United States Science Advisory Board EPA-SAB-IAQC-95-005

Environmental Washington, DC March 1995

Protection Agency 

 

AN SAB REPORT: 

HUMAN EXPOSURE ASSESSMENT:

A GUIDE TO RISK RANKING, RISK

REDUCTION AND RESEARCH

PLANNING

PREPARED BY THE INDOOR AIR

QUALITY/TOTAL HUMAN EXPOSURE

COMMITTEE
March 31, 1995

EPA-SAB-DWC-95-005

Honorable Carol M. Browner, Administrator

U.S. Environmental Protection Agency

401 M Street, SW

Washington, DC 20460

Subject: Human Exposure Assessment: A Guide to Risk Ranking,

Risk Reduction, and Research Planning

Dear Ms. Browner:

In July of 1993, through the Assistant Administrator of the Office of Office of

Policy, Planning and Evaluation (OPPE), the Administrator of EPA requested that

the Science Advisory Board (SAB) provide guidance to help EPA improve its

readiness to anticipate problems and to develop and implement appropriate

strategies for the protection of human health and the environment associated with

future risks. The Indoor Air Quality and Total Human Exposure Committee

(IAQTHEC) of SAB focused its report, which is attached, on anticipated

developments in the science of exposure assessment, and how these developments

can be advanced and harnessed to help EPA and the nation achieve substantial risk

reductions related to environmental hazards.

Exposure assessments are indispensable for the conduct of effective risk

assessments, for the success of epidemiologic research, for the surveillance of

environmental and health stresses in populations and ecosystems, and for

development and evaluation of the efficacy of risk management activities. Risk

management, in fact, almost invariably relies on management of exposures. 

Current exposure assessment capabilities, however, are hampered by technical

limitations in the currently available exposure measurement techniques, by severe

limitations of the currently available databases containing exposure and exposure-
2

relevant data, by reliance on numerous assumptions which have been proven

incorrect or are not supported by common experience and/or direct observations,

and by the current fragmentation and lack of coherence of available models for

different media, pathways, chemicals, etc.

As the attached report indicates, however, anticipated developments in both

the near-term and the long-term offer significant possibilities for advancement in

exposure assessment capabilities in the areas of databases of exposure and

exposure-related information, personal sampling technologies (e.g., sensor

technologies), biomarkers of exposure and effect, questionnaires, and improved and

validated exposure models.

By embracing a leadership role in catalyzing advances in these areas, the

Agency can create new capabilities that would allow it to more credibly model

population exposures from sources through health risks in support of its risk

assessments and risk management activities. Based on the Committee's

deliberations, the attached report makes five major recommendations to allow the

Agency to play this leadership role:

1) Develop more integrated research programs that develop, validate with

field data, and improve models for: a) measurements of total exposure and

their determinants; b) exposure distributions across different populations;

and c) exposure-dose relationships Biomarkers of exposure and dose will

play increasingly important roles in these activities.

2) Develop a robust database that reflects the status and trends in national

exposure to current and anticipated environmental contaminants. The

contents of this database must match the data needs for modeling

environmental fate and transport, for conducting exposure-dose assessments,

and for surveillance of environmental and health effects in populations and

ecosystems, and as measures of exposure control efficacy.

3) Develop sustained mechanisms and incentives to ensure a greater degree

of interdisciplinary collaboration in exposure assessment, and, by extension,

in the resulting risk assessment and risk management activities.

4) Develop a mechanism to support the research, validation and application

of: a) more sensitive and specific microsensors and other monitoring

technologies and approaches; b) the determinants of human exposures; and,
3

c) the determinants of susceptibility to adverse effects from environmental

exposures.

5) Take advantage of exploding capabilities in monitoring technology,

electronic handling of data, and electronic communications to establish earlywarning systems of developing environmental stresses, so that actions can be

taken that minimize environmental and health impacts.

In summary, we believe that an increased recognition of the importance of

the exposure paradigm as a fundamental approach for early identification of

environmental stresses and the human health and ecological problems they may

cause is necessary within EPA in order to effectively and efficiently prevent the

occurrence of adverse effects. To this end, we strongly recommend the adoption of

the above recommendations. 

Sincerely,

/Signed/

Dr. Genevieve M. Matanoski

Chair, Executive Committee

Science Advisory Board

/Signed/

Dr. Raymond Loehr, Chair

Environmental Futures Committee

Science Advisory Board

/Signed/

Dr. Joan M. Daisey, Chair

Indoor Air Quality/

Total Human Exposure Committee

Science Advisory Board
i

NOTICE

 This report has been written as a part of the activities of the Science Advisory

Board, a public advisory group providing extramural scientific information and

advice to the Administrator and other officials of the Environmental Protection

Agency. The Board is structured to provide balanced, expert assessment of

scientific matters related to problems facing the Agency. This report has not been

reviewed for approval by the Agency and, hence, the contents of this report do not

necessarily represent the views and policies of the Environmental Protection

Agency, nor of other agencies in the Executive Branch of the Federal government,

nor does mention of trade names or commercial products constitute a

recommendation for use.

Seven reports were produced from the Environmental Futures Project of the

SAB. The titles are listed below:

1) Environmental Futures Committee EPA-SAB-EC-95-007

[Title: "Beyond the Horizon: Protecting the Future with Foresight," Prepared

by the Environmental Futures Committee of the Science Advisory Board's

Executive Committee.]

2) Environmental Futures Committee EPA-SAB-EC-95-007A

[Title: Futures Methods and Issues, Technical Annex to the Report entitled

"Beyond the Horizon: Protecting the Future with Foresight," Prepared by the

Environmental Futures Committee of the Science Advisory Board's Executive

Committee.]

3) Drinking Water Committee EPA-SAB-DWC-95-002

[Title: " Safe Drinking Water: Future Trends and Challenges," Prepared by

the Drinking Water Committee, Science Advisory Board.]

4) Ecological Processes and Effects Committee EPA-SAB-EPEC-95-003

[Title: "Ecosystem Management: Imperative for a Dynamic World," Prepared

by the Ecological Processes and Effects Committee, Science Advisory Board.]

5) Environmental Engineering Committee EPA-SAB-EEC-95-004

[Title: "Review of Environmental Engineering Futures Issues," Prepared by

the Environmental Engineering Committee, Science Advisory Board.]
ii

6) Indoor Air and Total Human Exposure Committee EPA-SAB-IAQ-95-005

[Title: "Human Exposure Assessment: A Guide to Risk Ranking, Risk

Reduction and Research Planning," Prepared by the Indoor Air and Total

Human Exposure Committee, Science Advisory Board.]

7) Radiation Advisory Committee EPA-SAB-RAC-95-006

[Title: "Report on Future Issues and Challenges in the Study of

Environmental Radiation, with a Focus Toward Future Institutional

Readiness by the Environmental Protection Agency," Prepared by the

Radiation Environmental Futures Subcommittee of the Radiation Advisory

Committee, Science Advisory Board.]
iii

ABSTRACT

This report represents the findings of the Indoor Air Quality and Total

Human Exposure Committee study of the opportunities for advances in the field of

exposure assessment. The Committee noted that early recognition of the nature

and extent of potentially adverse exposures could be of immense benefit in avoiding

adverse effects to public health and ecological systems. Opportunities for

advancement were noted in key areas of microsensor and microprocessor

technologies, the development and understanding of biomarkers of exposure, and

the expansion of federal and industry databases on human exposures to toxic

substances in the work place and at home. They encourage the Agency to develop

integrated research programs to track and apply these developments to monitoring

and assessment models, to work constructively to improve and validate exposure

assessment models, to collaborate widely with governments and industry, to

promote improvements in databases, and employ interdisciplinary teams to address

exposure problems. The Committee anticipates that the importance of the exposure

paradigm will increase significantly in the future, leading to a combined approach

for human health and ecological resource protection.

KEY WORDS: Exposure Assessment, Exposure Monitoring, Biomarkers,

Epidemiological Data, Databases.
iv

ENVIRONMENTAL PROTECTION AGENCY

SCIENCE ADVISORY BOARD

INDOOR AIR QUALITY/

TOTAL HUMAN EXPOSURE COMMITTEE

Human Exposure Assessment: A Guide to Risk Ranking, 

Risk Reduction, and Research Planning

Chair

Dr. Joan Daisey, Indoor Environment Program, Lawrence Berkeley Laboratory,

Berkeley, CA

Members

Dr. Morton Lippmann, Institute of Environmental Medicine, New York University

Medical Center, Tuxedo, NY 10987

Dr. Paul Bailey, Mobil, Environmental Health and Sciences Laboratory, P.O. Box

1029, Princeton, NJ 08543-1029.

Dr. Robert Hazen, Chief, Bureau of Risk Assessment, State of New Jersey,

Department of Environmental Protection and Energy, CN 409, Trenton, NJ 08625-

0409.

Dr. Timothy Larson, Environmental Science and Engineering Program, Department

of Civil Engineering (FX-10), University of Washington, Seattle, WA 98195.

Dr. Brian Leaderer, John B. Pierce Foundation Laboratory, 290 Congress Avenue,

and Division of Environmental Health Sciences, Department of Epidemiology and

Public Health, Yale School of Medicine, New Haven, CT 06519.

Dr. Paul Lioy, Department of Environmental and Community Medicine, Robert

Wood Johnson School of Medicine, Piscataway, NJ 08854-5635.

Dr. Maria Morandi, University of Texas Health Science Center at Houston, School

of Public Health, Post Office Box 20186, Houston, TX 77225-0186.
 1

 Dr. Edward Bender provided staff support to complete this

report.

v

Mr. Roger Morse, ENTEK, Environmental and Technical Services, Inc., 125

DeFreest Drive, Troy, NY 12180.

Dr. Jonathan M. Samet, The University of New Mexico School of Medicine, New

Mexico Tumor Registry, 900 Camino De Salud NE, Albuquerque, NM 87131.

Mr. Ron White, Deputy Director, National Programs, and Director, Environmental

Health, American Lung Association, Suite 902, 1726 M Street, NW, Washington,

DC 20036-4502.

Science Advisory Board Staff

Mr. Manuel R. Gomez, Designated Federal Official, Science Advisory Board, U.S.

EPA, Washington, DC1

Mrs. Dorothy Clark, Secretary, Science Advisory Board, U.S. EPA,

Washington, DC
vi

TABLE OF CONTENTS

1. EXECUTIVE SUMMARY ....................................... 1

2. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1 The EFC Charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Preparation of the Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.3 The Scope of Human Exposure Assessment . . . . . . . . . . . . . . . . . . . . 5

2.3.1 Reasons for Performing Exposure Assessments . . . . . . . . . . . 5

2.3.2 Approaches to Performing Exposure Assessments . . . . . . . . 6

2.3.3 Selection of Approach for Specific Application . . . . . . . . . . . . 7

2.4 Human Exposure Assessment: Historical Development . . . . . . . . . . 8

2.5 Current Roles of Exposure Data and Exposure Models in the

Risk Assessment Process at EPA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.6 Emerging Opportunities in the Near Term . . . . . . . . . . . . . . . . . . . . 10

2.7 Anticipation of Longer-Term Developments . . . . . . . . . . . . . . . . . . . 14

3. CONSTRAINTS AFFECTING CURRENT HUMAN EXPOSURE

ASSESSMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.1 Constraints in Current Data Resources and Data Management

Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.2 Constraints in Current EPA Practices . . . . . . . . . . . . . . . . . . . . . . . . 18

3.2.1 Time-Activity Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.2.2 Exposure Pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.2.3 Lifestyle and Other Personal Factors . . . . . . . . . . . . . . . . . . . . 22

3.2.4 Residential Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2.5 Reliability of Predictive Models Based on Sources and

Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2.6 Temporal Variability at Specific Locations . . . . . . . . . . . . . . . 24

3.2.7 Independence of Exposures to Multiple Individual Agents

and Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.2.8 Infiltration of Outdoor Air into Indoor Spaces . . . . . . . . . . . . 25

3.2.9 Population Distributions of Exposure . . . . . . . . . . . . . . . . . . . 26

3.3 Currently Available Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4. DEVELOPMENTS AND PROSPECTS FOR IMPROVEMENTS IN

EXPOSURE ASSESSMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.1 Framework for the Science of Exposure Analysis . . . . . . . . . . . . . . . 34

4.1.1 Databases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
vii

4.1.2 Personal Samplers and Monitors . . . . . . . . . . . . . . . . . . . . . . . 39

4.1.3 Biomarkers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.1.3.1 Biomarkers of Exposure . . . . . . . . . . . . . . . . . . . 40

4.1.3.1 Biomarkers of Response . . . . . . . . . . . . . . . . . . . 41

4.1.3.3 Ethical Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.1.4 Questionnaires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.1.4.1 Source Characterization . . . . . . . . . . . . . . . . . . . 42

4.1.4.2 Diversity Pattern . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.1.4.3 Health Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.1.5 Improved and Validated Exposure Models . . . . . . . . . . . . . . . 43

5. FINDINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.1 The Near-Term (5-year) Perspective . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.1.1 Emerging Issues in Relation to Potentially Important

Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.1.2 Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.1.3 EPA's Recent Role and Activities . . . . . . . . . . . . . . . . . . . . . . . 46

5.1.5 Five Year Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.2 The Longer-Term (10-30 year) Perspective . . . . . . . . . . . . . . . . . . . . 47

5.2.1 Methods for Identifying Early Warning Signs of

Significant Exposures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.2.2 Technical Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6. RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

7. REFERENCES CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
1

1. EXECUTIVE SUMMARY

The Administrator of EPA, through the Assistant Administrator of the Office

of Office of Policy, Planning and Evaluation (OPPE) requested that the Science

Advisory Board develop a procedure for conducting a periodic scan of the future and

choose a few future developments for an in-depth examination of environmental

impacts. The SAB accepted the request, called it the Futures Project, and formed

an Environmental Futures Committee (EFC) to lead the effort. The objective of the

Project was to produce reports and guidance to help EPA improve its readiness to

anticipate problems and to develop and implement an appropriate strategy for the

protection of human health and the environment now and in the future.

