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OAR Box 1112 

Prepped by Ryan Dugan 

Document Number: 

31) II-C-2 

Docket Number: 

A-83-22 

Printed 4/16/2008 9:12:07AM Sheet 31 of 97 
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\ UNITED STATES ENVIRONMENTAL PROTECTION AGENCY JT , C - 2_ 

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 ^R Z ? Office of Air Quality Planning and Standards 

{SyT*~J Research Triangle Park, North Carolina 27711 

DEC 2 1 19*3 RECEIVEO^TnM 

ENVIRONMENTAL PROTECTION 

AGENCY 

JUL 12 1984 

Dr. Paul Kotin CENTRAL DOCKET 

4505 South Yosemite SECTION 

Denver, Colorado 80237 

Dear Dr. Kotin: 

Enclosed for your review and comment is the staff's identification 

and discussion of the major issues raised at the September 14-15, 1983 

Clean Air Scientific Advisory Committee (CASAC) Risk Assessment Subcommittee 

meeting. In addition to describing the major issues, we provided responses 

and/or outlined what efforts have been undertaken since the meeting to 

address the questions raised. 

The principal purpose of the enclosure is to make sure that we have 

properly characterized the Subcommittee's comments and have identified 

major areas of concern so they can be properly addressed during the 

development of the lead risk assessment program. If we have inadvertently 

misinterpreted comments made at the meeting or omitted any important 

issues, please let us know as soon as possible. We of course will make 

all such correspondence available to other members of the Subcommittee as 

well as the public (upon request) so everyone will be kept informed. 

Please direct any comments you may have to Tom McCurdy. If you 

have any questions, please feel free to call Tom at (919) 541-5655. 

In addition, I would like to thank Subcommittee members for 

participating in our September meeting, and especially appreciate 

Dr. Anita Curran's fine job of chairing the meeting in your absence. 

We look forward to hearing from you on these matters. 

Sincerely yours, 

/•* / 

Joseph Padgett 

Director 

Strategies and Air Standards Division 

Enclosure 

cc: Subcommittee Members 
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AMBIENT STANDARDS BRANCH STAFF IDENTIFICATION AND DISCUSSION 

OF ISSUES RAISED AT THE SEPTEMBER 14-15, 1983 MEETING OF THE 

CLEAN AIR SCIENTIFIC ADVISORY COMMITTEE SUBCOMMITTEE ON 

RISK ASSESSMENT 

Introduction 

ASB staff reviewed the unedited two-volume transcript of the Subcommittee 

meeting received from M.P. Currin and Associates, court reporters. Staff has 

grouped the various issues and concerns raised by Subcommittee members 

concerning the lead risk assessment effort into four major categories, as 

listed below. 

The following discussion is intended to foster a dialogue between EPA 

staff and interested parties on an appropriate methodology for assessment 

of health risks posed by exposure to airborne lead. In addition to 

summarizing issues raised by the Subcommittee, we have sought to respond 

to the major concerns expressed and to clarify points about which some 

misunderstanding was apparent at the meeting. In some cases, our comments 

reflect staff views on a topic, such as guaranteeing anonymity of the 

expert's probabilistic judgments and not forcing convergence of different 

expert's judgments. In other cases, we simply agree with a Subcommittee 

member's comment or explore alternatives for implementing a recommendation. 

Comments regarding this staff discussion should be addressed to: 

Thomas McCurdy 

Ambient Standards Branch 

U.S. Environmental Protection Agency 

MD-12 

Research Triangle Park, N.C. 27711 

(919) 541-5655 

(FTS) 629-5655 

A l l written communications wil l become part of the public record regarding 

the lead national ambient air qualit y standard (NAAQS) review. Al l verbal 

comments wil l be summarized and also wil l become part of the public record. 

Anyone interested in inspecting the public record on the lead air standard 

review, may see or copy material in Docket #A-83-22 available at: 

Central Docket Section 

U.S. Environmental Protection Agency 

401 M Street, SW 

Washington, D.C. 20460 
- ^ 

Table of Issues Addressed 

Issue Page 

1. Role of the lead risk assessment in the lead NAAQS review. 2 

2. The use of expert judgment in assessing health risk. 4 

3. Technical problems specific to a lead risk assessment. 10 

4. General issues. 24 

1. Role of the Lead Risk Assessment in the Lead NAAQS Review. 

These issues are relatively non-controversial and really involve Subcommittee 

member questions more than anything else. 

a. QUESTION: What role will the lead risk assessment play in the lead 

NAAQS review effort? 

RESPONSE: This point was addressed in part by Bob Kellam in his presentation 

to the Subcommittee (Vol I., p. 232 ff). It was also addressed in a more 

general way by Tom McCurdy (Vol I., p. 24 ff). The risk assessment generates 

health risk (probability) estimates associated with alternative air standards 

and assesses the nature and severity of possible adverse health consequences 

if they were to occur. The probabilistic approach is an explicit way of 

incorporating existing knowledge and major uncertainties into modeling what 

adverse health effects might result from alternative air standards, given all 

of the non-air exposures seen by children and others. The risk assessment 

permits a more meaningful and explicit treatment of the margin of safety 

issue, providing a means for equating an adequate margin of safety with a 

policy decision concerning what constitutes an acceptable level of risk for 

lead. This decision, of course, requires that risk estimates be provided. 

(Note that we distinguish between risk assessment and risk evaluation; we are 

undertaking a risk assessment only for lead. A formal risk evaluation probably 

will not be done as part of this exercise.) 

Results of the lead risk assessment will possibly be used in two major 

ways. First, risk estimates would be used in developing and assessing the 

"range of interest" for possible alternative air standards that is defined in 

the OAQPS "Staff Paper." Second, they would become part of the rationale for 

selection of the standard, specifically that part of the rationale focused on 

judging what constitutes an adequate margin of safety. This rationale appears 

in the Federal Register notice accompanying a proposed or promulgated NAAQS. 

J 
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b. QUESTION: What role will the CASAC Subcommittee on Risk Assessment 

play in the lead NAAQS review? 

RESPONSE: The Subcommittee is being asked to advise OAQPS on the conduct 

of the lead risk assessment effort, including such things as design of the 

project plan, experts to be consulted, development of the encoding protocol, 

and structuring of the various component models comprising the total effort. 

The Subcommittee also is part of CASAC's review of the lead criteria document, 

the staff paper, and some of the major supporting analyses (e.g., exposure 

analysis). 

2. Issues related to the use of expert judgment in assessing health risk. 

There was much discussion at the meeting on the use of expert judgments 

in the planned risk assessment. Much of it was general in nature, 

applying to all efforts using formal expert encoding to explicitly address 

major uncertainties in the data base. These general issues, as well as issues 

specifically addressing the lead effort, are discussed below. Since there 

is substantial overlap among the issues, the breakdown is somewhat artificial. 

a. COMMENT: Margin of safety considerations should be separated as much as 

possible from the risk assessment effort, especially the expert encoding part, 

so as to not "compound" the margin of safety. 

