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CHAPTER 2 INTEGRATIVE SUMMARY

2.1 ISA Development and Scope

1 This chapter summarizes and synthesizes the recently available scientific evidence and is 

2 intended to provide a concise synopsis of the ISA conclusions and findings that best 

3 inform the review of the NAAQS. The Integrated Review Plan (IRP) for the National 

4 Ambient Air Quality Standards for Lead (U.S. EPA, 2011c) identifies a series of 

5 policy-relevant questions (in Chapter 3) that provide the framework for this assessment, 

6 and which frame the entire review of the NAAQS for Pb, and thus are informed by both 

7 science and policy considerations. The plans and underlying questions for the ISA are 

8 included in the IRP. The ISA organizes, presents, and integrates the scientific evidence, 

9 which is considered along with findings from any risk analyses and policy considerations, 

10 to help the U.S. Environmental Protection Agency (EPA) address these questions during 

11 the NAAQS review for Pb. The ISA includes:

12  An integration of the evidence on the human health effects associated with Pb 

13 exposure, discussion of important uncertainties identified in the interpretation of 

14 the scientific evidence, and an integration across different scientific disciplines 

15 and across individual endpoints within major outcome categories.

16  An integration of the evidence on the ecological effects of Pb in aquatic and 

17 terrestrial ecosystems, discussion of endpoints common to plants, invertebrates 

18 and vertebrates and consideration of uncertainties in relating atmospheric Pb 

19 concentrations to ecological effects.

20  An integration of the effects associated with exposure to Pb across the scientific 

21 disciplines for health and ecology, focusing on common modes of action. 

22  Discussion of policy relevant considerations, such as potentially at-risk 

23 populations and concentration-response relationships. 

24 EPA has developed a process for evaluating the scientific evidence; and drawing 

25 conclusions and causal judgments regarding air pollution-related health and 

26 environmental effects. The ISA development process includes literature search strategies, 

27 criteria for selecting and evaluating studies, approaches for evaluating weight of the 

28 evidence, and a framework for making causality determinations. The process and 

29 causality framework are described in more detail in the Preamble to the ISA. This section 

30 provides a brief overview of the process for development of this ISA.
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1 EPA initiated the current review of the NAAQS in April 2010 with a call for information 

2 from the public (75 FR 20843). Literature searches were conducted routinely to identify 

3 studies published since the last review, focusing on studies published from 2006 (close of 

4 previous scientific assessment) through September 2011. References that were considered 

5 for inclusion or cited in this ISA can be found at http://hero.epa.gov/lead.

6 This ISA evaluates relevant epidemiologic, animal toxicological, and ecological and 

7 welfare effects studies, including those related to concentration-response relationships, 

8 mode(s) of action (MOA), and susceptible populations. Additionally, air quality and 

9 emissions data, studies on environmental fate and transport, and issues related to Pb 

10 toxicokinetics and exposure were considered for inclusion in the document. Previous 

11 AQCDs (U.S. EPA, 2006b, 1986b, 1977) have included an extensive body of evidence on 

12 these topics. In this ISA, the conclusions and key findings from previous reviews are 

13 summarized at the beginning of each section, to provide the foundation for consideration 

14 of evidence from recent studies. Results of key studies from previous reviews are 

15 included in discussions or tables and figures, as appropriate, and conclusions are drawn 

16 based on the synthesis of evidence from recent studies with the extensive literature 

17 summarized in previous reviews. 

18 The Preamble discusses the general framework for conducting the science assessment 

19 and developing an ISA, including criteria for selecting studies for inclusion in the ISA, 

20 evaluating and integrating the scientific evidence and developing scientific conclusions. 

21 In selecting the studies for inclusion in the Pb ISA, particular emphasis is placed on those 

22 studies most relevant to the review of the NAAQS. 

23 In drawing judgments regarding causality for the criteria air pollutants, evidence of health 

24 effects in the range of relevant pollutant exposures or doses is considered. With regard to 

25 the causal determinations drawn for human health effects of Pb, population-based 

26 epidemiology studies were emphasized over occupational studies. Recent occupational 

27 studies were considered insofar as they added to the evidence base for an outcome for 

28 which sufficient numbers of population-based studies were unavailable or, to the extent 

29 that they addressed a topic area that was of particular relevance to the NAAQS review 

30 (e.g., longitudinal studies designed to examine recent versus historical Pb exposure). 

31 Evidence from toxicological studies of effects observed in experimental animals at doses 

32 that were relevant to, or somewhat above, those currently experienced by the U.S. general 

33 population were emphasized. The extent to which studies of higher concentrations were 

34 considered varied by major outcome category, but generally studies with blood Pb levels 

35 within one order of magnitude above the upper end of the distribution of U.S. blood Pb 

36 levels were considered (i.e., toxicological studies reporting blood Pb levels less than 
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about 100 μg/dL).1 1 The majority of these studies reported blood Pb levels below 

2 approximately 30-40 μg/dL, however. Studies with higher blood Pb levels were 

3 considered to the extent that they provided useful information to inform modes of action 

4 or mechanisms or kinetics. For toxicological studies where blood Pb levels were not 

5 measured, judgments regarding how to distinguish high from the more relevant low doses 

6 were made considering the range of doses across the available body of evidence and 

7 emphasizing studies at the lower end of the range. 

8 Relevant concentrations for drawing causality judgments for ecological effects of Pb 

9 were determined considering the range of Pb concentrations in the environment and the 

10 available evidence for concentrations at which effects were observed in biota. Effects 

11 observed at or near ambient levels of Pb were emphasized and studies generally within 

12 one to two orders of magnitude above current conditions were considered in the body of 

13 evidence for terrestrial and aquatic ecosystems. Studies at higher concentrations were 

14 used to the extent that they informed modes of action and illustrated the wide range of 

15 sensitivity to Pb across taxa. For aquatic biota, generally, the number of studies available 

16 on freshwater organisms is greater than on saltwater organisms, covering more taxa as 

17 well as more endpoints. The causal determinations for aquatic biota are, therefore, based 

18 primarily on evidence from freshwater species. When available, data from saltwater 

19 ecosystems are discussed separately under the appropriate causal endpoint. For most of 

20 the endpoints under consideration, evidence is inadequate to establish causality in 

21 saltwater species. When sufficient evidence is available for marine organisms, data on 

22 concentrations at which effects are observed are presented in Section 7.4.2.

23 The causal statements for terrestrial and aquatic effects are arranged according to 

24 ecologically meaningful levels of biological organization (organism, population, 

25 community, ecosystem). As recognized in EPA's Framework for Ecological Risk 

26 Assessment (U.S. EPA, 1992), and in the adverse outcome pathway (AOP) framework

27 (Ankley et al., 2010) endpoints that are measured at one level of ecological organization 

28 may be related to an endpoint at a higher level. The AOP conceptual framework was 

29 proposed to link mechanistic data from initiating events at the molecular level through a 

30 series of higher order biological responses to survival and to developmental and 

31 reproductive endpoints that can be used in ecological risk assessment, i.e., at the 

32 population level and higher. Fecundity, growth, and survival are organism-level attributes 

33 that lead to population-level (e.g., abundance, production, extirpation), community-level

34 (taxa richness, relative abundance) and ecosystem-level responses (Ankley et al., 2010; 

 

1 Studies considered were generally within an order of magnitude above 10 μg/dL (i.e., 100 μg/dL), which was used 

to indicate the upper end of the blood Pb distribution in the U.S. population. Currently approximately 1.4% of U.S. 

children overall have blood Pb levels that exceed 10 μg/dL (Jones et al., 2009a); however, the proportion of 

individuals with blood Pb levels that exceed this concentration varies depending on factors including age and/or 

residence near a source of Pb exposure (Section 4.1).
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1 Suter et al., 2005). In the case of Pb, physiological stress, hematological effects and 

2 neurobehavioral alterations may decrease the overall fitness of an organism, even though 

3 their connection to effects at higher levels of biological organization may not have been 

4 characterized. Furthermore, the effects of Pb on ecosystems necessarily begin with some 

5 initial effects at the molecular level of specific organisms within the ecosystem (U.S. 

6 EPA, 1986b). There are many different molecular and cellular level effects, and toxicity 

7 of Pb in ecosystems may be attained through multiple modes of action. 

8 The ISA considers evidence of health effects for both short- and long-term pollutant 

9 exposures. Biomarkers are typically used in epidemiologic studies of the health effects 

10 associated with Pb as an index of exposure or dose. Consequently, the timing, frequency, 

11 level, and duration of Pb exposure(s) associated with the observed health effects are 

12 uncertain. Evidence from animal toxicological studies is also drawn upon to understand 

13 and interpret Pb exposures in epidemiologic studies. Animal toxicological studies that are 

14 relied upon to inform our understanding of the exposures needed to induce health effects 

15 in humans include chronic exposure studies (i.e., over 10% of the lifespan of the animal), 

16 long-term exposure studies (e.g., greater than 4 weeks in rodents) as well as acute or 

17 short-term exposure studies (e.g., less than 4 weeks in rodents). In this ISA, short-term 

18 human exposures are generally defined to include exposures of months (e.g., < one year) 

19 while long-term human exposures include those greater than one year in duration. In

20 addition, information including the age of the population studied, study period and study 

21 location can be used to aid in the interpretation of findings from epidemiologic studies 

22 because Pb exposures have declined over time and exposures vary depending on

23 proximity to Pb sources. 

24 This ISA uses a five-level hierarchy that classifies the weight of evidence for causation: 

25  Causal relationship 

26  Likely to be a causal relationship 

27  Suggestive of a causal relationship 

28  Inadequate to infer a causal relationship 

29  Not likely to be a causal relationship 

30 Beyond judgments regarding causality are questions relevant to quantifying health or 

31 environmental risks based on the understanding of the quantitative relationships between 

32 pollutant exposures and health or ecological effects. Once a determination is made 

33 regarding the causal relationship between the pollutant and outcome category, important 

34 questions regarding quantitative relationships include: 
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1  What is the concentration-response, exposure-response, or dose-response 

2 relationship in the human population?

3  What exposure conditions (dose or exposure, exposure pathways, duration and 

4 pattern) are important?

5  What populations and lifestages appear to be differentially affected i.e., at 

6 greater risk of Pb-related health effects?

7  What elements of the ecosystem (e.g., types, regions, taxonomic groups, 

8 populations, functions, etc.) appear to be affected or are more sensitive to 

9 effects?

10 This chapter summarizes and integrates the newly available scientific evidence that best 

11 informs consideration of the policy-relevant questions that frame this assessment. The 

12 organization of this chapter generally follows the organization of the document as a 

13 whole, with several additional sections including: (Section 2.1) a discussion of the 

14 assessment development and scope; (Section 2.8) an integration of the evidence across 

15 the disciplines of health and ecology; (Section 2.9) a discussion of policy-relevant 

16 considerations; and, (Section 2.10) an overall summary. This ISA itself is composed of 

17 six chapters including this integrative summary. Chapter 3 highlights key concepts or 

18 issues relevant to understanding the sources, ambient concentrations, and fate and 

19 transport of Pb in the environment. 0 summarizes key concepts and recent findings on Pb 

20 exposures, toxicokinetics, and biomarkers reflecting Pb exposure and body burden. 

21 Chapter 5 presents a discussion of the MOA of Pb and evaluates and integrates 

22 epidemiologic and animal toxicological information on health effects related to Pb 

23 exposures, including nervous system, cardiovascular, renal, immunological, reproductive 

24 and developmental, and cancer outcomes. Chapter 6 summarizes the evidence on 

25 potentially susceptible populations. Chapter 7 evaluates ecological effects evidence that 

26 is relevant to the review of the secondary NAAQS for Pb. 

2.2 Ambient Lead: Source to Concentration

2.2.1 Sources, Fate and Transport of Ambient Lead

27 The findings of this review with respect to sources of atmospheric Pb build upon those 

28 from the 2006 Pb AQCD (U.S. EPA, 2006b), which documented the decline in ambient 

29 air Pb emissions following the ban on alkyl-Pb additives for on-road gasoline. Pb 

30 emissions declined by 98% from 1970 to 1995 and then by an additional 76% from 1995 
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1 to 2008, at which time emissions were estimated at 970 tons per year. As was the case at 

2 the time of the last Pb NAAQS review, data from the 2008 National Emissions Inventory 

3 (NEI) (U.S. EPA, 2011a) illustrate that piston-engine aircraft emissions comprise the 

4 largest share (56%) of total atmospheric Pb emissions; the 2008 NEI estimated that 550 

5 tons of Pb were emitted from this source. Other sources of ambient air Pb, beginning with 

6 the largest, include metals processing, fossil fuel combustion, other industrial sources, 

7 roadway related sources, and historic Pb. Global atmospheric Pb deposition peaked in the 

8 1970s, followed by a decline. See Section 3.2 for additional information.

9 The atmosphere is the main environmental transport pathway for Pb. On a local scale 

10 atmospheric Pb is primarily associated with coarse particulate matter (PM), and on a 

11 global scale atmospheric Pb is primarily associated with fine PM. Both wet and dry 

12 deposition are important removal mechanisms for atmospheric Pb. Because Pb in fine 

13 particles is typically fairly soluble, wet deposition is more important for fine Pb. In 

14 contrast, Pb associated with coarse particles is usually insoluble and removed by dry 

15 deposition. Pb associated with coarse PM deposits to a great extent near local industrial 

16 sources, contributing to soil Pb concentrations in those locations, while fine Pb-bearing 

17 PM can be transported long distances, contributing Pb to remote areas. Depending on 

18 local conditions, once deposited particles may be resuspended and redeposited before 

19 reaching a site where further transport is unlikely, especially for dry deposition. See 

20 Section 3.3 for additional information.

21 Environmental distribution of Pb occurs mainly through the atmosphere, from which it is 

22 deposited into surface waters and soil. Surface waters act as an important reservoir, with 

23 Pb lifetimes in the water column largely controlled by deposition and resuspension of Pb 

24 in sediments. Substantial amounts of Pb from vehicle wear and building materials can 

25 also be transported by runoff waters to surface waters and sediments without becoming 

26 airborne. Pb containing sediment particles can be remobilized into the water column, and 

27 sediment concentrations tend to follow those in overlying waters. See Section 3.3 for 

28 additional information.

2.2.2 Monitoring and Concentrations of Ambient Air Lead 

29 The indicator for the Pb NAAQS is Pb in total suspended particles (Pb-TSP). The 

30 Pb-TSP indicator was retained in 2008 in recognition of the role of all PM sizes in 

31 ambient air Pb exposures. The Federal Reference Method (FRM) for Pb-TSP specifies 

32 that ambient air is drawn through a high-volume TSP sampler onto a glass fiber filter. 

33 The Pb-TSP sampler’s size selective performance is known to be affected by wind speed 

34 and direction, and collection efficiency has been demonstrated to decline with particle 
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1 size with an uncertain upper size limit (Wedding et al., 1977). In recognition of the steep 

2 spatial gradients associated with sources of ultra-coarse particles, ambient Pb sampled 

3 using the FRM for Pb-PM10 is allowed in certain instances where the expected Pb 

4 concentration does not approach the NAAQS and no sources of ultra-coarse Pb are 

5 nearby. A new FRM for Pb-PM10 has been implemented in which ambient air is drawn 

6 through an inertial particle size separator for collection on a polytetrafluoroethylene 

7 (PTFE) filter. The Pb-PM10 FRM is free of wind direction bias. Several FEMs have also 

8 been approved. The Pb-TSP FRM is based on flame atomic absorption spectroscopy 

9 (AAS), while the Pb-PM10 FRM is based on x-ray fluorescense spectroscopy (XRF). 

10 Inductively-coupled plasma mass spectrometry (ICPMS) is under consideration as a new 

11 FRM for Pb-TSP. See Section 3.4.1 for additional information.

12 Pb monitoring requirements have experienced several changes since publication of the 

13 2006 Pb AQCD (U.S. EPA, 2006b). The current Pb monitoring network design 

14 requirements include two types of FRM monitoring sites: source-oriented and 

15 non-source-oriented. Source-oriented FRM Pb-TSP monitoring sites are required near 

16 sources of air Pb emissions which are expected to or have been shown to contribute to 

17 ambient air Pb concentrations in excess of the NAAQS. At a minimum, monitoring is 

18 required near sources that emit 0.50 tons/year or more of Pb unless it can be determined 

19 that the source will not contribute to an ambient concentration exceeding 50% of the Pb 

20 NAAQS. With the December 2010 completion of action on regulatory requirement of Pb 

21 monitoring, one-year of Pb-TSP FRM monitoring is also required near 15 specific 

22 airports to gather additional information on the likelihood of NAAQS exceedances near 

23 airports due to combustion of leaded aviation gasoline. Non-source-oriented monitoring 

24 is also required at national core multipollutant monitoring network (NCore) sites in Core 

25 Based Statistical Areas (CBSA) with a population of at least 500,000. In addition to FRM 

26 monitoring, Pb is also routinely measured in smaller particle fractions in the chemical 

27 speciation network (CSN), interagency monitoring of protected visual environment 

28 (IMPROVE), and the national air toxics trends station (NATTS) networks. While 

29 monitoring in multiple networks provides extensive geographic coverage, measurements 

30 between networks are not directly comparable in all cases because there are differences in 

31 the methods, including the different particle size ranges sampled in the different 

32 networks. Depending on monitoring network, Pb is monitored in TSP, PM10, or PM2.5. 

33 For the purpose of analyzing data for the ISA, monitors reporting to the AQS were 

34 considered to be source-oriented if they were designated in AQS as source-oriented, or 

35 they were located within 1 mile of a 0.5 ton per year or greater source, as noted in the 

36 2005 NEI (U.S. EPA, 2008a) or the 2008 NEI (U.S. EPA, 2011a). See Section 3.4 for 

37 additional information.
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1 Ambient air Pb concentrations have declined drastically over the period 1980-2010. The 

median annual concentrations have dropped by 97% from 0.87 μg/m3 2 in 1980 to 0.03 

μg/m3 3 in 2010. While the sharpest drop in Pb concentration occurred during 1980-1990, a 

4 declining trend was observed between 1990 and 2010. There was an 84% reduction in the 

5 median annual source-oriented Pb concentration and a 85% reduction in the median 

6 annual non-source-oriented Pb concentration for 1990-2010. Recent estimates for the 

natural contribution to background Pb are ~0.3 to 1 ng/m3 7 . These estimates exceed 

8 estimates of natural background presented in the 1986 Pb AQCD by a factor of 2 to 20. 

9 The more recent estimate still indicates that background airborne Pb concentrations are 

10 well below current ambient concentrations. See Section 3.5 for additional information. 

11 AQS data for source-oriented and non-source-oriented FRM monitoring were analyzed 

12 for 2008-2010. For source-oriented monitors, the three-month rolling average was above 

13 the level of the NAAQS in twenty counties across the U.S. The three-month rolling 

14 average was never above the level of the NAAQS for any of the non-source-oriented 

15 FRM monitors. Pb concentrations, seasonal variations, inter-monitor correlations, and 

16 wind data were analyzed for six counties: Los Angeles County, CA; 

17 Hillsborough/Pinellas Counties, FL; Cook County, IL; Jefferson County, MO; Cuyahoga 

18 County, OH; and Sullivan County, TN. Spatial and temporal variability of Pb 

19 concentrations in each county were commonly high. Meteorology, distance from sources, 

20 and positioning of sources with respect to the monitors all appeared to influence the level 

21 of concentration variability across time and space. See Section 3.5 for additional 

22 information.

23 Size distribution of Pb-bearing PM varied substantially depending on the nature of Pb 

24 sources and proximity of the monitors to the Pb sources. Variation in the correlation of 

25 size fractionated Pb samples among different land use types may be related to differences 

26 in sources across land use types; for example, ultra-coarse Pb-PM has been observed near 

27 industrial sites. Additionally, Pb concentrations exhibited varying degrees of association 

28 with other criteria pollutant concentrations. Overall, Pb was moderately associated with 

29 PM2.5, PM10 and NO2. Pb was moderately associated with CO in fall and winter only. The 

30 poorest associations were observed between Pb and O3. Among trace metals, the 

31 strongest association was with Zn. Moderate associations with Pb concentrations were 

32 observed for Br, Cu, and K. Such correlations may suggest some common sources 

33 affecting the concentrations of various pollutants. See Section 3.5 for additional 

34 information.
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2.2.3 Ambient Lead Concentrations in Non-Air Media and Biota

1 Atmospheric deposition has led to measurable Pb concentrations observed in rain, 

2 snowpack, soil, surface waters, sediments, agricultural plants, livestock, and wildlife 

3 across the world, with highest concentrations near Pb sources, such as metal smelters. 

4 After the phase-out of Pb from on-road gasoline, Pb concentrations have decreased 

5 considerably in rain, snowpack, and surface waters. Declining Pb concentrations in tree 

6 foliage, trunk sections, and grasses, as well as surface sediments and soils in some 

7 locations, have also been observed (U.S. EPA, 2006b). In contrast, Pb is retained in soils

8 and sediments, where it provides a historical record of deposition and associated 

9 concentrations. In remote lakes, sediment profiles indicate higher Pb concentrations in 

10 near surface sediment as compared to pre-industrial era sediment from greater depth and 

11 indicate peak concentrations between 1960 and 1980 (when leaded on-road gasoline was 

12 at peak use). Concentrations of Pb in moss, lichens, peat, and aquatic bivalves have been 

13 used to understand spatial and temporal distribution patterns of air Pb concentrations. 

14 Ingestion and water intake are the major routes of Pb exposure for aquatic organisms, and 

15 food, drinking water, and inhalation are major routes of exposure for livestock and 

16 terrestrial wildlife. Overall, Pb concentrations have decreased substantially in media 

17 through which Pb is rapidly transported, such as air and water. Substantial Pb remains in 

18 soil and sediment sinks. Although in areas less affected by major local sources, the 

19 highest concentrations are below the surface layers and reflect the phase-out of Pb from 

20 on-road gasoline and emissions reductions from other sources. 

21 Information on ambient Pb concentrations in non-air media and biota is reported in 

22 Section 3.6, and concentrations considered in the interpretation of the ecological evidence 

23 are tabulated in Table 2-1. As noted in the preamble, the ecological causal determinations 

24 focus on studies where effects of Pb exposure are observed at or near ambient levels of 

25 Pb and studies generally within the range of one to two orders of magnitude above 

26 current or ambient conditions were also considered in the body of evidence. 
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Table 2-1 Ambient Pb Concentrations in Non-Air Media and Biota Considered 

for Ecological Assessment

Media Pb Concentration Years Data Obtained References

Soil National Average: 18.9 mg Pb/kg

Range of state averages: 5-38.6 mg Pb/Kg

1961-1976 U.S. EPA (2007d, 2006b, 2003b)

Sediment1 Median: 28 mg Pb/kg (dry weight) 1996-2001 Mahler et al. (2006), U.S. EPA (2006b)

Fresh Surface Water 

(Dissolved Pb)a

Median: 0.5 μg Pb/L; Max: 30 μg Pb/L

Range: 0.0003-0.075 μg Pb/L

2001-2003

2002-2007

U.S. EPA (2006b)

Field and Sherrell (2003), Landers et al. (2008)

Saltwaterb Range: 0.01 – 27 μg Pb/L

Range: 0.005-0.04 μg Pb/L

Dates not available Sadiq (1992)

Leland and Kuwabara (1985)

Vegetation Lichens: 0.3-5 mg Pb/kg

Grasses: 31% (percent of soil Pb in grass)

2002-2007

1980s-2000s

Landers et al. (2008)

Vandenhove et al. (2009)

Vertebrate Fish: Geometric Mean:

0.54 mg Pb/kg (dry weight) 

Maximum: 

23 mg Pb/kg (dry weight)

Fish: 0.0033-0.97 mg Pb/kg

Moose: 0.021-0.23 mg Pb/kg

2001-2003

2002-2007

U.S. EPA (2006b)

Landers et al. (2008)

a

Based on synthesis of NAWQA data reported in 2006 Pb AQCD (U.S. EPA, 2006b) b

Data from a combination of brackish and marine saltwater samples. In general, Pb in seawater is higher in coastal areas and estuaries since these locations are 

closer to sources of Pb contamination and loading from terrestrial systems.

2.3 Exposure to Ambient Lead

1 Exposure data considered in this assessment build upon the conclusions of the 2006 Pb 

2 AQCD (2006b), which found air Pb concentrations in the U.S. and associated biomarkers 

3 of exposure to have decreased substantially following the ban on Pb in on-road gasoline 

4 as well as an earlier ban on Pb in house-hold paints. Pb exposure is difficult to assess 

5 because Pb has multiple sources in the environment and passes through various media. 

6 The atmosphere is the main environmental transport pathway for Pb, and, on a global 

7 scale, atmospheric Pb is primarily associated with fine particulate matter, which can 

8 deposit to soil and water. In addition to primary emission of particle-bearing or gaseous 

9 Pb to the atmosphere, Pb can be suspended to the air from soil or dust. Air-related 

10 pathways of Pb exposure are the focus of this assessment. In addition to inhalation of Pb 

11 from ambient air, air-related Pb exposure pathways include inhalation and ingestion of Pb 

12 from indoor dust and/or outdoor soil that originated from recent or historic ambient air 

13 (e.g., air Pb that has penetrated into the residence either via the air or tracking of soil). 

14 Non-air-related exposures include occupational exposures, hand-to-mouth contact with t 

15 with dust or chips of peeling Pb-containing paint, or ingestion of Pb in drinking water 

16 conveyed through Pb pipes. Several study results indicate that exposure to Pb-containing 
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1 paint and home age (often a surrogate for the presence of Pb paint) are important 

2 residential factors that increase risk of elevated blood Pb (Sections 2.9.6.7 and 6.2.6). 

3 Most Pb biomarker studies do not indicate species or isotopic signature, and so non-air 

4 exposures are reviewed in this section because they can also contribute to Pb body 

5 burden. See Section 4.1 for additional information on exposure to ambient Pb.

6 A number of monitoring and modeling techniques have been employed for ambient Pb 

7 exposure assessment. Environmental Pb concentration data can be collected from 

8 ambient air Pb monitors, soil Pb samples, dust Pb samples, and dietary Pb samples to 

9 estimate human exposure. Exposure estimation error depends in part on the collection 

10 efficiency of these methods; collection efficiency for ambient air Pb FRM samplers is 

11 described in Section 3.4. Additionally, high spatial variability of the Pb concentrations in 

12 various media also can contribute to exposure error, as described in the 2009 PM ISA

13 (U.S. EPA, 2009). Models, such as the Integrated Exposure Uptake Biokinetic (IEUBK) 

14 model, simulate human exposure to Pb from multiple sources and through various routes 

15 including inhalation, ingestion, and dermal exposure. IEUBK model inputs include soil 

16 Pb concentration, air Pb concentration, dietary Pb intake including drinking water, Pb 

17 dust ingestion, human activity, and biokinetic factors. Measurements and/or assumptions 

18 can be utilized when formulating the model inputs; error in measurements and 

19 assumptions thus have the potential to propagate through the exposure models. 

20 Section 4.1 presents data illustrating potential exposure pathways. Soil can act as a 

21 reservoir for deposited Pb emissions and exposure to soil contaminated with deposited Pb 

22 can occur through resuspended PM as well as shoe tracking and hand-to-mouth contact, 

23 which is the main pathway of childhood exposure to Pb. Recent data by Yamamoto et al.

24 (2006) have shown that the size distribution of particles collected on children’s hands 

25 have a mode around 40 μm with the upper tail of the distribution extending to 

26 200-300 μm. Infiltration of Pb dust into indoor environments has been suggested, and Pb 

27 dust has been shown to persist in indoor environments even after repeated cleanings. 

28 Measurements of particle-bound Pb exposures reported in this assessment have shown 

29 that personal exposure measurements of Pb concentration are typically higher than indoor 

30 or outdoor ambient Pb concentrations. These findings regarding personal exposure to Pb 

31 dust may be related to the personal cloud effect. 

2.4 Toxicokinetics

32 The majority of Pb in the body is found in bone (roughly 90% in adults, 70% in children); 

33 only about 1% of Pb is found in the blood. Pb in blood is primarily (~99%) bound to red 

34 blood cells (RBCs). It has been suggested that the small fraction of Pb in plasma (<1%) 
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1 may be the more biologically labile and toxicologically active fraction of the circulating 

2 Pb. The relationship between Pb in blood and plasma is pseudo-linear at relatively low 

3 daily Pb intakes (i.e., <10 μg/day–kg) and at blood Pb concentrations <25 μg/dL, and 

4 becomes curvilinear at higher blood Pb concentrations due to saturable binding to RBC 

5 proteins. As blood Pb level increases and the higher affinity binding sites for Pb in RBCs 

6 become saturated, a larger fraction of the blood Pb is available in plasma to distribute to 

7 brain and other Pb-responsive tissues. See Section 4.2 for additional details.

