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Technical Support Document for U.S. EPA’s Proposed Rule for Addition of a 

Subsurface Intrusion Component to the Hazard Ranking System

Appendix E: Summary of Peer Review Comments and 

Responses
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Summary of Peer Review Comments and Responses on the Addition of a 

Subsurface Intrusion Component to the HRS

Table of Contents

Terms and Abbreviations ........................................................................................................................................5

Executive Summary..................................................................................................................................................6

1.0 Introduction ..................................................................................................................................................7

Hazard Ranking System .........................................................................................................................................7

Subsurface Intrusion Component ...........................................................................................................................8

Organization of Comments and Responses ............................................................................................................9

2.0 Peer Review Comments on Charge Questions and EPA’s Response .....................................................10

2.1 Charge Question #1: .................................................................................................................................10

2.1.a Equation for Potential for Exposure .....................................................................................................................11

2.1.b Structure Containment..........................................................................................................................................12

2.1.b.1 Structure Containment Factors....................................................................................................................12

2.1.b.2 Scientific Basis for Structure Containment Categories...............................................................................13

2.1.b.3 Consideration of Biogases and Inert Gases in Determining Structure Containment...................................14

2.1.c Depth to Contamination .......................................................................................................................................14

2.1.c.1 Definition of Depth to Contamination.........................................................................................................14

2.1.c.2 Scientific Basis for Depth to Contamination Factor....................................................................................15

2.1.c.3 Depth and Probability of Intrusion..............................................................................................................16

2.1.c.4 Method Used for Assigning Depth to Contamination Weighting Values...................................................17

2.1.d Vertical Migration ................................................................................................................................................17

2.1.d.1 Use of Term “Effective Porosity/ Permeability”.........................................................................................17

2.1.d.2 Effective Porosity/ Permeability Categories ...............................................................................................18

2.1.d.3 Scientific Basis for Vertical Migration Weightings....................................................................................18

2.1.d.4 Effect of Soil Type and Moisture Content on Vertical Migration Values...................................................19

2.1.d.5 Hydraulic Pumping .....................................................................................................................................19

2.1.e Vapor Migration Potential....................................................................................................................................20

2.1.f Biodegradation/Bioattenuation.............................................................................................................................20

2.1.g Consideration of Volatile Substances...................................................................................................................21

2.2 Charge Question #2: .................................................................................................................................22

2.2.a Support for Method ..............................................................................................................................................22

2.2.b Considering Waste Quantity above Areas of Subsurface Contamination ............................................................22

2.2.c Considering Waste Quantity of Multiple Structures ............................................................................................22

2.2.d Including Subsurface Contamination in the Waste Quantity................................................................................23

2.2.e Minimum Waste Quantity ....................................................................................................................................23

2.2.f Relevance of Waste Quantity ...............................................................................................................................23

2.2.g Consideration of Source Concentrations in Estimating Waste Quantity ..............................................................24

2.2.h Relative Risk of Airborne Contamination ............................................................................................................25

2.2.i Biodegradation and Waste Quantity.....................................................................................................................25
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2.2.j Impact of Indoor Sources on Waste Quantity ......................................................................................................26

2.2.k Applicability of the Hazardous Waste Quantity Calculation................................................................................26

2.2.l Consider Future Structures...................................................................................................................................26

2.2.m Adequacy of Site Inspection Data to Identify Subsurface Contamination ...........................................................27

2.2.n Need for Discussion and Reference Support in TSD ...........................................................................................27

2.3 Charge Question #3: .................................................................................................................................28

2.3.a Support for Weighting Targets.............................................................................................................................28

2.3.b Appropriate Benchmark Risk (Cancer and Non-Cancer).....................................................................................28

2.3.b.1 Number of Eligible Sites.............................................................................................................................29

2.3.b.2 Benchmark and Background (Understanding Overall HRS Process) .........................................................29

2.3.b.3 Cumulative Effects and Benchmarks..........................................................................................................30

2.3.b.4 Weighting Based on Exceeding Benchmark ...............................................................................................30

2.3.b.5 Benchmark Levels in Crawl Spaces............................................................................................................31

2.3.c Target Weighting..................................................................................................................................................32

2.3.c.1 Strength of Weighting Assumptions ...........................................................................................................33

2.3.c.2 Weighting Based on Benchmark Comparison ............................................................................................34

2.3.c.3 Weighting by Subsurface Conditions..........................................................................................................34

2.3.c.4 Weighting by Sample Type.........................................................................................................................35

2.3.c.5 Weighting Factor When Not Ten Times Benchmark..................................................................................35

2.3.c.6 Weighting Based on Subslab Samples and Ground Water Samples...........................................................36

2.3.c.7 Weighting Factors for Crawl Space Samples and Subslab Samples...........................................................36

2.3.d Consistency of Weighting Factors with Database/Guidance................................................................................36

2.3.d.1 Inconsistency of Attenuation Factors..........................................................................................................37

2.3.d.2 Analyzing Raw Data to Determine Weighting Factors...............................................................................37

2.3.e Consideration of Biodegradation of Hazardous Contaminants in Weighting Targets..........................................37

2.3.f Evaluating Vacant/Non-occupied Structures........................................................................................................38

2.3.g Evaluating Future Targets....................................................................................................................................38

2.4 Charge Question #4 ..................................................................................................................................39

2.4.a General Support for the Approach .......................................................................................................................39

2.4.b Consider Preferential Pathways Between Floors..................................................................................................39

2.4.c Inferring Level I Contamination...........................................................................................................................40

2.4.d Consider Discontinuous Contamination...............................................................................................................40

2.4.e Consideration of Lower Floors.............................................................................................................................40

2.4.f Weighting Based on Basement Samples..............................................................................................................41

2.4.g Evaluating Worker Exposure in Multi-Story Buildings.......................................................................................41

2.5 Charge Question #5 ..................................................................................................................................43

2.5.a Support for Worker Factor Value.........................................................................................................................43

2.5.b Indoor Air Exposure and Worker Risk.................................................................................................................43

2.5.c Rationale for Worker Factor Value ......................................................................................................................44

2.5.d Worker Factor Value Correlation.........................................................................................................................44

2.5.e Worker Benchmark Values..................................................................................................................................44

2.6 General Comments Affecting All Charge Questions ...............................................................................45

2.6.a Definitions............................................................................................................................................................45

2.6.b Consistency with Vapor Intrusion Guidance........................................................................................................45

3.0 Comments Outside the Scope of the Charge ............................................................................................46

3.1 Technical Comments Outside the Scope of the Charge ...........................................................................46

3.1.a General Comments...............................................................................................................................................46

3.1.b Likelihood of Exposure/Release...........................................................................................................................46

3.1.c Waste Characteristics: ..........................................................................................................................................47
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3.1.d Targets..................................................................................................................................................................47

3.2 Policy Comments Outside the Scope of the Charge .................................................................................47

4.0 Editorial Comments ..................................................................................................................................48

5.0 List of Commenters ....................................................................................................................................48
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Terms and Abbreviations

The following acronyms and abbreviations are used throughout the text: 

AOE Area of Observed Exposure 

ASC Area of Subsurface Contamination 

CERCLA Comprehensive Environmental Response, Compensation, and Liability Act of 1980, 42 

U.S.C. Sections 9601 et seq., also known as Superfund 

EPA U.S. Environmental Protection Agency, also USEPA

FR Federal Register

HI Hazard Index 

HRS Hazard Ranking System, Appendix A of the NCP 

HVAC Heating, Ventilation, and Air-Conditioning System

NCP National Oil and Hazardous Substances Pollution Contingency Plan, 40 C.F.R. Part 300 

NPL National Priorities List, Appendix B of the NCP

OSWER Office of Solid Waste and Emergency Response

OLEM Office of Land and Emergency Management

OUST Office of Underground Storage Tanks 

RfC Reference Concentration 

SAB Science Advisory Board

SsI Subsurface Intrusion 

TSD Technical Support Document 

VOC Volatile Organic Compounds 
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Executive Summary

The U.S. Environmental Protection Agency (EPA) is proposing to add a subsurface intrusion (SsI) component to 

the Hazard Ranking System (HRS). The HRS is the principal mechanism that EPA uses to evaluate sites for 

placement on the National Priorities List (NPL). This addition will add to the HRS the consideration of the threat 

posed by exposure to hazardous substances, pollutants or contaminants1 that enter regularly occupied structures 

through subsurface intrusion. The agency is not considering changes to the remainder of the HRS except for 

minor updates reflecting changes in terminology. 

During development of this proposed HRS update, external independent scientific peer review of proposed 

approaches for assigning HRS scores to specific situations at sites being evaluated was performed. For each of 

these proposed approaches the agency developed a specific charge question for consideration by the peer 

reviewers:

• Charge question 1 addressed the calculation of the potential for exposure factor value.

• Charge question 2 addressed the calculation of the hazardous waste quantity factor value.

• Charge question 3 addressed the evaluation of targets in areas of subsurface contamination.

• Charge question 4 addressed the evaluation of targets and hazardous waste quantity in multitenant buildings.

• Charge question 5 addressed the evaluation of worker targets.

This document explains the results of this external independent scientific peer review, including summaries of the 

peer reviewers’ comments, grouped according to the relevant charge question, and the responses to those 

comments. Many of the peer review comments were supportive of the methods used in the proposed addition, but 

requested modifications to specific aspects of the evaluation methodologies. In particular, changes were made in 

the evaluation of structure containment used in calculating the potential for exposure factor value, in the 

calculation of hazardous waste quantity to include occupied structures in areas of subsurface contamination, and 

in the weighting of targets in areas of subsurface contamination to reflect the type and depth of contamination 

instead of comparing concentrations to benchmarks. In addition, consideration of preferential pathways was 

emphasized in identifying targets in multi-tenant2

 buildings. Furthermore, additional changes were made in 

response to peer review comments to clarify the intended approaches of the proposed addition. These changes are 

also described in the following sections discussing the comments on the aforementioned charge questions. 

1 For the purpose of this document, the term “hazardous substances, pollutants or contaminants” will be referred to simply as 

“hazardous substances.” See section 1.1, of the 1990 HRS for the definition of a hazardous substance.

2 The proposed subsurface intrusion SsI addition to the HRS has undergone revision since it was submitted for peer review in 

2011. The current version of the proposed SsI addition has replaced the term “multi-tenant” with “multi-subunit”. This 

appendix reflects the use of “multi-tenant” in charge questions and peer review comments since that was the language used at 

the time of the peer review. In responses the new, “multi-subunit” term is used for consistency with the proposed addition. 

This change in terminology has no effect on the responses.
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1.0 Introduction

The U.S. Environmental Protection Agency (EPA) is proposing to add a subsurface intrusion (SsI) component to 

the Hazard Ranking System (HRS), which is the principal mechanism that EPA uses to evaluate sites for 

placement on the National Priorities List (NPL). This addition will add to the HRS the consideration of the threat 

posed by exposure to hazardous substances, pollutants or contaminants that enter regularly occupied structures 

through subsurface intrusion. The agency is not considering changes to the remainder of the HRS except for 

minor updates reflecting changes in terminology.

This proposed rule consists of narrow technical modifications to, and an expansion of the scope of, the 1990 HRS. 

The 1990 HRS was previously peer reviewed by the agency’s Science Advisory Board (SAB). The 1988 SAB 

review was comprehensive in scope and addressed the basic structure and concepts of the HRS. This proposed 

addition adheres to the basic structure and concepts of the 1990 HRS, and thus, is consistent with the 

recommendations of the SAB at that time. The 1988 SAB report (Review of the Superfund Hazard Ranking 

System) is available in the public docket for this proposed rulemaking. 

During development of the proposed addition of an SsI component to the HRS, several of the proposed 

approaches for evaluating and assigning HRS factor values warranted external independent scientific peer review. 

These approaches selected for review either represent the application of scientific concepts not addressed in the 

1990 HRS or approaches for which the scientific understanding of has improved since 1988. The review has been 

utilized to ensure the proposed addition of an SsI component to the HRS has a sound and credible basis. 

A peer review panel consisting of six individuals from the academic, government, and private sectors representing 

a range of expertise in public health risk assessments, contaminated soil and ground water investigations and the 

topic of vapor intrusion was assembled to evaluate these questions and provide comments to the EPA. The charge 

questions dealt with the following topics: 

• Charge question 1 addressed the calculation of the potential for exposure factor value.

• Charge question 2 addressed the calculation of the hazardous waste quantity factor value.

• Charge question 3 addressed the evaluation of targets in areas of subsurface contamination.

• Charge question 4 addressed the evaluation of targets and hazardous waste quantity in multi-tenant

buildings.

• Charge question 5 addressed the evaluation of worker targets.

For each element above, a specific charge question was developed for consideration by peer reviewers (these 

charge questions are detailed in section 2, Peer Review Comments on Charge Questions and EPA’s Response, of 

this response document). The proposed addition reflects modifications made as a result of EPA’s peer review 

process. Furthermore, the agency notes that the development of this proposed addition to the HRS underwent 

additional internal review by workgroup participants engaged in developing the proposed addition, EPA’s Office 

of Research and Development, agency senior management and the Department of Defense.

Hazard Ranking System 

The HRS is the principal mechanism used to place uncontrolled waste sites on the NPL. It is a numerically based 

screening system that uses information from initial, limited investigations (e.g., preliminary assessments and site 

inspections) to assess the relative potential of sites to pose a threat to human health or the environment. Placing a 

site on the NPL does not mean the site will require remediation only that based on the limited data available, the 

site poses sufficient risk to warrant further investigation. For further information see the preamble to the proposed 

addition to the HRS. Under the 1990 HRS, four pathways can be scored, which include ground water migration, 

surface water migration, soil exposure, and air migration. The HRS uses a structured value analysis approach to 
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scoring sites. This approach assigns numerical values to factors that relate to or indicate risk, based on conditions 

at the site. The factors are grouped into three categories: 

• Likelihood of Release: likelihood that hazardous substances have been released or have the potential to be

released into the environment;

• Waste Characteristics: characteristics of the waste (e.g., toxicity and waste quantity) that reflect toxicity

and mobility of the released substances and the magnitude of the release; and

• Targets: people or sensitive environments (receptors) actually or potentially affected by the release.

The total score for a site is the sum of all pathway scores. For further detail on the HRS scoring algorithm see

Section 2 of the 1990 HRS (55 FR 51587, December 14, 1990). 

Subsurface Intrusion Component 

The current approach for scoring the ground water, surface water or air migration pathways is not being altered by 

the proposed addition of a subsurface intrusion component. EPA is proposing to add the subsurface intrusion 

threat to the present (1990) soil exposure pathway, as this is the pathway that reflects the risk posed by direct 

exposure to targets. This pathway is proposed to be restructured and renamed the soil exposure and subsurface 

intrusion pathway. The restructured pathway will retain the existing two soil exposure threats (resident population 

and nearby population) in the pathway as one component. The threat posed by subsurface intrusion is proposed to 

be added as a new component (as Section 5.2 of the HRS). 

The present soil exposure pathway addresses direct contact with contamination outside of structures. The new 

subsurface intrusion component also addresses direct contact with contamination that has already been 

demonstrated to have entered into regularly occupied structures or where the contamination is present beneath the 

regularly occupied structures and is likely to enter or likely has already entered. 

The proposed subsurface intrusion component will be added as a new component of the restructured soil exposure 

and subsurface intrusion pathway. The scoring algorithm of the soil exposure component will remain the same as 

for the present soil exposure pathway, but the score will be assigned as a soil exposure component score, not a 

pathway score. The score assigned to the two components will be summed to produce the pathway score. 

The draft subsurface intrusion component, at the time of the peer review process, was proposed to be evaluated 

using the three factor categories that are also used in the 1990 HRS pathways: likelihood of exposure (also called 

likelihood of release for migration pathways), waste characteristics, and targets. These three factor categories, as 

incorporated in the current proposed HRS addition, include factor values, many of which are similar to those used 

in the 1990 HRS pathways and are outlined below: 

• Likelihood of exposure

o Observed exposure: Observed exposure may be established by “direct observation” based on a

solid, liquid or gaseous hazardous substance attributable to the site being observed or known to

have entered a regularly occupied structure from the subsurface (e.g., documented by sampling of

the medium as it intrudes). Or, observed exposure may be established by “chemical analysis”

based on a significant increase in a hazardous substance (relative to the background level)

measured in an indoor air sample taken from a regularly occupied structure, and that at least part

of the significant increase can be shown to be attributable to a release from the site being

evaluated.

o Potential for exposure: Potential for exposure may be evaluated for areas in which subsurface

contamination underlying regularly occupied structures (such as in surficial ground water or soil
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vapor) has been documented, but at which either sampling has not documented that subsurface 

contamination has entered an occupied structure or no sampling of indoor air has been 

undertaken. This evaluation assesses the probability of exposure based on factors such as the 

structural containment features of the occupied structure and the route characteristics within the 

subsurface.

