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Toolkit for
Exposure Assessment: Beginners Start Here
INTRODUCTION
What is an Exposure Assessment?
How is Exposure
Different from Dose?
How is Exposure
Different from Risk?
Who Performs
Exposure Assessments?
Further
Reading
What
is an exposure assessment?
Exposure characterization versus exposure
assessment
Many of the ongoing industry programs - such as the HPV
Challenge or Extended HPV program - anticipate the development
of an exposure characterization for a chemical product.
A characterization is viewed as a qualitative description
of the exposure potential from the chemical product. A characterization
would provide a general explanation as to how the product
is used, in what functions, and under what applications;
it would not necessarily include measured or calculated
values.
Some examples of qualitative exposure
reports are provided in the "Examples
of Qualitative Exposure Assessment" section of
the website.
An exposure assessment determines the
level, duration and extent of contact with a particular
chemical. In performing an exposure assessment, the following
questions should be addressed:
What
is the source of the potential exposure?
Examples: industrial operations, pesticide use, consumer
products
What potential media, or
point of exposure, does the chemical end up in?
Examples:air (indoor, outdoor), water (drinking
water, bathing water, river water), food, soil, sediment,
plants
Is the chemical changed before
it gets to these media?
By what route of exposure
does the chemical get to a receptor?
Examples: ingestion, dermal (skin) contact, inhalation
Who is the ultimate receptor
that is exposed?
Examples: worker, consumer, community resident,
ecological receptor
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The outcome of an exposure assessment
is an exposure dose, i.e., an estimate of the concentration
of the emitted chemical that is being contacted. Exposure
units are expressed as
- Mg/liter (mg/l) for liquids
- Mg/gram (mg/g) for solids
- Mg/cubic meter (mg/m3) for air/vapor/gas
For more about risk assessments, see
the National Library of Medicine's Web site at http://www.sis.nlm.nih.gov/toxtutor1/a61.htm.
How
is exposure different from dose?
The literature is not consistent in its definition of exposure.
Although most people agree that exposure means contact with
a chemical or agent, universal agreement has not been reached
as to whether that contact takes place at a person's exterior
or whether the the exposure means the chemical has penetrated
the boundaries and been absorbed through the skin, lungs,
or gastrointestinal tract. The following definitions were
set forth by EPA's National Center for Environmental Assessment
in their Final Guidelines for Exposure Assessment (reference
http://www.epa.gov/ncea/exposure.htm):
"The process of a chemical entering
the body can be described in two steps: contact (exposure),
followed by actual entry (crossing the boundary). Absorption...leads
to the availability of an amount of the chemical to biologically
significant sites within the body (internal dose or absorbed
dose)."
The definition of terms becomes increasingly
murky in the arena of "exposure models". Some developers
have viewed "exposure models" as models that determine environmental
concentrations that a person or other receptor may come
in contact with (external contact). Other developers of
"exposure models" are actually referring to models that
calculate a dose that the receptor has ingested, inhaled,
or has absorbed through the skin. This Web site does its
best to distinguish between the two families of models by
categorizing them as:
- FATE AND TRANSPORT MODELS that determine
environmental exposures (external contact) to ecological
receptors
- EXPOSURE models that determine predicted
doses to humans.
How
is exposure different from risk?
Exposure is a component of risk. Exposure occurs through
contact with a chemical, either by drinking, inhaling or
touching it. The amount of a chemical to which a receptor
is exposed does not indicate the risk; the hazard associated
with the chemical must also be known. A chemical's hazard
describes its potential to cause adverse effects to human,
animal or plant life. The risk of a chemical is a function
of both the level of exposure and the level of hazard. Often,
this is expressed as a "Margin of Exposure" (MOE). The MOE
is typically calculated from a threshold concentration for
an adverse effect, divided by the estimated exposure. Thus,
a very small exposure compared to the adverse effect level
results in an MOE > 1 and may represent a low risk situation.
Conversely, an MOE < 1 indicates that adverse effects
could occur.
Who
Performs Exposure Assessments?
Human exposure assessments are developed by toxicologists,
industrial hygienists, and others to help assess potential
risks associated with exposures to chemicals in consumer
products and via other sources. The "science" of exposure
assessment has advanced a lot over the past several years,
and continues to advance. Key "players" have included U.S.
EPA scientists, academicians, experts in consulting companies,
and exposure assessors in companies (often via the former
Chemical Manufacturers Association, now the American Chemistry
Council). Key professional societies include the International
Society of Exposure Analysis (ISEA: www.iseaweb.org/)
and the Society for Risk Analysis (SRA: www.sra.org/).
