Adult Lead Methodology's Default Baseline Blood Lead Concentration and Geometric Standard Deviation Parameters

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C, 2.0460

MAY 1 7 2017

OFFICE OF
LAND AND EMERGENCY
MANAGEMENT

formerly

OFFICE ur
SOL f;lp WASTE AND
5L:MERGENOV RBSHUNSf'

MEMORANDUM:

OLEM Directive 9285.6-56

SUBJECT:

FROM:

TO:

Transmittal of Update to the Adult Lead Methodology's Default Baseline Blood
Lead Concentration and Geometric Standard Deviation Parameters

Schatzi Fitz-James, Acting Director A!

Assessment and Remediation Division / \ (/

Office of Superfund Remediation and Technology Innovation

Superfund National Policy Mangers, Regions 1-10

The purpose of this memorandum is to transmit the document. Update of the Adult Lead
Methodology's Default Baseline Blood Lead Concentration and Geometric Standard Deviation
Parameters and I he Integrated Exposure Uptake Biokinetic Model's Default Maternal Blood
Lead Concentration at Birth Variable. The recommendations in this document provide the
technical basis for updating the default baseline blood lead concentration and default geometric
standard deviation input parameters of the Adult Lead Methodology and maternal blood lead
concentration in the Integrated Exposure Uptake Biokinetic Model. This document is primarily
intended for Regional risk assessors and others involved in assessing risk to lead at residential
and non-residential sites.

I he Adult Lead Methodology (ALM) is used to assess lead risks from the soil for non-residential
Superfund site scenarios. The Integrated Exposure Uptake Biokinetic Model (IEUBK) is used to
assess lead risks from soii at residential Superfund site scenarios. The baseline blood lead
concentration input parameter ol the ALM represents the geometric mean blood lead
concentration in women ot child-bearing age and the geometric standard deviation (GSD) input
parameter is a measure of the inter-individual variability in these concentrations. The Mother's
Blood Lead C oncentration at Childbirth (MatPb) allows the user to consider the impact of lead
transferred from the mother to the fetus in utero.

Default values for these input parameters were originally derived from an analysis of blood lead
data lor U.S. women 17-45 years of age, from Phase 1 (1988 to 1991) of the Third National
Health and Nutrition Examination Survey (NHANES III) as well as consideration of available
site-specific data on blood lead concentrations and GSD values. EPA prepared updated estimates
for these two parameters in 2002 (using data from NHANES 1988 to 1994) in 2009 (using data
from NHANES 1999 to 2004) and again in 2016 (using data from NHANES 2007 - 2012). The

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Recycled/Recyclable • Hamed with Vftflgtable Oil Based inks on 100% Poslconsumer. Ptogbss Gfttonne i rr?e Hficydw! Pnp|o
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proposed updated estimates for the ALM and IEUBK are based on the most recent six years of
PbB data (using data from NMANES 2009-2014).

1 his document and other ellorts related to addressing lead in soil can be found on the Internet
at https://www.epa.gov/superlund/lead-superfund-sites-guidance. ff you have any questions,
please contact me or have your staff contact Michele Burgess (Burgess.Michelef5Sepa.gov).

Attachment

1. "Update ol the Adult Lead Methodology's Default Baseline Blood Lead Concentration
and Geometric Standard Deviation Parameters and the Integrated Exposure Uptake
Biokinetic Model s Default Maternal Blood Lead Concentration at Birth Variable"

cc:

Barry N. Breen. OLEM
Nigel Simon, OLEM
Patrick Davis, OLEM
Reggie Cheatham. 01.EM/OEM
Barnes Johnson, OLEM/ORCR
David Lloyd, OLEM/OBLR
Charlotte Bertrand, OLEM/FFRRO
Carolyn Hoskinson, OLEM/OUST
Cyndy Mac key, OECA/OSRR
Sally Dalzell, OECA/FFEO
John Miehaud. OGC
OSRT1 Managers

Jill Lowe, Superfund Lead Region Coordinator. Region 3
NARPM Co-Chairs
TRW Committee Members

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FAQ:

Concerning appropriate use of the new NIIANES values (May 2017) for the Adult Lead
Methodology (ALM) and Integrated Exposure Uptake Biokinetie Model, this FAQ provides
some important caveats for using the updated values.

