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Executive Summary
INTRODUCTION
The Bush Administration and Congress are considering new approaches
intended to enhance the public health protection benefits of environmental
programs within available resources. One general approach being explored
would entail revising some federal strategies and priorities to place greater
emphasis upon reducing health risks instead of simply responding to statutory
requirements or reacting to public perceptions. Such an approach would need
to rely heavily upon the application of scientific knowledge in risk assessments.
The success of such an approach would therefore depend upon assessments
of human exposures to toxic substances, because the exposure component of
a risk assessment often entails greater uncertainty than the hazard component,
and because reducing human exposure is directly relevant to reducing health
risk for a given toxic substance.
Human exposure to air contaminants is one area in which a risk-reduction-
based approach could be beneficial. Traditionally, public concerns about air
pollution have focused on highly visible emission sources such as industrial
smokestacks and automobiles. Likewise, regulatory strategies mandated by
the Clean Air Act have emphasized controlling outdoor sources of air pollu-
tion. Since 1970, these strategies have substantially reduced outdoor concen-
trations for five of the six pollutants for which National Ambient Air Quality
Standards have been designated. It is important for these strategies to contin-
ue, because exposure to outdoor air pollutants continues to present significant
public health risks. For example, outdoor exposures to the sixth pollutant,
ozone, still threaten human health in many locations within the United States.
At the same time, air pollutants with major indoor sources are known to
cause adverse health effects and personal discomfort, as described in the 1981
National Research Council (NRC) report, Indoor Pollutants. In part because
the Clean Air Act has been interpreted as applying only to outdoor air, little
1
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2 ASSESSING HUMAN EXPOSURE
progress has been made during the past 20 years in reducing potentially harm-
ful human exposures to air pollutants in many indoor locations. But most
people In the United States spend far more time indoors than outdoors; thus
risk reduction strategies that address only outdoor air quality are only partially
effective. Such strategies need to be modified to better address the ~mpor-
tance of indoor exposures.
Benzene (a human carcinogen) provides an example In which the proper
application of exposure assessment methodology should be applied to identify
areas where more effective strategies are needed to achieve greater reduction
of risk. In response to its mandate in the Clean Air Act to control hazardous
air pollutants, the U.S. Environmental Protection Agency promulgated regula-
tions in August 1989 for industrial emissions of benzene to outdoor air. How-
ever, other large sources of exposure to benzene are not covered by that
rulemaking. Exposures from smoking, consumer products in the home, and
personal activities such as driving or painting have been estimated to account
for more than 80% of nationwide exposure to benzene. Therefore, actual
human exposures to benzene at the most significant concentrations and dura-
tions are likely to occur inside the home or while traveling within a motor
vehicle (i.e., from sources not currently subject to Clean Air Act regulations).
The issue of benzene exposure assessment is addressed in Chapter 7, under
Volatile Organic Compounds.
The following report describes a conceptual framework and methods for
assessing and analyzing the totality of exposures of an individual to air con-
taminants in the course of all activities over specified increments of time.
Accurate and realistic assessment of human exposures from all environmental
media can help to ensure that appropriate priorities are set to achieve optimal
reduction of human exposures to significant contaminants. E~osure-assess-
ment research should be supported by government programs according to
such priorities, commensurate with the importance of human exposures to
environmental contaminants, whether outdoors or indoors.
THE CHARGE TO THE COMMll1EE
The Committee on Advances in Assessing Human Exposure to Airborne
Pollutants was established in 1987 by the NRC's Board on Environmental
Studies and Toxicology to review important new developments in exposure-
assessment methods and instrumentation that have been produced in pollu-
tion-related research, occupational medicine, and other disciplines during the
past 10-15 years, especially as those developments apply to individual human
exposures to airborne to~cants. The committee included members with ex
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EXECUTIVE SUMMARY 3
pertise in chemistry, mathematical modeling, engineering, physics, air pollu-
tion, exposure assessment, medicine, biology, social science, statistics, and
environTnental policy. The committee's work was sponsored by the Agency for
Toxic Substances and Disease Registry of the U.S. Public Health Service,
whose mission ~ to prevent or mitigate adverse human health ejects and
diminution In quaky of life resulting from exposure to hazardous substances
in the environment.
