National Emission Standards for Hazardous Air Pollutants for Organic Hazardous Air Pollutants From the Synthetic Organic Chemical Manufacturing Industry, 34422-34446 [06-5219]

Agencies

• ENVIRONMENTAL PROTECTION AGENCY

[Federal Register Volume 71, Number 114 (Wednesday, June 14, 2006)]
[Proposed Rules]
[Pages 34422-34446]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 06-5219]



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Part II





Environmental Protection Agency





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40 CFR Part 63



National Emission Standards for Hazardous Air Pollutants for Organic 
Hazardous Air Pollutants From the Synthetic Organic Chemical 
Manufacturing Industry; Proposed Rule

Federal Register / Vol. 71, No. 114 / Wednesday, June 14, 2006 / 
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2005-0475; FRL-8181-3]
RIN 2060-AK14


National Emission Standards for Hazardous Air Pollutants for 
Organic Hazardous Air Pollutants From the Synthetic Organic Chemical 
Manufacturing Industry

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule; amendments.

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SUMMARY: In 1994, EPA promulgated National Emission Standards for 
Hazardous Air Pollutants (NESHAP) for the synthetic organic chemical 
manufacturing industry (SOCMI). This rule is commonly known as the 
hazardous organic NESHAP (HON) and established maximum achievable 
control technology (MACT) standards to regulate the emissions of 
organic hazardous air pollutants (HAP) from production processes that 
are located at major sources.
    The Clean Air Act (CAA) directs EPA to assess the risk remaining 
(residual risk) after the application of the MACT standards and to 
promulgate additional standards if required to provide an ample margin 
of safety to protect public health or prevent adverse environmental 
effect. The CAA also requires us to review and revise MACT standards, 
as necessary, every eight years, taking into account developments in 
practices, processes, and control technologies that have occurred 
during that time.
    Based on our findings from the residual risk and technology review, 
we are proposing two options (to be considered with equal weight) for 
emissions standards for new and existing SOCMI process units. The first 
proposed option would impose no further controls, proposing to find 
that the existing standards protect public health with an ample margin 
of safety and prevent adverse environmental impacts, as required by 
section 112(f)(2) of the CAA and would satisfy the requirements of 
section 112(d)(6). The second proposed option would provide further 
reductions of organic HAP at certain process units by applying 
additional controls for equipment leaks and by controlling some storage 
vessels and process vents that are uncontrolled under the current rule. 
This option would also protect public health with an ample margin of 
safety and prevent adverse environmental impacts, as required by 
section 112(f)(2) of the CAA and would satisfy the requirements of 
section 112(d)(6). Under this option, we are proposing that the 
compliance deadlines for additional promulgated requirements would be 
one to three years from the date of promulgation.

DATES: Comments. Written comments must be received on or before August 
14, 2006.
    Public Hearing. If anyone contacts EPA by July 5, 2006 requesting 
to speak at a public hearing, a public hearing will be held on July 14, 
2006.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2005-0475, by one of the following methods:
     Federal eRulemaking Portal: https://www.regulations.gov. 
Follow the on-line instructions for submitting comments.
     E-mail: a-and-r-docket@epa.gov.
     Fax: (202) 566-1741.
     Hand Delivery: Air and Radiation Docket, Environmental 
Protection Agency, 1301 Constitution Avenue, NW., Room B-108, 
Washington, DC 20014. Such deliveries are accepted only during the 
Docket's normal hours of operation and special arrangements should be 
made for deliveries of boxed information.
     Mail: EPA Docket Center (EPA/DC), Environmental Protection 
Agency, Mailcode 6102T, 1200 Pennsylvania Avenue, NW., Washington, DC 
20460.
    Please include a total of two copies. We request that a separate 
copy also be sent to the contact person identified below (see FOR 
FURTHER INFORMATION CONTACT).
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2005-0475. EPA's policy is that all comments received will be included 
in the public docket without change and may be made available online at 
https://www.regulations.gov including any personal information provided, 
unless the comment includes information claimed to be Confidential 
Business Information (CBI) or other information whose disclosure is 
restricted by statute. Do not submit information that you consider to 
be CBI or otherwise protected through https://www.regulations.gov or e-
mail. The https://www.regulations.gov Web site is an ``anonymous 
access'' system, which means EPA will not know your identity or contact 
information unless you provide it in the body of your comment. If you 
send an e-mail comment directly to EPA without going through https://
www.regulations.gov, your e-mail address will be automatically captured 
and included as part of the comment that is placed in the public docket 
and made available on the Internet. If you submit an electronic 
comment, EPA recommends that you include your name and other contact 
information in the body of your comment with a disk or CD-ROM you 
submit. 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. Electronic files should avoid the use of special 
characters, any form of encryption, and be free of any defects or 
viruses.
    Docket: All documents in the docket are listed in the https://
www.regulations.gov index. Although listed in the index, some 
information is not publicly available, e.g., CBI or other information 
whose disclosure is restricted by statute. Certain other material, such 
as copyrighted material, will be publicly available only in hard copy. 
Publicly available docket materials are available either electronically 
in https://www.regulations.gov or in hard copy at the Air and Radiation 
Docket, EPA/DC, EPA West, Room B102, 1301 Constitution Ave., NW., 
Washington, DC. The Public Reading Room is open from 8:30 a.m. to 4:30 
p.m., Monday through Friday, excluding legal holidays. The telephone 
number for the Public Reading Room is (202) 566-1744, and the telephone 
number for the Air and Radiation Docket is (202) 566-1742.
    Public Hearing: If a public hearing is held, it will be held at 10 
a.m. at the Environmental Research Center Auditorium, Research Triangle 
Park, NC, or at an alternate site nearby.

FOR FURTHER INFORMATION CONTACT: For questions about the proposed rule, 
contact Mr. Randy McDonald, EPA, Office of Air Quality Planning and 
Standards, Sector Policies and Programs Division, Coatings and 
Chemicals Group (E143-01), Research Triangle Park, NC 27711; telephone 
number (919) 541-5402; fax number (919) 541-0246; e-mail address: 
mcdonald.randy@epa.gov. For questions on the residual risk analysis, 
contact Mr. Mark Morris, EPA, Office of Air Quality Planning and 
Standards, Health and Environmental Impacts Division, Sector Based 
Assessment Group (C404-01), Research Triangle Park, NC 27711; telephone 
number (919) 541-5416; fax number (919) 541-0840; e-mail address: 
morris.mark@epa.gov.

