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7
Current and
Anticipalred Applications
INTRODUCTION
The previous chapters in this report dealt with the basic principles and
methodological elements of exposure assessment. To illustrate the state of the
science and its application to the mitigation of deleterious effects on health or
nuisance effects, this chapter analyzes some current and emerging problems
of exposure to environmental contaminants in the form of case studies: vola-
tile organic compounds, environmental tobacco smoke, polycyclic aromatic
hydrocarbons, lead, acidic particulate matter, substances in buildings that
cause occupancy complaints (sick-building syndrome), chemicals released from
manufacturing facilities, and radon. These do not represent all the important
issues but illustrate the state of the science in particular areas, such as biologi-
cal markers, multiroute exposure, and personal monitoring. Each section
addresses the completeness and results of the approaches in question, the
sophistication of the methods used, the requirement for improvement or
redirection, the misapplication (if any) of results, and the use of scientific
results in making regulatory decisions.
Discussions of several of the case studies in the context of exposure
through environmental media other than air, such as water, food, or soil,
relate to the general framework for exposure assessment discussed in Chapter
1. Accordingly, approaches to assess exposure through inhalation should be
considered within the framework of total exposure, which accounts for all
exposures a person has to a specific compound regardless of environmental
medium. Therefore, strategies to reduce air exposures to a given contaminant
should consider exposures due to other media. If other media are found to
contribute significantly to the total exposure even after air exposures are re-
duced, agencies responsible for or groups experienced with the other medium
should be apprised of the issue and play an active role in the development of
integrated exposure reduction strategies.
207
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208 ASSESSING HUMAN EXPOSURE
Unless the hazard of a contaminant is unique or the source of the contami-
nant exposure is well characterized, it is difficult to conduct an assessment on
one contaminant out of a group present in specific microenvironments. When
the contaminant does not have a unique health effect, it is necessary to identi-
fy those situations where populations have important 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 situa-
tions.
These case studies focus on the development of a new paradigm for e~o-
sure assessment in risk assessment, risk management, epidemiology, and the
application of clinical intervention. The conclusions focus on broad implica-
tions for the discipline of exposure assessment, notable advances, and remain-
ing needs. The committee hopes that these case studies will stimulate con-
tinued or accelerated development of basic principles of exposure assessment
and suggest ways to improve the investigations required for specific air con-
taminants and general problems.
1
VOLATILE ORGANIC COMPOUNDS
Introduction
Some volatile organic compounds (VOCs) such as benzene, formaldehyde,
and vinyl chloride-are classified as hazardous because of their role in human
carc~nogenicity. This discussion deals mainly with VOC exposure of the U.S.
population in general; occupational exposure is not specifically considered.
The discussion examines EPA's current approach to assessing exposure as part
of regulatory investigations of selected VOCs as air contaminants and the
advances made by EPA's Total Exposure Assessment Methodology (TEAM)
study in evaluating human exposure to VOCs. Benzene is used to examine an
eyposure-assessment dichotomy found between the TEAM study and EPA's
regulatory investigations.
Current Approaches to Exposure Assessment
Under the Clean Air Act
EPA is required, under Section 112 of the Clean Air Act, to establish
National Emission Standards for Hazardous Air Pollutants (NESHAP) that
provide an ample margin of safety to protect the public from harmful expo-
sure to VOC contaminants. NESHAPs are set by considering major source
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CURRENT AND ANTICIPATED APPLICATIONS 209
categories of emissions, determining exposures, calculating health risks associ-
ated with each contaminant, and focusing regulation on categories with the
greatest risk potential.
EPA's selection of source categories is often based on the assumption that
sources emitting the greatest amounts cause the greatest exposures. Outdoor
stationary sources (e.g., chemical plants and petroleum refineries) are usually
identified as the greatest contributors to exposure. The EPA approach to
exposure assessment relies heavily on modeling and uses little, if any, actual
monitoring data. The human-exposure model combines source emission rates
with atmospheric-dispersion equations to predict concentrations of VOC
contaminants at various receptor sites In the general population and test the
effectiveness of various emission-control strategies.
Modeling extremely long-term exposures, as is required for a NESHAP risk
assessment for exposure to carcinogens, presents several major difficulties.
The typical practice is to measure or model the concentration of a contami-
nant at one time and determine lifetime exposure by multiplying that concen-
tration by a fixed number of years, e.g., the average human lifetime. Model
input data are source locations and estimated emission characteristics, popula-
tion census data, and meteorological data. It is assumed that population
density remains unchanged for 70 years and that ambient concentrations are
constant for 24 hours/day throughout the assumed lifetime.
, ~
However, the nature of sources of exposure can change substantially over
a lifetime. Large facilities commonly have a design life of 30 years, so consid-
erable change can be anticipated in the sources over the 70-year human life-
t~me. In addition, individual time-activity patterns can vary substantially over
very long periods. In the United States, people change their place of resi-
dence often, and few live in the same place over a lifetime.