8 The burden of Pb in the body may be viewed as divided between a dominant slow 

9 (i.e., uptake and elimination) compartment (bone) and smaller fast compartment(s) (soft 

10 tissues). Pb uptake and elimination in soft tissues is much faster than in bone. Pb 

11 accumulates in bone regions undergoing the most active calcification at the time of 

12 exposure. During infancy and childhood, bone calcification is most active in trabecular 

13 bone (e.g., patella); whereas, in adulthood, calcification occurs at sites of remodeling in 

14 cortical (e.g., tibia) and trabecular bone (Aufderheide and Wittmers, 1992). A high bone 

15 formation rate in early childhood results in the rapid uptake of circulating Pb into 

16 mineralizing bone; however, in early childhood bone Pb is also recycled to other tissue 

17 compartments or excreted in accordance with a high bone resorption rate (O'Flaherty, 

18 1995). Thus, much of the Pb acquired early in life is not permanently fixed in the bone. 

19 The exchange of Pb from plasma to the bone surface is a relatively rapid process. Pb in 

20 bone becomes distributed in trabecular and the more dense cortical bone. The proportion 

21 of cortical to trabecular bone in the human body varies by age, but on average is about 

22 80% cortical to 20% trabecular. Of the bone types, trabecular bone is more reflective of 

23 recent exposures than is cortical bone due to the slow turnover rate and lower blood 

24 perfusion of cortical bone. Some Pb diffuses to deeper bone regions where it is relatively 

25 inert, particularly in adults. These bone compartments are much more labile in infants 

26 and children than in adults as reflected by half-times for movement of Pb from bone into 

27 the plasma (e.g., cortical half-time = 0.23 years at birth, 3.7 years at 15 years of age, and 

28 23 years in adults; trabecular half-time = 0.23 years at birth, 2.0 years at 15 years of age, 

29 and 3.8 years in adults) (Leggett, 1993). See Section 4.2 for additional details.

30 Evidence for maternal-to-fetal transfer of Pb in humans is derived from cord blood to 

31 maternal blood Pb ratios. Group mean ratios range from about 0.7 to 1.0 at the time of 

32 delivery for mean maternal blood Pb levels ranging from 1.7 to 8.6 μg/dL. Transplacental 

33 transfer of Pb may be facilitated by an increase in the plasma/blood Pb concentration 

34 ratio during pregnancy. Maternal-to-fetal transfer of Pb appears to be related partly to the 

35 mobilization of Pb from the maternal skeleton. See Section 4.2 for additional details.

36 The dominant elimination phase of Pb kinetics in the blood, exhibited shortly after a 

37 change in exposure occurs, has a half-life of ~20-30 days. An abrupt change in Pb uptake 
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1 gives rise to a relatively rapid change in blood Pb, to a new quasi-steady state, achieved 

2 in ~75-100 days (i.e., 3-4 times the blood elimination half-life). A slower phase of Pb 

3 clearance from the blood may become evident with longer observation periods following 

4 a decrease in exposure due to the gradual redistribution of Pb among bone and other 

5 compartments. See Section 4.3 for additional details.

2.5 Lead Biomarkers

6 Overall, trends in blood Pb levels have been decreasing among U.S. children and adults 

7 over the past 20 years (Section 4.4). The median blood Pb level for the entire U.S. 

8 population is 1.2 μg/dL and the 95th percentile blood Pb level was 3.7 μg/dL, based on 

9 the 2007-2008 NHANES data (NCHS, 2010). Among children aged 1-5 years, the 

10 median and 95th percentiles were slightly higher at 1.4 μg/dL and 4.1 μg/dL, 

11 respectively. 

12 Blood Pb is dependent on both the recent exposure history of the individual, as well as 

13 the long-term exposure history that determines body burden and Pb in bone. The 

14 contribution of bone Pb to blood Pb changes depending on the duration and intensity of 

15 the exposure, age, and various other physiological stressors that may affect bone 

16 remodeling (e.g., nutritional status, pregnancy, menopause, extended bed rest, 

17 hyperparathyroidism) beyond that which normally and continuously occurs. In children, 

18 largely due to faster exchange of Pb to and from bone, blood Pb is both an index of recent 

19 exposure and potentially an index of body burden. In adults and children, where exposure 

20 to Pb has effectively ceased or greatly decreased, a slow decline in blood Pb 

21 concentrations over the period of years is most likely due to the gradual release of Pb 

22 from bone. Bone Pb is an index of cumulative exposure and body burden. Even bone 

23 compartments should be recognized as reflective of differing exposure periods with Pb in 

24 trabecular bone exchanging more rapidly than Pb in cortical bone with the blood. This 

25 difference in the compartments making Pb in cortical bone a better marker of cumulative 

26 exposure and Pb in trabecular bone more likely to be correlated with blood Pb, even in 

27 adults. See Section 4.3 for additional details.

28 Sampling frequency is an important consideration when evaluating blood Pb and bone Pb 

29 levels in epidemiologic studies, particularly when the exposure is not well characterized. 

30 It is difficult to determine what blood Pb is reflecting in cross-sectional studies that 

31 sample blood Pb once, whether recent exposure or movement of Pb from bone into blood 

32 from historical exposures. In contrast, cross-sectional studies of bone Pb and longitudinal 

33 samples of blood Pb concentrations over time provide more of an index of cumulative 

34 exposure and are more reflective of average Pb body burdens over time. The degree to 
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1 which repeated sampling will reflect the actual long-term time-weighted average blood 

2 Pb concentration depends on the sampling frequency in relation to variability in 

3 exposure. High variability in Pb exposures can produce episodic (or periodic) oscillations 

4 in blood Pb concentration that may not be captured with low sampling frequencies. 

5 Furthermore, similar blood Pb concentrations in two individuals (or populations), 

6 regardless of their age, do not necessarily translate to similar body burdens or similar 

7 exposure histories. 

8 The concentration of Pb in urine follows blood Pb concentration, in that it mainly reflects 

9 the exposure history of the previous few months and therefore, is likely a relatively poor 

10 index of Pb body burden. There is added complexity with Pb in urine because 

11 concentration is also dependent upon urine flow rate, which requires timed urine samples 

12 that is often not feasible in epidemiologic studies. Other biomarkers have been utilized to 

13 a lesser extent (e.g., Pb in teeth). See Section 4.3.

2.6 Health Effects

14 This section evaluates the evidence from toxicological and epidemiologic studies that

15 examined the health effects associated with exposure to Pb and integrates that evidence 

16 across these disciplines. The results from the health studies are also considered in 

17 combination with the evidence from other disciplines (e.g., toxicokinetics) for the causal 

18 determinations (Section 2.1) made for the health outcomes discussed in this assessment. 

19 In the following sections a discussion of the causal determinations will be presented for 

20 the health effects for which sufficient evidence was available to conclude a causal or 

21 likely to be causal relationship (Table 2-2). A more detailed discussion of the underlying 

22 evidence used to formulate each causal determination can be found in Chapter 5 of this 

23 document.
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Table 2-2 Summary of causal determinationsa between exposure to Pb and 

health outcomes

Outcome Causality Determinationa

Nervous System Effects (Section 5.3.13) Causal Relationship

Cardiovascular Effects (Section 5.4.7) Causal Relationship

Renal Effects (Section 5.5.6) Causal Relationship

Immune System Effects (Section 5.6.8) Causal Relationship

Effects on Heme Synthesis and Red Blood Cell Function (Section 5.7.7) Causal Relationship

Reproductive and Developmental Effects (Section 5.8.10) Causal Relationship

Cancer (Section 5.10.6) Likely Causal Relationship

a

Based upon the framework described in the Preamble, a determination of causality was made for a broad outcome category (i.e., nervous 

system effects) by evaluating the coherence of evidence across disciplines and across a spectrum of related endpoints. However, the 

evidence on which the causal judgment is based, including the strength of evidence for the individual endpoints within the major outcome 

category, is characterized within the discussion. Causal determinations were made within approximately 1-2 orders of magnitude of 

current levels.

2.6.1 Nervous System Effects

1 Recent epidemiologic and toxicological studies substantiated the strong body of evidence 

2 presented in the 2006 Pb AQCD that Pb exposure is associated with nervous system 

3 effects. The weight of epidemiologic and toxicological evidence clearly supports 

4 associations of higher blood Pb levels with decrements in cognitive function in children, 

5 i.e., full-scale IQ and various measures of learning and memory. In epidemiologic 

6 studies, these associations were substantiated in children ages 1 to 11 years and in 

7 populations with mean blood Pb levels between 2 and 7 μg/dL. Observation of a 

8 supralinear concentration-response relationship and associations with mean (or quantile) 

9 blood Pb levels < 2 μg/dL do not provide evidence for a threshold for effects of Pb 

10 exposure on the developing nervous systems of children. Epidemiologic and 

11 toxicological evidence clearly demonstrates Pb-associated increases in behavioral 

12 problems, in particular, inattention and impulsivity (prior symptoms of ADHD). 

13 Associations are substantiated in children ages 1 to 12 years with mean concurrent blood 

14 Pb levels of 2 to 5 μg/dL. In epidemiologic studies, associations with cognitive function 

15 and behavior were observed after adjustment for a range of potential confounding 

16 variables, but most commonly, parental IQ, parental education, and other SES-related 

17 variables.
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1 In animals, the weight of evidence demonstrates effects on cognition and behavior with 

2 prenatal and early postnatal Pb exposures that resulted in blood Pb levels of 10 to 

3 40 μg/dL. In children, the weight of evidence supports cognitive function decrements and 

4 behavioral problems in association with concurrent blood Pb levels. Associations also are 

5 observed with prenatal, early childhood, and childhood average blood Pb levels, thus 

6 uncertainty remains regarding the lifestage of exposure within childhood that is 

7 associated with the greatest risk. The uncertainty regarding the frequency, timing 

8 duration and level of exposure contributing to the observed associations is greater in 

9 studies of older children compared to younger children. The weight of toxicological 

10 evidence demonstrates neurodevelopmental effects on cognition and behavior with 

11 prenatal and early postnatal Pb exposures that can have effects persisting to adulthood. 

12 The biological plausibility for epidemiologic and toxicological findings for effects on 

13 cognitive function and behavior is provided by evidence characterizing underlying 

14 mechanisms, including Pb-induced changes in neurogenesis, synaptogenesis and synaptic 

15 pruning, long term potentiation, and neurotransmitter function. Based most heavily on 

16 cognitive function decrements and inattention in children, the collective body of evidence 

17 integrated across epidemiologic and toxicological studies is sufficient to conclude that 

18 there is a causal relationship between Pb exposures and nervous system effects 

19 (Section 5.3.13).

20 New evidence demonstrates associations of blood Pb level with Attention Deficit 

21 Hyperactivity Disorder (ADHD) in children (8-17 years old) and this evidence is 

22 consistent with findings demonstrating the effect of Pb on inattention and impulsivity in 

23 children. Several new epidemiologic studies indicate associations between higher 

24 concurrent blood Pb level and higher prevalence or incidence of ADHD diagnosis and its 

25 contributing diagnostic indices, whereas previous evidence was inconsistent. The 

26 biological plausibility for associations with ADHD is strongly supported by the large 

27 epidemiologic and toxicological evidence base demonstrating Pb-associated increases in 

28 inattention and impulsivity, both of which are primary symptoms of ADHD.

29 The evidence for Pb-associated effects on other nervous system endpoints was also 

30 evaluated. Compared to the body of evidence on behavior and inattention, a smaller but 

31 equally consistent evidence base indicated associations of concurrent and early childhood 

32 blood Pb levels with social misconduct in children and delinquent behaviors in 

33 adolescents and young adults (Table 5-11). Associations of blood Pb levels with ADHD, 

34 misconduct, and delinquency were observed in populations of children with a wide range 

35 of blood Pb levels, 1 to 11 μg/dL, all similar in the strength of evidence. While the 

36 different behavioral indices are examined, Pb exposure also was found to affect behavior 

37 (decreased ability to escape predators or capture prey) in aquatic and terrestrial species 

38 (Section 7.2 and Section 7.3).
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1 A few new toxicological studies augmented the evidence for Pb-related effects to the 

2 visual system by demonstrating retinal changes in male rodent offspring in association 

3 with lower blood Pb levels (<15 μg/dL) than previously examined (20 to > 100 μg/dL) 

4 (Section 5.3.4.3). A small body of epidemiologic evidence together with a large historical 

5 base of toxicological evidence indicated associations of Pb biomarkers or exposure with 

6 impaired auditory function. Associations were found in children ranging from 4 to 

7 19 years in age and with mean concurrent blood Pb levels of 7-12 μg/dL 

8 (Section 5.3.4.1). While mood and emotional state have been examined less frequently 

9 compared with inattention and misconduct, several studies found associations of 

10 biomarkers of cumulative Pb exposure (i.e., tooth or childhood average blood Pb) and 

11 concurrent blood Pb levels with parental or teacher reports of withdrawn behavior or 

12 depression in children with mean blood Pb levels 8-28 μg/dL (Section 5.3.3.3). These 

13 findings in children are supported by a small body of toxicological studies in which 

14 prenatal plus lactational Pb exposure resulted in depression-like behavior in rodent 

15 models. Studies also reported associations of early childhood average and concurrent 

16 blood Pb levels with lower fine and gross motor function in children ages 3 to 17 years 

17 (Section 5.3.5). A common observation across studies was finding that biomarkers of Pb 

18 exposure were associated with decrements in multiple neurodevelopmental outcomes, 

19 including cognitive function, externalizing behaviors, internalizing behaviors, and motor 

20 function, within the same population. These findings in combination with previously 

21 discussed cognitive and behavioral effects indicate that Pb exposure affects a broad 

22 spectrum of neurodevelopmental effects in children.

23 In adults, the frequency, duration, timing and level of Pb exposure implicated in nervous 

24 system effects remain uncertain. Among occupationally-exposed adults, a spectrum of 

25 nervous system effects is associated with concurrent blood Pb level (≥ 14 μg/dL), which 

26 reflects both current and cumulative exposure. However, in adults without occupational 

27 exposure, cognitive performance is more strongly associated with tibia Pb levels than 

28 blood Pb levels, which indicates an effect of long-term, cumulative Pb exposures. Based 

29 on a smaller body of epidemiologic studies, blood and bone Pb levels were associated 

30 with essential tremor and PD, respectively, in adults (Section 5.3.7.1). However, in these 

31 case-control studies, it is difficult to establish temporality between Pb exposure and 

32 disease. Support for epidemiologic findings for PD is provided by toxicological evidence 

33 for Pb-induced decreased dopaminergic cell activity in the substantia nigra, which 

34 contributes to the primary symptoms of Parkinson’s disease. Whereas evidence for 

35 association with Alzheimer's disease in adults is weak, developmental Pb exposures of 

36 monkeys (early postnatal, PND 1-400) and rats (lactational) has been shown to induce 

37 formation of amyloid plaques, pathology that underlies Alzheimer's disease 

38 (Section 5.3.7.2). A small body of studies of behavior in adults examined and found 
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1 associations of blood and tibia Pb levels with depression and anxiety symptoms 

2 (Section 5.3.3.6). 

2.6.2 Cardiovascular Effects

3 The 2006 Pb AQCD concluded that there was a relationship between higher blood Pb and 

4 bone Pb and cardiovascular effects in adults, in particular increased blood pressure (BP)

5 and increased incidence of hypertension (U.S. EPA, 2006b). Building on this strong body 

6 of evidence, recent epidemiologic and toxicological studies substantiated the evidence 

7 that long-term Pb exposure is associated with cardiovascular effects in adults with the 

8 largest body of evidence demonstrating associations of Pb with increased BP and 

9 hypertension. The epidemiologic evidence is strengthened by several recent prospective 

10 studies that find associations between biomarkers of Pb and BP and hypertension and by 

11 effect estimates that are observed after adjustment for multiple potential confounding 

12 factors. The weight of epidemiologic evidence supported associations in adults with mean 

13 concurrent blood Pb levels less than 5 μg/dL. As these outcomes in epidemiologic studies 

14 were most often observed in adults with likely higher past than current Pb exposures, 

15 uncertainty exists as to the Pb exposure level, timing, frequency, and duration 

16 contributing to the observed associations. Recent epidemiologic studies found that bone 

17 Pb level, a metric of cumulative exposure, is strongly related to hypertension risk in 

18 adults with mean bone Pb levels greater than 20 μg/g. However, uncertainties also exist 

19 as to the specific Pb exposure conditions that contributed to the associations. The weight 

20 of animal evidence also demonstrates an increase in BP after long-term (i.e., greater than 

21 4 weeks in rodents) exposure to Pb. Whereas the majority of studies examined and found 

22 increases in BP in animals with mean blood Pb levels greater than 10 μg/dL (Table 5-18), 

23 a recent study found elevated BP in animals with a mean blood Pb level of 2 μg/dL (Tsao 

24 et al. 2000). Also, animal toxicological studies provided mechanistic evidence to support 

25 the biological plausibility of Pb-induced hypertension, including Pb-induced oxidative 

26 stress, activation of RAAS (renin-angiotensin-aldosterone system), altered sympathetic 

27 activity, and vasomodulator imbalance. Collectively, the evidence integrated across 

28 epidemiologic and toxicological studies as well as across the spectrum of other 

29 cardiovascular endpoints examined is sufficient to conclude that there is a causal 

30 relationship between Pb exposures and cardiovascular health effects (Section 5.4.7). 

31 Studies in the 2006 Pb AQCD (U.S. EPA, 2006b) also found associations between Pb 

32 biomarkers of exposure and other cardiovascular diseases such as IHD, cerebrovascular 

33 disease, peripheral vascular disease, and cardiovascular disease related mortality; 

34 however, the available evidence was limited. Recent epidemiologic and toxicological 

35 studies continue to provide evidence in adults for blood Pb-associated increased 
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1 atherosclerosis, thrombosis, IHD, PAD, arrhythmia, and cardiac contractility in 

2 populations with mean blood Pb levels >2.5 μg/dL (Table 5-19). Further, animal 

3 toxicological evidence continued to build on the evidence supporting the mechanisms 

4 leading to these cardiovascular system responses, as well as Pb-induced changes in BP 

5 and hypertension. Enhanced understanding of Pb-induced oxidative stress including  ̇NO 

6 inactivation, endothelial dysfunction leading to altered vascular reactivity, activation of 

7 the RAAS, and vasomodulator imbalance provides biological plausibility for the 

8 consistent associations observed between higher blood and bone Pb levels and greater 

9 cardiovascular effects. 

10 Recent epidemiologic studies of adults also investigated the interaction of Pb biomarkers 

11 with genetic variants in associations with cardiovascular effects. Evidence was presented 

12 for a larger blood Pb-associated increase in BP in carriers of the ALAD2 allele, which is 

13 associated with greater binding affinity for Pb in the bloodstream (Figure 5-26). 

14 Additionally, bone Pb concentration was associated with larger increases in PP, which is 

15 a good predictor of cardiovascular morbidity and mortality and an indicator of arterial 

16 stiffness, among adults with the HFE H63D and/or C282Y variant and there was 

17 evidence of HFE and transferrin gene variants, related to iron metabolism, impacting the 

18 associations of bone Pb levels with cardiovascular effects, evaluated by QT interval 

19 changes (Figure 5-26). The evidence for genetic factors to potentially increase the risk of 

20 Pb related health effects was limited (Table 6-4). New evidence extended the potential 

21 continuum of Pb-related cardiovascular effects by demonstrating associations of blood Pb 

22 and bone Pb with both cardiovascular and all-cause mortality with follow-up periods 

23 ranging between 8 and 12 years. The associations of Pb with cardiovascular morbidity 

24 observed in both epidemiologic and toxicological studies support recent epidemiologic 

25 findings of increased Pb-associated cardiovascular mortality. Since the populations 

26 enrolled in the epidemiologic studies were largely composed of adults with likely higher 

27 past than current Pb exposures, uncertainty exists as to the Pb exposure level, timing, 

28 frequency, and duration contributing to the observed associations.

2.6.3 Renal Effects

29 The 2006 Pb AQCD concluded that “in the general population, both circulating and 

30 cumulative Pb was found to be associated with a longitudinal decline in renal function,” 

31 evidenced by increased serum creatinine and decreased creatinine clearance or eGFR 

32 over follow-up of 4 to 15 years in association with higher baseline blood and bone Pb 

33 levels (U.S. EPA, 2006b). Uncertainty remained on the contribution of past Pb exposures 

34 to associations observed in adults, the renal effects of Pb in children, and the implication 

35 of hyperfiltration. Because blood Pb level in nonoccupationally-exposed adults reflects 
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1 both recent and past Pb exposures, the magnitude, timing, frequency, and duration of Pb 

2 exposure contributing to the observed associations remains uncertain. 

3 New epidemiologic and toxicological studies evaluated in the current review support or 

4 expand upon the strong body of evidence presented in the 2006 Pb AQCD (U.S. EPA, 

5 2006b). The weight of epidemiologic evidence demonstrates consistently a relationship 

6 between higher blood Pb level and kidney dysfunction (e.g., lower creatinine clearance, 

7 higher serum creatinine, and lower GFR [glomerular filtration rate]) in 

8 nonoccupationally-exposed adults with mean concurrent or baseline blood Pb levels of 

9 2-10 μg/dL. A few analyses find higher blood Pb levels to be associated with a greater 

10 longitudinal decrease in kidney function over time (4-15 years), suggesting that past Pb 

11 exposures may contribute to ongoing renal effects. The epidemiologic evidence is 

12 strengthened by associations between Pb biomarker levels and renal function that are 

13 observed after adjustment for multiple potential confounding factors such as age, sex, 

14 comorbid cardiovascular conditions, BMI (body mass index), smoking, and alcohol use. 

15 Coherence for epidemiologic findings is provided by observations in animal models that 

16 Pb exposure for greater than 6 months decreases GFR and increases serum creatinine. 

17 The weight of evidence in animal studies indicates that Pb induces histopathological 

18 changes, including tubular atrophy and sclerosis. Overall, reduced renal function and 

19 increased kidney damage in animals are observed with chronic Pb (> 4 weeks) exposure 

20 that results in blood Pb levels > 20 μg/dL. By demonstrating Pb-induced renal oxidative 

21 stress, inflammation, mitochondrial dysfunction, apoptosis, and glomerular hypertrophy, 

22 toxicological studies provide biological plausibility for the associations observed in 

23 epidemiologic studies between blood Pb levels and kidney dysfunction. Collectively, the 

24 evidence integrated across epidemiologic and toxicological studies as well as across the 

25 spectrum of renal outcomes is sufficient to conclude that there is a causal relationship 

26 between Pb exposures and renal health effects (Section 5.5.6).

27 In addition, data on the effects of Pb on the kidney in children were reported in a recent 

28 NHANES analysis of adolescents, ages 12-20 years, which observed an association 

29 between higher concurrent blood Pb (mean: 1.5 μg/dL) and lower cystatin C-based eGFR 

30 (Section 5.5.6). These findings are consistent with results from a rodent model study in 

31 which a low dose of Pb (50 ppm) administered from birth resulted in renal impairment 

32 (elevated serum creatinine as compared to control rats), but these observations require 

33 confirmation by measurement of GFR and renal pathology. This recent epidemiologic 

34 study along with several previous studies that included children with higher Pb exposures 

35 (due to residence near sources, Pb poisoning, or parental occupational exposure) provide 

36 evidence that renal function in children may be affected by Pb exposure. The NHANES 

37 adolescents, however, likely had higher Pb exposures earlier in childhood, thus, the 

38 magnitude, timing, frequency, and duration of Pb exposure contributing to the observed 
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1 association is uncertain. Research on the association of Pb with kidney function in the 

2 occupational setting is less consistent than that in environmentally exposed populations 

3 (Section 5.5.2.25.5.2.2). The observation of paradoxical or inverse associations (higher 

4 Pb dose with lower serum creatinine, and/or higher eGFR or calculated or measured 

5 creatinine clearance) in several of these studies reflects limitations of the study design.

2.6.4 Immune System Effects

6 The 2006 Pb AQCD presented consistent evidence for immune system effects associated 

7 with Pb exposure (U.S. EPA, 2006b). Rather than producing overt cytotoxicity or 

8 pathology, Pb exposure was found to be associated with alterations in several subclinical 

9 parameters related to cellular and humoral immunity (Figure 5-42). Recent toxicological 

10 and epidemiologic studies support the strong body of evidence presented in the 2006 Pb 

11 AQCD that Pb exposure may be associated with a broad spectrum of changes in both 

12 cell-mediated and humoral immunity that cumulatively promote a Th2 phenotype and 

13 hyperinflammatory state (U.S. EPA, 2006b). The principal findings are Pb-induced 

14 increased production of Th2 cytokines, suppressed production of Th1 cytokines, 

15 increased inflammation, and elevated IgE, with the weight of evidence provided by 

16 toxicological studies. Collectively, these findings are coherent with the observed effects 

17 of Pb exposure on decreasing responses to antigens (e.g., DTH, bacterial resistance) in 

18 animals. Both toxicological studies in animals and epidemiologic studies in children 

19 provide evidence for Pb-associated increases in IgE. The toxicological and epidemiologic 

20 findings for Th2 cytokines, IgE, and inflammation provide biological plausibility for the 

21 associations observed for blood Pb levels with asthma and allergic conditions in children. 

22 Associations with asthma and allergy were observed after considering potential 

23 confounding by several factors, including, SES and allergen exposure. Animal studies 

24 found a range of immune effects with prenatal exposure in juvenile animals and 

25 long-term postnatal (> 4 weeks) Pb exposures in adult animals. The blood Pb levels and 

26 Pb exposure lifestage, magnitude, frequency, and duration associated with immune 

27 effects are not well characterized in children or adults. Epidemiologic studies of children 

28 and adults primarily examined concurrent blood Pb levels. Little information was 

29 provided on concentration-response functions. In epidemiologic studies, higher IgE and 

30 higher asthma prevalence were examined and found in children with blood Pb levels > 

31 10 μg/dL. The consistency and coherence of findings across the continuum of related 

32 immune parameters that demonstrate a stimulation of Th2 responses in toxicological 

33 studies combined with the supporting epidemiologic evidence in children are sufficient to 

34 conclude that there is a causal relationship between Pb exposures and immune system 

35 effects (Section 5.6.8).
February 2012 2-22 Draft – Do Not Cite or Quote

1 The toxicological evidence for Pb-induced production of Th2 cytokines provides 

2 biological plausibility for the evidence linking Pb exposure with elevated IgE levels 

3 (Section 5.6.8). An increase in IL-4 from activated Th2 cells induces differentiation of B 

4 cells into antibody-producing cells, thereby amplifying B cell expansion to secrete IgE, 

5 IgA, and IgG. However, epidemiologic studies did not find blood Pb level to be 

6 associated consistently with B cell abundance. In addition to T cell responses, a 

7 prominent effect of Pb exposure, as demonstrated in an extensive historical toxicological 

8 evidence base, was the induction of macrophages into a hyperinflammatory state as 

9 characterized by enhanced production of ROS, suppressed production of NO, enhanced 

10 production of TNF-α, and excessive metabolism of arachidonic acid into 

11 immunosuppressive metabolites (e.g., PGE2). Consistent with these observations, a

12 previous epidemiologic study examined and found greater release of ROS and lower 

13 release of NO from macrophages, primarily in children with concurrent blood Pb levels 

14 10.31-47.49 μg/dL (Pineda-Zavaleta et al., 2004). Misregulated inflammation represents 

15 one of the major modes of action for Pb-induced immune effects. Toxicological studies 

16 provide evidence for the modulation of inflammatory cell function, production of 

17 pro-inflammatory cytokines and metabolites, enhanced inflammatory chemical 

18 messengers, and pro-inflammatory signaling cascades. In addition to the associations 

19 reported with IL-4, epidemiologic evidence for Pb effects on inflammation is limited to a 

20 few recent studies in nonoccupationally-exposed adults in which concurrent blood Pb 

21 level was associated with other indicators of inflammation such as CRP and IL-6 

22 (Section 5.6.5.1). The studies commonly adjusted for potential confounding by age, sex, 

23 BMI, and smoking status. However, because only concurrent blood Pb levels in adults 

24 were examined, there is uncertainty regarding the magnitude, timing, frequency, and 

25 duration of Pb exposures that contributed to the observed associations. 

26 The effects of Pb exposure on macrophages also suggest a role for the immune system in 

27 mediating Pb-associated effects in multiple other physiological systems. A small body of 

28 new toxicological studies indicated Pb-induced changes in specialized macrophages in 

29 nonlymphoid tissue such as alveolar macrophages, testicular macrophages, and brain 

30 microglia (Section 5.6.4.5); however, these studies primarily used the i.p. route to 

31 administer Pb. Thus, the relevancy of observations to those expected from typical routes 

32 of human exposure is not clear.