• Waste characteristics

o Toxicity: The toxicity factor reflects the toxicity of a hazardous substance associated with a

release or exposure, and considers the magnitude of each hazardous substance’s acute and chronic

toxicity to humans.

o Degradation: The degradation factor reflects the potential for a substance to degrade within the

subsurface prior to intruding into an overlying structure.

o Hazardous waste quantity: The hazardous waste quantity factor reflects the amount of hazardous

substances that people are currently or may be exposed to (i.e., the amount in regularly occupied

structures).

• Targets

o Exposed Individual: The reasonably maximally exposed individual factor reflects a person living,

attending school or day care, or working on a regular basis in a structure with observed exposure,

or living in a structure within an area of observed exposure (AOE) and/or area of subsurface

contamination (ASC). The assigned value reflects individuals in the eligible target population that

are expected to be exposed to the highest hazardous substance concentration of the released

hazardous substance(s).

o Population: The population factor reflects the number of people qualifying as an exposed

individual or a person living, working, or attending school or day care in a structure within an

AOE and/or ASC; this factor assigns a higher weight to a population exposed to hazardous

substances present, or likely to be present, at higher concentrations.

o Resources: The resources factor reflects the threat posed to populations regularly using structures,

but not accounted for in other target categories. (e.g., populations using libraries, churches, tribal

facilities, recreational structures), located in an AOE or ASC.

Organization of Comments and Responses

The remainder of this summary document is organized into four sections. 

• Section 2: Peer Review Comments on Charge Questions and EPA’s Response. This section contains EPA

responses to comments on subsurface intrusion-specific issues generated by the external independent

scientific peer review; these comments and responses are grouped into six subsections (2.1 through 2.6).

• Section 3: Comments Outside the Scope of the Charge. This section identifies those comments on topics

not included in the charge questions (e.g., comments on the impact of the proposed addition and

comments on changes to HRS Sections 2 and 5).

• Section 4: Editorial Comments.

• Section 5: List of Commenters. This section lists the peer reviewers and their credentials.

The actual charge question submission and accompanying documents and the submissions with the peer 

reviewers’ annotated comments are included in the public docket for the proposed SsI addition to the HRS.
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2.0 Peer Review Comments on Charge Questions and EPA’s Response

The following discussion is a summary of the peer review comments and the agency’s responses to each of the 

five charge questions submitted for peer review. Comments received that were applicable to each individual 

charge question are addressed in the subsections below. In cases where one comment could be considered 

applicable to more than one charge question, the comment has been addressed in the section where it was raised 

by the peer reviewer, and the other applicable subsections have been noted. 

2.1 Charge Question #1: 

Is the Environmental Protection Agency’s (EPA’s) proposed approach scientifically reasonable for 

evaluating and assigning proportional weightings for the following when evaluating potential for 

exposure? If no, what alternative would you recommend? 

a. Consideration of the construction/structure type for which sufficient site-specific information

could be collected during a site inspection for the occupied structure;

b. Distance between occupied structure and depth of contamination;

c. Evaluation of the vertical migration factor based on the thickness and porosity/permeability of

geologic materials in the interval between the lowest point of an occupied structure and the highest

known point of hazardous substances in the subsurface; and

d. Alternative approaches.

Charge question #1 is on the procedure for assigning the potential for exposure factor of the proposed subsurface 

intrusion component addition to the HRS. The potential for exposure factor is one of two ways to assign a value 

for the likelihood of exposure category. The likelihood of exposure reflects the probability that an individual may 

be exposed to hazardous substances via subsurface intrusion into regularly occupied structures. If such an 

exposure has been documented (observed exposure) a value of 550 is assigned. If an observed exposure is not 

documented, the potential for exposure is evaluated. Potential for exposure is based on the physical features of the 

structures and the subsurface materials as well as the characteristics of the volatile hazardous substances at the site 

being evaluated. More specifically, the potential for exposure at a site is evaluated at each area of subsurface 

contamination (ASC), which are areas in which subsurface contamination underlying regularly occupied 

structures (such as in surficial ground water or soil vapor) has been documented, but no observed exposure has 

been documented. 

This evaluation includes assessing the probability of exposure in the ASC based on factors such as structural 

containment features of the occupied structure and the route characteristics within the subsurface. The specific 

factors evaluated for potential for exposure include: structure containment, depth to contamination, vertical 

migration, and vapor migration potential. The equation for combining these factors to produce a potential for 

exposure factor value (subject to a maximum of 500) is: 

a x (b+c+d)

a = Structure containment factor value (Subject to a maximum of 10) 

b = Depth to contamination factor value (Subject to a maximum of 10) 

c = Vertical migration factor value (Subject to a maximum of 15) 

d = Vapor migration potential factor value (Subject to a maximum of 25) 
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This approach includes several factors for which sufficient site-specific information could be collected during a 

site inspection representing the greatest impact on the potential for exposure: structure containment, depth to 

contamination, vertical migration, and vapor migration potential. The 1990 HRS evaluates the potential for a 

release to occur within the ground water, surface water and air migration pathways. The potential for a release is 

determined in these pathways by the consideration of pathway specific factors that aid the site evaluator in 

predicting whether a release is likely to occur. 

Specific comments regarding the factors selected for use in assigning a potential for exposure factor value are 

addressed in the following sections:

• 2.1.a Equation for Potential for Exposure

• 2.1.b Structure Containment

• 2.1.c Depth to Contamination

• 2.1.d Vertical Migration

• 2.1.e Vapor Migration Potential.

2.1.a Equation for Potential for Exposure 

Comment(s): Peer reviewers commented that the draft subsurface intrusion (SsI) addition to the HRS uses 

incorrect or inconsistent factors when evaluating potential for exposure. Peer reviewers questioned:

• Why the factors used to evaluate the potential for exposure factor were selected. For example, the

reviewers also commented that the vapor migration potential should consider more factors than just vapor

pressure and Henry’s constant to better evaluate the greatest health threat at the site.

• The rationale for the relative weightings amongst the different factors.

• Why the potential for exposure equation was selected.

Response: The factors selected for assigning a potential for exposure factor value are deterministic factors 

identified in most proposed models for projecting vapor intrusion, and are likely to be quantifiable at the site 

screening stage when a Hazard Ranking System (HRS) evaluation is likely to be performed. The potential for 

exposure factor value reflects the probability that vapor intrusion into a regularly occupied structure will occur, 

but it does not directly reflect the concentration of the hazardous substances that may intrude. The projection of 

actual indoor concentrations cannot be accurately predicted based on information typically available at sites being 

evaluated using the HRS (i.e., the site inspection stage). 

The four factors included in the potential for exposure assessment are: a structure containment factor, a depth to 

contamination factor, a vertical migration factor and a vapor migration potential factor. The structure containment 

factor reflects whether a volatile substance can enter a regularly occupied structure after the substance has 

migrated from the subsurface to the base of the structure. The assigned containment factor depends primarily on 

the presence of structural features (e.g., preferential pathways such as cracks in a foundation) and mitigation 

actions that have been taken (e.g., active or passive ventilation systems). Two of the other three factors (depth to 

contamination and vertical migration) reflect the possibility of migration of the subsurface contamination to the 

base of the regularly occupied structure and are mainly dependent on the physical environment. The depth to 

contamination factor reflects the distance a hazardous substance has to travel to reach the regularly occupied 

structure; the vertical migration factor reflects the presence of vapor migration routes in the subsurface and 

reflects the effective porosity of the lowest conductive layer between the contamination and the structure, and the 

thickness of this layer. The fourth factor, the vapor migration potential factor, reflects the volatility of the 

hazardous substances and thus their ability to migrate as a vapor or gas. 

Regarding the relative weightings of the different factors, these weightings were assigned based on a sensitivity 

analysis evaluating the importance of these factors in modeling vapor migration through the vadose zone using 
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the Johnson and Ettinger (J&E) Vapor Intrusion Model. This model was selected because of its general 

acceptance as a reasonable predictor for subsurface vapor migration, although it may not accurately reflect sitespecific vapor intrusion rates. The accuracy of the projection may not be high on a site-specific basis but the 

model results are sufficient to reflect the relative likelihood of intrusion occurring at different locations. 

Regarding the structure of the formula for combining the factors into a single value, the structure is that of a first 

order transport equation and is used in all of the HRS pathways. The general form of the formula is used to 

determine potential for a hazardous substance to migrate to targets (and to the receptors associated with the target) 

through various media. The formula was reviewed and found acceptable for this purpose by the Science Advisory 

Board (SAB) when the HRS was revised in 1990; it has proven to be an effective and reasonable approach. 

These comments result in no change to the proposed addition to the HRS. 

2.1.b Structure Containment 

A structure containment value is assigned to a regularly occupied structure to indicate the likelihood for 

subsurface vapors to enter a structure once they have migrated through the subsurface to the base of the structure. 

The lower the assigned value, the less likely subsurface intrusion will occur. The draft SsI addition provided to 

the peer reviewers included a table with structures having different levels of containment. The table identifies 

containment factor values ranging from 0 to 10, with 0 corresponding to total containment (no intrusion likely). 

For example, a regularly occupied structure with an engineered, active vapor mitigation system with documented 

institutional controls or evidence of regular maintenance and inspection will be assigned a lower structure 

containment value than a regularly occupied structure with open preferential pathways from the subsurface. 

This section covers the following topics:

• 2.1.b.1 Structure Containment Factors

• 2.1.b.2 Scientific Basis for Structure Containment Categories

• 2.1.b.3 Consideration of Biogases and Inert Gases in Determining Structure Containment

2.1.b.1 Structure Containment Factors 

Comment(s): Several peer reviewers requested that Table 5-12, Structure Containment, of the draft SsI addition to 

the HRS be revised to include additional information related to factors that can influence the evidence of structure 

containment and the related assigned value. Reviewers also suggested that structure containment categories in 

Table 5-12 be revised to represent a broader range of structures that could be encountered, as typical home 

construction includes containment structure values that fall between the structure containment values represented

in the Table. 

Response: In response to peer review comments, EPA has modified Table 5-12, Structure Containment, in the 

proposed addition to the HRS. While the structure containment table has been revised to reflect peer review 

comments, given the wide variety of possible building structures encountered, site-specific conditions will always 

have to be considered and addressed as they arise. However, a new category has been added to the table and other 

categories were modified. A new Category No.8 has been added to include regularly occupied structures with 

unknown containment features and is assigned a value of 2 out of a possible 10. This value was assigned because, 

as noted in peer reviewer comments, recent studies show that most structures located over a significant subsurface 

contaminant plume are likely to experience subsurface intrusion unless they have specific design features that 

prevent it. Additionally structure containment categories in Table 5-12 have been modified to not consider 

differences in construction type; the peer reviewers’ indicated that studies have not found construction type to be 

a discriminating factor in whether subsurface intrusion can occur. However, regularly occupied structures with a 

foundation raised greater than six feet above ground still receive a structure containment factor value of 0. 
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Further, given the large number of possible combinations of containment features, it is impossible to include a 

single category in the table for each possible combination type. Therefore, interpretation of the table will be based 

on site-specific conditions. If a structure appears to fit multiple categories, the highest applicable structure 

containment factor value is assigned.

2.1.b.2 Scientific Basis for Structure Containment Categories 

Comment(s): One peer reviewer suggested that containment features in Table 5-12, Structure Containment, of the 

draft SsI addition to the HRS may not be as protective as the assigned values suggest and commented that, in the 

absence of definitive studies that show the effectiveness of containment features, Table 5-12 should be reduced to 

three structure containment values. The reviewer specifically commented that EPA’s Empirical Database does not 

show any significant impact of foundation type on indoor air quality. 

Another peer reviewer commented that the structure containment value assigned in Table 5-12 to vented crawl 

spaces, which have fairly small attenuation factors, is too high and should be lowered. 

Response: In developing the proposed structure containment table, after a review was done of available literature, 

EPA identified multiple structural and mitigation features as possibly influencing the likelihood that volatile 

hazardous substances present at the base of a regularly occupied structure could enter into the structure. These 

features include: documented evidence that gases or vapors have been entering the structure; construction 

characteristics such as the presence of a basement, a slab foundation, or an enclosed crawl space; the presence of 

preferential pathways such as foundation cracks, utility conduits, open sumps, and unsealed crawl space access

locations; and the presence of passive and active mitigation measures such as vapor barriers, passive and active 

ventilation systems, along with evidence that the systems are operating. 

The structure containment table submitted to the peer reviewers was developed using an approach that reflected 

the physical presence of features that theoretically could affect the occurrence of vapor intrusion. The higher the 

containment value assigned, the greater the likelihood that vapor intrusion would occur. Regularly occupied 

structures with evidence of intrusion or evidence of open preferential pathways are assigned the highest value, and 

structures with no actual migration route from the subsurface to the structure or structures containing effective 

active mitigation systems are assigned a lower value indicating the low likelihood of intrusion. As stated by a peer 

reviewer, no studies that focused on documenting the impacts of the features individually or in combination were 

identified or reviewed in the creation of Table 5-12, and for that reason, among others, this table was submitted 

for peer review input.

EPA has modified the assigned values in Table 5-12 of the proposed addition to the HRS regarding passive vapor 

mitigation systems. While there may be little published evidence on the effectiveness of passive systems, as these 

systems at least provide a possibly less resistant route for vapors to migrate, passive systems remain as a factor in 

the structure containment evaluation, and the agency considers their inclusion reasonable. The table has been 

modified to assign different values to regularly occupied structures depending on whether information is available 

to document that the passive systems are maintained, functioning or effective. The presence of a passive system 

alone does not result in the minimum containment value being assigned (i.e., that intrusion will not occur). 

Additionally, EPA has modified Table 5-12 to not consider foundation type, except for those structures with 

foundations raised greater than 6 feet from the land surface or that have been built and maintained in a manner to 

prevent subsurface intrusion, when assigning a structure containment factor value. The agency also removed 

classifications based on structure type (e.g., structure with enclosed crawl space). This reflects that, as pointed out 

by the peer reviewers, there has been little consistent correlation between construction type (e.g., structure with a 

crawl space) and subsurface intrusion in existing studies. Crawl spaces were considered as a possible special case, 

but the construction of crawl spaces and partially enclosed spaces and the efficiency of passive venting systems 
14

can be highly variable, therefore crawl spaces did not receive specific consideration in Table 5-12 of the proposed 

addition to the HRS. 

Instead of structure type, consideration is given to the presence of passive/active vapor mitigation/vapor migration 

barrier systems and open preferential pathways from the subsurface. (Note that crawl spaces are often 

accompanied by openings to the structure by hatches, vents, etc., that would be considered preferential pathways.) 

As mentioned previously, site-specific conditions will be addressed as they arise during an evaluation of the 

structure containment factor value. 

While Table 5-12 has been modified based on peer review comments, certain features of the table have been 

retained and the number of structure containment categories was not reduced as suggested by one peer reviewer. 

2.1.b.3 Consideration of Biogases and Inert Gases in Determining Structure Containment 

Comment(s): One peer reviewer questioned the usefulness of considering evidence of intrusion of biogases and 

inert gases, as referenced in Table 5-12 of the draft SsI addition to the HRS, to document structure containment. 

The reviewer added that evidence of these intrusions is not typically gathered during short inspections. 

Response: The text associated with Category No.1 in Table 5-12, Structure Containment, was modified to clarify 

that evidence of intrusion, including by bio or inert gases, provides support for evaluating a regularly occupied 

structure's containment value. EPA notes that evidence of intrusion by these gases (e.g., methane, radon) is often 

monitored in various regions of the country, and can be measured fairly rapidly using existing field equipment. 

While this information may not always be readily available during a typical site inspection, the agency considers 

it reasonable to use such information when it is available. When it is not available, the table identifies other 

features that can be used to assign a structure containment factor value.