The Environmental Futures Committee invited each of the SAB Standing

Committees to write a report on future developments in their areas of expertise. 

The Indoor Air and Total Human Exposure Committee (IAQTHEC) focused its

report on anticipated future developments in the science of exposure assessment,

and how these developments can be advanced and harnessed to help EPA and the

nation achieve substantial risk reductions related to environmental hazards.

The introductory chapter (chapter 2), summarizes the scope and recent

history of the science of exposure assessments and EPA's role in them, the four

major reasons why exposure assessments are conducted, i.e., risk assessment,

epidemiologic research, surveillance, and risk management; and the basic rationale

for the selection and use of the three current approaches to the conduct of

assessments, namely direct measurements, indirect measurements, and

mathematical modeling. For the near-term, i.e., 5 years, the report identifies rapid

technological advances in sensor technologies, computer capabilities and

communications networks as developments that will present many opportunities for

improved quantitative exposure measurements, for assembly and analysis of large

databases of exposure information (including time-activity pattern information),

and for more highly integrated computer frameworks for modeling population

exposures from source through health risks. For the longer-term, i.e., 20-30 years,

the report postulates that advances in monitoring technology could provide realtime information to institutions and individuals in feedback loops that will be able

to quickly affect controls and/or for individuals to modify personal behaviors

affecting exposures. Advances in exposure assessment will help to provide a sound

scientific basis for ranking environmental risk and for developing effective risk

management policies.
2

Chapter Three discusses the constraints affecting current human exposure

assessment. The weaknesses that are highlighted include the severe weaknesses of

the currently available databases containing exposure and exposure-relevant data,

the reliance by EPA on exposure assessments based on numerous assumptions

which have been proven incorrect, or are not supported by common experience and

systematic verification by direct observation, and the current fragmentation and

lack of coherence of available models for different media, pathways, chemicals, etc.

Chapter Four focuses on developments and prospects for improvements in

exposure assessment in five areas: databases, personal sampling, biomarkers,

questionnaires, and improved and validated exposure models. The report

emphasizes the need for more comprehensive exposure databases and outlines four

steps that should be taken: 1) making exposure monitoring reflect a theoretical

framework; 2) establishing an ongoing program of trend monitoring; 3) organizing

exposure research to more effectively integrate models and measurements; and, 4)

improving coordination between federal agencies. Targeted surveys and improved

uses of regulatory compliance data are identified as two complementary technical

approaches to meet some of the above goals. In the area of personal samplers and

monitors, the report highlights the need to obtain measures of breathing rates or

exercise patterns concurrent with air measurements, the need to develop personal

monitoring devices for dermal exposures, and the need for techniques to obtain 

better information about contaminant ingestion. Biomarkers of exposure and of

response are discussed as future tools for exposure assessment that will be

technically valuable yet ethically thorny. Major advances in questionnaire

technology are described as requiring substantial improvements in applications of

behavioral science, although increasingly automated methods involving computer

and video administration can be anticipated for collecting useful data on personal

activities. Finally, the chapter emphasizes the need to validate exposure models, to

link multiple models into comprehensive integrated models, and to improve the

efficacy of models as tools for prioritizing the major routes and media of concern.

Chapter Five presents a number of findings. First, the report uses some

emerging issues for which improved exposure measurement techniques are needed,

i.e., environmental estrogens, electromagnetic fields, and particulate matter to

illustrate exposure assessment issues. Using the inhalation route as an example,

the report discusses a number of areas where substantial progress in monitoring

capabilities can be anticipated, including passive monitors, electrochemical sensors,

and optically-based particle sensors. In this route, as in others, the report

emphasizes the recognition that monitoring of environmental concentrations always
3

needs to be accompanied by monitoring of individual characteristics relevant to the

uptake of environmental chemicals. Specific suggestions for EPA exposure research

activities are also made. In a 5-year time frame, the report suggests that the

Agency support a number of initiatives to facilitate advances in exposure

assessment. These include an increased recognition of the central role of the

exposure paradigm in risk assessment activities in general, greater linkage of

health concerns to research priorities, development of models that can iteratively

improve collection of useful data, a strengthening of the partnership between

academic and federal scientific communities, and the completion of a scientifically

credible design for the National Human Exposure Assessment Survey (NHEXAS). 

In a 10-30 year time frame, the report calls for developing methods to identify early

warning signs of significant exposures by creating and exploiting a broad-based

resource of computerized information about concentrations of contaminants and

pathogenic agents in environmental media, and for improvement in the capabilities

for measurement and analysis of inhalation exposures, especially through

miniaturized monitors and other innovative technologies and in microprocessing,

and other data storage and manipulation capabilities.

The final chapter of the report makes six fundamental recommendations. 

They are to : 1) establish a mechanism to develop, validate with field data, and

iteratively improve models that integrate measurements of total exposure and their

determinants, exposure distributions across different populations, and the most

current understanding available of exposure-dose relationships, including full

utilization of developments in the use of biomarkers of exposure and dose, which

are expected to provide significant capabilities in the next decades; 2) establish a

mechanism to validate with data, and iteratively improve, the available models for

environmental fate and transport of substances; 3) develop a robust database that

reflects the status and trends in national exposure and closely matches the data

needs for modeling environmental fate and transport, for conducting exposure-dose

assessments, and for surveillance for signs of environmental and health effects in

populations and ecosystems; 4) develop sustained mechanisms and incentives to

ensure a greater degree of interdisciplinary collaboration in exposure assessment; 5)

develop a mechanism to support the research, validation and application of

microsensors and other monitoring technologies and the determinants of human

exposures; and, 6) take advantage of exploding capabilities in monitoring

technology, electronic handling of data, and electronic communications, to establish

early-warning systems of developing environmental stresses, so that actions can be

taken that minimize environmental and health impacts. 
4

In summary, the IAQTHEC has undertaken an examination of prospects for

future developments in the field of exposure assessment over the next five to thirty

years, and the implications of the expected advances on the quality and utility of

procedures for risk assessments and risk management. We envision extensive and

highly significant developments during this time frame in the approaches, protocols

and technology for collecting and analyzing data on the exposure and related

dosimetric factors influencing human health and ecological sustainability.
5

2. INTRODUCTION

2.1 The EFC Charge

On July 16, 1993, Mr. David Gardiner, Assistant Administrator for the Office

of Policy, Planning and Evaluation (OPPE) at EPA, and EPA Administrator Carol

Browner requested that the Science Advisory Board develop a procedure for

conducting a periodic scan of the future horizon and also choose a few of the many

possible future developments for an in-depth examination of environmental

impacts.

The SAB accepted the request, called it the Futures Project, and formed an

Environmental Futures Committee (EFC) to lead the effort. The Futures Project

appeared to be a logical extension of the SAB's 1990 report, Reducing Risk, which

stressed the importance of identifying future potential risks to human health and

the environment.

The objective of the Futures Project was to produce reports and guidance to

help EPA improve its readiness to anticipate problems, to develop an appropriate

strategy for the protection of human health and the environment now and in the

future, and to implement that strategy. The Environmental Futures Committee

invited each of the SAB Standing Committees to consider writing a report on future

developments in their areas of expertise and their implications to EPA's abilities to

meet its mandates and responsibilities for environmental and public health

protection. The Indoor Air Quality and Total Human Exposure Committee

(IAQTHEC) accepted this challenge, and focussed its report on anticipated future

developments in the science of exposure assessment, and how these developments

can be advanced and harnessed to help EPA and the nation achieve substantial risk

reductions related to environmental hazards.

2.2 Preparation of the Report 

The IAQTHEC discussed the request of the EFC at its meeting on September

8 and 9, 1993, enumerated driving forces in the areas of indoor air and total human

exposure, and considered the options for reports in those areas. At its next meeting,

on October 28, 1993, the Committee decided to prepare a single report entitled: 

"Human Exposure Assessment: A Guide to Risk Ranking, Risk Reduction, and

Research Planning" as its major contribution to the Futures project. It would focus

on the anticipated developments in the science of human exposure assessment, and
6

the implications of these developments for EPA's abilities to improve and broaden

its mission and function. The Committee members accepted assignments for the

preparation of "think pieces" on various aspects of technological developments,

societal trends, and opportunities for improvements in exposure assessments and

their applications in risk assessment.

The Committee reviewed the original drafts of the "think pieces" at its

meeting on December 2, 1993, prepared a draft outline of its report, and agreed to

additional writing assignments on specific topics in the outline. The drafts were

reviewed at the next committee meeting on April 7 and 8, 1994, and additional

assignments were agreed to. This report is the product of the cumulative efforts of

the entire committee, as refined by correspondence and mail reviews of the

subsequent drafts.

2.3 The Scope of Human Exposure Assessment

2.3.1 Reasons for Performing Exposure Assessments

There are a variety of reasons for performing human exposure assessments. 

The major environmental health applications of exposure assessment are:

Risk Assessment: A risk assessment for a population is the product of a

hazard rating (e.g., unit risk for carcinogenesis, reference concentrations in

environmental media) and an exposure assessment (e.g., one distribution of

peak concentrations for acute toxicity and other distributions of long-term

averages of concentrations and/or uptakes for chronic disease development). 

Sizable contributions to the overall uncertainties in the resulting risk

assessment can typically arise from either the hazard rating or the exposure

assessment. In most current cases, however, the exposures estimates are

more uncertain and hence function as the limiting factor in the reliability of

risk assessments.

Environmental Epidemiology: Studies of exposure-response relationships in

human populations can provide the most direct and relevant data for

establishing hazard ratings for environmental agents affecting human

health. However, characterizing exposures of populations of interest to

environmental agents is a complex task because exposure is usually variable

in both time and space, and is influenced by the patterns of activities of the

individuals within the population. Furthermore, humans are often subjected
7

to other risk factors that influence their health status, such as cigarette

smoke, diet, genetic predisposition, and pre-existing disease. Because of such

complexities, and of resource limitations, exposure assessments for

epidemiology have often been simplistic. In many cases, they have been

limited to dichotomous comparisons between groups on the basis of known

exposure to the substance of interest vs. no known exposures, and have been

able only to determine if a hazard exists.

When epidemiology is to be used to obtain hazard ratings, it is

necessary to obtain reliable and accurate quantitative data regarding the

distribution of exposures to the agent(s) of interest, as is done for the

development of the National Ambient Air Quality Standards, using both

detailed measurement data and complex models for combining time-activity

data with airborne concentrations in numerous microenvironments.

Risk Management: When criteria for environmental exposures exist, the EPA

and others responsible for maintaining exposures within these criteria need

to conduct exposure assessments in order to determine if excessive exposures

are occurring and, if they are, to initiate programs that result in appropriate

reductions in exposure.

Baseline or Trends Analysis: When operators of potential sources of

environmental contaminants seek, and governmental agencies grant,

permission to operate processes and facilities, it is often prudent to conduct

baseline surveys and periodic re-surveys of exposure to the agents being

discharged to the surrounding populated areas. The purpose of such surveys

is to determine whether controls on emissions are adequate to avoid excessive

exposures in both: 1) the short-term (e.g., imminent exceedance of standards),

and 2) in the longer-term (e.g., trends indicating that continued operations

will result in eventual exceedance of existing standards) or exposures that

may warrant future standards.

2.3.2 Approaches to Performing Exposure Assessments

The three alternate approaches to performing exposure assessments are: 1)

direct measurements; 2) indirect measurements, such as biomarkers; and 3)

mathematical modelling. Modelling can be semi-empirical, with direct

measurements of concentrations in environmental media coupled with measured or

estimated exposures to those media through time-activity diaries or estimates, or by
8

the use of more conjectural estimation techniques using models relating source

strengths, dispersion models, and assumptions about contacts with environmental

media.

Direct measurement entails analysis of contaminant concentrations in air in

the breathing zone, the drinking water and diet consumed by the individual. These

are generally only feasible for small numbers of people and are only performed

when highly precise determinations of exposure or exposure potential are needed.

Indirect measurements consist of less direct estimations of exposure, such as

measurements of biological markers based on analyses of biological fluids for the

presence of environmental agents, their metabolites, adducts formed, or mediators

mobilized. Their use is complicated by uncertainties concerning metabolic rates

and pathways and the time intervals between environmental exposures an the

collection of the biological fluid that is analyzed.

Another indirect approach is micro-environmental modelling, in which

measured or available data on concentrations of the environmental agent of concern

in ambient air in the community drinking water supply, in representative

collections of diet components, etc., are combined with measured or estimated timeactivity patterns and standard assumptions about ingestion are used to calculate

peak and/or time-weighted average exposures. This approach assumes that the

person of interest is not a source or sink for the agent, and that his/her activities do

not perturb the local distribution of the agent. This assumption is clearly not

always true. Furthermore, the more one relies on assumptions about transport and

contacts with environmental media, the greater the uncertainty in the estimated

exposure.

The indirect approaches cited above involve some simple modelling, but

mathematical exposure models, as generally used and understood, require the use

of source strength data, environmental fate and transport models, and population

exposure distribution models. These models usually incorporate some broad

assumptions and rely on a variety of nominal or default assumptions concerning

estimated and derived constants for transport and transformation equations and

coefficients. This modelling approach, although widely used, is only substantially

justifiable for range-finding risk assessments, where one need to know the extent of

the exposures within one-to-two orders of magnitude.

2.3.3 Selection of Approach for Specific Application
9

When there is less margin for acceptable uncertainty, then either direct

measurement or indirect measurements with fewer degrees of uncertainty are

needed, and decisions must be made about what to measure, with what sensitivity

and specificity, as well as when and how often. The exposure assessor must be

prepared to justify the sampling and analytical protocols in relation to the

importance and value of the results as they relate to the decisions to be made using

them, as well as the accuracy and precision that will be required to meet the data

quality objectives.