33 In the large body of studies of adults (mostly males) with occupational Pb exposures 

34 (Section 5.6.2.5), the most consistent findings were decreased neutrophil functionality in 

35 workers with mean blood Pb levels 21-71 μg/dL. Recent epidemiologic studies provided 

36 new evidence in adults without occupational Pb exposures; however, each examined a 

37 different immune endpoint, for example, IgE, eNO, IL-6. These endpoints were 

38 associated with concurrent blood Pb levels in populations of adults with mean blood Pb 
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1 levels of 1.9 μg/dL to 7 μg/dL; however, there is uncertainty regarding the contributions 

2 of current Pb exposures and cumulative Pb stores in bone. The small body of 

3 epidemiologic studies of nonoccupationally-exposed adults examined different endpoints 

4 and found associations with concurrent blood Pb levels, which are influenced by current 

5 Pb exposures as well as cumulative Pb stores in bone (Section 5.6).

2.6.5 Heme Synthesis and Red Blood Cell Function

6 The 2006 Pb AQCD reported that Pb affects developing red blood cells (RBCs) in 

7 children and occupationally exposed adults as noted by anemia observed with blood Pb 

8 > 40 μg/dL. Pb-induced anemia is thought to occur due to decreased RBC life span and 

9 effects on hemoglobin (Hb) synthesis. The exact mechanism for these effects was not 

10 known, although Pb-induced changes on iron uptake or inhibition of enzymes in the heme 

11 synthetic pathway may be responsible. Pb was also observed to exert effects on heme 

12 synthesis through the inhibition of multiple key enzymes, most notably ALAD

13 (aminolevulinic acid dehydratase), the enzyme that catalyzes the second, rate-limiting 

14 step in heme biosynthesis (Figure 5-45 presents a schematic representation of the heme 

15 biosynthetic pathway). Decreased RBC ALAD activity was concluded to be the most

16 sensitive measure of human Pb exposure, in that measurement of ALAD activity is 

17 correlated with blood Pb levels. Oxidative stress was identified as an important potential 

18 mechanism of action by which Pb exposure induced effects on RBCs (U.S. EPA, 2006b).

19 Recent epidemiologic and toxicological studies support findings from the 2006 Pb 

20 AQCD. The principal finding regarding RBC survival and function are consistent 

21 Pb-induced alterations in several inter-connected hematological parameters such as Hb

22 (hemoglobin), Hct (hematocrit), and MCV (mean corpuscular volume) across multiple 

23 studies, with the weight of evidence provided by epidemiological studies in 

24 occupationally-exposed adult populations and children. In occupationally-exposed adults, 

25 these findings are most substantiated in populations with current blood Pb levels 

26 > 20 μg/dL, although effects on hematological parameters were observed in some 

27 occupationally-exposed populations at concurrent blood Pb levels in the range of 

28 5-7 μg/dL. The weight of evidence in adult rodents exposed long-term to Pb 

29 (i.e., ≥ 4 weeks) is coherent with epidemiologic studies regarding decrements in 

30 hematological parameters at blood Pb levels as low as 6.6-7.1 μg/dL in rats and mice. 

31 Regarding alterations in heme synthesis, the largest body of evidence again is provided 

32 by decreased ALAD activity in association with Pb exposure or blood Pb levels in 

33 occupationally-exposed adults or in children (3-12 years old), respectively. In the 

34 occupationally-exposed adult populations, the observation of decreased ALAD activity 

35 was most often observed in populations with concurrent blood Pb levels >15 μg/dL. 
February 2012 2-24 Draft – Do Not Cite or Quote

1 Animal toxicological studies also provide to the weight of evidence regarding altered 

2 ALAD activity, with effects seen in adult animals exposed for 3-4 weeks with blood Pb 

3 levels as low as 6.5 μg/dL. The weight of evidence for oxidative stress (i.e., increased 

4 lipid peroxidation or alterations in antioxidant enzyme levels) primarily comes from 

5 epidemiological studies in occupationally-exposed adults and children. The majority of 

6 evidence for increased oxidative stress in Pb-exposed adults comes from occupational 

7 cohorts with concurrent blood Pb levels >15 μg/dL. In children, concurrent blood Pb 

8 levels of 7-22 μg/dL were associated with measures of oxidative stress. The frequency, 

9 timing and duration of exposure necessary to alter RBC survival and function, heme 

10 synthesis, or the state of oxidative stress in RBCs is uncertain in both adults and children. 

11 The consistency of findings in epidemiologic studies investigating effects in 

12 occupationally-exposed adults and in children, and the coherence of findings in the 

13 toxicological literature and coherence across the disciplines, is sufficient to conclude that 

14 there is a causal relationship between Pb exposures and heme synthesis and RBC 

15 function (Section 5.7.7). 

2.6.6 Reproductive and Developmental Effects 

16 Recent toxicological and epidemiologic literature provides strong evidence that Pb 

17 exposure is associated with effects on reproduction and development supporting 

18 conclusions of the 2006 Pb AQCD and expanding evidence for additional endpoints. The 

19 weight of the evidence supports the association of Pb exposure with delayed onset of 

20 puberty in both males and females and detrimental effects on sperm and semen quality in 

21 occupationally-exposed males and in laboratory animals. In cross-sectional 

22 epidemiologic studies of girls (ages 6-18 years) with mean and/or median concurrent 

23 blood Pb levels less than 5 μg/dL consistent associations with delayed pubertal 

24 development (measured by age at menarche, pubic hair development, and breast 

25 development) were observed. Toxicological studies of rodents indicate that prenatal and 

26 lactational exposures to Pb can cause a delay in the onset of female puberty at blood Pb 

27 levels as low as 8 μg/dL. Recent studies show that pubertal onset is one of the more 

28 sensitive markers of Pb exposure with effects observed after maternal exposures leading 

29 to blood Pb levels in the pup of 3.5-13 μg/dL. In boys (ages 8-15 years), fewer studies 

30 were conducted but associations were observed in most. Male animal toxicology studies 

31 have reported delayed sexual maturity as measured with prostate weight, among other 

32 outcomes, seeing significant decrements at blood Pb levels of 34 μg/dL. There is 

33 uncertainty with regard to the exposure frequency, timing, duration and level that 

34 contributed to these observed association in these studies of adolescents. Multiple studies 

35 were performed in areas, which were contaminated with other chemicals as well as Pb. 
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1 Additionally, Pb exposure has been shown to have detrimental effects on sperm. These 

2 were observed in epidemiologic studies at population mean blood Pb levels of 30 μg/dL 

3 and greater among men occupationally exposed (mean blood Pb levels in study controls 

4 around 10 μg/dL) and in animal toxicological studies with rabbits exposed to 

5 subcutaneous Pb 3 times per week for 15 weeks with blood Pb levels of 20 μg/dL. The 

6 collective body of evidence integrated across epidemiologic and toxicological studies 

7 with a focus on the strong relationship observed with detrimental effects on sperm and 

8 delayed pubertal onset is sufficient to conclude that there is a causal relationship between 

9 Pb exposures and reproductive and developmental effects (Section 5.8.10).

10 The evidence on hormonal influences and fecundity are less consistent (Section 5.8.1, 

11 Section 5.8.2). Pb-mediated changes in levels or function of reproductive and growth 

12 hormones have been demonstrated in past and more recent toxicological studies; however 

13 the findings are inconsistent. Recent toxicological studies suggest that oxidative stress is 

14 a major contributor to the toxic effects of Pb on male and female reproductive systems. 

15 The effects of ROS may involve interference with cellular defense systems leading to 

16 increased lipid peroxidation and free radical attack on lipids, proteins, and DNA. Several 

17 recent studies showed an association between increased generation of ROS and germ cell 

18 injury as evidenced by destruction of germ cell structure and function. Co-administration 

19 of Pb with various antioxidant compounds either eliminated Pb-induced injury or greatly 

20 attenuated its effects. In addition, many studies that observed increased oxidative stress 

21 also observed increased apoptosis which is likely a critical underlying mechanism in 

22 Pb-induced germ cell DNA damage and dysfunction. 

23 Consistent conclusions in the 2006 Pb ISA recent studies of pregnancy outcomes were 

24 reported mixed results. Inconsistent evidence of a relationship with Pb was available for 

25 preterm birth and little evidence was available to study the associations with spontaneous 

26 abortions (Section 5.8.6, Section 5.8.3). The 2006 Pb AQCD included a few studies that 

27 reported potential associations between Pb and neural tube defects, but the recent 

28 epidemiologic studies found no association. Some associations were observed between 

29 Pb and low birth weight (Section 5.8.7) when epidemiologic studies used measures of 

30 maternal bone Pb or air exposures, but the associations were less consistent when using 

31 maternal blood Pb or umbilical cord and placenta Pb (maternal blood Pb or umbilical 

32 cord and placenta Pb were the biomarkers most commonly used in studies of low birth 

33 weight). Effects of Pb exposure during early development on toxicological studies 

34 included reduction in litter size, implantation, birth weight and postnatal growth 

35 (Section 5.8.4). Findings from epidemiologic studies of postnatal growth are inconsistent 

36 (Section 5.8.8). Toxicological studies demonstrated that the effects of Pb exposure during 

37 early development include impairment of retinal development and alterations in the 
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1 developing hematopoietic and hepatic systems (Section 5.8.9). Negative developmental 

2 outcomes were also noted including effects on the eyes and teeth (Section 5.8.9).

2.6.7 Cancer 

3 The previous epidemiologic studies included in the 2006 Pb AQCD (U.S. EPA, 2006b)

4 “provide[d] only very limited evidence suggestive of Pb exposure associations with 

5 carcinogenic or genotoxic effects in humans” and the studies were summarized as 

6 follows:

“The epidemiologic data ...suggest a relationship between Pb exposure and cancers of the 

lung and the stomach... Studies of genotoxicity consistently link Pb-exposed populations 

with DNA damage and micronuclei formation, although less consistently with 

chromosomal aberrations.”

7 The International Agency for Research on Cancer (IARC) classified inorganic Pb 

8 compounds as probable human carcinogens (Group 2A of IARC classifications) based on 

9 stronger evidence in animal studies than human studies, and organic Pb compounds as 

10 not classifiable (Group 3 of IARC classifications) (IARC, 2006a; Rousseau et al., 2005).

11 Additionally, the National Toxicology Program has listed Pb and Pb compounds as 

12 “reasonably anticipated to be human carcinogens” (NTP, 2004). The typical cancer 

13 bioassays used by IARC or NTP as evidence of Pb-induced carcinogenicity used rodents 

14 that were continuously exposed to Pb-acetate in chow or drinking water for 18 months to 

15 two years in duration. These two year cancer bioassays and the doses administered are 

16 typical of cancer bioassays used with other chemicals.

17 The animal toxicological literature continues to provide the strongest evidence for Pb 

18 exposure and cancer with some supporting evidence provided by the epidemiologic 

19 literature. Evidence from toxicological studies demonstrates an association between Pb 

20 and cancer, genotoxicity/clastogenicity or epigenetic modification. Carcinogenicity in 

21 historical animal toxicology studies with Pb exposure has been reported in the kidneys, 

22 testes, brain, adrenals, prostate, pituitary, and mammary gland, albeit at high doses of Pb. 

23 Epidemiologic studies of cancer incidence and mortality reported inconsistent results; one 

24 large epidemiologic study demonstrated an association between blood Pb and increased 

25 cancer mortality, but the other studies reported weak or no associations. In the 2006 Pb 

26 AQCD, Pb exposure was found to be associated with stomach cancer, but there was only 

27 one recent study on stomach cancer and Pb exposure, which reported mixed findings. 

28 Similarly, some studies in the 2006 Pb AQCD reported associations between Pb exposure 

29 and lung cancer (U.S. EPA, 2006b). More recent occupational studies of Pb exposure and 

30 lung cancer reported no associations. The majority of epidemiologic studies of brain 

31 cancer had null results overall, but positive associations between Pb exposure and brain 
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1 cancer were observed among individuals with certain genotypes (e.g., ALAD2). In 

2 toxicological studies, chromosomal aberrations after Pb exposure are most often reported 

3 with Pb chromate exposure, which is likely due to toxicity of the chromate moiety. 

4 Mechanistic understanding of Pb and its effect on cancer and genotoxicity is expanding 

5 through toxicological work focusing on antioxidants and other proteins that sequester Pb 

6 or reduce its bioavailability. The collective body of evidence integrated across 

7 toxicological and epidemiologic studies is sufficient to conclude that there is a likely 

8 causal relationship between Pb exposure and cancer (Section 5.10.6). 

2.7 Ecological Effects of Lead

9 This section evaluates the evidence from studies of ecological effects associated with 

10 exposure to Pb. Causal determinations are developed for the ecological outcomes 

11 discussed in this assessment, in combination with evidence from other disciplines 

12 (e.g., fate and transport) where relevant. Pb effects on terrestrial and aquatic systems 

13 (Section 2.7.1 and Section 2.7.2) are summarized from Chapter 7 in which the effects on 

14 terrestrial and aquatic ecosystems are presented separately. These sections are followed 

15 by a summary of the evidence for the causal determinations (Section 2.7.3) and 

16 consideration of atmospheric deposition of Pb as related to ecological effects 

17 (Section 2.9.3). The ecological causal determinations are integrated across endpoints 

18 (physiological stress, hematological effects, neurobehavioral effects, development and 

19 reproduction, growth, survival) common to both terrestrial and aquatic biota (Table 2-4). 

20 Where the causal determination varies substantially between types of organisms 

21 (typically between plants and other organisms or between aquatic and terrestrial biota), 

22 the divergence is noted. 

2.7.1 Summary of Effects on Terrestrial Ecosystems

23 Historically, Pb poisoning is one of the earliest recognized toxicoses of terrestrial biota, 

24 occurring primarily through ingestion of spent shot by birds (Section 7.4.2.3). At the time 

25 of the 1977 Pb AQCD few studies of Pb exposure and effects in wild animals other than 

26 birds were available. A limited number of rodent trapping studies and observations from 

27 grazing animals near smelters provided evidence for differences in Pb sensitivity among 

28 species and these findings were further supported in the 1986 and 2006 Pb AQCDs (U.S. 

29 EPA, 2006b, 1986b, 1977). ALAD was recognized as a sensitive indicator of Pb 

30 exposure in rats and waterfowl in the 1977 Pb AQCD and now is regarded as a biomarker 

31 of exposure in many terrestrial organisms. According to the 2006 Pb AQCD and 

32 supported by evidence from previous Pb AQCDs, commonly observed effects of Pb on 
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1 terrestrial organisms include decreased survival, reproduction, and growth, as well as 

2 effects on development and behavior. Furthermore, the toxicity of Pb to terrestrial 

3 vertebrates and invertebrates varies with bioavailability of the metal, species and 

4 lifestage, duration of exposure, form of Pb, and soil characteristics.

5 In plants, Pb effects have also been studied for several decades. At the time of the 1977 

6 Pb AQCD, it was understood that Pb uptake in plants was influenced by plant species and 

7 by the available Pb pool in the soils (U.S. EPA, 1977). Furthermore, most of the Pb taken 

8 up by plants from soil, remains in the roots and that distribution to other portions of the 

9 plant is variable among species (U.S. EPA, 1977). Plant growth was recognized as an 

10 endpoint of Pb toxicity in plants in the 1977 Pb AQCD and additional effects of Pb on 

11 growth processes were reported in subsequent Pb AQCDs (U.S. EPA, 2006b, 1986b, 

12 1977). A study reviewed in the 1977 Pb AQCD provided evidence for Pb effects on 

13 forest-nutrient cycling and shifts in arthropod community composition in the vicinity of a 

14 smelting complex. A number of ecosystem-level effects, including decreased species 

15 diversity, changes in floral and faunal community composition, and decreasing vigor of 

16 terrestrial vegetation have subsequently been reported near Pb-point sources (U.S. EPA, 

17 2006b, 1986b, 1977; Watson et al., 1976).

18 Pb in terrestrial ecosystems is either deposited directly onto plant surfaces, or 

19 incorporated into soil where it can bind with organic matter or dissolve in pore water. The 

20 amount of Pb dissolved in soil pore water determines the impact of soil Pb on terrestrial 

21 ecosystems to a much greater extent than the total amount present. It has long been 

22 established that the amount of Pb dissolved in soil solution is controlled by at least six 

23 variables: (1) solubility equilibria; (2) adsorption-desorption relationship of total Pb with 

24 inorganic compounds; (3) adsorption-desorption reactions of dissolved Pb phases on soil 

25 organic matter; (4) pH; (5) cation exchange capacity (CEC); and (6) aging. Since 2006, 

26 further details have been contributed to the understanding of the role of pH, CEC, organic 

27 matter, and aging. Smolders et al. (2009) demonstrated that the two most important 

28 determinants of both Pb solubility and toxicity in soils are pH and CEC. However, they 

29 had previously shown that experimental aging, primarily in the form of initial leaching 

30 following addition of Pb, decreases soluble metal fraction by approximately one order of 

31 magnitude (Smolders et al., 2009). Since 2006, organic matter has been confirmed as an 

32 important influence on Pb sequestration, leading to longer-term retention in soils with 

33 higher organic matter content, and also creating the potential for later release of deposited 

34 Pb. Aging, both under natural conditions and simulated through leaching, was shown to 

35 substantially decrease bioavailability to plants, microbes, and vertebrates.

36 There is evidence over several decades of research previously reviewed in Pb AQCDs 

37 and in recent studies reviewed in this ISA that Pb accumulates in terrestrial plants, 
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1 invertebrates and vertebrates. Studies with herbaceous plant species growing at various 

2 distances from smelters added to the existing strong evidence that atmospherically 

3 transported Pb is taken up by plants. These studies did not establish the relative 

4 proportion that originated from atmospheric Pb deposited in the soil, as opposed to that 

5 taken up directly from the atmosphere through the leaves. Multiple new studies showed 

6 that in trees, the latter is likely to be very substantial. One study attempted to quantify the 

7 proportion of atmospheric Pb up directly from the atmosphere, and suggested it amounts 

8 to 50% of the Pb contained in Scots Pine in Sweden (Klaminder et al., 2005). Studies 

9 with herbaceous plants found that in most species tested, soil Pb taken up by the roots is 

10 not translocated into the stem and leaves. Studies with trees found that soil Pb generally 

11 is translocated to other parts. 

12 Since the 2006 Pb AQCD, various species of terrestrial snails have been found to 

13 accumulate Pb from both diet and soil (U.S. EPA, 2006b). New studies with earthworms 

14 have found that both internal concentration of Pb and mortality increase with decreasing 

15 soil pH and CEC. In addition, tissue concentration differences have been found between 

16 species of earthworms that burrow in different soil layers. The rate of accumulation in 

17 each of these species may result from differences in interacting factors such as pH and 

18 CEC between layers. Because earthworms often sequester Pb in granules, some authors 

19 have suggested that earthworm Pb is not bioavailable to their predators. There is some 

20 evidence that earthworm activity increases Pb availability in soil, but it is inconsistent. In 

21 various arthropods collected at contaminated sites, recent studies found gradients in 

22 accumulated Pb that corresponded to gradients in soil with increasing distance from point 

23 sources.

24 There are a few new studies of Pb bioavailability and uptake in birds since the 2006 Pb 

25 AQCD (U.S. EPA, 2006b). Several found tissue levels in birds that indicated exposure to 

26 Pb, but none of the locations for these studies was in proximity to point sources, and the 

27 origin of the Pb could not be identified. A study at the Anaconda Smelter Superfund site 

28 found increasing Pb accumulation in gophers with increasing soil Pb around the location 

29 of capture. A study of swine fed various Pb-contaminated soils showed that the form of 

30 Pb determined accumulation. New studies were able to measure Pb in the components of 

31 various food chains that included soil, plants, invertebrates, arthropods and vertebrates. 

32 They confirmed that trophic transfer of Pb is pervasive, but no consistent evidence of 

33 trophic magnification was found.

34 Evidence in this review further supports the findings of the previous Pb AQCDs that 

35 biological effects of Pb on terrestrial organisms vary with species and lifestage, duration 

36 of exposure, form of Pb, and soil characteristics. In photosynthetic organisms,

37 experimental studies have added to the existing evidence of photosynthesis impairment in 
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1 plants exposed to Pb, and have found damage to photosystem II due to alteration of 

2 chlorophyll structure, as well as decreases in chlorophyll content in diverse taxa,

3 including lichens and mosses. Evidence of oxidative stress in response to Pb exposure has 

4 also been observed in plants. Reactive oxygen species were found to increase in broad 

5 bean and tomato plants exposed to increasing concentrations of soil Pb, and a 

6 concomitant increase in superoxide dismutase, glutathione, peroxidases, and lipid 

7 peroxidation, as well as decreases in catalase were observed in the same plants. Monocot, 

8 dicot, and bryophytic taxa grown in Pb-contaminated soil or in experimentally spiked soil 

9 all responded to increasing exposure with increased antioxidant activity. In addition, 

10 reduced growth was observed in some experiments, as well as genotoxicity, decreased 

11 germination, and pollen sterility.

12 In terrestrial invertebrates, evidence for Pb effects has included neurological and 

13 reproductive endpoints. Recently published studies have shown neuronal damage in 

14 nematodes exposed to low concentrations of Pb (2.5 μM), accompanied by behavioral 

15 abnormalities. Reproductive adverse effects were found at lower exposure in younger 

16 nematodes, and effects on longevity and fecundity were shown to persist for several 

17 generations. Increased mortality was found in earthworms, but was strongly dependent on 

18 soil characteristics including pH, CEC, and aging. Snails exposed to Pb through either 

19 topical application or through consumption of Pb-exposed plants had increased 

20 antioxidant activity, and decreased food consumption, growth, and shell thickness. 

21 Effects on arthropods exposed through soil or diet varied with species and exposure 

22 conditions, and included diminished growth and fecundity, endocrine and reproductive 

23 anomalies, and body malformations. Within each study, increasing concentration of Pb in 

24 the exposure medium generally resulted in increased effects, but the relationship between 

25 concentration and effects varied between studies, even when the same medium, e.g., soil, 

26 was used. Evidence suggested that aging and pH are important modifiers.

27 Effects on amphibian and reptiles included decreased white blood cell counts, decreased 

28 testis weight, and behavioral anomalies. However, large differences in effects were 

29 observed at the same concentration of Pb in soil, depending on whether the soil was 

30 freshly amended or field-collected from contaminated areas. As in most studies where the 

31 comparison was made, effects were smaller when field-collected soils were used. In some 

32 birds, maternal elevated blood Pb level was associated in recent studies with decreased 

33 hatching success, smaller clutch size, high corticosteroid level, and abnormal behavior. 

34 Some species evidenced little or no effect of elevated blood Pb level. Effects of dietary 

35 exposure were studied in several mammalian species, and cognitive, endocrine, 

36 immunological, and growth effects were observed. 
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1 New evidence reviewed in Section 7.2.3 and Section 7.2.4 demonstrates that exposure to 

2 Pb is generally associated with negative effects in terrestrial ecosystems. It also 

3 demonstrates that many factors, including species and various soil physiochemical 

4 properties, interact strongly with Pb concentration to modify those effects. In these 

5 ecosystems, where soil is generally the main component of the exposure route, Pb aging 

6 is a particularly important factor, and one that may be difficult to reproduce 

7 experimentally. Without quantitative characterization of those interactions, 

8 characterizations of exposure-response relationships would likely not be transferable 

9 outside of experimental settings. Since the 2006 Pb AQCD, a few studies of 

10 exposure-response have been conducted with earthworms, and results have been 

11 inconsistent (U.S. EPA, 2006b).

12 New evidence of effects of Pb at the community and ecosystem levels of biological 

13 organization include several studies of the ameliorative effects of mycorrhizal fungi on 

14 plant growth in the presence of Pb, attributed to decreased uptake of Pb by plants, 

15 although both mycorrhizal fungus and plant were negatively affected. Most recently 

16 published research on community and ecosystem-level effects of Pb has focused on soil 

17 microbial communities, which have been shown to be impacted in both composition and 

18 activity. Many recent studies have been conducted using mixtures of metals, but have 

19 tried to separate the effects of individual metals when possible. Soil microbial activity 

20 was generally diminished, but in some cases recovered over time. Species and genotype 

21 composition were consistently altered, and those changes were long-lasting or permanent. 

22 Recent studies have addressed differences in sensitivity between species explicitly, and 

23 have clearly demonstrated high variability between related species, as well as within 

24 larger taxonomic groupings. Mammalian no observed effect concentration (NOEC) 

25 values expressed as blood Pb levels were shown to vary by a factor of 8, while avian 

26 blood NOECs varied by a factor of 50 (Buekers et al., 2009). Protective effects of dietary 

27 Ca have been found in plants, birds, and invertebrates.

2.7.2 Summary of Effects on Aquatic Ecosystems

28 Pb effects on aquatic biota were previously assessed in the 1977 Pb AQCD, the 1986 Pb 

29 AQCD and the 2006 Pb AQCD (U.S. EPA, 2006b, 1986a, 1977). Evidence of toxicity of 

30 Pb and other metals to freshwater organisms goes back to early observations of 

31 contamination of natural areas by Pb mining leading to extirpation of fish from streams

32 (U.S. EPA, 1977). Observed responses of fish to Pb reported in the 1986 Pb AQCD and 

33 the 2006 Pb AQCD include inhibition of heme formation, alterations in brain receptors, 

34 effects on blood chemistry and hormonal systems, and decreases in some enzyme 

35 activities (U.S. EPA, 2006b, 1986a). In the 1986 Pb AQCD and 2006 Pb AQCD, 
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1 additional evidence for Pb toxicity was available for aquatic invertebrates. Inhibition of 

2 ALAD, and reduced reproduction, growth, and survival were reported. Few studies of Pb 

3 toxicity to saltwater organisms are reported in previous Pb AQCDs. The toxicity of Pb to 

4 saltwater and freshwater organisms varies with bioavailability of the metal, species and 

5 lifestage, duration of exposure, form of Pb, and water quality parameters (U.S. EPA, 

6 2006b, 1986a). As reviewed in the 2006 Pb AQCD, the biotic ligand model (BLM) 

7 constitutes an attractive means of quantifying factors that affect bioavailability, such as 

8 pH, dissolved organic carbon (DOC), and alkalinity (U.S. EPA, 2006b).

9 The toxicity of Pb to aquatic algae and plants has been recognized in earlier agency 

10 reviews of this metal. In the 1977 Pb AQCD, differences in sensitivity to Pb among 

11 different species of algae were reported and concentrations of Pb varied among genera 

12 and within a genus, an observation subsequently found across aquatic taxa (U.S. EPA, 

13 1977). At the time of the 1977 Pb AQCD, the information available on effects of Pb on 

14 aquatic plants was limited. For plants in general, Pb was recognized to affect 

15 photosynthesis, mitosis, and growth, but at concentrations higher than typically found in 

16 the environment (U.S. EPA, 1977). Findings from field studies of aquatic communities in 

17 the vicinity of Pb point sources include changes in species composition and species 

18 richness, predator/prey interactions, nutrient cycling and energy flow; however, Pb is 

19 often found coexisting with other metals and other stressors, which risk confounding the 

20 observed effects. 

21 Effects of Pb observed in aquatic organisms are tied to terrestrial systems via watershed 

22 processes (Section 3.3). Atmospherically-derived Pb can enter aquatic systems through 

23 runoff from terrestrial systems or via direct deposition over a water surface. Once Pb 

enters surface waters, its fate and bioavailability are influenced by Ca2+ 24 concentration, 

25 pH, alkalinity, total suspended solids, and dissolved organic carbon (DOC, including 

26 humic acids). In sediments, Pb bioavailability may be influenced by the presence of other 

27 metals, sulfides, Fe and Mn oxides and physical disturbance. In many, but not all aquatic 

28 organisms, Pb dissolved in the water can be the primary exposure route to gills or other 

29 biotic ligands. As recognized in the 2006 Pb AQCD and further supported in this review, 

30 chronic exposures to Pb may also include dietary uptake, and there is an increasing body 

31 of evidence showing that differences in uptake and elimination of Pb vary with species. 

32 Currently available models for predicting bioavailability focus on acute toxicity and do 

33 not consider all possible routes of uptake. They are therefore of limited applicability, 

34 especially when considering species-dependent differences in uptake and 

35 bioaccumulation of Pb. 

36 Recent evidence supports the 2006 Pb AQCD conclusion that processes such as Pb 

37 adsorption, complexation, and chelation alter bioavailability to aquatic biota. Given the 
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1 low solubility of Pb in water, bioaccumulation by aquatic organisms may preferentially 

2 occur via exposure routes other than direct absorption from the water column; which 

3 include ingestion of contaminated food and water, uptake from sediment pore waters, or 

4 incidental ingestion of sediment (U.S. EPA, 2006b).

5 There are considerable differences between species in both the amount of Pb taken up 

6 from the environment and in the amount retained in the organism. Closely related species 

7 can vary greatly in bioaccumulation of Pb and other non-essential metals. Recent studies 

8 on uptake of Pb by aquatic plants and algae support the findings of previous Pb AQCDs 

9 that all plants tend to sequester larger amounts of Pb in their roots than in their shoots, 

10 and provide additional evidence for species differences in compartmentalization of 

11 sequestered Pb and in responses to Pb in water and sediments. In invertebrates, Pb can be 

12 accumulated from multiple sources, including the water column, sediment, and dietary 

13 exposure. Since the last review, new studies using stable isotopes have enabled 

14 simultaneous measurement of uptake and elimination in several aquatic organisms to 

15 assess the relative importance of water versus dietary uptake. In uptake studies of various 

16 invertebrates, Pb was mainly found in the gills and digestive gland/hepatopancreas. There 

17 is more information now on the cellular and subcellular distribution of Pb in invertebrates 

18 than there was at the time of writing the 2006 Pb AQCD. Specifically, localization of Pb 

19 at the ultrastructural level has been assessed in several species.