The committee was charged to review new developments and developing
technologies in exposure assessment, identify technological gaps, and recom-
mend research and development priorities to fig those gaps The committee
was also charged to consider the value of various methods for est-imat~g
chemical exposures ~ risk assessment, risk management, pollution control,
and regulatory programs. As part of the information-gathering process, the
committee sponsored a 2-day symposium in October 1988, to obtain current
"formation on the uses of exposure analyses, measurement instrumentation,
analytical and survey techniques, biological markers, and study design
THE COMMIT SHE'S APPROACH TO ITS CHARGE
In response to its charge, the committee focused on human exposure to
airborne contaminants that can be inhaled or absorbed through the skin and
potentially can cause adverse health effects or discomfort for an individual.
The committee did not consider in detail exposure to airborne contaminants
that come in contact with humans only after the contaminants have transferred
to another environmental medium (e.g., deposition of airborne contaminants
into food or ingestion of contaminated soil or water). The committee ac-
knowledges that exposure to airborne contaminants is only part of total e~o-
sure, which includes all exposures a person has to a specific contaminant; this
includes all environmental media (air, water, food, and soil) and all routes of
entry (inhalation, ingestion, and dermal absorption). Assessment of total
exposure for specified contaminants requires the application of quantitative
techniques to all pertinent environmental media and all routes. A single-
medium exposure analysis, such as for air only, is appropriate only when
supported by convincing evidence that a single-medium exposure predominates
for the contaminants of concern.
The committee defines an exposure to a contaminant as an event consisting
of contact at a boundary between a human and the environment at a specific
environmental contaminant concentration for a specified interval of time; the
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4 ASSESSING HUMAN EXPOSURE
units to egress exposure are concentration multiplied by time. Exposure
assessment involves numerous techniques to identify the contaminant, contam-
inant sources, environmental media of exposure, transport through each medi-
um, chemical and physical transformations, routes of entry to the body, inten-
sity and frequency of contact, and spatial and temporal concentration patterns
of the contaminant. An array of techniques can be employed, ranging from
basic techniques for estimating the number of people exposed, to sophisticated
methodology employing contaminant monitoring, modeling, and biological
markers.
Art organizing construct was developed by the committee to facilitate its
evaluation of exposure-assessment techniques. The construct specified that
techniques be evaluated for their usefulness in reducing the uncertainty about
exposures and in reducing the invasiveness of the measurement techniques,
while improving the efficiency of ways to obtain data on the concentration and
duration of contaminant contact with the individual or population. Exposure
of the individual was considered key, because the committee determined that
knowledge of such exposures is essential to make inferences about the general
population.
The committee recognized that good exposure-assessment techniques do
not guarantee meaningful exposure data; techniques must be applied properly.
Therefore, exposure-assessment techniques also were considered for their
appropriateness in obtaining data within a scientifically sound conceptual
framework for exposure assessment. The conceptual framework for exposure
assessment defined by the committee is illustrated in Chapter 7 with a series
of critical analyses of the ways in which exposure assessments could be and
have been applied to specific public-health concerns. It was not within the
committee's charge to address in detail the proper application and further
development of techniques to assess exposure to specific contaminants.
RATIONALE FOR PERFORMING EXPOSURE ASSESSMENTS
Exposure assessment is an integral component of environmental epidemiol-
ogy, risk assessment, risk management, and disease diagnosis and treatment.
It is a multidisciplinary endeavor that frequently requires the combined exper
1Confusion often occurs with the use of the tenets "exposure" and "dose." Dose is
the amount of contaminant that is absorbed or deposited in the body of an exposed
individual over a specified time. Therefore, dose is different from and occurs as a
result of an exposure.
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EXECUTIVE SUMMARY 5
tise of engineers, environmental and industrial hygienists, toxicologists, epide-
miologists, chemists, physicians, mathematicians, and social scientists. E~o-
sure-assessment methodology employs direct and indirect techniques, including
measurement of environmental variables (e.ge, indoor-air exchange rates);
persona mowtor~g of contaminants ~ the breathing zone; and use of biolog'-
cal markers, questionnaires, and computational modeling. Exposure assess-
ment is an equal partner with "oncology in defining human health risk and
identifying e~osure-response relationships.
As relationships between particular agents and health effects have become
better understood r~sk-assessment methodology has evolved to estimate the
likelihood that exposure to a given pollutant will produce a given health effect.