OLEM recognizes adverse health effects at blood lead concentrations below 10 ug/dL1.
Accordingly, OLEM is updating the soil lead strategy to incorporate this new information.
However, the release date for the updated strategy is pending.

In the interim, the TRW Lead Committee is recommending the following considerations for all
non-residential risk assessments where lead is a contaminant of concern:

1. The updated NMANES values are appropriate for lead risk assessments for residential
and non-residential exposures both in assessing risk and in developing preliminary
remediation goals (PRCs) for your site.

2. Lead risk assessments should include a discussion of the most current toxicity
information and Centers for Disease Control and Prevention Reference level1.

3. Consistent with risk management bests practices, caution should be applied when
implementing cleanup levels based on the updated NI IANES values for non-residential
scenarios (PRGs are greater than 2000 ppm using default values). Ineffective controls or
incorrect land use assumptions could have potentially greater health consequences on
children who are exposed (e.g., by visiting, trespassing, or tracking the material to the
residence) to these high concentrations (especially given the new toxicity information).

Users are encouraged to contact the technical support hotline, TRW Lead Committee, or regional
risk assessor with any questions.

1 Sec 2006 Air Quality Criteria Document lor Lead (AQCD), 2012 Federal Advisory Committee on Childhood Lead
Poisoning Prevention to the Centers for Disease Control and Prevention (ACCLPP), 2012 National Toxicology
Program (NTP) Monograph: Health Effects of Low Level Lead, and 2013 Children's Health Protection Advisory
Committee (CHPAC) Letter to Acting Administrator Perciasepe concerning Childhood Lead Poisoning Prevention

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OLEM Directive 9285.6-56

Update of the Adult Lead Methodology's Default Baseline Blood Lead
Concentration And Geometric Standard Deviation Parameters
and the Integrated Exposure Uptake Biokinetic Model's Default
Maternal Blood Lead Concentration at Birth Variable

Overview

Since 1994, the Office of Land and Emergency Management (OLEM) (formerly known as the
Office of Solid Waste and Emergency Response) has recommended the Integrated Exposure
Uptake Biokinetic Model for Lead in Children (IEUBK model) as a risk assessment tool to
support environmental cleanup decisions at residential sites. The IEUBK model uses data from
a variety of scientific studies of lead biokinetics, contact rates of children with contaminated
media, and data on the presence and behavior of environmental lead to predict a plausible
distribution or geometric mean (GM) of blood lead (PbB) for a hypothetical child or population
of children. From this distribution, the IEUBK model estimates the risk (i.e., probability) that
the PbB concentration of an individual child or a population of children will exceed a specified
blood lead level.

Studies have demonstrated that there is no significant placental/fetal barrier for lead, since
fetal blood lead values are either equal to or slightly less than maternal blood lead values
(Goyer, 1990). The Mothers Blood Lead Concentration at Childbirth (MatPb) variable in the
I .EL BI< model allows the user to consider the impact of Pb transferred from the mother to the
fetus in utero. The Pb that is stored in the tissues of the newborn child in the IEUBK model is
calculated by entering the maternal PbB value at the time of birth.

In 1996, the Technical Review Workgroup for Lead (TRW) recommended the use of the Adult
Lead Methodology (ALM) (U.S. EPA, 1996) for assessing risks to adults from exposures to lead
in soil at non-residential Superfund sites.

The background blood lead concentration (PbBo) parameter in the ALM represents the
geometric mean (GM) blood lead concentration (PbB) (pg/dL) in US women of child-bearing
age1. The geometric standard deviation parameter (GSDi) is a measure of the inter-individual
variability in blood lead concentrations in a population whose members are exposed to the
same non-residential environmental lead levels. Default values for both PbBo and GSDi were
originally derived from an analysis of blood lead data for U.S. women 17-45 years of age, from
Phase 1 (1988 to 1991) of the Third National Health and Nutrition Examination Survey

1 The estimates do not include institutionalized women (e.g.. residents of nursing homes;
https://wwwn.cdc.gov/Nchs/Data/Series/sr02 162.pdI)

Page 1 of 9

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OLEM Directive 9285.6-56

(NHANES), as well as consideration of available site-specific data on PbBs and GSDs (U.S.
EPA, 1996). The TRW prepared updated estimates for these two parameters in 2002, 2009,
and 2016 using data from Phase 1 and 2 {1988 to 1994; 1999 to 2004; 2007 to 2012) of
NHANES (U.S. EPA, 2002, 2009, 2016 respectively).