Recent studies of exposures to some VOCs cast considerable doubt on the
NESHAP modeling approach and showed clearly that most people's exposures
depend far more on their activities than on whether they live near an industri-
al source of benzene emissions. The TEAM study has shown that in many
circumstances focusing on industrial sources is ineffective in determining
human exposure to select VOCs (Wallace, 1987~.
Total Exposure-Assessment Methodology Study
Overview
An assessment of human exposure to airborne VOCs has been carried out
through the TEAM study. The program originally intended to develop tech
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210 ASSESSII`JG HUMAN EXPOSURE
niques to measure total human exposure to a broad range of toxic chemicals,
including selected volatile and semivolatile organic compounds and metals, but
analysis of those chemicals in air, water, and food presented serious method-
ological problems except for a group of VOCs (Wallace, 1987~.
An implicit hypothesis was that the observed personal exposures to selected
VOCs could be related to point sources (e.g. from industry) and that the
farther one moved from these sources the smaller the observed exposures
would be. Stated another way, this implicit hypothesis was that there is no
difference between VOC exposure estimates made from stationary monitoring
networks and from direct personal-exposure measurements as made in the
TEAM program. For the small group of VOCs measured, the hypothesis has
been rejected.
The TEAM study measured exposure to selected VOCs directly with per-
sonal monitors that were worn by subjects. The monitors were designed to
be small, and to permit unobtrusive but accurate and precise sampling.
Monitoring of VOCs is complex, because VOCs are typically found at trace
levels. Contamination and artifact problems can affect the reliability of the
data, and the applied analytical methods generally require laboratory-based
instruments (Moschandreas and Gordon, in press). An extensive quality-
control and quality-assurance program was carried out to ensure the proper
interpretation of data. Sufficient sample size and probability sampling were
used to support inferences regarding the target population and to permit the
extrapolation of results to the general population. (Probability sampling is an
experimental design that provides unbiased estimates of statistics, including
precision, by weighting probability of selection, stratification, and clustering.)
The TEAM study measured 2=hour personal exposures to 20-35 target
VOCs in air and drinking water, including halogenated alkalies, alkenes, and
aromatic compounds. Subjects were monitored in urban (heavy and light
industry) and rural environments. In addition to personal samples, concurrent
outdoor samples were collected from the backyards of a subset of the subjects.
A comparison of matched indoor and outdoor samples showed that the con-
centrations of most of the chemicals were higher indoors. That conclusion has
been confirmed by other studies that analyzed for a comparable set of VOCs
(Molhave and Molter, 1979; Jarke et al., 1981; Seifert and Abraham, 1982; De
Bortoli et al., 1984; Gammage et al., 1984; Lebret et al., 1984; Monteith et al.,
1984~. In particular it should be noted that Molhave and Moller (1979) found
higher concentrations of benzene indoors than outdoors.
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CURRENT AND ANTICIPATED APPLICATIONS 211
Measurement Methods
The sampling system used a single-tube containing Tenax sorbent through
which a known volume of air was drawn with a personal sampling pump. The
adsorbent and pump were combined in a vest that was worn by the test sub-
ject. Two consecutive 12-hour samples were collected (6 a.m. to 6 p.m. and
6 p.m. to 6 a.m.~. While the subject slept and bathed, the vest was placed
carefully in a convenient location. Because most subjects remained at home
overnight, the overnight samples were considered indoor samples. Outdoor
samples were taken simultaneously near the house. The indoor-outdoor
relationships were then established. The disadvantages of Tenax are that it
will not retain very volatile compounds (vinyl chloride and methylene chloride)
well and it cannot be used to trap reactive compounds (such as formalde-
hyde). Samples were thermally desorbed from the Tenax onto a gas chro-
matograph, where the analyses were separated, and then detected using mass
spectrometry, which is highly specific and sensitive. Recently, the TEAM
study employed canisters for the indoor measurements.
Biological Markers
At the outset of the TEAM study, blood samples were taken at the end of
the sapling period and analyzed for the selected VOCs. However, the inva-
sive nature of the sampling and poor detection limits associated with the
analysis of blood led to the discontinuation of the technique. Fortunately,
breath samples were also taken at the end of the sampling period. Breath
sampling involved the use of a special spirometer in which the person exhaled
approximately 20 L of air into a Tedlar bag, the contents of which were
passed through the same type of Tenax traps as used in the air sampling. The
same analytical techniques were used for the breath samples as for the air
samples. The breath studies showed significant correlations with the personal-
monitoring analyses for all 11 prevalent chemicals and showed no correlation
with outdoor-air analyses (Wallace, 1987~.