In some cases where sample collection is relatively inexpensive, opportunities

for follow-up sampling uncertain, and analytical costs are high, it may be desirable

and cost-effective to collect many samples and submit only a small subset of

carefully selected samples for detailed analyses. The balance of the samples

collected can be archived for future reference and analyses, provided that sample

stability, access, and protection again loss can be assured. These considerations

apply to both environmental samples (air, drinking water, food) and biological

samples (blood, urine, tissues, etc.), although the latter may require more elaborate

storage and arrangements for access.

2.4 Human Exposure Assessment: Historical Development 

The principles of Exposure Assessment and their application to

environmental problems have evolved considerably over the past 40 years. The

basic practice of measuring human contact with a contaminant over a period of time

has matured, building upon its origins in the field of industrial hygiene. 

Historically, the concentrations of materials present in the community environment

usually have been much lower than those found in the work place. However, for

some pollutants, the duration of exposure in the community may be longer. These

differences have limited the direct transfer of occupational exposure measurement

techniques to community exposure assessments.

During the 1970s and early 1980s concerted efforts were made to improve the

methods used to measure personal exposures within the general population. At the

same time, it was recognized that individuals spend most of their time engaged in

activities in which air contaminants generated indoors may dominate their overall

exposures. Such contaminants could include unvented indoor combustion products,

residues of sprayed materials such as personal care products, pesticides, cleaning

materials, contaminated house dust, etc. There were also problems encountered

when trying to apply conventional ambient sampling and analytical protocols and
10

equipment to analyses of indoor environments, e.g., indoor air. A series of indoor

air monitors was developed for application to the estimation of exposure within

specific types of community "microenvironments," such as residences and public

buildings. Devices for measuring personal and microenvironmental exposures also

received attention for development and use within epidemiologic studies.

The application of risk assessment techniques to the identification and

control of environmental problems began at EPA during the mid-1970's. One of the

major elements of this process is exposure assessment. By 1980 it was realized that

adequate databases for completing reliable assessments did not exist for any route

of exposure, and that exposure assessment was one of the weakest links in the risk

assessment process. This led a National Academy of Sciences - National Research

Council (NAS-NRC) report to state, in 1983, that "current methods and approaches

to exposure assessment appear to be medium-and route specific ... exposure

assessment has very few components that could be applicable to all media." 

Fortunately, this point is no longer true today.

The first total human exposure assessments were led in the 1980's by the

EPA-sponsored Total Exposure Assessment Methodology (TEAM) studies. The

results of the TEAM studies pointed out that multi-disciplinary scientific analyses

were needed to inter-relate and prioritize the components of an exposure

assessment, including: time-activities, measurements, and biological markers. 

Further, it became clear to a number of individuals and organizations that, to truly

advance exposure assessment within the risk assessment process and other venues,

basic principles needed to be firmly established and presented in a systematic

fashion. Such concerns led to a series of National Research Council (NRC) reports

which outlined the needs and issues surrounding the development of biological

markers of exposure. A publication in 1991, known as the "white book", provided

one of the first comprehensive discussion of fundamental principles in exposure

assessment (NRC, 1991). The conceptual framework presented in that document

has led to mathematical presentations of individual and population based

exposures. It has also led to the development of theoretical models for exposure

assessment that link traditional environmental science with toxicology and the

expression of disease. This evolution has helped to prime the field. A new

professional society (the International Society of Exposure Analysis) was founded in

1989, and EPA has fostered the development of the technology and models,

encouraged the measurement of human exposure via multiple media, reduced

uncertainties in the exposure assessment process, and issued comprehensive

Guidelines for Exposure Assessment (EPA, 1992b). The EPA Office of Research and
11

Development has formed a Division of Exposure Assessment which is leading the

development of a National Human Exposure Assessment Survey (NHEXAS),

discussed elsewhere in this report.

2.5 Current Roles of Exposure Data and Exposure Models in the Risk

Assessment Process at EPA

When EPA must respond to risk related issues within quite limited

timeframes, there are seldom feasible options for exposure assessment other than

reliance on exposure models. At other times, there is reluctance to utilize exposure

measurement data because of the need to make interpretive judgements about such

data, or because of the extra time and effort required to do so in comparison to the

time and effort needed to use standard models. Since the models tend to be, by

design, conservative, the resulting exposure estimates tend to be greater,

sometimes much greater, than the actual exposures. However, the extent to which

exposures, and hence risks, are overestimated is unknown because of the almost

complete absence of comparisons of exposure estimates derived from models to

estimates based on direct measurements.

In this regard, the recent publication of the Guidelines for Exposure

Assessment by EPA (EPA, 1992b), which foster more direct measurements, may

help to create a suitable data base for model evaluation leading to validation or

revision. With an increased reliance on exposure assessments based on valid

measurements, progress can be made toward the goal of improving the accuracy of

risk assessments, as well as providing opportunities for applications in

epidemiology and intervention. It is also desirable to do follow-up exposure

assessment whenever a problem has been identified and remediation has occurred,

to ensure that the problem has been satisfactorily resolved. The additional data

collection can also provide opportunities to reduce uncertainties about the efficacy

of risk management procedures.

The use of exposure assessment in risk assessment has begun to include

applications of pharmacokinetic modeling for predicting doses or reconstructing

past exposures. This area holds promise as a way to reconcile the measurements of

external markers, which show the potential for delivery of a biologically effective

dose, with measurements of biological markers of exposure and/or effect.

2.6 Emerging Opportunities in the Near Term
12

During the last 10-15 years, there have been rapid technological advances in

sensor technologies, computer hardware, software, networks, and communications. 

These advances present many new opportunities for development of technologies

and methodologies for quantitative exposure measurements, for assembly and

analysis of large databases of exposure information (including time-activity pattern

information), and more highly integrated computer frameworks for modeling

population exposures from source through health effect risks.

In the area of chemical sensors, there are multiple possibilities for developing

automated and, in some cases, relatively inexpensive real-time microsensors that

can be used for measuring gaseous and particulate pollutants in air, soil gases, and

water, and for personal and microenvironmental measurements (see Table 1). New

materials and coating technologies have the potential for providing the chemical

specificity and selectivity needed for such sensors. These new technologies offer the

means to do near real-time measurements to understand the variability of

exposures over short and long time-periods. Such sensors could also be used to

directly reduce exposures by providing immediate exposure information to

monitored populations or through linkages to control systems, e.g., air quality

monitoring in buildings coupled with ventilation controls or ground water quality.

In many research applications, sensed data from field measurements are

already being transmitted over telecommunications lines directly to computer

systems for analysis. Such direct transmission reduces chances for errors in

recording data. Applications of this technology to measurements of human

exposures are now possible.
13

Table 1. A FEW EXAMPLES OF NEW SENSOR TECHNOLOGIES WITH

POTENTIAL APPLICATIONS TO AIR POLLUTANT EXPOSURE

ASSESSMENT

Ultrasonic Flexural Plate Wave (FPW) Devices for Chemical Multi-Array

Microsensors. Highly sensitive flexural plate wave devices are being developed for

in situ, real-time analyses of particles and volatile organic compounds in indoor and

outdoor air and clean rooms, and in emissions sources. FPW sensors can be batch

fabricated using well-developed and inexpensive silicon technology and interfaced

with microprocessors that record and analyze the sensed measurements.

Excimer Laser Fragmentation/Fluorescence Spectroscopy (ELFFS). Detection of

metals and organics in the ppb range is possible using ELFFS. The method is nonintrusive, fast, and can selectively detect and quantify many metals, metal species,

and organics. Major applications are likely to be for monitoring emissions from

sources; such information will be of value as input to transport and transformation

models to estimate concentrations of pollutants likely to reach human receptors.

Although ELFFS is unlikely to have the full selectivity and specificity of grab

samples analyzed by gas chromatography and mass spectrometry, such

continuously collected source emissions data would capture the variability in

emissions from sources that cannot be obtained from current grab-sampling

techniques.

Computer Tomography/Fourier-Transform Infrared Spectrometry. This emerging

technology will provide the means to characterize spatial distributions and

movements of air pollutants in three-dimensions in indoor and outdoor

environments. Recent breakthroughs in computer algorithms for the computer

tomography have made it possible for this technology to be commercially available

within three to five years.

Advances in technology should not blind us to the potential for developing

and deploying very inexpensive, passive monitors that can be used to survey

population exposures at relatively small costs. The California Department of

Health Services, for example, has used passive monitors for radon and for

formaldehyde to provide information on exposures of California populations. 

The development of microsensors for other media, however, needs significantly

more research to approach the level of those now available and used for air

quality analysis.
14

Many different kinds of exposure-related models, which take advantage of

computer capabilities and large databases of information, have already been

developed and are currently available. These include the many multimedia

transport and transformation models that provide estimates of pollutant

concentrations in exposure media (See Section 3.3), exposure models that

combine concentration data with time-activity patterns to estimate exposures,

physiologically-based pharmacokinetic models that describe the distribution and

metabolism of toxic chemicals (including biomarkers) in the body, and health

effects models (e.g., cancer risk models). Such models are typically developed as

single models, without consideration of linkages to other models, and are often

written in different computer languages and have system designs that are not

readily compatible with other models. For more fully integrated exposure

analysis, from sources to health effects, integrating frameworks must be

developed that more readily allow the output from one model to easily serve as

input into other models.

A significant problem in exposure analysis has been the lack of

communication and cooperation between modelers and the experimentalists. The

consequences have been that models did not get tested, modified, and validated

in a systematic way, and measurements are often made without reference to a

conceptual framework or hypothesis that could make them useful for producing

generalizable results. If we are to identify and effectively reduce the most

significant human exposures and risks, we must understand the relationship

between exposure and adverse health effects, as well as the cause of such

exposures and the control technologies that can reduce the exposures. This can

be accomplished only through more integrated and multidisciplinary research on

total exposure. Measurements and modeling must be integrated to provide a

single exposure analysis discipline, not many. New ways of organizing our

research and research institutions and rewarding multi-investigator research

will also be needed to foster interdisciplinary research.

Advances in computer and communications networks have already

provided greater access to existing databases useful for exposure assessment. 

Present shortcomings in existing databases, (as discussed in Section 3.1) often

reduce their usefulness in efforts to elucidate cause and effect or to plan effective

risk management steps. The advanced communications networks that are being

rapidly deployed could provide access to information collected for other purposes

that would be of value in assessing human exposure. For example, access to

computer databases of market information on food, consumer products, and time
15

spent on various activities such as hobbies or commuter travel, could be

extremely valuable for inferences about exposures when combined with

measurements of pollutants in food, water, consumer products, building

materials, etc. Gaining access to such information while ensuring privacy of

individuals could, however, be a challenge.

With the rapid interconnection of our society through the

"Communications Superhighway," databases, computer models and

environmental health information, and in some cases, misinformation, will

become more easily accessible to scientists, to regulators, and to the public. On

the positive side, rapid and widespread communications could enable EPA to

influence changes in exposure-related behaviors of the public through

information dissemination. To fulfill its mission of protecting the environment

and human health, EPA may have to develop new (non-regulatory) ways of

thinking about and communicating risks and control strategies to the public. On

the negative side, mis-information can also be easily and rapidly disseminated. 

An increasingly important need in this emerging era is to ensure the validity

and security of databases and information.

In the near future, new insights will inevitably come from combining

measurements of the personal environment with measurements of the

individual's capacity to interact with that environment. For example, it is

technically possible to record simultaneous real-time measurements of specific

airborne compounds in an individual's breathing zone, an individual's breathing

and exercise rates, and his or her geographic location. As these technologies

advance, it will become easier to make these measurements over increasingly

larger and more diverse populations. In addition, advances in the analysis of

small solid samples (such as food or soil) make it feasible to collect and analyze

food, water and soil samples from a given individual on a day-to-day basis. As

these analytical technologies advance and the costs are lowered, routine

monitoring of these environments will be possible.

2.7 Anticipation of Longer-Term Developments

In the longer term, the science of personal monitoring will provide 

important tools that could be used for assessing health risks of the population. 

In addition, the technology of personal monitoring could also provide on-line

important information to a given individual. Equipped with advanced sensor

technology, the individual will want to interact with it. As personal monitoring
16

technology develops, it will undoubtedly be combined with risk information

systems. These systems could be in the form of summary statements, warning

alarms, or access to guidance on actions to reduce personal risks. Whatever the

case, it is certain that individual risk avoidance behavior will occur. The extent

to which behavioral decisions will be informed and useful will depend in part on

the additional information provided with this sensor technology. EPA should be

involved in the development of these information systems, and provide the

public with useful information needed to make rational behavioral decisions

affecting personal exposures. 
17

3. CONSTRAINTS AFFECTING CURRENT HUMAN EXPOSURE

ASSESSMENT

3.1 Constraints in Current Data Resources and Data Management Practices

There are multiple needs for exposure data throughout the risk

assessment and risk management continuum. Exposure data are needed to: 1)

establish baseline exposure levels for populations and to observe trends in those

levels as a measure of the success or failure of risk management efforts (e.g.,

regulations); 2) perform surveillance functions, using the data to identify high

exposure/high risk groups where public health interventions might be

appropriate and effective before harmful effects occur; 3) investigate exposureresponse relationships for environmental health hazards (e.g., epidemiologic

research); and 4) better inform regulatory decisions by supporting population

exposure assessments, cost estimates and other aspects of regulatory analyses. 

The bottom line for these applications is the protection of public health. 

Each requires direct measures of exposure or the monitoring of environmental

factors that will permit an estimation of those exposures. To be useful,

repositories of exposure data should contain not only measurements of

contaminant concentrations in relevant media, but also information about the

circumstances that give rise to the concentrations and to potential exposures to

those media, i.e., simultaneous measurements of the activities of populations of

interest. This section discusses some salient aspects of current exposure

databases and the manner in which current assumptions and practices in

exposure monitoring affect the nature and usefulness of such data.