SUPPLEMENTARY INFORMATION: Regulated Entities. Categories and entities 
potentially regulated by the proposed rule are SOCMI facilities that 
are major sources of HAP emissions. The proposed rule would affect the 
following categories of sources:

[[Page 34423]]



------------------------------------------------------------------------
                                      NAICS \1\   Example of potentially
              Category                   code       regulated entities
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Industry............................        325  Chemical manufacturing
                                                  facilities.
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\1\ North American Industrial Classification Code.

    This table is not intended to be exhaustive, but rather provides a 
guide for readers regarding entities likely to be regulated by the 
proposed rule. To determine whether your facility would be regulated by 
the proposed rule, you should carefully examine the applicability 
criteria in 40 CFR 63.100 of the rule. If you have any questions 
regarding the applicability of the proposed rule to a particular 
entity, contact the person listed in the preceding FOR FURTHER 
INFORMATION CONTACT section.
    Submitting CBI. Do not submit this information to EPA through 
https://www.regulations.gov or e-mail. Clearly mark the part or all of 
the information that you claim to be CBI. For CBI information on a disk 
or CD-ROM that you mail to EPA, 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 claimed as CBI. In addition to one 
complete version of the comment that includes 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. 
Information so marked will not be disclosed except in accordance with 
procedures set forth in 40 CFR part 2.
    Public Hearing. Persons interested in presenting oral testimony or 
inquiring as to whether a hearing is to be held should contact Randy 
McDonald, Coatings and Chemicals Group, Sector Policies and Programs 
Division (Mail Code C504-04), U.S. EPA, Research Triangle Park, North 
Carolina, 27711, telephone number (919) 541-5402, electronic mail 
address mcdonald.randy@epa.gov, at least two days in advance of the 
potential date of the public hearing. Persons interested in attending 
the public hearing also must call Mr. Randy McDonald to verify the 
time, date, and location of the hearing. A public hearing will provide 
interested parties the opportunity to present data, views, or arguments 
concerning the proposed amendments.
    World Wide Web (WWW). In addition to being available in the docket, 
an electronic copy of the proposed rule is also available on the WWW 
through the Technology Transfer Network Web site (TTN Web). Following 
signature, a copy of the proposed rule will be posted on the TTN's 
policy and guidance page for newly proposed or promulgated rules at 
https://www.epa.gov/ttn/oarpg. The TTN provides information and 
technology exchange in various areas of air pollution control.
    Organization of this Document. The information presented in this 
preamble is organized as follows:

I. Background
    A. What is the statutory authority for regulating hazardous air 
pollutants?
    B. What are SOCMI facilities?
    C. What are the health effects of HAP emitted from SOCMI 
facilities?
    D. What does the HON require?
II. Summary of Proposed Revised Standards
III. Rationale for the Proposed Rule
    A. What is our approach for developing residual risk standards?
    B. How did we estimate residual risk?
    C. What are the residual risks from HON CMPUs?
    D. What is our proposed decision on acceptable risk?
    E. What is our proposed decision on ample margin of safety?
    F. What is EPA proposing pursuant to CAA section 112(d)(6)?
IV. Solicitation of Public Comments
    A. Introduction and General Solicitation
    B. Specific Comment and Data Solicitations
V. 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 and Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution, or Use
    I. National Technology Transfer Advancement Act
    J. Executive Order 12898: Federal Actions to Address 
Environmental Justice in Minority Populations and Low-Income 
Populations

I. Background

A. What is the statutory authority for regulating hazardous air 
pollutants?

    Section 112 of the CAA establishes a two-stage regulatory process 
to address emissions of HAP from stationary sources. In the first 
stage, after EPA has identified categories of sources emitting one or 
more of the HAP listed in section 112(b) of the CAA, section 112(d) 
calls for us to promulgate national performance or technology-based 
emission standards for those sources. For ``major sources'' that emit 
or have the potential to emit any single HAP at a rate of 10 tons or 
more per year or any combination of HAP at a rate of 25 tons or more 
per year, these technology-based standards must reflect the maximum 
reductions of HAP achievable (after considering cost, energy 
requirements, and non-air health and environmental impacts) and are 
commonly referred to as MACT standards. We published the MACT standards 
for SOCMI on April 22, 1994 at 59 FR 19402 (codified at 40 CFR part 63, 
subparts F, G, and H). The EPA is then required to review these 
technology-based standards and to revise them ``as necessary (taking 
into account developments in practices, processes and control 
technologies)'' no less frequently than every eight years, under CAA 
section 112(d)(6).
    The second stage in standard-setting is described in CAA section 
112(f). This provision requires, first, that EPA prepare a Report to 
Congress discussing (among other things) methods of calculating risk 
posed (or potentially posed) by sources after implementation of the 
MACT standards, the public health significance of those risks, the 
means and costs of controlling them, actual health effects to persons 
in proximity to emitting sources, and recommendations as to legislation 
regarding such remaining risk. The EPA prepared and submitted this 
report (Residual Risk Report to Congress, EPA-453/R-99-001) in March 
1999. The Congress did not act on any of the recommendations in the 
report, thereby triggering the second stage of the standard-setting 
process, the residual risk phase.
    Section 112(f)(2) requires us to determine for source categories 
subject to certain section 112(d) standards whether the emissions 
limitations protect public health with an ample margin of safety. If 
the MACT standards for HAP ``classified as a known, probable, or 
possible human carcinogen do not reduce lifetime excess cancer risks to 
the individual most exposed to emissions from a source in the category 
or subcategory to less than 1-in-1 million,'' EPA must promulgate 
residual risk standards for the source category (or subcategory) as 
necessary to provide an ample margin of safety to protect public 
health. The EPA must also adopt more stringent standards if necessary 
to prevent adverse environmental effect (defined in section 112(a)(7) 
as ``any significant and widespread adverse effect * * * to wildlife, 
aquatic life, or natural resources * * *.''), but must consider cost, 
energy, safety, and other relevant factors in doing so.