RESPONSE: While staff would not state the issue in the above manner, 

we agree with the comment. To us, what standard would provide an adequate 

margin of safety is obviously a value judgment that EPA policymakers should 

make. Such policy considerations should play no part in a risk assessment 

effort, which is analytic in nature. Addressing and attempting to minimize 

strategic and unintentional motivational bias is a standard part of the 

probability encoding process. Building "margins of safety" into judgments 

would be a bias in one direction just as judging degree of scientific 

confirmation instead of probability would be a bias in the other. Every 

effort will be made to minimize bias in either direction. 

One never knows for sure in any given case whether all motivational and 

cognitive biases have been eliminated. This is one of several reasons it is 

desirable to use a balanced sample of experts rather than just one. The 

staff is aware of the types of problems that can arise within an enterprise 

which has inherently subjective aspects, but emphasizes that the base of 

comparison is a less systematic approach with the same subjective aspects, 

not an unattainable objective ideal. 

More information regarding debiasing is contained in Drs. McNamee's 

and Wallsten's presentations at the meeting (Vol. I, p. 65 ff and p. 170 ff). 

b. ISSUE: Any technique that relies on opinions of experts is heavily 

influenced by the experts chosen. (Or, as another Subcommittee member put 

it, the neutrality of judgments to be obtained from the experts is questionable.) 
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RESPONSE: We agree. It is unfortunate that risk assessment is not an 

entirely objective matter. The important thing is that the approach elicits 

the right kinds of judgments from some of the best people to make them and 

makes use of them in the appropriate manner. 

We generally use the term 'judgment' rather than the term 'opinion' for a 

reason. The connotational difference is important. Reasoned probability 

judgments which incorporate both the available knowledge and the remaining 

uncertainty, obtained via tested elicitation techniques should not be equated 

with "opinions" (or point estimate "guesses") although opinions enter via the 

reasoning process which such judgments are made. Probability language is 

a flexible, yet precise, language in which the state of information, such as 

it may be, is represented in the most useful way. 

ASB staff and management recognize that the Agency should obtain a 

representative sampling of the entire spectrum of scientific thought on an 

uncertain topic in the lead risk assessment effort. This includes all aspects 

of the effort, and not just those components focused on dose-response 

relationships or on physiological interaction among body lead pools. However, 

there are definite practical limits to how much we can do. (See Issue 2.d 

below.) 

To obtain a diversity of scientific views on a particular health effect, 

ASB staff plans to continue its review of the lead criteria document, consult 

with ECAO staff working on it, and talk with CASAC Subcommittee members to 

identify experts who hold differing views on the scientific evidence suggesting 

an effect. For each health effect category that will be addressed in the 

risk assessment, we will then structure a list of experts representing the 

range of fully informed views. This list will be pared to match resources 

available for encoding. (Some "extra" people may be included in case some 

experts are unavailable at the time of encoding.) Generally, international 

travel will be avoided in the encoding effort due to resource limits. 

The pared list of experts to be encoded will be presented to the CASAC 

Subcommittee in a public meeting to allow all interested parties to review 

and comment upon our selection of experts. While ultimate responsibility for 

selecting experts rests with EPA, we would like the Subcommittee to determine 

whether or not we have included a reasonable spectrum and balance of scientific 

views. 

Not encoding all views on a topic does not undermine the approach; an 

omission may narrow the band of risk estimates that will ultimately come out 

of the exercise. This is particularly true if an extreme view is not encoded. 

c. COMMENT: The assessment obtained from an expert is markedly influenced 

by the way in which questions are asked. 

RESPONSE: The extent to which this is true is unknown. However, the 

extent to which the encoding protocol itself affects important judgments 

deserves close scrutiny. Since the expert also will have the protocol prior 

to his or her session, the expert becomes a self-monitor of the approach. 
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The Subcommittee and the interested public at large will have ample opportunity 

to review and comment upon the encoding protocol. 

d. ISSUE: The number of experts to be encoded is too small. OAQPS needs 

more than three experts per topic to obtain a useful range of judgments 

regarding most lead issues. 

RESPONSE: As ASB staff agreed at the meeting, it is desirable to encode 

more experts. However, each encoding requires resources which are limited. 

Each encoding session requires at least one trained probability assessor and 

a technical assistant to conduct the session and record information. The 

sessions are long, and no more than one expert a day can be encoded by one 

"team." Since experts are geographically dispersed, considerable travel is 

required so it is unlikely that more than two experts can be encoded per week 

per team. In addition, logistics of the effort are enormous. 

Encoding sessions are only part of the process. Preparation time is 

required by each encoding team. Protocols and training materials have to 

be developed, pre-testing has to be done, and revisions have to be made. 

All of this is time consuming and expensive. 

Current resources available to ASB for the lead encoding effort are 

estimated to be sufficient for a maximum of 30 encoding sessions. (For 

more discussion of this point, see Issue 4.a below.) We had originally 

planned on encoding 15 experts twice on at most five health end-points (or 

three experts per end-point). We did not anticipate a need for extensive 

encoding for the exposure and physiological components of the risk assessment 

effort. (We had planned to represent some of the most significant uncertain 

aspects of these two components via a combination of staff work and phone 

calls to experts). Given the Subcommittee's comments regarding the exposure 

and physiological models, however, it is apparent that encoding in these 

areas is needed. 

In the absence of additional funds for the lead risk assessment effort, 

undertaking encoding for the other two components will require a reallocation 

of the 30 encoding sessions. None of the reallocation options are particularly 

attractive. Three that immediately come to mind are: 

1. reduce the number of health end-points to be addressed. (The 

Subcommittee has already made such a recommendation, but see the 

discussion below of Issue 3.b.) 

2. reduce the number of experts encoded on any one end-point. 

3. eliminate the second encoding session for an expert for all or some 

of the end-points. 

Perhaps a combination of these strategies can be used to develop a more 

expansive but still viable encoding effort. We think it is important to 

formally and explicitly address the large uncertainties inherent in the 
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current state of information on lead. ASB staff welcomes suggestions from 

the Subcommittee on how we can best use the encoding resources available for 

this project. 

e. ISSUE: The number of experts to be encoded on a topic should vary 

depending upon the diversity of views in the scientific community or upon 

the significance or magnitude of subsequent health effects. 

RESPONSE: We distinguish this issue from the preceeding one in that the 

number of experts to be encoded on any topic conceivably could be more or less 

than three if an "importance" criterion is used. ASB staff agrees with the 

comment in general and will take it into consideration when allocating resources, 

f. COMMENT: Each expert is so narrow technically that her or his judgments 

become relatively valueless beyond the technical sense. 

RESPONSE: We will not be asking for synoptic information from an expert. 

We hope to elicit the expert's judgments concerning a focused scientific or 

technical matter that is inherently uncertain. We believe that the best 

place to obtain this information is from an expert on the topic. We will 

synthesize a particular expert's judgments with other judgments via mathematical 

models. No individual expert is expected to provide global judgments. And, 

as stressed above in staff's response to Issue 2.a, we do not want social 

value judgments from an expert as part of this exercise. 

Itis important to recognize that if a lead risk assessment were not done, 

at some point in the lead NAAQS review process someone (probably the proverbial 

"faceless bureaucrat") would have to make implicit or explicit decisions on 

most of the issues to be addressed in encoding sessions. If experts are 

involved, but in the ad hoc and informal manner as they are now, then the 

decision-making rationale is difficult to document and explain to the public. 

g. COMMENT: OAQPS should talk to experts before it structures the various 

models. This will allow staff to determine what is known about the problem so 

that realistic models can be developed. 