2.1.c Depth to Contamination

The depth to contamination factor represents the vertical distance between contamination (e.g., in soil gas or 

surficial ground water) and the lowest horizontal point of a regularly occupied structure (e.g., a basement floor). 

This distance represents how far a hazardous substance would have to travel through the subsurface to intrude into 

that structure. This distance is proportional to the time during which the concentration of the vapors can be 

reduced through diffusion. For example, a contaminated crawl space sample would be assigned the maximum 

factor value of 10, while a contaminated soil gas sample at a depth of 12 feet could receive a factor value of 8 

(depending on the presence (or absence) of channelized flow features). Hence, the probability of significant 

exposure decreases as this depth increases. The proposed depth to contamination factor values range from zero to 

ten, where increasing depth results in a lower factor value. A value for depth to contamination is assigned based 

on the least depth to contamination underlying a regularly occupied structure. 

Peer reviewers’ specific comments on depth to contamination are addressed in the following subsections: 

• 2.1.c.1 Definition of Depth to Contamination

• 2.1.c.2 Scientific Basis for Depth to Contamination Factor

• 2.1.c.3 Depth and Probability of Intrusion

• 2.1.c.4 Method Used for Assigning Depth to Contamination Weighting Values

2.1.c.1 Definition of Depth to Contamination 

Comment(s): One peer reviewer requested that a clarification of the meaning of “depth to contamination” be 

provided in the draft SsI addition to the HRS. 
15

Response: EPA has incorporated this suggestion by clarifying the meaning of "depth to contamination" in the 

proposed addition to the HRS. Depth to contamination is measured based on the distance between the lowest 

point of a regularly occupied structure to the highest known point of an eligible hazardous substance (see Section 

5.2.1.1.2.2 of the proposed addition to the HRS). Additionally, if the evaluation is based on semi-enclosed or 

enclosed crawl space samples, a depth to contamination factor value of 10 is assigned. If more than one depth to 

contamination distance can be estimated, the lowest depth measurement is used in the HRS evaluation. 

2.1.c.2 Scientific Basis for Depth to Contamination Factor 

Comment(s): Several peer reviewers raised questions about the scientific basis for including a depth to 

contamination factor and questioned the associated range of assigned values. One peer reviewer commented that, 

due to the information likely to be available at most sites, the depth to contamination factor will likely be based 

upon depth to ground water; the reviewer asserted that modeling and empirical data have not shown a correlation 

between the depth to ground water and the impact to vapor intrusion rates as it is suggested in Table 5-13, Depth 

to Contamination, of the draft SsI addition to the HRS. Reviewers specifically commented on the values assigned 

to the depths to contamination in Table 5-13 and included the following comments: 

• Depths to at least 50 feet in Table 5-13 should all be assigned the same value as there is no evidence that

there is a reduction in exposure potential below 20 feet.

• The maximum depth in Table 5-13 should be 100 feet and the assigned value should be 0 for

contamination below 100 feet.

Additionally, one peer reviewer commented that the depth to contamination is not as important a factor in 

predicting subsurface intrusion as whether or not a chemical is aerobically biodegradable. 

Response: The depth ranges selected for Table 5-13, Depth to Contamination, of the proposed addition, were 

based on projecting indoor air contaminant levels using the J&E Vapor Intrusion Model 3 and data from EPA’s 

Vapor Intrusion Database. The J&E Vapor Intrusion Model projects that depth to contamination is a significant 

factor in predicting vapor intrusion into an occupied structure. Furthermore, an EPA evaluation of EPA’s Vapor 

Intrusion Database4

, as of 2012, documents that depth to contamination is a significant factor. Therefore, depth to 

contamination is being retained as a factor in the potential for exposure evaluation in the proposed addition to the 

HRS. EPA notes however, that the likelihood of exposure factor is only one of many factors used in the HRS 

evaluation, but because it a multiplying factor in the HRS formula, even a small drop in this depth to 

contamination factor value requires a significant increase in other factors for a site to qualify for the NPL. 

Regarding biodegradation, EPA has incorporated a degradation factor into the proposed addition, but not in the 

evaluation of the potential for exposure factor. This degradation factor takes into account the depth to 

contamination, the half-life of the substance and the parent-daughter relationship of breakdown/decay products. 

Because degradation is both substance-specific and site-specific, it was determined that it would be appropriate to 

evaluate the degradation potential in the HRS consideration of contaminant characteristics (i.e., the waste 

characteristics factor category value). This addition of a degradation factor is consistent with EPA's general 

3 EPA On-line Tools for Site Assessment Calculations. Screening Level Implementation of the Johnson and Ettinger Vapor 

Intrusion Model. http://www.epa.gov/vaporintrusion/epa-spreadsheet-modeling-subsurface-vapor-intrusion. U.S. 

Environmental Protection Agency, Ecosystems Research, Athens, Georgia.

4 EPA’s Vapor Intrusion Database: Evaluation and Characterization of Attenuation Factors for Chlorinated Volatile Organic 

Compounds and Residential Buildings. Dr. Helen E. Dawson, U.S. Environmental Protection Agency's Office of Superfund 

Remediation and Technology Innovation (OSRTI), served as the principal author, with Dr. Richard B. Kapuscinski, USEPA. 

OSRTI, as coauthor. March 16, 2012.
16

approach for addressing vapor intrusion in both EPA's Office of Land and Emergency Management (OLEM)5

 and 

Office of Underground Storage Tanks (OUST)

6

 programs. 

Regarding the peer reviewers’ comments that the depth to contamination assigned values in Table 5-13 of the 

proposed HRS should be revised, sufficient evidence exists to predict that there is a difference in contaminant 

migration potential between the depth categories listed in Table 5-13. 

Regarding the reviewers’ concerns on the correlation of depth to ground water and the impact on vapor intrusion 

rates, the HRS evaluation must be based on data available at the time of a site inspection. At that time indoor air 

measurements, soil gas and ground water analytical data may not be available at all possibly threatened structures. 

While contaminated ground water may or may not be as representative of the vapor intrusion risk as soil gas, the 

presence of volatile substances in ground water, at a minimum, certainly represents a higher risk than sites with 

no such information. Therefore, both the depth to contamination and degradation factors have been retained. 

2.1.c.3 Depth and Probability of Intrusion

Comment(s): Several peer reviewers commented that the probability of intrusion will depend more on the media 

and the lithology of the subsurface (including whether preferential manmade and natural pathways are present) 

underlying a site than the actual depth to contamination. A reviewer recommended including the consideration of 

manmade preferential pathways in the draft SsI addition. One reviewer pointed out that if a ground water plume 

was present at the water table, the corresponding soil gas contamination measurements would always be 

shallower than ground water samples. The reviewer stated that this would affect the accuracy of values presented 

in Table 5-13. Additionally, one reviewer commented that the depth to contamination factor (specifically Table 5-

13) should prioritize the intrusion of liquids into structures over structures that do not have a documented

intrusion.

Response: The proposed depth to contamination factor reflects how much farther the contaminant has to travel to 

enter a regularly occupied structure, and does not reflect the sample media type. It is not used if there is evidence 

of intrusion into regularly occupied structures already collected at a site. The impact of lithology and media is 

considered in the assignment of a different HRS factor value (i.e., vertical migration factor) used in reflecting the 

probability of intrusion. The probability of intrusion is certainly relatively higher if, for example, there is evidence 

of contamination at a depth of 15 feet rather than 20 feet, regardless of sample media type, given there is less 

distance for the contamination to travel. If other sampling results become available, the site could be rescored. 

Regarding preferential pathways, EPA has incorporated the suggestion to include manmade preferential pathways 

in the consideration of channelized flow’s impact on the potential for vapor intrusion (see Section 1.1 of the 

proposed addition to the HRS). Regarding lithology, its impact on vapor migration is represented by a separate 

factor; the vertical migration factor. 

Regarding the prioritization of the intrusion of liquids into occupied structures, EPA notes that if a regularly

occupied structure under the commenter's scenario is documented to have an eligible hazardous substance(s) 

intruding via surficial ground water, the likelihood of exposure factor category would be scored based on the 

presence of an observed exposure. This would result in a higher likelihood of exposure factor category value than 

if it was scored using the potential for exposure factor category evaluation. 

5 United States Environmental Protection Agency. (2015). OSWER Technical Guide for Assessing and Mitigating the Vapor 

Intrusion Pathway from Subsurface Vapor Sources to Indoor Air. Office of Solid Waste and Emergency Response. June 

2015. (EPA Publication No. 9200.2-154). 6 United States Environmental Protection Agency. (2015).Technical Guide for Addressing Petroleum Vapor Intrusion at 

Leaking Underground Storage Tank Sites. Office of Underground Storage Tanks (OUST).
17

2.1.c.4 Method Used for Assigning Depth to Contamination Weighting Values 

Comment(s): One peer reviewer commented that the method for assigning weighting values to the different depths 

to contamination in Table 5-13 of the draft SsI addition to the HRS should be related to changes in diffusive flux 

based on Fick’s Law. 

Response: EPA considers the J&E Vapor Intrusion Model7

 to be sufficient to provide a relative projection of the 

importance of different depths to contamination in determining the potential for vapor intrusion into a regularly 

occupied structure. Furthermore, EPA has contrasted the effects of weighting depth to contamination factors using 

Fick's Law and the J&E Vapor Intrusion Model and compared the resulting predictions. The use of either model 

would result in the assignment of similar factor values, as both result in similar proportional values (e.g., both 

vary in the same direction with changes in common factors) and would result in similar relative rankings amongst 

sites. Therefore, the agency decided not to alter the basis for assigning weighting values to the range of depths 

presented in Table 5-13 of the proposed addition based on Fick’s Law. 

These comments result in no change to the proposed addition to the HRS. 

2.1.d Vertical Migration

The vertical migration factor considers the geologic makeup of materials between the base of a regularly occupied 

structure and the hazardous substance plume and the rate in which substances are likely to have moved through 

the geologic materials. The agency proposes to evaluate vertical migration based on two factors: effective porosity 

(or equivalently, permeability) of geologic materials and the thickness of the lowest porosity layer. 

The range of possible factor values for the effective porosity of geologic materials is based solely on the typical 

porosity of subsurface materials (e.g., gravel, sand, silt and clay). These factor values are used in conjunction with 

the thickness of the applicable lowest porosity layer to establish a vertical migration factor value. 

Specific peer reviewer comments on vertical migration are addressed in the following sections: 

• 2.1.d.1 Use of Term “Effective Porosity/ Permeability”

• 2.1.d.2 Effective Porosity/ Permeability Categories

• 2.1.d.3 Scientific Basis for Vertical Migration Weightings

• 2.1.d.4 Effect of Soil Type and Moisture Content on Vertical Migration Values

• 2.1.d.5 Hydraulic Pumping

2.1.d.1 Use of Term “Effective Porosity/ Permeability” 

Comment(s): One peer reviewer commented that the use of the term “effective porosity/permeability” to evaluate 

vertical vapor migration is unnecessary and the term “soil type” should be used instead. 

Response: Studies have demonstrated that effective porosity, which largely reflects moisture content, is an 

important factor in projecting vapor intrusion. However, long-term measurements of effective porosity are beyond 

the scope of a typical time-limited site inspection in that they vary with time, climate and depth. The HRS uses 

soil type as a surrogate for effective porosity, in that soil type reflects at least in part long-term moisture porosity 

7 A description of the basic model can be accessed at http://www.epa.gov/vaporintrusion/epa-spreadsheet-modelingsubsurface-vapor-intrusion
18

conditions. The term “soil type” was not adopted as a substitute name to reflect that it is only a surrogate for 

effective porosity.

This comment results in no change to the proposed addition to the HRS. 

2.1.d.2 Effective Porosity/ Permeability Categories 

Comment(s): One peer reviewer commented that Table 5-14, Effective Porosity/Permeability of Geologic 

Materials, of the draft SsI addition to the HRS contains two categories that are better categorized as one material

type. The reviewer stated that both the silt and sand material categories “show little difference in vapor migration 

across the soil types.” 

Response: The silt and sand categories have been retained. Calculations using an EPA version of the J&E Vapor 

Intrusion Model8 indicate there are differences in vapor migration rates due to effective porosity differences in silt 

and sand soil types. Furthermore, insufficient real-world data could be found to be confident that there is no 

difference in vapor intrusion rates due to the two soil types. Moreover, the soil type classification system in the 

proposed addition to the HRS has been used in HRS evaluations for the ground water migration pathway and the 

data needed to classify soil type at a location have been found to be readily available. 

This comment results in no change to the proposed addition to the HRS. 

2.1.d.3 Scientific Basis for Vertical Migration Weightings 

Comment(s): One peer reviewer commented that the vertical migration factor value associated with clays is too 

high relative to the values used to weight other geologic materials. The reviewer commented that “true clays and 

unfractured rocks” have extremely low porosities and are effective barriers to vapor transport, and recommended

lowering the migration factor values for clay materials in Table 5-15, Vertical Migration Factor Values, of the 

draft SsI addition to the HRS. 

Response: The difference in the present vertical migration weightings between typical clays and rock layers and 

other more porous geologic material layers is sufficient to reflect a relative difference in vertical migration rates 

and increases with the thickness of the layers. This difference between layer types has not been increased because 

not all clay and rock layers are competent horizontally under an entire site. Competent clay layers and other low 

porosity materials can be very effective vapor barriers, especially when they are thick and continuous over a large 

area. However, the horizontal continuity of the layers can often be incomplete (e.g., differential depositions during 

formation, erosion after deposition, or manmade preferential pathways limit the horizontal continuity) and allow 

for vapor migration through the resulting gaps. Similarly, bedrock often contains cracks and fractures that

function essentially as open channels for vapor migration. This phenomenon is especially important when these 

layers are relatively thin; therefore, Table 5-15, Vertical Migration Factor Values, of the proposed addition to the 

HRS assigns a high value in these situations. However, at greater thicknesses these layers tend to be more 

competent over a wider area, and thereby hinder vapor migration more effectively, and are assigned a lower value 

in these situations. Because the competence of the layers is not always known at the time of an HRS evaluation, 

the factor values were designed to consider this possibility. Regarding the values relative to those assigned to 

other geologic materials, these values were based on simulations run using the J&E Vapor Intrusion Model. While 

this model may not be site-specifically accurate, the predictions based on it are considered sufficiently accurate 

for relative comparisons between sites. 

8 Johnson and Ettinger (1991) Model for Subsurface Vapor Intrusion into Buildings. http://www.epa.gov/vaporintrusion/epaspreadsheet-modeling-subsurface-vapor-intrusion. U.S. Environmental Protection Agency
19

Furthermore, competent clay layers are likely to also influence the depth to contamination factor value as well as 

the vertical migration factor value. The presence of any low porosity layer would likely also correspond with a 

greater depth to contamination factor value and both together will further lower the potential for exposure factor 

value. 

These comments result in no change to the proposed addition to the HRS. 

2.1.d.4 Effect of Soil Type and Moisture Content on Vertical Migration Values 

Comment(s): Peer reviewers commented that differences in soil type (effective porosity/permeability) would not 

result in significant impacts on migration factor values. However, reviewers commented that soil moisture content 

or the presence of perched water should be accounted for in the vertical migration factor values as it can have a 

significant impact on the diffusive/advective flux. 

Response: EPA used a version of the J&E Vapor Intrusion Model to weight the assignment of vertical migration 

factor values based on effective porosity and thickness of the lowest porosity layer. Additionally, EPA’s Vapor 

Intrusion Database, as of 2012, shows that soil type is a significant factor in datasets that include this parameter.

EPA notes that soil moisture content can be a significant factor, but after discussion with subject matter experts, 

determined that soil moisture content over time could not be effectively represented by the limited data available 

from a typical short-term site inspection during the site assessment phase of the CERCLA process. It was decided 

that since soil type is a function of climate and is available for most locations, it is a practical (although imperfect) 

relative indicator of effective porosity and soil moisture over time. Additional consideration of how soil moisture 

can impact the potential for subsurface intrusion is discussed in the proposed HRS addition's Technical Support 

Document (TSD). Regarding perched water, which can also be a significant factor, given the short duration of 

sampling events for HRS studies; it will not be possible to predict the transient nature of perched water or its 

effects. Furthermore the possible effects of the presence of perched water on vapor migration are reflected in the 

potential for exposure equation in that the cause of the perched water, such as a clay layer, is reflected in the 

evaluation. 