Further
Reading
- WEBSITES
- PUBLICATIONS
WEBSITES
PUBLICATIONS
(Click on any title to expand the selection)
Byrd, D.M. and Cothern, C.R., "Introduction
to Risk Analysis" book, Government Institutes, a Division
of ABS Group Inc (2000).
This book (from the Preface)
"is the first comprehensive and integrated volume to provide
professionals with an introduction to all aspects of the
field of risk analysis. The three major areas of risk
analysis: risk assessment (exposure assessment and dose-response
curves), risk communication (it needs to be two-way) and
risk management (including values and ethics) are presented
in an integrated way along with many other topics."
Chapter 4 (pages 113-171) is
on "Exposure Assessment," with major sections on exposure
(an introduction and overview), planning an exposure assessment,
establishing the sampling strategy, sources of exposure,
fate and transport models, intake calculations, multimedia-multipathway
exposures and models, most important design elements in
multimedia-multipathway studies, National Human Exposure
Assessment Survey (NHEXAS), potential problems in multimedia-multipathway
studies, and sources of additional information.
Other chapters of key interest to
exposure assessors are Chapter 8 (pages 271-289) on "Risk
Characterization" and Chapter 13 (pages 375-413) on "Case
Studies" (two of the case studies are on "arsenic" via
various sources and routes, and "indoor air").
Hakkinen, P.J., Kelling, C.K., and Callender,
J.C., "Exposure Assessment of Consumer Products: Human Body
Weights and Total Body Surface Areas to Use, and Sources
of Data for Specific Products," Veterinary and Human Toxicology
33: 61-65 (1991).
Abstract: "A thorough understanding
of the routes and magnitudes of chemical exposures that
consumers experience during the use of a household product
is needed as part of a well-founded risk assessment for
that product and its components. This review describes
some sources of generic consumer data (e.g., relevant
body weight or total body surface area for a given human
age), and exposure-related data (e.g., task frequency
and duration) for specific product types needed for exposure
assessments. The review also contains a discussion of
the importance of statistical characterization of the
consumer data (e.g., does its range follow a normal, lognormal,
or other type of distribution?). The importance of examining
these data for correlative interactions is emphasized."
Paustenbach, D. J., "The Practice of
Exposure Assessment: A State-of-the-Art Review," Journal
of Toxicology and Environmental Health - Part B Critical
Reviews 3(3): 179-291 (2000). (Reprinted from the upcoming
Principles and Methods of Toxicology book, 4th edition,
2001).
Abstract: "Each of us encounters
hundreds of toxic agents everyday, without exposure and
subsequent absorption (uptake) however, there is no risk
of injury. Thus, exposure assessment is one of the three
legs of the stool (along with toxicity assessment and
dose-response assessment) upon which the practice of risk
assessment rests. The Field of exposure assessment has
evolved out of at least 3 other disciplines over the past
50 years, including health physics, industrial hygiene,
and epidemiology. Exposure assessments are a necessary
component to understanding the hazard posed by exposure
to naturally (e.g., aflatoxins in foods, radon in air)
and non-naturally occurring toxicants (e.g., benzene in
groundwater, MTBE in air, and food additives). This article
presents a thorough review of the field including a discussion
of the terminology used in exposure assessment, a description
of how to quantitatively estimate dose for the major sources
of exposure (food, water, air, and soil), and many of
the best sources of information. In addition, techniques
for assessing both variability and uncertainty are presented.
Lessons learned over the past twenty years are emphasized.
Some example calculations are included, nearly 400 references
are cited, and a glossary of terms is provided."
U. S. EPA Risk Characterization Handbook,
2000.
This handbook was prepared by the
Science Policy Council (SPC) for EPA staff and managers
and others as a guide to Risk Characterizatin. It provides
a single, centralized body of risk characterization implementation
guidance for Agency risk assessors and risk managers to
help make the risk characterization process transparent
and the risk characterization products clear, consistent
and reasonable (TCCR). This Handbook has two parts. The
first is the Risk Characterization guidance itself. The
second part comprises Appendices which contain the Risk
Characterization Policy that stimulated the effort, plus
several risk characterization case studies and references
U.S. EPA's Guidelines for Exposure Assessment
(1992; Federal Register Volume 57, Number 104, Pages 22888-22938).
"These guidelines establish
a broad framework for Agency exposure assessments by describing
the general concepts of exposure assessment including
definitions and associated units, and by providing guidance
on the planning and conducting of an exposure assessment.
Guidance is also provided on presenting the results of
the exposure assessment and characterizing uncertainty."
