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
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March 31, 1995
EPA-SAB-DWO95-005
Honorable Carol M. Browner, Administrator
U.S. Environmental Protection Agency
401M 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-
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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:
l) 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,
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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 early-
warning 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
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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:
I) 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.]
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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.]
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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.
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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
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.
IV
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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
1 Dr. Edward Bender provided staff support to complete this
report.
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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
VI
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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
Vll
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1. EXECUTIVE SUMMAEY
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 real-
time 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.
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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
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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 : l) 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.
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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.
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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
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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
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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: i) 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: I)
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
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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 time-
activity 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
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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 followup 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
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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 (NRG) 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 (NRG, 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
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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
Assessmenthy 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
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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 l). 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.
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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 non-
intrusive, fast, andean 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.
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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
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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
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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.
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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: l)
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 exposure-
response 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.
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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
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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
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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
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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
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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.
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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
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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:
I. 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
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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
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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.
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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.
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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 non-
reactive, 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:
I. the population exposure to any contaminant can be described by
one of the well characterized statistical distributions (e.g., log-
normal).
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.
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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-IAQO94-0096)
also recommended that EPA: i) 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.
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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: l) 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
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geometric standard deviation values established using the best information available on the
concentrations and the human activity patterns that lead to exposure.
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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
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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 offish 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 often 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
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.
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Protection Agency. Numerous pathways are considered (drinking water,
showering, ingestion offish 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.
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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.
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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.
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4. DEVELOPMENTS AND PROSPECTS FOR IMPROVEMENTS IN
EXPOSURE ASSESSMENT
EPA has: l) 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 integrateduse 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
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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.
Exposure Analys
Approaches
Methods Relate
to Individuals
I
±
Environmental Models
and Source Inventories
Methods Re I a tec
to Populations
Personal
Monitoring
Biological
Monitoring
Questionaires
and
Diaries
Mitigation
Factors
Individual
Exposure Model;;
Environmenta
Concentrations
Demographic!;
and
Life Style
Data
LJJ
I
Population
Exposure Model
Exposure Assessme
nts
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,
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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
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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:
I. 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.
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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
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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
April 3, 1995 3 The proceedings were published in the November/December
1992 issue of Archives of Environmental Health Vol. 47 (6).
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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
The proceedings of this conference will be published in the April 1995 issue of the Journal of App
Occupational and Environmental Hygiene 10(4).
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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
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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 follow-
up actions. They may also cause false alarms and needless stress for individuals
warned about the presence of uncertain signals.
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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 P attern
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
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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.
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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 time-
constants 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
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(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 B 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 cost-
effective 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
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particle sensors that rely on miniature laser diodes. The requisite sensitivity
currently exists to use these devices to measure particle size distribution in real-
time. 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 often 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:
I. 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.
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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
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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 pattern-
recognition 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:
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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
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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. one-
minute) 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
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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:
I. 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.
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6. RECOMMENDATIONS
Recommendation One:
Establish a mechanism to develop, validate with field data, and iteratively
improve models that integrate- I) 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
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human exposures, including relevant demographic characteristics, time-
activity 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.
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7. REFERENCES CITED
ASTM (1994). ASTM Guide for Risk-Based Corrective Action at Petroleum
Release Sites (Draft). ASTM, Race Street, Philadelphia, PA.
Cal Tox (1993). A Multimedia Total Exposure Model for Hazardous Waste
Sites (Draft). California Environmental Protection Agency, Sacramento,
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
Technology, Hoboken, NJ.
McKone, T. (1991). Estimating Human Exposure Through Multiple
Pathways from Air, Water, and Soil.
Regulatory Toxicology and Pharmacology 13:36-60.
NRC (1984). Risk Assessment in the Federal Government: Managing the
Process. National Academy Press,
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
Contaminated Soil: Identify Safe Concentrations for Seven of the Most
Frequently Encountered Exposure Scenarios. Regulatory Toxicology and
Pharmacology 16:21-56.
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Sanders, P.F. and Stern, A.H. In Press. Comparison of Two Models for
Calculation of Soil Cleanup Criteria for Carcinogenic Volatile Organic
Compounds as Controlled by the Soil-to-Building Interior Exposure
Pathway & Acceptable Total Doses (Short Communication).
Environmental Toxicology and Chemistry l_3j(8) 1994.
USEPA (1989). Risk Assessment Guidance for Superfund, Volume I'-
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 I'-
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 (l992a). Technical Support Document for Land Application of
Sewage Sludge, Volumes I & II, EPA 822/R-93-001 a & b.
USEPA (I992b). 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.
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