These comments result in no change to the proposed addition to the HRS. 

2.1.d.5 Hydraulic Pumping 

Comment(s): Peer reviewers commented that hydraulic pumping of vapors into structures should be considered in 

determining the vertical migration factor value. 

Response: It is likely that if significant hydraulic pumping is occurring in an area, the potential for intrusion 

would be greater than in other areas. However, at this time the relative importance of this phenomenon and factors 

that would influence its occurrence are not well documented, and so a special consideration for this occurrence 

has not been incorporated into the proposed addition to the HRS. The proposed potential for exposure formula 

would to some extent account for this situation, as hydraulic pumping impacts of vapor intrusion rates would 

likely be occurring in areas with shallow ground water (and thus shallow depths to contamination) and high 

porosity soils. Shallow ground water and high porosity soils are both factors corresponding to a high potential for 

exposure factor. 

These comments result in no change to the proposed addition to the HRS. 
20

2.1.e Vapor Migration Potential 

Comment(s): Several peer reviewers questioned the use of an average of vapor migration potential values in Table 

5-18, Vapor Migration Potential Factor Values, of the draft SsI addition to the HRS, to determine the assigned 

value for the vapor migration potential factor value. 

Response: The vapor migration potential factor is meant to reflect the volatility of a substance in the subsurface at 

a site. EPA has revised Section 5.2.1.1.2.4 of the proposed addition to the HRS by assigning the vapor migration 

potential factor value based on the most volatile substance associated with each ASC being evaluated. However, 

this value will be overridden with a higher value if the depth to contamination associated with the ASC being 

evaluated is documented to be 10 feet or less. The ASC would be assigned a vapor migration potential factor 

value of 25 (the maximum value). This value is assigned if the hazardous substance is both associated with an 

ASC (i.e., meets observed release criteria) and is present at a depth of 10 feet or less beneath a regularly occupied 

structure (i.e., at a depth where preferential pathways are likely present). This assignment reflects that the 

substance has migrated through the subsurface environment and is extremely likely to enter a regularly occupied 

structure.

2.1.f Biodegradation/Bioattenuation

Comment(s): One peer reviewer identified that biodegradation can be a significant factor in determining the 

potential for vapor intrusion and commented that biodegradation should be accounted for in the draft SsI addition 

to the HRS, either as a separate factor or as part of other potential for exposure factors. Specifically, it was

commented that there should be a distinction between chemicals that readily bioattenuate (i.e., biodegrade) and 

chemicals that do not bioattenuate. One reviewer suggested including a bioattenuation factor. 

Response: Biodegradation can play a major role in subsurface intrusion and, in response to peer review 

comments, EPA has added a degradation factor in the determination of the waste characteristics factor category 

value, but not in the potential for exposure evaluation. This addition of a degradation factor is consistent with 

EPA’s approach for addressing vapor intrusion in both EPA’s OLEM9 and OUST10 programs. Whether a 

substance will degrade depends on both the substance and site-specific conditions, e.g., while the ability of a 

substance to degrade in the environment can be predicted based on its characteristics, whether it actually degrades 

is dependent on site-specific conditions. The site-specific conditions that can affect the rate of degradation can be 

extremely variable in time and location (e.g., oxygen content, moisture, soil matrix) and it is beyond the scope of 

a short-term site inspection to measure or predict these conditions with accuracy. Thus, a degradation factor was 

added to the waste characteristics category, which reflects the chemical properties of the substances at a site.

The degradation factor is based on the substance being evaluated, whether there is sufficient depth to 

contamination at the site to ensure degradation of the substance, the half-life of the substance, and the parentdaughter relationship of breakdown and decay products. The degradation factor also considers the difference in 

half-lives of different hazardous substances, e.g., between petroleum or petroleum-like hazardous substances, 

which have the greatest potential to biodegrade, the presence of biologically active soil, and the depth to 

contamination. (See Section 5.2.1.2 and applicable subsections of the proposed addition to the HRS for further 

detail.) The inclusion of a degradation factor will result in a lower overall HRS site score for substances in an area 

of subsurface contamination (ASC) that are known to readily biodegrade at sites where the contamination is likely 

to be in a biologically active subsurface layer of sufficient depth to support degradation of the substance, or if a 

parent substance is present and is not constantly regenerating the daughter substance. 

9 See footnote 4.

10 See footnote 5.
21

2.1.g Consideration of Volatile Substances

Comment(s): One peer reviewer suggested that the draft SsI addition to the HRS be modified to consider the 

absence of vapor intrusion risks associated with high molecular weight semi-volatile compounds. 

Response: The recommendation to have a potential for exposure value of zero if no volatile substance is present 

has been implemented by adding the instruction to not assign a potential for exposure factor value if no volatile 

hazardous substances are present. With this change, only those substances with a vapor pressure greater than or 

equal to 1 torr or a Henry’s Law constant of 10-5 atmosphere-meter cubed per mole may be considered when 

evaluating the potential for exposure factor value; however, substances meeting observed exposure criteria will 

continue to be used for delineating an area of subsurface contamination. This achieves the recommendation but 

maintains the potential for exposure mathematical structure which is consistent with that used for equivalent 

calculations in other parts of the HRS (e.g., potential to release to ground water). Thus, the change also maintains 

the appropriate balance between the pathways and components. 
22

2.2 Charge Question #2: 

Is EPA’s approach of basing the hazardous waste quantity on the amount of contaminants that could 

be found in the intruded structures appropriate? If no, what alternative approach would you 

recommend? 

The hazardous waste quantity factor value in the proposed subsurface intrusion component reflects the amount of 

hazardous substances to which targets are exposed. The hazardous waste quantity is based on the amount of 

contaminants that could be found in the intruded structures, reflecting the subsurface intrusion component’s focus 

on exposure. This follows a similar approach to calculating hazardous waste quantity as the existing soil exposure 

pathway. In the 1990 HRS soil exposure pathway, the hazardous waste quantity does not include the total amount 

of hazardous substances in all site sources (as would be the case for the ground water, surface water, and air 

migration pathways), but rather is based on the amount of hazardous substances to which a target can be exposed 

where actual contamination has been documented.

2.2.a Support for Method

Comment(s): Peer reviewers agreed with the approach for calculating the hazardous waste quantity and agreed 

that it was a reasonably conservative approach. 

Response: The agency has retained the proposed approach that bases the hazardous waste quantity on the amount 

of contaminants that can be found in the intruded structures. However, in response to peer review comments, EPA

has included the consideration of the probable exposures within regularly occupied structures located within 

ASCs in estimating the site hazardous waste quantity. The procedure for estimating the hazardous waste quantity 

is the same as used in the other HRS pathways and components for estimating hazardous waste quantity. 

2.2.b Considering Waste Quantity above Areas of Subsurface Contamination

Comment(s): One peer reviewer commented that only the hazardous substances in areas of observed exposure are 

included in the calculation of the hazardous waste quantity and suggested that hazardous substances in areas of 

subsurface contamination be included in the calculation of the hazardous waste quantity. 

Response: The method for estimating the hazardous waste quantity factor value has been modified to reflect the 

amount of hazardous substances that may be entering regularly occupied structures within an ASC. The hazardous 

waste quantity factor value for the subsurface intrusion component is meant to reflect the amount of hazardous 

substances that populations are likely to be exposed to via subsurface intrusion. 

EPA agrees with the peer reviewer that populations are likely to come into contact with intruded hazardous 

substances in an ASC. Areas of subsurface contamination are delineated by sample locations meeting observed 

release criteria for subsurface intrusion, which document the presence of volatile hazardous substances, and EPA 

studies have shown it is possible, if not probable, that these substances will enter the structures in these areas. 

Therefore, EPA has included consideration of the probable exposures within these structures within an ASC in 

estimating the site hazardous waste quantity. Not including these areas may significantly underestimate the 

amount of hazardous substances that populations are exposed to from the subsurface. 

2.2.c Considering Waste Quantity of Multiple Structures 

Comment(s): One peer reviewer questioned the method for determining a source hazardous waste quantity value 

in the case where multiple homes were located above a large plume. The reviewer questioned how the structures 

above an ASC would be evaluated to determine the hazardous waste quantity. 
23

Response: All homes (and/or other regularly occupied structures) located in areas of observed exposure (AOE)

(and as modified to reflect peer review comments, in ASCs) are considered in estimating the hazardous waste 

quantity. The waste quantity assigned to each structure within all AOEs or ASCs is summed and the resulting 

value is used to determine the component hazardous waste quantity factor value. 

2.2.d Including Subsurface Contamination in the Waste Quantity

Comment(s): One peer reviewer commented that the hazardous waste quantity value calculation should reflect the 

quantity of hazardous substances in the subsurface as well as in buildings. 

Response: The hazardous waste quantity value for this proposed component reflects the amount of intruded 

hazardous substances that people may be exposed to when inside regularly occupied structures. Not all of the 

contamination in the subsurface will enter occupied structures, as most will be vented directly to the atmosphere. 

Thus, including the quantity of all hazardous substances in the subsurface could greatly overestimate the amount 

that will result in indoor exposure. Although the entire quantity of hazardous substances in the subsurface is not 

included in the estimation of hazardous waste quantity, the consideration of regularly occupied structures in areas 

of subsurface contamination in this calculation would reflect that some of the contamination would enter these 

structures. (EPA notes, however, that if the amount venting to the atmosphere is sufficient to pose a significant 

threat, this threat can be considered in a different HRS pathway evaluation, the air migration pathway, as it is a 

release directly to the atmosphere.) 

This comment results in no change to the proposed addition to the HRS. 

2.2.e Minimum Waste Quantity 

Comment(s): Peer reviewers questioned the relevance of using the ratio of subslab sample concentrations to 

health-based benchmarks in the calculation of a minimum hazardous waste quantity factor value. One reviewer 

suggested that subslab samples may be influenced by indoor air sources and are not necessarily a reliable 

indicator of vapor intrusion potential. Another reviewer suggested modifying the criteria for crawl spaces in the 

calculation of a minimum hazardous waste quantity factor value based upon EPA’s attenuation factor for crawl 

spaces. 

Response: EPA has removed the consideration of subslab and crawl space sample concentrations when 

determining a minimum hazardous waste quantity. Research has shown that the rate of vapor intrusion is 

extremely variable over time, and the variability of values from crawl space and subslab samples even from a 

single structure during a single sampling event is significant. Therefore the actual quantity of contamination via 

vapor intrusion into a structure may not be accurately represented using crawl space or subslab samples from a 

short-term sampling event such as those used for HRS evaluation purposes. 

Regarding the possibility of indoor air sources impacting subsurface contamination, this possibility is reflected in 

another part of the HRS evaluation: when identifying an actual observed exposure due to subsurface intrusion. 

This identification requires attributing the release to subsurface contamination, and not to indoor sources.

2.2.f Relevance of Waste Quantity 

Comment(s): One peer reviewer questioned the relevance of the hazardous waste quantity in determining the 

relative risk associated with hazardous substances migrating via the subsurface intrusion component. The 

reviewer commented that the hazardous waste quantity has little relevance to vapor intrusion impacts, and 

suggested that the HRS use the duration of time that a vapor source will produce vapor concentrations.
24

Response: The hazardous waste quantity factor serves as a surrogate for contaminant concentration and the extent 

of contamination. The decision to include waste quantity in all pathways and components in the HRS algorithm 

was made when the HRS was last revised in 1990 (see Section V.3 of the proposed 1988 HRS, 53 FR 51692, 

December 23, 1988; Section III.C of the 1990 HRS, 55 FR 51542, December 14, 1990). The decision was based 

on the concept that determining an accurate dose and length of exposure for the waste quantity estimate was 

beyond the scope of information available after a site inspection. Instead, hazardous waste quantity is used as a 

surrogate for these two factors. The HRS is only predicting that with a high hazardous waste quantity, it is more 

likely an exposure will occur and that people will be exposed to a high dose. 

The inclusion of hazardous waste quantity in the subsurface intrusion component is consistent with the other 

existing HRS pathway evaluations, allowing the ranking of the relative risk posed among different sites with 

different migration and exposure pathways (and thus consistent with the goal that the new component not affect 

the overall HRS site scoring algorithm). 

Furthermore, regarding the reviewer’s suggestion, it is not possible to predict with confidence the duration of a 

vapor intrusion episode based on a typical time-limited site inspection. However if an estimate with reasonable 

confidence can be developed for how much vapor intrusion has occurred over time, it can be used to estimate a 

wastestream quantity (see discussion of Tier B hazardous waste quantity estimate in Sections 2.4.2.1.2 and 

5.2.1.2.2 of the proposed addition to the HRS) that can in turn be used to assign the hazardous waste quantity 

factor value for the component. 

These comments result in no change to the proposed addition to the HRS. 

2.2.g Consideration of Source Concentrations in Estimating Waste Quantity

Comment(s): One peer reviewer commented that the calculation of the hazardous waste quantity should include 

consideration of source concentrations. 

Response: While it would result in a better estimate of risk to have the HRS algorithm reflect the concentration of 

hazardous substances at a site, particularly for subsurface intrusion, it is unlikely that representative site 

contaminant concentrations could be obtained using data collected during a typical time-limited site inspection.

Studies have shown that the rate of vapor intrusion varies significantly over time and this range cannot be 

estimated from a limited site inspection. Therefore, it is not likely that a measurement taken during a short time 

period would be accurate in predicting dose or actual waste quantity. 

However, if the concentration information necessary to calculate the actual mass of hazardous substances that 

individuals are exposed to is available, the HRS has a procedure to use this information to estimate the actual 

mass (see discussion of Tier A hazardous waste quantity estimate in Sections 2.4.2.1, 2.4.2.1.1 and 5.2.1.2.2 of 

the proposed addition to the HRS). 

EPA notes, however, that in another part of the HRS algorithm, the evaluation of targets, a higher weighting is 

assigned to targets (and, correspondingly, a higher site score) if the concentration of one or more hazardous 

substances found during a sampling event exceeds an appropriate health-based benchmark (see Sections 2.5 and 

5.2.1.3 [and all applicable subsections] of the proposed addition regarding Level I actually exposed targets). 

Regarding considering contamination in both the subsurface and in regularly occupied structures, the proposed 

addition is an exposure component (i.e., the threat of exposure to contamination where it is presently located is 

considered, as contrasted with the threat resulting from migration of the contamination, a migration component); 

therefore, the hazardous waste quantity factor value reflects the amount of contamination a target(s) may be 

exposed too. Including an estimation of subsurface contaminant sources in the calculation of hazardous waste 

quantity could overestimate the amount of hazardous substances a population is exposed to, as not all of the 
25

contamination will enter occupied structures (e.g., venting directly to the atmosphere instead). Although the 

quantity of hazardous substances in the subsurface is not included in the estimation of hazardous waste quantity, 

the consideration of regularly occupied structures in areas of subsurface contamination in this calculation would 

reflect that some of the contamination could enter these structures.

This comment results in no change to the proposed addition.

2.2.h Relative Risk of Airborne Contamination 

Comment(s): One peer reviewer questioned the method for calculating hazardous waste quantity, stating that it 

would underestimate the risk associated with the subsurface intrusion component relative to other pathways. The 

reviewer commented that it does not reflect that airborne contaminants pose a higher risk at lower concentrations 

than contaminants in other media. 

Response: The role of waste quantity in an HRS evaluation is not designed to reflect by itself the level of risk to 

targets. In an HRS analysis, waste quantity serves as a surrogate for the amount of a contaminant(s) that the 

population associated with a site might come in contact with. This factor is a surrogate for a measured dose used 

in a risk analysis. The higher the hazardous waste quantity value the more likely a population will be exposed and 

the higher the amount they will likely be exposed to. Hence the higher the hazardous waste quantity value the 

higher the HRS site score. 

The effect of the exposure, including the concentration, is considered in another part of the HRS algorithm, when 

assigning the target weightings. The lower the health-based benchmark associated with air exposure, the higher 

the HRS target factor category score will likely be. A higher weighting is given to target populations who can be 

documented to be exposed to hazardous substance levels at or above health-based criteria. The health-based

benchmarks EPA uses to assign the target weightings reflect the higher risk associated with airborne contaminants

than that for other media.

Therefore, the combination of hazardous waste quantity and weighting of targets based on benchmarks together 

accounts for the higher risk associated with exposure to airborne contaminants than to contamination in other 

media. 