Includes a "General Concepts in Exposure Assessment" section,
a "Planning an Exposure Assessment" section offering guidance
on screening-level and other types of approaches, a section
offering guidance on "Reviewing Exposure Assessments,"
and a "Glossary of Terms."
http://www.epa.gov/nceawww1/exposure.htm
U.S. EPA's Exposure Factors Handbook
(1997; EPA/600/P-95/002Fa,b,c. Hardcopy, or 21 PDF files
via the Worldwide Web or as a CD-ROM; CD-ROM version has
a 1999 publication year and is EPA/600/C-99/001)
EPA's stated purpose of this
book is to: 1) summarize data on human behaviors and characteristics
which affect exposure to environmental contaminants, and
2) recommend values to use for these factors. "The handbook
has strived to include full discussions of the issues
which assessors should consider in deciding how to use
these data and recommendations... is intended to serve
as a support document to EPA's Guidelines for Exposure
Assessment). Of particular interest for consumer exposure
assessors are Chapters 15 (Activity Factors), 16 (Consumer
Products), and 17 (Residential Building Characteristics).
The Handbook also includes a "Glossary" starting on Page
G-1. This handbook also discusses the recommendations
provided by AIHC's "Exposure Factors Sourcebook" (see
below).
http://www.epa.gov/ncea/exposfac.htm
Whitford, F. et al. (includes co-authors
from academia, industry, consulting companies, and U.S.
EPA), "Pesticides and Human Health Risk Assessment. Policies,
Processes, and Procedures," Purdue Pesticide Programs, Purdue
University Cooperative Extension Service (1999). This is
Document "PPP-48," available from Purdue's Media Distribution
Center (Phone: 888-398-4636 or Internet Web site: www.btny.purdue.edu/PPP/
Among this publication's sections
are ones providing overviews of approaches used for Dietary
Exposure Assessment, Occupational Exposure Assessment,
Residential Exposure Assessment, and Aggregate Risk Assessment
Under the Food Quality Protection Act.
Other publications providing general
training and other useful information include:
AIHC's (American Industrial Health Council's)
Exposure Factors Sourcebook (1994): AIHC no longer exists
as an organization; however, copies of this document are
still available via various industry contacts. The document
summarizes and evaluates the current scientific documentation
and statistical data for various exposure factors used
in risk assessments. Exposure factors are the variables
used in the risk assessment calculations that apply to
human activities and human physiological parameters.
Technical Report 79 on Exposure Factors
Sourcebook for European Populations (with Focus on UK
Data) is now available from the ECETOC secretariat (http://www.ecetoc.org/pages/FramesetHome.cfm):
This document summarises available exposure factor data
for use in risk-based decision making. It updates and
builds upon other available compendia of exposure factor
data - the AIHC Exposure Factors Sourcebook (EFS) and
the US EPA Exposure Factors Handbook (EFH). Whereas
the EFS and EFH have focused on US data, this document
focuses on data specific to Europe, in particular the
UK.
The exposure factors selected for
inclusion were those most relevant to risk-based decision
making for contaminated land sites. The factors and
data presented, however, are applicable to exposure
assessment and risk-based decision making in general.
The cited references may also serve as useful sources
of additional information on exposure factors and exposure
assessment. Data gaps have been identified and whilst
this document includes data for European countries in
general, its primary focus is the UK. Future expansion
of additional data for other countries would be useful
for improving the accuracy of exposure assessments for
other European populations.
Gad, S. C., "Hazard and Risk Assessment"
chapter, Pages 577-638 in: Product Safety Evaluation Handbook,
second edition, edited by S. C. Gad. Marcel Dekker, Inc.
(1999).
"...The starting point for
understanding the exposure and subsequent risk associated
with a chemical entity and its use is to understand both
the innate characteristics of the entity (physical state,
vapor pressure, boiling point, flash point, etc.) and
its life cycle (how it is made, transported, used, and
disposed of)..." Sections include an Introduction, Physical,
Chemical, and Manufacturing/Use Characteristics, Exposure
Identification and Assessment, etc. Specific topics include
Chemical and Physical Properties, Site-Specific Characteristics,
Monitoring Versus Modeling, Air Concentration Modeling,
Dermal Exposure Modeling, etc.
Residential Exposure Assessment Project's
(REAP's) Residential Exposure Assessment: A Source Book
(publication expected by mid-2001)
An editorial board and several expert
working groups have been developing a reference textbook
on residential exposure assessment as part of the Residential
Exposure Assessment Project (REAP). The REAP effort is
being funded by a cooperative agreement between the U.S.