To understand the relation of the breath analyses to the air measurements,
it is necessary to know the rates of absorption, distribution, metabolism, and
elimination of the analyses in the body (physical pharmacokinetics). In funda-
mental studies, subjects remained in an exposure chamber for a specified
period breathing selected VOCs at specified concentrations. The subjects
then left the chamber and their respired breath was analyzed repeatedly after
specific periods to establish the half-life of the VOCs in the blood. Half-lives
ranging from a few hours (benzene) to 21 hours (tetrachloroethylene) were
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212 ASSESSING lIUMAN EXPOSURE
observed (Gordon et al., 1985~. Similar results have been seen by Jo et al. (in
pressb) for chloroform. The half-lives can be used to determine the most
appropriate sampling time for the use of breath measurement as an indicator
of exposure.
Questionnaires
Two questionnaires were used. The first was a household questionnaire,
which included age, sex, occupation, household characteristics and activity
characteristics of the participant and other members of the household. The
"formation was used to obtain a probability sample of subjects and to ensure
the inclusion of highly exposed subjects in the studies. The second question-
naire involved a 2lhour recall and was administered immediately after the
end of the 2=hour monitoring period. The participants were asked whether
they had been exposed to potential sources of target chemicals. Monitoring
data were then compared with data from the second questionnaire. Variables
related to smoking, occupation, home characteristics, personal activities, and
automobile travel were found to be the most important determinants of expo-
sure. Benzene concentrations were 30-50% higher in homes of smokers than
in homes of nonsmokers. Subjects were heavily exposed to benzene (over 1
mg/m3) when filling automobile gas tanks; benzene exposure could often be
related to automobile use, which also includes time spent inside of an automo-
bile compartment (Wallace, 1989~.
Models
No models were used specifically to assess exposure in the TEAM study.
However, the use of pharmacokinetic models was considered essential for the
proper use and interpretation of breath measurements as indicators of eypo-
sure. A simple two-compartment model accounted for the effect of the initial
breath concentration and the residence time of VOC measured in the TEAM
study within the body (Wallace et al., 1983~. The model successfully predicted
the time needed for clearance of tetrachloroethylene from the body when
compared with the chamber studies mentioned earlier (Gordon, 1985~.
Benzene
Results of the TEAM study indicate that personal benzene-exposure con
OCR for page 213
213
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214 ASSESSING HUMAN EXPOSURE
centrations exceed ambient outdoor concentrations (Wallace, 1989~. Figure
7.1 shows industrial sources represent about 14% of total emissions of ben-
zene, but their contribution to exposure is relatively small~nly about 3% of
the total. Thus programs and regulations to reduce emissions from major
stationary point sources could affect, at most, 3% of total exposure nation-
wide. Nevertheless, a recent rule-making has established national emission
standards for benzene from industrial source categories: maleic anhydride
plants, ethy~benzene-styrene plants, benzene storage, equipment leaks, and
coke by-product recovery plants. Other larger indoor and personal sources
of exposure are not covered by this rule-making (EPA, 1988d). Exposures
from active smoking, involuntary smoking, 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. The sources of exposure la-
beled Motor vehicles (outdoor air)" do not include personal use, such as driving
or riding in an automobile; such uses are included in Motor vehicles (travel).~
(Note that the TEAM subjects were drawn from areas with little use of wood
stoves or kerosene heaters, which are potentially important sources of expo-
sure to benzene (Wallace, 1989~. These important sources of exposure must
be re-evaluated and considered for regulation and education. In addition,
similar types of integrated analyses are necessary for other VOC contami-
nants, which may have both indoor and outdoor sources.
Recommendations
To incorporate all significant exposure findings into future rule-makings for
other hazardous VOCs, exposure analysts and risk managers need to Interact.
Regulatory investigations should not be limited to some readily identifiable
and measurable point sources that might have insignificant impacts on expo-
sure. The findings of TEAM are at odds with conventional approaches used
to control VOC exposure. Therefore a major rethinking of the approaches
used to identify public health risk is warranted. Exposure analysts must con-
tinue to refine techniques that can identify important sources of contaminant
exposures, whether those sources are indoors or outdoors.
The VOCs examined in the TEAM study were almost exclusively in a
single exposure medium (air), were chemically stable, and had a volatility that
permitted their effective collection and concentration with the sorbent Tenax
New analytical techniques should be developed to broaden the range of ana-
lytes that can be collected and measured, so that the "T" in TEAM will actually
stand for "Total," and not for "Targeted compounds," as is now the case. In
particular, greater attention should be given to analyzing for highly reactive
OCR for page 215
CURRENT AND ANTICIPATED ~4PPLIC:ATIONS 215
compounds. Passive dosimeters (Lewis et al., 1985) that match the time
resolution of active monitors should continue to be developed, because they
are less expensive and usually more convenient to wear.
Better microenvironment monitoring data and time-activ~ty data, including
quality assurance aDd quality control, are needed to improve the modeling of
VOC exposures.
