p.1 

OAR Box 1128 

Prepped by Ollie Stewart 

Document Number: 

106)11-1-435 

Docket Number: 

A-83-22 

Printed 4/21/2008 11:07:15 AM Sheet 106 of 198 
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AEPA 

$-%?>' 

United States 

Environmental Protection 

Agency 

Office of Health and 

Environmental Assessment 

Washington DC 20460 

EPA/600/8-89/045 A 

March 1989 

External Review Draft 

Research and Development 

Evaluation of the 

Potential 

Carcinogenicity of 

Lead and Lead 

Compounds: 

Review 

Draft 

(Do Not 

Cite or Quote) 

1 

I 6 6 B f II. 

MAR 3 0 I992 

n Support of EPA AIR DOCKET 

Reportable Quantity 

Adjustments Pursuant 

to CERCLA Section 102 

NOTICE 

This document is a preliminary draft. It has not been formally 

released by EPA and should not at this stage be construed to 

represent Agency policy it is being circulated for comment on its 

technical accuracy and policy implications 
P.3 

DRAFT EPA/600/8-89/045A 

DO NOT QUOTE OR CITE March 1989 

External Review Draft 

EVALUATION OF THE POTENTIAL CARCINOGENICITY OF 

LEAD AND LEAD COMPOUNDS 

In Support of Reportable Quantity Adjustments 

Pursuant to CERCLA Section 102 

NOTICE 

THIS DOCUMENT IS A PRELIMINARY DRAFT. It has not been formally released by 

the U.S. Environmental Protection Agency and should not at this stage be 

construed to represent Agency policy. It is being circulated for comment on 

its technical accuracy and policy implications. 

Office of Health and Environmental Assessment 

Office of Research and Development 

U.S; Environmental Protection Agency 

Washington, D.C. 
P.4 

DISCLAIMER 

This document is an external draft for review purposes only and does not 

constitute Agency policy. Mention of trade names or commercial products does 

not constitute endorsement or recommendation for use. 

n 
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CONTENTS 

Tables v 

Preface vi 

Abstract ix 

Authors and Reviewers x 

1. WEIGHT OF EVIDENCE . 1 

1.1. HUMAN STUDIES 1 

1.1.1. Epidemiologic Studies 1 

1.1.2. Case Reports 17 

1.1.3. Discussion 18 

1.2. ANIMAL STUDIES 23 

1.2.1. Lead Acetate 23 

1.2.2. Lead Subacetate 46 

1.2.3. Lead Oxide 55 

1.2.4. Lead Phosphate 56 

1.2.5. Lead Nitrate 58 

1.2.6.. Lead Naphthenate 59 

1.2.7. Lead Dimethyldithiocarbamate 59 

1.2.8. Tetraethyl Lead 64 

1.2.9. Ambient lead 64 

1.2.10. Discussion 65 

1.3. SHORT-TERM TESTS 78 

1.3.1. Mutagenicity 78 

1.3.2. Cell Transformation 79 

1.4. TOXICOLOGIC EFFECTS RELEVANT TO CARCINOGENICITY 79 

1.4.1. Human Studies 79 

1.4.2. Animal Studies 82 

1.5. PHARMACOKINETIC PROPERTIES 84 

1.5.1. Absorption 84 

1.5.2. Distribution and Retention 88 

1.5.3. Excretion 92 

1.5.4. Discussion 93 

m 
CONTENTS (continued) 

. 1.6. WEIGHT-OF-EVIDENCE CLASSIFICATION 94 

2, POTENCY 104 

2.1. GENERAL METHODOLOGY 104 

2.2. DATA SELECTION 106 

2.3. POTENCY ESTIMATION AND CHARACTERIZATION 107 

2.4. DISCUSSION . Ill 

3. HAZARD RANKING . 118 

3.1. GENERAL METHODOLOGY 118 

3.2. . OVERALL CHARACTERIZATION 119 

REFERENCES '. 120 

iv 
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TABLES 

1-1 Summary of positive animal studies. . 

1-2 Summary of nonpositive animal studies 

24 

32 
PREFACE 

This report summarizes and evaluates information on the potential 

carcinogenicity of a hazardous substance defined under Section 101(14) of the 

Comprehensive Environmental Response, Compensation, and Liability Act of 1980 

(CERCLA). EPA's Office of Emergency and Remedial Response considers this 

report along with other information when adjusting reportable quantities under 

CERCLA Section 102. The general methodology for adjusting reportable 

quantities is described in the Technical Background Document to Support 

Rulemaking Pursuant to CERCLA Section 102, Volumes 1-3. The specific 

methodology for evaluating potential carcinogenicity is described in more 

detail in the Methodology for Evaluating Potential Carcinogenicity in Support 

of Reportable Quantity Adjustments Pursuant to CERCLA Section 102. Both 

methodology documents have been subjected to public review and comment as part 

of CERCLA rulemaking activities. 

Initial drafts of this report were researched and written by the staff of 

the Chemical Hazard Evaluation Program at the Oak Ridge National Laboratory 

under Interagency Agreement No. DW89933015. The discussions and conclusions 

of this report were developed and written by members of the Human Health 

Assessment Group in EPA's Office of Health and Environmental Assessment. 

Pertinent studies included in this report were identified through 

computer literature searches and from the reference lists of relevant 

publications. Every attempt has been made to use primary sources for the 

pertinent cancer studies instead of abstracts or data summaries contained in 

secondary sources such as review articles, surveys, and monographs. Computer 

searches went as far back in time as the data bases would allow. Studies 

vi 
P.9 

appearing in one of these sources up through 1988 have been included. The 

following data bases were used: 

CHEMLINE 

TOXLIT 

TOXLINE 

Subfiles: 

EMIC 

ETIC 

EPIDEM 

PESTAP 

HMTC 

TOXNET 

Subfiles: 

CCRIS 

. HSDB 

RTECS 

Chemical 

Abstracts (CA) 

BIOSIS 

International 

Pharmaceutical 

Abstracts (IPA) 

CIS Abstracts 

CRISP 

NTIS 

TSCATS 

National Library of Medicine 

National Library of Medicine 

National Library of Medicine 

Oak Ridge National Laboratory 

Oak Ridge National Laboratory 

Oak Ridge National Laboratory 

U.S. Environmental Protection Agency (Pesticide 

Abstracts) 

Department of the Army 

National Library of Medicine 

National Cancer Institute 

National Library of Medicine 

National Institute of Occupational Safety and 

Health 

Chemical Abstracts Service 

Biological Abstracts, Inc. 

American Society of Hospital Pharmacists 

International Labour Office, International 

Occupational Safety and Health Information 

Center 

National Institutes of Health 

National Technical Information Service 

U.S. Environmental Protection^Agency (Toxic 

Substances Control Act Test Submissions) 

* '" 

EPA's Air Quality Criteria for Lead (U.S. EPA, 1986c) has been used 

extensively as a reference. The Air Quality Criteria for Lead has had 

considerable peer review, including review by the Clean Air Scientific 

Advisory Committee of the Science Advisory Board in public sessions. 

Before beginning work on this report, EPA consulted several metal 

carcinogenicity experts regarding the evidence on the potential 

carcinogenicity of lead. The conclusions of this report are consistent with 

the recommendations of those experts. 

vn 
The conclusions of this report can also be regarded as a natural 

extension of the conclusions concerning the potential carcinogenicity of lead 

reached by the International Agency for Research on Cancer in 1980 and 1987 

and by the National Toxicology Program in 1985. 

This report is organized to reflect two factors—weight of evidence and 

potency—that the Agency considers important in characterizing potential 

carcinogens. Section 1 deveTops the weight of evidence (the strength of the 

evidence that a substance causes cancer) according to EPA's Guidelines for 

Carcinogen Risk Assessment. Information has been organized as described in 

the guidelines; accordingly, fection 1 has subsections dealing with human 

studies, long-term animal studies, short-term tests, toxicologic effects other 

than carcinogenicity that are relevant to the evaluation of carcinogenicity, 

and pharmacokinetic properties. Section 2 discusses the potency (the strength 

of a substance to cause cancer). Section 3 combines the weight of evidence 

and the potency into an overall hazard ranking for potential carcinogenicity. 

Finally, there is a list of references cited in this report. 

vm 
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ABSTRACT 

Information on the potential carcinogenicity of lead and lead compounds 

is summarized and evaluated under EPA's Guidelines for Carcinogen Risk 

Assessment. The human studies provide suggestive evidence of carcinogenicity, 

but, because of confounding exposures to other carcinogens and the lack of 

measurements of lead exposure, are inadequate to prove or disprove 

carcinogenicity. Numerous long-term animal studies, using several different 

forms of lead and routes of exposure, provide sufficient evidence. This 

combination of evidence, together with -Information from short-term tests, 

other toxic effects, and pharmacokinetic properties, yields a classification 

of lead and lead compounds as probable human carcinogens, Group B2. 

No specific cancer potency has been estimated for lead. Many factors 

influence lead-induced cancer, creating difficulties in selecting an 

appropriate measure of dose. Cross-species pharmacokinetic models are needed 

before the animal studies can be used to estimate a human cancer potency. 

Because it has been relatively high concentrations of lead that have increased 

the incidence of cancer in animals, it seems appropriate to characterize the 

cancer potency of lead as low, placing lead in potency Group 3 under EPA's 

Methodology for Evaluating Potential Carcinogenicity in Support of Reportable 

Quantity Adjustments Pursuant to CERCLA Section 102. 

Combining the weight-of-evidence group and the potency group, lead and 

lead compounds receive a low hazard ranking among potential carcinogens ranked 

for the purpose of adjusting CERCLA reportable quantities. The hazard ranking 

should be interpreted not as an absolute measure of concern, but as a ranking 

relative to other carcinogens. 

ix 
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AUTHORS AND REVIEWERS 

EPA's Office of Health and Environmental Assessment (OHEA) is responsible 

for this report. The project manager, with overall responsibility for 

coordinating and directing the research and production of this report is 

Vincent James Cogliano, Ph.D. 

Office of Health and Environmental Assessment 

U.S. Environmental Protection Agency 

Washington, D.C. 

Initial drafts of this report were researched and written by 

Kowetha A. Davidson, Ph.D. 

Chemical Hazard Evaluation Program 

Oak Ridge National Laboratory 

Oak Ridge, Tennessee 

Each section was reviewed and supplemented by OHEA scientists, who are 

responsible for the final draft and for developing and writing the discussions 

and conclusions. Each scientist is with OHEA's Human Health Assessment Group 

in Washington, D.C , unless otherwise indicated. 

Section 1.1. Aparna M. Koppikar, M.D., D.P.H., D.I.H., Ph.D. 

Srikumar Menon, M.D., M.P.H., Ph.D. 

Department of Psychiatry 

Albert Einstein Medical Center 

Philadelphia, Pennsylvania 

Section 1.2. William E. Pepelko, Ph.D. 

David H. Reese, Ph.D. 

Dharm V. Singh, D.V.M., Ph.D. 

Tables. Vincent James Cogliano, Ph.D. 
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Section 1.3. Vicki L. Dellarco, Ph.D. 

David H. Reese, Ph.D. 

Section 1.4. William E. Pepelko, Ph.D. 

Section 1.5. Jean C. Parker, Ph.D. 

J. Michael Davis, Ph.D. 

Environmental Criteria and Assessment Office 

Office of Health and Environmental Assessment 

U.S. Environmental Protection Agency 

Research Triangle Park, North Carolina 

Section 1.6. Vincent James Cogliano, Ph.D. 

Charles H. Ris, M.S., P.E. 

Section 2. Vincent James Cogliano, Ph.D. 

Section 3. Vincent James Cogliano, Ph.D. 

Overall review of this report was provided by 

Charles H. Ris, M.S., P.E. 

Deputy Director, Human Health Assessment Group 

Jean C. Parker, Ph.D. 

Acting Associate Director for Science, 

Office of Health and Environmental Assessment 

William H. Farland, Ph.D. 

Director, Office of Health and Environmental Assessment 

xi 
P.14 

1. WEIGHT OF EVIDENCE 

1.1. HUMAN STUDIES 

1.1.1. Epidemiologic Studies 

Over a dozen epidemiologic studies on lead exposure and cancer are 

reviewed. These studies were conducted in different occupational settings 

including: a cohort of lead battery workers in England (Dingwall-Fordyce and 

Lane, 1963, which was expanded and studied by Malcolm and Barnett, 1982, and 

then by Fanning, 1988); lead battery and smelter workers (Cooper and Gaffey, 

1975, and followed up later by Cooper, 1976, and Cooper et al., 1985); smelter 

workers (McMichael and Johnson, 1982; Selevan et al., 1985; Gerhardson et al., 

1986; Ades and Kazantzis, 1988); chemical plants producing tetraethyl lead 

(Robinson, 1974; Sweeney et al., 1986); ahd plumbers and pipe fitters (Cantor 

et al., 1986). Two other studies were designed to test the hypothesis that 

paternal occupational exposure to lead was associated with an increased risk 

of Wilms's tumor in offspring (Kantor et al., 1979; Wilkins and Sinks, 1984). 

In addition, two case reports of long-time lead workers having cancer are 

reviewed. 

The major objective in reviewing the epidemiologic data base is to 

determine if a causal association between lead exposure and cancer can be 

demonstrated or refuted. The presence and absence of positive results and the 

characterization of the study methodology along with confounding factors 

ultimately provides the basis for weighing the human evidence according to 

EPA's Guidelines for Carcinogen Risk Assessment (U.S. EPA, 1986a). 
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1.1.1.1. Studies in Lead Battery Workers--

1.1.1.1.1. Dingwall-Fordvce and Lane, 1963. In this retrospective survey of 

workers known to have been exposed to lead in several companies in England, 

mortality records for 425 pensioners were reviewed and the cause of death was 

obtained from death certificates. The cohort was defined as all men who 

became pensioners between 1926 and 1960 at the age of 65 years with at least 

25 years of service. Of the 425 pensioners, 184 had died during the review 

period. Expected deaths were calculated by applying the death rates for all 

males in England and Wales standardized by age. The mortality experience of 

men who had died between 1946 and 1961 while employed in one electric 

accumulator company was also studied providing 153 such deaths. Since the 

population at risk was unknown for this group, risk ratios were calculated 

using a proportional mortality analysis. Workers were placed in one of three 

categories depending on urine lead levels: (1) no exposure (grade A) , (2) 

negligible exposure (grade B), and (3) lead in urine between 100 and 250 ug/L 

(grade C) . 

The analysis demonstrated increased total cancer deaths among pensioners 

(standard mortality ratio [SMR] = 212) as well as among employed men 

(proportional mortality ratio [PMR] = 157) in grade B. In the grade A 

exposure group, an SMR of 123 and a PMR of 113 were observed in pensioners and 

employed men, respectively. None of the excesses were statistically 

significant. There was no evidence of any excess of deaths from malignancies 

among pensioners and employed men in grade C. 

This study is at best a very exploratory study with many limitations. 

These include small sample size, lack of lifetime occupational history, lack 
P.16 

df cause-specific death analysis, and absence of data on smoking and other 

confounders. 

1.1.1.1.2. Malcolm and Barnett, 1982. The mortality experience of 1898 

pensioners (1644 men and 254 women) from four lead and battery companies in 

England, which included the factories used by Dingwall-Fordyce and Lane in 

their study, was analyzed using a retrospective cohort design. A proportional 

mortality analysis of deaths among 553 employees who died while still employed 

was also done. The cohort of pensioners was defined as long-term employees 

who between January 1, 1925, and December 31, 1960, had a combined score of 

age at retirement plus years of service of 90 and 95 years for women and men, 

respectively. Causes of deaths were coded using the eighth revision of the 

International Classification of Deaths (ICD). Expected deaths were calculated 

by multiplying the number of individuals exposed at risk in each age group and 

quadrennial period by the appropriate national age and cause-specific rate of 

mortality for the same period. Principal occupation at work was used to 

classify workers as having no (group 1), light (group 2), or high (group 3) 

occupational lead exposure. 

The analysis showed no significant excess mortality from all cancers 

combined in any of the three groups among pensioners. Among, men dying in 

service, who had high exposure to lead, dying in service, an excess mortality 

from digestive tract cancer was observed (observed = 12, PMR = 167, 

p = 0.009). Most of this excess was confined from 1963 to 1966. Overall, no 

excess deaths from malignant diseases were found in any other groups. 

The absence of a consistent index of cumulative lead exposure is a major 

limitation of this study. The use of principal occupation at work rather than 

a summary measure of lifetime occupation to assess lead exposure is another 

3 
P.17 

drawback of this study. No data on confounding variables were available for 

analysis. 

1.1.1.1.3. Fanning, 1988. Using a case-control approach, the authors of this 

study reviewed the records of 867 men who died and were considered to have 

high or moderate lead exposure and compared it with the records of 1206 men 

who died and were identified as having little or no lead exposure. Battery 

factories used by Dingwall-Fordyce and Lane (1963) and Malcolm and Barnett 

(1982) to draw their population were included in the source population for 

this study. The study population was employed in battery factories and others 

producing plastics and electrical equipment. The case group comprised the 

cause-specific deaths of interest, and all other deaths served as the control 

group. Scrutiny of job titles by managers and physicians categorized workers 

into high and low exposure groups. As a general indication of the level of 

lead exposure of these two groups, the range of blood levels over the past 

20 years of monitoring (whenever carried out periodically) would have been 

between 40 and 80 ug/dL for high exposure while less than 40 ug/dL for low 

exposure. 

Although the study design was a case-control type, the results of the 

analysis were presented as tables of observed to expected values. The authors 

concluded that no excess risk was observed for malignant neoplasms. 

There are many questions related to the study design and analysis that 

have not been answered by the authors. Notably, in the methods section 

mention is made of constructing 2x2 tables and deriving odds ratios in a casecontrol design, but the results are discussed in terms of observed and 

expected deaths with no elaboration of how the expected deaths were 
P.18 

calculated. One is, therefore, unable to comment on the conclusions reached 

by the authors. 

1.1.1.2. Studies in Battery Plant and Smelter Workers--

1.1.1.2.1. Cooper and Gaffey, 1975; Cooper, 1976. Six lead production plants 

and 10 battery plants were selected for a study of mortality in workers who 

had been employed at least 1 year between January 1, 1946, and December 31, 

1970. A population of 7032 (2352 smelter workers and 4680 battery workers) 

was selected from a total of 24,494 workers. Between 1947 and 1970 there were 

1356 deaths (19 percent of the study population), with 342 deaths among 

smelter workers and 1014 among battery workers. The PMRs of 12 selected 

causes of death were determined and compared with those of U.S. males, and the 

SMR based on the number of observed and expected deaths was calculated using 

the general U.S. male population as the reference. Causes of death were coded 

according to the seventh (1955) revision of the ICD. Urinary and blood lead 

levels were available for some of the workers. The average urinary lead 

concentration was 173.2 ug/L in smelter workers and 129.7 ug/L in battery 

workers; the average blood lead concentration was 79.7 ug/dL in smelter 

workers and 62.7 ug/dL in battery workers. Because of the very low proportion 

of deceased workers with known values, no attempt was made to correlate the 

mortality with urinary or blood lead concentrations. 