The purpose of this report is to provide updated estimates for the PbBo and GSDi variables in
the ALM, as well as to identify an updated estimate for the MatPb variable in the IEUBK model
using more recent NIIANES survey data. The Centers for Disease Control (CDC) releases data
from the continuous NHANES in 2-year cycles; however, it is recommended to use four or
more years of data when estimating parameters for demographic sub-domains (Johnson et al.,
2013). The current estimates for the ALM and IEUBK model are based on the most recent six
years of PbB data (2009-2014) from the National Health and Nutrition Examination Survey
(NHANES) (CDC, 2012a,b, 2014a,b) and are presented in Table 1.

1 able 1. Updated estimates of the PbBo and GSDi for 17-45 year old women based on
NHANES (2009-2014)-

Parameter3

Estimate

90% Confic

ence Interval

Lower Confidence
Limit

Upper Confidence Limit

GM
(PbBo)

0.64

0.62

0.66

GSD
(GSDi)

1.80

1.76

1.85

!,Forty-seven (1.3% of the sample) of the blood lead measurements were below the detection
limit of 0.25 |Jg/dL and were assigned values of Vz the detection limit (0.125 ^g/dL). The
2013-2104 data used an updated detection limit of 0.07 jag/dL and all were detections.

This document provides the technical basis for updating the PbBo and GSDi parameters in the
ALM and the MatPb variable in the IEUBK model. This document details how the updated
estimates for the parameters were calculated. The intended audience for this document is risk
assessors who are familiar with using the ALM and IEUBK model. For background and further
detail on the use of the ALM or the IEUBK model in Superfund lead risk assessment, please
refer to U.S. EPA (2003,1994, respectively) or the TRW website
(https://www.epa.gov/superfund/Iead-superfund-sites-software-and-users-manualsL

Technical Analysis

Information on PbB for non-institutionalized U.S. women 17—45 years of age was extracted
from the NHANES database (CDC, 2012a, 2012b, 2014a, 2014b, 2017a, 2017b). Data from
three 2-year cycles of the continuous NHANES (2009~20i4) were used in this analysis in
accordance with CDC recommendations (Johnson et al., 2013).

Page 2 of 9

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OLEM Directive 9285.6-56

Estimates for MatPb, PbBo and GSDi were calculated using SAS® software. Version 9.4 of the
SAS System for Microsoft Windows2. Parameter estimates used the sample weights provided in
the NHANES demographic data files (CDC, 2012b, 2014b, 2017b). Standard errors for the GM
(MatPb and PbBo) and GSD were estimated using the sample weights and the masked variance
units (i.e., pseudo-strata and pseudo-primary sampling units which are also provided in the
NHANES demographic files). The sample weights account for the unequal probabilities of
selection of survey participants, the non-response of some participants, and are adjusted to
population controls. The masked-variance units account for the multistage sampling design
and are necessary to estimate accurate standard errors for parameter estimates. Standard
errors for the estimates of the GM were estimated using the Taylor linearization method in the
SURVEYMEANS procedure in SAS. The standard errors for the GSD were estimated using a
SAS macros that implements a jackknife method.

The detection limit for the NHANES 2013-2014 survey cycle data is 0.07 jug/dL; the 2013-2014
data do not include any non-detects. The detection limit for the 2009-2012 data is 0.25 jig/dL.
Results in the 2009-2012 data reported at less than the detection limit were assigned a value of
Va the detection limit (0.125 Pg/dL)- To evaluate the effect of the method used to handle non-
detects on the estimates, the PbBo and GSD; were also calculated using two alternate methods
for handling non-detects: assigning non-detects (i)V4 the detection limit and, (2) the
detection limit. The effect on the PbBo was approximately 0.005 pg/dL while the effect on the
GSDi was less than 0.05. An extensive sensitivity analysis performed with the 1999-2004
NHANES PbB data showed the estimated PbBo and GSDi were not sensitive to the method that
was used to treat the non-detects (U.S. EPA, 2009). Given the rate of non-detects in the 2009-
2014 PbB data (1.3%) is substantially lower than the rate of non-detects in the 1999-2004 PbB
data (2.2%), additional effort is not necessary to conclude that the method used to treat the
non-detects will not have a substantial effect 011 the estimates of the PbBo and GSDi.