This comment results in no change to the proposed addition to the HRS.

2.2.i Biodegradation and Waste Quantity

Comment(s): Peer reviewers commented that the sampling for subsurface intrusion and subsequent calculation of 

the hazardous waste quantity should differentiate between hydrocarbon volatile organic compounds (VOCs) and 

halogenated VOCs to consider biodegradation. The reviewer commented that paired subslab and indoor air 

samples showing similar contamination 80% of the time is incorrect because it does not differentiate between 

types of VOCs. 

Response: In response to peer reviewers’ comments, EPA has added a degradation factor to the evaluation of the 

waste characteristics factor category value for the subsurface intrusion component of the HRS. EPA determined 

that it would be more appropriate to evaluate a substance's degradation potential as a separate factor value and not 

within the hazardous waste quantity calculation. This is because many of the site-specific characteristics 

impacting a substance's degradation rate over time are beyond the scope of a typical site inspection to measure; 

therefore, it could result in a significant error in estimating the hazardous waste quantity for substances that might 

or might not biodegrade based on site-specific conditions. 
26

This degradation factor reflects the difference in degradation rates e.g., between readily degradable hydrocarbons 

such as straight chain or simply ring structures found in petroleum or petroleum-like hazardous substances, the 

presence of biologically active soil, and the depth to contamination. The agency notes, however, that in the HRS 

algorithm, the degradation factor is multiplied by the toxicity factor value, with the resulting factor value then 

multiplied by the hazardous waste quantity factor. The overall result is that if an ASC at a site is associated with 

substances that are likely to degrade, this factor will reduce the overall score for that site relative to other sites 

without degradable substances. 

2.2.j Impact of Indoor Sources on Waste Quantity

Comment(s): One peer reviewer commented that, in some cases, indoor air contaminant concentrations have been 

shown to result from an indoor source instead of subsurface intrusion. 

Response: The hazardous waste quantity estimate is designed to reflect the amount of hazardous substance 

intrusion from subsurface sources. In estimating hazardous waste quantity using the HRS tiers, the Tier A

estimate, called hazardous constituent quantity, actually represents an exact calculation of the mass of hazardous 

substances that has entered into a structure. This calculation would require subtracting out any indoor sources and 

thus would not include the quantity of contaminants originating from indoor sources. 

This comment results in no change to the proposed addition to the HRS. 

2.2.k Applicability of the Hazardous Waste Quantity Calculation

Comment(s): One peer reviewer questioned the applicability of the subsurface intrusion hazardous waste quantity 

calculation in the case where only an ASC exists. The reviewer questioned whether the waste quantity in this 

scenario would be better related to ground water or soil gas concentrations to more accurately estimate the 

quantity.

Response: The hazardous waste quantity factor value represents the amount of hazardous waste a target may be 

exposed to, which is not the same as the amount located in the subsurface. Much of the contamination in the 

subsurface will vent into the atmosphere and not enter into an overlying structure. Therefore, a minimum 

hazardous waste quantity value based on soil gas or ground water concentrations could significantly overestimate 

the amount a target could be exposed to inside a structure. However, the agency notes that regularly occupied 

structures in an area of subsurface contamination are now considered in determining hazardous waste quantity in 

the proposed addition. 

This comment results in no change to the proposed addition to the HRS. 

2.2.l Consider Future Structures 

Comment(s): One peer reviewer commented that the proposed structure of the draft SsI addition does not allow 

for the determination of a hazardous waste quantity for locations that exhibit significant contamination and 

currently have no structures, but where structures would or will be constructed. The reviewer suggested that an

approach be created to estimate the air concentrations if a structure were present in a location with VOC 

contamination based upon data collected from other sampled media. 

Response: The HRS scoring of the SsI component is based on current conditions, as demonstrated in the wording 

of the text in the proposed addition to the HRS. 
27

The agency also considers that the priority of a site without eligible structures would be lower than sites with 

eligible structures. However, if structures were later built on the site or the subsurface contamination migrates

beneath existing structures, the HRS evaluation could be revisited. 

This comment results in no change to the proposed addition to the HRS. 

2.2.m Adequacy of Site Inspection Data to Identify Subsurface Contamination

Comment(s): One peer reviewer questioned the ability to complete the necessary sampling within the time frame 

of a typical site inspection to estimate the hazardous waste quantity. It was suggested that prior sampling may be 

required to identify hazardous substances associated with subsurface contamination that should be targeted for 

analysis in indoor air and to gather the necessary information to estimate the hazardous waste quantity. 

Response: A phased approach may be necessary to collect the data required to score this component, if indoor air 

sampling is deemed necessary or has not been previously undertaken. In the first phase, data on ground water and 

soil gas samples and information on site-specific conditions are likely to be collected. If the information from the 

first phase investigation indicates a sufficient possibility of risk to populations, a second phase may occur. This

second phase will likely focus on sampling indoor air and collecting additional information on occupied 

structures, which could be used to estimate the hazardous waste quantity. Further, if subsequent sampling shows 

further contamination, the site can be re-evaluated for the NPL.

This comment results in no change to the proposed addition to the HRS.

2.2.n Need for Discussion and Reference Support in TSD 

Comment(s): One peer reviewer commented that there is too much uncertainty surrounding the assumptions used 

in evaluating the waste quantity in multi-story buildings and stated that there should be more discussion in the 

proposed addition’s TSD. 

Response: Regarding the evaluation of the waste quantity, the general approach for evaluating multi-story/multitenant buildings has been revised and is discussed in greater detail in the proposed addition to the HRS and its 

TSD. Furthermore, while this comment was received on charge question 4, which dealt with the evaluation of 

multi-story and multi-tenant structures, it was decided to group it with the other comments related to hazardous 

waste quantity.
28

2.3 Charge Question #3: 

Is it appropriate to consider individuals/populations located within the area of subsurface 

contamination (ASC) to be at a higher relative risk in situations when subslab contaminant 

concentrations are at least ten times greater than a health-based benchmark? If no, please explain.

In the subsurface intrusion component, targets subject to actual contamination receive greater weighting in 

scoring if the hazardous substance is present at or above a health-based benchmark. Targets subject to potential 

contamination are weighted less than those subject to actual contamination, and also receive lesser weightings 

based on the distance from a source of hazardous substances or possible dilution of the hazardous substances 

during migration from a source to a target. The subsurface intrusion component generally followed a similar 

target weighting structure as the one used in the 1990 HRS. In the 1990 HRS, all targets in all pathways are 

evaluated using two main categories: whether they are exposed to 1) actual contamination; or 2) potential 

contamination. If targets are exposed to hazardous substance levels meeting observed release criteria, they are 

identified as actually contaminated; however, if the targets are not exposed to a hazardous substance that meet the 

observed release criteria but are within the target area being evaluated, they are considered potentially 

contaminated. 

2.3.a Support for Weighting Targets

Comment(s): Several peer reviewers commented that it was appropriate to consider individuals to be at a higher 

relative risk when subslab contaminant concentrations are at least ten times greater than a health-based 

benchmark. One peer reviewer agreed that it was reasonable to consider those populations in occupied structures 

that have not been sampled but are surrounded by occupied structures with documented contamination (i.e., in an 

ASC), as being contaminated unless evidence indicates otherwise. Additionally, another peer reviewer 

commented that the proposed approach is consistent with the California vapor intrusion guidance. 

Response: The agency acknowledges that the reviewers generally agree with the differential weightings of targets 

based on projected indoor contaminant levels. However, based on peer review responses, the weighting of targets 

in the ASC (delineated in most situations based on subsurface samples) has been changed in the proposed addition 

to the HRS to not reflect a comparison to benchmark levels. The weighting of the populations based on the 

hazardous substance concentration in subslab and crawl space samples relative to benchmarks was changed so 

that it does not compare subsurface contaminant concentrations to health-based benchmarks when evaluating 

potential targets. This change was made because of studies that documented the unpredictability of vapor 

intrusion rates based on time-limited sampling events. However, the weighting based on sample type has been 

retained and expanded to also consider other sample media (both above and below ground) and the depth or

distance to contamination (see Table 5-20, Weighting Factor Values for Populations within an Area of Subsurface 

Contamination, of the proposed addition to the HRS for further information). 

2.3.b Appropriate Benchmark Risk (Cancer and Non-Cancer) 

Comment(s): One peer reviewer recommended increasing the risk level used to establish benchmarks for cancer 

and non-cancer risks. The reviewer suggested raising the risk level for benchmarks from greater than a one in a 

million risk (1 x 10-6) to one in ten thousand (1 x 10-4) for cancer effects and raising the non-cancer hazard index 

(HI) from 1 to 10. 

Response: The one in a million risk level for benchmarks has been maintained. Comments on this issue were 

solicited at the time the HRS was last revised (see Section III.H of the 1990 HRS, 55 FR 51547, December14, 

1990). The cancer risk of 10-6 for all HRS pathway evaluations was chosen because it is the lower end of the 

cancer risk range (i.e., 10-4 to 10-6) identified in the National Oil and Hazardous Substances Pollution Contingency 

Plan (NCP) and used by other EPA programs. A one in a million (1 x 10-6) risk level for cancer effects and a HI of 
29

1 for non-cancer effects are built into the overall HRS structure and used for all benchmarks in all HRS pathways 

and components. These risk levels are also the basis for EPA Regional Screening Levels which are also used as a 

screening tool to determine contaminants of concern at Comprehensive Environmental Response, Compensation, 

and Liability Act (CERCLA) sites. 

In addition, changing the benchmark risk levels for just the subsurface intrusion pathway would imbalance the 

HRS evaluation among pathways and components; using one benchmark risk level for subsurface intrusion and 

another for other HRS pathways and components would result in a lower qualifying risk level for sites scored on 

subsurface intrusion compared to sites scored using other pathways and components. Furthermore, given the large 

variability in vapor intrusion rates over time, assigning only those populations documented as exposed to a 10-4

based benchmark a higher weighting could significantly underestimate the contaminant concentration and the 

actual risk at a site. Existing studies indicate it is likely that short duration sampling, such as those used to support 

HRS evaluations, could underestimate the average exposure concentration and a higher risk-based benchmark 

would only magnify this underestimation of risk at the site.

These comments result in no change to the proposed addition to the HRS. 

More specific comments on benchmarks are addressed in the following subsections:

• 2.3.b.1 Number of Eligible Sites

• 2.3.b.2 Benchmark and Background (Understanding Overall HRS Process)

• 2.3.b.3 Cumulative Effects and Benchmarks

• 2.3.b.4 Weighting Based on Exceeding Benchmark

• 2.3.b.5 Benchmark Levels in Crawl Spaces

2.3.b.1 Number of Eligible Sites

Comment(s): One peer reviewer commented that assigning benchmarks based on one in a million (1 x 10-6) risk 

level is far too low and suggested increasing the benchmark to 1 in 10,000 (1 x 10-4) to avoid listing too many 

sites. 

Response: The number of sites on the NPL is not a factor considered in setting the risk level for HRS 

benchmarks; rather, the goal is to protect human health and the environment. A one in a million (10-6) risk level 

for cancer was chosen because it is the lower end of the cancer risk range (i.e., 10-4 to 10-6) identified in the NCP 

and used by other EPA programs. As a result, it is built into the overall HRS structure and used for all 

benchmarks in all HRS pathways and components. Comments on this issue were solicited at the time the HRS 

was last revised (see Section III.H of the 1990 HRS, 55 FR 51547, December 14, 1990). These risk levels are also 

the basis for EPA Regional Screening Levels which are also used as a screening tool to determine contaminants of 

concern at CERCLA sites. 

These comments result in no change to the proposed addition to the HRS. 

2.3.b.2 Benchmark and Background (Understanding Overall HRS Process) 

Comment(s): One peer reviewer suggested changing benchmarks for cancer and non-cancer risks and stated that if 

the benchmarks were raised, the issue of background samples containing contamination from indoor sources 

would be resolved. 

Response: The benchmark risk level is not used in any comparison to a background level in the HRS. In the HRS 

algorithm, benchmarks are used to increase the weighting of target populations that have been determined to be 
30

exposed to hazardous substances (i.e., if a target population is determined to be exposed to a hazardous substance 

at concentrations above an appropriate benchmark, then the target weighting will be higher than another target 

population that has been determined to be exposed to concentrations below an appropriate benchmark value). (See 

Section 5.2.1.3 and all applicable subsections of the proposed addition to the HRS.) 

As explained in section 2.3.b, Appropriate Benchmark Risk (Cancer and Non-Cancer), of this support document, 

a one in a million (10-6) risk level for cancer effects and a HI of 1 for non-cancer effects are built into the overall 

HRS structure and are considered appropriate for use in a screening tool. Assigning only those populations 

documented as exposed to a 10-4 benchmark in a higher weighting based on a limited site inspection sampling 

event could significantly underestimate the contaminant concentration and the actual risk at a site, given the large 

variability in vapor intrusion rates over time.

This comment results in no change to the proposed addition to the HRS. 

2.3.b.3 Cumulative Effects and Benchmarks

Comment(s): One peer reviewer questioned whether the additive effects of contaminants would be considered in 

the draft SsI addition to the HRS. 

Response: EPA has incorporated the suggestion of evaluating the additive effects of hazardous substances in the 

proposed addition to the HRS. With this change, populations within an AOE that are not exposed to hazardous 

substances above their individual health-based benchmarks can be weighted the same in certain situations as those 

exposed to hazardous substances documented to have exceeded their benchmark. This occurs if the ratios of the 

concentrations of multiple hazardous substances to their applicable benchmarks sum to greater than 1 for either 

the cancer or the non-cancer benchmarks. This is not done for weighting populations within an ASC because, 

given the large variability in vapor intrusion rates, it is unlikely that an actual exposure concentration can be 

projected based on subsurface samples with sufficient accuracy to justify the increased weighting. (See Section 

2.5.2 of the proposed addition to the HRS regarding I and J indices.) 

2.3.b.4 Weighting Based on Exceeding Benchmark

Comment(s): Several peer reviewers commented that the current weighting of targets, which is based on 

comparing contaminant levels to health-based benchmark levels, should be adjusted. Reviewers suggested that the 

requirement of being ten times above a health-based benchmark be revised to 50 times above the health-based 

benchmark when determining the population above an ASC. 

Response: Based on peer review comments, the weighting of targets in the ASC has been changed in the proposed 

addition to the HRS to not include comparisons of subslab or enclosed crawl space samples to benchmark levels. 

The weighting of the populations based on the hazardous substance concentrations in subslab and crawl space 

samples relative to benchmarks was changed so that it does not compare subsurface contaminant concentrations to 

health-based benchmarks when evaluating potential targets. However, the weighting based on sample type has 

been retained and expanded to also consider other sample media (both above and below ground) and the depth or 

distance to contamination (see Table 5-20, Weighting Factor Values for Populations within an Area of Subsurface 

Contamination, of the proposed addition to the HRS for further information). 

The comparison to benchmarks has been revised because recent studies have shown that the variability in vapor 

intrusion rates over time demonstrates that a simple arithmetic average exposure level cannot be projected with 
31

confidence based on limited sampling events. However, use of the 95th percentile alpha11 (i.e., attenuation) factor 

has been retained in that it was used to set the relative weightings for targets in the ASC and reflects a reasonable 

worst case scenario for projecting indoor concentrations due to vapor intrusion. Attenuation factors project that 

the higher the factor, the more likely it is that a greater portion of the contamination found in the subsurface and 

crawl space samples will actually enter structures above the contamination. Given that the HRS evaluation is 

designed to be performed when limited sampling has occurred, and studies have shown that contaminant levels 

from a time-limited sampling event are likely to underestimate the actual exposure levels, it is appropriate to 

consider the 95th percentile attenuation factors in weighting populations in an ASC. 

While EPA attenuation factors may not be sufficient to predict concentrations relative to a benchmark on a 

structure-specific basis, an analysis of the data in EPA’s Vapor Intrusion Database12 shows that there are 

statistically significant correlations by sample type and depth in the ratio of the concentration in subsurface 

samples and in indoor air samples. These findings have been used to develop a revised weighting system (see 

Table 5-20, Weighting Factor Values for Populations within an Area of Subsurface Contamination, of the 

proposed addition to the HRS for further information). This analysis shows the highest correlation in 

concentrations (i.e., the attenuation factor is closest to 1) is between crawl space and indoor air sample 

concentrations. These findings also show decreasing 95th percentile attenuation factors between indoor air and 

subslab samples, indoor air and subsurface soil or soil gas samples, and indoor air and ground water samples. 