As defined in the 1983 NRC report, Risk Assessment in the Federal Govem-
ment: Managing the Process, risk assessment has four components: hazard
identification, dose-response assessment, exposure assessment, and risk char-
acterization. Figure 1 illustrates the integral role that exposure assessment
plays in the risk-assessment and risk-management processes. Accurate expo-
sure data on contaminant concentrations at the point of contact between a
human and the environment are crucial to valid risk assessment.
Many advances in knowledge of exposure assessment have not been well
integrated into standard risk-assessment practice. A common approach to
regulation of contaminants has been to focus attention on a specific single
environmental medium (e.g., air) and to deal with the problems of contamina-
tion and human contact as a one-dimensional (single-medium) problem.
Although multimedia approaches to environmental problems have received
increasing attention in recent years (e.g., EPA's 1986 "Guidelines for Estimating
Exposures," which specify identification of the principal environmental path-
ways of exposure), risk-management practices generally remain dominated by
single-medium and single-contaminant approaches.
FRAMEWORK FOR ASSESSING EXPOSURES
TO AIR CONTAMINANTS
Exposure assessments for airborne constituents must be considered within
a framework that recognizes the potential contributions from other environ-
mental media. Furthermore, to achieve effective risk assessment, risk man-
agement, environmental epidemiology, and diagnosis and intervention in envi-
ronmental medicine, all media and routes of exposure should be assessed for
the relative magnitude of their contributions before an intensive assessment
of one medium is conducted. To maximize opportunities for risk manage-
ment, exposure assessments should include information on the specific sources
OCR for page 6
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OCR for page 7
EXECUTIVE SUMMARY 7
of contaminants, locations of human contact with the contaminant, and envi-
ronmental factors affecting the exposures to ensure that effective and appro-
priate mitigation measures can be formulated. Exposure data should be
collected over intervals consistent with the expected biological response time
to compounds; tub risque knowledge of con£am~nt to~ciW. Data on
individual exposures can be integrated by summing over time (time-integrated
exposure), over persons (population-integrated exposure), and over contami-
nants (contaminant-integrated exposure).
Studies to assess exposures to environmental contaminants, whether they
are intended to improve environmental epidemiology' disease diagnosis and
intervention, risk assessment, or risk management, need to consider the three
principal methods of exposure assessment: personal monitoring, biological
marker measurements, and indirect methods (e.g., microenvironmental con-
centration measurements coupled through models to time-activity data ob-
tained from questionnaires). These methods then can be incorporated into
the study design to the extent practical and necessary to meet the specific
objectives of the assessment. It is not always necessary to monitor contami-
nant concentration in the breathing zone of each individual in a potentially
exposed population. For example, if important contaminant sources and
exposure locations were known from previous studies, only fixed-site monitor-
ing data and knowledge of the frequency and duration of exposure of individu-
als at the contact location would be needed.
Exposure assessment has been practiced in several different disciplines, and
numerous definitions and methods of estimating exposure have been devel-
oped, including those in the 1988 EPA publication, Proposed Guidelines for
Exposure-Related Measurements.2 These guidelines define exposure by units
of mass. EPA's definition multiplies exposure (units of contaminant concen-
tration and time) by a contact rate (e.g., breathing in units of volume per
time). Tune and volume units cancel in the equation, leaving mass. This is
more appropriate as a definition of "dose" than of exposure, within the context
described by the NRC Committee on Biologic Markers.
Because standard definitions are critical to developing coherence in the
field of exposure assessment, the committee recommends that the scientific
and regulatory communities, including those responsible for reviewing articles
for scientific journals, use consistently the definitions of exposure and expo-
sure assessment recommended in this report to ensure standardization across
disciplines.
he EPA guidelines are being modified to incorporate new and improved ah
preaches to understanding exposure.
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8 ASSESSING HUMAN EXPOSURE
SAMPLING AND PHYSICAL CHEMICAL MEASUREMENTS
Quality Control and Quality Assurance
Using advanced measurement techniques in exposure studies does not in
itself ensure better data. Quality-assurance (QA) programs are critical com-
ponents of exposure studies; they must be established as part of initial study
designs, at which point it should be decided what precision and accuracy are
needed to test the study hypothesis. (Accuracy refers to the degree of agree-
ment of a measurement with an accepted reference or true value; precision
is a measure of the agreement among repeated individual measurements.)
