OERfc ^ES^'^
EPA #540-F-00-008
OSWER #9285.7-34
SHORT SHEET:
IEUBK MODEL MASS FRACTION OF
SOIL IN INDOOR DUST (MSD) VARIABLE
Office of Solid Waste and Emergency Response
U.S. Environmental Protection Agency
Washington, DC 20460
-------
NOTICE
Ibis document provides guidance to EPA staff. It also provides guidance to die public and to the
regulated community on how EPA intends to exercise its discretion in implementing the National
Contingency Plan. Hie guidance is designed to implement national policy on these issues. Hie document
does not, however, substitute for EPA's statutes or regulations, nor is it a regulation itself. Thus, it
cannot impose legally-binding requirements on EPA, Slates, or the regulated community, and may not
apply to a particular situation based upon the circumstances. EPA may change this guidance in the
future, as appropriate.
-------
U.S. ENVIRONMENTAL PROTECTION AGENCY
TECHNICAL REVIEW WORKGROUP FOR LEAD
The Technical Review Workgroup for Lead (TRW) is an interoffice workgroup convened by the U.S.
EPA Office of Solid Waste and Emergency Response/Office of Emergency and Remedial Response
(OSWER/OERR).
Region 8
Jim Luey
Denver, CO
CO-CHAIRPERSONS
NCEA/Washington
Paul White
MEMBERS
Region 1
Maiy Ballew
Boston, MA
Region 2
Mark Maddaloni
New York, NY
Region 4
Kevin Koporec
Atlanta, GA
Region 5
Patricia VanLeeuwen
Chicago, IL
Region 6
Ghassan Khoury
Dallas, TX
Region 7
Michael Beringer
Kansas City, KS
Region 10
Marc Stifelman
Seattle, WA
NCEA/Washington
Karen Hogan
NCEA/Cincinnati
Harlal Choudhury
NCEA/Research Triangle Park
Robert Elias
OERR Mentor
Larry Zaragoza
Office of Emergency and Remedial Response
Washington, DC
Executive Secretary
Richard Troast
Office of Emergency and Remedial Response
Washington, DC
Associate
Scott Everett
Department of Environmental Quality
Salt Lake City, UT
-------
IEUBK Model Mass Fraction of Soil hi Indoor Dist
(MJ Variable
STATEMENT OF THE ISSUE
The MSD is a variable in the dust lead (PbD) Multiple Source
Analysis module of die Integrated Exposure Uptake Biokinetic
(IEUBK) Model for Lead in Children (see IEUBK model data
entry window: Data Entry for Soil/Dost, Multiple Source
Analysis, Contribution of soil lead (PbS) to indoor house-
hold PbD [conversion factor]). The M,D represents the mass
fraction of house dust that is derived from outdoor soil. It is
used in Multiple Source Analysis to compute the contribution
of outdoor PbS to the indoor PbD concentration. The default
value for M^ recommended by EPA is 0.70 g soil/g dust. The
Multiple Source Analysis input screen for the IEUBK model
(version 0.99d; 3/8/94) and IEUBK Model Guidance Manual
(U.S. EPA, 1994; pp. 2-13) indicate that the M3D can be ex-
pressed as a dust lead/soil lead concentration ratio (PbD/PbS;
ugPb/g dust per ugPb/g soil). However, this guidance may be
subject to misinterpretation; therefore, this report clarifies and
supplements the IEUBK Model Guidance Manual.
MSD is equivalent to the PbD/PbS concentration ratio under
exposure scenarios where soil is the predominant source of
Pb in house dust and where there is no enrichment of the Pb
in soil materials transported indoors in comparison with
sampled soils (U.S. EPA, 1994; pp. 2-13). In scenarios where
non-soil sources are also important contributors to indoor
PbD (e.g., lead-based paint, non-soil airborne participates),
MSD represents the contribution of soil to dust; and total
PbD levels will exceed those resulting from the soil pathway
alone. In order to promote consistency in the use of site-
specific data to determine IEUBK model inputs, the M^
should be interpreted as the mass fraction of household dust
that is derived from soil; and the PbD /PbS ratio should be
viewed as a potentially useful estimator of MJD that is sub-
ject to various sources of bias.
BACKGROUND
Soil is a primary source of indoor dust in many residences.
Because of the potential for Pb in soil to be transported in-
doors and contribute to the concentration of Pb in dust, the
IEUBK model incorporates a soil-to-dust variable (M^). The
M8D is defined as the mass fraction of soil-derived particles in
indoor dust (g soil/g dust).
Values of the MgD are fractions bounded by the values 0
and 1. A relatively low value would reflect a scenario in which
soil contributes little to indoor dust mass, whereas a relatively
h|gh value would suggest that soil is the predominant source
of dust.