RESPONSE: We agree. As Subcommittee members who attended the meeting 

know, ASB staff were not allowed to substantively discuss the lead risk 

assessment effort with non-EPA scientists working on or reviewing the 

lead criteria document until public release of that document. While we 

did review the relevant literature, this certainly constrained what we 

were able to do in structuring our various component models since most 

lead experts are contributors to or are working on the CD. 

With release of the draft CD in October, we have begun a modest effort 

to discuss model structuring issues with various experts. This effort 

will expand in the near future. In addition, and as was suggested, we 
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will be using Subcommittee members as a "clearinghouse" for suggestions 

on who we should contact on particular model building issues. If any 

Subcommittee member would like to suggest experts on model-bui1 ding that 

we should consult with, please contact Tom McCurdy. (NOTE: Three exposure 

experts were recommended to us at the meeting: Drs. Spengler, Moschandreas, 

and Fugas. We will be contacting these and other individuals shortly. 

For the record, however, we have had contact with the first two experts 

and are familiar with the work of all three.) 

h. COMMENT: The best way to obtain information from experts is to develop 

an overall strategy for obtaining needed inputs and adapt it to each expert 

depending upon the circumstances. If the problem is over-structured, OAQPS 

will miss some specifics and opportunities that arise during the process. 

RESPONSE: ASB staff agree in general with the comment, however some 

discussion is warranted regarding flexibility of the model structure, 

especially at different stages in the risk assessment process. Our desire 

is to be as flexible as possible but there are three types of contraints 

on how flexible we can be. One constraint concerns the inherent logic of 

our situation, another concerns the goldfish bowl nature of the NAAQS 

setting process, and a third derives from our limited resources. These 

constraints are discussed below. 

Our overall strategy for obtaining information from experts is to 

first involve them in informally structuring the various models needed in 

the assessment and in identifying what kinds of judgments are most easily 

made by lead experts. We then want to formally encode experts on the 

form of functional relations and/or on values for particular variables in 

the relations. During the encoding sessions we plan to record and later 

present any observations the experts care to make. If two rounds of 

encoding are undertaken, the above will be repeated (and perhaps some 

model restructuring will also be accomplished). Finally, experts will 

review and comment upon outputs of the risk assessment project. 

With respect to the constraints, there are certain relationships that 

need to be captured in assessing risks associated with alternative NAAQS. 

As we see it, assuring that these relationships are incorporated into the 

risk model in a logical and defensible manner is one of the major 

responsibilities of an analyst in the assessment effort. Thus, once a 

model structure is derived—with expert assistance, of course— it cannot 

be changed arbitrarily to fit an expert's views. Thus, there is a limit 

to how flexible an analyst can be with respect to model structure. 

Using more than one model would increase flexibility, but this is probably 

infeasible due to resource limitations. By expanding and lengthening the 

process of interaction with various experts we could probably increase 

consensus among experts to be encoded concerning the best compromise 

model. (We mean consensus on the model and hence on inputs to be encoded -

not on probability judgments themselves.) However, available resources 

constrain the extent of consensus building we can feasibly undertake. (See 

the discussion of Issue 4.a on p. 24 for an elaboration.) 
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Finally, there are special circumstances surrounding most NAAQS standardsetting efforts that usually reduce the amount of flexibility available to 

us. The most important one from our experience in setting NAAQS is the need 

to extensively document for judicial review purposes what factors were considered 

in the decision-making process. This accountability mandate also applies to 

the encoding process, if one is undertaken. The need for accountability and 

the desire to avoid any appearance of manipulation of experts by EPA or its 

contractors forces us into a less flexible posture than might occur in a 

non-regulatory setting. It results in a fixed list of experts to be encoded 

and a "public" protocol available for review prior to the encodings. 

In the conduct of the lead NAAQS risk assessment effort, ASB and its 

contractors will be as flexible as possible within the constraints noted above. 

i. ISSUE: Should the encoded distributions be attributed to specific 

experts? 

RESPONSE: The Subcommittee itself was divided on this issue. Some 

members thought that every distribution should be identified with a 

particular expert; other members thought that anonymity was appropriate. 

ASB's plan was to not identify each encoded distribution with an expert, 

unless an expert desired it. Our preference was to simply label each 

curve as A, B, C, etc. The expert's reasoning behind her or his encoded 

judgments also would also be presented. In addition, we planned to 

include a list of all experts participating in the study, identified by 

health effect category. Thus, interested readers would know who was 

encoded on a particular health end point (or exposure model variable, 

etc.) but would not be told exactly who provided a particular encoding. 

Some Subcommittee members wanted explicit identification in order to 

promote accountability. The gist of their argument was that experts should 

be willing to defend their judgments. If they cannot, or if they were 

not comfortable being a participant in the encoding process, the experts 

should not participate. 

Other Subcommittee members thought that such identification would have a 

"chilling effect" on the expert's candor and on persuading many experts to 

participate. They felt that while scientists and other experts should be 

held accountable for their scientific conclusions, this reasoning did not 

apply to encoded judgmental probability distributions, which are judgments 

based on the available evidence, not scientific conclusions. 

ASB staff thinks that labeling each distribution with an expert's name 

might unnecessarily burden some of the experts with a concern about how their 

judgments will be received by various parties. We need the best judgments 

possible, and external pressure can only work against that aim. It is common 

understanding that anonymity generally enhances candor. The argument that 

anonymity would decrease the chance for total candor seems much weaker to us. 

If every distribution is attributed to an expert, some experts may not 

participate in the exercise on the grounds that participation would lead to 

involvement in a protracted public "discussion" (or, more likely, a heated 
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debate) of the merits of each encoded point in the expert's distribution. 

Neither the arena nor the subject matter would be scientific, and many scientists 

simply would not want to be involved in such a debate. 

Checks and balances on manipulation of the expert's distributions have 

been mentioned earlier. (See Issue 2.c.) In addition, the rationale behind each 

major encoded distribution will be provided along with the data so that 

readers can judge for themselves if the distribution makes sense (i.e., is 

based upon a fair review of the scientific evidence and is internally consistent), 

These checks should be sufficient to avoid making an individual's probability 

judgements the focus of public debate. 

Despite its merits, however, there remains the legal question of whether or 

not EPA can guarantee anonymity of encoded experts or make use of the resulting 

encoded distributions in a NAAQS risk assessment without attributing them to 

their source. EPA's Office of General Counsel (OGC) is currently investigating 

this issue and will make a recommendation to ASB on how to proceed. Their 

recommendation will be made available to the Subcommittee upon receipt by ASB 

staff. 

j. ISSUE: Expert judgment should not be aggregated into a combined, or 

single, distribution. 

RESPONSE: ASB staff agrees. Forcing the experts to converge or averaging 

the experts' results obscures situations where various researchers may have 

fundamentally different points of view and judgments concerning the phenomenon 

being encoded. Staff agrees with some Subcommittee members that the EPA 

Administrator should be given this information (as well as the reasons behind 

such disparate views). In addition, the "spread" among experts provides a 

vivid representation of the subjectivity involved in representing uncertainty 

associated with a variable or relationship being analyzed. The Administrator 

and public should be presented with this information as well. 