QA activities, such as interlaboratory comparisons and measurement system
audits, are carried out to ensure that the collected data achieve predetermined
levels of precision and accuracy. The committee considers QA to include
quality control, which comprises operational activities carried out before and
during the measurement process to ensure that the accuracy and precision of
data are sufficient to meet the needs of a study. An effective QA program is
costly (approximately 15-25% of total study expenses) and should be consid-
ered when establishing a project's budget.
A major deficiency in the field of exposure assessment is the general lack
of validation studies for most new samplers and analytical instruments. In this
regard, attention should be given to the availability of reliable chemical stan-
dards (sample compounds of known composition and concentration) to use as
validation references for measurement techniques. This is especially true for
highly reactive compounds. These compounds often are of concern from a
health perspective, and stable, known quantities can be difficult to prepare.
Sampling Techniques and Strategy
Most advances in exposure-assessment science have occurred in the devel-
opment of samplers and measurement techniques for fixed-site and personal
air monitor studies; the latter have focused on activities and sources that
contribute to individual and population exposures. However, new personal air
sampling techniques have been underused~specially to provide data to sup-
port regulatory decision making.
The choice of a sampling strategy and a measurement method hinges on
a strides specific aims and hypotheses. Physical, chemical, and biological
characteristics of a pollutant dictate the method chosen to sample and meas-
ure its airborne concentration. Because a contaminant can have very different
health effects in the vapor phase versus the condensed phase, care must be
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EXECUTIVE SUMMERY 9
exercised that the selected sampling procedure does not present a false picture
of a contaminant's physical state.
Airborne contaminants can be sampled actively or passively. Active sam-
pling uses a pump to pull airborne contaminants through a collection device.
Passive sampling relies on molecular diffusion to deliver airborne =~tam~-
nants to the collection medium. Passive monitors are well suited to collect
long-term integrated gas and vapor samples obtained over days or weeks, and
they can be extremely useful for personal or microenvironmental studies.
However, long-term sampling Bath passive monitors often places great con-
s~ain~ Qu the sorbent, which must retain the analyte of interest without
promoting unintended reactions with other adsorbed analyses. Sorbent im-
provements are needed to allow long-term sampling for a wide variety of
analyses.
Personal air monitoring is the most direct approach for assessing human
exposure to airborne pollutants. However, portability usually is attained at the
expense of sensitivity (compared with fixed-site microenvironmental monitor-
ing instruments). Personal monitors (active or passive) need to be developed
for many potentially harmful airborne contaminants, including certain metals,
polycyclic aromatic hydrocarbons and other semivolatile organic compounds,
polar volatile organic compounds (e.g., vinyl chloride), and radon progeny. In
some cases, personal monitors already exist but need to be refined, reduced
in weight and size, or validated (e.g., airborne particles and certain pesticides).
Certain pollutants produce effects only at concentrations greater than a
threshold value. Therefore, personal samplers of such pollutants are needed
that will continuously monitor only exposures to concentrations greater than
a designated threshold for community and occupational exposures.
Quiet and unobtrusive microenvironmental samplers are needed if f~xed-site
sampling is to be used widely; such samplers should be developed for the full
spectrum of air contaminants.
New sorbents for polar volatile organic, highly volatile, and very reactive
compounds are needed. Collection with sorbent sampling systems often is
associated with a compromise in analytic sensitivity, this results from the large
volumes used in liquid desorption techniques. Procedures such as supercritical
fluid extraction that permit Resorption with a minimum of dilution need to be
developed.
Instrumental Techniques
Many advances have been made in instrument design, operation, and exper-
imental deployment during the past 15 years. Liquid chromatography (LC)
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10 ASSESSING HUMAN EXPOSURE
techniques are being used to analyze for compounds not amenable to gas
chromatography (GC). In particular, ion chromatography is being used in-
creasingly to analyze highly polar air contaminants. LC combined with mass
spectrometry also is being developed into a technique to complement GC
combined with mass spectrometry. The development of new types of detector
configurations (e.g., ion trap mass spectrometer) has made mass spectrometry
a valuable air-analysis device. Simple advances have been made for other GC
and LC detectors.
Field applications of exposure-assessment methods are restricted by mstru-
ment limitations. Portable, reliable, and rugged instruments such as gas chro-
matographs, gas chromatograph/mass spectrometers, ion trap mass spectrom-
eters, and electrochemical sensors are needed. Sampling methods, instru-
ments, and software to function with these instruments also are needed to
permit unattended sample collection and analyses in field settings for extended
periods. A sensitive, highly specific detector applicable to numerous com-
pounds is needed for LC. Continued improvements in LC combined with
mass spectrometry are beginning to fill this gap.