20 Recent evidence also supports the 2006 conclusions that the gill is a major site of Pb 

21 uptake in fish, and that there are species differences in the rate of Pb accumulation and 

22 distribution of Pb within the organism are supported in this review. The anterior intestine 

23 has been newly identified as a site of uptake of Pb through dietary exposure studies. 

24 There are few new studies on Pb uptake by amphibians and mammals. At the time of the 

25 publication of the 2006 Pb AQCD, trophic transfer of Pb through aquatic food chains was 

26 considered to be negligible. Measured concentrations of Pb in the tissues of aquatic 

27 organisms were generally higher in algae and benthic organisms than in consumers at 

28 higher trophic levels, indicating that Pb was bioconcentrated but not biomagnified. Some 

29 studies published since the 2006 Pb AQCD support the potential for transfer of Pb in 

30 aquatic food webs, while other studies indicate that Pb concentration decreases with 

31 increasing trophic level (U.S. EPA, 2006b).

32 Evidence in this ISA further supports the findings of the previous Pb AQCDs that 

33 waterborne Pb is highly toxic to aquatic organisms, with toxicity varying with species 

34 and lifestage, duration of exposure, form of Pb, and water quality characteristics. Effects 

35 of Pb on algae reported in the 2006 Pb AQCD are further supported by evidence from 

36 additional species. They include decreased growth, deformation and disintegration of 

37 cells, and blocking of the pathways that lead to pigment synthesis, thus affecting 
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1 photosynthesis. Effects on plants supported by additional evidence in this review include 

2 oxidative damage, decreased photosynthesis and reduced growth. Elevated levels of 

3 antioxidant enzymes are commonly observed in aquatic plant, algae, and moss species 

4 exposed to Pb. 

5 Since the 2006 Pb AQCD, there is additional evidence for Pb effects on antioxidant 

6 enzymes, lipid peroxidation, stress response and osmoregulation in aquatic invertebrates. 

7 Studies of reproductive and developmental effects of Pb in this review provide further 

8 support for findings in the 2006 Pb AQCD. These new studies include reproductive 

9 endpoints for rotifers and freshwater snails as well as multigenerational effects of Pb in 

10 mosquito larvae and marine amphipods. As reviewed below, growth effects are observed 

11 at lower concentrations in some aquatic invertebrates since the 2006 Pb AQCD, 

12 especially in juvenile organisms. Behavioral effects of Pb in aquatic invertebrates 

13 reviewed in this ISA include decreased valve closing speed in scallops and slower 

14 feeding rate in blackworms.

15 Evidence in this ISA supports the findings of reproductive, behavioral, and growth effects 

16 in previous Pb AQCDs, as well as effects on blood parameters in aquatic vertebrates. 

17 Since the 2006 Pb AQCD, possible molecular targets for Pb neurotoxicity have been 

18 identified in fish and additional mechanisms of Pb toxicity have been elucidated in the 

19 fish gill and the fish renal system. In the 2006 Pb AQCD, amphibians were considered to 

20 be relatively tolerant to Pb. Observed responses to Pb exposure included decreased 

21 enzyme activity (e.g., ALAD reduction) and changes in behavior. Since the 2006 Pb 

22 AQCD, studies conducted at concentrations approaching environmental levels of Pb have 

23 indicated sublethal effects on tadpole endpoints including growth, deformity, and 

24 swimming ability. In fish, several recent studies on behavioral effects of Pb indicate 

25 decreased prey capture rate, slower swim speed and decline in startle response and visual 

26 contrast with Pb exposure.

27 Concentration-response data from plants, invertebrates and vertebrates are consistent with 

28 findings in previous AQCDs of species differences in sensitivity to Pb in aquatic systems 

29 (Section 7.3.17.4). In this ISA, as in previous AQCDs, aquatic plant growth was shown to 

30 be adversely affected by Pb exposure. The lowest EC50 for growth observed in marine 

31 microalgae and freshwater microalgae was in the range of 100 μg Pb/L. In the 2006 Pb 

32 AQCD, concentrations at which effects were observed in aquatic invertebrates ranged 

33 from 5 to 8,000 μg Pb/L. Several studies in this review have provided evidence of effects 

34 at lower concentrations. Among the most sensitive species, growth of juvenile freshwater 

35 snails (L. stagnalis) was inhibited at an EC20 of <4 μg Pb/L (Grosell and Brix, 2009; 

36 Grosell et al., 2006a). A chronic value of 10 μg Pb/L obtained in 28-day exposures of 

37 2-month-old Lampsilis siliquoidea juveniles was the lowest genus mean chronic value 
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1 ever reported for Pb (Wang et al., 2010e). In a series of 48 hour acute toxicity tests using 

2 a variety of natural waters across North America, LC50 values ranged from 29 to 180 μg 

3 Pb/L tests with the cladoceran Ceriodaphnia dubia (Esbaugh et al., 2011).

4 In the 2006 Pb AQCD, adverse effects were found in freshwater fish at concentrations 

5 ranging from 10 to >5,400 μg Pb/L, generally depending on water quality variables 

6 (e.g., pH, hardness, salinity). Additional testing of Pb toxicity under conditions of varied 

7 alkalinity, DOC, and pH has been conducted since the last review. However, adverse 

8 effects in fish observed in recent studies fall within the range of concentrations observed 

9 in the previous Pb AQCD.

10 Since the 2006 Pb AQCD, additional evidence for community and ecosystem level 

11 effects of Pb have been observed primarily in microcosm studies or field studies near 

12 point sources (mining, effluent) with other metals present. Ecological effects associated 

13 with Pb, reported in previous Pb AQCDs, include alteration of predator-prey dynamics, 

14 species richness, species composition, and biodiversity. New studies in this ISA provide 

15 evidence in additional habitats for these community and ecological-level effects, 

16 specifically in aquatic macrophyte communities and sediment-associated communities. 

17 Different species may exhibit different responses to Pb-impacted ecosystems dependent 

18 not only upon other environmental factors (e.g., temperature, pH), but also on the species 

19 sensitivity, lifestage, or seasonally-affected physiological state. Aquatic ecosystems with 

20 low pH and low DOM are likely to be the most sensitive to the effects of 

21 atmospherically-deposited Pb.
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2.7.3 Determinations of Causality for Effects on Ecosystems

Table 2-3 Summary of Pb causal determinations for plants, invertebrates and 

vertebrates

Effect Terrestrial Aquatica

Physiological Stress-All organisms Causal Causal

Hematological Effects-Invertebrates Inadequate Causal

Hematological Effects-Vertebrates Causal Causal

Neurobehavioral Effects-Invertebrates and Vertebrates Likely Causal Likely Causal

Developmental and Reproductive Effects-Plants Inadequate Inadequate

Developmental and Reproductive Effects-Invertebrates and Vertebrates Causal Causal

Growth-Plants Causal Causal

Growth-Invertebrates Inadequate Causal

Growth-Vertebrates Inadequate Inadequate 

Survival-Plants Inadequate Inadequate

Survival- Invertebrates and Vertebrates Causal Causal 

Community and Ecosystem Level Effects Likely Causal Likely Causal

a

Causal determinations for aquatic biota are based primarily on evidence from freshwater organisms.

2.7.3.1 Effects on Physiological Stress

1 Evidence is sufficient to conclude that there is a causal relationship between Pb 

2 exposures and physiological stress in terrestrial and aquatic plants, invertebrates and 

3 vertebrates (Sections 7.4.1.1 and 7.4.2.1). 

4 Endpoints associated with physiological stress received no consideration prior to the 

5 2006 Pb AQCD and this ISA. Studies published since the 2006 Pb AQCD support 

6 previous associations of Pb exposure with physiological stress. New evidence includes 

7 upregulation of antioxidant enzymes, production of reactive oxygen species and 

8 increased lipid peroxidation associated with Pb exposure in additional species of 

9 terrestrial and aquatic plants, invertebrates and vertebrates that support, and expand upon 

10 findings in the previous Pb AQCD (U.S. EPA, 2006b).

11 In this ISA and the 2006 Pb AQCD, there is strong evidence of upregulation of 

12 antioxidant enzymes and increased lipid peroxidation associated with Pb exposure in 

13 many species of plants, invertebrates and vertebrates. In plants, increases of antioxidant 
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1 enzymes with Pb exposure occur in algae, aquatic mosses, floating and rooted aquatic 

2 macrophytes, and terrestrial species. Most observations of antioxidant responses in plants 

3 typically occur at concentrations of Pb higher than found in the environment. However, 

4 in a few terrestrial plant species, increases of antioxidant enzymes occur at concentrations 

5 approaching the average Pb concentrations in U.S. soils and limited transplantation 

6 studies with aquatic plants indicate elevated antioxidant enzyme activity associated with 

7 Pb levels measured in sediments at polluted sites. There is considerable evidence for 

8 antioxidant activity in invertebrates, including gastropods, mussels, and crustaceans, in 

9 response to Pb exposure. Some recent evidence for invertebrate antioxidant responses in 

10 aquatic species indicates effects at Pb concentrations associated with polluted sites. 

11 Markers of oxidative damage are also observed in fish, amphibians and mammals, both in 

12 the laboratory and in exposed natural environments. Across all biota, there are differences 

13 in the induction of antioxidant enzymes that appear to be species-dependent.

14 Additional stress responses observed in terrestrial and aquatic invertebrates include 

15 elevated heat shock proteins, osmotic stress and decreased glycogen levels. Heat shock 

16 protein induction by Pb exposure has been observed in zebra mussels and mites. Tissue 

17 volume regulation is adversely affected in freshwater crabs. Glycogen levels in the 

18 freshwater snail Biomphalaria glabrata were significantly decreased following 96-hour 

19 exposures at near environmentally-relevant concentrations (50 μg Pb/L and higher)

20 (Ansaldo et al., 2006).

21 Upregulation of antioxidant enzymes and increased lipid peroxidation are considered to 

22 be reliable biomarkers of stress, and suggest that Pb exposure induces a stress response in 

23 those organisms, which may increase susceptibility to other stressors and reduce 

24 individual fitness. The oxidative stress responses associated with Pb exposure are 

25 consistent in terrestrial biota and in aquatic organisms. Furthermore, these responses are 

26 also observed in experimental animal studies, and in humans.

2.7.3.2 Hematological Effects

27 Based on observations in both terrestrial and aquatic organisms and additionally 

28 supported by toxicological and epidemiological findings in laboratory animals and 

29 humans, evidence is sufficient to conclude that there is a causal relationship between Pb 

30 exposures and hematological effects in aquatic invertebrates and terrestrial and aquatic 

31 vertebrates (Section 7.4.1.2, Section 7.4.2.2). The evidence is inadequate to conclude that 

32 there is a causal relationship between Pb exposures and hematological effects in 

33 terrestrial invertebrates.
February 2012 2-38 Draft – Do Not Cite or Quote

1 Recent studies add support to the strong body of evidence presented in previous Pb 

2 AQCDs that Pb exposure is associated with hematological responses in aquatic 

3 invertebrates and terrestrial and aquatic vertebrates. In environmental assessments of 

4 metal-impacted habitats, ALAD is a recognized biomarker of Pb exposure (U.S. EPA, 

5 2006b). ALAD activity is negatively correlated with total Pb concentration in bivalves. 

6 Lower ALAD activity has been significantly correlated with elevated blood Pb levels in 

7 fish and mammals as well. In the 1986 Pb AQCD, decreases in RBC ALAD activity 

8 following Pb exposure were well documented in birds and mammals (U.S. EPA, 1986a).

9 Further evidence from the 2006 Pb AQCD and this review suggests this enzyme is an 

10 indicator for Pb exposure across a wide range of taxa. Since the 2006 Pb AQCD, 

11 evidence of Pb effects on ALAD activity has been found in additional species of 

12 amphibians and fish, and has been identified in bacteria. New field studies of ALAD 

13 activity include observations in songbirds and owls near historical mining areas. In 

14 addition to consideration of ALAD activity, there is new evidence for deceased white 

15 blood cell counts in amphibians affected by Pb exposure. The consistency and coherence 

16 of these findings of effects on ALAD activity are also supported by some evidence of 

17 Pb-induced alterations of blood chemistry in fish reported in the 2006 Pb AQCD (U.S. 

18 EPA, 2006b). This evidence is strongly coherent with observations from human 

19 epidemiologic and animal toxicology studies where a causal relationship was identified 

20 between exposure to Pb and hematological effects in humans and laboratory animals 

21 (Section 2.6.5 and Section 5.7). 

2.7.3.3 Neurobehavioral Effects

22 Overall, the evidence from terrestrial and aquatic systems is sufficient to conclude that 

23 there is a likely causal relationship between Pb exposures and neurobehavioral effects in 

24 terrestrial and aquatic invertebrates and vertebrates (Section 7.4.1.3, Section 7.4.2.3). 

25 Evidence from laboratory studies reviewed in Chapter 7 and previous Pb AQCDs have 

26 shown adverse effects of Pb on neurological endpoints in both aquatic and terrestrial 

27 animal taxa. Studies that consider mode-of-action and molecular targets of Pb toxicity in 

28 biota are now available for a few species. New studies have continued adding to the 

29 evidence from both invertebrate and vertebrate studies that Pb adversely affects behaviors 

30 such as food consumption, avoidance and escape from predators, behavioral 

31 thermoregulation, and prey capture. These changes are likely to decrease the overall 

32 fitness of the organism. New evidence in this ISA includes reports of behavioral 

33 responses across a larger variety of organisms including reptiles and fish larvae born 

34 from Pb-exposed adults, while some impairments in feeding and escaping behaviors were 

35 reported for the first time. More evidence has become available for marine organisms.
February 2012 2-39 Draft – Do Not Cite or Quote

1 Central nervous system effects in fish recognized in previous Pb AQCDs include effects 

2 on spinal neurons and brain receptors. New evidence from this review identifies the 

MAPKs ERK1/2 and p38MAPK 3 as possible molecular targets for Pb neurotoxicity in 

4 catfish (Leal et al., 2006). Additionally, there is new evidence for neurotoxic action of Pb 

5 in invertebrates with exposure to Pb observed to cause changes in the morphology of 

6 GABA motor neurons in nematodes (Caenorhabditis elegans) (Du and Wang, 2009).

7 Decreased food consumption of Pb-contaminated diet has been demonstrated in some 

8 invertebrates (snails) and vertebrates (lizards, pigs, fish). Behavioral effects in grunt fish 

9 Haemulon scudderi, occupying the top level of a simulated marine food chain included 

10 lethargy and decreased food intake in a 42-day feeding study (Soto-Jiménez et al., 

11 2011a). These fish were fed white shrimp exposed to Pb via brine shrimp that were 

12 initially fed microalgae cultured at 20 μg Pb/L. In the same study, surfacing, reduction of 

13 motility, and erratic swimming were observed in the white shrimp after 30 days of 

14 exposure to Pb via diet. Pb may also decrease the ability of an organism to capture prey 

15 or escape predation. For example, Pb exposure has been demonstrated to adversely affect 

16 prey capture ability of certain fungal and fish species, and the motility of nematodes was 

17 adversely affected in Pb-contaminated soils (Wang and Xing, 2008). Prey capture ability 

18 was decreased in 10 day old fathead minnows born from adult fish exposed to 120 μg 

19 Pb/L for 300 days, then subsequently tested in a 21-day breeding assay (Mager et al., 

20 2010). In a laboratory study, Pb-exposed gull chicks exhibited abnormal behaviors such 

21 as decreased walking, erratic behavioral thermoregulation and food begging that could 

22 make them more vulnerable in the wild (Burger and Gochfeld, 2005). The chicks were 

23 exposed to Pb via injection to produce feather Pb concentration approximately equivalent 

24 to those observed in wild gulls. Lizards exposed to Pb through diet in the laboratory 

25 exhibited abnormal coloration and posturing behaviors. Other behavioral effects affected 

26 by Pb exposure include increased hyperactivity in fish and hypoxia-like behavior in 

27 frogs. 

28 These findings show strong coherence with findings from studies in laboratory animals 

29 described in Section 2.6.1 and Section 5.3 of the ISA that show that Pb induces changes 

30 in attention, increased response rates and motor function. The evidence presented in those 

31 sections is sufficient to conclude that there is a causal relationship between Pb exposure 

32 and neurobehavioral effects (Section 5.3). 

2.7.3.4 Effects on Development and Reproduction

33 Evidence in this review and the previous Pb AQCDs from invertebrate and vertebrate 

34 studies indicate that Pb is affecting reproductive performance in multiple species. Various 
February 2012 2-40 Draft – Do Not Cite or Quote

1 endpoints have been measured in multiple taxa of terrestrial and aquatic organisms to 

2 assess the effect of Pb on development, fecundity and hormone homeostasis, and they 

3 have demonstrated the presence of adverse effects. Reproductive effects are important 

4 when considering effects of Pb because impaired fecundity at the organismal level can 

5 result in a decline in abundance and/or extirpation of populations, decreased taxa 

6 richness, and decreased relative or absolute abundance at the community level (Suter et 

7 al., 2005; U.S. EPA, 2003a). The evidence is sufficient to conclude that there is a causal 

8 relationship between Pb exposures and developmental and reproductive effects in 

9 terrestrial and aquatic invertebrates and vertebrates (Section 7.4.1.4, Section 7.4.2.4). The 

10 evidence is inadequate to conclude that there is a causal relationship between Pb 

11 exposures and developmental and reproductive effects in plants (Section 7.4.2.4).

12 In terrestrial invertebrates, new developmental and reproductive endpoints shown to be 

13 affected by Pb include hatching success in collembolans, increased development time in 

14 fruit flies and aphids, and disrupted hormone homeostasis in moths; however, these 

15 studies have been conducted at Pb concentrations that exceed Pb soil concentrations 

16 found in most U.S. locations. In terrestrial vertebrates, new evidence for decreased sperm 

17 count and quality in deer at a location contaminated by mining, and for decreased testis 

18 weight in lizards, support previous associations between Pb exposure and reproductive 

19 and developmental effects. Recent studies in freshwater rotifers and cladocerans provide 

20 additional evidence for reproductive and developmental effects of Pb at concentrations at 

21 or near ambient Pb levels in some aquatic species. New evidence in frogs and fish 

22 continue to support developmental and reproductive effects of Pb in aquatic vertebrates 

23 reported in earlier Pb AQCDs. Few studies are available that specifically address 

24 reproductive effects of Pb exposure in plants. 

25 In terrestrial invertebrates, Pb can alter developmental timing, hatching success, sperm 

26 morphology, and hormone homeostasis. In fruit flies, Pb exposure increased time to 

27 pupation and decreased pre-adult development. Sperm morphology was altered in 

28 earthworms exposed to Pb-contaminated soils. Pb may also disrupt hormonal 

29 homeostasis in invertebrates as studies with moths have suggested. Evidence of 

30 multi-generational toxicity of Pb is also present in terrestrial invertebrates, specifically 

31 springtails, mosquitoes, carabid beetles, and nematodes where decreased fecundity in 

32 progeny of Pb-exposed individuals was observed. However, effects have only been 

33 studied in a small number of species and at concentrations that typically exceed Pb levels 

34 in U.S. soils. 

35 For aquatic invertebrates, reproductive effects were reported to begin at 19 μg Pb/L for 

36 the freshwater snail Lymnea palustris and 27 μg Pb/L for Daphnia sp. as reported in the 

37 1986 Pb AQCD (U.S. EPA, 1986b). Several new studies of snails, clams, and rotifers 
February 2012 2-41 Draft – Do Not Cite or Quote

1 support previous findings of adverse impacts to embryonic development. Reproductive 

2 effects have also been observed in multi-generational studies with aquatic invertebrates. 

3 Larval settlement rate and rate of population increase was decreased in rotifers and 

4 marine amphipods. Rotifers have a reduced fertilization rate associated with Pb exposure 

5 that appears to be due to decreased viability of sperm and eggs. 

6 In aquatic vertebrates there is evidence for reproductive and developmental effects of Pb. 

7 Pb-exposure in tadpoles has been demonstrated to delay metamorphosis, decrease larval 

8 size, produce subtle skeletal malformations, and to result in slower swimming speed. 

9 Previous Pb AQCDs have reported developmental effects in fish, specifically spinal 

10 deformities in larvae at a concentration of 120 μg Pb/L. In the 2006 Pb AQCD, decreased 

11 spermatocyte development in rainbow trout was observed at 10 μg Pb/L and in fathead 

12 minnow testicular damage occurred at 500 μg Pb/L. In fish, there is new evidence in this 

13 ISA of Pb effects on steroid profiles. Reproduction in fathead minnows was affected in 

14 breeding exposures following 300-day chronic toxicity testing. However, reproductive 

15 performance was unaffected in zebrafish Danio rerio exposed to Pb-contaminated prey. 

16 Additional observed impacts of Pb on reproductive endpoints in fish include decreased 

17 oocyte diameter and density in toadfish, associated with elevated Pb levels in gonad.

18 In terrestrial vertebrates, evidence from Chapter 7 and in previous Pb AQCDs indicates 

19 an association between Pb exposure and observed adverse reproductive effects. In 

20 mammals, few studies in the field have addressed Pb specifically: most available data in 

21 wild or grazing animals are from near smelters, where animals are co-exposed to other 

22 metals. Evidence obtained using mammals in the context of human health research 

23 demonstrates adverse effects of Pb on sperm, and on onset of puberty in males and 

24 females (Chapter 5), which is coherent with the partial evidence from mammals in the 

25 wild. Other reproductive endpoints including spontaneous abortions, pre-term birth, 

26 embryo development, placental development, low birth weight, subfecundity, hormonal 

27 changes, and teratology were also affected, but less consistently. New toxicological data 

28 support trans-generational effects, a finding that is also an area of emerging interest in 

29 ecology. The evidence presented in Section 5.8 is sufficient to conclude that there is a 

30 causal relationship between Pb exposure and reproductive effects in humans and 

31 laboratory animals. 

32 Many studies of effects on reproductive and developmental endpoints in terrestrial 

33 invertebrates and vertebrates have been conducted with soil Pb concentrations exceeding 

34 those found in most U.S. locations. Studies in this ISA include exposure-response 

35 experiments showing exposure-dependent responses with exposure increasing from 

36 background level to levels greater than near point sources. For some aquatic species, 
February 2012 2-42 Draft – Do Not Cite or Quote

1 recent evidence supports previous findings of reproductive and developmental effects of 

2 Pb and differential lifestage response at near ambient concentrations of Pb.

2.7.3.5 Effects on Growth 

3 Evidence is sufficient to conclude that there is a causal relationship between Pb 

4 exposures and effects on growth in aquatic and terrestrial plants and aquatic invertebrates 

5 (Section 7.4.1.5, Section 7.4.2.5). Evidence is inadequate to establish a causal 

6 relationship between Pb exposures and effects on growth in terrestrial invertebrates and 

7 in terrestrial and aquatic vertebrates (Section 7.4.2.5).

8 Alterations in the growth of an organism can impact population, community and 

9 ecosystem level variables. Evidence for effects of Pb on growth is strongest in aquatic 

10 and terrestrial plants. In invertebrates, evidence for effects of Pb on growth has been 

11 observed most extensively in aquatic taxa, with inhibition in sensitive species occurring 

12 near the current range of Pb concentrations in surface waters. Vertebrates, particularly 

13 terrestrial, have been the object of a comparatively much smaller number of studies of the 

14 effects of Pb on growth. Growth effects observed in both invertebrates and vertebrates, 

15 however, underscore the importance of lifestage to overall Pb susceptibility. In general,

16 juvenile organisms are more sensitive than adults. Evidence for growth effects of Pb in 

17 freshwater and terrestrial plant species is primarily supported by earlier Pb AQCDs. In 

18 aquatic invertebrates, the weight of the evidence continues to support growth effects of 

19 Pb with several new studies reporting effects at ≤ 10 μg Pb/L, specifically in snails and 

20 mussels. Also, growth effects in frogs are reported at lower concentrations in this ISA 

21 than in earlier reviews.

22 There is evidence over several decades of research that Pb inhibits photosynthesis and 

23 respiration in plants, both of which reduce growth (U.S. EPA, 2006b, 1977). Many 

24 laboratory and greenhouse toxicity studies have reported effects on plants. These effects 

25 are typically observed in laboratory studies with high exposure concentrations or in field 

26 studies near point sources. Pb has been shown to affect photosystem II with the 

27 hypothesized mechanism being that Pb may replace either Mg or Ca in chlorophyll, 

28 altering the pigment structure and decreasing the efficiency of visible light absorption by 

29 affected plants. Decreases in chlorophyll a and b content have been observed in various 

30 algal and plant species. Most primary producers experience EC50 values for growth in the 

31 range of 1,000 to 100,000 μg Pb/L with minimal inhibition of growth observed as low as 

32 100 μg Pb/L (U.S. EPA, 2006b).

33 Growth effects of Pb on aquatic invertebrates are reviewed in the draft Ambient Aquatic 

34 Life Water Quality Criteria for Pb (U.S. EPA, 2008b) and the 2006 Pb AQCD. In the 
February 2012 2-43 Draft – Do Not Cite or Quote

1 2006 Pb AQCD, the LOEC for growth of freshwater amphipods Hyalella azteca in 

2 42-day chronic exposure to Pb was 16 μg Pb/L (Besser et al., 2005). Recent studies 

3 provide additional evidence for effects on growth of aquatic invertebrates at ≤ 10 μg 

4 Pb/L. Growth of juvenile freshwater snails L. stagnalis was inhibited below the lowest 

5 concentration tested resulting in an EC20 <4 μg Pb/L (Grosell and Brix, 2009; Grosell et 

6 al., 2006a). In the same study, the NOEC was 12 μg Pb/L and the LOEC was 16 μg Pb/L. 

7 The authors indicated that the observed effect level for Pb was very close to the current 

8 U.S. EPA water quality criteria for Pb (3.3 μg Pb/L normalized to test water hardness)

9 (Grosell and Brix, 2009). In the freshwater mussel, fatmucket (L. siliquoidea) juveniles 

10 were the most sensitive Lifestage (Wang et al., 2010e). A chronic value of 10 μg Pb/L in 

11 a 28 day exposure of 2-month-old fatmucket juveniles was the lowest genus mean 

12 chronic value ever reported for Pb. Growth effects are also reported in marine 

13 invertebrates at higher concentrations of Pb than sensitive freshwater invertebrates.

14 In previous Pb AQCDs, a few studies have reported growth effects of Pb on vertebrates 

15 including fish (growth inhibition), birds (changes in juvenile weight gain), and frogs 

16 (delayed metamorphosis, smaller larvae). A new study reviewed in this ISA supports 

17 findings of growth effects in frogs and suggests that these effects may be occurring at 

18 lower concentrations: the growth rate of tadpoles of the northern leopard frog exposed to 

19 100 μg Pb/L from embryo to metamorphosis was slower than the growth rate of the 

20 controls (Chen et al., 2006b). In this study, Pb concentrations in the tissues of tadpoles 

21 were quantified and the authors reported that they were within the range of reported 

22 tissue concentrations reported in wild-caught populations. Reports of Pb-associated 

23 growth effects in fish are inconsistent. 

2.7.3.6 Effects on Survival

24 The evidence is sufficient to conclude that there is a causal relationship between Pb 

25 exposures and survival in terrestrial and aquatic invertebrates and vertebrates 

26 (Section 7.4.1.6, Section 7.4.2.6). The evidence is inadequate to conclude that there is a 

27 causal relationship between Pb exposure and survival in terrestrial and aquatic plants 

28 (Section 7.4.1.6, Section 7.4.2.6). In terrestrial vertebrates and invertebrates, evidence for 

29 Pb effects on survival is primarily supported by previous Pb AQCDs with no new studies 

30 reporting effects on survival at lower concentrations. For aquatic invertebrates recent 

31 studies support previous associations between Pb exposure and mortality at 

32 concentrations near the range of Pb measured in U.S. surface waters in cladocerans, 

33 amphipods and rotifers. In aquatic vertebrates, there is new evidence for effects in fish at 

34 <100 μg Pb/L. Pb is generally not phytotoxic to aquatic or terrestrial plants at 

35 concentrations found in the environment away from point sources, probably due to the 
February 2012 2-44 Draft – Do Not Cite or Quote

1 fact that plants often sequester large amounts of Pb in roots, and that translocation to 

2 other parts of the plant is limited. 

3 The relationship between Pb exposure and decreased survival rate has been well 

4 demonstrated in terrestrial and aquatic species, as presented in Section 7.2.5, 

5 Section 7.3.6, and Section 7.3.17.4 of this ISA (and in the previous Pb AQCDs). 

6 Toxicological studies have established LC50 values for some species of plants, 

7 invertebrates and vertebrates. From the LC50 data on Pb in this review and previous Pb 

8 AQCDs, a wide range of sensitivity to Pb is evident across taxa. LC50 values are usually 

9 much higher than Pb concentrations near point-sources, although physiological 

10 dysfunction that adversely impacts the fitness of an organism often occurs well below 

11 concentrations that result in mortality. 