[[Page 34424]]

B. What are SOCMI facilities?

    The SOCMI is a segment of the chemical manufacturing industry that 
includes the production of many high-volume organic chemicals. The 
products of SOCMI are derived from approximately 10 petrochemical 
feedstocks. Of the hundreds of organic chemicals that are produced by 
the SOCMI, some are final products and some are the feedstocks for 
production of other non-SOCMI chemicals or synthetic products such as 
plastics, fibers, surfactants, pharmaceuticals, synthetic rubber, dyes, 
and pesticides. Production of such non-SOCMI end products is not 
considered to be part of SOCMI production and, as a result, the current 
MACT standards do not (and the proposed standards would not) apply to 
downstream synthetic products industries, such as rubber production or 
polymers production, that use chemicals produced by SOCMI processes.
    The HON currently applies to chemical manufacturing process units 
(CMPUs) that: (1) Are part of a major source as defined in CAA section 
112; (2) produce as a primary product a SOCMI chemical listed in table 
1 of 40 CFR part 63, subpart F; and (3) use as a reactant or 
manufacture as a product, by-product, or co-product one or more of the 
organic HAP listed in table 2 of 40 CFR part 63, subpart F.
    The HON defines a CMPU as the equipment assembled and connected by 
pipes or ducts to process raw materials and to manufacture an intended 
product. For purposes of the HON, a CMPU includes air oxidation 
reactors and their associated product separators and recovery devices; 
reactors and their associated product separators and recovery devices; 
distillation units and their associated distillate receivers and 
recovery devices; associated unit operations; and any feed, 
intermediate and product storage vessels, product transfer racks, and 
connected ducts and piping. A CMPU includes pumps, compressors, 
agitators, pressure relief devices, sampling connection systems, open-
ended valves or lines, valves, connectors, instrumentation systems, and 
control devices or systems.
    A SOCMI plant site can have several CMPUs, which could produce 
totally separate and non-related products. In the background 
information document for the HON, it was estimated that there were 729 
CMPUs nationwide. Two hundred thirty-eight facilities have been 
identified as subject to the HON. These HON facilities were identified 
after extensive review of facility lists compiled by the EPA's Office 
of Enforcement and Compliance Assurance, EPA Regional Offices, and the 
American Chemistry Council (ACC).
    The five kinds of HAP emission points that are currently regulated 
by the HON are storage vessels, process vents, wastewater collection 
and treatment operations, transfer operations, and equipment leaks. 
Each emission source type is briefly described below.
1. Storage Vessels
    Storage vessels contain chemical raw materials, products, and co-
products. Different types of vessels are used to store various types of 
chemicals. Gases (chemicals with vapor pressures greater than 14.7 
pounds per square inch absolute (psia)) are stored in pressurized 
vessels that are not vented to the atmosphere during normal operations. 
Liquids (chemicals with vapor pressures of 14.7 psia or less) are 
stored in horizontal, fixed roof, or floating roof tanks, depending on 
chemical properties and volumes to be stored. Liquids with vapor 
pressures greater than 11 psia are typically stored in fixed roof tanks 
that are vented to a control device. Volatile chemicals with vapor 
pressures up to 11 psia are usually stored in floating roof tanks 
because such vessels have lower emission rates than fixed roof tanks 
within this vapor pressure range.
    Emissions from storage vessels typically occur as working losses. 
As a storage vessel is filled with chemicals, HAP-laden vapors inside 
the tank become displaced and can be emitted to the atmosphere. Also, 
diurnal temperature changes result in breathing losses of organic HAP-
laden vapors from storage vessels.
2. Process Vents
    Many unit operations at SOCMI facilities generate gaseous streams 
that contain HAP. These streams may be routed to other unit operations 
for additional processing (i.e., a gas stream from a reactor that is 
routed to a distillation unit for separation) or may be vented to the 
atmosphere. Process vents emit gasses to the atmosphere, either 
directly or after passing through recovery and/or control devices. The 
primary unit operations in a SOCMI unit from which process vents 
originate are reactor and air oxidation process units, and from the 
associated product recovery and product purification devices. Product 
recovery devices include condensers, absorbers, and adsorbers used to 
recover products or co-products for use in a subsequent process, for 
use as recycle feed, or for sale. Product purification devices include 
distillation operations. The HON applies only to process vents that are 
associated with continuous (non-batch) air oxidation, other reactor 
processes, or distillation unit operations within a SOCMI process unit.
3. Process Wastewater
    For some synthetic organic chemicals, the manufacturing process 
generates wastewater streams that contain HAP. Sources of wastewater 
include: Water formed during the chemical reaction or used as a 
reactant in a process; water used to wash impurities from organic 
products or reactants; water used to cool organic vapor streams; and 
condensed steam from vacuum vessels containing organics. Organic 
compounds in the wastewater can volatilize and be emitted to the 
atmosphere from wastewater collection and treatment units if these 
units are open or vented to the atmosphere. Potential sources of HAP 
emissions associated with wastewater collection and treatment systems 
include drains, manholes, trenches, surface impoundments, oil/water 
separators, storage and treatment tanks, junction boxes, sumps, basins, 
and biological treatment systems.
4. Transfer Operations
    Synthetic organic chemical products are often transported by 
railcars or tank trucks. Chemicals are transferred to these vehicles 
through a loading rack, which can have multiple loading arms for 
connection to several transport vehicles. Emissions can occur during 
loading operations when residual vapors in transport vehicles and 
transfer piping are displaced by chemicals being loaded.
5. Equipment Leaks
    Equipment leaks are fugitive releases of process fluid or vapor 
from process equipment. These releases occur primarily at the interface 
between connected components of equipment. The basic equipment 
components that are prone to develop leaks include pumps, compressors, 
process valves, pressure relief devices, open-ended lines, sampling 
connections, flanges and other connectors, agitators, product 
accumulator vessels, and instrumentation systems.

C. What are the health effects of HAP emitted from SOCMI facilities?

    Of the 131 organic HAP regulated by the HON (table 2 to subpart F 
of part 63), EPA lists four as known carcinogens, 33 as probable 
carcinogens, and 15 as possible carcinogens. The EPA classified agents 
as carcinogens based on the weight of evidence in long-

[[Page 34425]]

term human studies of the association between cancer incidence and 
exposure to the agent and in animal studies conducted under controlled 
laboratory conditions. After evaluating the evidence, the agents were 
placed into one of the following five categories: A--human carcinogen, 
B--probable human carcinogen, C--possible human carcinogen, D--not 
classifiable as to human carcinogenicity, and E--evidence of 
noncarcinogenicity for humans. Category B is divided into two 
subcategories: B1--indicates limited human evidence and B2--indicates 
sufficient evidence in animals and inadequate or no evidence in humans.
    With the March 2005 publication of revised Guidelines for 
Carcinogen Risk Assessment, EPA no longer uses the ``known, possible, 
probable'' nomenclature for classifying the weight of evidence for 
carcinogenicity of chemical compounds. Instead, EPA provides narrative 
descriptions of the weight of evidence for carcinogenicity, as well as 
the classifications ``carcinogenic to humans,'' ``likely to be 
carcinogenic,'' ``suggestive evidence of carcinogenic potential,'' 
``inadequate information,'' and ``not likely.'' In time, the older 
classification scheme described above will be replaced.
    The International Agency for Research on Cancer (IARC) also 
classifies carcinogens based on the ``strength of the evidence for 
carcinogenicity arising from human and experimental animal data.'' 
There are four groups under the IARC classification system: Group 1--
the agent is carcinogenic to humans, Group 2A--the agent is probably 
carcinogenic to humans, Group 2B--the agent is possibly carcinogenic to 
humans, Group 3--the agent is not classifiable as to its 
carcinogenicity to humans, and Group 4--the agent is probably not 
carcinogenic to humans. Of the 51 HON HAP classified by IARC, four are 
Group 1, 33 are Group 2, and 14 are Group 3.
    Additionally, many of the HAP regulated by the HON may result in 
noncarcinogenic effects at sufficient exposures. There is a wide range 
of effects due to chronic exposures to HON HAP, such as the 
degeneration of olfactory epithelium, peripheral nervous system 
dysfunction, and developmental toxicity. Effects from acute exposures 
range from mild to severe, and include skin, eye, and respiratory 
system irritation. More detail on the health effects of individual HON 
HAP may be found in numerous sources, including https://www.epa.gov/
iris.html, https://www.atsdr.cdc.gov/mrls.html, and https://
www.oehha.ca.gov/air/acute_rels/.