We also agree with some Subcommittee members that it is useful to have some 

type of feedback among the experts, _rf two rounds of encoding are done. This 

feedback would consist of showing the previous round's encodings along with 

an expert's reasoning for his or her distribution. This type of feedback 

would have value in that it would help experts become aware of other 

possible perspectives and judgments. 

This does not mean, however, that ASB staff thinks that a formal Delphi 

approach should be used to promote interaction among experts. We have no 

plans to use the Delphi or other formal processes designed to promote group 

consensus. 

k. ISSUE: Experts should be confined to material reviewed in the criteria 

document for the bases of their judgments. 

RESPONSE: The experts will be encouraged to be fully familiar with 

relevant material in the criteria document (CD). For both legal reasons and 
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reasons having to do with our view of how risk assessments are best done, we 

think it is important that scientific assessment of the state of knowledge 

given in the relevant criteria document be the scientific basis of the 

probabilistic risk assessment. Hence, to this extent we agree with the 

comment. 

However, as various meeting attendees recognized, it is also important to 

be realistic about the nature of probabilistic judgments. Even though they 

are based on a given scientific assessment of a data base they can also be 

influenced by other things. This includes the expert's training, history, 

temperament, background reading, and what he or she knows about ongoing or 

recently completed research. 

We cannot simultaneously ask an expert to give his or her best probabilistic 

judgment while constraining the expert to a particular scientific assessment. 

Probability judgments and scientific assessment judgments are related but 

different in important ways. It may make sense to undertake a scientific 

assessment of a given prescribed data base, but it is probably impossible to 

make a probability judgment that is influenced only by a prescribed data 

base. 

Please note that each expert will be encouraged to refer to the criteria 

document if it would help during the encoding procedure. Doing this allows 

the experts to refresh their memories on specific issues. In short, ASB staff 

certainly agrees that, interpreted realistically, the criteria document would 

provide the basis for the risk assessment. 

3. Technical issues that are specific to lead. 

The Subcommittee raised a number of issues relating to the technical 

feasibility of undertaking a risk assessment of alternative air lead 

standards. Most of the Subcommittee's concerns were focused on the 

exposure modeling component of the risk assessment. However, issues were 

raised about most components. This Section proceeds from the general to 

the specific, and within the specific issues part, from exposure to 

dose-response issues. 

a. ISSUE: Lead is too complex an issue to adequately analyze via a formal 

risk assessment method. (Specific complexities are noted below.) 

RESPONSE: The above statement was a theme voiced repeatedly in the 

two-day meeting. In response to this issue, ASB staff submits the following 

excerpt from a letter to Dr. Bernard Goldstein from Joseph Padgett dated 

May 4, 1983. 

oo 

g 

In the past, you have expressed concern that lead might be t 

controversial a pollutant for EPA's first attempt at incorporate 

formal risk assessment into the NAAQS standard-setting process. 

I agree that there is a great deal of difficulty surrounding the 

evaluation and interpretation of certain health effects attributed 
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to ambient lead exposures. Central nervous system impacts are a 

case in point. However, as you know, this is the case with most 

of our NAAQS pollutants; lead is not unique in that respect. 

We feel that the inherent uncertainty surrounding lead health 

effects makes lead a particularly good choice for doing a 

probabilistic risk assessment of alternative NAAQS. Our proposed 

method will explicitly model this uncertainty so that the impact 

of a weak information base on the estimation of health risks is 

clearly delineated. The methodology also permits comparison of 

health risks for many possible adverse health effects of ambient 

lead. In this manner, the controversy can be put into perspective. 

The quantitative description of uncertainty associated with all 

of the health effects categories will allow a more rigorous 

evaluation of the impacts of alternative lead air standards. 

(However, as you know, the lead risk assessment effort will not 

include a formal risk evaluation effort at this time. The 

quantitative integration of risk estimates into a formal evaluation 

method to further aid decision-making must await another time.) 

All of these virtues aside, as we mentioned at the meeting, if the 

Subcommittee feels that there are insurmountable problems associated with 

the conduct of a risk assessment for lead or if insufficient progress is 

made, we will discontinue the effort. 

b. ISSUE: Argonne presented a list of six health end points that could 

be used in the risk assessment. 

For three of the end points: renal function, immunological reactions, and 

reproductive/developmental effects there are little data but, they indicate 

adverse responses only at relatively high exposure levels. Therefore, 

these three end points should be dropped from the assessment effort. 

RESPONSE: The endpoints presented by Argonne were for illustrative 

purposes and do not necessarily represent health effects that ASB will 

include in the assessment. Evidence summarized in the criteria 

document indicates varying degrees of uncertainty associated with the 

entire spectrum of health responses to lead exposure. While the SubCommittee's advice to delete from consideration those effects which have 

been observed to occur in organs and other systems in children at relatively 

high levels is helpful, it is important to note that knowledge is incomplete 

and inconclusive, and the possibility of more general toxic effects 

cannot be ruled out. This seems particularly true for data, mostly from 

animal investigations, on alterations in fetal physiology and metabolism 

and post-natal delays in neurobehavioral development. Attempting to 

characterize uncertainties associated with such effects is the intent of 

the risk assessment. 

We are currently developing an encoding protocol for lead's interference 

with hemebiosynthesis and erythropoietic effects. This protocol will be 

initially reviewed by appropriate experts (e.g., hematologists), selected 
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12 

with the help of criteri a document authors and consultants. We plan on 

then sending the protocol to Subcommittee members for their review prior 

to our next meeting. 

c. ISSUE: There are numerous problems with the proposed lead NAAQS 

exposure model (lead-NEM). Various Subcommittee members raised the 

following issues concerning the proposed lead-NEM. In some cases, 

we group our response to several issues. 

ISSUE cl : There i s no good large set of air qualit y data that i s available, 

useable, or believable, especially in terms of the time resolution of 

exposure proposed in the lead-NEM. Data cannot be "manufactured" from 

the l-in- 6 days sampling of 24-hour air lead levels that is suitable for 

analyzing hourly or even daily air lead exposures. 

ISSUE c.2: Air lead monitoring stations are in the wrong place for estimating 

human exposures. 

ISSUE c.3: There are no monitoring data available to account for two factors 

that affect air lead exposures: the variation in chemical composition of 

lead particles and the variation in particl e size. 

In response, we agree that air data are lacking, do not full y represent 

human exposures, and are not well described as to particl e size or chemical 

composition. We wil l utiliz e time series analysis to fil l in missing values 

and use the neighborhood type, microenvironment concepts, and available 

praticl e size/composition data to better relate.ambient monitoring data to 

the type of air lead seen in human habitats*. Lead is not unique in having 

these shortcomings; al l NAAQS air quality information is deficient in at 

least some of the ways mentioned. 

Staff has reviewed al l lead air data in EPA's SAROAD system. From the 

hundreds of urban areas having data meeting EPA's acceptance criteri a for 

completeness, we picked 10 areas meeting certain population size and 

geographic criteri a that had the best data. 