12 Freshwater aquatic invertebrates are generally more sensitive to Pb exposure than other 

13 taxa, with survival adversely impacted in a few species at concentrations occurring near 

14 point-sources, or at concentrations near ambient levels. These impacted species may 

15 include endangered species or candidates for the endangered species list, such as the 

16 freshwater mussel Lampsilis rafinesqueana (Neosho mucket), The EC50 for foot 

17 movement (a measure of viability) for newly transformed juveniles of this species was 

18 188 μg Pb/L. Freshwater biota that exhibit sensitivity to Pb in the range of Pb 

19 concentrations measured in U.S. waters [median 0.50 μg Pb/L, range 0.04 to 30 μg Pb/L

20 (U.S. EPA, 2006b)], include some species of gastropods, amphipods, cladocerans, and 

21 rotifers although the toxicity of Pb is highly dependent upon water quality variables such 

22 as DOC, hardness, and pH. Other aquatic invertebrates such as odonates may be tolerant 

23 of Pb concentrations that greatly exceed environmental levels. 

24 Terrestrial invertebrates typically tolerate higher concentrations of Pb. In the 1986 Pb 

25 AQCD it was reported that Pb at environmental concentrations occasionally found near 

26 roadsides and smelters (10,000 to 40,000 μg Pb/g dry weight [mg Pb/kg]) can eliminate 

27 populations of bacteria and fungi on leaf surfaces and in soil. LC50 values for soil 

28 nematodes vary from 10-1,550 mg Pb/kg dry weight dependent upon soil OM content 

29 and soil pH (U.S. EPA, 2006b). In earthworms, 14 and 28 day LC50 values typically fall 

30 in the range of 2,400-5,800 mg Pb/kg depending upon the species tested.

31 Data on mortality of saltwater species associated with exposure to Pb is limited; however, 

32 in general, marine organisms are less sensitive to this metal than freshwater organisms 

33 and the highest toxicity is observed in juveniles. In one study, effects of Pb on survival at 

34 environmentally relevant concentrations of Pb in diet have been demonstrated though a 

35 simulated marine food chain in which the primary producer, the microalgae T. suecica, 

36 was exposed to 20 μg Pb/L and subsequently fed to brine shrimp Artemia franciscana, 

37 (mean Pb content 12 to 15 μg Pb/g) which were consumed by white-leg shrimp 
February 2012 2-45 Draft – Do Not Cite or Quote

1 Litopenaeus vannamei, itself consumed by grunt fish H. scudderi representing the top of 

2 the marine food chain (Soto-Jiménez et al., 2011a). Survival of brine shrimp was 25 to 35 

3 % lower than the control and both white shrimp and grunt fish had significantly higher 

4 mortalities than controls. 

5 In terrestrial avian and mammalian species, toxicity is observed in laboratory studies over 

6 a wide range of doses (<1 to >1,000 mg Pb/kg body weight-day) as reviewed for the 

7 development of Eco-SSL’s (U.S. EPA, 2005b). The NOAELs for survival ranged from 

8 3.5 to 3,200 mg Pb/kg • day.

9 In aquatic vertebrates there is considerable historic information on Pb toxicity to 

10 freshwater fish. New studies support findings in previous AQCDs of Pb effects on fish 

11 survival and indicate effects at lower concentrations in testing with juvenile lifestages. In 

12 a series of 96-hour acute toxicity tests with fathead minnow conducted in a variety of 

13 natural waters across North America, LC50 values ranged from 41 to 3,598 μg Pb/L and 

14 no Pb toxicity occurred in three highly alkaline waters (Esbaugh et al., 2011). Thirty day

15 LC50 values for larval fathead minnows ranged from 39 to 1,903 μg Pb/L in varying 

16 concentrations of DOC, CaSO4 and pH (Grosell et al., 2006a). In a recent study of 

17 rainbow trout fry at 2-4 weeks post-swim up, the 96-hour LC50 was 120 μg Pb/L at a 

18 hardness of 29 mg/L as CaCO3, a value much lower than in testing with older fish

19 (Mebane et al., 2008).

2.7.3.7 Community and Ecosystem Effects

20 More evidence for Pb toxicity to terrestrial and aquatic biota has been reported from 

21 single-species assays in laboratory studies than from whole ecosystem studies. The 

22 evidence is strong for effects of Pb on growth, reproduction and survival in very diverse 

23 species, but considerable uncertainties exist in generalizing effects observed under 

24 particular, small-scale conditions, up to the ecosystem level of biological organization. At 

25 the ecosystem level, the presence of multiple stressors, variability in field conditions, and 

26 differences in bioavailability of Pb make it difficult to measure the magnitude of effects, 

27 and to quantify relationships between ambient concentrations of Pb and ecosystem 

28 response. However, the cumulative evidence that has been reported for Pb effects at 

29 higher levels of ecological organization and for endpoints in single species with direct 

30 relevance to population and ecosystem level effects (i.e., development and reproduction, 

31 growth, survival) is sufficient to conclude that there is a likely causal relationship 

32 between Pb exposures and the alteration of species richness, species composition and 

33 biodiversity in terrestrial and aquatic ecosystems (Section 7.4.1.7, Section 7.4.2.7). 
February 2012 2-46 Draft – Do Not Cite or Quote

1 Ecosystem-level studies are complicated by the confounding of Pb exposure with other 

2 factors such as the presence of other trace metals and acidic deposition. In natural 

3 systems, Pb is often found co-existing with other stressors, and observed effects may be 

4 due to cumulative toxicity. In laboratory studies and simulated ecosystems, where it is 

5 possible to isolate the effect of Pb, this metal has been shown to alter competitive 

6 behavior of species, predator-prey interactions and contaminant avoidance. At higher 

7 levels of ecological organization, these effects may change species abundance and 

8 community structure. Uptake of Pb into aquatic and terrestrial organisms and its effects 

9 on survival, growth, physiological stress, blood, neurobehavior, and developmental and 

10 reproductive endpoints at the organism level are expected to have ecosystem-level 

11 consequences. Where evidence of effects is observed at the ecosystem level of 

12 organization, evidence from lower levels brings consistency and plausibility for causality.

13 Most direct evidence of community and ecosystem level effects is from near point 

14 sources where Pb concentrations are higher than typically observed in the environment. 

15 For terrestrial systems, evidence of impacts on natural ecosystems near smelters, mines, 

16 and other industrial sources of Pb has been assembled in previous decades. Those impacts 

17 include decreases in species diversity and changes in floral and faunal community 

18 composition. For aquatic systems, the literature focuses on evaluating ecological stress 

19 from Pb originating from urban and mining effluents rather than atmospheric deposition. 

20 In simulated ecosystems, where it is possible to isolate the effect of Pb, this metal has 

21 been shown to alter competitive behavior of species, predator-prey interactions and 

22 contaminant avoidance. For example, frogs and toads lack avoidance response while 

23 snails and fish avoid higher concentrations of Pb. New evidence, published since the 

24 2006 Pb AQCD indicates that some species of worms will avoid Pb-contaminated soils

25 (Langdon et al., 2005).These dynamics are likely to change species abundance and 

26 community structure at higher levels of ecological organization. 

27 Recent studies continue to demonstrate associations between Pb exposures and effects at 

28 higher levels of biological organization that were shown in field and microcosm studies 

29 in previous Pb AQCDs. New studies on plant and soil microbial communities and 

30 sediment-associated and aquatic plant communities increase the total number of types of 

31 ecological associations impacted by Pb.

32 In terrestrial ecosystems, most studies show decreases in microorganism abundance, 

33 diversity, and function with increasing soil Pb concentration. Specifically, shifts in 

34 nematode communities, bacterial species, and fungal diversity have been observed. 

35 Furthermore, presence of arbuscular mycorrhizal fungi may protect plants growing in 

36 Pb-contaminated soils. Increased plant diversity ameliorated effects of Pb contamination 

37 on a microbial community. 
February 2012 2-47 Draft – Do Not Cite or Quote

1 In aquatic ecosystems, Pb effects reviewed in the 2006 Pb AQCD included reduced 

2 species abundance, richness and diversity, decreased primary productivity, and altered 

3 predator-prey interactions. Since the 2006 Pb AQCD, there is further evidence for effects 

4 of Pb in sediment-associated communities. Exposure to three levels of sediment Pb 

5 contamination in a microcosm experiment significantly reduced nematode diversity and 

6 resulted in profound restructuring of the community structure (Mahmoudi et al., 2007).

7 Sediment-bound Pb contamination appears to differentially affect members of the benthic 

8 invertebrate community, potentially altering ecosystems dynamics in small urban streams

9 (Kominkova and Nabelkova, 2005). Although surface water Pb concentrations in 

10 monitored streams were determined to be very low, concentrations of the metal in 

11 sediment were high enough to pose a risk to the benthic community 

12 (e.g., 34-101 mg Pb/kg). These risks were observed to be linked to benthic invertebrate 

13 functional feeding group, with collector-gatherer species exhibiting larger body burdens 

14 of heavy metals than benthic predators and collector-filterers.

15 Changes to aquatic plant community composition have been observed in the presence of 

16 elevated surface water Pb concentrations. A shift toward more Pb-tolerant species is also 

17 observed in terrestrial plant communities near smelter sites (U.S. EPA, 2006b). Certain 

18 types of plants such as rooted and submerged aquatic plants may be more susceptible to 

19 aerially-deposited Pb resulting in shifts in Pb community composition. High Pb sediment 

20 concentrations are linked to shifts in amphipod communities inhabiting plant structures. 

2.8 Integration of Health and Ecological Effects

21 The health and ecological effects considered for causal determination are summarized in 

22 the Table 2-4. The health outcomes were nervous system, cardiovascular, renal, immune, 

23 effects on heme synthesis and RBC function, reproductive effects, and cancer. The 

24 ecological endpoints considered for causal determination were: physiological stress, 

25 hematological effects, neurobehavioral effects, developmental and reproductive effects, 

26 growth, survival, and community and ecosystem level effects. The evidence relating to 

27 specific ecological endpoints is also integrated across aquatic and terrestrial habitats. 

28 Further, the substantial overlap between the ecological and health endpoints considered in 

29 the causal determinations allowed the integration of the evidence across these disciplines.
February 2012 2-48 Draft – Do Not Cite or Quote

Table 2-4 Summary of Causal Determinationsa for Health and Ecological 

Effects

Outcome/Effect

Human Health 

Causal Determination

Ecological Receptors

Causal Determination

Neurological Effectsb Causal Relationship Likely Causal Relationship: Invertebrates and Vertebrates

Cardiovascular Effects Causal Relationship N/Ae

Renal Effects Causal Relationship N/Ae

Immune System Effects Causal Relationship N/Ae

Heme Synthesis and 

RBC Functionc

Causal Relationship Causal Relationship: Terrestrial Vertebrates, 

Aquatic Invertebrates and Vertebrates

Reproductive Effects and 

Birth Outcomed

Causal Relationship Causal Relationship: Invertebrates and Vertebrates

Inadequate to Infer Causal Relationship: Plants

Cancer Likely to be a causal relationship N/Ae

Mortality N/Ae (The strongest evidence of Pb-induced 

mortality in humans was observed for 

cardiovascular disease related mortality and this 

evidence was considered in determining the 

causal relationship between Pb exposure and 

cardiovascular effects.)

Causal Relationship: Invertebrates and Vertebrates

Inadequate to Infer Causal Relationship: Plants

Growth N/Ae (There is evidence from toxicological and 

epidemiologic studies of Pb effects on fetal and 

postnatal growth, which was considered in 

determining the causal association between Pb 

exposure and reproductive and developmental 

effects.) 

Causal Relationship: Plants and Aquatic Invertebrates

Inadequate to Infer Causal Relationship: Vertebrates and 

Terrestrial Invertebrates

Physiological Stress N/Ae (In Human Health, oxidative stress was 

considered as a upstream event in the modes of 

action of Pb, leading downstream to various 

effects. Ecological literature commonly uses 

oxidative stress as a proxy indicator of overall 

fitness, and thus treats it as an effect.)

Causal Relationship 

Community and 

Ecosystem Level Effects

N/Ae Likely Causal Relationship

a

Based upon the framework described in the Preamble, a determination of causality for health effects was made for a broad outcome category (i.e., nervous 

system effects) by evaluating the coherence of evidence across disciplines and across a spectrum of related endpoints. However, the evidence on which 

the causal judgment is based, including the strength of evidence for the individual endpoints within the major outcome category, is characterized within the 

discussion. Causal determinations were made within approximately 1-2 orders of magnitude of current levels. 

b

In ecological receptors, the causal determination was developed considering neurobehavioral effects that can be observed in toxicological studies of 

animal models and studies of ecological effects in vertebrates and invertebrates. The human epidemiologic evidence evaluated included a wider range of 

health endpoints such as cognition. 

c

The health hematological effects considered in the determination of causality were primarily heme synthesis and RBC function. The ecological evidence 

considered for the causal determination included heme synthesis, blood cell count, and altered serum profiles.

d Reproductive health effects, including effects on sperm, as well as birth outcomes such as spontaneous abortion, were considered in the causal 

determination. In the ecological literature, a wide range of endpoints, including embryonic development, multigenerational studies, delayed metamorphosis, 

and altered steroid profiles, was considered.

e

N/A, not applicable, i.e., for health effects no causal determination for this specific endpoint, considered within the causal determination for the larger 

outcome category; for ecological effects there was no comparable endpoint.
February 2012 2-49 Draft – Do Not Cite or Quote

2.8.1 Modes of Action Relevant to Downstream Health and Ecological 

Effects

1 The diverse health and ecological effects of Pb are mediated through multiple, 

2 interconnected modes of action. This section summarizes the principal

3 cellular/subcellular effects contributing to modes of action for human health endpoints 

4 associated with Pb exposure and the concentrations at which those effects are observed. 

5 Then, effects of Pb observed in aquatic and terrestrial species (Section 2.7) are evaluated 

6 along with evidence from epidemiological and laboratory animal studies to determine the 

7 extent to which common modes of action can be inferred from the observed effects. The 

8 rationale for this approach is that the mechanism of Pb toxicity is likely conserved from 

9 invertebrates to vertebrates to humans in some organ systems. 

10 Each of the modes of action discussed in Section 5.2 has the potential to contribute to the 

11 development of a number of Pb-induced health effects (Table 2-5). Evidence for the 

12 majority of these modes of action is observed at low blood Pb levels in humans and 

13 laboratory animals, between 2 and 5 μg/dL, and at doses as low as the picomolar range in 

14 animals and cells. The concentrations eliciting the modes of action (reported in Table 

15 2-5) are drawn from the available data and do not imply that these modes of action are 

16 not acting at lower exposure levels or that these doses represent the threshold of the 

17 effect. Also, the data in presented this table does not inform regarding the exposure 

18 frequency and duration required to elicit a particular MOA.
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Table 2-5 MOAs, their related health effects, and information on concentrations 

eliciting the MOAs

Mode of Action

[Related Health Effects 

(ISA Section)]

Concentrations or doses (Conditions)a

Blood Pb Dose

Altered Ion Status

[All Health Effects of Pb]

3.5 μg/dL

(mean in cord blood; association with cord blood Ca2+-

ATPase pump activity)

Huel et al. (2008)

0.00005 μM free Pb2+

(In vitro; 30 minutes; calmodulin activation assay)

Kern et al. (2000)

Protein Binding

[Renal (5.5), Heme Synthesis and RBC 

Function (5.7)]

17.0 μg/dL

(concurrent mean in adult workers with wildtype 

metallothionein expression; increased BP susceptibility) 

Chen et al. (2010a)

50 μM Pb glutamate

(In vitro; 24 hours; increased nuclear protein in 

neurological cell)

Klann and Shelton (1989)

Oxidative Stress

[All Heath Effects of Pb]

5.4 μg/dL

(concurrent mean in adult male workers; decreased CAT 

activity in blood) 

Conterato et al. (In Press)

0.1 μM Pb-acetate

(In vitro; 48 hours; decreased cellular GSH in 

neuroblastoma cells)

Chetty et al. (2005)

Inflammation

[Nervous System (5.3), Cardiovascular 

(5.4), Renal (5.5), Immune (5.6), 

Respiratory (5.6.4), Hepatic (5.9.1)]

2.5 μg/dL

(concurrent minimum in adult males; increased serum 

TNF-α and blood WBC count)

Kim et al. (2007)

0.01 μM Pb-acetate

(In vitro; 48 hours; increased cellular PGE2 in 

neuroblastoma cells)

Chetty et al. (2005)

Endocrine Disruption

[Reproductive and Developmental Effects 

(5.8), Endocrine System (5.9.3), Bone 

and Teeth (5.9.4)]

1.7 μg/dL

(concurrent minimum in women with both ovaries 

removed; increased serum FSH)

Krieg (2007)

10 μM Pb nitrate

(In vitro; 30 minutes; displaced GHRH binding to rat 

pituitary receptors) 

Lau et al. (1991)

Cell Death/Genotoxicity

[Cancer (5.10), Reproductive and 

Developmental Effects (5.8), Bone and 

Teeth (5.9.4)]

3.3 μg/dL

(concurrent median in adult women; increased rate of 

HPRT mutation frequency)

Van et al. (2004)

0.03 μM Pb-acetate

(In vitro; 18 hours; increased formation of micronuclei)

Bonacker et al. (2005)

a This table provides examples of studies that report effects with low doses or concentration; they are not the full body of evidence used to characterize the 

weight of the evidence. In addition, the levels cited are reflective of the data and methods available and do not imply that these modes of action are not acting at 

lower Pb exposure or blood Pb levels or that these doses represent the threshold of the effect. Additionally, the blood concentrations and doses (indicating Pb 

exposure concentrations from in vitro systems) refer to the concentrations and doses at which these modes of action were observed. While the individual modes 

of action are related back to specific health effects sections (e.g., Nervous System, Cardiovascular), the concentrations and doses given should not be 

interpreted as levels at which those specific health effects occur.

1 Ecosystem studies have presented evidence for the occurrence of many of these modes of 

2 action in animals, and to some degree in plants, however the connection to ecological 

3 outcomes must usually be inferred because ecological studies are typically not designed 

4 to address mode of action directly. The level at which Pb elicits a specific effect is more 

5 difficult to establish in terrestrial and aquatic systems due to the influence of 

6 environmental variables on Pb bioavailability and toxicity and substantial species 

7 differences in Pb susceptibility. 

The alteration of cellular ion status (including disruption of Ca2+ 8 homeostasis, altered ion 

9 transport mechanisms, and perturbed protein function through displacement of metal 

10 cofactors) appears to be the major unifying mode of action underlying all subsequent 
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1 modes of action in plants, animals, and humans (Figure 5-1). Pb can interfere with 

2 endogenous cation homeostasis, necessary as a cell signal carrier mediating normal 

cellular functions. Pb is able to displace metal ions, such as Zn, Mg, and Ca2+ 3 , from 

4 proteins due to the flexible coordination numbers and multiple ligand binding ability of 

5 Pb, leading to abnormal conformational changes to proteins and altered protein function. 

Disruption of ion transport leading to increased intracellular Ca2+ 6 levels is due in part to

the alteration of the activity of transport channels and proteins, such as Na+-K+ 7 ATPase 

and voltage-sensitive Ca2+ 8 channels. Pb can interfere with these proteins through direct 

9 competition between Pb and the native metals present in the protein metal binding 

10 domain or through disruption of proteins important in calcium-dependent cell signaling, 

11 such as PKC or calmodulin. 

12 This competition between metals has been reported not only in human systems, but also 

in fish, snails, and plants. Altered Ca2+ channel activity and binding of Pb with Na+-K+ 13

ATPase in the gills of fish disrupts the Na+ and Cl- 14 homeostasis, which may lead to 

ionoregulatory failure and death. Ca2+ 15 influx and ionoregulation has also been shown to 

16 be inhibited by Pb exposure in a sensitive species of snail, leading to a reduction in snail 

17 growth. In plants, substitution of the central atom of chlorophyll, Mg, by Pb prevents 

18 light-harvesting, resulting in a breakdown of photosynthesis. Pb-exposed animals also 

19 have decreased cellular energy production due to perturbation of mitochondrial function.

Disruption of ion transport not only leads to altered Ca2+ 20 homeostasis, but can also result 

21 in perturbed neurotransmitter function. Evidence for these effects in Pb-exposed 

22 experimental animals and cell cultures has been linked to altered neurobehavioral 

23 endpoints and other neurotoxicity. Neurobehavioral changes that may decrease the 

24 overall fitness of the organism have also been observed in aquatic and terrestrial 

25 invertebrate and vertebrate studies. There is evidence in tadpoles and fish to suggest Pb 

26 may alter neurotransmitter concentrations, possibly resulting in some of these 

27 neurobehavioral changes.

28 Altered cellular ion status following Pb exposure can result in the inhibition of heme 

29 synthesis. Pb exposure is commonly associated with altered hematological responses in 

30 aquatic and terrestrial invertebrates, experimental animals, and human subjects. The 

31 proteins affected by Pb are highly conserved across species accounting for the common 

32 response seen in human health and ecological studies. This evolutionarily conserved 

33 response to Pb is likely the result of the competition of Pb with the necessary metal 

34 cofactors in the proteins involved in heme synthesis. 

35 Although Pb will bind to proteins within cells through interactions with side group 

36 moieties, thus potentially disrupting cellular function, protein binding of Pb may 

37 represent a mechanism by which cells protect themselves against the toxic effects of Pb. 
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1 Intranuclear and intracytosolic inclusion body formation has been observed in the kidney, 

2 liver, lung, and brain following Pb exposure in experimental animals. A number of 

3 unique Pb binding proteins have been detected, constituting the observed inclusion 

4 bodies. The major Pb binding protein in blood is ALAD with carriers of the ALAD-2 

5 allele potentially exhibiting higher Pb binding affinity. Inhibition of ALAD activity is a 

6 widely recognized response to Pb in environments where Pb is present and is considered 

7 to be biomarker of Pb exposure in both terrestrial and aquatic biota. Additionally, 

8 metallothionein is an important protein in the formation of inclusion bodies and 

9 mitigation of the toxic effects of Pb. Protein binding of Pb is a recognized mechanism of 

10 Pb detoxification in some terrestrial and aquatic biota. For example, plants can sequester 

11 Pb through binding with phytochelatin and some fish have the ability to store 

12 accumulated Pb in heat-stable proteins. 

13 A second major mode of action of Pb is the development of oxidative stress, due in many 

14 instances to the antagonism of normal metal ion functions. Disturbances of the normal 

15 redox state of tissues can cause toxic effects and is involved in the majority of health and 

16 ecological outcomes observed after Pb exposure. The origin of oxidative stress produced 

17 after Pb exposure is likely a multi-pathway process. Studies in humans and experimental 

18 animals provide evidence to conclude that oxidative stress results from oxidation of 

19 δ-ALA, NAD(P)H oxidase activation, membrane and lipid peroxidation, and antioxidant 

20 enzyme depletion. Evidence of increased lipid peroxidation associated with Pb exposure 

21 exists for many species of plants, invertebrates, and vertebrates. Enhanced lipid 

peroxidation can also result from Pb potentiation of Fe2+ 22 initiated lipid peroxidation and 

23 alteration of membrane composition after Pb exposure. Increased Pb-induced ROS will 

also sequester and inactivate biologically active • 24 NO, leading to the increased production 

25 of the toxic product nitrotyrosine, increased compensatory NOS, and decreased sGC 

26 protein. Pb-induced oxidative stress not only results from increased ROS production but 

27 also through the alteration and reduction in activity of the antioxidant defense enzymes. 

28 The biological actions of a number of these enzymes are antagonized due to the 

29 displacement of the protein functional metal ions by Pb. Increased ROS are often 

30 followed by a compensatory and protective upregulation in antioxidant enzymes, such 

31 that this observation is indicative of oxidative stress conditions. A number of studies in 

32 plants, invertebrates, and vertebrates present evidence of increased antioxidant enzymes 

33 with Pb exposure. Additionally, continuous ROS production may overwhelm this 

34 defensive process leading to decreased antioxidant activity and further oxidative stress 

35 and injury.

36 In a number of organ systems Pb-induced oxidative stress is accompanied by 

37 misregulated inflammation. Pb exposure will modulate inflammatory cell function, 

38 production of proinflammatory cytokines and metabolites, inflammatory chemical 
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1 messengers, and proinflammatory signaling cascades. Cytokine production is skewed 

2 toward the production of proinflammatory cytokines like TNF-α and IL-6 as well as 

3 leading to the promotion of Th2 response and suppression of Th1cytokines and 

4 Th1-related responses.

5 Pb is a potent endocrine disrupting chemical. Steroid receptors and some endocrine 

6 signaling pathways are known to be highly conserved over a broad expanse of animal 

7 phylogeny. Pb will disrupt the HPG axis evidenced in humans, other mammals, and fish, 

8 by a decrease in serum hormone levels, such as FSH, LH, testosterone, and estradiol. Pb 

9 interacts with the hypothalamic-pituitary level hormone control causing a decrease in 

10 pituitary hormones, altered growth dynamics, inhibition of LH secretion, and reduction in 

11 StAR protein. Pb has also been shown to alter hormone receptor binding likely due to 

12 interference of metal cations in secondary messenger systems and receptor ligand binding 

13 and through generation of ROS. Pb disrupts hormonal homeostasis in invertebrates 

14 necessary for reproduction and development. Pb also may disrupt the HPT axis by 

15 alteration of a number of thyroid hormones, possibly due to oxidative stress. These 

16 studies have been conducted in humans and animals, including cattle; however the results 

17 of these studies are mixed and require further investigation.

18 Genotoxicity and cell death has been investigated after Pb exposure in humans, animals, 

19 plants, and cell models. High level Pb exposure to humans leads to increased DNA 

20 damage, however lower blood Pb levels have caused these effects in experimental 

21 animals and cells. Reports vary on the effect of Pb on DNA repair activity, however a 

22 number of studies report decreased repair processes following Pb exposure. There is 

23 some evidence in plants, earthworms, freshwater mussels and fish for DNA damage 

24 associated with Pb exposure. There is evidence of mutagenesis and clastogenicity in 

25 highly exposed humans, however weak evidence has been shown in animals and cells 

26 based systems. Human occupational studies provide limited evidence for micronucleus 

27 formation (>10 μg/dL), supported by Pb-induced effects in both animal and cell studies. 

28 Micronucleus formation has also been reported in amphibians. Animal and plant studies 

29 have also provided evidence for Pb-induced chromosomal aberrations. The observed 

30 increases in clastogenicity may be the result of increased oxidative damage to DNA due 

31 to Pb exposure, as co-exposures with antioxidants ameliorate the observed toxicities. 

32 Limited evidence of epigenetic effects is available, including DNA methylation, 

33 mitogenesis, and gene expression. Altered gene expression may come about through Pb 

34 displacing Zn from multiple transcriptional factors, and thus perturbing their normal 

35 cellular activities. Consistently positive results have provided evidence of increased 

36 apoptosis following Pb exposure. 
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1 Overall, Pb-induced health and ecological effects can occur through a number of 

2 interconnected and evolutionarily well conserved modes of action that generally originate 

3 with the alteration of ion status.

2.9 Policy Relevant Considerations 

2.9.1 Public Health Significance

4 The weight of the epidemiologic and toxicological evidence clearly supports the causal 

5 relationship of Pb with decrements in cognitive function and behavior in young children 

6 (Section 5.3.2 and Section 5.3.13). Associations of blood Pb level with higher IgE, 

7 asthma and allergy (Section 5.6.8) and delays in the pubertal development in children 

8 (Section 5.8.10), as well as associations of blood and bone Pb with increased blood 

9 pressure, hypertension (Section 5.4.7), and kidney function (Section 5.5.6) in adults are 

10 also well-substantiated in the evidence base. These endpoints were among those 

11 comprising the weight of the evidence for determining causal relationships for the organ 

12 system effects. 

13 The rationale for establishing the public health significance of the various health 

14 endpoints associated with Pb exposure is multifaceted. The 2006 Pb AQCD concluded 

15 that neurological effects in children and cardiovascular effects in adults were among the 

16 effects best substantiated as occurring at blood Pb levels as low as 5-10μg/dL (or possibly 

17 lower), and that these categories of effects were clearly of the greatest public health 

18 concern. Evidence of newly demonstrated immune and renal system effects in the general 

19 population was also noted to be of potential public health concern during the previous 

20 review (U.S. EPA, 2006b). No safe level of Pb exposure has been identified in current or 

21 previous assessments and any threshold for Pb neurotoxicity would have to exist at 

22 distinctly lower levels than those associated with the lowest blood Pb concentrations 

23 examined in the epidemiologic studies included in this assessment. Recent epidemiologic 

24 studies of children continue to find associations with several health endpoints in 

25 populations with lower mean blood Pb levels than previously reported (Chapter 5, Table 

26 2-9). 

