the private sector. 
59

The proposed rule imposes no enforceable duty on any State, 

local or tribal governments or the private sector. In any 

event, EPA has determined that the proposed rule does not 

contain a Federal mandate that may result in expenditures of

$100 million or more for State, local, and tribal 

governments, in the aggregate, or the private sector in any 

1 year. Because the proposed rule removes two subcategories

from the combustion turbine source category from regulatory 

consideration, it actually reduces the burden established 

under the CAA. Thus, today’s proposed rule is not subject 

to the requirements of sections 202 and 205 of the UMRA.

E. Executive Order 13132: Federalism

Executive Order 13132 (64 FR 43255, August 10, 1999) 

requires EPA to develop an accountable process to ensure 

“meaningful and timely input by State and local officials in

the development of regulatory policies that have federalism 

implications.” “Policies that have federalism implications”

is defined in the Executive Order to include regulations 

that have “substantial direct effects on the States, on the 

relationship between the national government and the States,

or on the distribution of power and responsibilities among 

the various levels of government.”

The proposed rule does not have federalism 

implications. It will not have substantial direct effects on
60

the States, on the relationship between the national 

government and the States, or on the distribution of power 

and responsibilities among the various levels of government,

as specified in Executive Order 13132. Thus, Executive 

Order 13132 does not apply to the proposal.

F. Executive Order 13175: Consultation and Coordination with Indian Tribal 

Governments

Executive Order 13175 (65 FR 67249, November 9, 2000) requires EPA 

to develop an accountable process to ensure “meaningful and timely input by 

tribal officials in the development of regulatory policies that have tribal 

implications.” The proposed rule does not have tribal implications, as specified 

in Executive Order 13175. The proposed action will eliminate control 

requirements for two subcategories from the combustion turbine source category 

and, therefore, reduces control costs and reporting requirements for any tribal 

entity operating a turbine contained in either of these subcategories. Thus, 

Executive Order 13175 does not apply to the proposed rule.

G. Executive Order 13045: Protection of Children from Environmental Health 

and Safety Risks

Executive Order 13045 (62 FR 19885, April 23, 1997) applies to any rule 

that: (1) is determined to be “economically significant” as defined under 

Executive Order 12866, and (2) concerns an environmental health or safety risk 

that EPA has reason to believe may have a disproportionate effect on children. 
61

If the regulatory action meets both criteria, the Agency must evaluate the 

environmental health or safety effects of the planned rule on children, and 

explain why the planned regulation is preferable to other potentially effective and 

reasonably feasible alternatives considered by the Agency.

The EPA interprets Executive Order 13045 as applying only to those 

regulatory actions that are based on health or safety risks, such that the analysis 

required under section 5-501 of the Executive Order has the potential to 

influence the regulation. The proposed rule is not subject to Executive Order 

13045 because it is not economically significant as defined in Executive Order 

12866, and because the Agency does not have reason to believe the 

environmental health or safety risks addressed by this action present a 

disproportionate risk to children. This determination is based on the fact that the 

noncancer human health values we used in this analysis (e.g., RfC) are 

determined to be protective of sensitive sub-populations, including children. 

Also, while the cancer human health values do not always expressly account for 

cancer effects in children, the cancer risks posed by turbines in these two 

subcategories are sufficiently low so as not to be concern for anyone in the 

population, including children. In addition, the public is invited to submit or 

identify peer-reviewed studies and data, of which the Agency may not be aware, 

that assesses results of early life exposure to the HAP emitted by lean premix 

gas-fired combustion turbines and diffusion flame gas-fired 

combustion turbines.
62

H. Executive Order 13211, Actions Concerning Regulations that Significantly 

Affect Energy Supply, Distribution, or Use

The proposed rule is not subject to Executive Order 13211 (66 FR 28355, 

May 22, 2001) because it is not a significant regulatory action under Executive 

Order 12866.

I. National Technology Transfer and Advancement Act

Section 112(d) of the National Technology Transfer and Advancement Act 

of 1995 (NTTAA), (Public Law No. 104-113, section 12(d) 915 U.S.C. 272 note), 

directs all Federal agencies to use voluntary consensus standards instead of 

government-unique standards in their regulatory activities unless to do so would 

be inconsistent with applicable law or otherwise impractical. Voluntary consensus 

standards are technical standards (e.g., material specifications, test

method, sampling and analytical procedures, business practices, etc.) that are 

developed or adopted by one or more voluntary consensus standards bodies. 

Examples of organizations generally regarded as voluntary consensus standards 

bodies include the American society for Testing and Materials, the National Fire 

Protection Association A), and the Society of Automotive Engineers. The 

NTTAA requires Federal agencies like EPA to provide Congress, through OMB, 

with explanations when an agency decides not to use available and applicable 

voluntary consensus standards. The 

List of Hazardous Air Pollutants, Petition Process, Lesser 

 Quantity Designations, Source Category List
63

Page 65 of 65

proposed rule does not involve technical standards. Therefore, EPA is not 

considering the use of any voluntary consensus standards.

List of Subjects in 40 CFR part 63

Environmental protection, Air pollution control, 

Hazardous substances, Reporting and recordkeeping 

requirements.

________________________

Dated:

_________________________

Michael O. Leavitt

Administrator
64