A nonsignificant excess in deaths due to all causes was observed among 

smelter workers but not among battery workers. Nevertheless, Cooper and 

Gaffey (1975) stated that this was an unfavorable finding, because most 

employed populations show SMRs below 100. The excess in deaths due to all 

malignant neoplasm's was statistically significant among smelter workers 

(p < 0.05) but not among battery workers. The SMRs for malignant neoplasms at 
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specific sites, however, were not statistically significant. It should also 

be noted that the SMRs for "other hypertensive diseases" (renal in origin) and 

for chronic or unspecified nephritis were increased in both cohorts but not 

significantly. The International Agency for Research on Cancer Working Group 

(IARC, 1980) used the Poisson method to recalculate the data presented by 

Cooper and Gaffey (1975) and reported no statistical significance based on 

95 percent confidence limits. Kang et al. (1980), believing that an error was 

involved in the statistical analysis by Cooper and Gaffey (1975), analyzed the 

data using the Chiang method (Chiang, 1961, as cited in Kang et al., 1980) and 

th? Poisson method and reported statistical significance for excess deaths due 

to all malignant neoplasms, neoplasms of the digestive organs, and neoplasms 

of the respiratory system for smelter workers. For battery workers, they 

reported statistical significance for excess deaths due to neoplasms of the 

digestive organs and respiratory organs but not for all malignant neoplasms. 

Gaffey (1980) stated that the error was made in the typing and not in their 

analysis. 

1.1.1.2.2. Cooper et al., 1985. A second follow-up study of the two cohorts 

was conducted; the observation period for mortality was extended to December 

31, 1980. The size of the study population, by strict editing of the data 

base, was reduced from 7032 to 6819, with 2300 smelter workers and 4519 

battery workers remaining in each cohort; 35 deaths counted earlier were 

eliminated. Causes of death were coded according to the seventh revision of 

the ICD. The same exposure data used in the previous study (Cooper and 

Gaffey, 1975) were presented in this report. 

Among battery workers, there was a significant excess in deaths due to 

all causes (SMR = 113, p < 0.01). Excess deaths due to all malignant 
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neoplasms (SMR = 114, p < 0.05), malignant neoplasms of the stomach 

(SMR = 168, p < 0.01), respiratory system (SMR = 124, p < 0.05), and trachea, 

bronchus, and lung (SMR = 124, p < 0.05) were significant. Nonsignificant 

excesses were observed for liver and laryngeal cancers, but both these 

excesses were based on a small number of deaths. Among smelter workers, there 

was a significant excess in deaths due to all causes (SMR = 107, p < 0.01) and 

due to all malignancies. Malignancies at specific sites were noted among 

smelter workers at the same sites as among battery workers. It should be 

noted that the SMRs for malignancies of the kidney were less than 100 in both 

cohorts, but deaths due to chronic kidney diseases were significantly elevated 

(p < 0.01) in both cohorts. Mortality could not be positively related to 

duration of employment in either cohort, because there was a trend toward 

decreasing SMRs with increasing cumulative years of employment. In the 

original study (Cooper and Gaffey, 1975) and the follow-up (Cooper et al., . 

1985), disease incidence could not be related to exposure due to insufficient 

numbers of workers with exposure values. 

Some of the methodologic limitations of these studies include lack of 

data on confounding factors, such as smoking habits, alcohol consumption, and 

exposure to other occupational chemicals, and absence of separate mortality 

analysis for whom the exposure data were available. Large sample size and a 

long follow-up period were among its strengths. 

1.1.1.3. Studies in Smelter Workers--

1.1.1.3.1. McMichael and Johnson, 1982. A proportional mortality analysis 

was conducted on the 140 deaths occurring among 241 male lead smelter workers 

in New South Wales, Australia, who were diagnosed as having lead poisoning 

between 1928 and 1959. At the time of diagnosis, each of these employees had 
P.21 

worked at the Braken Hill Associated Smelters operation for 1 to 30 years; the 

earliest date of hire was around 1910. A list of 241 employees with full 

names and dates of birth was available from company records and was crosschecked against death registration records in South Australia for the years 

1930 to 1944. A group of predominantly production workers free of known lead 

poisoning, together with a small number of office workers, totaling 695 

workers comprised the comparison group. Standardized proportional mortality 

analysis was done across four age strata: 30-44, 45-59, 60-74, and 75 and 

older. The expected number of deaths among lead-poisoned workers was 

calculated from the mortality experience of other workers. Proportional 

mortality rates were also calculated using the total Australian male 

population as the external comparison group. 

The analysis showed that lead poisoning did not increase the risk of 

cancer. The age-standardized PMR (SPMR) was 70. A modest excess of deaths 

was observed from chronic nephritis (SPMR = 306) and cerebral hemorrhage 

(SPMR = 199). 

The major limitations of proportional mortality analysis are that the 

derived ratios are not true measures of associations because a rise in the 

proportion of deaths due to a particular cause is always dependent on the 

deaths due to other causes. The population at risk is not known, so it is 

difficult to know whether the observed increase is a true rise in the actual 

rate of death or a fall in the rate from all other causes. The authors also 

do not provide any definition of "lead poisoning" or how it was measured, 

which was used as a surrogate for exposure. 

1.1.1.3.2. Selevan et al., 1985. Selevan et al. conducted a cohort mortality 

study of 1987 white male hourly workers, who had worked for at least 1 year at 

8 
P.22 

a primary lead smelter in Idaho, between January 1, 1940, and December 31, 

1965. The follow-up period was. through December 31, 1977. From this cohort, 

two nested subcohorts were selected. Those who had been subjected to mean 

airborne lead levels above 200 ug/m or those in which 50 percent or more 

jobs examined had mean airborne lead exposures greater than twice the standard 

were placed in the "high-lead" exposure group. Those in the high-lead group 

who had relatively low potential exposure to the other metals were placed in 

the "high-lead/low-others" group. Causes of death were coded according to the 

revision in effect at the time of death, and then converted to the seventh 

revision of the ICD. SMRs were calculated using the U.S. white male 

population as a reference. Statistical significance was presented as 95-

percent confidence intervals. 

Excess deaths due respiratory system cancers in the entire group 

(observed = 41, SMR = 111) and especially to kidney cancer in the entire group 

(observed = 6, SMR = 204) were observed in all cohorts. Both duration of 

exposure for more than 20 years and latency period of more than 20 years were 

positively associated with the cancers of the trachea, bronchus, and lung. 

None of these excesses were statistically significant. For kidney cancer, all 

but one case was found in both of the high-lead subcohorts. This finding 

approaches statistical significance in the high-lead/low-others group 

(SMR = 301, 95-percent confidence interval: 98 to 703). The SMR for chronic 

renal disease was highest in the high-lead group. Chronic renal disease was 

also positively correlated with duration of exposure, and the majority of 

excess was found after 20 years' latency. 
P.23 

This study had few methodologic limitations, one of them being lack of 

adjustment for exposure to other metals and smoking. In this study, cancer in 

humans was observed for the same site, namely the kidney, as in experimental 

animals, but the investigators pointed out that the levels of exposure to 

animals far exceeded the maximum doses tolerated by humans. 

1.1.1.3.3. Gerhardson et al., 1986. This is a retrospective cohort mortality 

study of 3832 male Swedish workers (cohort A) employed before 1967 at a copper 

smelter in Northern Sweden for at least 3 months and followed-up from 1950 to 

1981. A subcohort (B) of 437 workers employed at work sites with at least 

3 years' verified high-lead exposure from '950 to 1974 was also studied. The 

accumulated lead exposure for these workers was calculated as a cumulative 

blood lead dose obtained by a summation of the annual mean blood lead levels 

for each worker. Based on the median value of the cumulative blood lead dose 

(478.5 ug/dL), workers were categorized as high-exposure if the value exceeded 

the median and low-exposure if the value was lower. These subcohorts were 

designated as Bl (218 workers) and B2 (219 workers), respectively. Peak lead 

blood values were also used to divide workers into those who had at least once 

exceeded a blood lead concentration of 70 ug/dL (high peak value) and those 

who had never exceeded this level (low peak value). These subcohorts were 

called B3 (288 workers) and B4 (149 workers), respectively. Causes of deaths 

were coded by using the eighth revision of the ICD. Swedish national and 

county mortality rates by cause, sex, 5-year age groups, and calendar periods 

were used to calculate expected deaths and SMRs, and test-based 95-percent 

confidence limits were calculated according to Miettinen. 

A nonsignificant raised mortality for lung cancer (SMR = 160) was 

observed for subcohort B with high lead exposure. No consistent dose-response 

10 
pattern was seen when analysis was done by mean or peak blood lead values. 

Analysis using a latency period of 15 years did not provide additional 

information. 

The more precise assessment of lead exposure in this study is one of its 

major strengths. Since smelter workers have multiple exposures to other 

chemicals, a protective effect of some of them (e.g., selenium) cannot be 

ruled out. The sample size was small, and no analysis by smoking habits or 

other chemical exposures was available. 

1.1.1.3.4. Ades and Kazantzis, 1988. This is a retrospective cohort study 

that examined lung cancer mortality in a cohort of 4293 men employed at a 

zinc-lead-cadmium smelter in Great Britain. The cohort included individuals 

who were born prior to 1940, all hourly-paid male workers employed at the 

smelter on January 1, 1943, and those who subsequently started work before 

1970 and had worked for at least 1 year. At the end of the follow-up date of 

December 31, 1982, there were 4393 men. 

An excess of lung cancer was observed by using the comparison population 

of South West Urban Aggregates (observed = 182, SMR = 124, p < 0.005). A 

significant linear trend of increasing lung cancer SMRs with increasing 

duration of employment was observed (p < 0.001). The lung cancer SMRs for the 

total cohort were 190 (p < 0.005), 142 (p > 0.05), and 292 (p < 0.02) for the 

duration of employment of 20-29, 30-39, and 40+ years, respectively. 

A matched case-control analysis was also performed to determine the role 

of specific departments, processes, and contaminants in the smelter operation 

in causing cancer. A total of 4173 male smelter workers, of whom 174 died of 

lung cancer, were selected for analysis. Up. to 20 controls were chosen for 

each lung cancer case. In all there were 2717 controls. The follow-up period 

11 
was 10 years. Exposure estimates were obtained for cadmium, arsenic, lead, 

zinc, sulphur dioxide, and total dust. This was done by reducing the job 

codes to ten major categories and assigning values of 0 to 3 (to represent 

background to high exposure). These assessments were made by the plant 

hygienist without knowledge of the vital status or cause of death of the 

cohort members. SMRs were calculated using the Oxford person-years program, 

and the matched case-control study was constructed by selecting controls from 

the same cohort matched for date starting work, date of birth, and surviving 

the case. Cumulative exposures were calculated by summing the year-by-year 

exposure assessments over the period of employment up to 3 years before the 

• 

death of the case to whom controls were matched. Chi-square likelihood ratio 

statistics were derived by multiple logistic regression analysis preserving 

the matching. Relative risks were estimated for 10 years of employment at 

each exposure level independently and adjusted for the others. 

For lead alone, the chi-square likelihood ratio was 6.84, and the 

relative risk (R.R.) derived was 1.12 per level-decade (p < 0.01). For 

exposure to arsenic, the corresponding figures were chi-square = 6.31, 

R.R. = 1.22 (p < 0.025). A high correlation between exposure to arsenic and 

lead was observed (0.72), and a model yielding risks for each adjusting for 

the effects of the other showed no statistical significance. 

The investigators used a four-point rating scale to estimate exposures, 

but made no efforts to validate it, and did not discuss the assumptions and 

limitations of this approach. No data on smoking habits of the population 

were available. Lastly, while an increased risk of lung cancer was observed 

for lead, it was not possible to determine the contribution of simultaneous 

exposure to arsenic and other contaminants to this risk. 

12 
1.1.1.4. Studies in Tetraethyl Lead Production Workers--

1.1.1.4.1. Robinson. 1974. This study is a comparison of the mortality 

experience of workers having potential exposure to tetraethyl lead (TEL) with 

those having no exposure over a 20-year period. The TEL-exposed group 

comprised 592 white males (ages 20 to 58 years) who were actively employed in 

a production facility as of December 31, 1947. Their average length of TELarea service was 17.9 years. The non-TEL-exposed group consisted of all white 

male non-TEL operating and maintenance personnel at the same facility at the 

same date. There were 660 males in this group ranging in age from 21 to 

62 years and with an average length of service of 20.3 years. The mortality 

status of both populations was determined as of December 31, 1967. Crude and 

standardized death rates were calculated from the TEL group and the non-TEL 

groups. Standardized death rates were also calculated by applying race-sexage cause-specific death rates to the observed distributions of person-years 

of experience of the two groups. The expected numbers of deaths in the 

various categories of causes were derived. 

The analysis showed that both the TEL and non-TEL groups had no excess 

deaths from malignant neoplasms. 

The absence of specific quantitative information on TEL exposure and 

small sample size are two major limitations of this study. The low loss to 

follow-up rate and long follow-up period are among its strengths. 

1.1.1.4.2. Sweeney et al., 1986. This study is a retrospective cohort 

mortality study of 2510 males employed at an east Texas chemical plant to 

evaluate a suspected increase in deaths from multiple myeloma and brain 

cancer. The plant was opened in 1952 for the production of fuel antiknock 

additive TEL, and the cohort was defined as all male employees who worked at 

13 
P.27 

least 1 day at the plant between January 1, 1952, and December 31, 1977. 

Vital status through December 31, 1977, was ascertained through company 

sources and death certificates requested from State vital statistics offices. 

The National Institute of Occupational Safety and Health (NIOSH) modified 

life-table analysis system was used to accumulate person-years at risk for 

each cohort member. Vital status as of December 31, 1977, was ascertained for 

2493 (99%) of the 2510 men who comprised the cohort, and death certificates 

were ascertained for all but 2 of the 156 deaths. The seventh revision of the 

ICD was used to code the causes of deaths. The mortality for all causes was 

lower than the expected (SMR = 74). The SMR for all malignant neoplasms was 

slightly elevated (SMR = 102) due in part to statistically nonsignificant 

increases in mortality from carcinomas of the trachea, bronchus, and lung 

(SMR = 122), larynx (SMR = 426), and brain (SMR = 186). All these excesses 

were based on the small numbers. No increased risk of mortality was noted 

with increasing employment. 

The many limitations of the study include low power (0.27 for 

malignancies of specific sites), lack of complete workplace exposure data on 

lead compounds and other chemicals such as vinyl chloride monomers, ethylene 

dibromide, ethylene chloride, and dyes, and the absence of data on smoking and 

other lifestyle confounding variables. 

1.1.1.5. Study in Plumbers and Pipe Fitters--

1.1.1.5.1. Cantor et al., 1986. This is a proportional mortality study of 

7121 members and retirees of the United Association of Plumbers and Pipe 

Fitters in California who died between 1960 and 1979. The study population 

comprised all employed workers and retirees who remained union members of the 

United Association (UA), the leading labor union for plumbers and pipe fitters 

14 
P.28 

in North America. Death certificates from lists of deaths occurring between 

January 1, 1960, and December 31, 1979, were located from the- CaLifornic UA 

locals and State vital statistics offices and coded according to the eighth 

revision of the ICD. Ninety-seven percent of the death certificates were 

located. PMR was calculated using Monson's software by 5-year age and 5-year 

calendar periods. 

Among plumbers, the PMRs for all malignant neoplasms (PMR = 124); stomach 

cancer (PMR = 129); cancer of bronchus, trachea, and lung (PMR = 141); kidney 

cancer (PMR = 148); brain cancer (PMR = 150); and all lymphopoietic cancer and 

lymphosarcoma/reticulosarcoma (PMR = 124) were significantly elevated. The 

pipe fitters had a significantly elevated PMR for all cancers combined, 

primarily due to excess lung cancer. PMRs for lung cancer were consistently 

elevated when analysis was done by birth cohort and year of death. Lung 

cancer accounted for 185 of the 337 excess deaths from all malignant 

neoplasms. 

Since plumbers and pipe fitters encounter many hazardous materials 

including asbestos, solvents, metal fumes (lead and chromium), and gases from 

welding, it is impossible from this study to attribute any of the excess 

malignancies observed solely to lead exposure. No data were available to 

categorize and analyze data by degree of lead exposure. The significantly 

elevated PMR for kidney cancer among plumbers only (PMR = 1.48) and the 

experimental induction of renal tumors in animals is only suggestive of a 

possible association that merits further exploration in studies that assess 

individual exposures and work practices. 

15 
P.29 

1.1.1.6. Studies in Children with Paternal Occupational Lead Exposure--

1.1.1.6.1. Kantor et al., 1979. This study was designed to test the 

hypothesis that perinatal exposure to carcinogens may be associated with the 

subsequent occurrence of Wilms's tumor. A case-control study was conducted of 

149 Connecticut-born children with Wilms's tumor reported to the Connecticut 

Tumor Registry during the period 1935-1973. Cases ranged in age from the 

newborn to 19 years. The 149 controls were selected from the Connecticut 

State Health Department files of birth certificates and matched for sex, race, 

and year of birth. Sociodemographic information, including paternal 

occupation, was abstracted from birth certificates and the tumor registry 

files. Case-control comparisons were done for paternal individual occupations 

as well as for those categorized as lead exposure only, hydrocarbon exposure 

only, and the two combined. 

The occupations of fathers of 22 tumor cases and 6 controls were found to 

be lead related. The resulting odds ratio was 3.7 (p = 0.005). Cases and 

controls were found to be similar by mother's pregnancy history, parents' age 

and place of birth, child's birth weight, and length of gestation. The 

important limitations of this study are the total absence of data on maternal 

exposures, lack of validation of occupation history abstracted from birth 

certificates, lack of hereditary information of Wilms's tumor, and omission of 

results of tests of any measure of lead intoxication among study subjects. 

1.1.1.6.2. Wilkins and Sinks, 1984. This is an epidemiologic case-control 

study of paternal occupation and Wilms's tumor in offspring to test the 

hypothesis that lead exposure is a risk factor. The Columbus Childrens' 

Hospital (CCH) Tumor Registry was reviewed to identify children with Wilms's 

tumor between January 1, 1950, and October 31, 1981. For each case, two 

16 
P.30 

controls were selected at random using birth certificate files of Ohio 

residents as the sampling frame. Both controls were matched with the case by 

age, sex, and race, and one of them was also matched by mother's county of 

residence at the time of the child's birth. Controls were selected from 

counties within the CCH Wilms's tumor referral area. Birth certificates for 

study subjects were reviewed and information was abstracted on 

sociodemographic factors, including father's occupation at the time of child's 

birth. The occupation and exposure linkage system developed by Hoar et al. 

(1980) was used to identify job-related exposures of fathers of study 

subjects. Sixty-two cases and controls were available for comparison by 

paternal occupation. All forms of lead considered together gave an odds ratio 

of 1.0 (95-percent confidence interval of 0.5, 1.9). For exposure to lead and 

lead alkyls, the relative odds were 1.1 and 1.3, respectively, but neither was 

statistically significant. Thus, these data do not support the hypothesis 

that paternal exposure to lead is a risk factor for Wilms's tumor in the 

offspring. 