The PbB data were reviewed for the possible presence of extreme sampling weights, which
could have an undue influence on the estimates of the PbBo and GSD,. The maximum sampling
weight in the 2009-2014 NHANES PbB data was less than 5 multiples of the interquartile
range greater than the median and less than 5 times the average sampling weight; therefore,
there does not appear to be any need to truncate (trim) the sampling weights (Chowdhury et
al., 2007).

"SAS and all other SAS Institute Inc. product or service names are registered trademarks or trademarks of SAS
Institute Inc. in the USA and other countries.

^Confidence limits for the GM and GSD were estimated with a SAS macro that employs the 'leave one out'
jackknife method (e.g., Sarndal, et al. 1991).

Page 3 of 9

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OLEM Directive 9285.6-56

Implications for the Adult Lead Methodology (ALM)

Ihe PbBo and GSDi are estimated to be 0.6 ug/dL, and 1.8, respectively. Table 2 presents the
updated estimates as well as the estimates from the previous analyses.

Table 2. Geometric mean baseline blood lead concentration (PbBo, j.ig/dL) and geometric
standard deviation (GSDi) estimates and ALM calculation of Preliminary Remediation Goals
(PRGs) for the 5% probability that a fetus' blood lead level will not exceed a 5 ug/dL blood lead

ALM Output

NHANES Data
(ALM)

Detection
Limit

PfaB„
(M£/dL)

GSDi

P(PbBfelal>PbBt)

Soil PRG for PbBt =
5 Mg/dL

(ppm)

1988-1991 a

1.7-2.2
(i.95)f

1.8-
2.1

(1-95)

n/a

1988-1994 b

1.0

1-5

2.1

1999-2004 c

4.589

0.3

1.0

1.8

773

2007-2012 d

4,256

0.25

0.7

1.7

1126

2009-2014«

3,683

0.07, 0.25s

0.6

1.8

1050

JU.S. EPA, 1996
hU.S. EPA, 2003
CU.S. EPA, 2009
dU.S. EPA, 2015
'"Current Update

'Values 111 parentheses represent the midpoint between the upper and lower values.

^Detection limits for 2009-2012 and 2013-2014 are 0.25 (.ig/dL and 0.07 jig/dL, respectively (the 2013-2014 data do
not include any non-deteets).

Page 4 of 9

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OLEM Directive 9285.6-56

Implications for the IEUBK Model

The proposed geometric mean value for the MatPb variable is estimated to be 0.6 ng/dL. The
update for the IEUBK model recommends that the IEUBK model be used for the 12-71 month
age range, so changes to the MatPb variable have little impact on results.

Uncertainty

As blood lead levels continue to decline in the U.S. population, the number of non-detects in
the NHANES data has the potential to become an important source of uncertainty in estimates
of PbB and GSD. However, the detection limit for measuring lead concentration in blood has
also decreased from 1.0 jig/dL (1988-1994 NHANES) to 0.3 ng/dL (1999-2004 NHANES) and
to the current levels of 0.25 |ig/dL and 0.07 ng/dL (2007-2012 and 2013-2014 NHANES,
respectively). In addition, the rate of non-detects in the 2007-2012 NHANES data (1.1%) and
the 100% detection rate in the 2013-2014 NHANES are much lower than the rate of non-
detects in the 1999-2004 and 1988-1994 NHANES data (~2 and -21%, respectively). The
lower detection limit and lower-rates of non-detects removes a considerable source of
uncertainty that was present in previous estimates of the GM (U.S. EPA, 2009).

Page 5 of 9

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OLEM Directive 9285.6-56

Recommendations For the ALM

Consistent with U.S. EPA (2009), estimates of the PbBo and GSDi are provided for the
population of non-institutionalized U.S. women 17-45 years of age. Like the earlier analysis,
the TRW Lead Committee continues to recommend using a single national estimate. Based on
the analysis of the NHANES 2009-2014 data, the updated values for the PbBo and GSDi
parameters, 0.6 pg/dL and 1.8, respectively, are recommended for all applications of the ALM
where current and future use scenarios are assessed (see Table 3).

Table 3. Current and previous PbB0 and GSDi parameter values shown in the ALM PRG
calculation tab of the ALM spreadsheet. Calculations of PRGs 5% probability that a fetus' blood
lead level will not exceed a 5 |ig/dL blood lead target level.