ENVIRONMENTAL TOBACCO SMOKE
Introduction
The health herds associated with smoking have received extensive study
and are well letdown. Thus, it is not surprising that there is now a growing
concern that exposure to environmental tobacco smoke (ETS) might affect the
health and comfort of nonsmokers. The health and nuisance effects of so-
called involuntary smoking have been extensively reviewed in a National Re-
search Council report (NRC, 1986) and in a report of the Office of Smoking
and Health (1986~. Both reports concluded that exposure of nonsmokers to
ETS results in acute irritation of the eyes, nose, and throat; unacceptable
odor; upper-airway problems in children, including increased prevalence of
respiratory symptoms (cough, sputum production, and wheezing), decreased
lung function, increased lower-respiratory-tract illnesses, and increased rate
of chronic ear infections; and increased risk of lung cancer. The reports also
noted that other outcomes related to the growth and health of children had
positive associations In studies, including low birthweight and reduced growth
and development. However, the results of some of these studies continue to
be debated, and other related studies are ongoing. Thus, it is unportant to
examine ways to improve techniques to assess more accurately exposure to
ETS.
Until recently, epidemiological studies of the acute and chronic health
effects of ETS have been handicapped by limitations in assessing exposures
to ETS. Exposures occur at a u ide range of concentrations for highly variable
periods and in numerous indoor environments. Unlike active smoking, expo-
sure to ETS cannot now be easily assessed with standardized methods. Prev~-
ous epidemiological studies of the chronic effects of ETS, particularly lung
cancer, have determined exposure solely by questionnaires, which have not
been standardized or validated. The questionnaires have usually obtained
information on smoking habits of occupants of residences to permit assess-
ment of ETS exposures and have not adequately addressed the Impact of
occupational exposures. The use of such questionnaires might pose problems
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Representative terms from entire chapter:
exposure assessment
216 ASSESSING-HUMAN EXPOSURE
in misclassification of subjects by exposure status and obscure possible e~o-
sure-effect relationships.
In the past few years, new techniques have been developed that permit a
more accurate assessment of individual exposures to ETS (Leaderer, 1990~.
They are being applied to test hypotheses ~ epidemiological studies on the
relationships between ETS and acute and chronic health and nuisance effects.
The methods use advances in the applications of markers or profanes of ETS,
air monitoring, modeling, questionnaire survey, and biological markers.
Air
CURRENT AND ANTICIPATED APPLICATIONS 217
ETS-generated RSP in various indoor microenvironments (Repace and Low-
rey, 1980, 1982~. It is also being used to estimate ETS exposures retrospec-
tively and to assess risk (Repace and Lowrey, 1990~. As input, the model uses
known rates of RSP emission from tobacco combustion and data from several
sources, including measured and estimated smoking densities, infiltration and
ventilation rates, and deposition rates. The tapered element oscillating micro-
balance could be used to continuously monitor indoor concentrations of RSP
(Patashnick and Rupprecht, 1986~.
Biological Marlters
Physiological fluids can be analyzed for specific biological marker com-
pounds indicative of exposure to ETS. Thiocyanate, carboxyhemoglobin,
nicotine and cotinine, hydroxyproline, N-nitrosoproline, aromatic amines, and
protein or DNA abducts have all been considered as Indicators of dose of
tobacco smoke (NRC, 1986; Office of Smoking and Health, 1986~. Those
biological markers indicate that exposure has taken place, but might not be
directly related to the source or to the specific adverse effect under study.
Furthermore, a biological marker of exposure might not be specific for the
contaminant related to the effect, does not provide an exact measurement of
ETS exposure in a single environment, and does not provide information on
the environmental factors that affect the concentration in the environments in
which people spend time. Biological markers of ETS exposure can also vary
widely from person to person, because of differences in uptake, distribution,
and metabolism. Some markers are not specific for ETS exposure (e.g.,
carboxyhemogIobin); while others (e.g., thiocyanate) might be useful for active
smoke exposure, but not sensitive enough for ETS exposure. Cotinine and
nicotine measurements in the blood, urine, and saliva are specific for tobacco-
smoke exposure, and have been widely used as indicators of ETS exposure
(NRC, 1986~; they are valuable in determining the total or integrated short-
term (hours to days) dose of ETS across all locations in which a person
spends time.
Questionnaires
Questionnaires have been used extensively in epidemiological studies for
the classification of people into broad categories of ETS exposure on the basis
of reported exposure. Questionnaires are also used to obtain information on
the physical environments in which exposures take place, the factors affecting
246 ASSESSING HUMAN EXPOSURE
Models
The single compartment and multicompartment ma.cc-balance model dis-
cussed in Chapter 6 can be used to good purpose in designing and conducting
a study to elucidate the relationship between health effects and exposures to
airborne contaminants. Those models can provide a conceptual framework
for designing the sampling strategy to be used in various buildings, can help
to predict exposure concentrations from various sources or infernug source
strengths from concentration measurements, and can be useful In designing
controlled human exposure experiments in which concentrations of indoor
contaminants are varied.