The small sample size, lack of lifetime paternal occupational histories 

and exposure misclassification, absence of information on maternal exposures, 

and selection bias in selecting cases from one hospital only are among the 

major limitations of this study. 

1.1.2. Case Reports 

1.1.2.1. Baker et al., 1980--A 48-year-old male who had worked at a smelter 

for 22 years had elevated blood lead levels of 64 ug/dL. Exposure to other 

chemicals was not reported. Clinical tests indicated renal damage, and a mass 

was found on the right kidney which was removed by nephrectomy. By light 

microscopy, the mass was identified as an adenocarcinoma with a predominant 

17 
P.31 

tubular pattern and extensive necrosis. Sections of the renal cortex showed 

some glomerular sclerosis, swelling of proximal tubular epithelial cells, 

sparse intranuclear inclusion bodies, and focal areas of interstitial fibrosis 

and inflammation. The concentration of lead was 2.47 ug/g tissue in the 

tumor, 1.04 ug/g tissue in the renal cortex, and 0.78 ug/g tissue in the renal 

medulla. Normal values were reported as 0.27 to 1.27 ug/g tissue. The 

authors suggested that a recent chelation therapy may have lowered the lead 

concentration in normal renal tissue. 

1.1.2.2. Lilis, 1981--A 61-year-old male who had been employed in a secondary 

lead smelter for 34 years starting in 1939 required five episodes of chelation 

therapy and was repeatedly removed from his work because of excessive blood 

lead concentrations. Exposure to the other chemicals was not discussed. The 

patient denied significant alcohol consumption and had not been smoking for a 

year. Clinical tests in February and December 1973 indicated that the patient 

suffered from kidney damage, and blood lead concentrations were 83 and 

60 ug/dL, respectively. A tumor found on the right kidney was removed by 

partial nephrectomy and was identified as a clear cell adenocarcinoma. 

1.1.3. Discussion 

Epidemiologic studies of workers in occupations where the potential for 

lead exposure is usually greater than in the general population were evaluated 

to ascertain whether lead is causally associated with an increased occurrence 

of cancer. In general, no attempt was made by any investigator to consider 

different types of lead compounds or predominant route of exposure except for 

two studies dealing with tetraethyl lead. 

All the reviewed studies have one or more methodological limitations that 

preclude the definitive inference of a causal association between lead 

18 
P.32 

exposure and the occurrence of cancer in humans. Deficiencies in exposure 

assessments, especially the lack of information on cumulative lead exposure, 

are a major limitation in all the studies. Because of the lack of information 

on lead exposure it is difficult to tell whether the entire population studied 

really had the exposure and did not show any excess of cancer or whether the 

absence of an excess is due to the fact that there was little or no exposure 

to lead compounds. The known presence of other potentially carcinogenic 

agents in the workplace, in addition to lead, also makes it difficult to 

determine if the observed increased risk is due to the effect of lead alone or 

whether these excesses are influenced by the presence of other agents. 

Furthermore, several of the studies are proportional mortality studies, 

which by their nature can be used to generate a hypothesis about causality but 

cannot be used tp authenticate one. The interpretive limitations of 

proportional mortality studies cover five areas. First, a proportional 

mortality analysis is usually used when the information on the population at 

risk is not available. A proportional mortality analysis uses the deaths from 

a given setting but not the population exposed to that setting; hence, this 

type of study cannot provide any information about the population's risk of 

dying from any given cause. Second, a proportional mortality analysis assumes 

that overall mortality is equal in the two populations being compared. If 

this assumption is not true and the mortality rate of the study group is lower 

than the mortality rate of the comparison population for all causes of deaths, 

the proportional mortality ratio will inflate the cause-specific mortality. 

Third, proportional mortality ratios of two or more causes of death are 

interdependent rather than independent, since the sum of the observed numbers 

of deaths necessarily equals the sum of the expected numbers (100%); hence, a 

19 
P.33 

deficit in one cause-specific mortality will artificially force an excess in 

another cause-specific mortality. Fourth, a proportional mortality analysis 

also assumes that the classification and reporting of deaths in the study 

population and the comparison population are complete and comparable. Fifth,, 

when a proportional mortality analysis is used to compile causes of death, it 

is assumed that both populations are comparable. Any violation of these 

assumptions may result in excesses or deficits which are not authentic. 

That said, McMichael and Johnson (1982) studied only those individuals 

who had "lead poisoning"; other exposed workers were not included in the 

study. This wap a proportional mortality study with inherent limitations. 

Two other studies were conducted in children to evaluate the association 

of Wilms's tumor with paternal occupation in the lead industry (Kantor et al., 

1979; Wilkins and Sinks, 1984). While neither study identified a 

statistically significant association, both studies have limitations such as a 

lack of information on the genetic predisposition of children, maternal 

exposures, and small sample sizes. Therefore, these two studies are unable to 

provide either positive or negative evidence of transplacental carcinogenicity 

of lead and its compounds. 

Two studies were conducted among workers exposed to tetraethyl lead 

(Robinson, 1974; Sweeney et al., 1986). Both studies had low power due to 

small sample sizes and confounding by other exposures, hence precluding any 

conclusive inference for the carcinogenicity of tetraethyl lead. 

Among the remaining studies, nonsignificant excess of kidney cancer was 

observed in a cohort mortality study by Selevan et al. (1985), while a 

borderline significant excess of kidney cancer was reported by Cantor et al. 

(1986). These two studies, together with two case reports of kidney cancer 

20 
P.34 

among lead workers (Baker et al., 1980; Lilis, 1981), created some interest in 

certain scientific forums since, kidney cancer was also observed in animal 

studies. Among the four reports, the nonsignificant kidney cancer excess in 

the Selevan et al. (1985) study was based on only six cases. The Cantor et • 

al. (1986) study, which had borderline significance, is a proportional 

mortality study, which has inherent interpretive limitations. As there is no 

information available about the population at risk for the two case reports, 

these studies cannot be used for supporting or refuting any hypothesis. On 

the other hand, deficits in renal cancers were observed in both the battery 

and smelter worker cohort: of the Cooper studies. Therefore, the reported 

human kidney cancer excesses must be viewed in light of the varied results and 

attendant interpretive considerations in order to maintain a proper 

perspective. 

Studies by Cooper et al. (1985) in battery workers, Ades and Kazantzis 

(1988) in smelter workers, and a reanalysis by Kang et al. (1980) of the 

Cooper and Gaffey (1975) data in smelter workers showed statistically 

increased lung cancer mortality. In addition, nonsignificant excesses of lung 

cancer mortality among smelter workers were observed by Gerhardson et al. 

(1986), Cooper et al. (1985), and Selevan et al. (1985). Significant 

increases in stomach cancer mortality were also observed by Cooper et al. 

(1985) among battery workers and in the reanalysis by Kang et al. (1980) of 

the Cooper and Gaffey (1975) data on smelter workers. Although increased lung 

cancer mortality has been observed in several studies, there are 

methodological limitations such as confounding exposures to other agents in 

all these studies. The confounding includes exposure to smoking and some 

heavy metals such as arsenic and chromium, which have been proven to be 

21 
P.35 

pulmonary carcinogens. This possible confounding, however, does not lessen 

the relevance of exposure to lead, which may be contributing to the excesses 

in human cancer, and hence, cannot be ruled out. 

In conclusion, the epidemiologic evidence, although not strong enough to 

be sufficient or limited (i.e., to infer a causal relationship), should not be 

interpreted as negative. At this time the epidemiologic evidence for lead 

(and lead compounds) being a human carcinogen is considered inadequate 

according to the terminology of the EPA Guidelines for Carcinogen Risk 

Assessment (U.S. EPA, 1986a); nevertheless, it is important to note that the 

evidence is thought to be suggestive. In this instance "suggestive" means 

that although the available data are unable to provide the evidence for a 

causal association, they are positive enough to raise a concern about the 

human carcinogenicity of lead. A well-designed and well-conducted positive 

study may elevate the human evidence to the "limited" or even to the 

^sufficient" category, while a similar study showing negative findings in 

humans could support downgrading the classification. 

Additionally, a relatively large body of information is available on the. 

frequency of chromosomal aberrations in cultured lymphocytes of lead-exposed 

workers (see U.S. EPA, 1986c, for a review and references). Deficiencies can 

be found in both the positive and negative studies reported on lead (e.g., 

exposure to other chemicals, lack of appropriate controls, small number of 

workers evaluated, lack of dose-response relationships, protocol inadequacies 

in cytogenetic analysis, and/or lack of quantitative measurements of lead 

exposure). Nevertheless, when all of the studies are taken together, there is 

a suggestion that lead is capable of increasing the frequencies of chromosomal 

aberrations in peripheral blood lymphocytes of exposed workers. The positive 

22 
P.36 

cytogenetic studies in workers exposed to lead lend support to the "suggestive" 

epidemiologic cancer evidence. 

1.2. ANIMAL STUDIES 

A total of 35 experiments relevant to the carcinogenicity of lead in 

laboratory animals is reviewed. All except one are laboratory studies. The 

lone exception is a population study of free-living wild rats. Although lead 

acetate and lead subacetate have been investigated most extensively, other lead 

compounds were also tested, including lead oxide, lead phosphate, lead 

naphthenate, lead dimethyldithiocarbamate, and tetraethyl lead. Lead was 

administered in the food, by intraperitoneal injection, subcutaneous injection, 

intratracheal instillation, skin painting, and gavage. Lead subacetate was 

evaluated for tumor-promoting ability, while lead acetate and lead oxide were 

tested for cocarcinogenicity. At least some evidence of the carcinogenicity of 

lead or lead compounds was detected in 24 of these experiments. 

Summaries of the animal studies described in this section appear in 

Tables 1-1 and 1-2. 

1.2.1. Lead Acetate (CAS No. 301-04-2) 

1.2.1.1. Studies in Rats--

1.2.1.1.1. Boyland et al., 1962. Groups of 20 male Wistar rats were 

administered 1 percent lead acetate in the feed for 1 year. A control group 

was not included. Food intake was limited to 20 g per rat per day during the 

first month, 30 g per rat per day during the second month, and 40 g per rat per 

day during the 10 subsequent months. The average initial body weight was 

200 g, and terminal body weights ranged from 600 to 800 g. Following the 

23 
TABLE 1-1. SUMMARY OF POSITIVE ANIMAL STUDIES 

Compound Animal Dose and duration Total lead Tumor response Reference 

Lead acetate M Wistar rat 10,000 ppm in feed 34 q 

(12 mo) 

15/16 renal tumors Boyland et 

al., 1962 

Lead acetate M Wistar rat 

ro 

-p. 

Lead acetate 

F Wistar rat 

M/F Wistar 

rat 

Control 

3 mg in feed (2 mo), 

then 4 mg (16 mo) 

Control 

3 mg in feed (2 mo), 

then 4 mg (16 mo) 

3 mg in feed for 60, 

162, 307, or 504 d 

1 g 

0/19 

58/94 renal tumors, 

23/94 testicular tumors, 

23/94 adrenal tumors, 

22/94 prostate tumors, 

3/94 cerebra1 gliomas 

thyroid tumors; 

10/94 cerebral gliomas 

Zawirska and 

Medras, 1968 

i g 

0.1-

0.8 g 

0/13 

14/32 renal tumors 

pituitary, adrenal, 

lung, renal, prostate. 

Zawirska and 

Medras, 1972 

Lead acetate M/F Charles 

River CD rat 

0, 10, 50, 100, or 

1000 ppm lead in 

feed (22 mo) 

0.1-12 g Tumors observed, but 

incidence not reported 

API, 1971 

(continued on the following page) 
TAB-E 1-1. (cen tinned) 

Compound Animal Dose and duration Total lead Tumor response Reference 

Lead acetate 

rv> 

tn 

M rat 

(strain not 

specified) 

F rat 

(strain not 

specified) 

Lead acetate M F344 rat 

Lead acetate M Wistar rat 

3-5 ppm lead in 

18 ppm feed 

62 ppm (24 mo) 

141 ppm 

548 ppm 

1130 ppm 

2102 ppm 

3-5 ppm lead 

18 ppm 

62 ppm 

141 ppm 

548 ppm 

1130 ppm 

2102 ppm 

0 ppm lead in 

feed 

10,000 ppm (12 mo) 

0 ppm in feed 

500O ppm (6 mo) 

10,000 ppm 

0.06 

0.2 

0.8 

2 

7 

14 

26 

0.06 

0.2 

0.8 

2-

7 

1.4 

26 

g 

g 

0/120 

0/50 

0/50 

0/50 

5/50 

10/20 

16/20 

0/120 

0/50 

0/50 

0/50 

0/50 

0/20 

7/20 

renaI tumors 

renaI tumors 

Azar et al., 

1973 

None 

,3 g 

— 

9 g 

.7 

1/5 renal 

adenocarc inomas 

0/10 

0/12 

9/10 renal tumors 

Tanner and 

Lipsky, 1984 

Nogueira, 

1987 

(continued on the fallowing page) 
TABLE 1-1. (continued) 

Compound Animal Dose and duration Total lead Tumor response Reference 

ro 

Lead acetate M SpragueDawley rat 

Lead acetate F Swiss 

mouse 

0 ppm lead in 

water 

2600 ppm (18 mo) 

0 ppm lead with 

EU and MaNOj 

26 ppm lead with 

EU and NaNO2600 ppm lead with 

BJ and INaNC^ 

0 ppm lead in 

50 ppm water 

1.000 ppm (9 mo) 

— 

38 g 

— 

0.4 g 

38 

— 

0.1 g 

1.4 

0/7 

13/16 renal tumors 

C/8 

0/7 

6/10 renal tumors 

24/50 Lymphocytic 

34/50 leukemia 

38/50 

Koller et 

al., 1985 

Blakley, 

1987 

Lead 

subacetate 

Lead 

subacetate 

M Wistar rat 

F Wistar rat 

M Wistar rat 

o 

1000 

10,000 

0 

1000 

10,000 

0 

10,000 

ppm 

ppm 

ppm 

ppm 

ppm 

ppm 

ppm 

ppm 

in feed 

(24 mo) 

in feed 

(24 mo) 

in feed 

(life) 

— 

10 g 

97 

— 

10 g 

97 

— 

97 g 

Mane 

5/13 renal tumors 

6/7 

None 

6/13 renal tumors 

7/9 

1/20 

31/34 renal tumors 

Van Esch et 

al., 1962 

Mao and 

Molnar, 1967 

(continued on the following page) 
TABLE 1-1. (continued) 

Compound Animal Dose and duration Total lead Tumor response Reference 

Lead 

subacetate 

M Charles 

River CD rat 

5000-10,000 ppm in 

feed (7-17 mo) with 

ar without 2AAF 

14-69 g Renal tumors and 

cerebral gliomas 

Hass et a1., 

1965 

Lead 

subacetate 

M SpragueDawley rat 10,000 ppm in feed 

(18 mo) with: 

0V. calcium acetate 

0.37. 

17. 

37. 

67. 

73 g 

Kasprzak et 

al., 1985 

.13/29 renal tumors 

21/30 

18/29 

22/30 

22/28 

ro 37. calcium acetate 

only 

0/29 

Mei ther O/30 

(continued on the following page) 

c 
TABLE 1-1. (continued) 

Compound Animal Dose and duration Total lead Tumor response Reference 

Lead 

subacetate 

M Wistar rat 

ro 

co 

500 ppm EHEN in feed 2 g 

(2 wk) , then 

1000 ppm lead 

subacetate (20 wk), 

then basal diet 

(10 wk) 

500 ppm EHEN only 

1000 ppm EHEN in 2 g 

feed (2 wk), then 

1000 ppm lead 

subacetate (20 wk), 

then basal diet 

(IO wk) 

1000 ppm EHEN only 

100O ppm lead 2 g 

subacetate only 

Neither 

10/22 renal tumors Hiasa et 

al., 1983 

0/24 

17/17 renal tumors 

9/18 

0/24 

0/24 

(continued on the following page) 
TABLE "1-1. (continued) 

Compound Animal Dose and duration Total lead Tumor response Reference 

Lead 

subacetate 

M F344 rat lOOO ppm EHEN in 

wa ter (1 wk), then 

1000 ppm lead 

subacetate if feed 

(35 wk) 

5 g 13/25 renal tumors Shirai et 

al., 1984 

1000 ppm EHEN only 5/23 

1000 ppm lead 

subacetate only 

3 g 0/25, but 6/25 had 

possibl e preo.I'sor s 

ro 

Lead 

subacetate 

M Swiss 

mouse 

0 ppm in feed 

1000 ppm (24 mo) 

10,000 ppm 

2 g 

22 

None 

6/25 renal tumors 

Substantial mortality 

Van Esch and 

Kroes, 1969 

Lead 

subacetate 

F Swiss 

mouse 

M/F Strain 

A/Strong 

mouse 

O ppm in feed 

1000 ppm (24 mo) 

10,000 ppm 

15 i.p. injections 

totalling 30, 75, or 

150 mg/kg (7 mo) 

— 

2 g 

22 

0.0009-

0.0045 g 

None 

2/25 renal tumors 

Substantial mortality 

Lung tumor incidence and 

lung tumors per mouse 

Stoner et 

a). , 1976 

increased at 150 mg/kg; 

renal tumors not 

reported 

(continued on the fallowing page) 

ro 
TABLE 1-1. (continued) 

Compound Animal Dose and duration Total lead Tumor response Reference 

Lead oxide M/F Syrian 

hamster 

10 intratracheal 

instillations of 

1 mg with 1 mg B,a]P 

(14 mo) 

0.009 g 10/26 lung tumors Kobayashi 

and Okamoto, 

1974 

co 

o 

Lead 

phosphate 

Lead 

phosphate 

Lead 

naphthenate 

(mixture) 

M albino CB 

Wistar rat 

White rat 

M albino 

Schofield 

mouse 

Lead oxide only 

B[a]P only 

Neither 

s.c. or i.p. 

injections (8 mo) 

tota11ing: 

29 mg" 

145 mg 

450 mg 

s.c. injections 

totalling 40-760 mg 

lead (16 mo) 

Skin applications 

totalling 6 mL in 

207. solution in 

benzene (18 mo) 

O.0O9 g 

— 

— 

0.03 g 

0.1 

0.4 

0.04-

0.76 g 

N.C. 