For the most part, ambient lead is monitored for one 24-hour period 

every six days. This is no differen t than particulate matter, for which we 

have already developed a NAAQS exposure model. To obtain a ful l set of daily 

values, we fil l in missing data using time-series analytic techniques. This 

data-supplementing approach has been used on al l NAAQS pollutants analyzed 

to date. We would not attempt to develop one-hour data from 24-hour 

data. We had no plans to do thi s for lead, although that point probably 

was not made clear at the meeting. 

We are currently doing a detailed analysis of lead data having nearly 

complete annual data sets of 24-hour data. The City of Philadelphia has 

at least two stations reporting one or more years of complete daily 

averaged data. Apparently, there are a few other sites with the same 
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type of information. We are also conducting statistical analyses of the 

feasibility of replicating the original data distribution with various 

random samples of l-day-in-6 data sets. We are also comparing these 

sampled data sets supplemented by filled-in time series data with the raw 

data set to test how well filled-in data replicate raw data. These 

analyses are being done by PEDCo under contract to us. Written results 

will be sent to the Subcommittee as soon as we receive them. 

Other data problems related to the use of monitored data mentioned by the 

Subcommittee were known to us, such as the complex air flow and turbulence 

patterns characteristic of cities. This makes it difficult to relate monitor 

readings to ambient levels in localized hot spot conditions, such as in street 

canyons. We will attempt, however, to use available information contained in 

the criteria document on lead's vertical and horizontal dispersion over time 

within urban settings and on re-entrainment of road dust to develop transformation 

factors necessary to more accurately estimate potential exposures to airborne 

lead. 

Concern was raised at the meeting regarding the lack of attention we 

gave to lead particle size distribution and chemical composition of 

airborne lead. Both of these factors influence mobility in the environment, 

solubility, and the likelihood that an inhaled or ingested particle will be 

retained and absorbed. Urban lead particles are predominantly fine mode 

aerosols, with a mass median equivalent diameter consistently within a range 

of 0.23 to 0.30 ym (Robinson and Ludwig, 1969; Stephens et_ al_., 1978, 

1980, 1982). Lead aerosols collected near stationary sources tend to be 

larger (Landrigan et_ aj_., 1975), although the likelihood of their deposition 

into blood exchange regions of the respiratory tract or into the gut via 

the mucociliary escalator may be as high as for fine particles. Inorganic 

lead emitted from automobiles and point sources as lead halides and oxides 

appears to convert to lead sulfates (Ter Haar and Bayard, 1976; Biggins and 

Harrison, 1978, 1979; Dzubay and Stevens, 1973). Organic lead compounds 

(generally unmeasured) are photoreactive and their presence in urban atmosphere 

is transitory (Harrison et_ a_l_., 1979; Skogerboe, 1975). 

We had planned to adjust the lead monitor data via transformation 

factors (albeit based on limited information) in order to account for the 

variations in lead particle size and chemical composition. These 

adjustments, along with other considerations, will be more fully discussed 

in a preliminary ASB staff review of information relevant to the exposure 

model that will be sent to you in the next few weeks for review and 

comment. 

We feel compelled to use monitoring data as the bases of NAAQS analyses 

for both institutional and legal reasons. Attainment of a NAAQS standard 

is determined by what monitors sample, not what occurs in microenvironments 

used by people or by what people are actually exposed to. Implementation 

and control programs are developed so that air quality at the "critical" 

monitor (just) meets an applicable standard. Air quality experienced by 

people may or may not be better than what is officially recorded. Since 

attainment is predicated upon monitor readings, we must work from monitoring 

data. Otherwise, we cannot analyze the operational impact of what it means 

to just attain an alternative ambient air standard. Thus, predicating our 

analyses upon monitoring data is a logical constraint given our current 

regulatory philosophy rather than a matter of choice. 
P.16 

14 

The question then becomes, how do you best represent human exposures 

to airborne pollutants with ambient monitoring data? The NEM approach is 

one way of making that representation. We have no illusions that our 

exposure estimates precisely reflect reality and predict actual exposures. 

This is why we want to undertake probabilistic exposure analyses. We do 

think that estimates based on NEM will be reasonable and as good as can be 

made given the data base and resources available for analysis. The explicitness 

of our formulation allows us to make corrections as needed in various components 

of the NEM model, as was done successfully in the carbon monoxide version of 

NEM. To be able to correct for new information, however, means that you have 

to build in the necessary flexibility from the beginning. This, by necessity, 

results in a complex model with lots of components and variables (many of 

which zero out in the end). 

ISSUE c.4: There are too many cohort groups proposed to be used; many groups 

will have no children in them. 

As noted in the previous discussion of Issues c.l-c.3, we agree 

that many cohort groups may have no children in them. In fact, we welcome 

this occurrence as it reduces computer run time considerably. All previous 

applications of NEM had cohort groups that were eventually not used, but 

this fact was not known in advance of performing preliminary analyses. 

All cohort groups are there for a reason. For lead, the groups are 

defined primarily on the basis of suspected differences in response 

characteristics to lead exposure. However, practical considerations also 

play a role. Since cohort data generally come from U.S. Census Bureau files, 

it is often necessary to adopt their distinctions. (There is no other type 

of information available that is applicable nationwide.) Thus, there is some 

balancing of diverse criteria that has to occur in setting up cohort groups. 

ISSUE c.5: Some parts of the model have to be collapsed (suggestion: use 

only 3 children cohorts by age category rather than 7); other parts may 

have to be expanded. 

We have no problem with this suggestion. We had considered making a 

similar reduction prior to the meeting. It does make our analysis more 

manageable. However, said reduction should only be made if it makes 

sense from an exposure pattern, physiological impact, or health end point 

viewpoint. We solicit and welcome further input from the Subcommittee on 

preferred cohort categories and the rationale for their selection. 

(Having fewer cohort groups does save us computer run money. The savings 

are minimal, however, with respect to file set-up. Our procedure is 

highly automated and the number of categories used does not affect file 

development costs significantly.) 

ISSUE c.6: Air lead levels in urban areas do not correlate well with trends 

in gasoline lead. Traffic density is not a good indicator of lead fallout 

from motor vehicle traffic (citing an EPA report on correlation analyses in 

Dallas or Houston). 
P.17 

15 

We agree with the first part of this comment. Ambient lead comes 

from both primary sources (motor vehicle emissions, point source emissions) 

and secondary sources (re-entrained dusts, soil). We were not planning 

to use gasoline lead levels in our analysis, since our work is based on 

monitored data as noted above. (Or, in the case of point sources, on 

diffusion modeling of lead emissions.) 

We do not agree with the second part of the comment, however. A 

significant correlation between lead concentration in street dust and 

roadside soils and traffic density has been found (Rolfe et_ aj_, 1977; 

Quarles et_ a_U, 1974; Wheeler and Rolfe, 1979). These and other studies 

will be used in deriving appropriate relationships between air lead and 

soil and dust lead levels to approximate these pathways of lead exposure. 

ISSUE c.7: Housing type should be considered in the exposure model as 

well as housing age. From the lead paint viewpoint, old housing in one 

city is often very different than old housing in another due to wall 

covering practices. (The example given was wall paper use in Philadelphia 

vs. paint use in Chicago.) 