Empirical models constitute a second class of model that can often be used
as hypothesis-generating and testing tools. Such models are typically multivar-
iate. For an SBS study, some health end point would be related to env~ron-
mental variables in a stepwise multiple regression.
Conclusions
The SBS problem has surfaced only in the last decade. Thus, the methods
for understanding it have not had time or resources to be adequately devel-
oped by exposure analysts. However, the database obtained from ~nvestiga-
tions of BRI has suggested better approaches for the design of SBS studies
and a need to develop measures to reduce exposure and the Incidence of SBS.
Issues of technique include the development of more refined hypotheses; the
use of a broader range of physical, chemical, and biological measurements;
more complete and standardized health and activity questionnaires; and the
use of more sophisticated models of total exposure.
TOXICS RELEASE INVENTORY
Introduction
Title III of the Superfund Amendments and Reauthorization Act of 1986
(SARA), Public Law 99-499, is a free-standing statute titled "The Emergency
Planning and Community Right-To-Know Act of 1986". In the development
of SARA Section 313, it was acknowledged during discussions in Congress
that the extent of human exposure to toxic chemicals released by industry was
a major concern Congressional Record, H11205, December 5, 1985~. It was
also expressed in Congress that much work is needed before human exposure
CURRENT AND ANTICIPATED APPLICATIONS 247
to toxic chemicals can be effectively managed and that acquisition of informa-
tion is the next necessary step. Section 313 of SARA requires industrial facili-
ties that manufacture, process, or use toxic chemicals to report annual envi-
ronmental release information to the EPA. Initial requirements for submis-
sion of the information are specified by EPA in the Toxic Chemical Release
Reporting Final Rule (Fed. Reg., 1988a). The database resulting from the
information reported to EPA is referred to as the Toxics Release Inventory
(TRI). The TRI was seen as a means to gather information for three general
objectives: (a) to identify the chemicals of the greatest concern; (b) to identify
locations where the chemicals are manufactured, used, and released; and (c)
to determine the quantities released into the environment (Congressional
Record, S11772, September 19, 1985~.
The initial list of toxic chemicals for TRI reporting contains 308 specific
chemical compounds and 20 chemical categories and can be modified only by
a rule-making, such as the deletion of titanium dioxide ˘Fed. Reg., 1988b).
Information reported to the TRI includes routine releases (e.g., emissions
from stacks) and accidental releases to air, land, and water. The first reports
were filed on June 30, 1988. This case study examines the issues that should
be addressed in the TRI to make it useful for assessing exposure to toxic
chemicals.
The purpose of the TRI is to inform the public and government officials
about total releases of toxic chemicals. Section 313 of SARA requires EPA
to develop the TRI information into a computerized database for public ac-
cess. The information is intended, among other purposes, to assist research
and aid in the development of various regulations, guidelines, and standards
(EPA, 1988c). There are no requirements to perform risk assessments or to
regulate any TRI-listed chemicals. To minimize the burden of data-gathering
on industry, Section 313 of SARA allows release reports to be based on esti-
mates; monitoring data and other available information are not required, but
can be reported if available.
Applications to Exposure Assessment
Although the TRI provides useful information on estimated mass quantities
of chemical releases, it does little to assist in understanding the potential for
human exposure to those releases and resulting impacts on public health. The
inclusion of accidental and routine emissions in the total releases reported to
the TRI makes estimation of downwind concentrations and exposures techni-
cally infeasible. The separate types of releases involve different exposure
issues and require different analyses for determination of exposure and expo
248 ASSESSING HUMAN EXPOSURE
sure impact. The TRI database is useful In identifying chemicals of concern,
which may, with further analysis, provide data needed for exposure assess-
ment.
The TRI requires reporting of chemical quantities released directly into the
environment or transferred to off-site locations, identity of releasing facility,
geographical location (latitude and longitudes, identity of all sites to which the
reporting facility transports chemical wastes, how the reported chemicals are
used, and types and efficiency of on-site methods to treat chemical wastes.
The data, by themselves, are inappropriate for assessing either acute or chron-
ic exposures, because they are not linked specifically to the potential concen-
trations and locations of exposure of the general population (Levin and
Spence, 1989~. TRI data are only one type of input data for air-dispersion
models (see Chapter 6) used to estimate potential downwind concentrations,
which are then linked with human time-activ~ty data to assess potential e~o-
sures (see Chapter 5~.
Even the simplest dispersion models cannot be used to estimate downwind
concentrations of released toxic chemicals on the basis only of TRI data. The
TRI provides some data useful in determining downwind concentrations, such
as facility location, latitude and longitude (to assist in describing meteorolog~-
cal transport), and categorization of releases as either point sources (e.g.,
stack emissions) or fugitive releases. Additional data are needed for air-dis-
persion analyses. Values of various model parameters on individual sources
are needed: release temperature and discharge velocity, orifice diameter and
height of release; frequency and duration of releases; and nearby structure
characteristics likely to affect small-scale air movements. Because the TRI
does not collect those additional data, industry is not likely to obtain and store
them, so they cannot be obtained simply by calling the TRI coordinator at
each facility and requesting them. Some facilities have taken the initiative of
estimating potential exposure concentrations of released chemicals reported
to the TRI. Such information more fully prepares facilities to respond to
inquiries from the public about impacts of their toxic chemical releases on
public health and the environment.