The MSDmay be used to approximate the concentration of Pb
in indoor dust based on the concentration of Pb in nearby soil
if the following assumptions are valid at the site:
(1) Soil lead is the major source of indoor dust lead.
(2) The soil data are representative of that portion of the
soil fraction and matrix which contributes to indoor
dust. There is no enrichment or reduction of Pb in the
soil fraction that is transported to indoor dust.
(3) The areas where soil samples are collected coincide
with the major source areas for soil derived indoor
dust If the above assumptions are appropriate for a
site and the exposure scenarios under consideration,
then the dust Pb concentration can be estimated from
Equation 1:
rOQ = Myj'rOs Equation (1)
where:
PbD - indoor dust lead concentration (ugPb/gdust)
PbS = outdoor soil lead concentration (ugPb/gsoil)
= mass fraction of soil in dust (g soil/g dust)
When there are also other significant sources of Pb in indoor
dust, the M,D is useful for estimating the contribution mat soil
makes to house dust levels. This estimate will often be impor-
tant to soil cleanup goals, where M^ can be used to estimate the
amount by which PbD concentrations can be reduced through
soil cleanup. However, where there are other significant sources
of Pb in dust, attempts to use measured Pb concentration data
for soil and dust to estimate MSD become more problematic.
The default value of 0.70 for Mso in the IEUBK model reflects
an analysis of empirical relationships between soil and dust Pb
concentrations measured in a variety of residential communi-
ties (U.S. EPA, 1994; pp. 2^t2). An intuitive understanding of
the movement of outdoor soil is that the contribution to the
total indoor dust mass from outdoor soil is also approximately
of the same magnitude or greater when mere are no other sig-
nificant sources of indoor dust; that is, the primary contributor
to the indoor dust mass is outdoor soil. However, further stud-
ies are needed to confirm the magnitude of the mass movement
Produced by the Technical Review Workgroup for Lead (TRW)
-------
-------
with a unit change in PbS concentration. If non-soil sources
are suspected of contributing significantly to indoor PbD,
multiple regression analysis of data on the contributing path-
ways can be considered. However, the development of statis-
tical models to predict the impact of paint Pb levels and paint
condition on Pb in house dust is likely to be problematic.
It should be kept in mind that even in scenarios in which soil
appears to be the major contributor to Pb in indoor dust, a
regression analysis of outdoor PbS and indoor PbD concen-
trations may result in a biased (low) estimate of the MSD due to
the effects of measurement error in fitting these models. Stan-
dard statistical theory for regression analysis assumes that
independent variables (in the present case PbS levels) are
known without error. When there is significant error in the
independent variables, regression slopes will underestimate
the true relationships (and intercept terms will be artificially
elevated). In this context, measurement error represents the
variability in empirical measurements of the soil levels or other
Independent" exposure variables, compared to the "actual,"
but unobserved, levels contacted by children. This type of
measurement error is a broader concern wan errors that may
occur in sample collection or in the processing and chemical
analysis of the samples. Examples of typical sources of mea-
surement error mat should be considered when exploring PbS
and PbD relationships include: (l)the soil particle size fraction
contributing to indoor dust may not be the same fraction that
is sampled and sieved to estimate PbS concentrations; (2) the
soil sampling locations in a residential yard may not be the
same locations where children (and pets) primarily contact soils
which are subsequently transported indoors; and (3) neigh-
boring properties and other community soil sources may con-
tribute to indoor dust. The above sources of measurement
error will introduce a negative bias into the estimate of the Mm
derived from a regression analysis; that is, they will contribute
to a reduction in bom the regression slope and correlation
estimates for indoor PbD and outdoor PbS concentration rela-
tionships. Use of an underestimate of the Mw in the IEUBK
Multiple Source Analysis would result in an underestimate of
indoor PbD concentrations (Equation 1).
Lead Speciation
Chemical speciation and physical examination of particulates
in house dust may support more direct approaches to deter-
mining the contribution of soil derived Pb in house dust. Sev-
eral methods involving different levels of complexity can be
considered. While these methods would require further re-
search and evaluation, they have much potential to strengthen
site specific estimates of MSD.
• Analyses using tracer elements and/or particle speciation
could be developed to directly estimate fractional amount
of soil derived dust mat is present in house dust without
reliance on Pb measurements.
• Particle identification and micro analytical techniques
could be applied to estimate the contribution of dust from
1 lead-based paint to the total Pb content of house dust. If
1 lead-based paint is the major non-soil contributor to house
1 dust, the residual Pb would be attributable to soils.
* Source apportionment of indoor PbD may be possible in
some residential scenarios by comparing elemental fin-
gerprints of lead-bearing particles in indoor dust to char-
acteristic fingerprints of source materials (Hunt et ai.