EPA and the Society for Risk Analysis (SRA), and includes
participation by and funding from industry, including
Procter and Gamble and the American Chemistry Council
(ACC). The REAP book will contain chapters that include
discussions of monitoring methods, exposure estimation
techniques (deterministic and stochastic), available surrogate
data, sources of data, and software for exposure-pathway
specific analyses, indoor and outdoor consumer products,
combustion sources, microbial agents, and other issues
related to residential exposure assessment. Of special
interest to ACA members is this book's "Appendix B, Exposure
Models/Software Bibliography" by Dr. Muhilan Pandian.
Twenty-seven models/software products useful for residential-type
exposure assessments are each discussed by Dr. Pandian,
with citations (see Key Software and Databases section
below note: this chapter was finalized in early 1999;
the information on software elsewhere in this ACA Web
site contains updated information for most of the software
products noted by Dr. Pandian).
Lioy, P., "Human Exposure Assessment:
a Graduate Level Course," JOURNAL OF EXPOSURE ANALYSIS AND
ENVIRONMENTAL EPIDEMIOLOGY 1: 271-281 (1991).
"... the problems provided
to the students for homework...give the student quantitative
perspective on the concepts, range in values, variables,
and uncertainties necessary to complete an assessment.
In addition, the development of the mathematical and conceptual
continuum for placing exposure assessment in the context
of toxicology, environmental science, epidemiology, and
clinical intervention provides a basic framework for the
discipline."
Olin, S. S. (ed.), "Exposure to Contaminants
in Drinking Water. Estimating Uptake Through the Skin and
by Inhalation" book, International Life Sciences Institute
(ILSI) Press (1999).
This book was developed via a cooperative
agreement with U.S. EPA's Office of Water, and provides
guidance, exposure factors, and other information needed
to assess dermal and inhalation exposures to contaminants
(volatilized and/or aerosolized) in household (tap) water.
Examples of potential residential exposures leading to
dermal and inhalation exposures include showering, bathing,
cooking, washing clothes or dishes, and swimming. Although
the information was developed for water contaminants,
the approaches and information also lend themselves to
assessment of other residential volatiles and aerosols.
The key sections of this book include ones on contaminant
characteristics, exposure characteristics, developing
exposure estimates, respiratory uptake, dermal uptake,
and a case study (the case study demonstrates the application
of the methods recommended in this book). Also noted in
the book are data gaps and research needs judged to be
important by the expert-working group that authored the
book.
Paustenbach, D. J. (ed.), "The Risk Assessment
of Environmental Hazards. A Textbook of Case Studies" book,
John Wiley & Sons (1989).
This book includes contributions
from 50 risk assessment experts. Included among the twenty-two
case studies are ones covering water contaminants, hazardous
waste sites, air contaminants, occupational hazards, and
consumer products. A new edition of this book may be available
within the next several years.
Whitmyre, G.K., Driver, J.H., and Hakkinen,
P.J. "Assessment of Residential Exposures to Chemicals"
chapter, Pages 125-141 in: Fundamentals of Risk Analysis
and Risk Management, edited by V. Molak. CRC Lewis Publishers
(1997).
Summary: "Individuals in and around
residences come in contact with a variety of chemicals
from various potential sources, including outdoor sources
that enter the residence, and from combustion sources
and consumer products. Among the factors that determine
the extent of exposure to a chemical are human exposure
factors (e.g., body weight, types, frequencies and duration
of various daily activities) and residential exposure
factors (e.g., design and properties of a residence, including
air exchanges per hour for the residence or the area of
interest within the residence). The goal of this chapter
is to provide readers with an overview of the assessment
of residential exposures to chemicals. The chapter is
organized as follows: Key Words, Introduction, Overview
of General Issues, Lessons from the TEAM Studies, Assessment
of Inhalation Exposures in the Residence, Assessment of
Dermal Exposures in the Residence, Assessment of Ingestion
Exposures in the Residence, Assessment of Exposures to
Chemicals in Indoor Sources: Principles and Case Studies,
Assessment of Exposures to Chemicals in Outdoor-Use Products:
Principles and Case Studies, Data Sources for Residential
Exposure Assessment, Discussion and Conclusions, References,
Questions for Students to Answer."
Hakkinen, P. J. and Leep, C. J., "Industry's
Use of Risk, Values, Perceptions, and Ethics in Decision
Making" chapter, Pages 73-81 in: Handbook for Environmental
Risk Decision Making: Values, Perceptions, and Ethics, edited
by C. R. Cothern. CRC Lewis Publishers (1996).
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