ISSUE c.8: Household cleanliness is another factor that affects lead 

exposure; this is correlated with socio-economic standing. To get a 

handle on this issue (and related exposure issues), talk with managers of 

lead-poisoning prevention programs. 

In response to Issues c.7 and c.8, we understand that indoor lead is 

partly a function of the items mentioned and intended to use two variables 

as surrogates for them: pre-1950 housing expressed as a percentage, and 

average family income expressed as the percentage of families below the 

official poverty line. Alternative possible surrogates do exist. (Wall 

paper use data will be a problem, however. U.S. Census data are not 

available on this item.) We will investigate further the Subcommittee's 

suggestions expressed in the two comments, including contacting lead 

poisoning program officials in cities like Chicago and Phildelphia. 

ISSUE c.9: The exposure modeling effort should be based upon material 

contained in the lead criteria document. 

Staff agree and fully intend to base exposure modeling work on 

material contained in the criteria document (CD). All of the transformation 

factors used in the model are based on a review of studies contained in 

the CD. The same is true for other model variables, such as non-air 

routes of lead uptake and microenvironment additive factors (used to 

account for lead generated in a location, not related to ambient lead 

levels). 
P.18 

16 

To provide the Subcommittee with an impression of the extent to which 

ASB ties its exposure model to the CD, we enclose two working memos from 

PEDCo discussing various issues and relating them to various pages in the 

draft lead CD. There are other memos on the subject that we will make 

available to the Subcommittee if it so desires. The enclosed memos are: 

1. "Routes of Lead Uptake." T. Johnson to T. McCurdy, October 13, 1982. 

2. "Lead Uptake Rates by Source." T. Johnson to T. Feagans and 

T. McCurdy, July 5, 1983. 

ISSUE c.10: ASB should review other lead exposure modeling work to see what 

has been done in the past (especially those efforts using simple techniques). 

Staff concur and have and will again review other lead exposure 

models. As mentioned earlier, we will also talk with lead exposure 

modeling experts on the issues noted above (and below). 

ISSUE oil . ASB fit an existing model to lead instead of developing a 

new model; lead is a unique air pollutant, and ASB needs a unique model 

for it. 

We feel that the NEM logic and structure is applicable to all NAAQS 

pollutants. Basically, it relates monitored air quality to human 

microenvironments, accounts for indoor generated air pollutants, uses the 

best information available on population location, and accounts for population 

movements through time and space. Simulation runs can test the impact on 

outputs of alternative values of input variables of interest. In addition, 

NEM is structured toward doing (but has not to date been so used) probabilistic 

analyses of exposures to address uncertainty. 

The major new factors specific to lead that are not already in the 

logic of NEM are indirect and non-air routes of exposure. We have known 

from the beginning that these routes would have to be explicitly handled 

in the lead-NEM. We have devoted considerable time and thought on incorporating 

non-air pathways into the model but agree that we need to spend a lot more 

effort in this area. 

We will attempt to incorporate information on all major pathways into the 

lead exposure model, and represent the most significant uncertainties. 

We recognize that we did not communicate our thinking or progress 

on these issues to the Subcommittee very well. We will attempt to do so 

at the next Subcommittee meeting, and in writing before that. We are 

conscientiously reevaluating our entire exposure modeling approach and 

continue to learn about lead; hopefully this will be better communicated 

in the future. 
P.19 

17 

ISSUE c.12: ASB should minimize non-air sources of lead in the modeling 

effort to avoid "excessive background noise" due to lead-based paint, 

pica, etc. Look at the "cleanest" cohorts only. 

ISSUE c.13: There is too much uncertainty in each of the component models; 

there is too much disaggregation using an idealized framework for which 

no data exists to either do the analysis or to evaluate plausibility of 

the model itself. There are no data to verify the model. 

ISSUE c.14: There are too many compartments in the model that are not 

needed to estimate the impact that different air lead levels have on 

children's health. Do away with unessentials at first and add compartments 

as needed to address question as they arise. (Or as another Subcommittee 

put it: NEM is an "over-arching conceptualization" that is being forcefit to the science.) 

ASB staff's response to these three Subcommittee comments builds 

upon our previous discussion. We see the three comments as reflecting 

the scientific perspective of understanding phenomena, whereas we feel 

compelled by the very nature of the exercise to adopt the analytic perspective. 

Comment 12 reflects the scientific view of experimental design, where 

confounding variables are controlled so that relationships of interest 

are isolated. We use, rather than generate, any relevant information 

that has been generated by well designed experiments or by the application 

of statistical procedures that control as well as possible (usually not 

very well) for extraneous influences of nonexperimental data. Our role 

is not to generate such information but rather to use whatever information 

is available at the time a decision must be made to assess the difference 

meeting alternative Pb standards would make in health risks incurred. 

Non-air sources of Pb contribute to the levels that cause this risk as 

well as air sources. In other words we would commit the fallacy of 

attributing a single cause to a phenomena for which there are multiple 

causes if we were to minimize non-air sources of Pb in the modeling 

effort. The resulting risk assessment would be logically unsound and 

therefore tend to produce misleading outputs. 

Comments c.13 and c.14 have been addressed in part in the response above. 

We think a lot of disaggregation is needed, especially at first, in order 

to properly structure a complex situation. In addition, it is usually 

easier to aggregate in modeling than to decompose after a structure is 

finalized. Lack of data to adequately define a variable is one good 

reason to aggregate. With each aggregation, however, one less uncertain 

factor remains that can be addressed explicitly. One less variable also 

remains that can be exercised in order to determine what impact varying 

it has on model outputs. 

From our past experience, we can anticipate the following kinds of 

questions regarding lead exposures. If variables do not exist in the 

model to address them, we are remiss as analysts. 
P.20 

18 

Dietary lead makes up a significant portion of total lead ingested 

by the typical child. What is the distribution of dietary lead 

used in the model, how is the relationship between airborne and 

dietary lead levels derived, and what impact does dietary lead 

have on elevated blood lead levels when alternative lead NAAQS 

are just attained? 

2. If smelters reduced their emissions by 50% (or some other proportion) 

on average, what is the estimated impact on blood lead distributions 

in children less than six years old who live in smelter communities? 

What might it be after ten years, when dust and soil lead levels 

are subsequently reduced? 

Staff anticipates that many questions like these will be raised during 

the lead NAAQS review. We want to be able to formally address them. To 

do so, we probably need a complex model. As mentioned at the meeting, 

there is now an "inescapable demand" for exposure modeling of NAAQS 

pollutants. This point was echoed by one of the Subcommittee members in 

Comment c.15 (below), and staff concur. 

As mentioned, we are reviewing the entire exposure model (as well as 

other modeling components) in order to simplify where possible. However, 

the Subcommittee should be aware that there is a strong counterforce 

toward complexity—not for its own sake, but because it is in the only 

way to analyze issues that industrial and environmental reviewers of our 

work are as well as EPA management interested in. If the underlying data 

base is highly uncertain, this will be reflected in our results. 

ISSUE c.15. All problems aside, you still "have to come up with exposure 

estimates" of airborne lead to analyze air lead standards. 