The last two decades have seen a sea-change in the availability of

methods for sampling and analysis of environmental exposures, as well as in the

means to store and manipulate data through ever cheaper and more efficient

computers. Coupled with the regulatory legislation of the same period, these

changes have triggered an explosion in the volume of quasi exposure-related

data collected and stored. Substantial financial, human and technological

resources are devoted to this task throughout the nation. However, the quality,

availability and usefulness of the resulting data resources leave much to be

desired for their application to long-term assessment of exposure and changes in

exposure. Present data collection approaches are neither comprehensive nor

cost-effective, and the collected data have not been optimally utilized.
18

Government agencies, particularly the EPA and FDA, are the major

collectors of human exposure data, with the notable exception of the

occupational environment, where OSHA, NIOSH and MSHA have primary

domain. EPA typically monitors contaminants in source emissions (i.e.,

industrial stacks or discharge pipes), in the environment (i.e., air and drinking

water) and, much less frequently, in human tissue or blood (i.e., the former

National Human Adipose Tissue Survey). The data, however, are frequently

inadequate for estimating exposure in various exposure assessment applications. 

The monitoring is largely driven by regulatory or legal mandates related to

compliance with source emission or environmental standards. Environmental

monitoring focused on measuring contaminants in the air, water, food and soil,

does not provide actual measures of human exposure. 

In addition to meeting regulatory mandates,, government agencies are

frequently forced to deal with crises (i.e., "contaminant of the day") forcing them

to expend a considerable amount of their time, effort and resources on programs

that are re-active rather than pro-active. Furthermore, as a result of the current

regulatory structure, the measurement of environmental contaminants is highly

compartmentalized among and within various government agencies (OSHA,

EPA, FDA, etc.). This compartmentalization frequently results in artificial

barriers that limit the ability of any one Agency or industry to assess total

human exposure and develop effective mitigation strategies. 

Assessing exposures to air contaminants provides a good example of the

problems posed by the current regulatory structure. EPA has regulatory

responsibility for monitoring community air quality, and OSHA and the

regulated industry have responsibilities for monitoring air in the work place. 

Indoor residential sources, despite their importance (particularly for the

susceptible segments of the population), are not assessed on any routine basis by

any agency, and are not considered in establishing exposure standards. The

ability to foresee and effectively address emerging environmental issues will

require such regulatory barriers to be removed, minimized, or adapted in ways

that promote multiple uses of the data.

Regulatory efforts frequently separate contaminants by whether they are

found in these different media, whether they are ingested, inhaled or absorbed

through the skin, and by the environment in which the exposure occurs (e.g.,

work place, home). Contaminants in multi-environmental media and

multi-human exposure pathways, such as pesticides, automotive fuels, polycyclic
19

aromatic hydrocarbons, and heavy metals, are not monitored in a systematic

way such that the media and pathway of exposures can be assessed or the actual

exposures quantified. A 1992 survey of the availability and utility of existing

federally sponsored databases (Sexton et al. 1992), with excluded occupational

exposure databases, identified 67 such databases. Of these, 41 collected data in

one media and 9 in two media. Databases with data collected in more than one

media or pathway are typically associated with special studies rather than

ongoing programs. The focus of environmental monitoring needs to shift to an

integrated approach. Exposures need to be considered across sources, media,

pathways and environments, then tied to population activity patterns. The

ongoing National Human Exposure Assessment Survey (NHEXAS) will

eventually develop into the first national study to provide estimates of

exposures across different media for a variety of environmental contaminants. 

This integrated approach needs to be further developed as a tool for EPA risk

assessments. The results will provide baseline and trend data for incorporation

into the process of developing policy and regulatory strategies.

A primary justification for human exposure assessment is to support

actions that protect public health and welfare. Seldom, however, are

environmental exposure data gathered in combination with measures of dose or

effects. The National Health and Nutrition Examination Surveys (NHANES) of

the National Center for Health Statistics are some of the few national health

surveys that have gathered biomarker indicators of exposure in combination

with health outcome data. These studies, however, have relied mostly on

questionnaires to assess exposures, with some utilization of environmental

monitoring data that were collected independently. Exposure monitoring needs

to be linked to indicators of dose (i.e. ,biomarkers) and health or comfort

outcomes, as well as to information about the sources and circumstances that

gave rise to the exposures. In many ways NHEXAS is being developed in a

fashion that parallels, and significantly improves on, the limited exposure

metrics that can reasonably be employed in NHANES.

Since environmental contaminants, whether found in the air, water, food

or soil, are generally found as part of a complex mix, current efforts to monitor

environmental contaminants that focus on single compounds (specific pesticides,

individual air contaminants, specific heavy metals, etc.), may not reflect the

complex nature of the mix or the toxicity from synergistic interactions among the

components. If monitoring efforts are to be tied to effects they must take into

account the complex nature of contaminant mixtures
20

Environmental monitoring networks are designed to assess

environmental concentrations of regulated contaminants in areas of suspected

high concentrations and media where the potential for large scale exposure

exists (i.e., ambient air, soil, and water quality monitoring networks). While

these monitoring networks may meet the mandated regulatory needs, they

usually do not provide data for the direct measurement of human exposure, or

data necessary to estimate human exposure, characterize exposure distributions,

or identify high-risk groups. Monitoring locations are rarely selected to be

statistically representative of populations or geographic areas. Population

time-activity patterns are not measured or considered in site selections, or in the

interpretation of data.

 

Numerous environmental monitoring databases have been established by

a variety of federal and state agencies to serve an assortment of needs. Each

database has associated with it potential sources of bias, error and

inconsistencies resulting from all aspects of the data gathering effort (site

selection, frequency of sampling, sampling and analytical methods utilized, etc.).

Quality control and quality assurance procedures vary greatly among these

databases. Seldom is the documentation for the data presented and the

limitations and inconsistencies identified. No standardized procedures exist for

the collection or reporting of environmental data. The lack of such procedures

introduces considerable difficulties in combining existing monitoring data for the

estimation of human exposures.

Furthermore, much of the available environmental data are maintained in

formats that are not easily accessible. The systems through which these data

are stored, retrieved, analyzed and reported are not designed to be easily

accessible and useable by government or nongovernment groups interested in

assessing human exposures. Those databases that are more easily accessible

frequently do not contain the necessary documentation.

3.2 Constraints in Current EPA Practices

Although EPA's research has pioneered and helped to establish much of

the framework and methodology of environmental exposure assessment, these

concepts and principles have only very slowly permeated the practice of exposure

and risk assessment by the Agency as a whole. To a large extent, exposure

assessment for application in risks assessments is still performed under a

number of assumptions which have been proven incorrect, or are not supported
21

by common experience and systematic verification by direct observation. These

assumptions and their shortcomings are summarized in Sections 3.2.1 to 3.2.9

below.

3.2.1 Time-Activity Patterns

Consideration of the time spent by individuals in different locations and

their activities have been incorporated to a very limited extent in exposure

assessments conducted for risk assessment purposes. Current practice is

generally based on the following questionable assumptions:

1. time-activity patterns are invariant during an individual's lifetime,

2. there are no significant differences in time-activity patterns of the

population as a function of age, gender, socioeconomic status, or

ethnic origin which may affect exposures, and

3. there are no significant differences in time-activity patterns of the

population in relation to regional variability, as well as urban,

suburban, or rural place of residence.

In actuality, the amount of time spent in different microenvironments

often varies significantly over an individual's lifetime. Young children are likely

to spend more time outdoors and to be engaged in physically demanding (play)

activities than adults. As the individual ages, the place of activities shifts

towards school and work, largely indoors. After retirement, a shift may occur

again towards spending more time outdoors, but in leisure activities.

Time-activity patterns at any given age can also change significantly over

time due to societal trends. For example, children used to remain at home until

school age. With more women working outside the home, however, more infants

and preschoolers spend most of their day in child care settings. There is also a

trend towards lengthening the school year, which could result in children

spending less time in the home or outdoors, or in changed times of the year for

school vacations. Children and adolescents are also increasingly involved in

structured, competitive sports activities, either indoors or outdoors, for

significant amounts of time. The duration and place where these activities are

undertaken could impact children's exposures and their inhalation doses

associated with elevated ventilation rates. The trend towards telecommuting
22

and the increased number of home-based businesses increases the amount of

time spent at home, as compared to a separate work place, for at least some

fraction of the adult population. A large segment of the adult population also

engages in sports or exercise activities, both indoors and outdoors, during 

significant amounts of time, for health maintenance or cosmetic reasons. As

such adults age, and have more free time, they are likely to engage in less

strenuous, longer duration exercise and leisure activities (e.g., daily walks, golf)

performed outdoors, resulting in less time spent indoors. Consideration of these

societal trends means that activity patterns cannot be considered fixed in time.

Although research in the U.S. and abroad has indicated that time-location

budgets are rather consistent (i.e., approximately 90% of the time is spent

indoors), there has been inadequate consideration of time-activity pattern

variability as a function of socioeconomic status, gender, ethnic origin, or

location of residence. For example, population subgroups in lower socioeconomic

strata with significant levels of unemployment would be expected to have very

different time-activity patterns than those in the middle or upper socioeconomic

strata. They are also less likely to engage in the health maintenance and leisure

activities previously described. The type of employment also can influence the

outdoors vs. indoors time budget (e.g., construction, lawn maintenance). Gender

also affects individual time-activity patterns. Even when over half the women

with school age children work away from the home, a significant number still

remain within the home environment most of the day. There is also limited

information on differences in activity patterns which may affect exposures as a

function of the region of the country, or urban vs. suburban vs. rural

populations, due at least in part, to the lack of activity in behavioral science

research. This is a critical gap in exposure analysis.
23

3.2.2 Exposure Pathways

Although EPA exposure assessments, as applied to risk assessment, have

incorporated the consideration that certain exposure sources and pathways may

be more important for some population subgroups (e.g., ingestion of

contaminated soil by children), current practice is, by and large, based on the

following questionable assumptions:

1. source emissions and concentrations of contaminants in media,

foodstuffs, and consumer products to which an individual may be

exposed are invariant during an average lifetime of 70 years, and

2. the relative importance of different exposure pathways does not

change over an individual's lifetime (exception: soil ingestion by

children).

3. there are no significant differences in relevant exposure pathways

as a function of gender, socioeconomic status, or ethnic origin.

Reformulation of consumer products, e.g., cosmetics, cleansing agents,

personal care products, occurs continuously, both in terms of the types and

relative concentrations of the chemical constituents of such products. Emissions

from mobile sources in particular, but those from industrial point sources as

well, have shown a consistent trend of reductions during the last four decades,

largely as a result of improvements in technology and regulatory pressures. The

types of emissions have also changed in many cases. For example, benzene and

other aromatic compounds have replaced tetraethyl lead as engine antiknock

additives, with the consequent changes in airborne lead and benzene

concentrations, as well as the exposures associated with mobile sources. 

Changes in the types and relative consumption of different fuels as well as

improvements in combustion technology have also resulted in reductions of

airborne concentrations of some pollutants such as sulfur oxides and particulate

matter. As a result, exposures have changed both qualitatively and

quantitatively well within the generally assumed 70-year life span, and current

models do not account for increasing life expectancy.

The types and relative proportions of different foodstuffs in the diet and,

consequently, food consumption-related exposures are generally assumed to be
24

constant over a lifetime after infancy. However, food preferences can vary

significantly with the age of the individual, as well as with societal trends. 

Young children may consume proportionally larger amounts of fruits and milk in

their diet as compared to adults. Adolescents may consume significantly larger

amounts of "fast" or "junk" foods. As adults, they may become more aware of the

importance of a healthy diet and reduce their intakes of such foods, fat, and

meat, and revert to an increased consumption of fruits and vegetables. Advice

regarding what constitutes a healthy diet also changes over time, therefore

affecting exposures as well. In addition, a significant fraction of the population

at any given time may be on special diets (e.g.; weight reduction diets).

Dietary sources of exposure also vary over time as a result of changes in

agriculture and food production practices. For example, pesticides, herbicides,

and other chemicals used in the production, preservation and processing of foods

vary as a result of regulatory and market pressures. New food products are

introduced into the market place constantly, affecting dietary exposures. The

levels of contaminants present in locally grown foods may not be a good indicator

of dietary exposures for a particular population. Foods grown in one area of the

country are distributed nationally; food imports have also increased significantly

as a result of increased trade. The types and relative amounts of foods

consumed also vary according to socioeconomic status, ethnic origin, and gender.

3.2.3 Lifestyle and Other Personal Factors

Current EPA practice in exposure assessment assumes that:

1. in general, lifestyle and/or other personal factors do not impact

exposures significantly.

2. lifestyle and/or other personal factors which might impact

exposures do not vary significantly throughout an individual's

lifetime or across the population.

Lifestyle factors will significantly affect exposures. The most notorious 

behavioral factor with a strong influence on exposures to a wide range of

contaminants is, obviously, smoking. But other lifestyle factors are also

potentially important. Dietary intakes of benzo(a)pyrene may not only be

strongly affected by food preferences, but can also be strongly affected by the

method used to cook the food. Any lifestyle factor affecting time-activity
25

patterns will also affect exposures. For example, the choice of using a personal

motor vehicle or public rail for work-related commuting to and from the suburbs

could significantly impact the motor-vehicle related exposure of commuters, or

the use of pesticides or herbicides at home can lead to significant personal

exposures to children by dermal contact, ingestion, and inhalation.

3.2.4 Residential Mobility

The typical current assumption by EPA is that individuals reside in the

same location throughout their lifetime.

The reality is, however, that the U.S. population is highly mobile, with the

average family changing residences every 5 to 7 years on average. There may

also be differences in mobility associated with socioeconomic status. 