Acute toxicity is the primary concern for assessment of exposure to acci-
dental releases. To identify possible carcinogenic impacts, analysis of lifetime
exposure to routine emissions is required. Even if all the necessary model
data were provided for each release source, the results would be of little use
for exposure assessment, because of the combination of data in the TRI on
routine emissions and accidental releases.
For example, at low concentrations, hydrogen cyanide (HCN) gas, an acute-
ly toxic agent, can be detoxified by the body. However, at high concentrations,
HCN causes breathing loss and death. Reporting all emissions of HCN on an
CURRENT AND ANTICIPATED APPLICATI0115 249
annual basis could give a false impression of potential exposures that had
acute health outcomes. The estimation of exposure on an annual basis might
be acceptable for long-term effects, but even a single breath of HCN at more
than 2,300 ppm~v) would result in death.
Only time will tell whether the TRI database will be applied incorrectly to
Closure assessment activities. It is clear today, on the information submitted
to the TRI database, that industry has committed resources to reduce emis-
sions (Steyer, 1988) and EPA is expected to move more rapidly to develop
regulations for several specific hazardous air pollutants.
Implications
The TRI reporting requirement will, in all probability, provide tangible
environmental benefits. Data on releases to all media are important for
understanding the impact of a chemical release on total human exposure and
the global environment, but releases to air warrant special attention. Releases
to air probably result in the most immediate, and perhaps the most important,
exposures of the public living near an industrial facility that produces or uses
toxic chemicals. Exposure to airborne toxic chemicals can occur directly
through inhalation of contaminated air or through ingestion of food or water
that contains contaminants deposited from the air.
In the future, acutely and chronically toxic chemicals should be reported
separately to allow proper focus of resources on the most important exposure
issues. A source and receptor database needed for the proper exposure as-
sessment for both acutely and chronically toxic chemicals should be carefully
considered for inclusion in the data-collection effort In any revision of SARA
Section 313. Industry burden of providing the additional information is an
important aspect of the considerations.
Because the technology used for exposure assessment is changing rapidly,
it would be appropriate to define data needs in regulations, rather than in
specific laws. Regulations could then be changed as necessary to respond to
advances ~ exposure assessment without the need to amend laws.
RADON
Introduction
Exposure of the general public to radon and its decay products appears to
constitute an important naturally occurring environmental health risk. Radon
250 ASSESSING HUhlAN EXPOSURE
decay products have clearly produced lung cancers in exposed underground
miners (NRC, 1987a). However, there are considerable uncertainties in how
the risks identified in the miner studies can be extrapolated to the general
public. No clearly identified lung-cancer mortality in the general population
ran yet be specifically linked to exposure to radon decay products (NCRP'
1984a,b). Four relatively small case-controlled studies have suggested a
possible relationship between lung cancer and building construction or
residential radon exposure (Axelson et al., 1979; Edling et al., 1984; Lees et
al., 1987; Svensson et al., 1987), but there are no unequivocal measurements
of the Inug-cancer risk associated with indoor radon. Because the estimated
risks are higher than those associated with many other environmental agents
suspected of having adverse health effects, there has been considerable inter-
est In looking for clear evidence of radon-related lung cancer in the general
population.
The problem of protecting the public health has been exacerbated by the
uncertainties in the exposures and the corresponding risk estimates. Risk-
management decisions of EPA have suggested radon concentrations in indoor
air that should trigger mitigation action, and those action concentrations if too
low would result in unnecessary expenditures and concern. EPA reported in
a September 1988 press conference that radon causes 2O,000 lung-cancer
deaths a year in the United States. However, that estimate is at the high end
of the range estimated by the National Council on Radiation Protection and
Measurements (NCRP, 1984a,b), so the risk estimates are not in good agree-
ment. Major factors affecting the uncertainty in risk estimates are related to
the measurement of the proportion of exposure that is environmental.
There are major difficulties in assessing exposure to natural airborne radio-
activity, particularly to those radionuclides of greatest health-effect potential.
It is universally agreed that the short-lived decay products of radon (hippo'
2~4Pb, alibi, and 2~4po) cause the presumed health effects, but radon Is gener-
ally measured as a surrogate for these other radionuclides, because it is rea-
sonably easy and inexpensive to measure the indoor radon concentration. One
must be careful in extrapolating short-term screening measurements made
under nontypical conditions (e.g., in a basement in a closed house during
winter) to annual average exposures, although these nontypical-condition
measurements may represent a maximum exposure condition. Methods for
measuring long-term, average exposure to radon require further development,
and better communication is necessary to explain the risk uncertainties to the
public.