We agree. 

d. ISSUE: The Subcommittee thought that there were a number of problems 

associated with the proposed use of a Rabinowitz-type of compartmental 

model to analyze distribution and kinetics of lead in children. Comments 

referred to the following: 

ISSUE d.l. Without developed physiological models on children, the exercise 

is doomed. There are many limitations associated with using models 

developed for adults to children because of differences related to lead 

metabolism. Among these differences are the greater gastrointestinal 

absorption and retention rates in children of ingested lead, and the 

rapid growth of tissues in the developing child and thus increased transfer 

rates between body compartment. 
P.21 

19 

ISSUE d.2. The Rabinowitz et_ a\_. work is good but it was done on adults 

only. The pathway constants are much different for adults than for 

children. Perhaps recent primate data may help in addressing the adult 

vs. children metabolism issue. Perhaps human autopsy studies may be 

another source of compartment lead levels at differing ages. 

ISSUE d.3. The relationship between absorbed lead and bone lead is nonlinear. It is not a first-order process. 

RESPONSE: As the criteria document notes, "in young children, blood lead 

is much less a reliable indicator" of total body burden because of their 

large labile pools of soft tissue and bone lead (CD, p. 10-28). Thus, 

their blood lead levels reflect recent as well as past exposures. 

Mathematical descriptions of physiological lead kinetics that are being 

considered for our model allow a quantitative understanding of the sequence 

of changes in levels of lead in various body pools and tissues. This is 

essential to estimating and interpreting changes in blood lead distribution 

associated with alternative air lead standards, given past air and nonai r exposures. 

Hopefully, available models such as Rabinowitz's can be used or 

modified for this purpose. Whatever model is developed will not be any 

more complex than necessary. 

The Ph.D. dissertation on lead uptake and biokinetics in infant and 

juvenile primates given to ASB staff by Dr. Lippmann promises to yield 

important information. Staff will investigate the usefulness of human 

autopsy studies in providing ASB with compartmental lead information in 

children. 

While relationship between blood lead and air lead (Hammond et al., 

1980), diet (United Kingdom Central Directorate on Environmental Pollution, 

1982) and tap drinking water (Sherlock e1*_ al_., 1982) are all non-linear in 

mathematical form, most of the availabble compartmental model of lead 

metabolism are governed by a system of linear first-order differential 

equations with constant fractional transfer coefficients Holtzman, 

1978; Rabinowitz et al., 1976; Batschelet et_ a]_., 1979). There is 

evidence, however, that non-linear kinetic mechanisms exist between 

toxicologically active lead and the blood lead pool including: partially 

rate-limited gut absorption of lead (Aungst et_ al_., 1982); rate-limited 

and site-limited kinetics in the distribution of lead between plasma and 

erythrcyte pools (De Silva, 1981); increased urinary excretion of lead 

(Chamberlain et_ aj_., 1974); increased tissue absorption of blood lead 

(Indraprasit et_ aj_., 1974). Increased mobilization of lead stored in the 

Skelton, especially during childhood, and; transport of lead through bone 

tissues by process (Barryl 1982). Marcus (1983, in press) formally tested 

non-linear models for several of these mechanisms and did not find 

significantly superior predictions of blood kinetics over linear models. 
P.22 

20 

It is clear that understanding non-linear mechanisms for lead kinetics 

in man requires far more detailed experiments than have to date been 

carried out. 

ASB will continue to assess the available literature to address the 

problems raised by Subcommittee members in this, as well as other areas 

of physiological modeling. We will also talk to relevant experts about 

the possibility of modifying the Rabinowitz model so it can be used for 

children, given all of the problems noted by the Subcommittee. We will 

relay our findings to the Subcommittee as soon as possible. 

It should be noted that if we have no acceptable way of relating air 

and other media lead exposures to blood (or bone) lead, there is little 

sense in developing dose-response relationships. In order to do a lead 

risk assessment this relationship must be represented. The subject area 

is obviously very important to the entire assessment effort. 

What would be acceptable? ASB staff wishes that a scientifically 

valid model was available to just "plug in" and obtain physiological 

impact estimates associated with alternative lead air standards. This is 

not the situation, obviously. What, then, is the alternative to 

physiological modeling? If there is none, our only option seems to be to 

use the best information available and explicityly represent major 

uncertainties in our procedure. Again, staff is interested in receiving 

Subcommittee input on this issue. 

ISSUE d.4. OAQPS should do research on how well the Rabinowitz model compares 

to data on children. 

RESPONSE: OAQPS is not the organization within EPA that would do the 

research suggested. ASB will convey the recommendation to the appropriate 

group in EPA's Research and Development Office. 

ISSUE d.5. There are too many compartments in the lead physiological model. 

ASB should eliminate non-essential compartments initially and add them as 

needed. The only compartment for which a good data base exists is blood 

lead and since blood lead is known, there is no need to compute it. 

RESPONSE: Staff concurs with the concept that the physiological model 

not have any more compartments beyond those necessary to "understand" and 

describe the distribution and equilibrium dynamics of the lead body 

burden. As the Subcommittee is aware, blood lead levels are a function 

of recent lead exposures as well as past lead exposures. To estimate 

what changes occur in blood lead distributions of children when an alternative 

air lead standard is just attained requires us to model major functional 

relations among body pools of lead. Hopefully, the Rabinowitz model can 

be used or modified for this purpose. That model, or any other, will not 

be any more complex than needed to do the job. 
P.23 

21 

It should be noted that some Subcommittee members did not feel that the 

model contained too many compartments. They noted that the documented 

transfer of lead among compartments and the influence of total body burden 

of lead on the absorption and distribution of newly introduced lead, 

argue for a model that sufficiently addresses this dynamic situation. 

ASB staff concur. 

A subcommittee member also commented that modeling blood lead was 

unnecessary given the wealth of blood lead data. We do have blood lead 

data that will be used to the fullest extent possible. At the minimum, 

it will be used to check blood lead distributions estimated via the 

physiological model. The same is true of any bone lead data, or shed 

teeth data (if such a compartment appears in the model), that are available. 

Staff will make a concerted effort to obtain all relevant data on body 

lead distributions. 

However, having blood and other tissue lead data is not sufficient 

for standard-setting purposes. Often, exposure information associated 

with blood lead data is lacking, and an exposure-physiological link 

cannot be made. In addition, blood lead data are not associated with the 

hypothetical situation of just attaining an ambient air standard. Thus, 

such data do not provide an understanding of how blood lead levels may 

change over time in a sensitive population group experiencing air lead 

levels expected at attainment (given various assumptions about lead 

exposure due to non-air routes). 

ISSUE d.6. Modeled outputs from the compartmental model should be compared 

to blood lead information that is available in selected cities. (However, 

one Subcommittee member noted that blood lead data is not a good surrogate 

for total lead exposure as blood lead levels change too rapidly.) 

RESPONSE: This issue has been addressed in the previous discussion. 