27 The concept of population risk is relevant to the interpretation of findings for the 

28 continuously-distributed subclinical health endpoints frequently studied in association 

29 with Pb biomarkers in the assessment of their public health significance. The World 

30 Health Organization definition of “health” is “the state of complete physical, mental and 

31 social well-being and not merely the absence of disease or infirmity” (WHO, 1948). By 
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1 this definition, even decrements in health status that are not severe enough to result in the 

2 assignment of a clinical diagnosis might be undesirable if they reflect a decrement in the 

3 well-being of an individual. Further, deficits in subtle indices of health or well-being may 

4 not be observable except in aggregate, at the population level, so the critical distinction 

5 between population and individual risk is essential for interpreting the public health 

6 significance of study findings. The American Thoracic Society (ATS) discusses concepts 

7 related to understanding the shift in a population distribution of a health endpoint (ATS, 

8 2000). As shown in Figure 2-1, a seemingly small increase in the mean of a continuously 

9 distributed health index may push the most susceptible group in the population above a 

10 critical cut point on the continuum of disease development, such that their condition 

11 meets the clinical definition of a disease. Moreover, small changes at the population level 

12 could translate into large numbers of clinical events if a large population is affected. 

Figure 2-1 The effect of a small shift in population mean on the proportion of 

individuals in the population diagnosed with clinical disease 

(i.e., the proportion to the right of the “Critical Line.”)

13 As shown in Table 2-6, the size of the Pb-associated effects observed in epidemiologic 

14 studies may be small relative to the effect sizes that are considered clinically relevant. 

15 Still, these small shifts in population means are often significant from a public health 

16 perspective. For example, small changes in IQ or inattention are often significant at the 

B

Critical 

Line

Shift in Population Mean

Percent of Population

A

Health Outcome
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1 population level, especially in the two tails of the outcome distribution. Further, a small 

2 shift in the population mean may result in a substantial increase in the number of 

3 individuals functioning in the low range of the IQ distribution who may fall into the IQ 

4 range associated with increase risk of educational, vocational, and social failure after 

5 experiencing a Pb-associated decrement in IQ. In cohorts of children where Pb-associated 

6 decrements in cognition and attention were observed, lower academic performance, 

7 antisocial behavior, or delinquent behavior assessed in adolescence or in early adulthood 

8 were also observed in association with blood Pb levels. Studies also find that higher 

9 blood Pb level is associated with measures of inattention and hyperactivity as well as 

10 with ADHD or other diagnostic indices used in a clinical setting (Section 5.3.3.1). A 

11 downward shift in the mean IQ value can also reduce the proportion of the population 

12 achieving very high IQ scores. It is shown that interventions that shift the population 

13 mean, in a beneficial direction, by an amount that is without clinical consequence for an 

14 individual produce substantial decreases in the proportion of individuals with clinically 

15 significant disorders. This evidence, which informs the public health significance of the 

16 evidence on the nervous system effects of Pb, is summarized in detail in Section 5.3.12.

17 It is also important to note that the change in a population mean observed in an 

18 epidemiologic study may be small compared to its standard error of measurement for the 

19 test. As noted in the 2006 Pb AQCD, statistics that pertain to individual-level data should 

20 not be used to draw inferences about group-level data. Measurement error affects the 

21 likelihood of detecting an association and is not relevant to the size of the association that 

22 is detected. If a study is large enough it will have adequate statistical power to detect 

23 small changes. Bias may be introduced if the measurement error of the outcome is highly 

24 correlated with the exposure and there is no evidence indicating that individuals with 

25 higher blood Pb levels test systematically lower than their true IQ. 

26 Pb-associated changes in other subclinical indices also increase an individual’s risk for 

27 health effects that are of greater clinical consequence, thus of greater public health 

28 concern. Small increases in blood pressure or decreases in GFR that are associated with 

29 Pb biomarkers, may shift the population mean resulting in a larger proportion of the 

30 population that is diagnosed with hypertension or CKD, respectively. Results from the 

31 Framingham Heart Study show that higher levels of blood pressure, even within the 

32 nonhypertensive range, impose increased rates of cardiovascular disease (Kannel, 2000a, 

33 b). A continuous graded increase in cardiovascular risk is observed as blood pressure 

34 increases, with no evidence of a threshold value. Most events arise not in the most severe 

35 cases, but mainly in those with high normal blood pressure (i.e., mild hypertension). 

36 Kannel (2000a) emphasized that systolic blood pressure exerts a strong influence on more 

37 serious cardiovascular events, as it is the primary cause of hypertension and its adverse 

38 cardiovascular sequelae. In addition to the small increases in blood pressure associated 
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1 with Pb, Pb-associated effects on cardiovascular morbidity outcomes such as ischemic 

2 heart disease (Section 5.4.3.5) and mortality (Section 5.4.5) have been observed. The 

3 high correlation between blood pressure and clinical cardiovascular outcomes combined 

4 with the high prevalence of cardiovascular disease in the U.S. adult population translate 

5 into a large increase in the prevalence of conditions in the population. In addition to the 

6 small changes in markers of kidney function, which are potentially significant from a 

7 public health perspective, Navas-Acien (2009) and Munter et al. (2003) both report 

8 increased risk of clinically significant CKD and albuminuria in association with Pb 

9 among NHANES III participants. CKD results in substantial morbidity and mortality, 

10 and, even at earlier stages than those requiring kidney dialysis or transplantation, is an 

11 important risk factor for heart disease. As kidney dysfunction can increase BP and 

12 increased BP can lead to further damage to the kidneys, Pb-induced damage to either or 

13 both renal or cardiovascular systems may result in a cycle of further increased severity of 

14 disease. 
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Table 2-6 Illustrative examples contrasting the effect sizes observed in 

epidemiologic studies to effect sizes considered significant in a 

clinical setting.a

Organ System Endpoint and Findings from Epidemiologic 

Studies Clinical Significance Context 

Nervous

IQ: 1 μg/dL increase in blood Pb is typically associated 

with small decrement in IQ (e.g., < 3 point decrement 

in FSIQ score) 

IQ below 70 is considered a disability by the Social Security Disability 

Insurance Administrationb 

Cardiovascular

Systolic Blood Pressure (SBP): Doubling of blood Pb 

is associated with an approximate 1 mmHg increase in 

blood pressure (Nawrot et al., 2002)

Hypertension is defined as systolic over diastolic blood pressure that 

is greater than 140/90. Pre-hypertension is blood pressure that is 120 

to 139/80 to 89. Hypertension is a risk factor for other cardiovascular 

diseases and mortality.

Renala

Glomerular Filtration Rate (GFR): Increased blood Pb 

level is associated with reductions in GFR 

(e.g., Akesson et al. (2005) reports a decrease of -2.0 

mL/min (95% CI: -3.2, -0.9) per 1 μg/dL increase in 

blood Pb)

GFR <60mL/min/1.73m2 for ≥ 3 months indicates Chronic Kidney

Disease (CKD). Estimated GFR (eGFR) can be calculated using 

several different equations. An equation using serum cystatin C rather 

than serum creatinine may be a more sensitive marker of early kidney 

damage.

Serum Creatinine: Increased blood Pb is associated 

with increased serum creatinine (e.g., (Kim et al., 

1996) observed an increase 0.030 mg/g (95% CI: 

0.011, 0.049) among those with peak blood Pb levels 

< 10 μg/dL).

Creatinine ≥ 30 mg/g creatinine indicates albuminuria. 

Creatinine Clearance: [Increased blood Pb is 

associated with decreased creatinine clearance 

(e.g., Akesson et al. (2005) reports a -1.8 mL/min 

(95% CI: -3.0, 0.7) per 1 μg/dL increase in blood Pb)

Normal values range from 97 to 137 mL/min (males) and 88 to 128 

mL/min (females). Abnormal results indicate poor kidney function.

a

Effect sizes based on general population studies

b

Information on Adult Mental Disorders can be found at: http://www.socialsecurity.gov/disability/professionals/bluebook/12.00-MentalDisordersAdult.htm

1 Rather than producing overt cytotoxicity or pathology, blood Pb level is associated with 

2 alterations in several subclinical parameters related to cellular and humoral immunity 

3 (Figure 5-42). Increases in IgE may contribute to more consequential health conditions 

4 including allergy and asthma and the small changes observed in epidemiologic studies 

5 may result in a population shift and increased prevalence of these conditions. Several 

6 studies found associations between blood Pb levels and asthma and allergy in children 

7 (Section 5.6.4.2). Immune changes may also be associated with autoimmune diseases 

8 later in life as well as reduced capacity to combat certain viral infections and cancer. 

9 Although the public health significance of a delay in onset of puberty is less clear, there 

10 is some limited evidence that delay in puberty is associated with other health outcomes

11 (Gilsanz et al., 2011; Naves et al., 2005).

12 There is a new body of literature suggesting there may be relationship between maternal 

13 bone Pb, cord Pb and/or concurrent blood Pb and cardiovascular effects in children 

14 including increased blood pressure, Total Peripheral Resistance (TPR) responses to acute 

15 stress tasks, and acute stress-induced autonomic and cardiovascular dysregulation 

16 (Section 5.4.4). This evidence, however, is difficult to interpret with respect to the risk 
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1 for health effects because cardiovascular and renal effects are associated with long-term 

2 exposures to Pb and there is uncertainty regarding the frequency, timing, duration and 

3 level of exposure associated with the effects. There is some evidence that the 

4 pathogenesis of CVD begins in childhood (Kapuku et al., 2006). Although compensatory 

5 mechanisms may be more active in children compared to adults, these early 

6 cardiovascular effects may persist and worsen if Pb exposure persists over the long-term. 

7 A recent study that strengthens the evidence of an association between blood Pb and 

8 altered renal function in children is included in this review. This NHANES analysis in 

9 adolescents found that association between higher blood Pb and lower cystatin C-based 

10 eGFR (Section 5.5.6). This association is reported in a population with mean blood Pb 

11 levels less than 5 μg/dL; however, those enrolled in the study were likely to have higher 

12 past than recent Pb exposure and the pattern and level of their exposure was not assessed 

13 in the study. 

14 In addition to the long-term effects of Pb, early life exposure to Pb (postnatally) may 

15 permanently alter normal development of the CNS to increase risk of pathology in adults 

16 (Section 5.3.2.2). For example connections between developmental exposure to Pb with 

17 early life programming and the resulting inflammation-associated DNA damage with 

18 neurodegeneration have been demonstrated in rats. Additionally, there is animal 

19 toxicological evidence for a Pb exposure contribution to Alzheimer’s disease pathology 

20 through the generation of neuronal plaques is in early life (Section 5.3.7.2). Early life Pb 

21 exposure is also associated with impaired auditory function (Section 5.3.4.3).

2.9.2 Air Lead-to-Blood Lead Relationships

22 The 1986 Pb AQCD described epidemiological studies of relationships between air Pb 

23 and blood Pb. Much of the pertinent earlier literature described in the 1986 Pb AQCD 

24 was drawn from a meta-analysis by Brunekreef (1984). Based on the studies available at 

25 that time that considered multiple air-related Pb exposure pathways in the aggregate, the 

26 1986 Pb AQCD concluded that “the blood Pb versus air Pb slope β is much smaller at 

27 high blood and air levels.” This is to say that the slope β was much smaller for 

28 occupational exposures where high blood Pb levels (>40 μg/dL) and high air Pb levels 

(much greater than 10 μg/m3 29 ) prevailed relative to lower environmental exposures which 

showed lower blood Pb and air Pb concentrations (<30 μg/dL and <3 μg/m3 30 ). For those 

31 environmental exposures, it was concluded that the relationship between blood Pb and air 

32 Pb “...for direct inhalation appears to be approximately linear in the range of normal 

ambient exposures (0.1-2.0 μg/m3 33 )” (pp 1–98 of the 1986 Pb AQCD). In addition to the 

34 meta-analysis of Brunekreef (1984), more recent studies have provided data from which 

35 estimates of the blood Pb-air Pb slope can be derived for children (Table 2-7, Table 
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4-11). The range of estimates from these studies is 2-9 μg/dL per μg/m3 1 , which

2 encompasses the estimate from the Brunekreef (1984) meta-analysis of (3-6 μg/dL per 

μg/m3 3 ). Most studies have described the blood Pb-air Pb relationship as either log-log

4 (Schnaas et al., 2004; Hayes et al., 1994; Brunekreef, 1984), which predicts an increase in 

5 the blood Pb-air Pb slope with decreasing air Pb concentration or linear (Hilts, 2003; 

6 Tripathi et al., 2001; Schwartz and Pitcher, 1989), which predicts a constant blood Pb-air 

7 Pb slope across all air Pb concentrations. These differences may simply reflect model 

8 selection by the investigators; alternative models are not reported in these studies. 

9 The blood Pb-air Pb slope may also be affected in some studies by the inclusion of 

10 parameters (e.g., soil Pb) that may account for some of the variance in blood Pb 

11 attributable to air Pb. Other factors that likely contribute to the derived blood Pb-air Pb 

12 slope include differences in the populations examined and Pb sources, which varied 

13 among individual studies. See Section 4.5.1 for a detailed discussion of studies that 

14 inform air Pb-to blood-Pb relationships.
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Table 2-7 Summary of Estimated Slopes for Blood Pb to Air Pb Relationships 

in Humans

Reference Study Methods Model Description Blood Pb–Air Pb 

Slopea

Children Populations

Brunekreef et al. (1984) Location: Various countries

Years: 1974-1983

Subjects: Children (varying age ranges, n>190,000)

Analysis: Meta analysis of 18 studies

Model: Log-Log

Blood Pb: 5-41 μg/dL 

(mean range for studies)

Air Pb: 0.1-24 μg/m3

(mean range for studies)

All children:

4.6 (1.5)c

Children <20 

μg/dL:

4.8 (0.54)d

Hayes et al. (1994) Location: Chicago, IL

Years: 1974-1988

Subjects: 0.5-6 yr (n = 9,604)

Analysis: Regression of quarterly median blood Pb and 

quarterly mean air Pb 

Model: Log-Log

Blood Pb: 12-30 μg/dL 

(annual median range)

Air Pb: 0.05-1.2 μg/m3

(annual mean range)

8.2 (0.62)e

Hilts et al. (2003) Location: Trail, BC

Years: 1989-2001

Subjects: 0.5-6 yr (Estimated n = 220-460, based on 292-536 

blood Pb measurements/yr with 75-85% participation).

Analysis: Regression of blood Pb screening and community air 

Pb following upgrading of a local smelter

Model: Linear

Blood Pb: 4.7-11.5 μg/dL 

(annual geometric mean range)

Air Pb: 0.03-1.1 μg/m3

(annual geometric mean range)

6.5 (0.48)f

Ranft et al. (2008) Location: Germany

Years: 1983-2000

Subjects: 6-11 yr (n = 843) 

Analysis: Pooled regression 5 cross-sectional studies

Model: Multivariate Log-Linear 

Blood Pb: 2.2-13.6 μg/dL 

(5th-95th percentile)

Air Pb: 0.03-0.47 μg/m3

(5th-95th percentile)

3.2 (0.1)g

Schnaas et al. (2004) Location: Mexico City

Years: 1987-2002

Subjects: 0.5-10 yr (n = 321)

Analysis: Regression of lifetime blood Pb from longitudinal 

blood Pb measurements and annual average air Pb data

Model: Log-Log

Blood Pb: 5-12 μg/dL 

(annual GM range)

Air Pb: 0.07-2.8 μg/m3

(annual mean range in yr of 

birth)

2.2 (0.4)h

Schwartz and Pitcher

(1989), U.S. EPA (1986a)

Location: Chicago, IL

Years: 1976-1980

Subjects: Black children, 0-5 yr (n = 5,476)

Analysis: Chicago blood Pb screening, gasoline consumption 

data, and Pb concentrations in gasoline

Model: Linear

Blood Pb: 18-27 μg/dL 

(mean range)

Air Pb: 0.36-1.22 μg/m3

(annual maximum quarterly 

mean)b

8.6 (0.75)i

Tripathi et al. (2001) Location: Mumbai, India

Years: 1984-1996

Subjects: 6-10 yr (n = 544)

Analysis: Regression of blood Pb and air Pb data

Model: Linear

Blood Pb: 8.6-14.4 μg/dL 

(regional GM range)

Air Pb: 0.11-1.18 μg/m3

(regional GM range)

3.6 (0.45)j
February 2012 2-62 Draft – Do Not Cite or Quote

Reference Study Methods Model Description Blood Pb–Air Pb 

Slopea

Adult Populations

Rodrigues et al. (2010) Location: New England, U.S.

Years: 1994-1995

Subjects: Adult bridge painters (n=84, 1 female)

Analysis: Regression analysis of blood Pb and air Pb data 

(personal monitors) collected during work performing various 

job-related tasks

Model: Log-log

Blood Pb: 16.1 μg/dL 

(GM, 1,7 GSD)

Air Pb: 58 μg/m3

(GM, 2.8 GSD)

0.01 (58)k

Mixed Child-Adult Populations

Schwartz and Pitcher

(1989), U.S. EPA (1986a)

Location: U.S.

Years: 1976-1980

Subjects: 0.5-74 yr (n = 9,987)

Analysis: NHANES blood Pb, gasoline consumption data and 

Pb concentrations in gasoline

Model: Linear

Blood Pb: 11-18 μg/dL 

(mean range)

Air Pb: 0.36-1.22 μg/m3

(annual maximum quarterly 

mean) 

9.3 (0.75)l

a

Slope is predicted change in blood Pb (μg/dL per μg/m3) evaluated at ± 0.01 μg/m3 from central estimate of air Pb for the study 

(shown in parentheses) b

Based on data for U.S. (1986 Pb AQCD). c

ln(PbB) = ln(PbA) × 0.3485 + 2.853

d

ln(PbB) = ln(PbA) × 0.2159 + 2.620

e

ln(PbB) = ln(PbA) × 0.24 +3.17 f

PbB = PbA × 6.5

g

PbB =1.5 × EXP(0.9361 × (PbA-0.1)/0.44), where 1.5 μg/dL is the background PbB, and 0.1 μg/m3 is the median PbA for the study; model also 

adjusted for soil Pb concentration, which may reduce estimated slope

h

ln(PbB) = Ln(PbA) × 0.213 +1.615 for the 1987 cohort, see text in Section 4.5.1 for more study details. i

PbB = PbA × 8.6

j

PbB =Pb A × 3.6

k

ln(PbB) = ln(PbA) × 0.05 +2.12

l

PbB = PbA × 9.63

GM, geometric mean; GSD, geometric standard deviation; PbB, blood Pb concentration (μg/dL); PbA, air-Pb concentration (μg/m3)

2.9.3 Ecological Effects and Corresponding Lead Concentrations

1 There is limited evidence to relate ambient air concentrations of Pb to levels of deposition 

2 onto terrestrial and aquatic ecosystems and to subsequent movement of 

3 atmospherically-deposited Pb through environmental compartments (e.g., soil, sediment, 

4 water, biota). The proportion of observed effects of Pb attributable to Pb from 

5 atmospheric sources is difficult to assess due to a lack of information not only on 

6 bioavailability, as affected by the specific characteristics of the receiving ecosystem, but 

7 also on deposition, and on kinetics of Pb distribution in ecosystems in long-term exposure 

8 scenarios. Therefore, the connection between air concentration and ecosystem exposure 

9 continues to be poorly characterized for Pb, and the contribution of atmospheric Pb to 

10 specific sites is not clear. 

11 Furthermore, the level at which Pb elicits a specific effect is difficult to establish in 

12 terrestrial and aquatic systems, due to the influence of other environmental variables on 

13 both Pb bioavailability and toxicity, and also to substantial species differences in Pb 

14 susceptibility. Current evidence indicates that Pb is bioaccumulated in biota; however, 
February 2012 2-63 Draft – Do Not Cite or Quote

1 the sources of Pb in biota have only been identified in a few studies, and the relative 

2 contribution of Pb from all sources is usually not known. There are large differences in 

3 species sensitivity to Pb, and many environmental variables (e.g., pH, organic matter) 

4 determine the bioavailability and toxicity of Pb. 

2.9.4 Concentration-Response Functions for Health Effects

5 With each successive assessment to-date, the epidemiologic and toxicological study 

6 findings show that progressively lower blood Pb levels or Pb exposures are associated 

7 with cognitive deficits and behavioral impairments (Section 5.3.9). C-R functions have 

8 been examined across different populations and the interpretation of these functions 

9 depends on several factors including lifestage and Pb biomarker used for the analysis. 

10 Compelling evidence for a steeper slope for the relationship between blood Pb level and 

11 children’s IQ at lower blood Pb levels was presented in the 2006 Pb AQCD based on the 

12 international pooled analysis of seven prospective cohort studies by Lanphear et al.

13 (2005), a subsequent reanalysis of these data focusing on the shape of the concentration 

14 response function (Rothenberg and Rothenberg, 2005), and several individual studies 

15 noted in Figure 2-2. The majority of the epidemiologic evidence from stratified analyses 

16 comparing the lower and the higher ends of the blood Pb distributions indicates larger 

17 negative slopes at lower blood Pb levels. 
February 2012 2-64 Draft – Do Not Cite or Quote

Note: Studies are presented in order of increasing mean blood Pb level. 

a

Strata refer to peak blood Pb level measured in child at any point during follow up. 

FSIQ = full-scale IQ, MDI = mental development index. Effect estimates are standardized to a 1 μg/dL increase in blood Pb level. 

Black symbols represent effect estimates among all subjects or in highest blood Pb stratum. Blue symbols represent effect estimates in lower 

blood Pb strata. 

Figure 2-2 Comparison of associations between blood Pb and cognitive 

function among various blood Pb strata.

1 Relatively few studies examined the shape of the concentration-response relationship 

2 between Pb in blood or bone and effects in adults. There is uncertainty regarding the 

3 frequency, duration, timing and level of exposure contributing to the blood Pb or bone Pb 

4 levels in the adult population studied. Some of the populations examined (e.g., NHANES, 

5 NAS) are likely to have higher past than recent Pb exposure. Other populations 

6 (e.g., worker populations) studied have ongoing exposure to Pb. As described elsewhere 

7 in the document (Section 4.3, Section 5.3, Section 5.4, Section 5.5), the interpretation of 

8 the study findings depends on the exposure history and the choice of the biomarker in the 

9 context of what is known about that exposure history. 

10 In NHANES analyses describing the C-R function for blood Pb level and neurocognitive 

11 effects in adults, only log-linear models were used to fit the data (Krieg et al., 2010; 

12 Krieg and Butler, 2009; Krieg et al., 2009). With regard to C-R relationships for 

13 cumulative exposure metrics, nonlinearity in the BMS and NAS cohorts of 

Study

Lanphear et al. (2000)

Bellinger and Needleman

(2003)

Canfield et al. (2003)

Tellez-Rojo et al. (2006)

Jusko (2011)

Kordas et al. (2006)

Lanphear et al. (2005)

Schwartz (1994)

Blood Pb Timing

Concurrent

Peak

Lifetime avg

Concurrent

Peak

Concurrent

Concurrent

Early childhood

Outcome

Reading score

FSIQ

FSIQ

Bayley MDI

FSIQ

Math score

FSIQ

FSIQ

Blood Pb

stratum (μg/dL)

All

<10

<7.5

<5

<2.5

>10

<10

All

<10a

≥10

<10

5-10

<5

All

< 10

all

<10

≥10a

<10a

≥7.5a

<7.5a

>15

≤15

◄

◄

◄

►

Sample size

4853

4681

4526

4043

2467

NR

NR

172

101

90

294

101

193

174

96

532

293

1089

244

1230

103

NR

NR

-4 -3 -2 -1 0 1

Change in Cognitive Function Score per 1 μg/dL

increase in blood Pb level (95% CI)
February 2012 2-65 Draft – Do Not Cite or Quote

1 community-dwelling older adults was examined with the use of quadratic terms, 

2 penalized splines, or visual inspection of bivariate plots (Bandeen-Roche et al., 2009; 

3 Weisskopf et al., 2007a; Shih et al., 2006). There was some evidence for nonlinearity for 

4 some but not all cognitive tests or subjects in the NAS cohort (Section 5.3.10). In 

5 analyses of the BMS cohort, observations of a statistically nonsignificant quadratic term

6 (Shih et al., 2006) or spline (Bandeen-Roche et al., 2009) for tibia Pb indicated that a 

7 linear model adequately fit the relationship between tibia Pb level and various tests of 

8 cognitive performance.

9 Concentration-response relationships were examined in several epidemiologic studies of 

10 blood pressure and mortality (Section 5.4.5). With regard to the concentration-response 

11 relationship, meta-analysis of human studies found that each doubling of blood Pb level 

12 (between 1 and >40 μg/dL measured concurrently in most studies) was associated with a 

13 1 mmHg increase in systolic BP and a 0.6 mmHg increase in diastolic BP (Nawrot et al., 

14 2002). However, there is uncertainty in the concentration-response relationship to 

15 cardiovascular endpoints at lower blood Pb levels since most studies model a linear 

16 relationship. Weaver et al. (2010) provided the results of further analysis of this Korean 

17 worker cohort, with a focus on determining the functional form of the 

18 concentration-response relationships. The coefficient indicated that every doubling of 

19 blood Pb level was associated with a systolic BP increase of 1.76 mmHg. The J test, a 

20 statistical test for determining which, if either, of two functional forms of the same 

21 variable, provides superior fit to data in non-nested models. Davidson and MacKinnon

22 (1981) returned a p-value of 0.013 in favor of the natural log blood Pb level, over the 

23 linear blood Pb level specification. This analysis indicates that systolic BP increase in this 

24 cohort is better described as a logarithmic function of blood Pb level within the blood Pb 

25 level range of the study than by a linear function. Animal toxicological studies provide 

26 support for this concentration response relationship. Few studies that focused on 

27 Pb-induced hypertension in experimental animals have included more than two exposure 

28 concentrations; however these few studies appear to have a supralinear (concave 

29 downward) dose response. 

30 Studies investigating both all-cause and cardiovascular mortality report both linear and 

31 non-linear relationships (Section 5.4.5). Findings from NHANES analyses were mixed 

32 with Schober et al. (2006), reporting a linear association between the relative hazard for 

33 all cause mortality with blood Pb, and Menke et al. (2006), reporting a non-linear 

34 relationships between blood Pb level and the hazard ratio for all cause, MI, stroke, and 

35 cancer mortality. There is uncertainty regarding the frequency, duration, timing and 

36 magnitude of exposure contributing to the blood Pb levels among the NHANES 

37 population studied; individuals in this population are likely to have higher past than 

38 recent exposures. Weisskopf et al. (2009) examined the concentration-response 
February 2012 2-66 Draft – Do Not Cite or Quote

1 relationships of patella and tibia Pb with mortality. Concentration-response relationships 

2 were approximately linear for patella Pb on the log HR scale for all CVD, but appear 

3 nonlinear for IHD (p <0.10). The peak HR is shown around 60 μg/g, beyond which the 

4 HR tends to decrease. It is important to note the wide confidence limits, which increase 

5 uncertainty at the lower and upper bounds of patella Pb levels. The strongest associations 

6 were observed between mortality and baseline patella Pb concentration. Baseline tibia Pb 

7 levels were more weakly associated with CVD mortality. Tibia bone Pb level is thought 

8 to reflect a longer cumulative exposure period than is patella bone Pb level because the 

9 residence time of Pb in trabecular bone is shorter than that in cortical bone.

10 Concentration response information was provided in a small number of studies of 

11 Pb-related nephrotoxicity in the occupational setting (Weaver et al., 2003b; Ehrlich et al., 

12 1998). Data in 267 Korean Pb workers in the oldest age tertile (mean age = 52 years) 

13 revealed no threshold for a Pb effect (beta = 0.0011, p = <0.05; regression and lowess 

14 lines shown), however the mean blood Pb level in this population was 32 μg/dL (Weaver 

15 et al., 2003b).

16 Non-linear concentration/exposure response relationships or attenuation of these 

17 relationships at higher exposure levels is reported in the occupational literature for a 

18 variety of effects. Explanations for this phenomenon include greater exposure 

19 measurement error, competing risks, and saturation of biological mechanisms at higher 

20 exposure levels, and exposure-dependent variation in other risk factors (Stayner et al., 

21 2003). With respect to Pb exposure, different biological mechanisms may operate at 

22 different exposure levels and/or there may be a lower incremental effect of Pb due to 

23 covarying risk factors such as low SES, poorer caregiving environment, and other higher 

24 environmental exposures. The 2006 Pb AQCD also considered the explanation for the 

25 supralinear concentration response function postulated by Bowers and Beck (2006), who 

26 stated that “a supralinear slope is a required outcome of correlations between a data 

27 distribution where one is lognormally distributed and the other is normally distributed.” 