D. What does the HON require?

    The HON was proposed December 31, 1992 (57 FR 62608), and the final 
rule was published April 22, 1994 (59 FR 19402). Subsequently, several 
revisions to the rule have been issued: the first dated September 20, 
1994 (59 FR 48175) and the last dated December 23, 2004 (69 FR 76859).
    The HON regulates organic HAP emissions from five types of emission 
points: Storage vessels, process vents, wastewater collection and 
treatment systems, transfer operations, and equipment leaks. For 
storage vessels, process vents, process wastewater streams, and 
transfer operations, the HON establishes applicability criteria to 
distinguish between Group 1 emission points and Group 2 emission 
points. Controls are required only for emission points meeting the 
Group 1 criteria. Group 2 emission points are subject to recordkeeping 
requirements only. Before implementation of the HON, total HAP 
emissions were estimated to be 570,000 tons per year (tpy). We 
estimated that after implementation of the HON, total HAP emissions 
would be 66,000 tpy.
    The HON provides many different control options, but the primary 
control requirements are summarized below.
1. Storage Vessels
    The HON requires that Group 1 vessels be equipped and operated with 
an internal or an external floating roof, or reduce organic HAP 
emissions by at least 95 percent. A Group 1 vessel has a capacity 
greater than or equal to 40,000 gallons and contains a HAP with a vapor 
pressure greater than or equal to 0.75 psia. A vessel is also Group 1 
if it has a capacity greater than or equal to 20,000 gallons and less 
than 40,000 gallons and contains a HAP with a vapor pressure greater 
than or equal to 1.9 psia.
2. Process Vents
    The HON requires that the organic HAP emissions from Group 1 
process vent streams be reduced by at least 98 percent by weight or 
achieve an outlet concentration of 20 parts per million by volume 
(ppmv) or less. A Group 1 process vent stream has a total organic HAP 
concentration of greater than or equal to 50 ppmv and a total resource 
effectiveness (TRE) of less than or equal to 1.0. Facilities also have 
the option of sending the process vent to a flare or maintaining a TRE 
index greater than 1.0. The TRE index is a measure of how costly a 
particular process vent is to control (the higher the TRE index, the 
more costly the control).
3. Process Wastewater
    The HON requires that Group 1 wastewater streams be treated to 
reduce the HAP mass in the streams. Group 1 wastewater streams are 
streams that meet one of several minimum flow and HAP concentration 
criteria in the rule. The required mass removals are HAP-specific and 
range from 31 percent (e.g., for methanol) to 99 percent (e.g., for 
benzene). Emissions from collection and management units must be 
suppressed from the point of generation to the treatment device. Air 
emissions from treatment systems (except for open biological treatment 
systems which have different requirements) must be collected in a 
closed vent system and conveyed to a control device that reduces HAP 
emissions by 95 percent (or achieves an outlet concentration of 20 ppmv 
or less for combustion devices).
4. Transfer Operations
    The HON requires control of Group 1 transfer racks to achieve a 98 
percent reduction of organic HAP or an outlet concentration of 20 ppmv. 
Alternatively, facilities can use vapor balancing systems. A Group 1 
transfer rack is a transfer rack that annually loads greater than or 
equal to 0.17 million gallons of liquid products that contain organic 
HAP with a rack weighted average vapor pressure greater than or equal 
to 1.5 psia.
5. Equipment Leaks
    The HON requires equipment and work practice standards (in the form 
of a leak detection and repair program) to reduce equipment leak 
emissions. The equipment leak provisions apply to all equipment 
components that are associated with a process subject to the HON and 
that are in organic HAP service for 300 hours per year or more. The HON 
requires valves to be monitored once per month (or implementation of a 
quality improvement program) at each process unit with two percent or 
greater leaking valves. The monitoring frequency may be decreased as 
the percentage of leakers decreases or if the equipment leaks standards 
are met over consecutive periods.

II. Summary of Proposed Revised Standards

    This proposal provides two options that we expect to choose between 
for revising the HON rule. The first option is to retain the current 
HON rule. The second option is to revise subparts F, G, and H to 
require more stringent standards for process vents, storage vessels, 
and equipment leaks that emit

[[Page 34426]]

or store certain HAP. As explained below, we propose that either option 
would meet the requirements of both section 112(f)(2) and 112(d)(6). 
Their difference results from how we weigh certain risk factors 
(specifically, maximum individual lifetime cancer risk versus cancer 
incidence, and their relative relationship to costs) within our 
determination of what is necessary to protect public health with an 
ample margin of safety under section 112(f)(2), and of what changes are 
necessary under section 112(d)(6).

A. Summary of Option 1

    Under this option, the control requirements of 40 CFR subpart F, G, 
and H would remain the same as under the current rule, and we would not 
revise applicability criteria to require currently uncontrolled storage 
vessels and process vents to control emissions, nor would we reduce the 
percentage of leaking valves.