3.2.5 Reliability of Predictive Models Based on Sources and Transport

Current EPA practice in exposure assessment is based on the assumption

that:

exposures can be adequately assessed by using source emissions or

concentrations of contaminants in media and environmental transport models,

and a limited number of exposure situations (i.e., scenarios).

The sophistication and complexity of models used for exposure assessment

have increased notably over the last decade and has also led to the development

of new models that attempt to link exposure to the internal dose of a chemical at

a critical target site. However, there is a significant lack of validation of such

models with actual data.

All models are based on explicit and implicit assumptions about how

contaminants move into, through, and across environmental compartments, as

well as on some conceptual framework about the most significant routes of

exposure. The weakness of this approach is that, frequently, neither the

assumptions nor the model results are validated with actual exposure data. The

TEAM results are an excellent example in which measurements of volatile

organic compounds in outdoor, indoor, and personal air demonstrated that the

long-held assumption that populations living near industrial sources experience

higher exposures to these compounds than those living in less industrialized
26

areas was incorrect. The study showed, in fact, that indoor sources dominate

exposures to many volatile organic compounds, and that exposures to the study

populations were similar and not dominated by the presence of nearby sources. 

Another example is the assumption that ingestion is the dominant route of

exposure for chemicals in potable water. In many cases, major contributions to

internal dose derive from dermal contact in bathing or inhalation of vapors and

aerosols released in showers and sprays. 

Another limitation in exposure model development, as currently done, is

that available information on parameters or variables necessary to estimate

concentrations and transport through compartments may be lacking, very

limited, or even inappropriate for the specific application, adding further

uncertainty to the reliability of model outcomes. In these cases, efforts should be

made to obtain the appropriate data. By limiting assessment of exposures to

specific scenarios and pathways of exposure without validating their importance,

these models may fail to recognize important exposure pathways as well as

variability across the diverse U.S. population. They do not take in consideration

factors such as time-activity patterns which can directly and significantly

influence exposures.

3.2.6 Temporal Variability at Specific Locations

Although EPA occasionally includes consideration of long-term

accumulation of contaminants in environmental compartments, the Agency often

assumes that: the concentration of contaminants in media remains constant

over time.

As a result of this very questionable assumption, exposures associated

with those contaminants and compartments are also assumed to be invariant

over time. To a large extent, this assumption is related to the heavy reliance on

environmental transport and fate models which are generally based on steady

state condition assumptions. Each source has its own attributable effects,

independent of other sources. In reality for agents with the threshold-type

responses, the responses may differ substantially when multiple sources impact

a receptor. Furthermore, concentrations of contaminants in media can either

increase or decrease over time due to changes in environmental conditions or

human activities which may result in either enhancing or diminishing the

release and bioavailability of the contaminant in specific compartments.
27

3.2.7 Independence of Exposures to Multiple Individual Agents and Sources 

Current EPA practice is based on the assumption that: exposures can be

assessed on a chemical by chemical basis, or, conversely, that there are no

significant interactions between simultaneous exposures to more than one agent. 

There is, however, significant evidence that suggests that this assumption

is not appropriate, since effects from a combination of agents may be more or

less than additive. More information is needed on multiple chemical exposures

and their effects. For those cases in which such interactions are known to be

important, the implications for public policy can be substantial. For example, in

the case of the risk from radon exposure for smokers, which are almost one order

of magnitude higher than for non-smokers, a more cost-effective means of

reducing lung cancer, as compared to reducing average population exposures to

radon through radon control in all homes with radon >4pC/L might be to reduce

the number of smokers. This would have the added benefit of reducing other

health risks associated with smoking, e.g., heart and non-malignant lung

disease. The ability to undertake other than chemical-by-chemical exposure

assessment is highly dependent upon the state of knowledge of such

interactions. Although possible, it is obviously not practical to measure

exposures to all potential chemicals simultaneously. However, determination of

simultaneous exposures to more than one chemical could be done and would be

useful when there is toxicologic information on their combined effects and to

guide toxicologic research.

3.2.8 Infiltration of Outdoor Air into Indoor Spaces

Although there has been some recognition that indoor environments are

more significant contributors to solvent and NO2 exposures that outdoor air,

current EPA exposure assessment practice is still largely based on the following

questionable assumptions:

1. buildings do not significantly affect exposures to outdoor airborne

pollutants, and

2. buildings are static and indoor concentrations of airborne

contaminants can be modeled using steady state assumptions.
28

As previously indicated, most of the population spends about 90% of their

time indoors. As a result, indoor environments are typically more important in

determining exposures to some airborne contaminants than ambient air. Indoor

exposures to airborne pollutants are the sum of the fraction of the outdoor

concentration which penetrates into a building plus the contribution from indoor

sources. The extent of infiltration of outdoor air and contaminants into a

structure depends on a number of variables related to the building itself (e.g,

type of construction, type of ventilation system and its maintenance), 

environmental conditions (e.g., season of the year, temperature differences

between outdoors and indoors), the type of contaminant (e.g., reactive or nonreactive, small or large particles), and human factors (e.g., opening of windows,

obstruction of air intakes to maintain higher temperatures indoors, use of the

space for purposes other than the original design). As a result, infiltration of

outdoor contaminants can vary significantly from building to building as well as

over time for the same building. Steady state conditions are, therefore, unlikely

to occur.

3.2.9 Population Distributions of Exposure

Inherent in current EPA practice on exposure assessment are the

assumptions that:

1. the population exposure to any contaminant can be described by

one of the well characterized statistical distributions (e.g., lognormal).

2. the distribution of exposures in the population is not affected by

factors such as socioeconomic status, gender, ethnicity, and lifestyle

factors.

EPA generally assumes that population exposure distributions behave

similarly because concentrations of contaminants in media generally approach

one of the simpler statistical distributions (i.e.,typically a unimodal, log-normal

distribution). The idea that, for risk assessment purposes, an acceptable

exposure can be established at a certain percentile in the upper tail of such

distribution is also based on the assumption of a "well behaved" statistical

distribution of exposures.
29

Figure 1, from EPA's Guidelines for Exposure Assessment (FR 57(104)

5/29/94, 22888-22938) shows the current terminology for describing exposures at

the upper end of the population distribution. This guidance replaced armchair

and uninterpretable terminology, such as a maximally exposed individual. 

When total exposures occur via multiple media, and especially when indirect

exposures to contaminants emitted into and transported through the

atmosphere are involved, the situation can become quite complex. In its recent

review of EPA's Draft "Addendum to the Methodology for Assessing Health

Risks Associated with Indirect Exposure to Combustion Emissions," the SAB

advised the Administrator that Addendum was not ready for release as an "EPA

Methodology" for routine regulatory assessments. At this stage of its

development it can be useful as an analytical tool to identify the chemicals most

likely to accumulate in the environment, the environmental compartments most

at risk of excessive accumulations, and the exposure pathways most likely to

lead to aggregate risks of concern. The SAB report (EPA-SAB-IAQC-94-0096)

also recommended that EPA: 1) develop and implement a strategic plan to

collect critical input data for the models and to validate the methodology; and 2)

establish a framework to ensure that the entire range of potential risks from

stationary combustors are addressed holistically. Implementation of these

recommendations would make it possible to validate and use reliable total

human exposure analysis models.
30

Figure 1 

Recommended Alternative to Figure 5.1 of the Draft EPA Document

Primary features of Figure 1 are the following:

a. Separation of the semi-quantitative measures of exposure from the quantitative

estimators of exposures depicted in the figure.

b. Emphasis on determining or estimating a distribution of population exposure (not

concentration) and selection of a default distribution when the actual distribution is not available

or too little information can be obtained to estimate the distribution.

c. Identification of several statistical estimators of exposure: 1) 50th percentile; 2) 90th

percentile, the "High End", 3) 95th percentile, 4) 98th percentile and a range for bounding

estimates.

d. The Bounding Estimate is an estimate of individual exposure or dose where the

estimate is intentionally constructed to be higher than the individual in the distribution having

the 99.9th percentile exposure. A bounding estimate can be useful in constructing statements

that the exposure is "not greater than .....".

 * Measured Distribution of Exposure.

** The Default Distribution - in the absence of sufficient data to establish the form of the

distribution of exposure (not concentrations) for the population of interest, a default distribution

using a log-normal format should be employed. It should be defined on the basis of median and
31

geometric standard deviation values established using the best information available on the

concentrations and the human activity patterns that lead to exposure.
32

presented in Chapter 6 correctly recognizes that uncertainty analysis cannot be done by following

a formula, that such a process can range from a very simple to a quite complex process, and that

the process requires scientific judgment. Both qualitative (choice of model or measurement

method, underlying assumptions, etc.) and quantitative aspects of uncertainty are recognized and

clearly presented. The types of uncertainties that must be considered have been clearly

identified and the various approaches which may be taken to evaluate and/or estimate

uncertainty are scientifically correct and adequate.

While data on population exposures and their distributions are very

limited, there is some evidence that population exposure distributions are often

skewed, i.e., they may be multimodal because the overall distribution of

population exposures is a juxtaposition of different population subgroups

exposures). A major reason for this is the influence of societal factors and sex in

determining exposures, as previously described. Socioeconomic status (which is

also linked to ethnicity, education, gender, and age in the U.S.), for example, is a

powerful determinant of where people live, the food they consume, their

activities, and their personal habits, all variables that affect exposures. The

population subgroups that experience higher exposures as a result of their living

conditions and lifestyles may also be more prone to experience the negative

health outcomes from such exposures, either because of genetic susceptibility, or

as a result of poor health status. Current approaches to exposure and risk

assessment cannot either identify or adequately protect these subpopulations.

3.3 Currently Available Models

Up to now, the most thoroughly developed and validated models used by

EPA are the recent versions of the NEM models used in support of the National

Ambient Air Quality Standards (McCurdy, ISEA, ISEE, 1994). They are also the

most data intensive models, and the resources that have gone into the

development and validation are not likely to be available for exposure

assessment of other environmental exposures.

The NEM models have used measurements of personal exposure as

indicators of the potential for the development of future chronic disease. In their

absence, environmental fate and transport models can be used to estimate

future personal exposures. A comprehensive suite of models that could credibly

predict movement, degradation, and accumulation of chemicals in the

environment could serve as the common metric for regulatory decisions in the

areas of standard development, permitting, site specific risk assessments, site

ranking, and cleanup. Unfortunately, there are, as yet, no validated models that

meet these goals.

Current models for human exposure assessment provide estimates of

chemical concentrations in environmental media, as well as mass flux from
 2

 This summary is abstracted from material presented by the New Jersey Department of Environmental

Protection and Energy to the Environmental Risk Assessment and Risk Management Study Commission in

March 1994.

33

sources, through environmental compartments, to human receptor target

organs, tissues or cells. Most of the modeling effort has been focussed on the

solution of chemical fate and transport equations for the prediction of

concentrations in air, food, water and soil. Common pathways of concern

include: drinking water, incidental ingestion of soil, inhalation of vapors and

particulate matter, and ingestion of fish and garden vegetables. 

The following is a brief summary of some of the current applications of

fate and transport modeling:2

Non-Governmental Models

A recent review article compared the output of ten soil standard models

when applied to one site in Vancouver. The authors observed that the models

fell into two categories; those with one cleanup number for each chemical

regardless of site conditions (absolute approach) and those which would accept

site specific conditions as determinants of the chemical specific cleanup number

(relative approach). [Jessiman, 1992]. 

In a summary table comparing significant pathways of exposure across all

models, ingestion accounted for 99% of all exposure to inorganics. The other

tabulated pathways were dust inhalation, vapor inhalation, and dermal uptake

from soil and water. In contrast for organics, dermal contact with soil and water

accounted for about 50% of the dose, with ingestion making up most of the

remainder. Crop exposure was most often the largest contribution to ingestion

for the 23 chemicals examined.

A multimedia chemical transport and transformation model, GEOTOX,

was used by McKone (1991) to estimate concentrations of contaminants in air

(particulate and gas phase), soil, drinking water and surface water. The model

has different exposure pathways pertaining to each of the media when they are

considered as sources. Test simulations found that the ingestion pathways of

fruit, vegetables, and grains were important contributors to total exposure and

dose.

Risk Assistant, developed by Hampshire Research Institute Inc.

(Hampshire 1991) with EPA support, is a microcomputer-based software system

that contains formulas for 14 pathways of exposure. This software development

was funded by U.S. EPA with contributions and reviews by the New Jersey

Department of Environmental Protection and the California Environmental
34

Protection Agency. Numerous pathways are considered (drinking water,

showering, ingestion of fish and shellfish and homegrown food products,

inhalation of vapors and particulates inside and outside the residence, and

others), along with numerous media (groundwater, surface water, biota, soil, air,

and sediment).

State Models

The State of New Jersey proposed rules for the cleanup of contaminated

sites in 1992 (NJR 1992). Numerical soil standards were proposed for over 100

contaminants. The pathways considered were soil ingestion, contamination of

ground water, inhalation, and contamination of surface water bodies and the

food chain due to erosion of soil. Vapor migration into structures was

approached with a trigger level soil gas concentration which would require

further investigation. 

The State of California model (CalTOX 1993), which is in the Beta-test

stage, is potentially the most advanced of all of the models reviewed with respect

to exposure, although the transport components are simplified for ease of

handling. Intake equations are the same as those used by the USEPA with two

modifications. First, there is a multimedia total exposure model. Second, it is

used stochastically (instead of a single risk level, a Monte Carlo derived

distribution of risk is presented). CalTOX contains fugacity based multimedia

fate and transport equations. 

USEPA Models (Superfund)

The Risk Assessment Guidance for Superfund, Part B (USEPA 1991),

reflects current EPA guidance for developing soil cleanup levels. Pathways of

exposure are suggested for two land-use scenarios, residential and

commercial/industrial. Default equations are only provided for some of the

pathways. It is the responsibility of the risk assessor to find appropriate

equations for the other pathways in these two generic exposure scenarios.