USE OF BIOLOGICAL MARKERS IN ASSESSING
HUMAN EXPOSURE TO AIRBORNE CONTAMINANTS
Biological markers in an exposed individual can provide information about
an original contaminant, a metabolite of a contaminant, or the product of an
interaction between a contaminant agent and some target molecule or cell.
As one progresses from markers of exposure to markers of health effect,
variability associated with the individual becomes an increasingly significant
problem.
Biological markers have been studied as a part of research on exposure to
air pollutants for a limited number of compounds. Critical issues are associat-
ed with marker specificity and sensitivity to an exposure contaminant. In
some cases, biological markers cannot be used without adjustments for expo-
sures to background concentrations of contaminants from the same or other
media and adjustment for seasonal or regional variation.
The use of biological markers in exposure assessment should normally be
done in conjunction with personal or microenvironmental exposure-measure-
ment techniques. Biological markers integrate all routes and sources of expo-
sure and can provide measures of the actual dose of a contaminant an individ-
ual has received when appropriate metabolic data are available and when the
relationships between times of exposure and sample collection are adequately
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EXECUTIVE SUMMARY 11
defined. However, the actual routes and sources of exposure cannot be de-
tected without information on environmental contaminant concentrations.
Analytical techniques with improved chemical specificity and sensitivity for
biologically significant markers are needed for exposure assessments; especial-
ly needed me flexible assays that ~ analyze several markers simultane,Qusly
or be readily adapted to analyze numerous markers sequentially. For such
techniques, validation studies are needed to link biological markers conclusive-
ly to causative agents.
Pharmacokinetics is the quantitative description of the rates of absorption,
dutribut;~, metabolism? and elimination of a contact taken into a bio-
log~cal system. Better pharmacokinet~c data are needed for an increasing
number of chemicals. These data are needed to further the development and
validation of sophisticated biological marker models and to further under-
standing of how to model multiple metabolic pathways as a function of expo-
sure level.
SURVEY RESEARCH METHODS
AND EXPOSURE ASSESSMENT
Although there have been major advances in personal monitoring equip-
ment and in conducting sociological studies of time use, exposure-related data
on human activity patterns are in short supply. The techniques normally
employed are questionnaires and recall diaries to obtain information on loca-
tion and duration of human activities. Elementary principles of statistical
survey design, sample selection, and question format often are ignored in
questionnaires and surveys used by exposure analysts. Collaboration of social
scientists having expertise in survey statistics with exposure analysts can help
to develop more appropriate questionnaires, to limit the effects of biased
questions, and to validate survey instruments before their use in the field.
Far more attention needs to be given to the design of survey research in
exposure studies. Exposure analysts need to employ the expertise of special-
ists in survey statistics and the measurement of human time-activity patterns
through the use of questionnaires. Improvements are needed in the reliability
(precision) and validity of survey results, especially with regard to estimates
of frequency or duration of exposure. The estimation of long-term exposures
presents challenges yet to be addressed by survey researchers.
In addition to resource-intensive surveys of large populations, far more use
should be made of well-designed, but less-expensive, sequential or one-time
studies done at the community level. Such surveys could be especially impor-
tant for personal-exposure monitoring studies. In addition, a series of small
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12 ASSESSING HUMAN EXPOSURE
benchmark surveys of"normal" situations or environments are needed to help
establish baselines for other studies (e.g., studies of time activity in "health/'
buildings for comparison with similar studies in ~sick" buildings).
MODELS USED IN ASSESSING
HUMAN EXPOSURE TO AIRBORNE CONTAMINANTS
Mathematical models use systems of equations to quantify and explain the
relationships between air-pollutant exposure and important variables, such as
emission rates from contaminant sources, as well as for estimating exposures
in situations where direct measurements are unavailable. These models are
derived from assumptions and approximations that permit complex physical-
chemical-behavioral problems to be represented by mathematical formula-
tions.
Models considered by the committee were classified into two broad catego-
ries: those that predict exposure (in units of concentration multiplied by time)
and those that predict concentration (in units of mass per volume). Exposure
models obviate extensive environmental- or personal-measurement programs
by providing estimates of population exposures that are based on small num-
bers of representative measurements. Although concentration models are not
truly exposure models, the output of concentration models can be used to
estimate exposures when combined with information on human time-activity
patterns.