B. Summary of Option 2

    Under this option, the control requirements of 40 CFR subpart G 
would remain the same as under the current rule, but the applicability 
criteria for Group 1 storage vessels and process vents would be revised 
so that additional emission points would be required to control 
emissions. For equipment leaks, the first option would reduce, in 
subpart H, the percentage of leaking valves.
    The existing applicability criteria for equipment leaks and Group 1 
criteria for storage vessels and process vents would continue to apply. 
After the rule becomes effective, an additional criterion would be 
added. The additional criterion would apply only to emission points 
that emit maleic anhydride, methyl bromide, acrolein, and any HAP for 
which inhalation cancer unit risk estimates (UREs) have been 
developed.\1\ A list of these HAP is given in proposed table 38 of 40 
CFR, part 63, subpart G. This list may be amended over time as more 
information indicates that some HAP should be added or removed.
---------------------------------------------------------------------------

    \1\ The URE is the upper-bound excess lifetime cancer risk 
estimated to result from continuous exposure to an agent at a 
concentration of 1 microgram per cubic meter ([mu]g/m3) 
in air. For example, if a URE of 1.5 x 10-6 per [mu]g/
m3 is reported, then 1.5 excess cancer cases are expected 
to develop per 1,000,000 people if exposed daily for a lifetime to 1 
ug of the chemical in 1 cubic meter of air.
---------------------------------------------------------------------------

    The proposed changes to the standards, based on the second control 
option, are summarized below:

------------------------------------------------------------------------
       Emission source          Proposed changes to standards (Option 2)
------------------------------------------------------------------------
Storage vessels..............  A group 1 storage vessel means a Group 1
                                storage vessel as currently defined in
                                Sec.   63.111 to subpart G of part 63.
                                On or after [DATE THE FINAL RULE IS
                                PUBLISHED IN THE FEDERAL REGISTER], a
                                group 1 storage vessel also includes
                                storage vessels that store one or more
                                HAP listed in table 38 to subpart G of
                                part 63, and have a combined HAP
                                emission rate greater than 4.54
                                megagrams per year (5.0 tons HAP per
                                year) on a rolling 12-month average.
Process vents................  A group 1 process vent means a Group 1
                                process vent as currently defined in
                                Sec.   63.111 to subpart G of part 63.
                                On or after [DATE THE FINAL RULE IS
                                PUBLISHED IN THE FEDERAL REGISTER], a
                                group 1 process vent also includes
                                process vents for which the vent stream
                                emits one or more HAP listed in table 38
                                to subpart G of part 63, and the TRE
                                index value is less than or equal to
                                4.0.
Equipment leaks..............  For CMPUs containing at least one HAP
                                listed in table 38 to subpart G of part
                                63, on or after [DATE THE FINAL RULE IS
                                PUBLISHED IN THE FEDERAL REGISTER],
                                monthly monitoring of equipment
                                components is required until the process
                                unit has fewer than 0.5 percent leaking
                                valves in gas/vapor service and in light
                                liquid service.
------------------------------------------------------------------------

    For storage vessels, emissions would be computed using the 
procedures in Sec.  63.150. Group 2 storage vessels that contain table 
38 HAP would be required to maintain records of rolling 12-month 
average HAP emissions. For equipment leaks, the frequency of monitoring 
could be reduced to quarterly, semi-annually, and annually if 
successive monitoring periods show that facilities are able to maintain 
less than 0.5 percent leakers. Monthly monitoring would be required if 
the percent leakers exceeds 0.5 percent.
    Under Option 2, we are also proposing compliance dates for sources 
subject to the proposed revised standards pursuant to section 112(i) of 
the CAA. When Congress amended the CAA in 1990, it established a new, 
comprehensive set of provisions regarding compliance deadlines for 
sources subject to emissions standards and work practice requirements 
that EPA promulgates under section 112. However, as discussed later in 
this section of this preamble, Congress also left in place other 
provisions in section 112(f)(4) that in certain respects are redundant 
or conflict with the new compliance deadline provisions. These 
provisions also fail to accommodate the new State-administered air 
operating permit program added in title V of the amended CAA.
    For new sources, section 112(i)(1) requires that after the 
effective date of ``any emission standard, limitation, or regulation 
under subsection (d), (f) or (h), no person may construct any new major 
source or reconstruct any existing major source subject to such 
emission standard, regulation or limitation unless the Administrator 
(or State with a permit program approved under title V) determines that 
such source, if properly constructed, reconstructed and operated, will 
comply with the standard, regulation or limitation.'' Section 112(a)(4) 
defines a ``new source'' as ``a stationary source the construction or 
reconstruction of which is commenced after the Administrator first 
proposes regulations under this section establishing an emission 
standard applicable to such sources.'' Under sections 112(e)(10) and 
112(f)(3), any section 112(d)(6) emission standards and any residual 
risk emission standards shall become effective upon promulgation. This 
means generally that a new source that is constructed or reconstructed 
after this proposed rule is published must comply with the final 
standard, when promulgated, immediately upon the rule's effective date 
or upon the source's start-up date, whichever is later.
    There are some exceptions to this general rule. First, section 
112(i)(7) provides that a source for which construction or 
reconstruction is commenced after the date an emission standard is 
proposed pursuant to subsection (d) but before the date a revised 
emission standard is proposed under subsection (f) shall not be 
required to comply with the revised standard until 10 years after the 
date construction or reconstruction commenced. This provision ensures 
that new sources that are built in compliance with MACT will not be 
forced to

[[Page 34427]]