0/20 

0/27 

0/57 

0/23 renal tumors 

14 /z3 

2/3 

19/29 renal tumors 

5/59 renal tumors 

Roe et al., 

1965 

Zol1inger, 

1953 

Baldwin et 

al., 1964 

(continued on the foilowing page) 
TABLE 1-1. (continued) 

Compound Animal Dose and duration Total lead Tumor response Reference 

Lead 

dimethyldithiocarbamate 

Tetraethyl 

lead 

M B6C3F1 

mouse 

M/F Swiss 

ICR/Ha mouse 

46.4 mg/kg/d by 

gavage (1 mo), then 

130 ppm in feed 

(17 mo) 

s.c. injections 

(12 mo) totalling: 

0.6 mg 

0.2 g 

0.0004 g 

5/16 reticulum cell 

sarcomas, significant 

over control (12/146) 

BRL, 1968 

5/41 1ymphomas, 

significant over 

tricaprylin control 

(1/89) 

Epstein and 

Mantel, 1968 

CO 1.2 mg 0.0008 9Z/. mortality 

before weaning 

.0 mg 0.0013 1007. mortality 

Total lead adjusts for the lead content of each compound (lead acetate is 207/379 lead; lead subacetate, 

621/807; lead oxide, 207/223; lead phosphate, 722/812; lead dimethyldithiocarbamate, 207/447; tetraethyl 

lead, 207/323; and lead nitrate, 207/331). Total lead from food or wr.er exposure assumes that rats eat and 

drink 57. and 7.87., respectively, of their 350-g weight each day, and that mice eat and drink 137. and 177., 

respectively, of their 30-g weight each day. 

N.C. - No calculation. 
TABLE 1-2. 9JMMARY OF NONPOSITIVE ANIMAI.. STUDIES 

Compound Animal Dose and duration Total lead Comments Reference 

Lead acetate M/F Long- 5 ppm lead in water 0.1 g 

Evans rat for life 

MTD was not reached Kanisawa and 

Schroeder, 

1969 

Lead acetate F Swiss 

mouse 

O. 50, 200, or 

100O ppm lead in 

water for 15 wk with 

1 i.p. injection of 

urethan 

0.03-

0.5 g 

No effect on urethan- Blakley, 

induced pulmonary tumors 1987 

CO 

ro 

.ead acetate Beagle dogs 2, 16, 57, 155, or 

576 ppm in feed for 

24 mn 

N.C. Study duration is 

inadequate for a cancer 

study in dogs 

Azar et al., 

1973 

Lead acetate M/F Beagle 

dogs 

O, 10, 50, 100, or 

1000 ppm in feed for 

22 mo 

N.C. Study duration is 

inadequate for a cancer 

study in dogs 

API, 1*771 

Lead acetate M/F Rhesus 

monkeys 

0, 1.25, or 

25 mg/kg/d by gavage 

for 22 mo 

N.C. Study duration is 

inadequate for a cancer 

study in monkeys 

API, 1971 

Lead 

subacetate 

Golden 

hamsters 

0, 1000, or 50OO ppm 

in feed for 24 mo 

M.C. High mortality, with few 

survivors after 60O days 

Van Esch and 

Kroes, 1969 

(ccritinued on the following page) 
TABLE 1-2. (continued) 

Compound Animal Dose and duration Total lead Comments Reference 

Lead 

subacetate 

M Rabbits (3 

strains) 

CO 

CO 

Lead nitrate 

Lead 

dimethyldithiocarbamate 

M Long—Evans 

rat 

M/F F344 rat 

5000 or 10,000 ppm 

in feed for 18 mo 

with or without 

2AAF, 1inseed oi1, 

cholesterol, 

chloroform, carbon 

tetrachloride, or 

vitamin D 

25 ppm lead in water 

for life with 1 ppm 

trivalent chromium 

0, 25, or 50 ppm in 

feed for 24 mo 

N.C. 

0.5 g 

0.4 

Study duration is 

inadequate for a cancer 

study in rabbits 

Hass et a1., 

1965 

MTD was not reached 

No toxic signs, this 

indicates that the MTD 

was not reached; renal 

lesicns were observed at 

higher doses in the 

range-finding study; 3 

gliomas were seen 

Schroeder et 

al., 1970 

NCI, 1979 

Lead 

dimethyldithiocarbamate 

F B6C3F1 

mouse, M/F 

B_A<F1 mouse 

46.4 mg/kg/d by 

gavage for 1 mo, 

then 130 ppm in feed 

for 17 mo 

0. No tumors observed at 

this low dose 

BRL, 1968 

(continued cn ttie following page) 
TABLE 1-2. (continued) 

Compound Animal Dose and duration Total lead Comments Reference 

M/F B6C3F1 

mouse, M/F 

B6AKF1 mouse 

One s.c. injection 0.02 g 

of 1000 mg/kg 

No tumors observed at 

this low dose 

Lead 

dimethyldithiocarbamate 

M/F B6C3F1 0, 25, or 50 ppm in 0.04-

mouse feed for 24 mo 0.08 g 

No toxic signs, this 

indicates that the MTD 

was not reached; renal 

lesions were observed at 

higher doses in the 

range-finding study; 1 

glioma was seen 

NCI, 1979 

CO 

Total lead adjusts for the lead content of each compound (lead acetate is 207/379 lead; lead subacetate, 

621/807; lead oxide, 207/223; lead phosphate, 722/812; lead dimethyldithiocarbamate, 207/447; tetraethyl 

lead, 207/323; and lead nitrate, 207/331). Total lead from food or water exposure assumes that rats eat and 

drink 57. and 7.87., respectively, of their 350-g weight each day, and that mice eat and drink 137. and 17%, 

respectively, of their 30—g weight each day. 

N.C. - No calculation. 

3 
P.48 

exposure period, the animals were killed when moribund or when palpable tumors 

were detected. 

Renal lesions, but no tumors, were seen in four animals that died within 

6 months of the start of exposure. After 6 months, all rats developed chronic 

cystic nephritis due to a cystic dilatation of the renal tubules accompanied by 

slight interstitial fibrosis. The first neoplastic lesion was detected in the 

kidney at 11 months. Of the 16 animals that survived, 15 (94 percent) 

developed renal tumors. Almost all renal tumors were described as bilateral 

cuboidal cell carcinomas. 

1.2.1.1.2. Zawirska and Medras, 1968. The experiments were carried out with a 

total of 126 Wistar rats (94 males, 32 females); an additional 32 rats served 

as controls (19 males, 13 females). The animals were fed a daily ration of 

20 g of feed containing lead acetate for 18 months. The daily dose was 3 mg 

per rat for the first 2 months, and for the remainder of the exposure period 

(16 months) the dose was raised to 4 mg per rat. Assuming an average body 

weight of 0.35 kg, the weight-normalized dose of lead acetate was 8.6 mg/kg'd 

for the first 2 months and 11.4 mg/kg'd thereafter. The total dose of lead 

acetate was 2131 mg per rat at the end of 18 months of exposure. Controls were 

fed a diet without lead. All animals were examined once per week. The kidneys 

and other organs in the abdominal cavity as well as the testes of males were 

palpated. All animals were examined histologically. Forty animals were killed 

at various time intervals following exposure; the remainder were killed at 

26 months from the start of the experiment. Of 94 male rats, 81 survived 18 to 

20 months; of 32 females, 9 survived 18 to 20 months. 

35 
P.49 

Lead acetate induced numerous benign and malignant tumors at multiple 

sites. The main target organs were the kidneys, testes, and other endocrine 

glands. Most of the tumors appeared between the 18th and 20th month (i.e., 

after cessation of treatment). The incidence of renal adenomas was 46 percent 

and 38 percent in males and females, respectively; the incidence of renal 

carcinomas was 16 percent and 6 percent in males and females, respectively. As 

a result of numerous cysts and tumorous changes, the kidneys were always 

enlarged. The longest surviving animals displayed fibrous changes of the 

kidneys. Many kidney carcinomas metastasized to the lungs, lymph nodes 

(particularly to those located in the peritoneum), and liver but rarely to the 

spleen. 

Other tumors and the overall incidence based on 126 exposed animals (males 

and females combined) were as follows: 26 unilateral adenomas, 5 bilateral 

adenomas, and 1 carcinoma of the adrenal gland; 5 pituitary adenomas; 

3 adenomas, and 2 carcinomas of the liver; 4 adenomas and 1 carcinoma of the 

thyroid; and an unspecified number of lung adenomas. In the males (94 animals 

total) there were 14 unilateral and 9 bilateral tumors of the testes, 21 

adenomas and 1 carcinoma of the prostate, 2 neurinomas and 2 carcinomas of the 

seminiferous duct, 3 gliomas, and 2 skin carcinomas. Females (32 animals 

total) developed 1 adenoma and 2 carcinomas of the mammary gland and 1 

carcinoma of the forestomach. The percent response is not provided here 

because the number of affected animals could not be determined from the data 

presented (number of unilateral and bilateral sites). With the exception of 

one adenoma (site unspecified) and one mammary carcinoma, there were no tumors 

in the control group. Historical controls developed no spontaneous tumors in 

kidneys, other organs, or endocrine glands. This study indicates that, at a 

36 
P.50 

relatively low dose, lead acetate induces tumors at multiple sites involving 

the kidneys and endocrine organs. 

1.2.1.1.3. Zawirska and Medras, 1972; Zawirska, 1981. A total of 94 Wistar 

rats (47 males and 47 females) were divided into four exposure groups, and an 

additional 94 rats (47 males and 47 females) served as controls. The animals 

were fed a daily ration of 20 g of feed per day containing 3 mg of lead 

acetate. Assuming an average body weight of 0.35 kg, the weight-normalized 

dose of lead acetate was 8.6 mg/kg'd. Group I (20 rats) was given dietary 

lead acetate for 60 days; six animals (three of each sex) were killed 

immediately after the exposure period ended, and the remain.ng rats were left 

to die naturally. Group II (20 rats) was given dietary lead acetate for 

162 days; six animals were killed immediately after the exposure period ended, 

and the remaining rats were left to die naturally. Group Ila (8 rats) was also 

given dietary lead acetate for 162 days; one was killed when moribund, and the 

remaining rats were left to die naturally. Group III (20 rats) was given 

dietary lead acetate for 307 days; six animals were killed immediately after 

the exposure period ended, and the remaining rats were left to die naturally. 

Group IV (26 rats) was given dietary lead acetate for 504 days; seven rats (3 

females and 4 males).were killed immediately after the exposure period ended, 

and the remaining animals were left to die naturally. 

The survival of the animals left to die naturally was 38 to 505 days for 

group I, 95 to 398 days for group II, 178 to 296 days for group Ila, 4 to 344 

days for group III, and 324 to 572 days for group IV. Of the 94 control 

animals used, six were killed after 60 days and six after 162 days. Animals in 

groups I and II were killed in equal numbers at the same time. The survival of 

37 
P.51 

the remaining animals ranged from 300 to 800 days. No neoplastic or 

nonneoplastic lesions were found in any of the control animals. The incidence 

of kidney adenomas in exposed animals was 1/20 (5 percent) in groups I and II, 

1/8 (13 percent) in group Ila, 3/20 (15 percent) in group III, and 6/26 

(23 percent) in group IV. The overall incidence for all groups combined was 

12/94 (12.8 percent). The incidence of gliomas was 2/20 (10 percent) in groups 

I, II, and III, 0/8 in group Ila, and 4/26 (15 percent) in group IV. The 

overall incidence for all groups combined was 10/94 (11 percent). Therefore, 

in these animals, gliomas, which are not often found in lead-treated rats 

occurred at an incidence almost as high as that of kidney tumors, which are 

usually found in lead-exposed animals. Other tumors and the overall incidence 

based on 94 treated animals were as follows: 15 lung adenomas (16 percent), 17 

pituitary adenomas (18 percent), 11 thyroid adenomas (12 percent), 

4 parathyroid adenomas (4 percent), 15 adenomas of the adrenal cortex/medulla 

(16 percent), 11 prostate adenomas (12 percent), 8 mammary adenomas 

(9 percent), 3 reticulosarcomas (3 percent), and 1 leukemia (1 percent). A 

total of 109 tumors were found in 94 animals treated with lead. The 

multiplicity of tumor types found in these animals is unusual for animals 

treated with dietary lead. In one group of animals (group IV animals killed 

after 504 days of dietary lead), 10 different types of tumors were found in 

seven animals. 

The lack of a detailed description of pathology in current control animals 

(or historical controls), considering that some animals lived as long as 

800 days, is a major weakness in this study. It is unlikely that 82 animals 

could survive as long as the current controls (up to 800 days) and not develop 

any reportable pathology. The absence of this information considerably weakens 

38 
P.52 

the value placed on this study (which shows that lead induces tumors at 

multiple sites at doses lower than those reported in other studies) for 

evaluating the carcinogenicity of lead. The number of animals used in each 

exposure group is small; thus, meaningful results requires pooling data from 

the various groups. 

1.2.1.1.4. American Petroleum Institute, 1971. Chronic toxicity studies were 

conducted for 22 months in rats. Male and female Charles River CD rats (50 per 

group per sex) were exposed to lead acetate in their diets at concentrations of 

0, 10, 50, 100, or 1000 ppm. The rats were killed at the end of the exposure 

period. 

Intranuclear inclusion bodies and cytomegaly were found in most animals. 

Atypical nodular or adenomatous renal epithelial hyperplasia was observed in 

one male receiving 50 ppm (number examined not reported), 4/14 males receiving 

100 ppm, and 7/15 males and 3/15 females receiving 1000 ppm. According to the 

report, renal neoplasia of the cortical epithelium was observed, but incidence 

data were not presented. 

1.2.1.1.5. Azar et al., 1973. Lead acetate was fed to rats (strain not 

specified) at concentrations of lead measured at 5 (basal diet), 18, 62, 141, 

548, 1130, or 2102 ppm for 2 years. There were 100 rats of each sex in the 

control group and 50 rats of each sex in each exposure group receiving the four 

lowest doses. The two highest doses were started at a different time and 

consisted of 20 rats of each sex in the control and exposed groups. Body 

weights, food consumption, and lead content in the blood, urine, feces, and 

tissues were measured; clinical appearance and behavioral changes were 

observed. All animals were examined for gross and microscopic lesions. 

39 
No clinical toxicity or behavioral changes were observed in rats, even at 

the highest concentration of lead. Body weight gain was reported to be reduced 

in animals receiving the two highest concentrations. Mortality was increased 

in male rats receiving 548 or 2102 ppm, compared with their respective 

controls. An unexplained high mortality rate was observed in the second group 

of control rats; consequently, the mortality rate in males receiving 1130 ppm 

was not increased significantly. The mortality rate was not increased in any 

of the female groups. Kidney tumors, most of them adenomas arising from the 

tubular epithelium, were induced in male rats receiving the three highest doses 

and in female rats receiving the highest dose only. The tumor incidence was 

10, 50, and 80 percent in males receiving 548, 1130, and 2102 ppm, 

respectively, and 35 percent in females receiving 2102 ppm. No tumors were 

observed in controls. The lead content in the kidney increased with dose up to 

13.2 and 13.37 ug/g of tissue in rats fed 548 and 1130 ppm, respectively, with 

a slight decrease to 11.60 ug/g of tissue at 2102 ppm at the end of 24 months. 

Thus, the increase in the tumor incidence at the higher doses did not correlate 

with a similar increase in lead content in the kidney. However, other 

measurements, specifically, urine, liver, and bone lead content, continued to 

increase with dose. Lead content was highest in bone, which had levels more 

than 10 times that of other tissues, and its level increased throughout all 

dietary concentrations. The number of stippled red blood cells increased at 

the 18-ppm dose and the ALA-D was decreased at 62 ppm; the hemoglobin and 

hematocrit, however, were not depressed in the rats until they received a dose 

of 1130 ppm. At 1130 and 2120 ppm lead, 21-day-old weanling rats showed no 

tumors but did show histological changes in the kidney comparable to those seen 

in adult rats receiving 548 ppm or more lead in their diet. 

40 
P.54 

There were several weaknesses in this study. The description of 

methodology was very scanty. Body weights and food consumption data were not 

presented. Only the terminal mortality rate was presented, and time to first 

tumor was not given; tumor incidence could not be related to numbers of animals 

at risk (number of survivors at latency). Furthermore, the data were not 

analyzed statistically. The report of the study comes from the proceedings of 

the International Symposium on the Environmental Health Effects of Lead 

convened in Amsterdam in October 1972. The proceedings were published 7 months 

later in May 1973. A telephone call to the Haskell Laboratory in Newark, 

Delaware, revealed that the raw data from the study could not be located and 

thus was not available for review. There is no evidence that this study has 

undergone a peer review in the conventional sense of being published in a peerreviewed journal. The strengths of the study include the use of multiple dose 

levels, the 2-year duration of exposure, and the numbers of animals per dose at 

the lower dose levels. 

1.2.1.1.6. Tanner and Lipsky, 1984. Male Fischer 344 rats were exposed to 

lead acetate in their diets at a concentration of 10,000 ppm for up to 

52 weeks; controls were fed a basal diet without added lead acetate. Survivors 

were killed at 16, 24, 36, or 52 weeks from the start of exposure. Although 50 

animals per group were exposed at the start of the study, as a result of 

scheduled deaths and early mortality, only 5 rats remained after 52 weeks. 

None of the controls developed tumors. 

No tumors were found in animals (5 to 10 rats for each time period) killed 

at 16, 24, or 36 weeks. One exposed animal, of five surviving, developed an 

adenocarcinoma of the kidney at 52 weeks. Nonneoplastic renal lesions were 

also observed. Karyomegaly (enlarged nuclei) was the first morphologic change 

41 
in proximal tubule epithelial cells, appearing as early as week 4 in leadexposed animals and increasing from weeks 16 to 52. Hyperplastic lesions of 

the kidney cortex were present at week 16 in 4/8 rats and in 10/10 rats at week 

36. According to the investigators, there was no correlation between 

karyomegaly and the later appearance of hyperplasia or carcinomas. The 

weaknesses of the study were the use of only one dose level and the small 

number of animals surviving to termination. 

1.2.1.1.7. Noqueira, 1987. Male Wistar rats were administered dietary lead 

acetate at concentrations of 0.5 percent or 1 percent for 24 weeks, or the 

basal diet without lead acetate (control). The animals were killed at the end 

of the 24-week exposure period. Neoplastic and nonneoplastic lesions were 

identified by microscopic examination. All rats survived until detection of 

the first tumor. 

Two types of renal tumors were found, those composed of basophilic cells 

and those composed of chromophobic cells. Both tumor types were claimed to 

have originated from the proximal tubule. In 2/10 controls, "oncocytic" 

changes were noted in the walls of the collecting duct. None of the rats 

receiving 0.5 percent lead acetate developed tumors at any site, whereas 2/10 

rats receiving 1 percent lead acetate developed basophilic renal tumors and 

7/10 developed chromophobic tumors. Nonneoplastic renal tubular lesions 

included karyomegaly, yellow cytoplasmic pigment, and nuclear inclusions. The 

limitations of this study were small group size, short duration of exposure and 

of the study, and use of one sex only. 

1.2.1.1.8. Kanisawa and Schroeder, 1969. Male and female random-bred 

Long-Evans strain rats (50 per sex) were administered lead acetate at a 

concentration of 5 ppm (5 mg/L) in drinking water for their lifetime. The life 

42 
P.56 

span of the rats administered lead was shortened compared with controls; 

75 percent mortality was observed at 903 and 925 days in exposed male and 

female rats, respectively, compared with 979 and 1050 days in control male and 

female rats, respectively. Survival time, however, was considered to be 

adequate, even if somewhat shortened. 

