Region 8

HH: Evaluation of Risks from Lead

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Introduction

In the case of lead, human exposure and risk are characterized using a somewhat different approach than that described under the main Human Health Risk Assessment page. This is because lead exposure is evaluated using a biokinetic model and risk is interpreted in terms of blood lead concentration rather than a Hazard Quotient. EPA's Technical Review Workgroup (TRW) for lead has developed extensive guidance on how to evaluate risks from lead, and all of this information is available at the TRW website. The following sections provide an overview of the toxicity of lead, along with the methods and tools recommended by EPA for performing risk assessment calculations for lead.

Toxicity of Lead

Excess exposure to lead may cause a wide variety of adverse effects in humans, including neurotoxicity, developmental delays, hypertension, impaired blood synthesis, and male reproductive impairment. In many cases, these effects may be present without overt signs of toxicity. General descriptions of the potential effects of lead on human health may be found on the EPA's Technical Review Workgroup introductory page on Lead and Human Health and in the CDC's 1991 report Preventing Lead Poisoning in Young Children. Detailed descriptions of key studies on the toxicity of lead may be found in the Agency for Toxic Substances and Disease Registry (ATSDR)'s Toxicological Profile for Lead (1999).

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Blood Level of Concern

Young children (aged 6-84 months) are usually the sub-population of chief concern for lead exposure. This is because: 1) young children tend to have higher intakes of environmental media per unit body weight than adults (especially for soil and dust), 2) young children tend to absorb a higher fraction of ingested lead than adults, and 3) young children are inherently more susceptible to the adverse effects of lead, since their nervous systems are still developing.

It is currently difficult to identify what degree of lead exposure, if any, can be considered safe in young children. Some studies report subtle signs of lead-induced neurobehavioral effects in children beginning at blood lead levels around 10 μg/dL or even lower, with population effects becoming clearer and more definite in the range of 30-40 μg/dL (see Preventing Lead Poisoning in Young Children and Toxicological Profile for Lead).

After a thorough review of all the data, EPA has identified 10 μg/dL as the blood lead level at which effects that warrant avoidance begin to occur, and has set as a goal that there should be no more than a 5 percent chance that any child will have a blood lead value above 10 μg/dL (Guidance Manual for the Integrated Exposure Uptake Biokinetic Model for Lead in Children (EPA 9285.7-15-1, February 1994); Revised Interim Soil Lead Guidance for CERCLA Sites and RCRA Corrective Action Facilities (PDF) (18 pp, 156 K) (Memorandum from E. P. Laws, July 1994, OSWER Directive #9355.4-12). The Centers for Disease Control (CDC) also established a guideline of 10 μg/dL in preschool children, which is believed to prevent or minimize lead-associated cognitive deficits Preventing Lead Poisoning in Young Children. CDC has recently re-evaluated this level and has determined that it is not appropriate to revise the value at this time (see CDC, 2004).

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Use of the IEUBK Model to Assess Risks to Children

In residential locations and other areas where children are exposed to lead, EPA recommends the use of the Integrated Exposure Uptake Biokinetic Model for Lead in Children (IEUBK) model to evaluate exposures from lead-contaminated media and to predict the likely effects of those exposures on blood lead levels.

The IEUBK model requires input of environmental lead concentration levels, and also allows the user to input relevant exposure and absorption parameters if reliable site specific data are available. Given this input, the IEUBK model performs a series of computations that quantify the intake, absorption, distribution, and excretion of the lead over time and predicts the blood lead level in a typical child. To account for variability between different children, the IUEBK model assumes that the distribution of blood lead values across different children is lognormal and generates this distribution based on a geometric standard deviation (GSD) supplied as input to the model. Once the full distribution is generated, the probability of a child having a blood lead level above 10 μg/dL can be computed and compared to EPA's health-based goal (no more than a 5 percent chance of exceeding 10 μg/dL). The IEUBK model software and associated guidance documents can be found on the TRW's Software and Users' Manuals page. The EPA's Technical Review Workgroup (TRW) web site also provides answers to a number of frequently asked questions about the IEUBK model.

In addition, the TRW has prepared guidance and recommendations for the assessment of lead risks when exposures are not continuous and chronic (e.g., day care, recreation, or trespass scenarios). This guidance is intended to augment existing guidance pertaining to the IEUBK and Adult Lead Methodology, and includes some case study examples of how the guidance is to be applied. This guidance can be obtained at the following page:

Assessing Intermittent or Variable Exposures at Lead Sites (PDF) (OSWER 9285.7-76, November 2003) (46 pp, 441 K)

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Use of the Adult Lead Model to Assess Risks to Adults

When young children are not expected to be present at a site (e.g., a workplace), then the population of concern is the adult (e.g., a worker). While both males and females are susceptible to adverse effects from excess lead exposure, the female of child-bearing age is the sub-population of chief concern, since exposure of the pregnant female can result in exposure of the fetus in utero. The EPA has developed the Adult Lead Methodology (ALM) for evaluating the potential risks from lead in pregnant females. 