undergo modifications to comply with a residual risk rule unreasonably 
early.
    In addition, sections 112(i)(2)(A) and (B) provide that a new 
source which commences construction or reconstruction after a standard 
is proposed, and before the standard is promulgated, shall not be 
required to comply with the promulgated standard until three years 
after the rule's effective date, if the promulgated standard is more 
stringent than the proposed standard and the source complies with the 
proposed standard during the three-year period immediately after 
promulgation. This provision essentially treats such new sources as if 
they are existing sources in giving them a consistent amount of time to 
convert their operations to comply with the more stringent final rule 
after having already been designed and built according to the proposed 
rule.
    For existing sources, section 112(i)(3)(A) provides that after the 
effective date of ``any emission standard, limitation or regulation 
promulgated under this section and applicable to a source, no person 
may operate such source in violation of such standard, limitation or 
regulation except, in the case of an existing source, the Administrator 
shall establish a compliance date or dates * * * which shall provide 
for compliance as expeditiously as practicable, but in no event later 
than 3 years after the effective date of such standard[.]'' This 
potential 3-year compliance period for existing sources under section 
112(i)(3) matches the 3-year compliance period provided for new sources 
subject to section 112(d), (f), or (h) standards that are promulgated 
to be more stringent than they were proposed, as provided in sections 
112(i)(1) and (2).
    As for new sources, there are exceptions to the general rule for 
existing sources under section 112(i)(3), the most relevant being 
section 112(i)(3)(B) allowance that EPA or a State title V permitting 
authority may issue a permit granting a source an additional one year 
to comply with standards ``under subsection (d)'' if such additional 
period is necessary for the installation of controls. As explained 
below, EPA now believes that this reference to only subsection 112(d), 
rather than to section 112 in general, was accidental on Congress' part 
and presents a conflict with the rest of the statutory scheme Congress 
enacted in 1990 to govern compliance deadlines under the amended 
section 112.
    Even though, in 1990, Congress amended section 112 to include the 
comprehensive provisions in subsection 112(i) regarding compliance 
deadlines, the enacted CAA also included provisions in section 112(f), 
leftover from the previous version of the Act, that apply compliance 
deadlines for sources subject to residual risk rules. These deadlines 
differ in some ways from the provisions of section 112(i). First, 
section 112(f)(4) provides that no air pollutant to which a standard 
``under this subsection applies may be emitted from any stationary 
source in violation of such standard * * *'' For new sources, this is a 
redundant provision, since the new provisions added by Congress in 
sections 112(i)(1), (2), (3), and (7)--which explicitly reach standards 
established under section 112(f)--already impose this prohibition with 
respect to new sources and provide for the allowable exceptions to it. 
In contrast, for new sources, the prohibition in section 112(f)(4) 
provides for no exception for a new source built shortly before a 
residual risk standard is proposed, makes no reference to the new title 
V program as an implementation mechanism, and, where promulgated 
standards are more stringent than their proposed versions, makes no 
effort to align compliance deadlines for new sources with those that 
apply for existing sources. From the plain language of section 112(i), 
it is clear that Congress intended in the 1990 amendments to 
comprehensively address the compliance deadlines for new sources 
subject to any standard under either subsections 112(d), (f), or (h), 
and to do so in a way that accommodates both the new title V program 
added in 1990 and the fact that where circumstances justify treating a 
new source as if it were an existing source, a substantially longer 
compliance period than would otherwise apply is necessary and 
appropriate. It is equally clear that the language in section 112(f)(4) 
fails on all these fronts for new sources.
    In addition, for existing sources, section 112(f)(4)(A) provides 
that a residual risk standard and the prohibition against emitting HAP 
in violation thereof ``shall not apply until 90 days after its 
effective date[.]'' However, section 112(f)(4)(B) states that EPA ``may 
grant a waiver permitting such source a period up to two years after 
the effective date of a standard to comply with the standard if the 
Administrator finds that such period is necessary for the installation 
of controls and that steps will be taken during the period of the 
waiver to assure that the health of persons will be protected from 
imminent endangerment.'' These provisions are at odds with the rest of 
the statutory scheme governing compliance deadlines for section 112 
rules in several respects. First, the 90-day compliance deadline for 
existing sources in section 112(f)(4)(A) directly conflicts with the 
up-to-3-year deadline in section 112(i)(3)(A) allowed for existing 
sources subject to ``any'' rule under section 112. Second, the section 
112(f)(4)(A) deadline results in providing a shorter deadline for 
ordinary existing sources to comply with residual risk standards than 
would apply under section 112(i)(2) to new sources that are built after 
a residual risk standard is proposed but a more stringent version is 
promulgated. Third, while both section 112(i)(1), for new sources 
subject to any section 112(d), (f), or (h) standard, and section 
112(i)(3), for existing sources subject to any section 112(d) standard, 
refer to and rely upon the new title V permit program added in 1990 and 
explicitly provide for State permitting authorities to make relevant 
decisions regarding compliance and the need for any compliance 
extensions, section 112(f)(4)(B) still reflects the pre-1990 statutory 
scheme in which only the Administrator is referred to as a decision-
making entity, notwithstanding the fact that even residual risk 
standards under section 112(f) are likely to be delegated to States for 
their implementation, and will be reflected in sources' title V permits 
and need to rely upon the title V permit process for memorializing any 
compliance extensions for those standards.
    While we appreciate the fact that section 112(i)(3)(B) refers 
specifically only to standards under subsection 112(d), which some 
might argue means that subsection 112(i)(3), in general, applies only 
to existing sources subject to section 112(d) standards, we believe 
that Congress inadvertently limited its scope and created a statutory 
conflict in need of our resolution. Notwithstanding the language of 
subparagraph (B), section 112(i)(3)(A) by its terms applies to ``any'' 
standard promulgated under ``section'' 112, which includes those under 
subsection 112(f), in allowing up to a three year compliance period for 
existing sources. Moreover, Congress clearly intended the section 
112(i) provisions applicable to new sources to govern compliance 
deadlines under section 112(f) rules, notwithstanding the language of 
section 112(f)(4). This is because sections 112(i)(1) and (2) 
explicitly reach standards under section 112(f). To read section 
112(i)(3)(B) literally as reaching only section 112(d) standards, with 
section 112(f)(4)(B) reaching section 112(f) standards, leaves the 
question as to whether there can be compliance extensions for section

[[Page 34428]]

112(h) standards completely unaddressed by the statute, even though it 
may in fact be necessary in complying with a section 112(h) work 
practice standard to install equipment or controls. A narrow reading of 
the scope of section 112(i)(3) also ignores the fact that in many 
cases, including that of this proposed rule, the governing statutory 
authority will be both section 112(f)(2) and section 112(d)(6)--the 
only reasonable way to avoid a conflict in provisions controlling 
compliance deadlines for existing sources in these situations is to 
read the more specific and comprehensive set of provisions, those of 
section 112(i), as governing both aspects of the regulation.
    Nothing in the legislative history suggests that Congress knowingly 
intended to enact separate schemes for compliance deadlines for 
residual risk standards and all other standards adopted under section 
112. Rather, comparing the competing Senate and House Bills shows that 
each bill contained its own general and/or specific versions of 
compliance deadline provisions, and that when the bills were reconciled 
in conference the two schemes were both accidentally enacted, without 
fully modifying the various compliance deadline provisions in accord 
with the modifications otherwise made to the section 112 amendments in 
conference.
    We recognize that our existing regulations in the part 63 General 
Provisions currently reflect the dual scheme presented by sections 
112(f)(4) and 112(i) (See 40 CFR 63.6(c)(2), 63.6(i)(4)(ii)). In the 
near future, we intend to revise those regulations to comport with our 
interpretation, as explained above, to avoid confusion and situations 
where a rule incorporates those provisions by reference such that 
compliance deadlines are inconsistent with our interpretation. In the 
meantime, notwithstanding the part 63 General Provisions, we are 
proposing a compliance deadline for existing sources, under Option 2, 
of three years for process vents and storage vessels and one year for 
equipment leaks. The proposed compliance deadline for existing sources 
of three years for process vents and storage vessels is realistic for 
any affected facility that has to plan their control strategy, purchase 
and install the control device(s), and bring the control device online. 
Less time is required for compliance with the new equipment leak 
requirements, but plants will have to identify affected equipment and 
modify their existing leak detection and repair program to meet the new 
requirements for monitoring frequency.