The amount of airborne radon decay products in a room depends on sever-
al factors, including the amount of radon to produce them, the concentration
of airborne particles to which they can become attached, and the aerodynamic
CURRENT AND ANTICIPATED APPLICATIONS 251
processes that contribute to the deposition of radioactivity on surfaces in the
room (walls, ceilings, furniture, etc). Thus, the actual concentration of a~r-
boruc radioactivity is a complicated function of several environmental vari-
ables.
. ~
The health effects of radon decay products also depend heavily on their
aerodynamic behavior in the indoor atmosphere. Particularly for Typo, parti-
tioning between the unattached state and the attached forms (i.e., combined
with pre-existing aerosol particles) has an important impact on the calculation
of the dose to the lung from a given airborne decay-product concentration.
In the dose models commonly used to relate tissue dose to airborne radioac-
tiv;ity concentrations (Jacob; and Eisfeld, 1980; James et al., 1980), a substan-
tially increasing effective dose to the target tissue is predicted with decreasing
particle size down to about 3 nm. The increase in dose is due to the increase
in effective deposition through molecular diffusion as particle size approaches
that of free molecules. Small changes In particle size in this range result in
large changes In the diffusion coefficient and in depositional behavior, particu-
larly in regard to the location of deposition in the tracheobronchial tree.
These models of delivered dose of alpha radiation to Jung tissue show radon
to be a reasonable surrogate for exposure to the `decay products because
several of the opposing factors in the exposure cancel each other.
Hypothesis and Study Design
The hypothesis of interest is that increased exposure to radon decay prod-
ucts In the indoor environment increases the risk of induction of lung cancer.
Exposure to tobacco smoke and differential residential mobility are substantial
confounding factors in the estimation of health risk.
Two epidemiological studies are attempting to relate lung cancer to envi-
ronmental radon and decay-product exposure through retrospective measure-
ment of indoor radon concentrations. One is being conducted by the state of
New Jersey and the other by Argonne National Laboratory. Both are con-
cerned with obtaining better risk estimates related to exposure of the general
population to radon decay products and are using cases of lung cancer in
white women as the subjects of case-controlled studies
The New Jersey study (Schoenberg et al., 1987) is an earlier extension of
a statewide population-based case-controlled interview study of New Jersey
women. The cases include all of the female residents of New Jersey whose
histologically confirmed primary cancers of the lung were newly diagnosed in
the period from August 1982 to September 1983. For cancer patients who
were interviewed, age- and race-matched controls were chosen from New
252 ASSESSING HUMAN EXPOSURE
Jersey drivers-license files and from Health Care Financing Administration
files for Medicare enrollees. For next-of-kin interviews, matched controls
were selected from state death-certificate files. For the 1,306 cases identified,
994 patients or next-of-kin were interviewed; of the 1,449 controls chosen, 995
were interviewed. Some 53% of the interviews were with the patients, and the
rest were with next-of-kin.
The study began without consideration of indoor radon, and residential
housing information had been collected only on the towns in which the sum
jects lived. The subjects or next-of-kin were therefore recontacted to obtain
street-address information. It was assumed that there is a minimal 10-year
latency period between exposure and onset of cancer. Only one house was
tested per subject because of resource limitations, so the study focused on
subjects who lived for at least 10 years at an address in New Jersey during the
period 1953-1972, about 10-30 years before the case diagnosis or control
selection. It was found that 17% of the subjects had not lived in New Jersey
for at least 10 years dunng 1953-1972 and that 10~o had not lived at any
address for at least 10 years during the critical period. In another 2% of the
cases, it was not possible to determine specific street addresses. It was possi-
ble to obtain addresses for 1,216 subjects that met the criteria. Of those
addresses, 82 no longer existed or were dwellings in upper floors of apart-
ments, trailers, or other situations in which radon exposure would be expected
to be negligible; that left 1,134 addresses. Short-term charcoal-canister meas-
urements were made for a quick screening. For a better determination of the
annual average concentrations, two alpha-track detectors were deployed in the
1,134 dwellings. In 10% of the dwellings, a third track-etch detector was
collocated with one of the other detectors for quality assurance.
The Argonne study provides a good example of a potentially useful study
design. The study population comprises white females born in Pennsylvania
who lived in eastern and central Pennsylvania, excluding Philadelphia and
Pittsburgh, and died of lung cancer between 1970 and 1987. Controls will be
chosen from white females born in Pennsylvania in the same years as cases,
selected by random-digit dialing and random selection from vital statistics. A
large number of lung cancer cases are available (more than 6,000 through
19843 in an area where there are likely to be high indoor radon concentra-
tions. Separate case series will be defined by histopathological type of lung
cancer and by smoking status. The first case series of about 500 cases in-
cludes all lung cancers and categories of smokers. For each of the dwellings
that the subjects have occupied and that can be identified, both short-term
charcoal and long-term track-etch measurements will be made for all levels
in the dwellings. When current occupants are willing, two sets of sequential
6-month track-etch samplers will be left and picked up by project personnel,
CURRENT AND ANTICIPATED APP~C4TIONrS 253
to ensure adequate response. Charcoal-canister measurements will be used
to screen the dwellings to make preliminary assignments as to radon exposure.