To improve the development and validation of models, measurements are
needed of the concentrations of airborne pollutants in workplaces, homes, and
other microenvironments. Simultaneous measurements of critical independent
variables, such as source-emission rates and indoor-ventilation rates, are also
needed for models. Concentration gradients within defined microenviron-
ments also need to be accurately measured.
Validation studies are needed for many existing models. In particular,
immediate efforts are needed to validate the models used for decision making
by EPA, ATSDR, and others about public and occupational health and to
tailor the models to relate to the actual situations that can result in large
population exposures. Valid emission-rate models are needed to provide
better estimates for multicomponent mixtures. Validated outdoor-dispersion
models are needed to predict concentrations in complex terrain and to provide
accurate exposure estimates for down- and up-gradient terrain conditions.
The same data set used to develop a model cannot be used to refine and
validate it; new, independent data are required. In addition, all assumptions
used in developing a model should be documented explicitly. Care should be
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EXECUTIVE SUMMARY 13
taken by investigators in any field-monitoring program to integrate measure-
ments with modelers' information needs so that the requisite model input data
are obtained or the measurement results can be used to test, refine, or vali-
date appropriate models.
Little work has been done to model very short-term exposures (peak expo-
sures) and gradients for dispersion, deposition, and ventilation In indoor envi-
ronments. Measuring and modeling the temporal patterns of source strength
as a function of readily identifiable or measurable source characteristics is a
critical step In that process. In addition, more work is needed to model the
relationship of indoor-air quality to the composition of the outdoor air.
Concentration Models
New developments in outdoor-dispersion models have improved prediction
of the average and time-varying concentrations to which individuals are ex-
posed. Receptor models can be compared with dispersion models as a means
of checking the predictions of both models. They also can be used to identify
sources of exposure. In many cases, however, data describing the source
characteristics are not available on the time scale for which the model predic-
tions are needed. Such mismatches of the time scale of the measurements
with the time scale of the models impede model development, validation, and
application to new exposure scenarios.
Source-emission models are available to predict mass-emission rates for a
variety of dynamic and steady-state emission problems. The available emis-
sion models allow the estimation of downwind exposure for continuous and
catastrophic releases of pure compounds or binary mixtures. However, these
models have not been validated. Dense cloud dispersion models are available
to estimate downwind exposure for heavier-than-air vapor or aerosol releases;
they also have not been validated.
Exposure Models
Models for predicting exposures to populations have been developed but
have not been adequately validated. Limited validation studies of the EPA's
"Simulation of Human Air Pollution Exposure Model," for example, show that
the average exposure values are well predicted but also show substantial dis-
crepancies in the tails of the distribution. Further development and validation
of these models are warranted. Exposure models used for regulatory decision
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14 ASSESSING HUAlAN EXPOSURE
making should be developed and validated for microenv~ronments where
significant exposures occur.
FUTURE DIRECTIONS FOR EXPOSURE ASSESSMENT
Air-exposure reduction strategies for a given contaminant should first
consider exposures due to all media. If other media are found to contribute
significantly to the total exposure even after air exposures are reduced, agen-
cies responsible for or experienced with the other media should be apprised
of the issue and be actively involved in the development of ir~tegra;ted expo
sure-reduct~on strategies.
Unless the health effect of a contaminant is unique or the source of the
contaminant exposure is well characterized, it is difficult to relate an exposure
to a health effect for one of a group of contaminants present in specific micro-
environments. When a contaminant does not have a unique health effect, it
is necessary to identify those situations where populations can have substantial
exposures. Once the exposure is assessed, that information should be used
to perform studies to establish the magnitude of the health outcome from
exposure in those situations.
Attempts to assess human exposure to air contaminants have achieved
varying degrees of success. It is easy to find flaws with any exposure study
when we are only beginning to understand the ways in which human activity
affects exposures of individuals and populations. It is clear, however, that the
field of air contaminant exposure assessment has advanced significantly as a
result of indoor-a~r pollution studies. Further progress will be achieved as
exposure study designs more completely address and rank potential contribu-
tions from all environmental media to all relevant microenv~ronments.
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A ^~n #~
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Representative terms from entire chapter:
human exposure