Kidney tumors were not found, and the incidence of tumors at other sites 

was not significantly increased in the exposed groups. The dose, however, was 

very low. 

1.2.1.1.9. Koller et al., 1985. One group of 16 male Sprague-Dawley rats 

received lead acetate alone at a concentration of 2600 ppm of lead in deionized 

drinking water for 76 weeks. Other groups received 26 or 2600 ppm of lead in 

combination with ethyl urea and sodium nitrite. The controls were not exposed. 

The weight-normalized high dose calculated from the fraction of the body weight 

consumed as water (0.078 L/kg'day) (U.S. EPA, 1986b) was 195 mg of lead per kg 

per day. The rats were killed when moribund or immediately after termination of 

exposure (76 weeks). 

Tumors were detected only in the kidneys. They were identified as tubular 

cell carcinomas. The first tumor appeared at 72 weeks in rats receiving lead 

only; the incidence at the end of the study was 13/16 (81 percent). Renal 

neoplasms in three rats metastasized to the lungs, adrenal glands, and 

lymphatics. Kidney tumors (6/10) also appeared in rats receiving ethyl urea 

and sodium nitrite in addition to 2600 ppm lead, but not in rats receiving 

26 ppm of lead in combination with ethyl urea or sodium nitrite. No tumors 

developed in controls at any site. The results indicate that lead acetate is 

carcinogenic, but there is no evidence for cocarcinogenicity with 

43 
P.57 

ethylnitrosourea precursors. The limitations of the study include small group 

size and use of only one sex. 

1.2.1.2. Studies in Mice--

1.2.1.2.1. Blakley, 1987. To study the effect of lead on the development of 

urethan-induced pulmonary adenomas, lead acetate was administered to 3-week-old 

female Swiss mice (number of animals per dose was not given) in acidified 

drinking water at 0, 50, 200, or 1000 ppm of lead for 15 weeks. Three weeks 

after initiating exposure, the mice were given an intraperitoneal 

(intraperitoneal) injection of urethan (1.5 mg/g body weight). The animals 

we^e killed immediately after terminating exposure. Neither size nor number of 

tumors per mouse was affected by lead (the number of animals affected is not 

reported). 

In another experiment by the same authors, designed to study the effect of 

lead on the spontaneous incidence of lymphocytic leukemia of thymic origin in 

Swiss mice, 8-week-old female mice (50 animals per dose) were-administered lead 

in drinking water for 280 days at concentrations of 0, 50, or 1000 ppm of lead. 

The incidence of animals dying from lymphocytic leukemia of thymic origin 

increased from 24/50 in controls to 34/50 in the low-dose group and 38/50 in 

the high-dose group. The median survival time was reduced due to lymphocytic 

leukemia in mice exposed to lead. Measurement of the lead content in the liver 

and kidney showed an exposure-related increase in tissue concentrations of 

lead, but the magnitude of the response between the two exposure groups did not 

correspond with either exposure concentration or tissue concentration. The 

authors interpreted these results as indicative of an immunosuppressive effect 

of lead with regard to the spontaneous induction of lymphocytic leukemia, but 

not to the induction of pulmonary adenomas by urethan. 

44 
1.2.1.3. Studies in Doqs--

1.2.1.3.1. Azar et al., 1973. Lead acetate was fed to. beagle dogs at measured 

concentrations equal to 2 (basal diet), 16, 57, 155, or 576 ppm of lead for 

2 years. Four beagle dogs of each sex were used for each group. Body weights, 

food consumption, and lead content in blood, urine, feces, and tissues were 

measured; clinical appearance and behavioral changes were observed. All 

animals were examined for gross and microscopic lesions. 

There *as no gross or microscopic damage in dogs, except slight renal 

damage in male dogs receiving 576 ppm. This study had several limitations. 

The group size was small, and the 2-year exposure duration was inadequate for 

detecting cancer in dogs with life spans up to 15 years. Body weights and food 

consumption data were not presented. 

1.2.1.3.2. American Petroleum Institute, 1971. Chronic toxicity studies were 

conducted for 22 months in dogs. Six male and six female beagle dogs per group 

were administered lead acetate in their diets at concentrations of 0, 10, 50, 

100, or 1000 ppm. One male and one female were killed after 1 year, the 

remainder after 22 months. No gross lesions were observed; minor microscopic 

lesions characterized as thickening of the glomerular tufts were observed in 

females receiving 1000 ppm. The lack of effects was reflected by the blood 

lead levels, which were only slightly elevated relative to controls. The study 

duration was inadequate to detect cancer in dogs. 

1.2.1.4. Studies in Monkevs--

1.2.1.4.1. American Petroleum Institute, 1971. Male and female rhesus monkeys 

(two per sex per group) were given lead acetate in distilled water by gavage, 

7 days per week for 22 months at dose levels of 0, 1.25, or 25.0 mg/kg'day. 

45 
P.59 

The control group consisted of two male and two female monkeys intubated with 

water alone. One male from the 25-mg/kg group died after 9 months due to 

respiratory disease; the rest of the animals were killed at the end of the 

exposure period. Intranuclear inclusion bodies and degenerative lesions in the 

tubular epithelium were observed in animals receiving 25 mg/kg. Because of 

their long life span (35 years), the study duration is considered to be 

inadequate to detect cancer in rhesus monkeys. 

1.2.2. Lead Subacetate (CAS No. 1335-32-6) 

1.2.2.1. Studies in Rats--

1.2.2.1.1. Van Esch et al., 1962. Male and female Wistar rats were 

administered either 0.1 percent lead subacetate (14 male and 15 female control 

rats, 16 male and 16 female treated rats) in their diets for 29 months or 

1 percent lead subacetate (13 male and 13 female control rats, 13 male and 11 

female treated rats) for 24 months. Moribund animals were killed, and all 

surviving animals were killed when the exposure period ended. Body weights 

were measured weekly, and hematology tests were performed at 14 weeks on 

animals receiving the 0.1 percent diet and at 37 weeks on animals receiving the 

1 percent diet. 

Average body weights were significantly reduced after 10 weeks in both 

male and female rats receiving the 1 percent diet. The average survival time 

was reduced in rats receiving the 1 percent diet; six male and two female rats 

died prior to the appearance of the first tumor, leaving seven males and nine 

females at risk for developing renal tumors. In rats receiving 0.1 percent 

lead acetate in the diet, the early mortality rate was lower than in the 

corresponding controls; 3 treated rats of each sex died prior to the appearance 

of the first tumor, leaving 13 rats of each sex at risk for developing renal 

46 
P.60 

tumors. The first kidney tumor appeared 18 to 24 months after initial exposure 

in animals receiving 0.1 percent lead subacetate in the diet and 6 to 12 months 

in females or 12 to 18 months in males receiving 1 percent in the diet. The 

incidence of kidney adenomas and carcinomas was 5/13 in males and 6/13 in 

females receiving 0.1 percent lead subacetate. The incidence was 6/7 in males 

and 7/9 in females receiving 1 percent lead acetate. Combining the sexes 

results in an incidence of 11/26 in rats administered the 0.1 percent diet and 

13/16 in rats administered the 1 percent diet. Although the number of animals 

per dose group was small, both sexes were used, the duration of exposure was at 

least 2 years, and tumor induction was dose-related. 

1.2.2.1.2. Mao and Molnar, 1967. Forty male Wistar rats were administered 

1 percent lead subacetate in the feed for their lifetimes. Thirteen animals 

were killed between 213 and 593 days of feeding; the remaining animals died 

between 162 and 677 days. Controls consisted of 20 rats fed the basal diet 

without lead subacetate. The kidneys of all animals were subjected to 

microscopic examination for detection of tumors not grossly visible. 

Six animals died or were killed prior to the appearance of the first 

microscopic kidney tumor on day 305 (10 months) of feeding. Kidney adenomas or 

carcinomas were found in 31/34 (91 percent) animals surviving 305 days, 24/25 

(96 percent) animals surviving at least 400 days, and 12/12 (100 percent) 

animals surviving at least 500 days. The strengths of this study are the 

lifetime duration of exposure and microscopic examination of the kidney. 

1.2.2.1.3. Hass et al.. 1965. Twenty-four male Charles River CD rats were 

administered dietary lead subacetate plus indole, linseed oil, or 

N-2-fluorenylacetamide (2AAF) for 30 to 74 weeks. Indole was used to reduce 

the toxicity of 2AAF, and linseed oil was used as a carrier. Group I received 

47 
P.61 

0.06 percent 2AAF and 1.6 percent indole; group II received 0.5 to 1.0 percent 

lead subacetate and indole; group III received lead subacetate, indole, and 

linseed oil; and group IV received lead subacetate, indole, linseed oil, and 

2AAF. A control group was not included. All animals dying spontaneously, 

killed when moribund, or killed when exposure was terminated were processed for 

gross and histopathologic examination. The data were analyzed based on tumor 

incidence after either 30 to 52 weeks or 5? to 74 weeks of exposure. 

Renal tumors were not reported in group I. Among group II animals 

examined between 30 and 52 weeks of exposure, renal adenomas were found in 8/13 

(62 percent) and adenocarcinomas in 3/13 (23 percent). Among those examined 

between 52 and 74 weeks, 11/11 animals (100 percent) had renal adenomas and 

8/11 (73 percent) had adenocarcinomas. The overall incidence was 19/24 

(79 percent) for adenomas and 11/24 (46 percent) for carcinomas. In group III 

rats, the reported incidence was 0/11 for renal adenomas and 1/11 (9 percent) 

for adenocarcinomas between 30 and 52 weeks and 20/39 (51 percent) for adenomas 

and 13/39 (33 percent) for adenocarcinomas between 52 and 74 weeks, giving an 

overall incidence of 20/50 (40 percent) for adenomas and 14/50 (28 percent) for 

adenocarcinomas. In group IV animals, the incidence was 10/32 (31 percent) for 

renal adenomas and 11/32 (34 percent) for adenocarcinomas between 32 and 

52 weeks and 19/42 (45 percent) for adenomas and 14/42 (33 percent) for 

adenocarcinomas between 52 and 74 weeks, giving an overall incidence of 29/74 

(39 percent) for adenomas and 25/74 (34 percent) for adenocarcinomas. Renal 

cystomas were found with an overall incidence of 0/64 among group I, 22/24 

(92 percent) among group II, 30/50 (60 percent) among group III, and 37/74 

(50 percent) among group IV animals. Renal tumors induced by lead subacetate 

were confined to the cortex; they were slow growing and had a low tendency to 

48 
metastasize. This study showed that (1) renal adenomas and carcinomas were 

induced in rats given dietary lead subacetate, (2) the incidence of both 

increased with duration of exposure, and (3) 2AAF did not potentiate the 

effect. 

Nonneoplastic renal toxicity was shown by the accumulation of 

intracytoplasmic pigment, cytomegaly, and intranuclear inclusion bodies in 

tubular epithelial cells. These changes were evident after 2 to 3 months of 

exposure. The inclusion bodies tended to decrease during the late phase of 

exposure and were not found in renal tumor cells. 

Tumors were also found at other sites. One papilloma of the renal pelvis 

was found in group II and one cholangiocarcinoma in group III. Cerebral 

gliomas were reported in these animals with overall incidences of 3/64 

(5 percent) in group I, 2/24 (8 percent) in group II, 4/50 (8 percent) in group 

III, and 8/74 (11 percent) in group IV. Since controls were not included in 

the study, the statistical significance of these findings is uncertain. 

In another study, Oyasu et al. (1970) also reported the incidence of 

gliomas from data collected from various experiments using rats administered 

2AAF, lead subacetate, or both. In animals given 1 percent lead subacetate 

only, the incidence of gliomas was 2/17 (12 percent), compared with 3/41 

(7 percent) in animals given lead subacetate and indole, and 6/72 (8 percent) 

when 2AAF was added. In animals given 2AAF without lead subacetate, the 

incidence ranged from 0 to 6.8 percent, with an overall incidence (all 

experiments pooled) of 12/487 (2.5 percent), compared 1/325 (0.3 percent) among 

pooled controls. According to the authors, the difference in tumor incidences 

between controls and animals receiving either 2AAF, lead subacetate, or both 

was statistically significant. The significance of the results of this study 

49 
are questionable for at least two reasons. First, the results reported were 

obtained by pooling the data from several experiments with possibly differing 

protocols. Second, the average survival times for the two groups of controls 

were 58 and 60 weeks. The authors reported that, in rats administered 2AAF in 

combination with other substances, development of gliomas was usually delayed 

until after 60 weeks of age. Consequently, whether the controls died or were 

killed, it is doubtful that most survived long enough for gliomas to appear. 

Had they lived longer, the incidence of gliomas in control rats may have been 

higher. 

1.2.2.1.4. Kasprzak et al., 1985. Male Sprague-Dawley rats (3C per group) 

were placed on diets containing 1 percent lead subacetate, with or without 0.3, 

1, 3, or 6 percent calcium acetate, for 79 weeks. The animals were weighed 

weekly. All animals were necropsied. At the end of the study, body weights 

were significantly reduced in all exposure groups: by 19 percent in the group 

receiving lead subacetate alone, by 7 percent in those receiving calcium 

acetate alone, and by 26 to 46 percent in those receiving lead subacetate plus 

0.3 to 6 percent calcium acetate. Relative weights of the kidneys and liver 

were increased in rats fed lead subacetate alone; the addition of calcium 

acetate to the diet caused further increases in relative kidney weights but not 

in relative liver weights. Mortality was not significantly increased; 29/30 

rats receiving lead subacetate alone were alive when the first tumors appeared 

at 58 weeks, and 23 rats were alive at the end of the study. Of the animals 

receiving lead acetate plus calcium acetate, 28 to 30 were alive at 58 weeks, 

and 26 to 28 were alive at the end of the study. 

Among animals exposed to lead subacetate alone, adenomas were found in 11 

(38 percent) and adenocarcinomas in 2 (7 percent), for an overall incidence of 

50 
P.64 

13/29 (45 percent), compared with 0/30 in unexposed animals. This increase was 

highly significant (p < 0.001). The incidence of adenocarcinomas was increased 

to 17, 24, 23, or 25 percent in animals receiving lead subacetate plus 0.3, 1, 

3, or 6 percent calcium acetate, respectively. The total incidence of renal 

tumors (adenomas plus adenocarcinomas) increased to 70, 62, 73, and 78 percent 

respectively, in the latter four groups. The increase in the total incidence 

of renal tumors in animals given lead acetate plus 3 or 6 percent calcium 

acetate was statistically significant (p < 0.03 or p < 0.01, respectively), 

compared with those receiving lead subacetate alone. No other tumors were 

found in animals given lead subacetate only, and no other exposure-related 

tumors were found in the other groups. This study demonstrates both the 

carcinogenicity of lead subacetate and the enhancing effect of calcium acetate 

on carcinogenesis. 

Kasprzak et al. (1985) also measured the lead content in the kidneys at 

79 weeks. They reported that in animals receiving lead subacetate alone the 

lead content was 572 ug/g of dry tissue. Addition of calcium acetate resulted 

in a significant decrease in the lead content in the kidney, to 162 ug/g of dry 

tissue in animals receiving 6 percent calcium acetate. There was no change in 

the lead content in the liver, suggesting, in contrast to other reports 

(Meredith et al., 1977; Conrad and Barton, 1978), that the decrease in lead in 

the kidney was not due to a calcium-induced decrease in absorption of lead from 

the gastrointestinal tract. The decrease in lead content in the kidney, 

coincident with an increase in the incidence in renal tumors, suggests an 

absence of a correlation between the concentration of lead in the kidney and 

the incidence of renal tumors. 

51 
1.2.2.1.5. Hiasa et al., 1983. An initiation-promotion study was carried out 

in which male Wistar rats were administered 500 or 1000 ppm of 

N-ethyl-N-hydroxyethylnitrosamine (EHEN) (a known carcinogen) in the diet for 

2 weeks, followed by 1000 ppm of lead subacetate for 20 weeks, and a basal diet 

for an additional 10 weeks. Other groups received the basal diet, 500 or 

1000 ppm of EHEN alone for 2 weeks, or 1000 ppm lead subacetate for 20 weeks. 

Survivors were killed 32 weeks after the start of the study. 

The incidence of renal tumors (adenomas and adenocarcinomas) in the 

various groups was as follows: 0/24 in rats receiving the basal diet, lead 

subacetate alone, or 500 ppm EHEN alone, 9/18 in rats receiving 1000 ppm EHEN 

alone, 10/22 in rats receiving 500 ppm EHEN and lead subacetate, and 17/17 in 

rats receiving 1000 ppm EHEN and lead subacetate. The increase in incidence of 

kidney tumors in rats receiving lead subacetate and EHEN was significantly 

elevated compared with animals that received EHEN alone. Kidney tumors found 

in rats receiving EHEN followed by lead subacetate were larger than those found 

in rats receiving EHEN alone. The primary noncancer pathology was minimal and 

was in the form of dysplastic lesions, which appear to be forerunners of the -

tumors. This study indicates that lead subacetate may act as a kidney tumor 

promoter. 

1.2.2.1.6. Shirai et al., 1984. An initiation-promotion study was carried out 

in which male Fisher 344 rats were administered 0.1 percent EHEN in their 

drinking water for 1 week, followed by O.I percent lead subacetate in the diet 

for 35 weeks. Control groups included rats given 0.1 percent EHEN or 

0.1 percent lead subacetate only. 

No renal tumors were found in the 25 rats receiving lead subacetate alone, 

although effects suggestive of early neoplastic changes were observed in 6/25 

52 
rats. In rats receiving EHEN alone 5/23 (22 percent) developed renal tumors. 

Combination of EHEN plus lead acetate induced renal tumors in 13/25 

(52 percent) of the rats. These results indicate that lead subacetate can act 

as a kidney tumor promoter or possibly as a cocarcinogen, and confirms the 

results of Hiasa et al. (1983). 

1.2.2.2. Studies in Mice--

1.2.2.2.1. Van Esch and Kroes, 1969. Male and female Swiss mice (25 of each 

sex) were administered diets containing 0 percent, 0.1 percent, or 1 percent 

lead subacetate for 2 years. All surviving animals were killed 2 years after 

initial exposure. The 1 percent diet was reduced to 0.5 percent on day 92 for 

males and day 114 of feeding for females; nevertheless, by this time the 

survival rate was too low (about 10 female and 2 male survivors) for an 

analysis of long-term effects. The mortality rate in animals receiving the 

0.1 percent die't was not significantly different from controls, with about 

80 percent surviving at least 1 year. 

No renal tumors were detected in control mice. Among animals receiving 

0.1 percent lead subacetate, kidney adenomas were found in one female and twomales. Renal carcinomas were seen in four males. Among the surviving animals 

receiving the high dietary concentration, a kidney carcinoma was found in one 

female. While the exposure duration and initial group size was adequate, 

excessive mortality in the high-dose group and uncertainty regarding the number 

of animals at risk limited the strength of this study. 