The ALM model is mathematically simpler than the IEUBK model, but the basic concept is the same: information on lead exposure from all environmental sources is used to predict the blood lead level in a typical adult female, and this is converted into a lognormal distribution based on a specified geometric standard deviation (GSD). Based on studies of maternal/fetal blood lead ratios, the blood lead of the fetus is assumed to be 90 percent of the value in the mother. Decision criteria for the ALM are usually similar to that for young children (no more than a 5 percent chance that the blood lead level in a fetus will exceed a value of 10 μg/dL). Links to download the ALM guidance documents and example calculation spreadsheets, as well as answers to frequently asked questions about the ALM, can be found on the EPA's Technical Review Workgroup (TRW) Software and Users' Manuals page.

In addition, the TRW has prepared guidance and recommendations for the assessment of lead risks when exposures are not continuous and chronic (e.g., day care, recreation, or trespass scenarios). This guidance is intended to augment existing guidance pertaining to the IEUBK and Adult Lead Methodology, and includes some case study examples of how the guidance is to be applied. This guidance can be obtained at the following link:

Assessing Intermittent or Variable Exposures at Lead Sites (PDF) (OSWER 9285.7-76, November 2003) (46 pp, 441 K)

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Data Collection and Evaluation

Sampling Environmental Media

As noted above, both the IEUBK model and the ALM model may consider lead exposure from a wide variety of environmental media, including outdoor soil, indoor dust, drinking water, food, air, and (sometimes) leaded paint. Thus, measurements of lead in environmental media are needed to support model calculations. Often the greatest emphasis is placed on characterizing lead levels in soil, since this is the medium most often addressed in Superfund remediation projects. However, because the models consider exposure from other media as well, it is often valuable to collect data on lead levels in other media (e.g., dust, water, air, food, paint), as well as the relationships between those media. The following documents provide valuable guidance on strategies and techniques for collection and analysis of environmental samples for lead analysis:

Superfund Lead-Contaminated Residential Sites Handbook (PDF) (OSWER 9285.7-50, August 2003) (124 pp, 1 MB)

Residential Soil Lead Sampling Guidance Document (PDF) (Region 8 Superfund Program, April 2000 Draft Final)

Draft Sampling Manual for IEUBK Model (PDF) (Region 8, December 1996)

Evaluating the Bioavailability of Lead in Soil and Soil-Like Media

The effect of an ingested dose of lead on blood lead levels depends on how much of the ingested lead is absorbed from the gastrointestinal tract into the blood. For children, the IEUBK assumes a default value of 30 percent for lead in soil and dust, and the ALM model assumes a default value of 12 percent for adults. However, lead in soil can occur in a wide variety of chemical and physical forms, and there may be important differences in the degree to which the different forms are absorbed after ingestion. More detailed information and data on the absorption of lead from soil is provided on the Bioavailability page.

Blood Lead Sampling and Analysis

In some cases, biomonitoring of blood lead levels may be a useful tool to help evaluate current exposure levels at a site (assuming that a population of humans is present at the site). In general, the results of the biomionitoring may be compared to other (reference) populations to help understand the magnitude of the site-related lead exposure, and/or may be compared to health-based guidelines for the maximum level of exposure that is considered acceptable. In addition, blood lead biomonitoring may also be useful in some cases to help improve the accuracy of IEUBK or ALM model calculations of exposure and risk. Useful documents on the collection and interpretation of blood lead data are provided below:

Criteria for Evaluating Blood Lead Data Quality and Use (PDF) (Region 8 Superfund Technical Guidance RA-07, September 1995)

Sample Analysis and Quality Assurance Plan for Urinary Arsenic and Blood Lead Among Residents of VBI70 Neighborhoods (PDF) (Region 8, June 2002)

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Guidance on Establishing Cleanup Levels for Lead

EPA has established guidance for default screening levels of lead in soil and other media, along with methods for deriving site-specific cleanup levels, as described in the following documents:

Revised Interim Soil Lead Guidance for CERCLA Sites and RCRA Corrective Action Facilities (PDF) (Memorandum from E. P. Laws, July 1994, OSWER Directive #9355.4-12) (18 pp, 156 K)

Note: This interim directive replaces all previous directives on soil lead cleanup for CERCLA and RCRA programs, including OSWER Directive #9355.4-02 (Longest & Diamond memorandum, September 1989) and the update to 9355.4-02 (Clay memorandum, August 1991).

Review of the National Ambient Air Quality Standards for Lead: Exposure Analysis Methodology and Validation (PDF) (QAQPS Staff Report, EPA-450/2-89-011, June 1989) (188 pp, 9.3 MB)

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