Similarly, studies show that there is a significant correlation between indoor air concentrations and depth to 

contamination. 

Therefore, for example, in the new proposed weighting system, the highest target weighting of 0.9 will be

assigned to those populations in regularly occupied structures associated with contaminated crawl space samples

meeting observed release criteria. The next highest target weighting of 0.4 will be assigned to populations 

associated with contaminated samples (meeting observed release criteria) in any media located within a distance 

of 5 feet of a regularly occupied structure’s foundation (including subslab samples). The weightings continue to 

decrease for other contaminated sample media up to 30 feet in depth. A minimum target weighting (i.e., 0.1) is 

assigned for contaminated samples at greater than 30 feet in depth. (A minimum weighting is used to reflect that 

at the time of an HRS evaluation, a profile of concentration with depth may not be available and only ground 

water samples may be available.) For further categories regarding the weighting of targets see Table 5-20, 

Weighting Factor Values for Population within an Area of Subsurface Contamination, of the proposed addition. 

2.3.b.5 Benchmark Levels in Crawl Spaces 

Comment(s): One peer reviewer suggested modifying the criteria for determining the population value above 

crawl spaces. The reviewer recommended that since the attenuation factor for crawl spaces is 1, if a crawl space 

sample has contamination above a health-based benchmark (in place of the requirement to be ten times above a 

health-based benchmark) the residents of the structure should be assigned the weighting factor reflecting exposure 

above the benchmark in the evaluation of targets. 

11 The term “alpha” is used interchangeably with the term “attenuation” and both are referred to in this document as factors. 

This factor refers to the reduction in concentration of volatile substances that occurs during vapor migration in the subsurface 

(as a result of diffusion, advection, sorption, transformation reactions and other processes), coupled with the dilution that can 

occur when the vapors enter a building and mix with indoor air. The quantification of the value can be defined as the ratio of 

the indoor air concentration arising from vapor intrusion to the subsurface vapor concentration at a point or depth of interest 

in the vapor migration pathway. 

12 EPA’s Vapor Intrusion Database: Evaluation and Characterization of Attenuation Factors for Chlorinated Volatile Organic 

Compounds and Residential Buildings. Dr. Helen E. Dawson, U.S. Environmental Protection Agency's Office of Superfund 

Remediation and Technology Innovation (OSRTI), served as the principal author, with Dr. Richard B. Kapuscinski, USEPA 

OSRTI, as coauthor. March 16, 2012.
32

Response: Based on the peer review comments and a 2012 EPA evaluation of EPA’s Vapor Intrusion Database, 

the methodology for evaluating the populations within an ASC has been modified so as to not compare a 

contaminated sample concentration in any non-indoor sample to a benchmark. The weighting assigned based on 

the ratio of the contaminant concentration in subslab and crawlspace samples to the applicable benchmark was 

removed because, as pointed out by the peer reviewers, the EPA’s Vapor Intrusion Database as of 201213 shows 

that there is inadequate correlation, due to high variability, to predict with confidence whether a benchmark is 

likely to be exceeded even when extensive site-specific data are available. This correlation would be even more 

tenuous if the projection were based on the limited sampling data collected during a typical short duration site 

inspection (i.e., the data normally available during an HRS evaluation). 

In addition, the weighting system for targets in the ASC has been revised based on peer review comments and is 

now also based on sample type (e.g., crawl space, soil gas) and the depth or distance to contamination. The 

revised weighting system is included in the proposed addition to the HRS (see Table 5-20, Weighting Factor 

Values for Population within an Area of Subsurface Contamination, of the proposed addition). See section 2.3.c.7, 

Weighting Factors for Crawl Space Samples and Subslab Samples, of this support document for further 

explanation of the specific weighting values associated with crawl spaces.

2.3.c Target Weighting 

Comment(s): Peer reviewers commented that the weighting of individual targets in an ASC does not appear 

consistent with EPA’s compilation of paired subslab and indoor data. Reviewers commented that rationales for 

using specific weighting values were not provided and that these inconsistencies in the weighting factors could 

underestimate the target score at a site. Additionally, one reviewer commented that the paired subslab and indoor 

air data may allow for an observed exposure to be predicted based on subslab samples. 

Response: While multiple weighting scenarios were originally considered, the assigned weighting values were 

determined to most appropriately reflect the relative threat posed to target individuals in a regularly occupied 

structure based on the paired subslab/indoor air data reviewed. However, based on the peer review comments and 

an analysis of EPA’s Vapor Intrusion Database as of 2012, the methodology for evaluating the populations within

an ASC has been modified. The weighting assigned based on the ratio of the contaminant concentration in subslab 

and crawlspace samples to the applicable benchmark was removed because, as pointed out by the peer reviewers, 

this analysis shows that there is inadequate correlation to predict with confidence whether a benchmark is likely to 

be exceeded even when significant site-specific data are available. 

Additionally, using the paired subslab and indoor data to predict an observed exposure has not been adopted 

because new data suggest extreme variability of vapor intrusion rates, and representative site contaminant 

concentrations are unlikely to be predicted with any precision based on data collected during a typical short 

duration site inspection (i.e., the data normally available during an HRS evaluation). 

While EPA attenuation factors may not be sufficient to predict concentrations relative to a benchmark on a 

structure–specific basis, the 2012 analysis of the data in EPA’s Vapor Intrusion Database shows that there are 

statistically significant correlations by sample type and depth in the ratio of the concentration in subsurface 

samples and in indoor air samples. These findings have been used to develop a revised weighting system (see 

Table 5-20, Weighting Factor Values for Population within an Area of Subsurface Contamination, of the proposed 

addition to the HRS for further information). This analysis shows the highest correlation in concentrations (i.e., 

the attenuation factor is closest to 1) between crawl space and indoor air sample concentrations. It also shows 

decreasing 95th percentile attenuation factors between indoor air and subslab samples, indoor air and subsurface 

13 U.S. EPA Vapor Intrusion Database: Attenuation Factor Analysis. Accessible at: http://www.epa.gov/vaporintrusion
33

soil or soil gas samples, and indoor air and ground water samples. Similarly, studies show that there is a 

significant correlation between indoor air concentrations and depth to contamination. 

Therefore, for example, in the new proposed weighting system, the highest target weighting of 0.9 will be 

assigned to those populations in regularly occupied structures associated with contaminated crawl space samples

meeting observed release criteria. The next highest target weighting of 0.4 will be assigned to populations 

associated with contaminated samples (meeting observed release criteria) in any media located within a distance 

of 5 feet of a regularly occupied structure’s foundation (including subslab samples). The weightings continue to 

decrease for other contaminated sample media up to 30 feet in depth. A minimum target weighting (i.e., 0.1) is 

assigned for contaminated samples at greater than 30 feet in depth. (A minimum weighting is used to reflect that 

at the time of an HRS evaluation, a profile of concentration with depth may not be available and only ground 

water samples may be available.) For further categories for the weighting of targets see Table 5-20, Weighting 

Factor Values for Population within an Area of Subsurface Contamination, of the proposed addition. 

More specific weighting value comments are addressed in the following subsections:

• 2.3.c.1 Strength of Weighting Assumptions

• 2.3.c.2 Weighting Based on Benchmark Comparison

• 2.3.c.3 Weighting by Subsurface Conditions

• 2.3.c.4 Weighting by Sample Type

• 2.3.c.5 Weighting Factor When Not Ten Times Benchmark

• 2.3.c.6 Weighting Based on Subslab Samples and Ground Water Samples

• 2.3.c.7 Weighting Factors for Crawl Space Samples and Subslab Samples

2.3.c.1 Strength of Weighting Assumptions 

Comment(s): One peer reviewer questioned the assumptions underlying weighting of potential targets (i.e., those 

populations in an ASC). The reviewer commented that the approach for weighting potential targets assumes that 

areas of subsurface contamination have a ten times lower relative impact than areas with observed exposure. The 

reviewer questioned the assumption that subslab soil gas samples are reliable indicators of vapor intrusion. 

Additionally, the reviewer commented that without actual samples there is no way to confidently predict which 

occupied structures within an ASC might be contaminated. 

Response: The HRS weighting of targets in an ASC does not predict contamination concentrations in structures 

within the ASC, but instead the HRS site score reflects the possibility of vapor intrusion occurring. The HRS 

score is a relative ranking between sites and the quantitative impact on potential targets (i.e., those in the ASC) is 

not assumed to be ten times lower; a score does not correspond to actual risk levels at a site. The weightings are 

assigned to allow differentiation in site scores to reflect the relative risk among sites. The tenfold difference in 

weighting populations potentially exposed and those documented to be exposed (actually exposed) is used in all 

HRS pathways to emphasize the presence of actually exposed targets at sites. 

Consistent with the NCP, an HRS evaluation of a site is based on the data available at the time of the evaluation, 

and is designed to be performed after an initial time-limited site inspection has been completed. Since the HRS is 

used to prioritize sites for further investigation and was directed in CERCLA to put emphasis on actually exposed 

populations, it is reasonable to assign a higher score to a site where populations are known to be exposed to 

indoor contamination, than to a site with no documented exposures. While the difference may be simply that one 

site has not been as thoroughly investigated as another site or different samples were collected, this situation is 

beyond the scope of the HRS regulation (specifying sampling requirements in a regulation could easily result in 

the regulation being outdated when new or improved sampling procedures are adopted). Any ranking system is 
34

subject to the quality of the information available; however, based on peer review comments, the weighting of 

populations in areas of subsurface contamination has been revised to consider other sample types. 

Furthermore, the weighting of populations located within an ASC has been modified to also consider the distance

or depth to contamination from regularly occupied structures. (Sampling procedures for monitoring vapor 

intrusion in general is a topic being addressed in other EPA efforts.) 

Regarding the assumption that subslab soil gas samples are reliable indicators of vapor intrusion and not the result 

of indoor sources, this is assumed only in situations where the subsurface contamination meets observed release 

criteria. Observed release criteria include both that the subslab hazardous substance concentrations are 

significantly above background and that the increase can be attributed at least in part to the release from the site 

being evaluated and not entirely from anthropogenic indoor sources. 

This comment results in no change to the proposed addition to the HRS.

2.3.c.2 Weighting Based on Benchmark Comparison 

Comment(s): One peer reviewer suggested an alternative scheme for weighting targets within an ASC, based on 

both contaminant concentrations and health-based benchmarks. The reviewer recommended assigning a weighting 

factor of 0.1 to occupants with contaminant concentrations less than 50 times the health-based benchmark; a

weighting factor of 1 to occupants with contaminant concentrations between 50 times and 500 times the healthbased benchmark; and a weighting factor of 10 to occupants with contaminant concentrations greater than 500 

times the health-based benchmark. 

Response: This alternative weighting approach has been considered, but because of the peer reviewers’ comments 

on the large variability in vapor intrusion rates, consideration of this level of quantitative detail on projecting 

indoor contaminant concentrations is not recommended. Representative site contaminant concentrations may not

be available based on the data collected during a typical short duration site inspection; therefore, this approach to 

weighting potential targets has not been implemented. However, based on additional peer review responses, the 

weighting of potential targets has been updated in the proposed HRS addition to reflect the contaminant media 

both above (e.g., crawl space) and below ground (e.g., soil gas vs. ground water) and the depth or distance to 

contamination. (Please see section 2.3.c, Target Weighting, of this response document regarding the proposed 

alternative weighting system.)

2.3.c.3 Weighting by Subsurface Conditions 

Comment(s): One peer reviewer suggested that the potential for exposure in one part of a site should be 

considered the same as in another part of the same site, in areas of similar subsurface conditions, unless extensive 

data collected have indicated otherwise. The reviewer stated that there is no basis for assigning a lower risk to an 

occupied structure with similar subsurface conditions to nearby structures that have observed exposure, just 

because impacts have not been observed. 

Response: The agency considers it reasonable to assign different weighting factors to target populations 

depending on whether the populations are documented to be actually exposed to subsurface intrusion. 

Documented exposure to a hazardous substance based on a limited sampling event indicates that a site poses 

higher relative risk, and warrants further investigation, than sites with no documented exposure to a hazardous 

substance. EPA acknowledges that a potentially exposed target may be subject to exposure at different time 

periods than that during a site inspection. However, an HRS evaluation is based on currently available data and a 

site may be re-evaluated if additional data are obtained in the future. The proposed HRS addition contains a 

provision for inferring that contamination is entering structures surrounded by other contaminated structures, and 
35

for assigning a weighting of targets in the structures with inferred contamination unless information indicates 

otherwise. 

This comment results in no change to the proposed addition to the HRS.

2.3.c.4 Weighting by Sample Type

Comment(s): Several peer reviewers commented that the method of weighting targets may result in a low bias to 

population risks. The reviewer commented that the method is dependent on the type of samples collected, and 

noted that low bias might particularly happen at sites where subslab samples are taken but no indoor samples are 

collected. Additionally, reviewers commented that the method for weighting targets did not fully consider subslab 

data and undervalued subslab data relative to indoor data. 

Response: A site with limited subsurface and indoor sampling might receive a lower HRS score than a site with 

more extensive sampling, especially if a more extensive sampling event documents indoor exposures or 

contamination at shallower depths. It has also been well documented that due to the variability in vapor intrusion 

rates with time, it is possible that short duration sampling events may misidentify whether subsurface intrusion is 

occurring or will occur under different conditions. Therefore, when an ASC has been identified (i.e., no actual 

exposures documented in a specific area but sufficient sampling has been performed to demonstrate subsurface 

contamination is present), EPA is proposing to evaluate target populations in this area as potentially exposed. In 

the case of an ASC delineated at least in part by indoor air samples meeting observed release or observed 

exposure criteria, target populations within the ASC would also be evaluated as potentially exposed. 

Regarding the possibility of conservative bias, these potentially exposed populations are being proposed to be 

weighted at a higher value than potential targets in other HRS pathways and components, due to the high 

variability in vapor intrusion rates and the difficulty in obtaining representative samples in a short duration 

sampling event. In other HRS pathways and components, the maximum weighting factor is 0.1 for potential 

targets. In the subsurface intrusion component, the maximum weighting proposed is 0.9 and the minimum factor 

is 0.1. 

Furthermore, if further sampling performed at a later time shows a greater degree of contamination, the HRS site 

score can be re-evaluated and the decision regarding whether to place the site on the NPL can be reconsidered. 

2.3.c.5 Weighting Factor When Not Ten Times Benchmark 

Comment(s): One peer reviewer commented that in weighting potential targets in structures where soil vapor or 

shallow ground water samples contain contaminants but subslab or enclosed crawl space samples do not exceed 

ten times the health-based benchmark, the residents should be multiplied by 0 in place of 0.1. 

Response: The proposed addition has been modified to not compare subsurface contaminant concentrations to 

health-based benchmarks when evaluating potential targets (see section 2.3.b.4, Weighting Based on Exceeding 

Benchmark, of this response document for additional information). However, assigning a weighting greater than 

zero to potential targets is consistent with the overall HRS structure reflecting the limited sampling events prior to 

an HRS evaluation. Using a non-zero value reflects the potential for intrusion into a regularly occupied structure 

to have occurred or to occur in the future. Furthermore, a site with potential targets poses a greater relative risk 

than a site containing no potential targets; therefore, a factor value of zero is not appropriate because it would not 

account for differences among sites that either do or do not have potential targets. 
36

2.3.c.6 Weighting Based on Subslab Samples and Ground Water Samples

Comment(s): One peer reviewer commented that when determining the population in an ASC, applying a higher 

weight to subslab samples than to ground water samples assumes a higher confidence in subslab sample data. 

Response: The agency does have more confidence that populations located in an ASC will be exposed to 

subsurface intrusion of a hazardous substance if a subslab sample documents contamination compared to only if 

ground water contamination is documented. If the contamination is in subslab samples in an ASC, volatilization

from the ground water has already occurred and contamination has moved through the subsurface media 

underneath a structure. Additionally, the 95th percentile attenuation factor from the 2012 EPA evaluation of EPA’s 

Vapor Intrusion Database is higher for subslab samples than it is for ground water samples, indicating that more 

of the contamination in subslab samples is more likely to enter structures than that in ground water samples.

This comment results in no change to the proposed addition to the HRS. 