Variable

Description of
Variable

Units

Current

GSDi and
PbBo from
Analysis of
NHANES:
2009-2014

GSDi and PbBn
from Analysis of
NHANES 1999-
2004

PbBtoi 0.9>

95lh percentile PbB in
fetus

Hg/dL

Rfetal/malemal

Fetal/maternal PbB ratio

0.9

BKSF

BioMnetic Slope Factor

Ug/dL per
MK/day

0.4

GSDi

Geometric standard
deviation PbB

1.8

PbB,!

Baseline PbB

pg/dL

0.6

1.0

IRs

Soil ingestion rate
{including soil-derived
indoor dust)

g/day

0.050

AFs, D

Absorption fraction
(same for soil and dust)

0.12

F.Fs, d

Exposure frequency
(same for soil and dust)

days/yr

219

ATs. u

Averaging time (same for
soil and dust)

days/yr

36.5

365

PRG

Preliminary
Remediation Goal
Soil Lead
Concentration where
PbB, = 5 we/dL

ppm

1050

773

Page 6 of 9

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OLEM Directive 9285.6-56

Recommendations For the IEURK Model

Based on the analysis of the NHANES 2009-2014 data, 0.6 (ig/dL is recommended as the
updated value for the Mother's Blood Lead Concentration at Childbirth (MatPb) variable. This
default value is appropriate for all applications of the IEUBK model where current and future
residential scenarios are being assessed (see Figure 1). The TRW Lead Committee does not
recommend changing this value unless representative site-specific information is available that
meet the Data Quality Objectives of the site.

The empirical validation effort for the IEUBK model did not include data specific to the
disposition of maternally supplied lead in a young child, and the IEUBK model predictions
during the first few months of life related to this should be interpreted with caution.

Maternal Data

Mother's Blood Lead Concentration at n gi T~

Childbirth (jig Pb/dL): —

Cancel

Reset

Help?

TRW Homepage:

http:;'y'vrow.epa. gc^/superfund/health/eontaminarrts/lead/index, htfri

Figure 1. Proposed Mother's Blood Lead Concentration at Childbirth (MatPb) default value for
the IEUBK model.

References

Bowers, T.S., Beck, B.D., Karam, U.S. 1994. Assessing the relationship between environmental
lead concentrations and adult blood lead levels. Risk Anal. 14(2): 183-189. Available online at:
http:// www, ncbi. n I m. n i h. gov/pub med / 8 0 o 8 Q27.

Centers for Disease Control and Prevention (CDC). 2005. Blood Lead Levels - United States,
1999-2002. Available online at:

h ttp: / / YvwYv.cdc.Hov/mm wr / n review/ mm wrh tm I / m 111542035. htm.

Page 7 of 9

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OLEM Directive 9285.6-56

Centers for Disease Control and Prevention (CDC). 2010. National Health and Nutrition
Examination Survey. 2007-2008 Demographics File. Retrieved 9/29/10 from
http://www.cdc.gov/nchs/nhanes/nhanes2007-20o8/demoQ7 o8.htm

Centers for Disease Control and Prevention (CDC). 2011. National Health and Nutrition
Examination Survey. 2007-2008 Laboratory File. Retrieved 12/8/11 from
http://mvw.cdc.gov/nchs/nhanes/nhanes2007-2008/lab07 08.htm

Centers for Disease Control and Prevention (CDC). 2012a. National Health and Nutrition
Examination Survey. 2009-2010 Laboratory File. Retrieved 2/1/12 from
http://www.cdc.gov/nchs/nhanes/nhanes2QOQ-20io/laboQ io.htm

Centers for Disease Control and Prevention (CDC). 2012b. National Health and Nutrition
Examination Survey. 2009-2010 Demographics File. Retrieved 2/26/12 from
http://www.cdc.gov/nchs/nhanes/nhanes200Q-2010/dem00Q io.htm

Centers for Disease Control and Prevention (CDC). 2014a. National Health and Nutrition
Examination Survey. 2009-2010 Laboratory File. Retrieved 7/14/14 from
http://www.cdc.gov/nchs/nhanes/20011-2012/PbCd G.htm