1.2.2.2.2. Stoner et al., 1976. Strain A/Strong male and female mice (20 per 

group) were administered intraperitoneal injections of lead subacetate 

(dissolved in tricaprylin) three times per week for 5 weeks. The total doses 

were 150 mg/kg (maximum tolerated dose), 75 mg/kg, and 30 mg/kg; controls 

53 
received the vehicle tricaprylin three times per week for 8 weeks or were left 

untreated. All animals were killed 30 weeks after initial exposure. The lungs 

were examined for nodules; other organs, including the kidneys, were examined 

for abnormal lesions; and grossly abnormal tissues were examined 

histologically. The results were presented as the percentage of mice with lung 

tumors and the average number of tumors per mouse. 

The response in the different groups was as follows: untreated controls, 

6/19 (31. percent), 0.28 + 0.07 tumors/mouse (mean + standard error); 

tricaprylin controls, 8/18 (44 percent), 0.50 + 0.12 tumors/mouse; 30 mg/kg, 

6/17 (35 percent), 0.35 tumors/mouse; 75 mg/kg, 5/12 (42 percent), 0.50 ± 0.14 

tumors/mouse; 150 mg/kg, 11/15 (73 percent), 1.47 + 0.38 tumors/mouse. The 

lung tumor response was significantly increased only at the highest dose. 

Renal tumors were not described. If gross lesions were not observed, then the 

kidneys were not examined for microscopic lesions. This study shows that lead 

subacetate is capable of inducing tumors at another site and by another route 

of exposure. Although the kidneys were apparently not examined microscopically, 

positive responses in this organ were unlikely due to the small doses, 

approximately 5 mg per mouse at the highest level. 

1.2.2.3. Studies in Hamsters--

1.2.2.3.1. Van Esch and Kroes, 1969. Golden hamsters (45 or 46 per group, 

about equally divided between sexes) were administered 0.5 percent dietary lead 

subacetate, 0.1 percent, or a basal diet without lead subacetate for 2 years. 

All surviving animals were killed when exposure was terminated. The mortality 

rate was increased in both exposed groups; approximately 50 percent of the 

animals receiving the 0.1 and 0.5 percent diet survived at least 400 days and 

300 days, respectively, and a few animals survived 600 days. 

54 
P.68 

No renal tumors or early indications of tumor growth were found in either 

exposed or control animals. Intranuclear inclusion bodies and other signs of 

renal toxicity were observed. In addition, "oval cell type" bile duct 

proliferation was seen in two males and two females receiving the 0.1 percent 

diet, and in four females and three males receiving the 0.5 percent diet. 

1.2.2.4. Studies in Rabbits--

1.2.2.4.1. Hass et al., 1965. Male New Zealand albino rabbits and also a few 

German Checker and Belgian Hare strains (sex unspecified, 5 to 35 per dose 

group) were given 0.5 to 1 percent dietary lead subacetate alone, or in 

combination with 2AAF, linseed oil, cholesterol, chloroform, carbon 

tetrachloride, or vitamin D for 3 to 78 weeks. Renal tumors were not found in 

any rabbits receiving lead subacetate either alone or in combination with other 

substances. 

Cytoplasmic pigment, cytomegaly, intranuclear inclusion bodies, cellular 

degeneration, and anaplasia were seen in the kidneys. The earliest changes 

occurred about 8 to 12 weeks after initial exposure; permanent damage, which 

became evident after about 28 to 36 weeks, increased in severity up to about : 

55 weeks, then leveled off. Mild to moderate hemosiderosis was observed in the 

livers of animals given lead subacetate. Under the conditions of this study, 

lead was not shown to be carcinogenic in rabbits; however, the duration of 

exposure was probably inadequate for detecting cancer in rabbits, since this 

species normally lives up to 7 years. 

1.2.3. Lead Oxide (CAS No. 1317-36-8) 

1.2.3.1. Studies in Hamsters--

1.2.3.1.1. Kobayashi and Okamoto. 1974. Groups of 15 male and 15 female 

Syrian hamsters were administered 1 mg of lead oxide, 1 mg of benzo[a]pyrene, 

55 
P.69 

or a combination of 1 mg of each suspended in 0.2 mL of 0.5 percent 

carboxymethylcellulose-isotonic saline solution by intratracheal instillation 

weekly for 10 weeks. Controls were administered the vehicle or were untreated. 

The animals were killed 60 weeks after initiation of exposure or when moribund. 

The animals were autopsied, and various tissues were processed for microscopic 

examination. 

Both compounds and the combination induced alveolar metaplasia in the 

lungs, but lead oxide was the most effective. Neither compound alone induced 

tumors in the lung, but the combination induced adenomas in nine animals and an 

adenocarcinoma in another. The tumors were located in the peripheral area of 

the lung, indicating bronchiolar-alveolar origin. Untreated or vehicle 

controls developed no tumors. The study showed that lead oxide may act as a 

cocarcinogen. This study provides some evidence for the carcinogenicity of 

another lead compound by another route of exposure. 

1.2.4. Lead Phosphate (CAS No. 7446-17-7) 

1.2.4.1. Studies in Rats--

1.2.4.1.1. Roe et al., 1965. Male albino CB Wistar rats were given 

subcutaneous (subcutaneous) or intraperitoneal injections of technical grade 

lead orthophosphate (in water) over a period of 34 weeks, according to the 

following protocols: group A received a total dose of 450 mg per rat given in 

doses of 25 or 12.5 mg each; group B received 145 mg per rat in doses of 5 mg 

each; group C received 29 mg per rat in doses of 1 mg each. Because of 

insufficient information, daily weight-normalized doses could not be 

calculated. The animals were killed 200 days from the start of exposure, when 

moribund, or when with obvious neoplasms. Renal tumors originating in the 

renal cortex were found in groups A and B. In group A, 3/24 rats survived 

56 
P.70 

200 days, with 2 developing renal tumors. In group B, 23/24 rats survived 

200 days, with 14 developing renal tumors (7 adenomas, 6 adenocarcinomas, and 1 

undifferentiated malignant tumor). In group C, 23/24 rats survived 200 days, 

with r;one developing renal tumors. One control rat developed an 

undifferentiated malignant tumor in the kidney, and another developed a 

transitional-cell carcinoma arising in the renal pelvis. Additional groups of 

rats and appropriate controls were injected with either testosterone propionate 

or xanthopterin, but neither influenced the incidence of renal tumors. Lead 

treatment was not associated with tumors of other organs or tissues. Chronic 

nephritis was present in all treated rats, but there was no correlation between 

the severity of the condition and the tumor response. This study provides good 

evidence for the carcinogenicity of lead phosphate. 

1.2.4.1.2. Zollinger, 1953. Groups of 10 white rats (270 animals total; sex 

and strain not reported) were administered subcutaneous injections of 1 mL of a 

2 percent lead phosphate suspension (20 mg lead phosphate) once per week for up 

to 16 months. According to the authors, the total lead dose varied from 40 to 

760 mg per animal depending on time of death. Forty rats served as controls. 

Some of the animals were killed during the exposure period, others at an 

unspecified time period after the exposure period. The rats were autopsied, 

and the kidneys of 112 were processed for microscopic examination. The 

mortality was high. 

Renal cortical tumors were observed in 21 animals. The earliest tumor was 

found between the third and fourth months of exposure. Among animals surviving 

10 months or longer, adenomas, papillomas, or cystadenomas of the renal cortex 

were detected in 19/21 animals. The majority of the tumors could be palpated. 

Carcinomas were observed in three rats, two of which metastasized to the 

57 
P.71 

peritoneum or renal pelvis. Only 5/18 animals examined between the 15th and 

16th months of exposure were tumor free. The lowest dose that induced a tumor 

was 120 mg. A direct relationship between the lead dose and tumor incidence 

could not, however, be established. The tumor incidence in controls was not 

reported. Nonneoplastic changes occurring in treated animals included renal 

cortical cysts of varied size and appearance in the early phase of treatment 

and in the later phase, atypical as well as hyperplastic changes of the tubular 

epithelium. Most animals had enlarged livers. The study provides additional 

evidence for the carcinogenicity of lead phosphate. Weaknesses of the study 

include inadequate reporting of the dose administered, small number of animals 

surviving to the end of the study, and a lack of examination of tissues other 

than the kidney to determine if tumors may have been induced at more than one 

site. The data were not evaluated statistically. 

1.2.5. Lead Nitrate (CAS No. 10099-74-8) 

1.2.5.1. Studies in Rats--

1.2.5.1.1. Schroeder et al., 1970. A group of 52 male Long-Evans rats were 

exposed to 25 ppm of lead nitrate in their drinking water. The water also 

contained 1 ppm trivalent chromium and other metals as nutritional supplements. 

(A previous study of chromium-deficient rats given the same dose of lead showed 

early mortality and a shortened life span, which was corrected by chromium 

supplements [Schroeder et al., 1965].) This was the only source of drinking 

water for the entire lifetime of the animals. Controls received unsupplemented 

water. 

The results showed that body weights and survival were not affected by 

lead. The investigators attributed this observation to the protective effects 

of chromium against lead toxicity. Mean lead content in kidney was 2.65 ug/g 

58 
P.72 

of tissue in treated animals compared with 0.50 ug/g of tissue in controls. 

According to the authors, lead nitrate was "not tumorigenic as evidenced by 

visible tumors at necropsy." Microscopic examination of the kidneys was not 

performed. Furthermore, the dose of lead was low. 

1.2.6. Lead Naphthenate (CAS No. 61790-14-5) 

1.2.6.1. Studies in Mice--

1.2.6.1.1. Baldwin et al.. 1964. The lead naphthenate fraction of an engine 

oil fraction was tested fo" carcinogenic activity in the skin of male albino 

(Schofield strain) mice. The fraction, which was used as a 20 percent (by 

volume) solution in benzene, was applied to the dorsal skin of the mice once or 

twice per week for 12 months. The total dose of lead naphthenate administered 

was equal to 6 mL. The animals were killed at 18 months. Skin tumors were 

observed in only 2 of 50 animals treated with lead naphthenate, but extensive 

renal damage along with the presence of four renal adenomas and one renal 

carcinoma was reported. A solvent control was not included in this study; 

therefore, the carcinogenic or potentiating effect of benzene could not be 

assessed. A statistical analysis was not performed. Nevertheless, since 

kidney lesions induced by lead naphthenate were identical to those induced by 

other lead compounds administered orally, and since the critical target organ 

for benzene is not the kidney, the results are strongly suggestive of a 

carcinogenic effect. 

1.2.7. Lead Dimethyldithiocarbamate (CAS No. 19010-66-3) 

1.2.7.1. Studies in Rats--

1.2.7.1.1. National Cancer Institute, 1979. A comprehensive 2-year 

carcinogenicity study of lead dimethyldithiocarbamate (also known as ledate) 

was conducted in male and female Fischer 344 rats. The compound was 98 percent 

59 
pure and contained 45 percent lead. The concentration of lead used in the 

study was estimated from a 7-week range-finding study in which rats were given 

dietary lead dimethyldithio- carbamate at doses ranging up to 500 ppm (the 

lower dose range was not reported). Toxicity was assessed by relevant 

histopathologic findings. Nephrotoxicity was seen as hypertrophy of the 

epithelium of the proximal tubules accompanied by nuclear swelling and the 

presence of eosinophilic nuclear inclusion bodies. These lesions were moderate 

in rats at 500 ppm, slight to moderate at 250 ppm, mild at 125 ppm, minimal at 

62 ppm, and absent at lower doses. Hematopoietic changes were slight to mild 

in rats given 125 to 500 ppm. 

Based on these results, 25 and 50 ppm were selected as the low and high 

doses for the chronic studies. All exposed groups were composed of 50 animals 

of each sex, and age-matched control groups were composed of 20 animals per 

sex. The rats remained on the diets for 104 weeks. Body weights and clinical 

examinations were performed at regular intervals; gross and histopathologic 

examinations were performed on all animals. Mortality and tumor incidence data 

were analyzed statistically. Body weight gain was not significantly affected, 

and 2-year survival was equal to or greater than 70 percent in males and 

females at both doses. Therefore, survival rates were sufficient for analysis 

of late-developing lesions. 

Neoplasms found in these animals occurred in equal frequency in both 

exposed and control animals. No renal tumors occurred in either exposed or 

control animals. Gliomas were found in one high-dose female and in one highand one low-dose male rat. Nonneoplastic renal lesions were not induced and 

intranuclear inclusion bodies were not seen. Hyperplasia of the renal pelvic 

epithelium was seen in one low-dose male. 

60 
This study was generally well designed, although data on food consumption 

was lacking, and lead content in selected tissues was not analyzed. The 

weight-normalized doses with respect to the amount of lead, however, were very 

low compared with other studies showing nephrotoxicity and carcinogenicity. 

Nevertheless, the appearance of renal lesions in rats given higher doses in the 

range-finding study shows that, like other carcinogenic lead compounds, lead 

dimethyldithiocarbamate is capable of inducing nephrotoxicity, and higher doses 

may have elicited a carcinogenic response. The authors concluded that the 

absence of any toxic signs in the exposed animals as well as the lack of 

significant mortality and weight depression suggested that the animals may have 

tolerated higher doses, that is, the maximum tolerated dose (MTD) was not 

reached. Therefore, this study is considered to be inadequate for definitively 

evaluating the potential carcinogenicity of lead dimethyldithiocarbamate. 

1.2.7.2. Studies in Mice--

1.2.7.2.1. Bionetics Research Labs, Inc., 1968. An evaluation of the 

potential carcinogenicity of lead dimethyldithiocarbamate was performed in two 

strains of mice, B6C3F1 and B6AKF1, administered the test compound either by 

subcutaneous injection or orally. In the oral study, lead 

dimethyldithiocarbamate suspended in 0.5 percent gelatin was administered to 

the mice daily from 7 to 28 days of age, via gavage, at a dose of 

46.4 mg/kg'd. Dosing was continued after day 28 by administering the compound 

in the diet at 130 ppm. The animals were exposed until they were 18 months of 

age at which time they were killed and necropsied, and selected tissues were 

processed for histopathologic examination. Animals exposed subcutaneously were 

injected once at 28 days of age with 1000 mg/kg of lead dimethyldithiocarbamate 

61 
suspended in 0.5 percent gelatin. These animals were also killed when they 

were 18 months of age; they were necropsied and the tissues were processed for 

histopathologic examination. Vehicle and untreated control groups were formed 

by pooling the controls from all other chemicals tested at the same time. The 

combined controls for all test compounds given orally were as follows: 146 

male and 171 female B6C3F1 mice and 175 male and 166 female B6AKF1 mice. Among 

orally-treated male B6C3F1 mice, 5 of 16 animals (31 percent) surviving to 

18 months developed type A reticulum cell sarcomas, whereas only 12 of 146 

controls (8.2 percent) developed the same tumor. No other neoplasms occurred 

in more than one or two animals given the compound orally. The total number of 

controls for test compounds injected subcutaneously was as follows: 54 male 

and 62 female B6C3F1 mice and 56 male and 60 female B6AKF1 mice. Among animals 

injected subcutaneously with lead dimethyldithiocarbamate, no exposure-related 

neoplasms were found. Only 1 of 15 (7 percent) exposed and 5/54 (9 percent) 

control B6C3F1 males surviving to 18 months developed type A reticulum cell 

sarcomas. The only other tumors found were pulmonary adenomas in two male and 

two female B6C3F1 mice and one male and one female B6AKF1 mouse. This study, 

shows that lead dimethyldithiocarbamate administered orally may also induce 

reticulum cell sarcomas in mice. Since the group size was small and effects 

were not seen in females or in mice exposed subcutaneously, the results are 

considered to be only suggestive. 

1.2.7.2.2. National Cancer Institute (NCI), 1979. A comprehensive 2-year 

carcinogenicity study of lead dimethyldithiocarbamate was conducted in male and 

female B6C3F1 mice. The compound was 98 percent pure and contained 

45.2 percent lead. The concentration of lead used in the study was estimated 

from a 7-week range-finding study in which rats and mice were given dietary 

62 
P.76 

lead dimethyldithiocarbamate at doses ranging up to 500 ppm (the lower dose 

range was not reported). Toxicity was assessed by histopathologic findings. 

Intranuclear inclusion bodies and vacuolar degeneration of the proximal tubular 

epithelium were observed. These lesions were slight to moderate at 250 ppm and 

slight at 62 and 125 ppm. 

Based on these results, 25 and 50 ppm were selected as the low and high 

doses for the chronic studies. All exposed groups were composed of 50 animals 

of each sex and age-matched control groups were composed of 20 animals per sex. 

The mice remained on the diets for 105 weeks. Body weights and clinical 

examinations were performed at regular intervals; gross and histopathologic 

examinations were performed on all animals that died or were killed when 

moribund or at termination. Mortality and tumor incidence data were analyzed 

statistically. Body weight gain was not significantly affected, and survival 

was equal to or greater than 75 percent for all dose groups. Therefore, 

survival rates were sufficient for analysis of late-developing lesions. 

Neoplasms found in these animals occurred with approximately equal 

frequency in exposed and control animals; no renal tumors were found in any ..... 

group. One glioma appeared in one male given the low dose. Other neoplastic 

lesions were of single or low occurrence. Nonneoplastic renal lesions or 

intranuclear inclusion bodies were not reported. 

This study satisfied almost all the requirements used to evaluate studies 

for carcinogen risk assessment, with the exception of adequate doses. It 

appears that the MTD was not reached, because the doses of lead 

dimethyldithiocarbamate did not result in significant weight depression, 

mortality, or pathology. Doses were also less than those used in the oral BRL 

(1968) study. Therefore, this study is considered to be inadequate for 

63 
definitively evaluating the potential carcinogenicity of lead 

dimethyldithiocarbamate. 

1.2.8. Tetraethyl Lead (CAS No. 78-00-2) 

1.2.8.1. Studies in Mice--

1.2.8.1.1. Epstein and Mantel, 1968. Male and female random-bred Swiss mice 

(ICR/Ha) were administered subcutaneous injections of tetraethyl lead dissolved 

in tricaprylin. The mice were injected according to the following protocol: 

group 1 (124 mice) received tricaprylin alone; group 2 (69 mice) received one 

injection of 2.0 mg of tetraethyl lead on day 1 after birth; group 3 (92 mice) 

received 0.2 mg on days 1 and 7, and 0.4 mg on days 14 and 21; group 4 (109 

mice) received 0.1 mg on days 1 and 7, and 0.2 mg on days 14 and 21. The study 

was terminated after 49 to 51 weeks. Mortality was yery high with 100, 92, and 

20 percent of the animals in groups 2, 3, and 4, respectively, dying prior to 

weaning. 

Lymphomas were observed in female mice in group.4, with an incidence of 

5/41 (compared with 1/89 among controls, p < 0.02) at 36 weeks. Unlike other 

lead compounds, tetraethyl lead did not induce nephrotoxic lesions or renal -.% 

tumors. A limitation of this study was the high early mortality rate, leaving 

very few animals, or none, for detection of late lesions. While renal tumors 

were not detected, the duration of this study may have been inadequate. This 

study did show that tetraethyl lead can induce lymphomas in mice. 