28 The 2006 Pb AQCD determined that, while the conclusions drawn by Bowers and Beck 

29 may be true under certain conditions, their assumptions (e.g., that IQ are scores forced 

30 into a normal distribution) were not generally the case in the epidemiologic analyses 

31 showing a supralinear concentration response function. To support this conclusion, the 

32 2006 Pb AQCD cited Hornung et al. (2006), which provided evidence that the IQ data 

33 used in the pooled analysis of seven studies by Lanphear et al. (2005) were not 

34 normalized and a log-linear model (a linear relationship between IQ and the log of blood 

35 Pb) provided the best fit. 

36 The factors contributing to the supralinear relationship between blood Pb levels and 

37 neurocognitive function in children has not been examined widely in epidemiologic 
February 2012 2-67 Draft – Do Not Cite or Quote

1 studies to date. However, in several populations, higher blood Pb levels have been 

2 measured in susceptible groups such as those with higher poverty, greater exposure to 

3 tobacco smoke, lower parental education, and lower birth weight (Lanphear et al., 2005; 

4 Lanphear et al., 2000). It has been suggested that in populations of low SES, poorer 

5 caregiving environment, and greater social stress, the incremental effect of Pb exposure 

6 may be attenuated due to the overwhelmingly larger effects of these other risk factors

7 (Schwartz, 1994). Several studies found significant associations of these 

8 sociodemographic risk factors with neurocognitive deficits, and Miranda et al. (2009)

9 found that indicators of SES (i.e., parental education and enrollment in a free/reduced fee 

10 lunch program) accounted for larger decrements in EOG scores than did blood Pb level 

11 (Figure 5-7). Few studies have compared Pb effect estimates among groups in different 

12 sociodemographic strata, and the limited data are mixed. Greater Pb-associated 

13 neurocognitive deficits were reported in low-SES groups by Bellinger et al. (1990). In a 

14 meta-analysis of eight studies, Schwartz (1994) found a smaller decrement in IQ per 

15 1 μg/dL increase in blood Pb level for studies in disadvantaged populations (-2.7 points 

16 [95% CI: -5.3, -0.07]) than for studies in advantaged populations (-4.5 points [95% CI: 

17 -5.6, -2.8]). It is important to note that blood Pb level is associated with deficits in 

18 neurocognitive function in both higher and lower SES groups; however, it is unclear what 

19 differences there are between groups in the decrement per unit increase in blood Pb and 

20 whether these differences can explain the nonlinear concentration-response relationship.

21 In support of epidemiologic findings, toxicological studies provided some evidence that 

22 compared with control or higher Pb exposures (e.g., 150 ppm), lower Pb exposures 

23 (e.g., 50 ppm) during gestation and lactation (possibly with offspring stress co-exposure) 

24 impaired learning and memory as indicated by increased FI responses on schedule 

25 controlled behavior tests (Rossi-George et al., 2011; Cory-Slechta, 1994). The 2006 Pb 

26 AQCD did not identify a biological mechanism for a steeper slope at lower than at higher 

27 blood Pb levels but such a mechanism was not ruled out. In fact, several lines of 

28 toxicological evidence support the possibility of lower and higher Pb exposures acting 

29 through differentially activating mechanisms underlying cognition. 

30 As detailed in Section 5.3.10, the shape of the C-R relationship for dopamine varied 

31 across studies. However, relative to higher Pb exposures, lower Pb exposures were found 

32 to reduce LTP (Gilbert et al., 1999) and glutamate release in the hippocampus (Lasley 

33 and Gilbert, 2002). LTP is one indication of synaptic plasticity (Section 5.3.8.4) and is 

34 considered to mediate learning and memory. Glutaminergic neurotransmission via its 

35 NMDA receptor has been implicated in learning and memory (Section 5.3.8.8). Thus, 

36 these differential responses to lower versus higher Pb exposures may provide mechanistic 

37 understanding and additional biological plausibility for the nonlinear associations of Pb 
February 2012 2-68 Draft – Do Not Cite or Quote

1 exposure or blood Pb levels with neurodevelopmental outcomes observed in animals and 

2 children. 

3 While epidemiologic studies have not examined widely the shape of the 

4 concentration-response relationship for other nervous system effects, as detailed in 

5 Section 5.3.10, toxicological studies have found nonlinear C-R relationships for diverse

6 outcomes such as rod and retinal effects in the visual system (Giddabasappa et al., 2011; 

7 Fox et al., 2010), motor function (Leasure et al., 2008), and hippocampal neurogenesis

8 (Fox et al., 2008; Gilbert et al., 2005).

9 The supralinear concentration-response relationship widely documented for Pb is 

10 consistent with the lack of a threshold for Pb-associated nervous system effects in 

11 children because a smaller effect estimate would be expected at lower blood Pb levels if a 

12 threshold existed. However, an important limitation of previous studies in terms of 

13 identifying whether a threshold exists, is the limited examination of effects in populations 

14 or blood Pb strata with blood Pb levels more comparable to the current U.S. population 

15 mean. While Schwartz (1994) did not find evidence for a threshold in the Boston study 

16 data, the mean blood Pb in that population was 6.5 μg/dL, and 56% of subjects had a 

17 blood Pb level >5 μg/dL. Recent studies indicate a downward shift in the distribution of 

18 blood Pb levels (i.e., 50% of subjects in the 2001-2004 NHANES population had a blood 

19 Pb <1 μg/dL (Braun et al., 2008). Additionally, more sensitive quantification methods 

20 have improved the detection limits, for example, from 0.6 μg/dL in 1999-2002 NHANES 

21 to 0.025 μg/dL in 2003-2004 NHANES allowing categorization of children in multiple 

22 blood Pb quantiles below 1 μg/dL (Braun et al., 2008). Consequently, the examination of 

23 populations with large proportions of subjects at very low blood Pb levels has improved 

24 the ability to discern a threshold for Pb-associated nervous system effects. In the 

25 2001-2004 NHANES population, Braun et al. (2008) found higher odds ratios for 

26 conduct disorder and ADHD among children with blood Pb levels 0.8-1.0 μg/dL (2nd 

27 quartile) compared with children with blood Pb levels 0.2-0.7 μg/dL (1st quartile). 

28 However. the large proportions of adolescents in NHANES analyses who were born the 

29 1970s may have had higher past Pb exposures that contributed to associations observed 

30 with concurrent blood Pb levels. Nonetheless, several recent studies reported associations 

31 between blood Pb levels and deficits in cognitive and behavioral endpoints in children 

32 ages 8-11 years with mean or quantile blood Pb levels <2 μg/dL (Cho et al., 2010; Kim et 

33 al., 2009b; Miranda et al., 2009). In comparisons of various quantiles of blood Pb, 

34 Miranda et al. (2009) reported lower EOG scores in children in North Carolina with 

35 blood Pb levels of 2 μg/dL compared with children with blood Pb levels of 1 μg/dL. 

36 Collectively, these new findings in children, as summarized in this document, do not 

37 provide evidence for a threshold for the nervous system effects of Pb in the ranges of 

38 blood Pb levels examined to date.
February 2012 2-69 Draft – Do Not Cite or Quote

2.9.5 Pb Exposure and Nervous System Effects in Children

1 As discussed in various sections across this document, there are uncertainties related to 

2 the frequency, timing, duration and level of Pb exposure contributing to the health effects 

3 observed in epidemiologic studies of adults and older children who are likely to have 

4 higher past than recent Pb exposure. Both blood Pb and bone Pb, which are the most 

5 common metrics of exposure/dose used in epidemiologic studies, are integrative 

6 measures and do not allow these aspects of exposure to be distinguished. Bone Pb is a 

7 measure of cumulative exposure in adults and is not typically used in studies of children. 

8 As discussed in Section 4.3.5, blood Pb may reflect both recent exposures and Pb 

9 released from the bone; the relative proportion of blood Pb from recent versus past 

10 exposure is uncertain in the absence of information on exposure timing and duration. 

11 Young children, however, do not have lengthy exposure histories and consequently the 

12 interpretation of associations with blood Pb levels for this age group may be less 

13 complicated compared to older age groups. Several lines of evidence, which are 

14 discussed below, inform the interpretation of study findings as they relate to aspects of 

15 exposure that can be attributed to the neurocognitive and behavioral effects of Pb 

16 observed in young children.

17 A common limitation of epidemiologic studies is the potentially high correlation between 

18 Pb exposure metrics at different ages in childhood and with maternal Pb exposure 

19 metrics. In longitudinal studies of the effect of Pb in children, blood Pb levels remained 

20 relatively stable over time. Thus, it is difficult to distinguish effects of past and 

21 concurrent exposures. Further, concurrent blood Pb levels in children, although highly 

22 affected by recent exposure, are also influenced by their past exposure (including prenatal 

23 exposure) due to the rapid growth-related bone turnover in children. Thus, concurrent 

24 blood Pb level in children also may reflect cumulative dose (Section 4.3.5.1). There is 

25 some evidence that the influence of maternal Pb levels on postnatal blood Pb level is 

26 substantially reduced soon after birth. Simon et al. (2007) followed a cohort of 13 

27 children living near an Australian smelter from birth through 36 months. In general, 

28 immediately after birth, blood Pb levels fell for 1-2 months to approximately 47% of 

29 birth blood Pb level. After this initial fall, all infants' blood Pb levels rose with age until 

30 approximately 12 months old. There was a good correlation between child blood Pb level 

and child hand Pb loading (R2 31 = 0.70) in this study, indicating the influence of concurrent 

32 Pb exposures on blood Pb during the early childhood years. 

33 In epidemiologic studies, associations of cognitive function and behavior have been

34 observed with prenatal, early-childhood, lifetime average, and concurrent blood Pb levels 

35 as well as with childhood tooth Pb levels and it is difficult to ascertain which lifestage 

36 within childhood is associated with the greatest risk of Pb-associated effects on cognition 
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1 (Section 5.3.9, Table 5-13, Table 5-14). Epidemiologic studies of Bayley MDI following 

2 children beginning in utero and through three years of age indicate that short-duration 

3 exposures in utero or during early childhood are associated with cognitive decrements in 

4 young children. Among studies with prenatal and concurrent blood Pb measurements, 

5 some found stronger associations for prenatal blood Pb levels (Hu et al., 2006; Bellinger 

6 et al., 1984), and other found stronger associations for concurrent blood Pb levels

7 (Wasserman et al., 1998; Wasserman et al., 1992). Studies that found associations with 

8 concurrent blood Pb levels also tended to find associations with prenatal cord or maternal 

9 blood Pb levels. Thus, both postnatal child and maternal Pb exposures may contribute to 

10 lower cognitive function in young children. In addition, exposures that are reflected by 

11 concurrent blood Pb measured when children are older, by cumulative blood Pb levels or 

12 by tooth Pb levels have also been demonstrated to be associated with neurodevelopmental 

13 deficits throughout school-age and into adolescence. These findings are consistent with 

14 the understanding that the nervous system continues to develop throughout childhood. 

15 However, the weight of epidemiologic evidence supports associations of concurrent 

16 blood Pb level with neurodevelopmental effects (i.e., cognition and behavior) in children 

17 (Sections 5.3.2.1 and 5.3.3.1). 

18 The persistence of neurodevelopmental effects from low-level Pb exposure was also 

19 considered in the 2006 Pb AQCD, with some evidence suggesting that the effects of Pb 

20 on neurodevelopmental outcomes persisted into adolescence and young adulthood. The 

21 toxicological evidence continues to support a range of effects with prenatal and early 

22 postnatal Pb exposures which persist to adulthood (Sections 5.3.2.1 and 5.3.2.4). A 

23 number of mechanisms, including changes in neurogenesis, synaptogenesis and synaptic 

24 pruning, long term potentiation, and neurotransmitter function have been identified that 

25 provide biological plausibility for epidemiologic and toxicological findings of persistent 

26 cognitive and behavioral effects that result from short-term Pb exposures during prenatal 

27 and early childhood periods. The persistence of effects appears to depend on the duration 

28 and window of exposure as well as other factors that may affect an individual’s ability to 

29 recover from an insult. 

30 Toxicological studies in the 2006 Pb AQCD highlighted the importance of Pb exposure 

31 during early life in promoting Alzheimer’s like pathologies in the adult rodent brain, with 

32 Pb-induced neurodegeneration and formation of neurofibrillary tangles in aged animals in 

33 which blood Pb levels had returned to control levels after an earlier life Pb exposure

34 (U.S. EPA, 2006b). Recent toxicological studies continue to point to an early life window 

35 in which Pb exposure can contribute to pathological brain changes consistent with 

36 Alzheimer’s disease. The same Pb exposure (i.e., dose and duration equivalent to the 

37 early life exposure, to adult rodents) did not induce neurofibrillary tangles in the aged 

38 animals. However, blood Pb is not generally associated with Alzheimer’s disease in 
February 2012 2-71 Draft – Do Not Cite or Quote

1 epidemiologic studies of adults. In other studies, recent evidence indicates associations 

2 between early life ALAD activity, a biomarker of Pb exposure, and schizophrenia later in 

3 adulthood when schizophrenia typically manifests. Consistent with these findings, 

4 toxicological studies have shown NMDA pathway disruption with Pb exposure; 

5 manipulation of this pathway is known to be associated with schizophrenia. There is also 

6 toxicological evidence for Pb-induced emotional changes in males and the occurrence of 

7 depression changes in females. Sensitive windows of early life Pb exposure have been 

8 associated with persistent changes in adulthood as demonstrated with animal models of 

9 neurodegeneration, i.e., neurofibrillary tangle formation, and with epidemiologic findings 

10 for the psychotic disorder schizophrenia. These effects are not reflective of concurrent 

11 blood-Pb levels at the age of manifestation of the pathology but instead are associated 

12 with an earlier life Pb exposure.

2.9.6 Populations Potentially At-Risk for Health Effects 

13 The NAAQS are intended to provide an adequate margin of safety for both the population 

14 as a whole and those groups with unique factors that make them potentially at increased 

15 risk for health effects in response to ambient air pollutants. Interindividual variation in 

16 human responses to air pollution exposure suggests that some populations are at 

17 increased risk for detrimental effects of ambient exposure to an air pollutant. 

18 Epidemiologic studies reporting results from stratified analyses designed to identify 

19 effect measure modifiers comprise the overall weight of evidence for conclusions related 

20 to the increased risk of specific populations to the Pb-related health effects presented in 

21 Section 6.3. Supporting evidence from toxicological studies provide biological 

22 plausibility for conclusions and summarizes factors that potentially influence Pb levels 

23 within the body (Section 6.1) and factors related to differential Pb exposures 

24 (Section 6.2). The factors that were evaluated are listed in Table 2-8.
February 2012 2-72 Draft – Do Not Cite or Quote

Table 2-8 Factors evaluated that may determine populations potentially at 

increased risk from lead

Physiological Factors that Influence the 

Internal Distribution of Lead

Population Characteristics Potentially 

Related to Differential Lead Exposure

Factors Potentially Related to Increased 

Risk of Lead Induced Health Effects

Age (Section 6.1 and Section 4.4) Age (Section 6.2.1) Age (Section 6.3.1, and Section 5.3.8)

Sex (Section 6.2.2) Sex (Section 6.3.2)

Genetics (Section 6.1, Section 4.1, Section 4.2) Genetics (Section 6.3.3)

Pre-existing Disease (Section 6.3.4)

Race and Ethnicity (Section 6.2.3) Race and Ethnicity (Section 6.3.6)

Socioeconomic Status (Section 6.2.4) Socioeconomic Status (Section 6.3.7)

Proximity to Pb Sources (Section 6.2.5, 

Section 3.2, Section 3.3, Section 3.5, and 

Section 4.1)

Residential Factors (Section 6.2.6)

Lifestyle Factors 

Smoking (Section 6.3.5)

Body Mass Index (Section 6.3.8)

Alcohol Consumption (Section 6.3.9)

Nutrition (Section 6.1 and Section 4.2) Nutrition (Section 6.3.10)

Stress (Section 6.3.11)

Cognitive Reserve (Section 6.3.12)

Co-exposure (Section 6.1 and Section 4.2) Other metals (Section 6.3.13)

1 Studies are included in Section 6.3 only if analyses were conducted to identify the 

2 presence or absence of effect measure modification. By virtue of their design some cohort 

3 studies, including cohort studies of pregnant women or other populations or lifestages 

4 with no comparison group, are discussed in the endpoint-specific sections rather than in 

5 Chapter 6. This integrative summary, however, draws on evidence relating to potentially 

6 at-risk populations and lifestages appearing throughout this document. Also, 

7 physiological factors that influence the internal distribution of Pb, population 

8 characteristics potentially related to differential Pb exposure and factors potentially 

9 related to increased risk of Pb induced health effects are considered together in this 

10 integrative summary. 
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2.9.6.1 Age and/or Lifestage

Infancy and Early Childhood

1 It is recognized that Pb can cross the placenta to affect the developing nervous system of 

2 the fetus (Section 4.2.2.4). Further, elevated blood Pb levels among mothers present a 

3 potential exposure to their children through breast milk. This transfer of Pb from mother 

4 to fetus is observed to occur partly from the remobilization of the mother’s bone stores 

5 (Section 4.2.2.4). Other biokinetic factors that vary by age, including bone turnover and 

6 absorption, also affect blood Pb levels. Typically, children have increased exposure to Pb 

7 compared with adults because children’s behaviors and activities include increased hand8 to-mouth contact, crawling, outdoor play, and poor hand-washing that typically result in 

9 increased ingestion compared with adults. Blood Pb among different age groups are 

10 shown in Table 6-1. Blood Pb levels are highest among the young children and decrease 

11 with increasing age of the child. 

12 There is evidence of increased susceptibility to the neurocognitive effects of Pb exposure 

13 during several lifestages throughout childhood (Section 5.3.2.1 to Section 5.3.8, and 

14 Section 6.3.1.1). Exposure to environmental toxicants during prenatal and/or early 

15 postnatal development may alter the normal course of morphogenesis and maturation that 

16 occurs in utero and early in life, resulting in changes that affect structure or function of 

17 the central nervous system via altered neuronal growth and/or synaptogenesis/pruning 

18 structure (Section 5.3.9). Synaptic pruning, which is active throughout early childhood 

19 (ages 1-4 years), may underlie the elevated risk of young children to environmental 

20 exposures. Overall evidence indicates early childhood as a lifestage of increased risk for 

21 Pb-related health effects with epidemiologic studies report associations with cognition 

22 (Section 5.3.9) among the youngest age groups (6 months to 3 years). Toxicological 

23 studies provide support for the younger age groups being especially sensitive to Pb 

24 exposure. Toxicological studies have reported that younger animals, whose nervous 

25 systems are developing (i.e., laying down and pruning neuronal circuits) and whose 

26 junctional barrier systems in the brain (i.e., the blood brain barrier) and GI system 

27 (i.e., gut closure) are immature, are more at risk from the effects Pb exposure (Fullmer et 

28 al., 1985). Further, blood Pb level is associated with effects to the immune system 

29 (Section 5.6) of children. Findings of a recent study indicate that very young children 

30 may be at increased risk for Pb-associated activation of humoral immune responses and 

31 perturbations in cell-to-cell interactions that underlie allergic, asthma, and inflammatory 

32 responses (Section 5.6.2.1 and Section 5.6.3). 
February 2012 2-74 Draft – Do Not Cite or Quote

Adolescents

1 The 2007-2008 NHANES data suggest that sex-based differences in blood Pb levels are 

2 not substantial until adolescence at which time blood Pb levels of boys are higher than 

3 girls. Anatomical understanding of brain development in the typically or normally 

4 developing child also helps to inform findings with associated pathologies by providing 

5 grounds for comparison. Findings from MRI studies have been used to detail anatomical 

6 changes within separate brain segments in the normally developing child and adolescent, 

7 from age 3-30 (Section 5.3.9). Throughout adolescence, normal brain development 

8 remains dynamic. Volumes of specific regions vary largely by sex and age, but also 

9 include inter-individual variation. Studies have linked concurrent blood Pb level as well 

10 as other blood Pb metrics) in adolescents to decrements in cognitive function, delinquent 

11 or criminal behavior (Section 5.3.3.1). Delays in puberty onset (Section 5.8.1.3) and renal 

12 effects (Section 5.5.1.1) are also observed in association with concurrent blood Pb level 

13 in cross-sectional studies of adolescents. However, the populations of older children 

14 studied generally had higher past exposures, which may have influenced the findings of 

15 these studies. 

Adulthood

16 There is uncertainty regarding the frequency, duration, timing and magnitude of exposure 

17 contributing to the blood Pb levels among the adult population studied. Associations of 

18 both blood Pb and/or bone Pb with blood pressure, hypertension and mortality, renal, 

19 immune, hematological, and reproductive effects. Blood Pb and bone Pb levels tend to be 

20 higher in older adults (>65 years) compared with the general population. Higher average 

21 and median bone and blood Pb levels among older adults could potentially be due to a 

22 shared experience of higher historical Pb exposures stored in bone in conjunction with 

23 remobilization of stored Pb during bone loss (Section 4.2). In recent studies, age was 

24 specifically examined as an effect modifier of the association of Pb with mortality 

25 (Section 5.4.5), cognition (Section 5.3.2.5) and blood pressure (Section 5.4.2) in adults. 

26 Results for age-related modification of the association between Pb and mortality were 

27 mixed and no difference by age was observed for the associations between Pb and 

28 cognitive function or blood pressure. Toxicological studies have shown increases in 

29 Pb-related health effects by age that may be relevant to nervous system effects in humans 

30 (Section 5.3.1). 
February 2012 2-75 Draft – Do Not Cite or Quote

2.9.6.2 Sex 

1 Multiple associations between Pb and various health endpoints have been examined for 

2 effect measure modification by sex (Section 6.3.2). Some studies reported differences

3 between the associations for males and females; overall, the findings were mixed. For 

4 example, studies on cognition from the Cincinnati Lead Study cohort and a study in 

5 Poland reported males to be an at-risk population, whereas studies from Australia pointed 

6 to females as an at-risk population. Toxicological studies continue to demonstrate 

7 increased susceptibility of males to Pb for specific endpoints such as sensory function, 

8 balance, liver hyperplasia, obesity, memory and gross motor skills. Males and females 

9 show differential susceptibility in other toxicological studies across a wide array of 

10 endpoints including of behavior, stress hormone homeostasis, and depression with some 

11 endpoints having greater effects in one sex versus the other based on the exposure 

12 paradigm.

2.9.6.3 Genes

13 The 2006 Pb AQCD observed that genetic polymorphisms s have been implicated as 

14 influencing the absorption, retention, and toxicokinetics of Pb in humans, with the 

15 majority of discussion focused on the aminolevulinate dehydratase (ALAD) and vitamin 

16 D receptor (VDR) polymorphisms (U.S. EPA, 2006b). Studies published since then 

17 continue to indicate that variants of the VDR and the ALAD polymorphism have been 

18 associated with varied internal Pb concentrations. Overall, studies of ALAD observed 

19 increased Pb-related health effects associated with certain gene variants. Other genes, 

20 such as VDR, MTHFR, APOE, HFE, DRD4, GSTM1, TNF-α, and eNOS, may also 

21 affect the risk of Pb-related health effects but conclusions are limited due to the small 

22 number of studies (Section 6.3.3). 

2.9.6.4 Pre-existing Conditions

23 Studies have also been performed to examine whether certain morbidities increase an 

24 individual’s risk of Pb-related effects on health. Recent studies have explored 

25 relationships for autism, atopy, diabetes, and hypertension; however, there were generally 

26 few studies available that evaluated conditions such as autism and atopy. The 2006 Pb 

27 AQCD concluded that diabetes and hypertension may result in increased risk of 

28 Pb-associated declines in renal function (U.S. EPA, 2006b). However, a recent study 

29 indicates that diabetes was not related to increased risk of Pb-related cardiovascular 

30 outcomes, and there were mixed results regarding the potential for hypertension to 
February 2012 2-76 Draft – Do Not Cite or Quote

1 increase the risk of Pb-related effects on renal function and cardiovascular effects 

2 (Section 6.3.4). 

2.9.6.5 Race and Ethnicity 

3 Higher blood Pb and bone Pb levels among African Americans has been well 

4 documented (U.S. EPA, 2006b). Recent studies are consistent with those previous 

5 findings (Section 6.2.3). Greater tibia Pb, but not patella Pb or blood Pb levels, was 

6 reported in the Baltimore population studied by Theppeang et al. (2008b). This finding 

7 may indicate greater historical exposure among African Americans compared to 

8 Caucasians. In addition, a higher proportion of blacks compared with other race-ethnicity 

9 groups has been reported in Census blocks where soil Pb concentration is higher, whereas 

10 the proportion of whites was higher in low soil Pb Census blocks for a study of New 

11 Orleans soil exposure (Campanella and Mielke, 2008). The results of recent 

12 epidemiologic studies suggest that there may be race/ethnicity-related increased risk for 

13 cardiovascular effects and delayed puberty, although the overall understanding of 

14 potential effect measure modification by race/ethnicity is limited by the small number of 

15 studies. Additionally, these results may be cofounded by other factors, such as 

16 socioeconomic status (Section 6.3.6). 

2.9.6.6 Socioeconomic Status

17 Socioeconomic factors have sometimes been associated with Pb biomarkers, although 

18 these relationships have not always been consistent (U.S. EPA, 2006b). On a national 

19 level, the gap between income levels with respect to blood Pb has been decreasing. 

20 However, blood Pb level of children remains higher depending on income, enrollment in 

21 Medicaid and poverty income ratio (PIR), which is the ratio of family income to the 

22 poverty threshold appropriate for a given family size (Section 6.2.4). Although there are a 

23 limited number of studies, there is some evidence that Pb-associated neurocognitive 

24 effects may be larger in magnitude among lower SES populations (Section 6.3.7). There 

25 is also evidence that some cognitive effects of prenatal Pb exposure may be transient and 

26 that recovery is greater among children reared in households with more optimal 

27 caregiving characteristics and in children whose concurrent blood Pb levels were low

28 (Bellinger et al., 1990). In a meta-analysis, Schwartz (1994) found that in studies in 

29 higher SES populations, blood Pb was associated with a greater decrement in IQ than in 

30 low SES populations
February 2012 2-77 Draft – Do Not Cite or Quote

2.9.6.7 Proximity to Pb Sources and Residential Factors

1 Proximity to an industrial Pb source or Superfund site likely contributes to higher Pb 

2 exposures (U.S. EPA, 2006b). Recent studies show that soil Pb concentrations may be 

3 higher in some urban areas as a result of contemporaneous and/or historical Pb sources 

4 (Section 6.2.5). Consistent with findings of the 2006 Pb AQCD (U.S. EPA, 2006b), child 

5 blood Pb level was significantly associated with renovation activities in older homes as 

6 well as housing factors (e.g., year of construction, floor and windowsill condition). It has 

7 also been observed that windowsill dust Pb was significantly associated with the presence 

8 of deteriorated indoor paint. Other studies of renovation activities involving removal of 

9 Pb paint have included measurements of highly elevated blood Pb among children and 

10 adult workers. Living in a home built prior to 1950, where old home age is a surrogate for 

11 presence of Pb paint, has also been shown to be a significant predictor of blood Pb. 

12 (Sections 4.1.3.2 and 6.2.6). 

2.9.6.8 Lifestyle Factors

13 Body mass index (BMI), obesity and alcohol consumption have been examined in 

14 epidemiologic and toxicological studies. Modification of associations between Pb with 

15 mortality and HRV was not observed by BMI or obesity (Section 6.3.8). There is limited 

16 evidence on the potential for alcohol consumption as a modifier of Pb-related effects 

17 (Section 6.3.9) and mixed findings on whether smoking modifies the relationship 

18 between Pb and health effects (Section 6.3.5). 

2.9.6.9 Nutritional Factors

19 It is well established that diets sufficient in minerals such as calcium, iron, and zinc offer 

20 some protection from Pb exposure by preventing or competing with Pb for absorption in 

21 the GI tract (Section 6.1). The 2006 Pb AQCD included studies that indicated individuals 

22 with iron-deficiency and malnourishment had greater inverse associations between Pb 

23 and cognition/intellect (U.S. EPA, 2006b), and recent epidemiologic and toxicological 

24 studies continue to observe greater Pb-related health effects among individuals with low 

25 levels of iron (Section 6.3.10). Calcium intake from diet and vitamin D supplement can 

26 modify blood Pb levels in women of various ages. Calcium supplementation during 

27 pregnancy and lactation may decrease the amount of Pb to which the developing fetus of 

28 infant is exposed. The evidence for this seems especially strong for protection during 

29 pregnancy and more mixed for protective effects of calcium during lactation. There is 

30 little recent evidence available on the potential for zinc or other as modifiers of Pb-related 
February 2012 2-78 Draft – Do Not Cite or Quote

1 health effects associations. A recent study in China reported that children who regularly 

2 consumed breakfast had lower blood Pb levels than those children that did not eat 

3 breakfast (Liu et al., 2011b). Recent toxicological studies r indicate that diets designed to 

4 limit or reduce caloric intake and induce weight loss have been associated with increased 

5 blood Pb levels and an epidemiologic analysis reported that regular consumption of 

6 breakfast reduced blood Pb levels in children, compared with children who did not eat 

7 breakfast (Section 6.1).

2.9.6.10 Stress and Cognitive Reserve

8 Animal toxicology findings described in the 2006 Pb AQCD demonstrated interactions 

9 between Pb exposure and stress. Pb-exposed animals reared in cages with enriched 

10 environments (toys) perform better in the Morris water maze than their Pb-exposed 

11 littermates who were reared in isolation. New findings indicate a potentiating effect of 

12 stress on behavior and memory at low-dose Pb exposures. Although examined in a 

13 limited number of studies, recent epidemiologic studies observed modification of the 

14 association between Pb and various nervous system and cardiovascular effects by 

15 stress-level and also indicated that maternal self-esteem may attenuate the negative 

16 effects observed of Pb on MDI and PDI scores. Toxicological studies have demonstrated 

17 that early life exposure to Pb and maternal stress can result in altered responses in 

18 multiple systems, including altered corticosterone and neurotransmitter levels. 