2.3.c.7 Weighting Factors for Crawl Space Samples and Subslab Samples

Comment(s): Peer reviewers commented that the proposed addition to the HRS evaluates and weights crawl space 

data and subslab data similarly, but notes that EPA generally considers the attenuation factor between crawl 

spaces and indoor spaces to be 1. Reviewers commented that crawl spaces and subslab samples should receive 

different weighting factors based on the most recent EPA guidance. 

Response: Based on the peer review comments and a 2012 EPA evaluation of EPA’s Vapor Intrusion Database, 

the methodology for evaluating the populations within an ASC has been modified. These changes reflect the most 

recent EPA vapor intrusion guidance. As mentioned in section 2.3.c, Target Weighting, of this response 

document, the analysis of EPA’s Vapor Intrusion Database as of 2012 shows that there are statistically significant 

correlations by sample type and depth in the ratio of the concentration in subsurface samples and in indoor air 

samples. These findings have been used to develop a revised weighting system (see Table 5-20, Weighting Factor 

Values for Populations within an Area of Subsurface Contamination, of the proposed addition to the HRS for 

further information). In this revised weighting system, crawl space and subslab samples are associated with 

different weightings for targets in an ASC. Populations associated with regularly occupied structures containing 

semi-enclosed or enclosed crawl space samples meeting observed release criteria would be assigned a target 

weighting of 0.9. 

2.3.d Consistency of Weighting Factors with Database/Guidance

Comment(s): One peer reviewer commented that in estimating a population in an ASC, the requirement of being 

ten times above a health-based benchmark is inconsistent with EPA vapor intrusion guidance. 

Response: Based on additional peer review responses, the weighting of potential targets has been updated in the 

proposed addition to reflect the contaminant media both above (e.g., crawl space) and below ground (e.g., soil gas 

vs. ground water) and the depth or distance to contamination. In addition, the proposed addition to the HRS was 

modified to not compare subsurface contaminant concentrations to health-based benchmarks when evaluating 

potential targets (see section 2.3.b.4, Weighting Based on Exceeding Benchmark, of this response document for 

additional information). 

EPA also notes that the workgroup developing this proposed addition to the HRS has worked with EPA’s vapor 

intrusion guidance workgroup to ensure consistency between the two documents. However, in some cases, 

because the HRS is applied at a different stage in the Superfund process when only limited data are available, the 

HRS methodology may be different than that generically discussed in other guidance documents designed for 

different stages in the risk assessment and remediation process. 
37

More specific comments on the relationship between weighting factors and existing data are addressed in the 

following subsections: 

• 2.3.d.1 Inconsistency of Attenuation Factors

• 2.3.d.2 Analyzing Raw Data to Determine Weighting Factors

2.3.d.1 Inconsistency of Attenuation Factors 

Comment(s): One peer reviewer commented that the current attenuations factors in the draft SsI addition for 

weighting targets are inconsistent with previous analysis of vapor intrusion data. 

Response: The attenuation factors being used to develop the proposed addition have been updated to the 2012 

EPA values14. These 2012 values have been used to revise the weighting system being proposed. While recent 

studies have shown that the variability in vapor intrusion rates over time demonstrates that a simple arithmetic 

average exposure level cannot be projected with confidence, the 95th percentile attenuation factor does reflect a 

reasonably worst case scenario for projecting indoor concentrations due to vapor intrusion. Given the HRS is a 

screening tool used to identify sites for further investigation, the large variation in intrusion rates and that the HRS 

is not predicting actual intrusion rates but rather only a relative difference in rates under different circumstances, it 

is reasonable to use the 95th percentile factor when identifying sites for further investigation. 

2.3.d.2 Analyzing Raw Data to Determine Weighting Factors 

Comment(s): One peer reviewer recommended that the raw data from EPA’s Empirical Database [Vapor Intrusion 

Database] should be reviewed when determining the weighting factors to be applied to potential targets. 

Response: The agency has reexamined the raw data from EPA’s Vapor Intrusion Database (as of 2012) in 

response to the peer review comments. Based on additional peer review comments, the weighting of potential 

targets has been updated in the proposed addition to reflect the contaminant media both above (e.g., crawl space) 

and below ground (e.g., soil gas vs. ground water) and the depth or distance to contamination. EPA's Vapor 

Intrusion Database was also reevaluated and used to determine the relative weightings used in the revised target 

weighting system (see Table 5-20, Population Weighting Factor Values for Populations within an Area of 

Subsurface Contamination, of the proposed addition to the HRS for further information). 

2.3.e Consideration of Biodegradation of Hazardous Contaminants in Weighting Targets

Comment(s): One peer reviewer commented that the study serving as the basis for the tenfold difference used to 

assign a higher potential target weighting was based only on halogenated VOCs and therefore should not 

necessarily be used for hydrocarbon VOCs. 

Response: The agency agrees that hydrocarbon VOCs may have a different fate in the subsurface than 

halogenated VOCs. Biodegradation of hydrocarbon VOCs can play a major role in subsurface intrusion and, in 

response to peer review comments, EPA has added a degradation factor in the proposed addition to the HRS. The 

degradation factor considers the difference in half-lives of different hazardous substances, e.g., between 

petroleum or petroleum-like hazardous substances, which have the greatest potential to biodegrade, the presence 

of biologically active soil, and the depth to contamination. However, because a substance will degrade at a 

14 Please see EPA’s 2012 evaluation of its Vapor Intrusion Database written by Dr. Helen E. Dawson and Dr. Richard B. 

Kapuscinski. March 16, 2012.
38

specific site depends on both the substance and site-specific conditions, it was determined that it would be more 

appropriate to include this factor in the evaluation of the HRS waste characteristics factor category value and not 

in the weighting of targets. Please see Section 5.2.1.2 (and all applicable subsections) of the proposed addition to 

the HRS and the proposed addition’s TSD for additional information. 

EPA also notes that hydrocarbon VOCs associated strictly with petroleum releases are excluded from 

consideration under CERCLA. 

2.3.f Evaluating Vacant/Non-occupied Structures

Comment(s): One peer reviewer commented that regularly occupied structures that have been vacated due to siterelated contamination, be evaluated based on the probability that the structure will be occupied again. 

Response: The definition in the HRS of "occupied structures" contains the phrase, "present on a regular basis;" 

therefore for a structure to be eligible for evaluation, people must be regularly present in the structure. Specific 

criteria for meeting the “regular basis” requirement are not set in the HRS and are evaluated on a site-specific 

basis. The criteria or guidelines used in the evaluation of “regular basis” will be consistent with those for 

identifying targets in other HRS pathways (e.g., the criteria for identifying the number of people served by a 

residential well). These criteria have included considerations that relate to the probability of exposure and the 

duration of exposure to contamination, such as the frequency and duration of occupancy, seasonality of 

occupancy and county occupancy averages obtained from the U.S. Census Bureau. However, because of the large 

number of possible site-specific situations, it is unrealistic to develop specific requirements that will address all 

possible situations. 

This comment results in no change to the proposed addition to the HRS. 

2.3.g Evaluating Future Targets 

Comment(s): A peer reviewer commented that the current structure of the draft SsI addition to the HRS does not 

consider future target structures or populations above an ASC where structures are not currently present; the 

reviewer also suggested adding default structure properties that could be scored in the absence of actual 

structures. 

Response: Consistent with the HRS being used as a tool to identify sites that most warrant further investigation, 

an HRS evaluation is focused on current conditions that exist at a site. Sites with regularly occupied structures 

above subsurface contamination are a higher priority for further investigation than sites where future structures 

could be built above subsurface contamination or to which subsurface contamination might migrate in the future. 

The agency did consider reflecting the possible migration of hazardous substances through the subsurface in an 

HRS evaluation, but based on the current state of the science of horizontal subsurface vapor migration modeling, 

the agency considered the available scientific knowledge insufficient to support such an evaluation. Similarly, to 

predict the construction of future structures is speculative and not defensible. However, if situations change at a 

particular site in the future, the site can be re-evaluated using any new information. 

This comment results in no change to the proposed addition to the HRS. 
39

2.4 Charge Question #4 

When calculating waste quantity and population, is it scientifically reasonable for populations in 

multi-story or multi-tenant structures to be evaluated based on using only the population inhabiting 

the intruded space and the population on the floor above and below the intruded space (For example, 

if contamination is found on the first floor, population from the basement and second floor would be 

used in the calculation. If contamination is found on the fifth floor, population from the fourth floor 

and sixth floor would be used in the calculation.)? If no, what alternative approach would you 

recommend? 

The SsI component, at the time of peer review, generally evaluated targets and hazardous waste quantity in multistory/multi-tenant buildings based on those areas and individuals that reasonably can be exposed to subsurface 

intrusion. That is, for the purposes of calculating hazardous waste quantity and the target population for the 

subsurface component, targets are counted as follows: if there is no observed exposure within the structure, only 

those targets in the lowest story would be counted; or, if there is observed exposure in any story, those targets in 

that story, one story above and all stories below the story would be counted. This is similar to the approach for

identifying targets and hazardous waste quantity in the 1990 soil exposure pathway, in that in the 1990 HRS soil 

exposure pathway, targets must be able to come into contact with areas of contamination for both them and the 

contamination to be considered eligible for consideration in the soil exposure evaluation. 

*Note: The proposed HRS addition has undergone revision since it was submitted for peer review in 2011. The

current version of the proposed HRS addition has replaced the term “multi-tenant” with “multi-subunit”. This

appendix reflects the use of “multi-tenant” in charge questions and peer review comments since that was the

language used at the time of the peer review. In responses, the new “multi-subunit” term is used for consistency

with the proposed addition. This change in terminology was made to reflect that tenant spaces within a structure

may not actually have different features such as dividing walls or different heating, ventilation and air

conditioning (HVAC) systems, but has no effect on the responses to the peer review comments.

2.4.a General Support for the Approach

Comment(s): Peer reviewers commented that the overall approach for evaluating the waste quantity and 

population in multi-story or multi-tenant structures is reasonable and appropriate. 

Response: While the reviewers generally agreed with the approach for evaluating multi-story/multi-subunit 

regularly occupied structures, the consideration of other peer review comments resulted in modifications of the 

proposed addition. 

2.4.b Consider Preferential Pathways Between Floors 

Comment(s): Peer reviewers commented that the draft SsI addition should consider preferential pathways (e.g., 

pipe/electrical chases, elevators) between floors when evaluating multi-story and multi-tenant structures. The 

reviewers commented that if a preferential pathway to a higher floor exists and there is a possibility of 

contaminant transport, then those targets on the affected floors should be considered in the scoring unless 

information exists to indicate otherwise. 

Response: To allow for consideration of situations on a site-specific basis, such as where contaminant transport 

within a structure might be impacted by specific building features (i.e., preferential pathways), the phrase "unless 

available information indicates otherwise" has been incorporated into the text of the proposed addition (see 

Section 5.2.0 of the proposed addition to the HRS). Therefore, if site-specific conditions document the presence

of effective preferential pathways, such as elevator shafts and centralized HVAC systems that enhance migration 
40

of contamination throughout the structure, then the weighting of the populations in different parts of the structure 

can reflect the site-specific situation and be considered in the HRS evaluation.

EPA acknowledges that elevators and/or conduits have been demonstrated to represent a preferential pathway for 

intruded vapors in multi-story (multi-level) buildings. However, depending on the design of the structures and the 

location of the preferential pathways, these pathways may not allow even distribution of contamination 

throughout the entire building; therefore, consideration of these site-specific conditions will be addressed as they 

arise.

2.4.c Inferring Level I Contamination

Comment(s): One peer reviewer commented that in the case where multi-tenant residences contained confirmed 

Level I targets in either multiple basement residences or in residences directly above the basements, then all 

basement residences should be considered Level I targets due to indoor air quality variability. 

Response: Target populations in multi-story or multi-subunit structures will not be inferred to be exposed to levels 

at or above the appropriate benchmarks (Level I) unless site-specific information indicates otherwise. Due to the 

documented potential for spatial and temporal variability in contaminant concentrations in the subsurface 

environment and the variation of the movement of contamination from the subsurface environment into occupied 

multi-story/multi-subunit structure (as well as within the structure), Level I concentrations are only assigned to 

units or subunits with indoor air samples confirming that targets are exposed to contamination above appropriate 

benchmarks. However, target populations may be inferred to be exposed to Level II concentrations if they are 

located in an area of observed exposure. This approach is consistent with how target populations are evaluated in

the residential threat of the current soil exposure pathway in the 1990 HRS. 

This comment results in no change to the proposed addition to the HRS. 

2.4.d Consider Discontinuous Contamination

Comment(s): One peer reviewer commented that the example exposure scenario presented in charge question #4, 

where contamination is measured only on the 5th floor of a multi-story structure, is highly unusual and 

inconsistent with the proposed addition to the HRS. The reviewer commented that all residences in a multistory/multi-tenant building beneath a floor where contamination has been found should be factored into the waste 

quantity and target population calculation, not just the one floor above and below the observed exposure. 

Response: The reviewer is correct that the example exposure scenario presented in the charge question is 

inconsistent with the method described in the proposed addition. The method presented in the proposed addition 

to the HRS, not in the charge question, is the intended approach. All populations on floors below a documented 

actual exposure will be considered actually exposed (within an AOE) and the population one floor above will be 

considered potentially exposed (within an ASC) unless information indicates otherwise. Furthermore, Sections 

5.2.0 and 5.2.1.3.2.3 of the proposed addition to the HRS have been modified to clarify how multi-story/multisubunit buildings are evaluated. This example of an observed exposure on the 5th floor of a structure may not be 

common, but this example was provided to illustrate a generic approach.

2.4.e Consideration of Lower Floors 

Comment(s): Peer reviewers questioned why a residence (or floor), which has not been sampled, in a multistory/multi-tenant structure would ever be discounted if the residence (or floor) is located beside or beneath a 

documented observed exposure. Reviewers commented that all residences (or floors) in a multi-story/multi-tenant 

building beneath a residence where an observed exposure has been documented should be considered as actually 

exposed and not discounted. 
41

Response: The proposed addition to the HRS has been clarified to identify that all targets on and below floors (or 

levels) with a documented actual exposure are considered to be actually exposed. However, due to the 

documented potential for spatial and temporal variability in contaminant concentrations in the subsurface 

environment, and the variation of the movement of contamination from the subsurface environment into an 

occupied multi-subunit structure (as well as within the structure), Level I concentrations are only assigned to units 

with indoor air samples confirming that targets are exposed to actual contamination above appropriate 

benchmarks. 

In the proposed addition, unless available information indicates otherwise, target populations residing on lower 

floors within a regularly occupied structure where subsurface intrusion is found are also evaluated as actually 

exposed to contamination (at Level II concentration levels). The population residing one floor above will be 

considered potentially exposed unless information indicates otherwise. 

The proposed addition's instructions have also been amended to include the phrase "unless information indicates 

otherwise" to allow site-specific conditions to be considered in assigning levels of exposure. Furthermore, 

Sections 5.2.0 and 5.2.1.3.2.3 of the proposed addition have been modified to clarify how multi-level/multisubunit buildings are evaluated. 

2.4.f Weighting Based on Basement Samples

Comment(s): One peer reviewer commented that in a multi-story/multi-tenant structure where only an unoccupied 

basement has been sampled and found to be contaminated, the weighting of the occupants on the first level of the 

building should be based on the concentration found in the basement. 

Response: The suggested weighting system provided by the peer reviewer has not been adopted for use in the 

proposed addition. Contamination in a multi-story/multi-subunit structure cannot be assumed to move from the 

lower floor, including a basement, to an upper floor without some amount of dispersion or dilution. Thus, intruded 

vapors cannot be assumed to reach the occupied units in an above floor and spread evenly into each unit or at 

comparable rates, unless site-specific information indicates otherwise. 

However, the modification in the proposed addition to the target weightings for populations in an area of 

subsurface contamination allow for the population residing on the floor above an observed exposure to now be 

weighted at a 0.9 value (as compared to a weighting of 1.0 for actually exposed targets); this value reflects 

attenuation factors presented in an EPA evaluation of EPA's Vapor Intrusion Database as of 2012. The 

contaminated sample, collected in the basement in this scenario, is considered conceptually to be equivalent to a 

crawl space sample collected from a single family residence, and is therefore weighted the same as a crawl space 

sample in an HRS evaluation. Furthermore, populations residing within regularly occupied tenant spaces located 

above a level with a gaseous indoor air sample meeting observed release criteria but no residential tenants on this 

lower level would also be assigned a weighting factor value of 0.9. 