Draft Soil Screening Level Guidance, (USEPA 1993) is a response to the

Administrator's request for a 30-day study to outline options for accelerating the

rate of cleanups at Superfund sites. There are changes in this guidance from the

pre-existing guidance (USEPA 1991), such as those involving soil saturation by a

chemical as a trigger level for cleanup, and different integration of childhood soil

ingestion. A quantitative approach for the soil to groundwater pathway is given. 

Monte Carlo analysis is suggested for use in this guidance, but only for the

groundwater pathway. 
35

Assessing Potential Indoor Air Impacts for Superfund sites, September

1992 (USEPA 1992) was developed to assess the risk for occupants of buildings

near Superfund sites. The model accounts for contaminated water or soil

leading to volatilization of chemicals and movement through the soil to building

interiors because of withdrawal of soil gas to interior living spaces. Several tiers

of screening equations lead to quantitative estimates of the relationship between

soil and or groundwater contamination to soil gas and, thereby, interior

concentrations of contaminants. The only pathway considered in this document

is soil gas to building interiors.

USEPA Models (Non-Superfund)

Methodology for Assessing Health Risks Associated with Indirect

Exposure to Combustor Emissions, January 1990 (USEPA 1990) is an Interim

Final EPA report with quantitative models sufficient for a multi-pollutant,

multimedia human exposure assessment. The focus of this document is risk

assessment of several pathways arising from of stationary source combustion

facilities. as noted earlier, a revised version of this model was reviewed by

IAQC/SAB and judged to be inadequate for quantitative exposure assessment.
36

 The Sludge Risk Assessment Branch of the Office of Science and

Technology of the Office of Water developed the Standards for the Use or

Disposal of Sewage Sludge (40 CFR Part 503) (USEPA 1992a). In these

regulations there is a basis and background which describes 5 pathways of

human exposure to land applied sewage sludge, including a quantitative

approach and the data requirements for the pollutant fate and transport

analysis.
37

4. DEVELOPMENTS AND PROSPECTS FOR IMPROVEMENTS IN

EXPOSURE ASSESSMENT

EPA has: 1) conducted many field studies which have contributed to our

understanding of human exposure; and 2) has developed many models for

human exposure assessment. Under the current organization of scientific

research at EPA, however, modeling and field measurement activities are often

compartmentalized and there is little or no interaction between scientists

involved in these two activities. The consequences are that models do not get

tested, modified, and validated in a systematic way and measurements are often

done without some conceptual framework or hypothesis that would allow

generation of more generalizable results.

Development of effective policies for the protection of human health

requires a sound scientific understanding of complex systems. The scientific

process of hypothesis generation and testing requires integrated use of exposure

models and field measurements to develop such understanding. Well-developed

and validated models clearly have predictive powers that are useful for

evaluating the outcome of alternative proposed policies. However, they also

provide the means to integrate field measurement data into coherent theoretical

frameworks that allow us to quantitatively link the sources and dynamics of

exposure to exposure, dose, and health effect, and to develop control policies that

are targeted to the largest sources of exposure and the most significant

environmental factors controlling exposure. Models also provide the means to

design more strategic and cost-effective field studies for hypothesis testing. 

Field measurements, in turn, are absolutely essential to the validation of

models. Comparison of field measurements to model predictions often reveals a

lack of understanding of a given exposure pathway and the need to modify the

model. Furthermore, well-designed and conducted field measurements provide

the "ground truth" for exposure analysis.

In an era of constrained budgets and increased demand for sound

environmental policies, it will be essential to integrate the measurement and

modeling activities to provide a single integrated science of exposure analysis.

4.1 Framework for the Science of Exposure Analysis

The development, in this report, of a fundamental framework for

conducting exposure analyses to be applied in exposure assessments for

epidemiology, risk assessment, or risk management purposes has identified two

basic approaches for obtaining data on human contact with environmental

chemicals: i.e., direct and indirect. An overall schematic diagram of the

categories which are included within each approach are shown in Figure 2. 

Present techniques can reliably obtain information for one or more categories
38

associated with each route of exposure. However, there are numerous gaps, and

improvements are needed, especially for indirect approaches, such as biological

markers for all media and routes of entry to the body. As discussed in the

following, identification of exposures of significance to human health above

natural or baseline levels and norms necessitates the collection of data on

general population norms.

Figure 2. Possible Approaches for Analysis of Contaminant Exposures.

For the indirect approaches, needs have been identified for validation of

exposure/dose and microenvironmental models, biomarkers survey instruments,
39

and microenvironmental measurement techniques. The technological needs are

quite varied with respect to both sophistication and reliability for each route of

exposure. Some devices are relatively well developed for measurements of

environmental quality. Unfortunately, there are still some significant problems

with the interpretation of measurements made for assessing exposure, since

little information is collected on the potential for actual human contact. This

discussion will focus on some serious issues related to achieving better

techniques for both environmental quality and exposure related problems and

how to access and store such data.

The fundamental framework evolving for exposure analysis is also

challenging the field to validate models, integrate across routes of exposure, and

use pharmacokinetics in quantitative attempts to determine dose from single

and multiroute exposure. These types of information are essential for the

continued evolution of the measurement technology that can feed back

information on the gaps in data indicated by the models, and help reduce

uncertainty. Microsensors for all media can be used in such applications.

4.1.1 Databases

Changes in a broad range of domains affecting environmental exposures

(fuel usage, consumer products, population demographics, source and type of

foods, transportation, type and location of work, population time-activity

patterns, environmental monitoring technology, etc.) will greatly influence the

nature and extent of effects of environmental contaminants on human health

and environmental quality over the next several decades . The ability to

anticipate and assess population exposures will be critical in EPA's effort to

assess potential risks associated with emerging problems and to develop

effective mitigation strategies to reduce or eliminate those risks. The EPA and

other federal and local agencies will have to make fundamental changes in their

current rationales for environmental monitoring if they are to meet the

challenges of the future. The widely accepted approach of highly

compartmentalized sampling efforts that are single contaminant, single media,

and single pathway based with no clear relationship to the time-activities of

individuals, to other exposure determinants, or to at risk populations will not be

adequate for addressing future needs. This is especially true for chemicals in

consumer products, such as those in motor vehicles, fuels, solvents, and

pesticides.

The lack of a comprehensive system for the collection, storage and use of

exposure data, as outlined in Section 3.1, should come as no surprise. The

patchwork character of our environmental legislation and regulation, and the

resulting exposure monitoring and data systems, are a natural consequence of

current Agency mandates and organization. The challenge today is to re-orient
40

our environmental protection infrastructure so that it can become more coherent

and cost-efficient. The improved collection and use of exposure data, as key

elements in risk assessment and management, are integral parts of this overall

goal. The focus will always remain on protecting the public health and welfare,

but a greater emphasis should be placed on establishing links between total

exposure and its determinants, doses and effects. The capability of EPA to

assess exposures will in large part be determined by the framework within

which it approaches to exposure assessment. Among some of the steps that

should be taken to position the Agency to meet the future challenges in human

exposure assessment posed by emerging issue are the following:

1. BROADEN THE RATIONALE FOR ENVIRONMENTAL

MONITORING TO INCLUDE EXPOSURE ANALYSIS:

Environmental monitoring efforts currently underway are generally

for regulatory purposes and not guided by an exposure assessment

framework. Efforts should be made to restructure or augment the

data collection process to assess exposures to complex contaminant

mixtures and relate those exposures to dose and ultimately the

health, comfort, or ecological endpoint. Limitations or gaps in

information should be identified and methods of supplementing

current data gathering protocols to make them more relevant to

exposure assessment should be identified and incorporated into

ongoing monitoring efforts.

2. ESTABLISH AN ONGOING PROGRAM OF TREND

MONITORING: Monitoring programs that are explicitly directed

toward assessing time trends in micro-environmental

concentrations, factors impacting concentrations, receptor

time-activity patterns, and time trends in exposures should be

established, using both existing data resources and new programs

to fill critical data gaps. For human health effects, an objective

should be to obtain representative U.S. population distributions of

exposure, with emphasis on populations at high risk groups (young,

low socio-economic status, minorities, etc.). A stable, long-term

monitoring system (production volumes, concentrations, emissions,

time-activity patterns, personal exposures, etc.) is crucial for

identifying emerging trends (identifying new contaminants,

determining the factors impacting transport through the

environment, identifying high exposure populations, determining

exposures of sensitive populations, etc.), following the time course

of current and past environmental contaminants, providing

exposure data for risk based priority setting, developing and

validating exposure models, developing effective mitigation policies,

and tracking progress toward meeting exposure reduction goals.
41

3. ORGANIZE EXPOSURE RESEARCH TO INTEGRATE

MODELING AND MEASUREMENTS. EPA has numerous

opportunities to utilize large data sets containing information on

concentrations of chemicals in environmental media collected for

specific research studies and enforcement efforts, for dispersion

model evaluation and/or validation. The exposure assessment

group in EPA should be directed to identify and utilize existing

environmental data resources for testing EPA dispersion and

exposure models, and to look for opportunities to "boot-strap" some

additional data collection onto forthcoming monitoring activities in

order to provide unique opportunities for model evaluation. These

efforts could lead to new models and modifications of existing

models that would enhance the credibility and utility of risk

assessment and risk management decisions.

4. IMPROVE COORDINATION BETWEEN FEDERAL AGENCIES: 

Numerous federal agencies are responsible for a variety of

environmental monitoring efforts. These efforts are typically

conducted independently. Approaches to fostering a greater level of

cooperation between these agencies in the design and conduct of

their monitoring programs is needed. Cooperation should be sought

in standardizing procedures for collecting, storing, analyzing,

reporting and providing access to databases for all relevant

exposure data. This standardization should include the

establishment of a common set of criteria under which

environmental monitoring should be conducted and evaluated. 

Cooperation should also extend to exploring the establishment of

integrated data bases where data collected by various agencies is

presented in a format suitable for use in a wide range of exposure

assessment applications. Such data bases will enhance efforts in

risk assessment, risk-management and environmental

epidemiology and in determining trends and result in the

formulation of more cost effective environmental and health policy. 

For example, geographical mapping of measured or estimated

exposure could be used in combination with effects data to identify

potential environmental and health risks and develop testable

hypotheses.

Two complementary approaches to accomplish many of these objectives

have emerged in the recent scientific literature. One approach seeks to

implement schemes that collect, de novo, information about total human

exposures from the nation as a whole or from particular subpopulations. 

Naturally, any such project must limit the number of contaminants, populations

and exposure routes that are examined, or it becomes unmanageable. The EPA
April 3, 1995 3 The proceedings were published in the November/December

1992 issue of Archives of Environmental Health Vol. 47 (6).

42

is now in the pilot stages of the National Human Exposure Assessment Survey

(NHEXAS). The results of this program should shed much light on the viability

and utility of this approach to provide the long-term multi-pollutant and

multiple data route needed to assess exposures in coming decades.

A complementary approach seeks to maximize the usefulness of the very

large volume of compliance data that are already collected under the mandates

of the various statutes and programs. This represents a substantial target of

opportunity, given the vast resources that are already being spent on the

collection of such data, and because compliance data will continue to make up

the bulk of the exposure-related data that will be collected in any systematic

way in the future.

It will not, however, be easy to modify routine compliance protocols to

enable such data to serve both the practical and short-term needs of fulfilling

multiple legal requirements, as well as the longer-term needs of risk assessors

and managers to consider exposures. There are challenges at many levels. At

the simplest level, it will be necessary to collect more and different information

about the circumstances that influence the raw exposure measurements than is

now the case. Those who collect the data, but have no immediate need for such

additional information, may tend to resist the collection of the critical accessory

data. On the other hand, temptations will also exist for excessive data

collection, with consequent drain on resources. Also, if multiple compliance data

resources are to achieve the coherence necessary to help evaluate total human

and ecological exposures, they must be sufficiently compatible and standardized,

which is no trivial task. Finally, there are major scientific questions about

precisely what additional information is needed, along with raw measurements,

to establish closer links between exposure data that are routinely collected and

those needed for effects evaluations, dose estimations (e.g., biomarkers),

exposure estimates for specific populations or targets of interest, or surveillance,

as well as links to sources and pathways of exposure.

Efforts have already begun in EPA and elsewhere to improve the

character of the exposure data to be collected in the future, so that it can

contribute more fully to the resolution of environmental dilemmas. Two

landmark conferences focussed attention on these challenges and produced

recommendations to develop mechanisms to tackle them. A workshop designed

to examine exposure-related databases in the traditional EPA media--air, water,

soil--was held in 19923

, and an International Conference on Occupational
 4 The proceedings of this conference will be published in the April 1995 issue of the Journal of App

Occupational and Environmental Hygiene 10(4).

43

Exposure Databases was held in 19934. Both gatherings resulted in important

recommendations for the improvement of data resources to meet the broader

needs of the future. The ability of the sponsoring organizations and others in

the field to form a critical mass and implement these recommendations will have

important implications for the 5-30 year time frame covered in the reports on

Environmental Futures.

While a danger exists that the current scenario of uncoordinated collection

of data will become even more entrenched, the almost certain advances in the

technologies used to monitor exposures and the technical abilities to manage

vast amounts of data, discussed elsewhere in this report, set the stage for

qualitative advances in the exposure data resources that will be available to

support environmental and health protection needs.

4.1.2 Personal Samplers and Monitors

The historical development of personal inhalation exposure monitoring

has focused on measurements of the environment surrounding the individual. 

This is still true in most cases. For example, personal monitors of airborne

contaminants provide information on concentrations of a variety of compounds

near the individual's breathing zone. However these measurements are rarely

accompanied by concurrent measures of breathing rates or exercise patterns,

even though these measurements are technically feasible. Finally, we know of

no commercially available personal air sampler that measures the accompanying

composition of an individual's exhaled air - information relevant to the

respiratory uptake of the compound(s) of interest. 