19 Additionally, toxicological studies have demonstrated that immune stress also affects 

20 associations with Pb (Section 6.3.11). 

2.9.6.11 Co-exposure of Lead with Metals and Chemicals 

21 Other exposures have also been studied to assess how they affected the uptake and 

22 absorption of Pb. Recent toxicological studies that have examined the addition of arsenic 

23 (As) to Pb and Cd mixtures report increases in bioavailability of Pb (Section 6.1). The 

24 2006 Pb AQCD reported that the majority of studies examined other metals as potential 

25 confounders rather than effect measure modifiers (U.S. EPA, 2006b). Recent 

26 epidemiologic studies have, however, begun to explore the possible interaction between 

27 Pb and other metals. These studies report some stronger associations between Pb and 

28 various health endpoints with co-exposure to Cd, As and Mn (Section 6.3.13); some 

29 studies indicate that coexposures may increase bioavailability of Pb (Section 6.1). Since 

30 Pb is acid soluble, fluoridation may increase Pb concentration in water through leaching 

31 from pipes and Pb solder (Section 4.1.3.3)
February 2012 2-79 Draft – Do Not Cite or Quote

2.9.6.12 Summary of At-Risk Populations

1 As shown in Table 2-9 (Table 6-4) the most well-substantiated at risk population for the 

2 effects of Pb exposure is children. Evidence for all other factors was limited. Among 

3 children, the youngest age groups were observed to be most at risk of elevated blood Pb 

4 levels, with levels decreasing with increasing age of the children. Recent epidemiologic 

5 studies of infants/children detected increased risk of Pb-related health effects, and this 

6 was supported by toxicological studies. Synaptic pruning, which is active throughout 

7 early childhood (ages 1-4 years), may underlie the elevated risk of neurodevelopmental 

8 effects in young children to environmental exposures. Other factors that have been 

9 studied with regard to the influence on Pb-related risk are listed in Table 2-9 below.

Table 2-9 Summary of evidence for factors that potentially increase the risk of 

lead-related health effects

Factor Evaluated Potentially Related to Increased Risk

Age (Section 6.3.1) Children

Sex (Section 6.3.2) Males,a Femalesa

Genetics (Section 6.3.3) ALADa, VDRa*, DRD4a*, GSTM1a, TNF-α a, eNOSa, APOEa, HFEa

Pre-existing Disease (Section 6.3.4) Autisma, Atopyab, Hypertensionb

Smoking (Section 6.3.5) Smokersa

Race/Ethnicity (Section 6.3.6) Non-Hispanic Blacksa, Hispanicsa

Socioeconomic Status (SES) (Section 6.3.7) Low SESa

Nutrition (Section 6.3.10) Iron deficiencya

Stress (Section 6.3.11) High stressa

Cognitive Reserve (Section 6.3.12) Low cognitive reserveab

Other Metals (Section 6.3.13) High/co-exposure to Cda, Asa, Mna

a Evidence for this factor was limited.

b

Possible mediator
February 2012 2-80 Draft – Do Not Cite or Quote

2.10 Summary

1 Table 2-10 summarizes the main conclusions from the 2006 Pb AQCD and from this 

2 assessment, including causality determinations, regarding the health and ecological 

3 effects of Pb. 

Table 2-10 Summary of evidence from epidemiologic, animal toxicological and 

ecological studies on the effects associated with exposure to Pb

Outcome/Effect Conclusions from the 2006 Pb AQCD Conclusions from the (2012-2nd Draft) Pb ISA

Health Outcomes:

Nervous System 

Effects

The 2006 Pb AQCD did not integrate the evidence 

across specific endpoints within the nervous system to 

make conclusions regarding role of Pb in causing effects 

for the organ system as a whole. 

Based most heavily on cognitive function decrements and 

behavioral problems in children, the collective body of evidence is 

sufficient to conclude that there is a causal relationship between 

Pb exposures and nervous system effects.

Neurocognitive 

Function 

and Learning

Children:

The overall weight of the available evidence provides 

clear substantiation of neurocognitive decrements being 

associated in young children with blood-Pb 

concentrations in the range of 5-10 μg/dL, and possibly 

lower. Prenatal, early childhood, lifetime average, and 

concurrent blood Pb levels were associated with 

decrements in IQ and specific indices of learning and 

memory; however, concurrent blood Pb level was the 

strongest predictor.

Recent epidemiologic studies in children continue to 

demonstrate associations of concurrent blood Pb level with 

IQ decrements; most new evidence emphasizes 

associations of concurrent blood Pb levels with specific 

indices of neurocognitive function (e.g., verbal skills, 

memory, learning visuospatial processing). The weight of 

evidence supports associations in populations with mean 

blood Pb levels 2-7 μg/dL.

Adults:

Among environmentally-exposed adults, bone Pb levels 

but not blood Pb levels were associated with poorer 

cognitive performance. These findings point to an effect 

of long-term cumulative Pb exposure.

Consistent with previous studies, the weight of evidence in 

new studies indicates associations of cumulative Pb 

exposure metrics with cognitive performance in 

environmentally-exposed adults. However, as these 

outcomes are observed in adults with likely higher past Pb 

exposures, uncertainty exists as to the Pb exposure level, 

frequency, duration, and timing contributing to the observed 

associations

Neurobehavioral 

Effects

Children:

Several epidemiologic studies reported associations 

between Pb exposure and effects that ranged from 

inattention and aggression in children to delinquent 

behaviors and criminal activities in adolescents and 

young adults. Most studies examined blood Pb levels 

measured earlier in childhood (means ~ 10 μg/dL), tooth 

Pb, or bone Pb. There was little examination of 

concurrent blood Pb levels. Uncertainty remained 

regarding whether Pb exposure was an independent 

predictor of neurobehavioral effects. Results from studies 

of ADHD were inconclusive. Suggestive relationship for 

both blood and bone Pb with withdrawn behavior and 

anxiety symptoms in children.

Recent studies in children continue to support associations 

of Pb exposure with a range of effects, largely inattention 

and hyperactivity, and also misconduct delinquent behavior. 

In new studies, the weight of evidence supports 

associations with concurrent blood Pb in populations with 

lower mean blood Pb levels (2–5 μg/dL) than those in 

previous studies. New evidence indicates associations of 

concurrent blood Pb levels with ADHD diagnosis and 

contributing diagnostic indices in populations with mean 

blood Pb levels 2-4 μg/dL. 
February 2012 2-81 Draft – Do Not Cite or Quote

Outcome/Effect Conclusions from the 2006 Pb AQCD Conclusions from the (2012-2nd Draft) Pb ISA

Neurobehavioral, 

Mood and 

Psychiatric Effects

Adults:

Associations of Pb exposure with behavioral outcomes 

were not widely examined in environmentally-exposed 

adults.

In the few available new studies of environmentallyexposed adults, blood and bone Pb levels are associated 

with symptoms of depression and anxiety.

Sensory Organ 

Function

The selective action of Pb on retinal rod cells and bipolar 

cells is well documented in earlier AQCDs. There was 

coherence between the extensive animal and the limited 

available human literature reporting associations 

between concurrent blood Pb levels (population means 

7-12 μg/dL) and decreased auditory function in children. 

The limited available new epidemiologic studies on sensory 

organ function in children examined children with high 

blood Pb levels ((means > 30 μg/dL. A recent toxicological 

study found retinal effects in male rodents who had been 

exposed at gestation through 10 days of age at exposures 

resulting in lower blood Pb levels (~12 μg/dL) than those at 

which effects had been previously reported.

Neurodegenerative 

Diseases

In the limited body of epidemiologic studies, blood and 

bone Pb levels were not associated with Alzheimer’s 

disease or Amyotrophic Lateral Sclerosis among 

environmentally-exposed adults. Each study had 

sufficient limitations.

A few case-control studies report associations between 

bone Pb levels and PD in environmentally-exposed adults. 

Recent toxicological evidence suggests that early life Pb 

exposure may be associated with neurodegeneration in 

adult animals. 

Cardiovascular Effects

The 2006 Pb AQCD did not integrate the evidence 

across specific endpoints within the cardiovascular 

system to make conclusions regarding role of Pb in 

causing effects for the organ system as a whole.

The weight of the evidence continues to support and 

expand upon the strong body of evidence that Pb exposure 

is associated with increased blood pressure and 

hypertension in adults. The evidence is sufficient to 

conclude that there is a causal relationship between Pb 

exposures and cardiovascular effects; however, as these 

outcomes are observed in adults with likely higher past Pb 

exposures, uncertainty exists as to the Pb exposure level, 

frequency, duration, and timing contributing to the observed 

associations. 

Blood Pressure

A meta-analysis of numerous epidemiologic studies 

estimated that a doubling of blood Pb level (e.g., from 5 

to 10 μg/dL) was associated with a 1 mmHg increase in 

systolic BP and a 0.6 mmHg increase in diastolic BP.” 

Recent epidemiologic and toxicological studies continue to 

support associations between long-term Pb exposure and 

increased BP. Associations of increased BP with blood and 

bone Pb concentrations are observed in populations with 

lower mean blood Pb levels. 

Hypertension

Epidemiologic studies consistently demonstrated 

associations between Pb and incidence of hypertension

with suggestive evidence that bone Pb may be 

associated with hypertension. Animal studies 

demonstrated that long-term exposure to Pb resulted in 

hypertension that persisted after cessation of exposure.

Recent studies, including those using bone Pb as a metric 

of cumulative exposure, continue to demonstrate 

associations of hypertension with Pb levels in adults at 

lower population Pb concentrations. Recent studies have 

emphasized the interaction of cumulative exposure to Pb 

with other factors including stress.

Cardiovascular 

Mortality

The evidence for an association of Pb with 

cardiovascular mortality is limited but supportive. 

Recent studies address limitations of previous studies and 

provide additional evidence for an association of Pb with 

cardiovascular mortality in adults. Specific causes of 

mortality that were associated with Pb could be related to 

increased BP and hypertension.

Renal Effects

The 2006 Pb AQCD did not integrate the evidence 

across specific endpoints within the renal system to 

make conclusions regarding role of Pb in causing effects 

for the organ system as a whole. Circulating and 

cumulative Pb was associated with longitudinal decline in 

renal function in adults. Toxicological studies 

demonstrated that initial accumulation of absorbed Pb

occurred primarily in the kidneys and noted a 

hyperfiltration phenomenon during the first 3 months of 

exposure, followed by decrements in kidney function. 

Recent epidemiologic and toxicological studies evaluated 

in the current review support or expand upon the strong 

body of evidence indicating that Pb exposure is associated 

with kidney dysfunction (e.g., lower creatinine clearance, 

higher serum creatinine, and lower GFR) in 

nonoccupationally-exposed adults. The evidence is 

sufficient to conclude that there is a causal relationship 

between Pb exposures and renal health effects; however, 

as these outcomes are most often observed in adults with 

likely higher past Pb exposures, uncertainty exists as to the 

Pb exposure level, frequency, duration, and timing 

contributing to the observed associations.
February 2012 2-82 Draft – Do Not Cite or Quote

Outcome/Effect Conclusions from the 2006 Pb AQCD Conclusions from the (2012-2nd Draft) Pb ISA

Immune System 

Effects

The 2006 Pb AQCD did not integrate the evidence 

across specific endpoints within the immune system to 

make conclusions regarding role of Pb in causing effects 

for the organ system as a whole.

The consistency and coherence of findings across the 

continuum of related immune parameters that demonstrate 

a stimulation of Th2 responses in toxicological studies 

combined with the supporting epidemiologic evidence in 

children are sufficient to conclude that there is a causal 

relationship between Pb exposures and immune system 

effects.

Children:

Several epidemiologic studies suggested that Pb 

exposure may be associated with effects on cellular and 

humoral immunity in children. The principal effects 

demonstrated were decreases in T cell abundance and 

increases in serum immunoglobulin E levels with 

concurrent blood Pb levels > 10 μg/dL. Toxicological 

evidence supported these findings with extensive 

evidence for prenatal and early postnatal Pb exposures

skewing toward Th2 cytokine production and affecting 

downstream events such as inflammation and decreased 

responses to antigens (e.g., delayed type 

hypersensitivity, bacterial resistance).

A small body of new studies supports the previous 

evidence that Pb exposure is associated with immune 

effects in children. New studies in children added to the 

evidence for associations of blood Pb levels with asthma, 

allergy, and IgE. The consistency and coherence of 

findings among related immune effects that support a shift 

from a Th1 to a Th2 phenotype establishes the biological 

plausibility for epidemiologic observations of associations 

with asthma, allergy and inflammation-related effects in 

other organ systems. 

Adults:

In the large body of studies in occupationally-exposed 

adults, the most consistent findings were reduced 

neutrophil functionality in workers with blood Pb levels > 

30 μg/dL. Pb exposure-associated immune effects were 

not widely examined in environmentally-exposed adults.

Several toxicological studies found a Pb-induced shift to 

Th2 cytokine production and a hyperinflammatory 

phenotype of macrophages in adult animals with longterm (> weeks) exposure.

A small body of available studies provides new evidence 

for immune effects in environmentally-exposed adults. 

Specific endpoints varied among studies but were 

consistent with increased inflammation. Associations were 

observed in populations with a wide range of mean 

concurrent blood Pb levels (3-22 μg/dL). However, as these 

outcomes are observed in adults with likely higher past Pb 

exposures, uncertainty exists as to the Pb exposure level, 

frequency, duration, and timing contributing to the observed 

associations. A few available toxicological studies find Pbassociated immune effects in adult mice.

Heme Synthesis and 

RBC Function

Children:

Pb exposure was associated with disruption in heme 

synthesis with increases in blood Pb levels of 

approximately 20 μg/dL sufficient to halve ALAD activity 

and inhibit ferrochelatase. Risk of clinical anemia in 

children becomes apparent at high blood Pb levels: 10% 

probability of anemia was estimated to be associated 

with ~ 20 μg/dL Pb at 1 year of age, 50 μg/dL at 3 years 

of age, and 75 μg/dL at 5 years of age.

Recent epidemiologic studies provide strong evidence that 

exposure to Pb is associated with numerous deleterious 

effects on the hematological system in children, including 

altered hematological parameters (Hb, MCV, MCH, RBC 

count), perturbed heme synthesis mediated through 

decreased ALAD and ferrochelatase activities, and 

oxidative stress. Associations were observed in populations 

with mean (or median) blood Pb concentrations as low as 

approximately 5 μg/dL. 

Adults:

Pb exposure was associated with disruption in heme 

synthesis with increases in blood Pb levels of 

approximately 20 μg/dL sufficient to halve ALAD activity 

and inhibit ferrochelatase. Exposures to Pb resulting in 

blood concentrations < 40 μg/dL appear to be tolerated 

without decreases in blood hemoglobin or hematocrit, 

however changes in erythropoiesis do occur at these 

blood levels.

Recent epidemiologic studies provide strong evidence 

exposure to Pb is associated with numerous deleterious 

effects on the hematological system, including altered 

hematological parameters (Hb, MCV, MCH, RBC count), 

perturbed heme synthesis mediated through decreased 

ALAD and ferrochelatase activities, decreased 

erythropoiesis, and oxidative stress. 

Reproductive Effects 

and Birth Outcomes

The 2006 Pb AQCD did not integrate the evidence 

across specific endpoints within the reproductive system 

to make conclusions regarding role of Pb in causing 

effects for the organ system as a whole.

The collective body of evidence integrated across 

epidemiologic and toxicological studies with a focus on the 

strong relationship observed with negative effects on sperm 

and delayed pubertal onset is sufficient to conclude that 

there is a causal relationship between Pb exposures and 

reproductive and developmental effects.

Children:
February 2012 2-83 Draft – Do Not Cite or Quote

Outcome/Effect Conclusions from the 2006 Pb AQCD Conclusions from the (2012-2nd Draft) Pb ISA

No epidemiologic studies of delayed onset of puberty 

were reviewed.

Recent toxicological and epidemiologic studies provide 

strong evidence for delayed onset of puberty in males and 

females. The weight of evidence for delayed onset of 

puberty was among children ages 6-18 years with 

mean/median blood Pb levels less than 5 μg/dL. 

Adults:

Epidemiologic evidence suggested small associations 

between Pb exposure and male reproductive outcomes 

including perturbed semen quality and increased time to 

pregnancy. Associations between Pb exposure and male 

reproductive endocrine status were not observed in the 

occupational populations studied. Toxicological studies 

provided evidence that Pb produced effects on male and 

female reproductive junction and development and 

disrupts endocrine function.

Recent toxicological and epidemiologic studies provide 

strong evidence for effects on sperm. Evidence on 

pregnancy outcomes was inconsistent and less coherent 

across disciplines for preterm birth, spontaneous abortion, 

low birth weight, birth defects, hormonal influence and 

fecundity. 

Cancer

Epidemiologic studies of highly exposed occupational 

populations suggest a relationship between Pb and 

cancers of the lung and the stomach; however the 

evidence is limited by the presence of various potential 

confounders, including metal coexposures (e.g., to 

arsenic, cadmium), smoking, and dietary habits. The 

2003 NTP and 2004 IARC reviews concluded that Pb 

and Pb compounds were probable carcinogens, based 

on limited evidence in humans and sufficient evidence in 

animals. Based on animal data and inadequate human 

data Pb and Pb compounds would be classified as likely 

carcinogens according to the EPA Cancer Assessment 

Guidelines for Carcinogen Risk Assessment.

The toxicological literature continues to provide the 

strongest evidence for Pb exposure and cancer with 

supporting evidence provided by the epidemiologic 

literature. Epidemiologic studies of cancer incidence and 

mortality reported inconsistent results.The evidence i is 

sufficient to conclude that there is a likely causal

relationship between Pb exposure and cancer. 

Ecological/Welfare Effects:

Physiological Stress

Terrestrial Organisms:

Pb exposure may cause lipid peroxidation and changes 

in glutathione concentrations. There are species 

differences in resistance to oxidative stress.

Recent studies continue to support previous associations 

of Pb exposure with physiological stress. New evidence 

includes upregulation of antioxidant enzymes, production of 

reactive oxygen species and increased lipid peroxidation 

associated with Pb exposure in additional species of 

terrestrial plants, invertebrates and vertebrates. 

Experimental exposures increasing from background 

concentrations to concentrations higher than near point 

sources result in increasing effects. The evidence is 

sufficient to conclude a causal relationship between Pb 

exposure and physiological stress in terrestrial plants, 

invertebrates and vertebrates.

Aquatic Organisms:

Pb exposure associated with alterations in enzymes 

involved in physiological stress responses.

Recent studies continue to support previous associations 

of Pb exposure with physiological stress. New evidence 

includes upregulation of antioxidant enzymes, production of 

reactive oxygen species and increased lipid peroxidation 

associated with Pb exposure in additional species of 

aquatic plants, invertebrates and vertebrates and 

decreased glycogen levels in freshwater snails. Observed 

effects generally occurred at concentrations that typically 

exceed Pb levels in U.S. waters with limited evidence for 

effects associated with Pb at polluted sites. The evidence is 

sufficient to conclude a causal relationship between Pb 

exposure and physiological stress in aquatic plants.
February 2012 2-84 Draft – Do Not Cite or Quote

Outcome/Effect Conclusions from the 2006 Pb AQCD Conclusions from the (2012-2nd Draft) Pb ISA

Hematological Effects

Terrestrial Organismps:

Pb effects on heme synthesis were documented in the 

1986 Pb AQCD and continue to be studied in terrestrial 

biota. Changes in ALAD are not always related to 

adverse effects but may simply indicate exposure. The 

linkage between effects of Pb on blood parameters is 

well documented, however, the linkage between 

hematological indicators and ecologically relevant effects 

is less well understood. 

Consistent with previous studies, the weight of the 

evidence in new studies continues to support findings of Pb 

effects on heme synthesis and ALAD enzyme activity. New 

studies in birds near historical mining areas and altered 

serum profiles and blood cell counts in vertebrates provide 

evidence for additional species and hematological 

endpoints potentially effected by Pb. The evidence is 

sufficient to conclude a causal relationship between Pb 

exposure and hematological effects in invertebrates and 

vertebrates in terrestrial ecosystems.

Aquatic Organisms:

In metal impacted habitats, ALAD is a recognized 

biomarker of Pb exposure. Changes in ALAD are not 

always related to adverse effects but may simply indicate 

exposure. In fish, Pb effects on blood chemistry have 

been documented with Pb concentrations ranging from 

100 to 10,000 μg Pb/L.

Consistent with previous studies, the weight of the 

evidence in new studies continues to support findings of Pb 

effects on ALAD and expands this evidence to additional 

species of bacteria, invertebrates, and vertebrates as well 

as new studies on altered blood cell counts in vertebrates. 

The evidence is sufficient to conclude a causal relationship 

between Pb exposure and hematological effects in 

invertebrates and vertebrates in aquatic ecosystems.

Neurobehavioral 

Effects

Terrestrial Organisms:

Exposure to Pb in laboratory studies and simulated 

ecosystems may alter species competitive behaviors, 

predator-prey interactions and contaminant avoidance 

behaviors.

Recent studies continue to support previous evidence that 

Pb exposure is associated with behavioral alterations. New 

studies identify possible molecular targets for Pb 

neurotoxicity in invertebrates and there is new evidence in 

a few invertebrate and vertebrate species for behavioral 

effects associated with Pb exposure (i.e., feeding and 

escape behaviors). The evidence is sufficient to conclude 

that there is a likely causal relationship between Pb 

exposure and neurobehavioral effects in terrestrial 

invertebrates and vertebrates.

Aquatic Organisms:

Exposure to Pb has been shown to affect brain receptors 

in fish and may alter avoidance behaviors and predatorprey interactions.

Recent studies continue to support previous evidence that 

Pb exposure is associated with behavioral alterations. New 

studies identify possible molecular targets for Pb 

neurotoxicity in fish and provide additional evidence for Pb 

effects on behaviors in aquatic organisms that may impact 

predator avoidance (swimming). The evidence is sufficient 

to conclude that there is a likely causal relationship 

between neurobehavioral effects in aquatic invertebrates 

and vertebrates.
February 2012 2-85 Draft – Do Not Cite or Quote

Outcome/Effect Conclusions from the 2006 Pb AQCD Conclusions from the (2012-2nd Draft) Pb ISA

Developmental and 

Reproductive Effects

Terrestrial Organisms:

No information on reproduction in plants. There are an insufficient number of studies that consider 

Pb effects on plant reproduction. Thus the evidence is 

inadequate to infer a causal relationship for terrestrial 

plants

Limited evidence in invertebrates and vertebrates. Recent studies in a few taxa expand the evidence for Pb 

effects on developmental and reproductive endpoints for 

invertebrates and vertebrates at concentrations that 

generally exceed Pb levels in U.S. soils. In some 

organisms, exposure-dependent responses in development 

and reproductive outcomes are observed in experiments 

where exposure increases from background level to levels 

greater than near point sources. Data on terrestrial species 

is supported by toxicological data from mammals in the 

context of human health research. The evidence is 

sufficient to conclude a causal relationship in terrestrial 

invertebrates and vertebrates.

Aquatic Organisms:

No reviewed studies on reproductive effects in aquatic 

plants. 

Reproductive and developmental effects reported in a 

few species of invertebrates at <50 μg Pb/L and in fish at 

<150 μg Pb/L

Recent evidence supports previous findings of reproductive 

and developmental effects of Pb in aquatic biota and 

differential lifestage response at near ambient 

concentrations of Pb in some organisms. The evidence is 

inadequate to conclude a causal relationship for plants, 

and sufficient to conclude a causal relationship in 

invertebrates and vertebrates. 

Growth

Terrestrial Organisms:

Pb inhibits photosynthesis and respiration in plants.

Limited evidence for growth effects in soil invertebrates, 

avian and mammalian consumers.

Recent studies support previous findings of Pb effects on 

plant growth with some evidence for exposure-dependent 

decreases in some plant species’ biomass in soil amended 

with 30 mg Pb/kg soil. The evidence is adequate to 

conclude a causal relationship in plants. 

Limited studies considered effects on growth in 

invertebrates and vertebrates. The evidence is inadequate 

to conclude a causal relationship for terrestrial 

invertebrates and vertebrates.

Aquatic Organisms:

Evidence for growth effects in algae, aquatic plants and 

aquatic invertebrates

Most primary producers experience EC50s for growth in 

the range of 1,000 to 100,000 μg Pb/L

The weight of the evidence continues to support growth 

effects of Pb in aquatic plants and invertebrates.Recent 

studies on growth in invertebrates find effects of Pb at 

lower concentrations than previously reported. The 

evidence is sufficient to conclude a causal relationship 

between Pb exposure and growth in aquatic plants and 

invertebrates. Growth inhibition in one species of 

freshwater snail was observed at <4 μg Pb/L in juveniles.

Lowest genus mean chronic value for Pb reported at 10 μg 

Pb/L in a freshwater mussel.

The evidence is inadequate to conclude a causal 

relationship between Pb exposure growth effects in aquatic 

vertebrates.
February 2012 2-86 Draft – Do Not Cite or Quote

Outcome/Effect Conclusions from the 2006 Pb AQCD Conclusions from the (2012-2nd Draft) Pb ISA

Survival

Terrestrial Organisms:

No information on mortality in plants. Effects of Pb on 

invertebrates and vertebrates include decreased survival. 

In terrestrial and avian species toxicity was observed in 

laboratory studies over a wide range of doses 

(<1 to >1,000 mg Pb/kg body weight•day) (U.S. EPA, 

2005b).

Recent studies in invertebrates and vertebrates support 

previous associations between Pb exposure and mortality. 

The evidence is inadequate to conclude a causal 

relationship between Pb exposure and survival for 

terrestrial plants. The evidence is sufficient to conclude a 

causal relationship in terrestrial vertebrates and 

invertebrates.

Aquatic Organisms:

No studies reviewed on mortality in plants at current 

concentrations of Pb in the environment.

Pb impacted survival of some aquatic invertebrates at 

<20 μg Pb/L dependent upon water quality variables 

(i.e., DOC, hardness, pH).

Range of 96-hour LC50 values in fathead minnow: 810-

>5,400 μg Pb/L

The weight of evidence continues to support Pb effects on 

survival of aquatic invertebrates and vertebrates and 

indicates that there are effects in a few species at lower 

concentrations than previously reported.

New evidence for effects in a few invertebrates: at 

<20 μg Pb/L

New evidence in fish for impacts to survival at <100 μg 

Pb/L dependent upon water quality parameters and 

lifestage

96- hour LC50 values as low as 41 μg Pb/L in fathead 

minnows tested in natural waters from across the U.S. 

The evidence is inadequate to conclude a causal 

relationship between Pb exposure and survival for aquatic 

plants. The evidence is sufficient to conclude a causal 

relationship between Pb exposure and survival for aquatic 

invertebrates and vertebrates.

Community and 

Ecosystem Level 

Effects

Terrestrial Ecosystems:

Effects of Pb difficult to interpret because of the presence 

of other stressors including metals. The 1986 Pb AQCD 

reported shifts toward Pb-tolerant communities at 500 to 

1,000 mg Pb/kg soil. 

In the 2006 Pb AQCD, decreased species diversity and 

changes in community composition were observed in 

ecosystems surrounding former smelters.

New evidence for effects of Pb in soil microbial 

communities add to the body of evidence for effects at 

higher levels of biological organization. However, most 

evidence for Pb toxicity to terrestrial biota is from singlespecies assays. Uncertainties exist in generalizing effects 

observed under small-scale, predicted conditions up to 

effects at the ecosystem level however, uptake of Pb into 

terrestrial organisms and subsequent effects on 

reproduction, growth and survival at the species level is 

likely to lead to effects at the population, community and 

ecosystem level. The evidence is sufficient to conclude that 

there is a likely causal relationship between Pb exposure 

and the alteration of species richness, species composition 

and biodiversity in terrestrial ecosystems.

Aquatic Ecosystems:

Most evidence of community and ecosystem level effects 

is from near point sources, usually mining effluents. 

Effects of Pb difficult to interpret because of the presence 

of other stressors including metals.

Generally, there is insufficient information available for 

single materials in controlled studies to permit evaluation 

of specific impacts on higher levels of organization 

(beyond the individual organism).

New evidence for Pb effects on sediment-associated and 

aquatic plant communities add to the body of evidence of 

effects at higher levels of biological organization. However, 

most evidence for Pb toxicity to aquatic biota is from singlespecies assays. Uncertainties exist in generalizing effects 

observed under small-scale, predicted conditions up to 

effects at the ecosystem-level however, uptake of Pb into 

aquatic organisms and subsequent effects on reproduction, 

growth and survival at the species level is likely to lead to 

effects at the population, community and ecosystem level. 

The evidence is sufficient to conclude that there is likely to 

be a causal relationship between Pb exposure and the 

alteration of species richness, species composition and 

biodiversity in aquatic ecosystems.

1
February 2012 2-87 Draft – Do Not Cite or Quote

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