Centers for Disease Control and Prevention (CDC). 2014b. National Health and Nutrition
Examination Survey. 2009-2010 Demographics File. Retrieved 7/14/14 from
http://www.cdc.gov/nchs/nhanes/2011-2012/ demo G.XPT

Centers for Disease Control and Prevention (CDC). 2017a. National Health and Nutrition
Examination Survey. 2009-2010 Laboratory File. Retrieved 2/6/14 from
http://www.cdc.gov/nchs/nhancs/20Qii-20i2/PBCD H.htm

Centers for Disease Control and Prevention (CDC). 2017b. National Health and Nutrition
Examination Survey. 2009-2010 Demographics File. Retrieved 2/14/17 from
http://www.cdc.gov/nchs/nhanes/2011-2012/ DEMO H.XPT

Chowdhury, S., Khare, M. and K. Wolter. 2007. Weight Trimming in the National
Immunization Survey. Paper presented at the 2007 American Statistical Association meeting.

Goyer, R.A. 1990. transplacental transport of lead. Environ. Health Perspect. 89: 101—105.
Available online at: http: //www.ncbi.nlm.nih.gov/pubmed/2088735.

Johnson CL, Paulose-Ram R, Ogden CL, et al. National Health and Nutrition Examination
Survey: Analytic guidelines, 1999-2010. National Center for Health Statistics. Vital Health Stat
2(161), 2013.

Sarndal, C.E., Swenson, B. and J.Wretman, 1991. Model Assisted Survey Sampling. Springer-
Verlag New York Inc.

U.S. Environmental Protection Agency (U.S. EPA). 1994. Guidance Manual for the Integrated
Exposure Uptake Biokinetic Model for Lead in Children. U.S. Environmental Protection
Agency, Office of Solid Waste and Emergency Response: Washington, DC. PB93-963510,
OSVVER 9285.7-15-1. February. Available online at: https://www.epa.gov/superfund/lead-
superfund-sites-software-and-users-manuals#guidance.

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OLEM Directive 9285.6-56

U.S. Environmental Protection Agency (EPA). 1995. A TRW Report: Review of a Methodology
for Establishing Risk-Based Soil Remediation Goals for the Commercial Areas of the California
Gulch Site. Technical Review Workgroup for Lead, October, 1995.

U.S. Environmental Protection Agency (EPA). 1996. Recommendations of the Technical
Review Workgroup for Lead for an Interim Approach to Assessing Risks Associated with Adult
Exposures to Lead in Soil. Available from: https://www.epa.gov/superfund/lead-superfund-
sites-software-and-users-manuals#guidance

U.S. Environmental Protection Agency (U.S. EPA). 1998. OSWER Directive: Clarification to the
1994 Revised Interim Soil Lead (Pb) Guidance for CERCLA Sites and RCRA Corrective Action
Facilities. U.S. Environmental Protection Agency, Office of Solid Waste and Emergency
Response: Washington, DC. EPA/540/F-98/030. PB98-963244. OSWER Dir #9200.4-27?.
August. Available online at: http://epa.gov/superfund/lead/miidance.htm.

U.S. Environmental Protection Agency (EPA). 2002. Blood Lead Concentrations of U.S. Adult
Females: Summary Statistics from Phases ] and 2 of the National Health and Nutrition
Examination Survey (NHANES). OSWER #9285.7-52. March. Available from:
http://epa.gov/superfund/lead/Droducts/nhanes.Ddf

U.S. Environmental Protection Agency (EPA). 2003. Recommendations of the Technical
Review Workgroup for Lead for an approach to assessing risks associated with adult exposures
to lead in soil. EPA-540-R-03-001. January. Available from:
http://epa.gov/superfund/lead/trw.htm

U.S. Environmental Protection Agency (EPA). 2009. Update of the Adult Lead Methodology's
Default Baseline Blood Lead Concentration and Geometric Standard Deviation Parameters.
OSWER 9200.2-82. June. Available from: https://www.epa.gov/superfund/lead-superfund-
sites-guidance

U.S. Environmental Protection Agency (EPA). 2016. Update of the Adult Lead Methodology's
Default Baseline Blood Lead Concentration and Geometric Standard Deviation Parameters and
the Integrated Exposure Uptake Biokinetic Model's Default Maternal Blood Lead
Concentration at Birth Variable. OLEM Directive 9285.6-55, August. Available from:
https://www.epa.gov/superfund/lead-superfund-sites-guidance

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