An analogous, if less characterized, human exposure pathway is dermal

exposure. Measurements of the surface composition of skin or of devices

attached to the skin can provide useful information on the time-averaged

concentrations of compounds of interest. However there is currently no routine

personal monitoring of the adhesion to or uptake of compounds through the skin. 

Further, the lack of time-activity data precludes identifying the factors that

reduce adhesion of particles or compounds on the skin.

Whereas inhalation and dermal exposure assessment lacks information on

the individual compared with the environment, ingestion exposure assessment

historically lacks information on the environment compared with the individual. 

Instead of a physical sampling device, the instrument of choice is the

questionnaire. In many cases, questionnaire surveys can adequately estimate

what and how much food and water an individual has ingested (but not how
44

much soil). However information on the specific chemical composition of this

ingested material is lacking. We do not have a personal monitor that senses the

time averaged composition of ingested food, water or soil. Duplicate diet

techniques hold some promise for obtaining information on contaminant

ingestion, but the lack of well-designed and non-intrusive protocols precludes

implementation within large populations at this time.

4.1.3 Biomarkers

The term "biomarkers" is now widely used to refer to measurements that

can be made in biological materials; the quantities measured may be indicators

of exposure, dose, susceptibility, pre-clinical disease, and biological injury and

disease processes. Biomarkers offer the promise of serving as indicators for

these endpoints that can be more biologically appropriate than those utilized in

present research approaches. While validated biomarkers have already proven

valuable as research tools, their promise as practical indicators of population

susceptibility, exposure, or response has not yet been fulfilled. This failure, in

spite of intensive research in the last decade, reflects the unanticipated

complexity of validating biomarkers; we also lack strategies for efficiently and

quickly moving new biomarkers through the sequence that begins with

development, continues with validation, and ends with applications in

populations. These limitations apply to both biomarkers of exposure and of

response.

4.1.3.1 Biomarkers of Exposure

Biomarkers of exposure are indicators of prior contact with an

environmental agent, including the agent itself or its metabolites, markers of

immune response to the agent or metabolites, or other changes indicative of

exposure. Exposure biomarkers are considered complementary to, and more

biologically informative than, indirect and qualitative measures of exposure,

such as questionnaires. There are diverse types of biomarkers, ranging from

simple to complex in measurement requirements and indexing remote to recent

exposures. There is also a range of biological relevance among exposure

biomarkers: some provide indices that are directly biologically relevant, e.g.,

level of carbon monoxide in end-tidal air samples and risk of myocardial

ischemia, whereas others may not cover the temporally appropriate exposure

window, e.g., nicotine levels in biological fluids and lung cancer risk from smoke

exposure.

What is the potential of exposure biomarkers for future research and

assessment and control of environmental risks? For the near-term, extensive

development of new biomarkers relevant to malignant and non-malignant

diseases can be anticipated. However, these new exposure biomarkers remain to
45

be validated, and few will be ready for translation to the population over the

short-term. Their most immediate application will be in the context of

epidemiologic studies of environmental agents. Biomarkers will increasingly

become components of the exposure assessment strategies used in

environmental epidemiology; efficient research strategies will apply biomarkers

to samples of subjects to validate more broadly used exposure assessment

methods, and to provide better estimates of dose.

For the longer term, biomarker measurements should become one of the

approaches used for monitoring the exposures and health risks of population

groups. Exposure biomarkers may be applied to groups with unique exposure or

susceptibility patterns, to monitor the population in general, and to document

the consequences of exposure assessment strategies designed to reduce

population exposures. Exposure biomarkers validated against the endpoint of

disease risk, and used in conjunction with other measurements and metrics of

exposure, should prove particularly effective in risk assessment.

4.1.3.1 Biomarkers of Response

Biomarkers of response are indicators of injury induced by environmental

agents. They may be reflective of the earliest stages of the process initiated by

exposure, even at the molecular level, or markers of changes that presage

disease at its earliest phases. As for biomarkers of exposure, the near-term need

is for basic developmental research. For chronic diseases, such as cancer, only

lengthy studies can provide the needed validation against the occurrence of

disease as the most relevant "gold standard." For the longer-term, biomarkers of

response may serve the same population monitoring purposes listed above for

exposure biomarkers. However, the validation process may not be completed for

many biomarkers of response over the 30-year time frame considered in this

report.

4.1.3.3 Ethical Issues

The ethical issues inherent in applying biomarkers are already a subject

of considerable research and discussion, largely centered around use of genetic

markers of susceptibility (such ethical issues have been considered in a rapidly

enlarging literature; see for example, Kevies and Hood, 1992). However,

biomarkers of exposure and response may also pose complex ethical challenges

and new and unanticipated ethical dilemmas. Information gained from

biomarkers of exposure, response and susceptibility may provide an early

warning of high risk or pre-clinical disease; capability for early warning will

require a high level of, and an accepted social-regulatory framework for followup actions. They may also cause false alarms and needless stress for individuals

warned about the presence of uncertain signals.
46

As biomarkers of exposure are developed over the short-term, a parallel

research program is needed on the ethical issues that will inevitably follow

application of the markers. Anticipating the longer term is more difficult, but it

seems likely that increasingly sensitive exposure markers and increasingly

comprehensive understanding of genetic determinants of response will lead to

increasingly difficult ethical issues.

4.1.4 Questionnaires

Questionnaires have been a central tool in environmental epidemiology,

used to assess exposure, susceptibility, and response. This central role led to an

early recognition of the need for standardization and validation. As a result,

many of the questionnaires currently used in environmental epidemiology have

been characterized as to their accuracy. Major advances in questionnaire

technology would require substantial advances in applications of behavioral

science, although increasingly automated methods of administration involving

computer and video administration can be anticipated for collecting useful data

on personal activities. Biomarkers will prove to be a useful tool for validating

questionnaires, and vice versa.

4.1.4.1 Source Characterization

For the purposes of exposure assessment, questionnaires have primarily

obtained information on contact with sources, e.g., living in a home with a gas

stove releasing invented combustion products. Some have incorporated items on

activity patterns that may influence exposures, including source use and source

avoidance. For the short-term, further advances in use of questionnaires for

characterizing sources and exposures should be anticipated. Standardized

instruments are likely to be developed that capture key sources and activities in

a uniform manner. If applied in representative samples, data from such

instruments could begin to better characterize population patterns of exposure. 

For the longer term, major advances in automated technology, such as activity

loggers and factors affecting contaminant release from sources can be

anticipated.

4.1.4.2 Diversity Pattern

We are increasingly recognizing the relevance of the population's

heterogeneity in environmental epidemiology. Race, gender, ethnicity,

education, and income are powerful determinants of both susceptibility and

environmental exposures in the work place, at home, and in the neighborhood. 

Until recently, methods for capturing the population's diversity have received

little attention in environmental health research. With the emergence of the

concept of "environmental justice and equity" and the recognition of the
47

substantial stratification of our society in the extent of environmental exposures,

rapid advances can be anticipated in questionnaire-based methods for assessing

racial, cultural, and socioeconomic factors that influence patterns of

environmental exposure. For the short-term, application of these instruments

should quickly provide new insights into previously overlooked determinants of

exposure.

4.1.4.3 Health Status

We are gaining new insights of the need to assess a broad range of

indicators of population response to environmental agents. While the focus of

research and risk management approaches has largely been on medically

relevant outcomes or on overt disease, increasingly sophisticated instruments

are becoming available to assess quality of life and health status. Those more

holistic data collection approaches have received limited application in regard to

environmental exposures; on the other hand, the relevance of loss of well-being

and comfort has been made clear by the emerging problem of indoor air

pollution. Changing societal expectations in regard to the environment will also

force wider application of questionnaires directed at quality of life and health

status.

For the short-term, it is likely that environmental health researchers will

increasingly incorporate existing instruments into research protocols. However,

instruments appropriately tuned for investigating the environment are needed,

and they will likely be developed over the short-term.

4.1.5 Improved and Validated Exposure Models

Over the past few years numerous state and federal agencies have

proposed comprehensive exposure models for the purpose of regulating

discharges to the environment and cleanup of hazardous waste sites. Most of

these efforts have proceeded independently of each other. There is much to be

gained by reviewing all the approaches for estimating exposure to contaminants

for common pathways and reaching some consensus on the best approaches. 

This could provide a common logic and justification for regulating the many

hundreds of chemicals in the environment. By testing the pathway analysis for

many chemicals from many sources it would be expected that some chemicals by

some pathways of exposure will be predicted to result in unacceptable doses to

people. An effort could then be made to validate the critical pathway models. 

Most of the exposure models in current use have not been validated and also

need to be linked within comprehensive models that attempt to characterize

exposure and dose at target sites. Further, attempts should be made in the

near-term to improve exposure models as tools for prioritizing the major routes

and media of concern.
48
49

5. FINDINGS

5.1 The Near-Term (5-year) Perspective

5.1.1 Emerging Issues in Relation to Potentially Important Risks

There are numerous emerging issues confronting EPA and the nation for

which much of the current uncertainty is related to the extent and nature of

human and ecosystem exposures. Some examples are:

1. Environmental estrogens - There is considerable uncertainty

concerning which of the numerous chemicals with estrogenic

activities are most likely to produce adverse effects, and their timeconstants for accumulation in environmental biota, food-chains, and

human tissues. Thus, there is considerable uncertainty about what

human contacts are, what environmental media to sample, how

much to sample, over what sampling intervals, and what analytical

protocols to employ to yield sufficient sensitivity and precision for

meaningful analyses.

2. Electromagnetic fields (EMF) - A number of statistically significant

associations between excesses in a variety of cancers and proximity

to electrical power lines have been observed, causing serious public

health concerns for people living near high-voltage power

transmission lines and for people using powered appliances

generating strong local fields (electric blankets, electric shavers,

etc.). Unfortunately, the field properties most directly related to

possible cancer causation are not known. Candidate factors include

field intensity, specific frequency windows, local physical

configurations leading to standing waves, etc. In view of the lack of

knowledge of the basic biomedical mechanisms by which EMF can

contribute to the expression of cancer, it is not yet possible to

specify what aspects of EMFs should be measured in exposure

assessments.

3. Particulate Matter (PM10) - There is an ever-increasing array of

epidemiological evidence linking daily ambient air particulate

matter (PM) concentrations to daily mortality rates and indices of

morbidity such as hospital admissions for respiratory diseases,

emergency room visits for respiratory disease, exacerbation of

asthma, and absences from school or work. Various metrics of PM

have been used for such studies, including PM10 (particles < 10 m

in aerodynamic diameter), PM2.5 aka fine particles (particles < 2.5

m in aerodynamic diameter) and sulfate ion chemical equivalents
50

(sum of sulfuric acid, ammonium bisulfate, and ammonium sulfate -

all particulates originating from the oxidation of sulfur dioxide

vapor). The strongest correlations with health effects are generally

with PM2.5 and SO4

=

. It is important to identify the inorganic and/or

organic constituents within ambient PM that drive the associations

with health effects in order to guide control efforts by the most costeffective means.

5.1.2 Capabilities

Market forces will be a major driver with respect to the demand for

increasingly more powerful personal monitoring devices. This market will

primarily be directed at characterizing work place exposures, as companies

recognize the increasing importance of documenting workers' exposures. These

monitoring capabilities can be broadly classified by route of exposure -

inhalation, ingestion or dermal. As an example of the anticipated progress in

exposure assessment, we discuss the inhalation route in some detail, although

this does not mean that the other routes are of any less significance or concern.

As mentioned earlier, near-term progress is expected in our knowledge of

inhalation exposures with the recognition that monitoring of airborne

concentrations needs to be accompanied by monitoring of breathing

characteristics. This will provide more accurate information on the extreme

exposures within the population being studied and will help to improve models

of exposure at the extremes. 

In addition to this anticipated progress, there are currently a number of

rapidly evolving technologies for monitoring the composition of air in an

individual's breathing zone. These include active samplers that collect the

entire air sample, or move air past a sensor or through a collection medium. 

They also include passive samplers that rely on collection of chemical vapors by

diffusion onto a sampling substrate. EPA researchers have recently developed a

prototype personal whole air sampler for collection of volatile organic

compounds. This sampling approach, combined with modern GC/MS technology,

will continue to expand our knowledge of the breathing zone concentrations of a

large number of compounds. An increasing number of electrochemical sensors

have been developed for real-time measurement of inorganic vapors in the work

place. As this technology improves its sensitivity, either through improved

sensors or development of pre-concentrating capabilities, we will be able to

document short-term exposures variations in breathing zone concentrations of a

number for gaseous compounds.

Measurement of particle levels in the breathing zone also shows promise

of dramatic advancement with the development of personal optically based
51

particle sensors that rely on miniature laser diodes. The requisite sensitivity

currently exists to use these devices to measure particle size distribution in realtime. In fact, modern clean room counters are now employing such miniaturized

laser diode technology. Coupled with advanced microprocessor capabilities, we

will see real-time optical particle sizing for personal exposure assessment in the

near future. There is also a serious need for improvement of data on the

inorganic and organic constituents of the mass collected or detected by such

devices.

In summary, immediate progress is expected in our knowledge of personal

exposures with the recognition that monitoring of environmental concentrations

needs to be accompanied by monitoring of individual characteristics relevant to

the uptake of environmental chemicals. This is true for all routes of intake. 

Using the inhalation route as an example, it is easily seen that application of

current technologies will dramatically enhance our knowledge of personal

environments in the near future.

5.1.3 EPA's Recent Role and Activities

In Future Risk (1988), EPA's Science Advisory Board recommended the

creation of a human exposure research program as one of ten strategic research

priorities for the next decade. Noting the fundamental lack of total exposure

research and measurements for virtually all microenvironments and pollutants,

the SAB stressed the importance of the exposure paradigm in reducing

uncertainty in risk assessments and in strengthening the scientific basis for

regulatory decision-making.