III. Rationale for the Proposed Rule

A. What is our approach for developing residual risk standards?

    Following our initial determination that the individual most 
exposed to emissions from the category considered exceeds a 1-in-1 
million individual lifetime cancer risk, our approach to developing 
residual risk standards is based on a two-step determination of 
acceptable risk and ample margin of safety. The first step is the 
consideration of acceptable risk. The second step determines an ample 
margin of safety to protect public health, which is the level at which 
the standards are set (unless a more stringent standard is required to 
prevent adverse environmental effect after the consideration of costs, 
energy, safety, and other relevant factors).
    The terms ``individual most exposed,'' ``acceptable level,'' and 
``ample margin of safety'' are not specifically defined in the CAA. 
However, CAA section 112(f)(2)(B) refers positively to the 
interpretation of these terms in our 1989 rulemaking (54 FR 38044, 
September 14, 1989), ``National Emission Standards for Hazardous Air 
Pollutants: Benzene Emissions from Maleic Anhydride Plants, 
Ethylbenzene/Styrene Plants, Benzene Storage Vessels, Benzene Equipment 
Leaks, and Coke By-Product Recovery Plants (Benzene NESHAP),'' 
essentially directing us to use the interpretation set out in that 
notice. See also ``A Legislative History of the Clean Air Act 
Amendments of 1990,'' volume 1, p. 877 (Senate debate on Conference 
Report). We notified Congress in a report on residual risk that we 
intended to utilize the Benzene NESHAP approach in making CAA section 
112(f) residual risk determinations (see Residual Risk Report to 
Congress, March 1999, EPA-453/R-99-001, p. ES-11).
    In the Benzene NESHAP (54 FR 38044, September 14, 1989), we stated 
as an overall objective: * * * in protecting public health with an 
ample margin of safety, we strive to provide maximum feasible 
protection against risks to health from hazardous air pollutants by (1) 
protecting the greatest number of persons possible to an individual 
lifetime risk level no higher than approximately 1-in-1 million; and 
(2) limiting to no higher than approximately 1-in-10 thousand [i.e., 
100-in-1 million] the estimated risk that a person living near a 
facility would have if he or she were exposed to the maximum pollutant 
concentrations for 70 years.''
    The Agency also stated that, ``The EPA also considers incidence 
(the number of persons estimated to suffer cancer or other serious 
health effects as a result of exposure to a pollutant) to be an 
important measure of the health risk to the exposed population. 
Incidence measures the extent of health risk to the exposed population 
as a whole, by providing an estimate of the occurrence of cancer or 
other serious health effects in the exposed population.'' The Agency 
went on to conclude that ``estimated incidence would be weighed along 
with other health risk information in judging acceptability.\2\'' As 
explained more fully in our Residual Risk Report to Congress, EPA does 
not define ``rigid line[s] of acceptability,'' but considers rather 
broad objectives to be weighed with a series of other health measures 
and factors (EPA-453/R-99-001, p. ES-11).
---------------------------------------------------------------------------

    \2\ In the benzene decision, the Agency considered the same risk 
measures in the ``acceptability'' analysis as in the ``margin of 
safety'' analysis, stating: ``In the ample margin decision, the 
Agency again considers all of the health risk and other health 
information considered in the first step. Beyond that information, 
additional factors relating to the appropriate level of control will 
also be considered, including costs and economic impacts of 
controls, technological feasibility, uncertainties, and any other 
relevant factors. Considering all of these factors, the Agency will 
establish the standard a level that provides an ample margin of 
safety to protect the public health, as required by section 112.''
---------------------------------------------------------------------------

B. How did we estimate residual risk?

    The Residual Risk Report to Congress provides the general framework 
for conducting risk assessments to support decisions made under the 
residual risk program. As acknowledged by the report, the design of 
each risk assessment would have some common elements, including a 
problem formulation phase, an analysis phase, and the risk 
characterization phase.
    The primary risk assessment for the SOCMI source category focused 
on inhalation exposures, both chronic and acute, to HAP emissions from 
CMPUs that are subject to the HON. The primary risk assessment was 
reviewed by Agency scientists before being used for this proposed 
rulemaking. The emissions estimates used in the primary risk assessment 
represented actual emissions that remain after the application of MACT, 
not emissions at the rate allowed by the HON requirements 
(``allowable'' emissions) that may be higher than actual emissions. 
Some of the emission points subject to the HON may be controlled to a 
higher level than required by the rules and some Group 2 points may be 
controlled even though the rule does not require them to be. This may 
be due to some State or local rules that are more stringent than the 
HON, or because some facilities may reduce emissions for reasons other 
than regulatory requirements. This means that the

[[Page 34429]]