2.4.g Evaluating Worker Exposure in Multi-Story Buildings 

Comment(s): One peer reviewer commented that the proposed addition should consider preferential pathways 

between floors when evaluating the workers factor value. 

Response: In response to this comment, and other comments, EPA has removed the separate factor the worker 

factor value from the proposed addition to the HRS. Workers are now included in the same category as other 

exposed individuals, and are therefore evaluated in a similar manner as an exposed individual or population 

within a multi-story building. The evaluation still reflects that workers are exposed to indoor air at the workplace 
42

for a different time period than residents at home. The contribution of workers to the population factor value also 

varies depending on whether they are full- or part-time employees. 
43

2.5 Charge Question #5 

Because of longer exposure times indoors, is it reasonable to assume that the relative risk is three 

times greater for workers exposed to subsurface intrusion within the workplace than it is for workers 

exposed to outdoor surface contamination at the workplace? If no, please explain. 

The draft SsI addition to the HRS, at the time of peer review, assigned a target factor value to workers based on 

the number of workers actually exposed or inferred to be exposed to contamination, as is performed in an 

evaluation using the soil exposure pathway in the 1990 HRS. In the 1990 HRS, for the resident population threat 

of the soil exposure pathway, a factor value for workers is assigned based on the total number of workers present 

within the area of observed contamination. These factor values range from 0 to15, depending on the number of 

workers exposed to outdoor contamination. However, it is anticipated that workers spend at minimum a larger 

percentage of their time indoors. Therefore, these workers may be exposed longer to contamination resulting from 

subsurface intrusion than would workers exposed to outdoor soil (or other surface) contamination. EPA has 

changed the method of evaluating workers in the proposed addition to the HRS based upon comments received. 

The previous version of the proposed HRS addition assigned a factor value to workers in a similar method to the 

existing soil exposure pathway which included a maximum factor value. The proposed addition to the HRS now 

includes workers in the population target factor value ; this change enables workers to be evaluated by the 

proposed addition to the HRS using health-based benchmarks, and removes the maximum limit that workers can 

contribute to the site score.

2.5.a Support for Worker Factor Value

Comment(s): Peer reviewers agreed that it is reasonable to weight workers exposed to subsurface intrusion in 

regularly occupied structures at three times the relative risk to workers exposed to outdoor contamination in an 

area of observed contamination because of longer exposure times indoors. 

Response: As noted above, EPA has changed the method in which workers are evaluated in the proposed addition 

to the HRS since the peer review. The comment was comparing the method of evaluating workers in a previous 

version of the proposed addition to the HRS to the method of evaluating workers in the existing soil exposure 

pathway. The current proposed addition to the HRS no longer evaluates workers in manner similar to the existing 

soil exposure pathway. 

2.5.b Indoor Air Exposure and Worker Risk

Comment(s): One peer reviewer commented that the current method for assigning factor values to weight indoor 

versus outdoor worker exposure would underestimate the relative risk of exposure for indoor workers. The peer 

reviewer commented that an indoor worker is exposed to contaminated air in a limited volume compared to 

outdoor air that is not constrained and is diluted by atmospheric air. The peer reviewer suggested that the draft SsI 

addition’s TSD contain support to justify the weighting factors used and the increased relative risk for indoor 

workers. 

Response: As discussed above, workers are now included in the population factor value, and are therefore 

evaluated against health-based benchmarks in areas of observed exposure (AOEs). This method will help to more 

accurately assess the relative-risk to workers, whereas the previous method of using a separate worker target 

factor value, which did not consider if the exposure was above health-based benchmarks, could potentially have 

underestimated the risk to workers.
44

2.5.c Rationale for Worker Factor Value

Comment(s): One peer reviewer commented that, while it is reasonable to assume that the risk is greater for 

indoor workers, the basis for using a factor of three to score indoor workers higher has not been explained and 

should be included in the proposed addition. 

Response: The basis for the threefold increase in the weighting of worker targets, as discussed in the proposed 

addition's companion TSD, was based on the projected length of exposure to indoor air contaminants being at 

least three times greater for those working in an indoor area compared to workers exposed to contaminated soil in 

general. Given the diversity of possible exposure scenarios, it was determined that a threefold increase in the 

assigned values was a conservative, defensible projection. The rationale for this threefold difference has been 

reconsidered, and instead EPA has weighted workers based on the difference in exposure time for full-time

workers of 8 hours a day and part-time workers of 4 hours a day versus the typical exposure scenario for residents 

of as high as 24 hours a day. The agency recognizes that this ratio is not exact for all situations, and that to 

determine the most appropriate ratio of exposure would require site-specific information beyond that collectable 

during a typical site inspection. If a more universally-accepted ratio is identified during public comment on this 

issue, it will be considered at that time. 

In addition, as discussed above, workers are now included in the population target factor value, and are therefore 

evaluated against health-based benchmarks in AOEs. The comment provided was based upon the previous version 

of the proposed HRS addition and is no longer applicable to the current version. 

2.5.d Worker Factor Value Correlation

Comment(s): One peer reviewer questioned the comparison of indoor vapor inhalation to outdoor soil exposure 

for workers and stated that a fixed correlation between the two is inappropriate. 

Response: As noted above in section 2.5.c, EPA has changed the method of evaluating workers in the proposed 

HRS addition based upon comments received. The method for evaluating workers in the current version of the 

proposed HRS addition has changed to reflect that the length of exposure to a worker in a workplace as compared 

to exposure scenarios for residents at home is considerably less. 

This comment results in no change to the proposed addition to the HRS.

2.5.e Worker Benchmark Values

Comment(s): One peer reviewer commented that the benchmarks for workers should be based on industrial or 

commercial screening values and not residential values. 

Response: As noted above, EPA has changed the method of evaluating workers in the proposed HRS SsI addition 

based upon comments received. In the current version of the proposed HRS addition workers are included in 

population. The suggested HRS factor value for workers in the version of the draft version of the SsI addition 

presented for peer review was not weighted based on a comparison of the level of contamination to a health-based 

benchmark, calculated based on residential exposure scenarios. The benchmarks for comparison for populations 

in the proposed HRS addition are the oral cancer slope factor and the reference concentration for inhalation 

exposures. However the benchmarks to be used are for the worst case exposure scenario regardless of land use, as 

is done in all other pathways and components of the HRS. It would be beyond a screening evaluation to precisely 

establish a site-specific benchmark reflecting different exposure scenarios (Note: The relative weighting for fulland part-time workers is 1/3 and 1/6, respectively, which reflects a difference in a typical daily exposure length 

for workers and residents.)

This comment results in no change to the proposed addition to the HRS.
45

2.6 General Comments Affecting All Charge Questions 

The following comments apply generally to the draft SsI addition to the HRS as well as each of the 5 charge 

questions presented above. 

2.6.a Definitions

Comment(s): One peer reviewer questioned whether the terms “gas” and “vapor” have the same meaning or if the 

proposed addition to the HRS intends to define them differently than one another. 

Response: The terms “gas” and “vapor” are used as synonyms in many EPA guidance documents; however, 

technically they can hold different meanings. Scientifically, gases refer to a substance in a gas phase while a vapor 

is a suspension in a gas. Because of this issue, definitions of these terms have been included in the proposed 

addition's TSD. The agency also notes that for the purposes of the proposed addition, the term "vapor intrusion," 

includes the intrusion of both vapors and gases and that the two terms are used interchangeably for this purpose. 

2.6.b Consistency with Vapor Intrusion Guidance

Comment(s): One peer reviewer commented that the draft SsI addition to the HRS is in general inconsistent with 

the current EPA- Office of Solid Waste and Emergency Response (OSWER)

15 and EPA-OUST vapor intrusion 

guidance documents. 

Response: EPA workgroup developing this proposed addition to the HRS has worked with the vapor intrusion

guidance workgroups to ensure consistency among the three documents. However, in some cases, because the 

HRS is applied at a different stage in the Superfund process when only limited data are available, the HRS 

methodology may be different than the methodologies in other guidance documents that are designed for different 

stages in the risk assessment and remediation processes. 

15 During the peer review period, EPA’s OLEM was known as the Office of Solid Waste and Emergency Response. 
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3.0 Comments Outside the Scope of the Charge

In addition to comments directly related to the five charge questions, peer reviewers submitted comments on 

topics not included in the charge questions. These comments are acknowledged in the following subsections, and 

have been divided into technical and policy comments. While these comments are not responded to in this 

document, they were considered in developing the proposed addition. 

3.1 Technical Comments Outside the Scope of the Charge 

Of the peer reviewers’ comments on topics outside the scope of the charge questions, some were technical in 

nature—that is, they are comments on the rationale for the approaches used in the Hazard Ranking System (HRS) 

or in the implementation of the HRS. The U.S. Environmental Protection Agency (EPA) reviewed these 

comments and, where applicable, made changes to the draft subsurface intrusion (SsI) addition. An individual 

response is not provided for these comments in this document; as they were related to sections of the 1990 HRS 

that are not being modified or were beyond the original charge put to the peer reviewers. However, because the 

comments involve the technical content of the proposed addition, they are summarized below. 

3.1.a General Comments

General comments on HRS-related terminology and the equation used to derive an HRS site score were provided 

and are summarized below: 

• Suggested modifying the terminology used in the draft SsI component (HRS Section 2.1.3) to identify

sources or areas of contamination.

• Agreed with the form of the HRS site score equation and how the proposed subsurface intrusion

component was incorporated into it.

• Questioned the definition of the draft SsI component as a single threat evaluation if defined to include

intrusion by liquids (direct contact and inhalation) and vapors (inhalation). Additionally, it was noted that

subsurface intrusion threats could include the potential for explosions.

3.1.b Likelihood of Exposure/Release

Several comments were submitted on aspects of HRS observed release criteria and the establishment of an 

observed exposure as described in the draft SsI addition at the time of peer review. These comments are 

summarized below: 

• Questioned the identification and use of solid or liquid substances as a means to identify an observed

exposure by direct observation.

• Inquired if there was a criterion for establishing an observed exposure in the draft SsI addition to the

HRS.

• Questioned the suitability of the criteria used to establish an observed release of a hazardous substance

under the HRS for evaluating the “vapor intrusion pathway.”

• Suggested restricting observed release evaluations to using ground water and soil gas data and not indoor

air data.

• Questioned how conflicting lines of evidence (e.g., detections in ground water but not soil gas) would be

evaluated.

• Suggested that if the background level is elevated above an applicable benchmark, any indoor air result

greater than that level should be considered elevated under the draft SsI component.
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• Inquired what the evaluation procedures would be for a situation where “health levels of concern are at or

below background levels.”

• Recommended the use of reference values instead of nearby sampling data to establish background levels.

• Recommended either including additional discussion on the appropriate use of background levels in the

draft SsI component or references to supplementary discussions of this topic in other rulemaking

documents.

• Noted that the rulemaking documents for the draft SsI component did not discuss the type of values to be

used in setting background levels (e.g., average concentration, maximum concentration). If a discussion

was not provided, it was suggested that the agency provide a reference to an appropriate guidance

document.

• Noted that as indoor air can enter the subsurface, the identification of a hazardous substance in both the

subsurface and indoor air does not “conclusively prove that indoor concentrations are resulting from SsI.”

3.1.c Waste Characteristics:

Peer reviewers submitted comments regarding the evaluation of waste characteristics as described in the draft SsI 

addition at the time of peer review. These comments are summarized below: 

• Questioned the use of categories A, B and C as weight-of-evidence classifications for carcinogenicity.

• Questioned why the changes in reference concentration (RfC) values in HRS Table 2-4 of the draft SsI

addition to the HRS do not increase at the same rate as the assigned toxicity values.

• Recommended that to “capture true risk” to not assign the toxicity factor value based on only one

substance.

3.1.d Targets

Peer reviewers submitted comments on the evaluation of targets as described in the draft SsI addition at the time 

of peer review. These comments are summarized below: 

• Questioned assigning a value of 20 for the exposed individual factor value if at least one eligible receptor

is located in an occupied structure within an area of subsurface contamination (ASC), as there is no

evidence that this individual is exposed.

• Proposed an alternative weighting system for actually exposed targets that assigned one of three potential

weightings based on comparison to a benchmark.

• Suggested using long duration or multiple targeted shorter-term ambient air sampling events to better

represent long-term indoor air exposure levels.

• Agreed with the agency’s rationale for not including sensitive environments as an eligible target category.

3.2 Policy Comments Outside the Scope of the Charge 

Peer reviewers submitted several comments on policy related to the 1990 HRS and the HRS evaluation described 

in the draft SsI addition. Such comments are beyond the scope of the charge, and therefore neither summaries of 

these comments nor responses are provided. However, where applicable, modifications in response to these

suggestions were incorporated into the proposed addition. 
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4.0 Editorial Comments 

Peer reviewers submitted several comments on the 1990 Hazard Ranking System (HRS) and the draft subsurface 

intrusion (SsI) addition that were editorial in nature. Such comments are beyond the scope of the charge, and 

therefore neither summaries of these comments nor responses are provided. However, where applicable,

modifications in response to these suggestions were incorporated into the proposed addition. 

5.0 List of Commenters

The peer review panel for the U.S. Environmental Protection Agency’s (EPA’s) Proposed Rule: Addition of a 

Subsurface Intrusion Component to the Hazard Ranking System included six individuals from the academic, 

government (both federal and state), and private sectors representing a range of expertise in public health risk 

assessments, contaminated soil and ground water investigations and the topic of vapor intrusion. The six members 

of the peer review panel are listed below:

Tonia R. Burk, Ph.D., Environmental Health Scientist, U.S. Department of Health and Human Services, Agency 

for Toxic Substances and Disease Registry, Atlanta, Georgia. Dr. Burk’s current responsibilities include serving 

as the agency’s expert on vapor intrusion and making recommendations on actions that serve to protect the public 

health. 

Blayne Hartman, Ph.D., Vapor Intrusion Assessments of methane and volatile organic compounds and Soil Gas 

Support, Hartman Environmental Geoscience, California. Dr. Hartman specializes in vapor intrusion support 

services, with many years of experience conducting vapor intrusion assessments and soil gas sampling events. Dr. 

Hartman has served as a contributing author and editor to vapor intrusion and soil gas related guidance documents 

for federal and state agencies and private organizations. Dr. Hartman has also participated in multiple state, 

federal and private workgroups related to the field of soil vapor investigation methods. 

Paul C. Johnson, Ph.D., Dean & Professor, Fulton Schools of Engineering, Arizona State University, Tempe, 

Arizona. Dr. Johnson is recognized for his contributions to the fields of soil and ground water remediation and 

risk assessment. Dr. Johnson has also authored multiple professional journal articles related to the sciences of 

vapor intrusion and subsurface contaminant monitoring and remediation. 

Kimiko A. Klein, Ph.D., Staff Toxicologist Emerita, California Department of Toxic Substances Control, 

Berkeley, California. Dr. Klein was a part of the Department of Toxic Substances Control’s working group to 

develop guidance for evaluating the intrusion of subsurface vapors into indoor air pathway. 

Dot Lofstrom, M.S., Section Chief, Geological Services Unit, California Environmental Protection Agency – 

Department of Toxic Substances Control, Sacramento, California. Ms. Lofstrom’s current responsibilities include 

serving as a supervisor of geologists working on investigations of and remediation activities of soil, soil gas and 

ground water contamination within the State of California. Ms. Lofstrom also serves as the vapor intrusion/soil 

gas collection specialist within the Department of Toxic Substances Control and was the lead author of the 

department’s Vapor Intrusion Mitigation Advisory and technical editor for the department’s Guidance for the 

Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air (Vapor Intrusion Guidance). 

Henning Larsen, R.G. and, M.S., Senior Hydrogeologist, Oregon Department of Environmental Quality, 

Portland, Oregon. Mr. Larsen’s current responsibilities include serving as an agency expert on contaminant fate 

and transport modeling, developing and providing senior level review of hydrogeological investigations, 

developing statewide policy, guidance and rule recommendations for the State of Oregon Cleanup Program. Mr. 

Larsen also was the coauthor/primary author of the State of Oregon’s Guidance for Assessing and Remediating 

Vapor Intrusion in Buildings.