EPA's Office of Research and Development responded to this

recommendation later that same year by creating a human exposure research

program with strong commitment to cooperative research with the extramural

research community. This included the establishment of an Exposure

Assessment Research Division within the ORD laboratory in North Carolina.

Although exposure research funding has been modest, it has facilitated research:

1. to advance the state of the science in exposure measurement and

associated statistical methods;

2. to develop activity pattern data as well as single-and multi- media

exposure models; and

3. to characterize human exposures to: motor vehicle emissions;

particles, aerosol acidity, and PAH (polycyclic aromatic

hydrocarbons) in urban areas; lead in urban residential areas; etc.
52

In addition, EPA's ORD human exposure research community has begun

a partnership with federal and academic research institutions to develop

protocols that would permit national-scale assessments of total human exposure

to be conducted. This program, called the National Human Exposure

Assessment Survey (NHEXAS), is supporting pilot and scoping studies through

competitive cooperative agreements. They will study exposure at both

community and regional levels and will test the scientific validity of current

protocols over a period of five years. This work is being done in conjunction with

the Exposure Assessment Group in ORD Washington. This group in EPA has

advanced data and modeling needs for EPA exposure assessments by publishing

the Exposure Assessment Guidelines and an Exposure Factors Handbook.

EPA is poised to expand its exposure research commitment. On July 27,

1994, EPA Administrator Carol Browner presented a plan to Congress that

would reorganize EPA's research program into four national laboratories. The

specific mission of one of these four laboratories would be exposure analysis.

5.1.5 Five Year Goals

In a 5-year time frame, initiatives should be promoted to facilitate:

1. an increased recognition of the importance of the exposure

paradigm as a fundamental approach for investigating

environmental and human health problems;

2. creation of a growing body of exposure and dose data and peer

reviewed literature for the scientific and regulatory communities;

3. development of an approach that links the health concerns to the

research priorities established for pollutants that affect people as a

result of exposures in both single and multiple media.

4. development of exposure models that can be used to prioritize

single and multiple route exposures, and provide information for

improvement of the exposure measurements for current and future

chemicals.

5. a strengthening of the partnership with the academic and federal

communities to conduct basic and applied research on exposure

assessment. These include model validation, instrumentation

development and study design issues;

6. the training and availability of increasing numbers of Ph.D.

graduates with expertise in exposure principles; and
53

7. the completion of a scientifically credible design for implementing

the National Human Exposure Assessment Survey (NHEXAS), to

establish long-term trends in exposure to airborne and waterborne

pollutants, pesticides, and household exposures.

8. increased linkages between human behavior, biomarkers and

external exposure markers.

5.2 The Longer-Term (10-30 year) Perspective

5.2.1 Methods for Identifying Early Warning Signs of Significant Exposures

Looking beyond the horizon for characterizing exposures, and the

resulting human health and ecological risks resulting from such exposures, is a

daunting challenge. Issues clearly needing further study during the next five

years, such as those identified in Section 5.1, i.e., environmental estrogens and

reproductive health effects on wildlife and humans, electromagnetic fields and

possible increases in cancer, and airborne fine particles and associated increases

in human mortality and morbidity, were not anticipated five years ago.

The only realistic approach for anticipatory exposure assessment for

emerging issues in the decades to come is to create and exploit a broad-based

data resource of concentrations of contaminants and pathogenic agents in

environmental media (air, drinking water, soil and sediments, foods, etc.);

emissions data (e.g., toxic release inventories by industry, stack and tailpipe

emissions for fossil fuel usage, etc.); time-activity patterns of populations of

interest (because of the susceptibilities or likelihood of high-end exposures);

concentrations of biomarkers in biological fluids and tissues (environmental

chemicals, their metabolites, protein and DNA markers, etc.); species

distributions and diversity (in ecosystems under environmental stresses); and

health surveillance data (mortality and morbidity rates by cause).

Effective exploitation of the environmental data resources will require

further development and validation of: fate and transport models; exposure

assessment models; toxicokinetic models; and population dynamics models for

ecosystems, as well as implementation and refinement of quality-assurance

protocols for all essential data elements, and advances in techniques for patternrecognition for analysis of significant data trends.

The EPA is the natural home for such an environmental data resource,

and needs to take the lead in establishing it. In order to create the basis for the

establishment, maintenance, and utilization of this resource, EPA should:
54

a. Create, within the ongoing Environmental Futures Office, a core

group responsible for the planning and organization of the

environmental data resource.

b. Establish a mechanism to facilitate coordination in environmental

data collection and the distribution within and among the various

EPA programs, other federal agencies, state agencies, foreign

governments, international agencies, university researchers, and

private companies and trade associations.

c. Establish a mechanism to support investigator-initiated research

for the development, validation, and application of: sensitive and

specific environmental microsensors and biomarkers; models for

environmental fate and transport, exposure assessment,

toxicokinetics, etc; techniques for pattern recognition, especially

with regard to trends analysis for exposures and responses of target

populations of concern.

d. Establish a mechanism for communicating, on a continuing basis,

to all interested stakeholders, in EPA and elsewhere; the nature

and extent of the environmental data resource; the results of

analyses, by EPA and others, of the data from the resource; the

results of the coordinated research program, in order to inform data

generators and users of new opportunities for the collection of data

that would extend technical capabilities to: new levels of sensitivity;

new compounds; new populations of concern; etc. The

communication channels should also describe new challenges for

data analysis raised by the outlook panel affiliated with the ongoing

Environmental Futures program in EPA.

5.2.2 Technical Capabilities

In this section we extend our assessment of the development of inhalation

exposure capabilities to the 10 to 30 year time-frame (see Section 5.2 for

discussion of the 5 to 10 year time-frame).

The development of miniaturized vapor sensors will continue to progress. 

The number of miniaturized GC columns and sensors will continue to expand, as

will their detection and resolution capabilities. In addition, new adsorbent

materials and coatings will allow better pre-separation of complex, reactive

mixtures on the same sensor platform. This detector miniaturization will mean

that smaller air volumes can be characterized which, in turn, means that air

moving devices will be able to run unattended for longer periods of time. 

Pushing this latter trend is the rapid development of energy storage devices
55

with increasing energy densities. There is a need to increase the research that

can provide microsensors for the other routes of exposure. 

Vapor detectors based upon laser spectroscopy are currently employed

over relatively long path lengths (e.g. 100-1000 m). Within the next few decades,

laser based gas sensor technologies will improve dramatically. Diode pumped

lasers requiring neither dyes nor gases have already been developed primarily

for military applications. The civilian conversion of this technology will

constitute a technology driver. Solid state lasers now exist that are continuously

tunable in the 0.2 to 12 m wavelength range with narrow bandwidths

(resolving the finest rotational-vibrational features for light molecules). This

high degree of spectral resolution allows for a differential absorption LIDAR

technology in the mid-infrared spectrum. Within the next few decades, this

laser technology will be miniaturized to the point where extremely powerful

personal air samplers become feasible.

In addition, microprocessing and data storage capabilities continue to

increase. In the future, we will be able to store short-term averages (e.g. oneminute) of hundreds of measured gaseous compounds over weeks or months. In

addition, concurrent advances in microprocessor technology will allow real time

compound analysis from on-line compound libraries. This could be extended to

include on-line source signature libraries as well. Voice recognition technology

will also allow the capability for personal interaction with these sensors. The

development of personal data communication devices will provide the ability to

not only link personal monitors to global positioning systems, but also to link a

population of these devices to a central computing facility. This will mean that

time-location patterns could accompany chemical sensor data, additional

information that would be needed to assess the sources of exposure.

Looking ahead, the future will bring powerful personal monitors that will

be able to sense hundreds of compounds in real-time. This information along

with personal characteristics (e.g. breathing patterns, location) will be

telemetered to central computing facilities for additional analyses. On-board 

computers will also process individual data and will communicate judgements

about risk avoidance to the individual. The near-term prospects are most

promising for the inhalation route of exposure. In the longer term, it is essential

to promote comparable developments for monitoring dermal and ingestion

exposures.

5.2.3 Thirty Year Goals

The attainment of the thirty-year goals outlined below will depend, at

least in part, on the leadership and resource commitment provided by EPA. If

EPA leadership can envision the technical and programmatic advances that
56

opportunities in the art and science of exposure assessment can provide to

environmental risk recognition and prevention, then the Agency can set a high

standard for foresight and accomplishment in the federal government as well as

serve the public interest.

In a thirty-year time frame and with the emphasis on the importance of

the exposure paradigm that is recommended above, a more sophisticated

program of exposure assessment should become well established within EPA and

research should advance the state of science to the point of:

1. developing a robust data base on the status and trends in national

exposure, to current and anticipated contaminants, and the

relationship of exposures (and environmental regulations) to

indicators of health effects; and

2. demonstrating technologies and approaches that can integrate

measures of exposure and dose within a single biomonitoring

device.

3. providing exposure and dose models which can anticipate potential

problems associated with the introduction of new chemical and

biological toxins.

4. establishing enhanced credibility with industry and the public that

provides a firmer basis for maintaining and enhancing progress on

cost-effective environmental risk reduction.
57

6. RECOMMENDATIONS

Recommendation One:

Establish a mechanism to develop, validate with field data, and iteratively

improve models that integrate: 1) measurements of total exposure and

their determinants; 2) exposure distributions across different populations;

and 3) the most current understanding available of exposure-dose

relationships, including full utilization of developments in the use of

biomarkers of exposure and dose, which are expected to provide more

significant capabilities in the next decades. The determinants of exposure

may need to include, as appropriate, factors as diverse as building

materials, food consumption, time-activity patterns, behavior and

lifestyle.

Recommendation Two:

Establish a mechanism to validate with data and iteratively improve the

available models for environmental fate and transport of substances that

may pose environmental or health hazards when released to the

environment.

Recommendation Three:

Develop a robust database that reflects the status and trends in national

exposure, to current and anticipated environmental contaminants. The

contents of this database must be designed to closely match the data

needs for modeling environmental fate and transport, for conducting

exposure-dose assessments, and for surveillance for signs of

environmental and health effects in populations and ecosystems and

evaluation of exposure control efficacy.

Recommendation Four:

Develop sustained mechanisms and incentives to ensure a greater degree

of interdisciplinary collaboration in exposure assessment, and, by

extension, in the resulting risk assessment and risk management

activities.

Recommendation Five:

Develop a mechanism to support the research, validation and application

of: 1) more sensitive and specific microsensors and other monitoring

technologies and approaches to measure exposures; 2) the determinants of
58

human exposures, including relevant demographic characteristics, timeactivity patterns, location, behavioral and lifestyle factors, and others; 3)

the determinants of susceptibility to adverse effects from environmental

exposures, including poverty, genetic background, proximity to sources of

exposure, etc.

Recommendation Six:

Take advantage of exploding capabilities in monitoring technology,

electronic handling of data, and electronic communications, to establish

early-warnings of developing environmental stresses, so that actions can

be taken that minimize environmental and health impacts. Such

mechanisms might, for example, act to better inform societal choices of

fuels and transportation, behavioral patterns and lifestyles. They would

rely on exposure databases, relevant models, and increased understanding

of health and environmental effects for existing as well as new hazards.
59

7. REFERENCES CITED

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Release Sites (Draft). ASTM, Race Street, Philadelphia, PA.

Cal Tox (1993). A Multimedia Total Exposure Model for Hazardous Waste

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CA.

Hampshire (1991). Risk Assistant Users Guide, Hampshire Research

Institute, Alexandria, VA.

Jessiman, B. (1992). A Quantitative Evaluation of Ten Approaches to

Setting Site Specific Clean-Up Objectives.

Journal of Soil Contamination 1:39-59.

Korfiatis, G.P. and Talimcioglu, N.M. 1991. Model for Evaluation of the

Impact of Contaminated Soil on Groundwater. Report to the New Jersey

Dept. of Environmental Protection & Energy, Stevens Institute of

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NRC (1984). Risk Assessment in the Federal Government: Managing the

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Washington, D.C.

NRC (1991). Human Exposure Assessment for Airborne Pollutants:

Advances and Opportunities. National Academy Press, Washington, D.C.

NYDEC (1993). Incineration 2000 Phase II Report, Division of Air

Resources, New York State Department of Environmental Conservation.

NJR (1992) New Jersey Register, Volume 24, Monday February 3, 1992.

cite 24 NJR 373. Site Remediation Program Cleanup Standards for

Contaminated Sites Proposed New Rules: N.J.A.C. 7:26D, p. 373-403.

Paustenbach, D. (1992). A Proposed Approach to Regulating

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Sanders, P.F. and Stern, A.H. In Press. Comparison of Two Models for

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Compounds as Controlled by the Soil-to-Building Interior Exposure

Pathway & Acceptable Total Doses (Short Communication). 

Environmental Toxicology and Chemistry 13:(8) 1994.

USEPA (1989). Risk Assessment Guidance for Superfund, Volume 1:

Human Health Evaluation Manual, (Part A) Interim Final

EPA/5401-89-002.

USEPA (1990). Methodology for Assessing Health Risks Associated with

Indirect Exposure to Combustor Emissions. Interim Final

EPA/600/6-90/300.

USEPA (1991). Risk Assessment Guidance for Superfund, Volume 1:

Human Health Evaluation Manual, (Part B, Development of Risk-based

Preliminary Remediation Goals) EPA/540/R-92/003-

USEPA (1992). Assessing Potential Indoor Air Impacts for Superfund

Sites, EPA-451/R-92-002.

USEPA (1992a). Technical Support Document for Land Application of

Sewage Sludge, Volumes I & II, EPA 822/R-93-001 a & b.

USEPA (1992b). Guidelines for Exposure Assessment. EPA/600/Z-92/001. 

Risk Assessment Forum. Office of Research and Development. 

Washington, D.C. May 29, 1992.

USEPA (1993). Draft Soil Screening Level Guidance, Office of Solid

Waste and Emergency Response. 
61

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