1.2.9. Ambient Lead 

Kilham et al. (1962) studied tissues of wild Norway rats trapped at a 

trash dump. This dump burned or smoldered continuously. As a result, the 

animals at this location lived under a constant pall of smoke. The authors 

considered that a major portion of the exposure to any toxicants present was 

64 
P.78 

the result of smoke inhalation. Intake of toxicants via contaminated ground 

water, however, cannot be ruled out. 

A variety of pathological effects was seen in the kidneys of all 62 adult 

rats examined upon capture. These included the presence of nuclear inclusion 

bodies, cellular destruction, and focal hyperplasia in the proximal convoluted 

tubules of the kidneys. Additionally, kidney carcinomas were found in about 

5 percent of the rats. These tumors were histologically and biologically 

malignant. They often invaded extensively, destroying adjacent renal tissue. 

Several lines of evidence indicated that the toxicant responsible for 

these effects was lead. First of all, lead was present in much of the refuse 

in such landfills (for example, storage batteries). Secondly, the nature of 

the pathological effects, especially the presence of nuclear inclusion bodies, 

is typical of lead exposure. These responses were also shown to be related to 

exposure in that first-generation offspring of these rats, which were 

maintained in clean air, did not have inclusion bodies. Finally, kidney lead 

levels were greatly increased in rats examined soon after trapping. 

1.2.10. Discussion 

Thirteen studies were reported in which the carcinogenicity of lead 

acetate was tested. In seven of these studies the lead acetate was 

administered to rats in the feed. The cumulative dose of lead administered in 

these sets of experiments ranged from less than 0.1 g to about 63 g. Increases 

in kidney tumor incidences were generally detected at lifetime doses in excess 

of 7 g or more. In the studies conducted by API (1971) the dose level inducing 

tumors was not reported, but based on pathology, probably occurred in the highdose group receiving a lifetime dose of 6 g. In the Zawirska and Medras (1972) 

studies, kidney tumors were reported at cumulative doses of slightly less than 

65 
P.79 

1 g. In this study, however, the significance of the results are rendered 

somewhat uncertain by a lack of control data. 

Positive results were reported in one of two experiments with rats exposed 

to lead acetate via drinking water. In the positive study (Koller et al., 

1985) the cumulative lead dose was over 30 g, compared with only 0.05 g in the 

negative study (Kanisawa and Schroeder, 1969). Possible induction of 

lymphocytic leukemia occurred in mice dosed with as little as 0.1 g of lead via 

the drinking water (Blakley, 1987). Negative results in monkeys and dogs were 

likely due to inadequate exposure durations (Azar et al., 1973; API, 1971). 

Among the lead acetate studies, the most rigorous one was conducted by 

Azar et al. (1973). Fifty rats per sex were exposed to six different lead 

concentrations in the feed, resulting in cumulative doses of 0.1 to 14 g. The 

investigators observed clinical appearance and behavior; recorded food 

consumption, growth and mortality; assessed reproduction; and analyzed blood 

specimens for red cell counts, hemoglobin, hematocrit, stippled cell count, 

prothrombin time, alkaline phosphatase, urea nitrogen, glutamic-pyruvate 

transaminase, and albumin-to-globulin-ratio. The activity of the enzyme deltaaminolevulinic acid dehydrase (ALA-D) in the blood and the excretion of its 

substrate delta-aminolevulinic acid (d-ALA) in the urine were also determined. 

A thorough necropsy, including both gross and histologic examination, was 

performed on all animals. Reproduction was also assessed. In this study 

tumorigenic responses were seen at cumulative doses of 7 g or more. 

The carcinogenicity of lead subacetate was tested in 10 separate 

experiments. Of the six rat studies in which the agent was administered in the 

feed, four were positive; the other two tested positive for tumor promotion, 

but were negative for direct induction of tumors in 32 to 35 weeks. In the 

66 
P.80 

latter two studies (Hiasa et al., 1983; Shirai et al., 1984), cumulative doses 

of 2 to 3 g were used compared with 10 g or more in the other four positive 

studies. Significant increases in kidney tumor incidences were reported for 

mice administered cumulative doses of lead subacetate of 2 g or more in the 

feed (Van Esch and Kroes, 1969), but much lower doses were successful when mice 

were administered the agent by intraperitoneal injection (Stoner et al., 1976). 

Tumor induction was not reported in hamsters administered lead subacetate in 

the feed at a lifetime dose of about 4 g (Van Esch and Kroes, 1969). Negative 

results were reported in a 78-week rabbit study at concentrations of up to 

1 percent in the diet (Hass et al., 1965). Since rabbits may live for 6 to 

7 years, however, the exposure duration was considered inadequate for 

evaluating the effects of lifetime exposure. 

Among the lead acetate or subacetate studies in rats, the lifetime doses 

inducing detectable increases in tumor incidences, with the exception of the 

Zawirska and Medras (1972) study, ranged from 7 g to more than 60 g. At a 

cumulative dose of 2 to 3 g, evidence for tumor promotion was also seen in rats 

treated with lead subacetate. In two of three studies with mice dosed orally, 

the effective levels did not differ greatly from that of rats if differences in 

body weight are taken into account. In the Stoner et al. (1976) study, in 

which effective doses were very small, not only was a genetically susceptible 

strain used, but the agent was administered via intraperitoneal injection, with 

possibly more efficient uptake. 

The lack of response in hamsters given a total dose of about 4 g, an 

amount as large or larger on a mg/kg body weight basis to that used in many of 

the positive rat studies, suggests that the hamster may be less sensitive than 

rats or mice to the carcinogenic effects of lead. However, it should be noted 

67 
P.81 

that hamsters were tested much less extensively than rats. It should also be 

emphasized that a lack of detectable responses at lower lifetime doses is not 

proof of noncarcinogenicity at these levels, or a basis to demonstrate a 

threshold, but merely suggests that the study may lack the sensitivity for 

detection of positive responses (e.g., the likely case for a low potency 

carcinogen). 

A variety of other lead compounds, while less extensively tested, have 

also been shown to induce tumors in laboratory animals. These include lead 

phosphate administered via subcutaneous or intraperitoneal injection in rats at 

cumulative doses of less than 1 g (Roe et al., 1965; Zollinger, 1953), lead 

naphthenate via skin painting in mice, total lead dose unknown (Baldwin et al., 

1964), tetraethyl lead via subcutaneous injection in mice at cumulative doses 

as low as 0.4 mg (Epstein and Mantel, 1968), and in mice treated orally with 

lead dimethyldithiocarbamate at a cumulative dose of about 0.2 g (BRL, 1968). 

Lead oxide, about 0.01 g, also acted as a cocarcinogen with benzo[a]pyrene in 

the induction of lung tumors in Syrian hamsters (Kobayashi and Okamoto, 1974). 

Some of these other lead compounds were successful in inducing cancer at 

lower doses than lead acetate or subacetate. In several of the studies 

administration was carried.out by injection of lead phosphate, tetraethyl lead, 

or lead oxide. It is likely that the effectiveness of the lower doses was due 

to a greater absorbed fraction, since as will be discussed later, absorption of 

lead is fairly low via the ingestion route. Since neither lead acetate nor 

lead subacetate have been tested by injection, it is uncertain if the 

effectiveness of these compounds at lower doses is due to more efficient uptake 

by the injection routes, if compounds such as lead phosphate, lead oxide, or 

tetraethyl lead are more soluble and thus more efficiently taken up by any 

68 
P.82 

route, or if these compounds are inherently more potent than lead acetate or 

lead subacetate. 

Although inhalation can be a major route^ of human exposure to lead 

compounds, none of the animal cancer bioassays were conducted using this 

exposure route. Data from a variety of related studies, however, indicate that 

lead is likely to be as effective at equal or lower doses by inhalation as by 

ingestion in food or drinking water. Deposition of lead particles of varying 

sizes in the human respiratory tract has been shown to range from 23 percent to 

as high as 80 percent (Chamberlain et al., 1978; Gross, 1981; Morrow et al., 

1980; Mehani, 1966; Nozaki, 1966). Of the amount deposited in the deep lung, 

more than 90 percent was absorbed regardless of the chemical form of lead. 

Moreover, lead was not accumulated in the lungs, indicating that it was taken 

up into the bloodstream and transported to distant target sites following 

absorption (Chamberlain et al., 1978; Morrow et al., 1980). In contrast, 

absorption from the gastrointestinal tract in nonfasting adults was shown to 

vary from 6 to 15 percent (Chamberlain et al., 1978; Rabinowitz et al., 1974, 

1980; Kehoe, 1961a,b,c). Finally, the increase in blood lead concentration 

with dose in humans appears to be greater following exposure via inhalation 

than following ingestion (U.S. EPA, 1986c). 

The likelihood that the bioavailability of lead by the inhalation route of 

exposure is high compared with ingestion is supported by the only inhalation 

toxicology study reported to date. Prigge and Greve (1977) found that in 

pregnant rats exposed during gestation to yery low doses of lead aerosol (2 to 

about 20 mg), fetal weights and hematocrits were reduced in the high-dose 

groups, while inhibition of aminolevulinic dehydrogenase activity in both dams 

and fetuses occurred in a dose-related manner. Kilham et al. (1962) claimed 

69 
that kidney pathology and kidney cancer in wild rats trapped at a trash dump 

were induced by inhalation of lead-containing smoke. If true, it would show 

that inhalation of lead can result in carcinogenic effects even under ambient 

conditions, albeit under extremely polluted ambient conditions. However, 

intake of lead in contaminated ground water or the possibility of other 

contaminants inducing similar effects cannot be ruled out. 

While the kidney is a primary target organ for carcinogenesis in the rat, 

tumor induction was also noted at other sites. For example, in the study by 

Zawirska and Medras (1968), an apparent increase in adrenal, testicular, and 

prostate tumors occurred. Increased tumor incidences at multiple sites were 

also reported in a later rat study by Zawirska (1981). Increased numbers of 

gliomas in rats were reported by Hass et al. (1965) and Oyasu et al. (1970). 

Lung tumors were significantly increased in Strain A mice injected with lead 

subacetate (Stoner, 1976) and in hamsters injected with benzo[a]pyrene plus 

lead oxide compared with benzo[a]pyrene alone (Kobayashi and Okamoto, 1974). 

Increases in lymphocytic leukemia were reported in mice administered lead 

acetate (Blakley, 1987) or tetraethyl lead (Epstein and Mantel, 1968). 

Significant increases-in reticulum cell sarcomas were reported in mice exposed 

to lead dimethyldithiocarbamate (BRL, 1968). 

The evidence for tumors at other sites is weaker than for kidney tumors. 

For example, in the studies by Zawirska (1981), control incidences were not 

reported. The significance of results reported by Hass et al. (1965) and Oyasu 

et al. (1970) are also rendered less clear-cut by factors such as a lack of 

controls and pooling of results of several experiments with possibly differing 

protocols. In the Blakley (1987) study using lead acetate, the increases in 

leukemia occurred against an already high background, suggesting that lead may 

70 
be merely potentiating an ongoing process. In the BRL (1968) study, the group 

size was small. Increases in reticulum cell sarcomas were seen only in females 

and with oral, but not subcutaneous, dosing. Thus, while it appears that lead 

compounds are capable of inducing tumors at sites other than the kidney, the 

presence of confounding factors increases the uncertainty somewhat. 

The relationship between pathological changes in the kidneys of animals 

exposed to lead compounds and tumor induction is uncertain, despite 

considerable speculation. Some of the effects commonly seen include proximal 

tubule cell cytomegaly and swollen mitochondria with increased numbers of 

lysozomes (Fowler et al., 198C;. Spit et al., 1981) and interstitial fibrosis 

(Hass et al., 1964; Goyer, 1971). The primary localization of effects in the 

straight (S3) segments of the proximal tubule indicates that not all cell 

types of the kidney are equally involved in the toxicology of lead (Hass et 

al., 1964; White, 1977; Fowler et al., 1980). Although Nogueira (1987) has 

claimed that the kidney tumors arise from the same segments of the proximal 

tubules, this conclusion requires independent confirmation, since he did not 

establish quantitative temporal and spatial relationships between dose and 

response. 

Since many of the cancer bioassays were carried out using large doses, the 

threshold for pathological effects could seldom be determined. Among the 

multidose studies, in which lower doses were tested, API (1971) reported 

hyperplasia in a few animals at concentrations in the food as low as 100 ppm. 

NCI (1979) reported mild necrosis in rats subchronically exposed to 125 but not 

62 ppm. Azar et al. (1973) noted pathological changes at 500 ppm, but not 

100 ppm. Tumors were generally not detected at these levels. Pathological 

71 
P.85 

effects, however, were frequently described in animals administered 1000 to 

10,000 ppm in the food, the same concentrations shown to induce kidney cancer. 

Another pathological end point seen in most lead exposure studies is the 

presence of lead inclusion bodies. Inclusion bodies are commonly found in 

cells of the kidney, liver, bone, and brain of lead-exposed animals. In the 

kidney they occur most frequently in the proximal portion of the tubules (pars 

recta) and appear as dense spherical homogeneous eosinophilic bodies. They can 

be differentiated from viral inclusions by being acid-fast when stained with 

the Ziehl Nielson's technique. Ultrastructurally, the bodies have a dense 

central core and outer fibrillary region. The bodies are composed of nonhistone acidic protein, rich in glutamic and aspartic acids, glycine and little 

cystine. The lead binds loosely to carboxyl groups of the acidic amino acids. 

The formation of inclusion bodies appears to be a universal response to 

the presence of intracellular lead and is probably an attempt to immobilize it. 

Nuclear inclusion bodies occur not only in animals but also in plants that are 

tolerant of lead. For example, they have been observed in the nuclei of the 

leaf cells of moss grown in soil containing lead. Inclusion-bearing cells have 

also been found in the renal tubular epithelium of workers exposed 

occupationally to lead. (For further details see Goyer, 1971, 1983.) 

While the pathological effects of lead exposure were reported to occur at 

relatively large doses, the presence of nuclear inclusions is one of the 

earlier signs of lead toxicity and occurs at considerably lower exposures. For 

example, Choie et al. (1975) reported the presence of nuclear inclusion bodies 

in mice 8 hours after injection with as little as 150 ug of lead. 

Although the mechanism(s) of cancer induction by lead is currently 

unknown, the presence of nuclear inclusion bodies in the kidneys may be 

72 
P.86 

involved, even though their formation is considered to be a mechanism to 

immobilize lead and therefore to protect the cell. The interaction of lead 

with key nonhistone chromosomal proteins in the nucleus to form the inclusion 

bodies, or the mere presence of inclusion bodies in the nucleus, may 

sufficiently alter genetic function to cause cell transformation. While such a 

hypothesis is interesting, it is still speculative. 

Another particularly interesting mechanism, which has the potential for 

providing an explanation for the carcinogenic properties of lead compounds, 

involves activation of the enzyme protein kinase C (PKC). Markovac and 

Goldstein (1988) have shown recently that picomolar concentrations of lead 

acetate can activate partially-purified PKC from rat brain to enzyme levels 

which normally require micromolar concentrations of calcium, the normal 

activator of the enzyme. If this very recent observation can be corroborated, 

then this unique effect of lead on PKC becomes highly significant in view of 

the known properties of the enzyme and its putative function. 

PKC is a calcium- and phospholipid-dependent enzyme which is found widely 

distributed in tissues and organs. The highest concentrations of the enzyme -

have been reported in brain tissue. In addition to calcium and phospholipid 

(probably exclusively phosphatidylserine), the plasma memorane lipid, 

diacylglyceol, also plays an important role in activating PKC. By 

significantly increasing the affinity of the enzyme for calcium, diacylglycerol 

causes PKC to be fully activated without a net increase in free calcium above 

the normal cellular concentration (Nishizuka 1986; Kikkawa and Nishizuka, 

1986). Presumably, the concentration of free calcium available in most resting 

cells is inadequate for PKC activation in the absence of diacylglycerol which 

exists only transiently in the plasma membrane. The enzyme, therefore, is 

73 
tightly regulated by the availability of both calcium and diacylglycerol. 

Tumor promotors such as phorbol esters, mezerein, teleocidin, and Aplasia toxin 

can substitute for diacylglycerol in activating PKC. At least in the case of 

the phorbol esters, tumor promotors perform the same function as 

diacylglycerol, i.e., they increase the affinity of PKC for calcium. PKC is 

now considered to be the primary cellular target for the phorbol esters 

(Castagna et al., 1982; Kikkawa and Nishizuka, 1986). Once activated, PKC is 

capable of phosphorylating a number of cellular protein substrates including 

growth factor receptors and proto-oncogenes. Through protein phosphorylation, 

PKC is thought to play a pivotal role in both the transduction of external 

signals which are essential for activating various normal cellular functions, 

including the control of cell proliferation, and in mediating the effects of 

tumor promotors (Nishizuka 1986; Kikkawa and Nishizuka 1986). 

Because PKC appears to play such an important role in mediating the 

complex sequence of events associated with tumor promotion, the unusually 

higher sensitivity of the enzyme for activation by lead strongly suggests that 

the carcinogenic effects of lead compounds demonstrated in the animal studies, 

may be mediated, at least in part, through PKC. The positive evidence from 

animal studies indicating that some lead compounds have tumor promoting or 

cocarcinogenic activity (Hiasa et al, 1983; Shirai et al, 1984; Kobiyashi and 

Okamato, 1974) and the study by Choie and Richter (1974) showing that single 

injected doses of lead acetate as low as 1 ug/g body weight can cause a 

significant increase in kidney cell proliferation in the mouse are certainly 

consistent with the PKC mechanism and suggest the possibility that lead either 

functions as a promotor or facilitates promotion. It is possible, for example, 

that the high affinity of PKC for lead (compared to that of calcium) causes the 

74 
P.88 

requirement for endogenous diacylglycerol and/or environmental promotors to be 

lowered such that PKC becomes inappropriately activated. One result of this 

activation would be the initiation of cell proliferation in cells not normally 

scheduled for replication. Lead compounds are capable of stimulating 

proliferation in renal tubular epithelial cells and are considered to be potent 

mitogens. 

It was stated in the 1986 Air Quality Criteria for Lead (U.S. EPA, 1986c) 

that the "... evidence is accumulating to suggest that lead and its compounds 

are complete carcinogens possessing both initiating and promoting activity." 

With the new evidence for the involvement of lead as an activator of PKC, the 

above quote takes on an added significance. 

It should be noted that in addition to its possible involvement with PKC 

in carcinogenesis, lead may also function through PKC to cause some of the 

neurotoxicological effects. PKC is found in high concentrations in the brain, 

localized to a large extent in the presynaptic membranes, and is thought to 

play a major role through protein phosphorylation in presynaptic function 

(Kikkawa and Nishizuka, 1986). The inappropriate activation of PKC by lead in 

the nervous system may have serious consequences for normal neural function. 