1993). If lead-bearing particles in dust are found to be
characteristic of soil sources, this approach could be used
to estimate the relative contributions of soil sources to
total indoor PbD, and, accordingly, support estimation of
M$D. The approach would require the development of a
library of characteristic fingerprints for possible sources
of indoor PbD at the site, in addition to soil sources, since
there will be some overlap in the elemental compositions
of various sources. Complications will arise where differ-
ent site soils diverge in their mineralogical composition.
Applications of the Mu to Baseline Lead Risk As-
The TRW recommends using measured indoor PbD concen-
trations in risk assessments for estimating Pb intake from ex-
posure to indoor dust If adequate PbD concentration data are
available, these data may be used as inputs to the IEUBK
model rather than using the Multiple Source Analysis menu
options (including the MSD). In situations where indoor dust
sampling is precluded or data gaps exist, calculations using
M,D may be used to estimate likely PbD concentrations under
appropriate circumstances. However, site risk assessments
should consider all potential sources of indoor PbD such as
exterior and interior lead-based paint, deposition of non-soil
airborne particulates, and outdoor soil.
Applications of the Mw to Soil Lead Cleanup Goals
Ujnlike Pb risk assessments, in which measured indoor PbD
concentrations are preferentially used over estimates based
oft the MSD, PbS cleanup goals require assumptions about the
soil-to-dust transport pathway, and other pathways that may
contribute PbD after remediation is completed. The default
value of 0.70 for the MSD is intended to be representative of the
mass fraction of soil in indoor dust for typical residences. As
described previously hi the Background section, there are
njany factors that may influence the transport and deposition
of soil-derived dust into the indoor environment, and there is
usually limited information on the effects that soil remediation
might have on these factors. For example, at most sites, it
Would be difficult to predict what impacts remediation would
have on the transport of soil into homes, unless the quantita-
tive impacts of remediation on the principal factors that influ-
ence soil transport could be assessed. If site specific esti-
mates of M80 are developed, careful consideration needs to be
gjiven to the methodological issues that were previously dis-
cjissed. In setting remediation goals, consideration also needs
to be given to the impact of non-soil sources of dust lead. Soil
lead exposures will generally present additional risks to chil-
Produced by the Technical Review Workgroup for Lead (TRW)
-------
dren who also have Pb exposures from other sources (e.g.,
dust from lead-based paint). The Superfund Lead Directive
(OSWER Directive #9355.4-12) provides important guidance
relevant to these issues.
RECOMMENDATIONS
The TRW recommends that measurements of Pb concentra-
tions be used as inputs to the IEUBK model when conducting
residential Pb risk assessments. In the absence of site-specific
data on PbD concentrations, the Multiple Source Analysis
may be used with a default estimate forMso of 0.70. The TRW
recognizes that the MSD value may vary depending on site-
specific factors that control the transport, deposition, and
removal of soil and other sources of indoor dust. If there is
compelling evidence to suggest that the mass fraction of soil
in indoor dust differs from 0.70, this information should be
presented in the risk assessment. Given the complexity of the
dust exposure pathway, the TRW further recommends that, for
the time being, EPA assessors and managers seek TRW review
of site MSD values when considering using values other than
the IEUBK model default of 0,70 (for use in either baseline risk
assessment or for estimating soil cleanup levels). At the present
time information is limited regarding both the practical applica-
tions of techniques to estimate MSD and the range of valid
values for this parameter. The review of additional site data will
also assist the TRW in providing further guidance regarding a
plausible range of values for MSD The TRW will appreciate
receiving feedback from model users regarding the technical
guidance provided in this document as well as their experiences
in addressing MSD at Pb sites.
REFERENCES
Hunt, A., D.L. Johnson, I. Thornton, and J.M. Watt. 1993.
Apportioning the sources of lead in house dusts in the Lon-
don borough of Richmond, England. Sci. Total Environ
138:183-206.
Roberts, J.W., D.E. Camann, and T.M. Spinier. 1991. Reducing
lead exposure from remodeling and soil track-in older homes.
In: Proceedings of the Annual Meeting of the Air and Waste
Management Assoc., Vancouver, BC. Pittsburgh, PA, Air and
Waste Management Assoc., Paper No. 15:134.2.
Roberts, J.W. and P. Dickey. 1995. Exposure of children to
pollutants in house dust and indoor air. Rev. of Environ.
Contam. & Tax. 143:59-78.
U.S. EPA. 1994. Guidance Manual forthe Integrated Exposure
Uptake Biokinetic model for lead in children. U.S. Environmen-
tal Protection Agency, EPA/540/R-93/081, PB93-963510.
Produced by the Technical Review Workgroup for Lead (TRW)
-------
U S EPA Headquarters Library
Mail code 3404T
1200 Pennsylvania Avenue NW
Washington, DC 2Q460
202-566-055B
------- |