Staff will compare our estimated blood lead distributions with available 

data. We agree that blood lead data do not provide the entire picture 

concerning total lead exposure, especially for long time periods. For 

this reason we have proposed the use of a compartmental model of lead 

pool interaction as the basis for our physiological impacts procedure. 

e. ISSUE: The Subcommittee had few comments regarding the dose-response 

portion of the lead risk assessment project. Some of the Subcommittee's 

discussion of this portion related to output measures (discussed below) 

or to health end points (discussed above at 3b). Subcommittee concerns 

regarding the proposed planned dose-response effort were focused on the 

probability surface and sensitive population group "cutpoint" concepts 

presented by Argonne National Laboratory. These concerns were: 
P.24 

22 

ISSUE e.l. The idea of developing individual dose-response functions for 

different "cutpoints" in the sensitive population group is the F/B 

formulation. 

ISSUE e.2. A three dimensional dose-response relationship is too complex. 

Slice it down to 2 so that experts can be comfortable that the judgment 

will be properly communicated. 

RESPONSE: The first comment concerns Argonne's proposal to generate a 

probability surface by encoding health experts at, say, the first, fifth, 

tenth, twenty-fifth etc. percentiles for the most sensitive group (children). 

This approach, while first formulated by Tom Feagans and Dr. Biller, is 

not an idea peculiar to a particular approach to risk assessment. Rather, 

it captures the logic of the situation we must deal with: namely, that 

the shape and location of relevant dose-response relationships are uncertain 

and that this uncertainty affects risks to the more sensitive members of 

the most sensitive groups. We must deal with this inherent logic at 

OAQPS, as we have in the past, whether we do a formal risk assessment or 

not. 

As the members of the Subcommittee know, empirical evidence shows 

that not all sensitive people respond to a pollutant in the same way. 

Some sensitive people are not as sensitive as others. Neither this fact 

nor the uncertainty that exists about the shape and location of resultant 

dose-response relationships can be ignored. It is clear from the 

legislative history of the Clean Air Act and its amendments that a NAAQS 

should protect almost all members of the most sensitive group against 

adverse health effects with an adequate margin of safety (Senate Committee 

on Public Works A Legislative, History of the Clean Air Act Amendments of 

1970; serial 93-18 (1974)). (As an aside, both CO reports presented at 

the meeting treated the sensitive population group as a homogeneous 

category of people and this was a fundamental problem with their approaches. 

In both cases it resulted in logically indefensible risk numbers.) 

ASB staff believes that, as well as headcount risk measures, a risk 

measure that will give risk estimates for types of individuals at various 

percentile "cutpoints" of the most sensitive group is required. Which 

percentile cutpoints would be focused on is a policy choice, of course, 

that ASB staff does not make. Once encodings have been obtained for a 

set of cutpoints that span the most sensitivve group, risk estimates for 

cutpoints chosen by policy-makers are calculable by interpolation. 

The Subcommittee's second comment concerns whether a 3-dimensional 

dose-response surface is too complex for experts to work with. Dr. 

Wallsten's research indicates that health experts can understand and work 

with 3-dimensional surfaces. As Drs. Whitfield and Wallsten indicated at 

the meeting (Vol. II, p. 121 ff), the three surfaces are obtained conditionally 

on one dimension so that an expert only has to deal with two dimensions 

at any one time. 
P.25 

23 

In addition, we submit that three dimensions are necessary for a 

probabilistic assessment. By definition, two dimensions are dose (expressed 

in physical units) and response (usually expressed as a percent of the 

exposed group that responds to some specified health effect level). The 

uncertainty concerning what the true dose-response relationship is for 

the group being analyzed results from the fact that not all persons were 

investigated nor were all possible exposure conditions taken into account. 

To address uncertainty in the population dose-response relationship, we 

will encode experts to obtain their probabilistic judgments regarding 

response rate for the population given a particular dose (or dose given a 

specified response rate). These probabilistic judgments consitutes the * 

third dimension. Without them you have to assume a particular dose-response 

curve. We do not think that an approach that ignores uncertainty in the 

dose-response relationship is the way to conduct a risk assessment of 

alternative ambient air standards whose main purpose is to formally deal 

with significant uncertainties so that policymakers can most meaningfully 

decide which standard, in their judgment, provides an adequate margin of 

safety. 

During the CO risk assessment method development project, Woodward 

and Clyde Consultants (WCC) developed a three-dimensional dose-response 

relationship for three major carbon monoxide health effects. (SRI did 

not.) For the additional angina attack effects category, WCC developed 

the probability dimension by fitting a normal distribution around the 

log-odds dose-response relationship that the firm derived from encoded 

expert data. WCC assumed that the mean of the fitted normal distribution 

was the same as the mean of the log odds regression model (-3.65 + 3.11 

In (COHb)) with a standard deviation of 0.50, which came from encoded 

data. A similar procedure was used to account for uncertainty regarding 

dose-response relationships for the other two major health effects of CO. 

While ASB does not necessarily agree with how WCC derived the probabilistic 

dimension in its work, the firm showed that it can be done--as did 

Dr. Wallsten. 

Our conclusion, then, is that a 3-dimensional surface is both feasible 

and desirable for probabilistic analyses of uncertain dose-response 

relationships. We solicit Subcommittee comment on this conclusion. 

4. General Issues 

These issues do not fit well into any of the previous categories. 

They involve output measures and ASB analytic resources. 

a. COMMENT: ASB should expend more resources to undertake the lead 

risk assessment. The economic impacts of a lead NAAQS warrant spending 

sufficient resources on the pollutant. 

RESPONSE: OAQPS will be expending on the order of $750,000 on the 

lead risk assessment effort. This figure includes the contractual work 

with Argonne National Laboratory (and its consultants and subcontractors); 
P.26 

24 

air quality data analysis and file development work by PEDCo; part 

of Dr. BiTier's contractural work with Strategies and Air Standards 

Division (SASD); exposure modeling computer charges at EPA's National 

Computer Center; part of the point source modeling work by GCA Technology; 

and'in-house staff time. 

This figure represents a sizable amount of SASD's analysis budget for 

NAAQS pollutants. Besides lead, that budget must be divided among ozone, 

particulate matter, sulfur dioxide, nitrogen dioxide, and carbon monoxide-' 

the pollutants that are currently being reviewed. These pollutants have 

a societal economic impact at least as large as lead. 

SASD's budget for regulatory analyses is determined at higher levels 

of EPA management. NAAQS reviews compete with many other programs for 

resources. Under our current budget, providing additional funds for lead 

risk assessment activities (such as more encoding) may mean that the 

regulatory analysis budget for one of the other NAAQS pollutants will 

have to be reduced. The zero-sum nature of the competition for available 

resources requires us to stay within our risk assessment budget if at all 

possible. 

b. COMMENT: The output measures used in the lead risk assessment 

effort should include prevalence types of measures. Impact on both ALA-D 

and protoporphyrin in the sensitive group should be used as output measures 

of the physiological model. .'•''.

 v*:'"' 

RESPONSE: Staff agree on both counts. We plan to provide peopleoccurrences output measures as well as the number of people affected. 

We intend to use changes in erythrocyte protoporphyrin levels as an 

output measure since it does reflect somewhat longer term exposures than 

does blood lead. The use of ALA-D inhibition will be considered as well. 

Both of these indices are also being considered as possible health 

endpoints in the risk assessment.