estimated risks based on allowable emissions would be higher than the 
risks estimated using actual emissions.
    For some HON emission points, we could estimate allowable 
emissions; for others, it is nearly impossible. For equipment leaks, 
because the standards are work practice standards the actual emissions 
and allowable emissions are likely the same for equipment in the leak 
detection and repair program required by the HON. More frequent 
monitoring of equipment components (for example, monthly instead of 
quarterly) could result in actual emissions being lower than allowable 
emissions, but few, if any, sources monitor more frequently than 
required by the HON. For wastewater and process vents, if a facility 
chooses to control an emission point (to the level required in the 
HON), there is no requirement to determine whether the point is 
actually required to be controlled. A requirement to determine the 
applicability of controls for such emission points was intentionally 
not included in the HON because it was seen as an unnecessary burden 
for points that would be controlled anyway. Consequently, there are 
some emission points for which there is no readily available data that 
can be used to determine the applicability of control requirements. 
Without such data, there is no accurate way to determine allowable 
emissions under the current rule. In addition, HAP emissions from 
wastewater sources are likely controlled to a greater extent than the 
rules require, but this overcontrol is impossible to estimate. 
Emissions from transfer operations are small relative to the emissions 
from other points, with emissions from controlled points nationally 
accounting for less than one percent of total HON HAP emissions. Given 
the small contribution to total emissions from transfer operations, any 
differences between actual and allowable emissions would not be 
significant relative to the total emissions from all HON emission 
points.
    While we acknowledge that there is some uncertainty regarding the 
differences between actual and allowable emissions, we believe that 
there is neither a substantial amount of overcontrol of Group 1 sources 
nor control of Group 2 sources so that actual emissions are a 
reasonable approximation of allowable emissions. Basing this analysis 
on actual emissions provides an acceptable approach to determining the 
remaining risks to public health and the environment after application 
of the MACT standards. Indeed, in this case, given the impossibility of 
definitively estimating allowable emissions, we have no choice but to 
rely upon the best available alternative information for assessing 
remaining risks after application of MACT, industry supplied actual 
emissions data. Uncertainty in the use of this data can be considered 
in the selection of the standards as appropriate.
    Screening level assessments were also conducted to examine human 
health and ecological risk due to multipathway exposure and to examine 
the risks from entire plant sites (i.e., HON CMPUs and other HAP-
emitting processes). A full discussion of the primary and screening 
level assessments is provided in the risk characterization document in 
the public docket.
1. How did we estimate the atmospheric dispersion of HAP emitted from 
HON CMPU sources?
    To estimate the dispersion of HAP emitted from HON CMPUs for the 
inhalation and multipathway assessments, we used the Human Exposure 
Model, version 3 (HEM-3), which incorporated the Industrial Source 
Complex Short-term model, version 3 (ISCST-3). The ISCST3 dispersion 
model is one of EPA's recommended models for assessing pollutant 
concentrations from industrial facilities. The ISCST3 model handles a 
wide range of different source types that may be associated with an 
industrial source complex, including stack sources, area sources, 
volume sources, and open pit sources.
    Inputs to the HEM-3 include source data to characterize the 
emissions from the facility, the emission sources at the facility, and 
the location of the facility. For the inhalation and multipathway 
assessments, we used site-specific information for the base year 1999 
for 104 of the 238 existing HON facilities. These data were collected 
by the ACC through a voluntary survey and provided to EPA. These data 
consisted of organic HAP emissions from five types of emission points 
subject to the HON and included stack parameters, emission rates, and 
location coordinates. Data were provided for 271 HON CMPUs in the 1999 
data collection. When scaled to 238 HON facilities, 732 HON CMPUs would 
be estimated for the industry. In the background information for the 
HON, it was estimated that there were 729 HON CMPUs nationwide. The 
similarities in the structure of the industry indicate that the 1999 
collected data provide a reasonable picture of post-compliance 
emissions of organic HAP, and that the process unit information used in 
the residual risk analysis is representative of the CMPUs for the 
entire industry.
    We recognize that the 1999 survey data have some uncertainties 
regarding the sources responding to a voluntary data request and the 
emissions reported. It is unclear the amount of bias that may exist in 
the data and the extent to which the 104 facilities in the survey are 
representative of the risks posed by the remaining facilities (see 
section III.C.1. of this preamble for additional discussion). However, 
the 1999 survey data are still the most detailed and comprehensive data 
available, and we conclude that the data are appropriate for use in 
conducting this residual risk assessment. Uncertainty in the use of 
this data can be considered in the selection of the standards as 
appropriate.
    Some inorganic HAP, such as hydrochloric acid and chlorine, may be 
emitted from HON sources. However, these compounds were not considered 
in this risk assessment because data were not available to characterize 
emissions of those HAP. The HON regulates emissions of organic HAP only 
and the 1999 ACC data provided information on organic HAP emissions 
only. As discussed below in III.B.4, an additional analysis was 
conducted using information in the National Emissions Inventory (NEI) 
to estimate the risk from the entire plant site at which the HON CMPU 
are located. The NEI information contained information on both organic 
and inorganic HAP emitted from each facility. A comparison between the 
analyses using the two different data sets showed that there were no 
cases where the concentration of an inorganic HAP emitted from a HON 
CMPU exceeded its reference value. Therefore, we concluded that not 
including inorganic HAP in the primary risk assessment does not affect 
the results of the analysis and that no further assessment of inorganic 
HAP emissions is necessary.
2. How did we assess public health risk associated with HAP emitted 
from HON CMPUs?
    The primary tool used to estimate individual and population 
exposures in the inhalation and multipathway assessments was the Human 
Exposure Model, Version 3 (HEM-3). The HEM-3 incorporates the ISCST3 
air dispersion model and 2000 Census data, along with HAP dose response 
and reference values, to estimate chronic and acute human health risks 
and population exposure. This model is considerably more sophisticated, 
and less conservative, than tools traditionally associated with 
scoping-type analyses

[[Page 34430]]

(such as use of the Human Exposure Model, version 1.5). More 
information on HEM-3 is available from the HEM-3 User's Guide.
    The HEM-3 performs detailed analyses of acute and chronic air 
pollution risks for populations located near industrial emission 
sources. The HEM-3 performs three main operations: dispersion modeling, 
estimation of human health risks, and estimation of population 
exposure. In order to perform these calculations, HEM-3 draws on three 
data libraries provided with the model: A library of meteorological 
data for over 60 stations, a library of census block internal point 
locations, populations, and elevations to provide the basis for human 
exposure calculations, and a library of pollutant unit risk factors and 
reference concentrations used to calculate risks.
    In our assessment of public health risk associated with HAP emitted 
from HON CMPUs, we considered risks of cancer and other health effects. 
Cancer risks associated with inhalation exposure were assessed using 
lifetime cancer risk estimates (i.e., assuming 70 years of exposure 24 
hours a day for all individuals in a given location). The noncancer 
risks were characterized through the use of hazard quotient (HQ) and 
hazard index (HI) estimates. The HQ and HI also assume continuous 
lifetime exposures. An HQ compares an estimated chemical intake (dose) 
with a reference level below which adverse health effects are unlikely 
to occur. Within the context of inhalation risk, EPA uses a ``Reference 
Concentration (RfC)''. An RfC is an estimate (with uncertainty spanning 
perhaps an order of magnitude) of a continuous inhalation exposure to 
the human population (including sensitive subgroups) that is likely to 
be without an appreciable risk of deleterious effects during a 
lifetime. It can be derived from a NOAEL, LOAEL, or benchmark 
concentration, with uncertainty factors generally applied to reflect 
limitations of the data used. An HQ is calculated as the ratio of the 
exposure concentration of a pollutant to its health-based reference 
concentration. If the HQ is calculated to be less than 1, then no 
adverse health effects are expected as a result of the exposure. 
However, an HQ exceeding 1 does not translate to a probability that 
adverse effects will occur. Rather, it suggests the possibility that 
adverse health effects may occur. An HI is the sum of HQ for pollutants 
that target the same organ or system. As with the HQ, values that are 
below 1.0 are considered to represent exposure levels with no 
significant risk of adverse health effects.
3. How did we assess multipathway impacts of HAP emissions from HON 
CMPUs?
    The HON CMPUs at six of the 238 facilities emit HAP that are of 
concern for potential adverse health impacts from pathways other than 
inhalation (e.g., soil or fish ingestion). These HAP are often termed 
persistent bioaccumulative toxics (PBTs). Whe