When current occupants are not willing to allow measurements, radon concen-
tration will be based on the age and construction lope of the dwelling and its
geological setting. Although data on a number of dwellings already permit
building of a predictive model for indoor radon, the results from the coopera-
tive dwellings with occupants win improve the database on which the models
are built.
The researchers also plan to measure radon-decay product concentrations
to assess exposures to radon progeny more directly. There is no apparent
plan to measure the radioactive particle size distributions. Thus, it will not be
possible to assess the potential for deposition, and the analyses will have to
include estimates of the effectiveness of the measured concentrations In pro-
ducing specified doses.
Measurement Methods
Short-term charcoal and long-term track-etch detectors will be used. In
both cases, it is assumed that current radon concentrations reflect past radon.
If there have not been changes In the insulation, heating system, or general
nature of a dwelling, the assumption should be reasonable. However, with the
extensive energy-conservation efforts many homeowners made in the late
1970s and early 1980s, many homes might have been modified. Estimation of
prior concentrations would then constitute a considerable problem.
Another important problem is the concentration of decay products relative
to the radon concentration. If, for example, one or more occupants smoked
and then quit, indoor particle concentrations might be much lower now than
in the past. Higher particle concentrations result In higher decay-product
concentrations, but lower the concentrations of more diffusive unattached
decay products and thus result in a lower average dose per unit of airborne
radioactivity. Similarly, if a gas stove were traded for an electric unit, particle
concentrations resulting from cooking would be lower; this could change the
effective exposure to decay products. Air cleaners can substantially increase
the unattached fraction, so EPA does not recommend the use of air cleaners
to mitigate the effects of radon decay products.
Models
The choice of radon as the measured entity suggests the possibility of an
254 ASSESSING HURON E~OSU~
implicit use of dose models that make the following prediction: the inverse
relationship between the concentration of airborne particles, the total decay
products, and the unattached fraction cancels out the effects of particle con-
centration on dose (James, 1988~. Therefore, exposure can be adequately
measured by determining the integrated, average radon concentration. Such
calculations have been presented by Vanrnarcke et al. (1985~.
The capability to predict indoor radon concentrations Is central to the
development of radon exposure models. Considerable effort is being devoted
to the development of models of radon entry into houses as a function of soil
characteristics (e.g., radium content and permeability), cInnatic conditions, and
house characteristics (e.g., substructure type, type of heating system, and air-
leakage area). Indoor radon concentrations are predicted by combining the
models of radon entry into basements (Loureiro, 1987; Mowris and Fisk,
1988), generally with steady-state, two- or three-dimensional numerical codes
that model the convective (pressure-driven) entry of soil gas (containing ra-
don) through openings In the substructure. These models are being upgraded
at Lawrence Berkeley Laboratory to account for diffusive entry of radon,
spatial variability of soil properties, simultaneous transport of soil moisture,
and transient effects. A smaller effort has been devoted to the entry of radon
into houses with craw! spaces (Mowris and Fisk, 19~) and to the development
of simplified closed-form or statistical models (Mowris and Fisk, 1988; Rev-
zan, 1989~. None of these models has been adequately validated, although a
current experimental effort by Lawrence Berkeley Laboratory should provide
critical data on radon entry into basements during the next few years.
Advances
Both of the large studies discussed here are making direct measurements
in at least one of the dwellings occupied by each of many lung-cancer subjects
over a long period and thus should yield a reasonable estimate of radon con-
centrations to which they have been exposed. In addition, information has
been obtained on smoking behavior and mobility to try to account for these
strongly confounding variables. The Argonne study will be partially supple-
mented by direct measurement of concentrations of radon decay products
although the particle size distributions and their potential influence on dose
are not explicit parts of either study.
New measurement methods have recently been developed that permit the
determination of both concentrations and size distributions of radon decay
products. The use of single screens in nonconventional diffusion batteries
(graded screen arrays) and measurement of the radioactivity that passes
CURRENT AND ANTICIPATED APPLIC4TIOli/S 255
through each screen permit one to obtain size distribution over the range of
0.5-500 rim (Reineking and Porstend~orfer, 1986; Holub and Knutson, 1987;
Ramamurthi and Hopke, 1988~. A new system can provide hourly measure-
ments of concentrations and size distributions of each decay product, so it is
now possible to measure directly the species that are responsible for hearth
effects without resorting to assumptions and models (Ramamurthi, 1989~.
This system can be used soon to test the variability of concentrations in ~ffer-
ent size ranges directly so that a better understanding of the dynamics of
inducer radon decay products will be possible.