Despite the uncertainty regarding mechanisms of action, the evidence to 

date, while not conclusive, indicates that the lead moiety is the most likely 

common element responsible for tumor induction. For example, four different 

lead compounds, lead acetate, lead subacetate, lead phosphate, and lead 

naphthenate have been shown to induce similar types of tumors in the same 

target organ, the kidney, in either rats or mice. The non-oncogenic pathology 

was also similar among these four compounds (Baldwin et al., 1964; Zollinger, 

1953). Finally, NCI (1979) also observed the same type of renal pathology 

75 
P.89 

along with nuclear inclusion bodies in mice or rats exposed subchronically to 

lead dimethyldithiocarbamate, even though the chronic bioassays, conducted at 

very low doses, were negative for carcinogenesis. While the type of 

nephropathy reported cannot presently be linked to the carcinogenic process, 

the fact that a variety of lead compounds can induce similar oncogenic as well 

as non-oncogenic pathological changes suggests that the lead itself is the 

active agent. 

In summary, lead acetate was shown to be carcinogenic, following oral 

dosing in seven of nine studies with rats and in one of two studies with mice. 

A lack of detectable tumorigenic responses in dogs and monkeys is considered to 

be due to inadequate duration of the experiments. Lead subacetate was shown to 

be carcinogenic in six of ten studies including four with rats and two with 

mice. Two other studies were positive for tumor promotion only. With the 

exception of a chronic bioassay with hamsters, which may be less sensitive than 

rats, the negative results occurred in experiments using either low doses or 

short exposure durations. Several other lead compounds were also shown to 

induce carcinogenic effects including lead phosphate, lead naphthenate, lead v 

dimethyldithiocarbamate, and tetraethyl lead. Lead subacetate tested 

positively for tumor promotion, while lead oxide was cocarcinogenic with 

benzo[a]pyrene. Based on these results, it is concluded that the evidence for 

carcinogenicity of lead in animals is adequate, although positive responses 

were generally seen only at high doses (generally in excess of a cumulative 

dose of 1 g) . It should be noted, though, that in some studies where 

administration was by injection, tumor induction occurred at lower doses 

indicating absorption efficiency may be an important factor in potency. 

76 
P.90 

The mechanism of action of lead or lead compounds is as yet unknown, 

although it has been hypothesized in the literature that lead inclusion bodies 

or protein kinase C are involved. The evidence to date, however, suggests that 

the lead moiety itself is responsible for the tumorigenic responses. For 

example, lead inclusion bodies are seen following exposure to most lead 

compounds, including organic as well as inorganic ones. Non-oncogenic 

pathological responses are similar for most lead compounds tested to date. 

Several different lead compounds, including organic and inorganic compounds, 

have been shown to be capable of inducing kidney cancer. All lead compounds 

tested have been shown to be capable of increasing the body burden of lead. 

From a qualitative weight-of-evidence perspective, therefore, all lead 

compounds are considered to be potentially carcinogenic. 

Although lead was most commonly administered in the food, positive results 

were also seen following administration via drinking water, intraperitoneal 

injection, subcutaneous injection, and skin painting. While no inhalation 

cancer bioassays are available, there is ample supplemental evidence for the 

bioavailability of lead following exposure by this route. Lead is, therefore,, 

considered to be potentially carcinogenic by any route. 

In the majority of cases the critical target organ for oncogenic effects 

was the kidney. While tumors have been reported in lead exposed animals at a 

variety of other target sites, due to weaknesses in experimental design, 

uncertainties in reporting, etc., the evidence for tumor induction at these 

other sites is considered to be less certain, but would still contribute to 

some degree to the weight of evidence. 

77 
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1.3. SHORT-TERM TESTS 

1.3.1. Mutagenicity 

The genotoxicity of lead has already been evaluated in depth in the Air 

Quality Criteria for Lead (U.S. EPA, 1986c).1 The current report does not 

attempt to conduct a new evaluation; rather, it summarizes the Air Quality 

Criteria for Lead and discusses recent information published since the 1986 

report. It should be noted that recent studies lend support to the conclusions 

reached in the Air Quality Criteria for Lead. This document concluded that 

lead has "genotoxic properties." 

Lead has been evaluated for its ability to produce both gene mutations and 

chromosomal anomalies. Briefly, studies for point mutations in bacteria have 

been negative (see U.S. EPA, 1986c, and IARC, 1987, for a review). However, 

bacterial systems generally are regarded as inappropriate for assaying metal 

ions. With the exception of a recent study on V79 cells (Zelikoff et al., 

1988), lead-induced gene mutations in cultured mammalian cells have been 

observed only at toxic concentrations (Hsie et al., 1980; Oberly et al., 1982). 

Both positive and negative responses have been reported for chromosome 

aberrations in higher organisms (Sharma and Talukder, 1987; U.S. EPA, 1986c). 

The positive responses, however, appear to depend on factors such as harvest 

time following exposure, duration and route of exposure, and test system. 

Furthermore, the in vivo chromosome breaking activity of lead is also affected 

by diet. It has been shown that lead-exposed animals on calcium-deficient 

diets exhibit a higher incidence of chromosomal aberrations than do lead-

^ h e Air Quality Criteria for Lead (U.S. EPA, 1986c) has had considerable peer 

review, including review by the Clean Air Scientific Advisory Committee of the 

Science Advisory Board in public sessions. 

78 
P.92 

exposed animals on standard diets (O'Riordan and Evans, 1974; Deknudt et al., 

1977b). 

The mechanism(s) by which lead induces chromosomal aberrations and 

mutations is unknown. The pattern of responses (e.g., lack of clear dose 

responses in certain studies, concentrations required to elicit a response, and 

culture conditions) observed in the various genotoxicity studies suggest, 

however, that lead compounds may not directly damage the genetic material but 

rather may act by an indirect mechanism(s). For example, lead has been shown 

to disturb or inhibit enzyme functions important for DNA replication and repair 

(Popenoe and Schmaeler, 1979; for a review see U.S. EPA, 1986c). Reviews and 

additional references on the mutagenicity of lead can be found in the U.S. EPA 

Air Quality Criteria for Lead (U.S. EPA, 1986c) and in the International Agency 

for Research on Cancer monograph (IARC, 1987). 

1.3.2. Cell Transformation 

Lead compounds have been reported to induce cell transformation in 

Balb/3T3 mouse cells and Fischer 344 rat embryo cells infected with the 

Rauscher murine leukemia virus (Dunkel et al., 1981). In studies on Syrian 

hamster embryo cells, both positive (Zelikoff et al., 1988; DiPaolo et al., 

1978) and equivocal (Dunkel et al., 1981) results have been obtained. Lead 

compounds have also been reported to enhance the viral transformation of Syrian 

hamster embryo cells (Casto et al., 1979). 

1.4. TOXICOLOGIC EFFECTS RELEVANT TO CARCINOGENICITY 

1.4.1. Human Studies 

The nonneoplastic effects due to overexposure to lead have been 

extensively reviewed by the U.S. EPA (1986c). Some of the effects discussed in 

79 
P.93 

this document include the following: subcellular effects, effects on heme 

biosynthesis and erythropoiesis/erythrocyte physiology, neurotoxicity, 

nephrotoxicity, reproductive and developmental toxicity, immunotoxicity, and 

toxic effects on the cardiovascular, hepatic, gastrointestinal, and endocrine 

systems. The renal effects (lead nephropathy) of lead, however, are of 

particular interest, because of a possible, although not established, 

correlation with renal carcinogenicity. Goyer (1985) discussed two types of 

lead nephropathy, acute (early nephropathy) and chronic (late nephropathy). 

The characteristic features of acute nephropathy are as follows: (1) the 

proximal tubular epithelium as the focus of injury; (2) structural features 

including formation of nuclear inclusion bodies, ultrastructural changes in 

mitochondria, and cytomegaly; (3) function changes, such as increased 

aminoaciduria, glucosuria, phosphaturia, and increased sodium and decreased 

uric acid excretion; (4) absence of minimal glomerular effects; and 

(5) reversibility of the above effects. Chronic nephropathy or late lead 

nephropathy is seen as nephrosclerosis or chronic interstitial nephritis. The 

characteristic of chronic nephropathy are as follows: (1) morphologic features 

including progressive interstitial fibrosis, dilatation of tubules, atrophy or 

hyperplasia of,tubular epithelial cells; (2) clinical features including 

reduction in glomerular filtration rate and azotemia, tubular disfunction 

disproportionately greater than indicated by the decrease in glomerular 

filtration rate followed by absence of tubular dysfunction at later stages; and 

(3) absent or sparse nuclear inclusion bodies. According to Goyer (1985), it 

is difficult to attribute the etiologic factor of chronic lead nephropathy to 

lead itself because of the absence of the inclusion bodies. Consequently, the 

association of lead with chronic nephropathy is based on circumstantial 

80 
P.94 

evidence. In humans, the absence of nuclear inclusion bodies may be due to the 

use of chelation therapy, which could mobilize the lead sequestered in 

inclusion bodies, thereby eliminating evidence of their presence by light 

microscopy. Animal studies have shown that EDTA can mobilize renal lead 

(Goyer, 1985). 

Blood lead levels are used as an indicator of lead intoxication. Goyer 

(1985) suggested that blood levels in excess of 70 ug/dL are required for 

induction of nephropathy, and Buchet et al. (1980, as cited in Goyer, 1985) 

found that blood levels not exceeding 62 ug/dL and an average duration of 

exposure of 13 years do not influence renal function. Wedeen et al. (1975, 

1979, as cited in Goyer, 1985) found that blood lead levels correlated poorly 

with the degree of renal toxicity. The U.S. EPA (1986c) concluded that 

. . . it is possible to estimate at least roughly the range of lead 

exposure associated with detectable renal dysfunction in both human 

adults and children. Numerous studies of occupationally exposed 

workers have provided evidence for lead-induced chronic nephropathy 

being associated with blood lead levels ranging from 40 to more than 

100 ug/dL, and some [studies] are suggestive of renal effects 

possibly occurring even at levels as low as 30 ug/dL. In children, 

the relatively sparse evidence available points to the manifestation 

of renal dysfunction only at quite high blood lead levels (usually 

exceeding 120 ug/dL). (p. 12-190) 

Blood lead levels, however, may not reflect the degree of renal toxicity, and 

urinary lead levels in response to EDTA chelation therapy, which measures past 

lead exposure, may be a better indicator of the potential for renal toxicity. 

The U.S. EPA (1986c) further concluded that. 

. . . the EDTA lead-mobilization test is the most reliable technique for 

detecting persons at risk for chronic nephropathy; blood lead measurements 

are a less satisfactory indicator because they may not accurately reflect 

cumulative absorption some time after exposure to lead has terminated, 

(p. 12-191) 

81 
With regard to the general relationship between internal lead levels and 

adverse effects, the U.S. EPA (1986c) concluded that. 

In living human subjects, direct determination of tissue lead burdens 

or how these relate to adverse effects in target tissues is not 

possible. Some accessible indicator (e.g., lead in a medium such as 

blood or a biochemical surrogate of lead such as erythrocyte 

protoporphyrin), must be employed. While blood lead still remains 

the only practical measure of excessive lead exposure and health 

risk, evidence continues to accumulate that such an index has some 

limitations in either reflecting tissue lead burdens or changes in 

such tissues with changes in exposure, (p. 10-69) 

At present, the measurement of plumburesis [lead excreted in urine] 

associated with challenge by a single dose of a lead-chelating agent 

such as CaNa2EDTA is considered the best indicator of the mobile, 

potentially toxic fraction of body lead. Chelatable lead is 

logarithmically related to blood lead, such that an incremental 

increase in blood lead is associated with an increasingly larger 

increment of mobilizable lead. (p. 10-70) 

Therefore, although blood lead may not be the best indicator of adverse effects 

including nephropathy, it can be correlated with other indicators of lead 

toxicity, which in turn can be correlated with nephropathy. 

1.4.2. Animal Studies 

A number of animal studies have shown that nephrotoxicity can be induced 

with various lead compounds administered either orally or topically. These 

include lead acetate (Boyland et al., 1962; API, 1971; Tanner and Lipsky, 1984; 

Nogueira, 1987), lead subacetate (Hass et al., 1965; Van Esch and Kroes, 1969; 

Oyasu et al., 1970; Kasprzak et al., 1985), lead naphthenate (Baldwin et al., 

1964), and lead dimethyldithiocarbamate (NCI, 1979). Most of these studies 

were conducted in rats and mice, but lead compounds have also been shown to 

induce nonneoplastic renal lesions in hamsters (Van Esch and Kroes, 1969), 

rabbits (Hass et al., 1965), dogs and monkeys (API, 1971). 

82 
P.96 

Lead compounds are capable of stimulating proliferation in renal tubular 

epithelial cells and are considered to be potent mitogens (Choie and Richter, 

1980). A single intraperitoneal injection of 40 ug of lead per g body weight 

stimulated a 40-fold increase in cell proliferation in rats as measured by 

autoradiography, or when combined with unilateral nephrectomy, a 65-fold 

increase (Choie and Richter, 1972b, as cited in Choie and Richter, 1980). A 

single intracardiac injection of lead acetate given at various doses ranging 

from 0.01 to 100 ug of lead per g stimulated DNA synthesis in mouse kidneys as 

measured by incorporation of H-thymidine into DNA (Choie and Richter, 1974). 

DNA synthesis, which peaked at 33 hours, was followed by a peak in the mitotic 

index at 39 hours; each fell to control levels by 2 and 4 days, respectively. 

Maximum DNA synthesis was elevated 15-fold over controls, and the maximum 

mitotic index was 45-fold greater than controls. The authors concluded that 

the stimulation in cell proliferation was not due to a regenerative response, 

because tubular necrosis was not seen by light microscopy. Prolonged treatment 

with lead for 6 months also stimulated cell proliferation and focal hyperplasia 

of tubular cells. The magnitude of the proliferative response was less afters 

6 months than after a single treatment (Choie and Richter, 1972a, as cited in 

Choie and Richter, 1980). Similar effects on cell proliferation and 

hyperplasia were seen in the liver of rats given a single intravenous injection 

of lead nitrate (Columbano et al., 1983, 1984) or repeated injections over a 

30-day (four injections) or 80-day period (nine injections) (Ledda-Columbano et 

al., 1983). After a single injection, the peak in DNA synthesis (incorporation 

of H-thymidine into DNA) occurred 36 hours after administration of lead and 

the peak in the number of cells entering mitosis occurred 48 hours after 

83 
administration of lead. The absence of necrotic cells as seen by light 

microscopy prompted the investigators to conclude that the stimulation in cell 

proliferation was a mitogenic response rather than a regenerative response 

(Columbano et al., 1983). The magnitude of the proliferative response after 

repeated administration of lead showed a progressive decrease, such that the 

increase was 16-, 6-, or 4-fold after 1, 4, or 9 injections, respectively 

(Ledda-Columbano et al., 1983). Additional studies showed that the mitogenic 

effect of lead was ineffective in initiating hepatocarcinogenesis, as 

determined by the induction of enzyme-altered foci in the liver (Columbano et 

al., 1987a, b). Roomi et al. (1986) demonstrated that lead nitrate induced 

biochemical changes (drug-metabolizing enzymes) characteristic of liver nodules 

(foci). The significance of this observation is unclear, considering that lead 

was shown not to induce liver foci. 

1.5. PHARMACOKINETIC PROPERTIES 

The pharmacokinetics of lead have already been reviewed in great depth in 

the Air Quality Criteria for Lead (U.S. EPA, 1986c, Volume III). The current, 

report does not attempt to conduct a new evaluation; rather, it quotes 

extensively from the Air Quality Criteria for Lead as the primary authority on 

the subject. This reference has had considerable peer review, including review 

by the Clean Air Scientific Advisory Committee of the Science Advisory Boa^d in 

public sessions. 

1.5.1. Absorption 

1.5.1.1. Human Studies — Lead is absorbed into the body following inhalation or 

ingestion exposure: dermal exposure to certain lead compounds can also result 

84 
P.98 

in notable absorption. Gastrointestinal absorption of ingested lead is 

influenced by several factors including age and dietary factors. 

The role of nutrients in lead absorption has been reported in several 

metabolic balance studies for both adults and children. .. . A number of 

reports have documented the association of lead absorption with suboptimal 

nutritional states for iron and calcium, with reduced intake being 

associated with increased lead absorption. . . . Various surveys have 

indicated that iron, calcium, zinc, and vitamin deficiencies are 

widespread among [children], particularly the poor. 

(U.S. EPA, 1986c, pp. 1-76, 10-43) 

In adult humans, the absorption of ingested lead has been estimated to range 

from 10 to 15 percent, but fasting humans may absorb 21 percent to about 

63 percent. Several studies have focused on differences in gastrointestinal 

absorption rates between adults and children. Experimental studies of children 

measured an average absorption rate of approximately 50 percent for ingested 

lead as compared to the 10 to 15 percent seen in adults. According to a review 

of the available literature on humans (U.S. EPA, 1986c), the absorption rates 

of different chemical farms of ingested lead appear to be about equal. Among 

the primary determinants of the rate of absorption are the conditions under 

which lead is ingested. As stated above, lead taken in a fasting state is 

generally more readily absorbed than lead taken with food. Also, the presence 

of minerals or other substances in the food may reduce the absorption of 

ingested lead. 

Under ambient conditions, the fraction of inhaled lead particles deposited 

in the lungs is about 30 to 50 percent (deposition of lead partir.les of various 

sizes ranges from 23 to 80 percent; see section 1.2.10.), depending on particle 

size and ventilation rate. The absorption of lead deposited in the lower 

respiratory tract, which appears to be greater than 90 percent, is apparently 

not markedly influenced by the chemical form of lead compounds; therefore, the 

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overall pulmonary lead uptake is governed by the concentration of lead in the 

air, the ventilation rate, and the fraction deposited. However, regional 

deposition factors such as lung clearance in the upper respiratory tract must 

also be considered when assessing absorption of inhaled lead. 

The rate of absorption of lead through the skin is generally much lower 

than absorption via the gastrointestinal and respiratory tracts, although 

dermal exposure to alkyl leads, lead soaps, and certain other lead compounds 

can result in significant absorption. 

1.5.1.2. Animal studies—Animal studies add support to human data. 

While experimental animal data for quantitative assessment of lead 

deposition and absorption from the lung and upper respiratory tract are 

limited, available information from the rat, rabbit, dog, and nonhuman 

primates supports the findings that respired lead in humans is extensively 

and rapidly absorbed. (U.S. EPA, 1986c, p. 10-61) 

Young developing animals absorb a much higher percentage of lead from the gut 

than adult animals, although the percent absorption among animal species varies 

somewhat. In laboratory animal experiments, lead absorption from the gut 

generally resembles that of humans. For example, in male albino Wistar rats, a 

comprehensive study of factors that affect the absorption and excretion of lead 

was conducted using radioactive lead acetate or lead nitrate as tracers and 

lead chloride as a carrier in distilled water (Conrad and Barton, 1978). Four 

hours after dosing with 0.001 and 10 ug of lead chloride (by gavage) the 

absorption rate was 4 to 5.5 percent of administered dose; the rate decreased 

to about 1 or 2 percent at doses of 100 and 1000 ug. Likewise, in other animal 

studies lead absorption as a percentage of dose decreased, suggesting a 

saturation phenomenon for lead transport across the gut wall (U.S. EPA, 1986c). 

Most of the lead was absorbed from the duodenal region of the small intestine. 

Factors that affected absorption were age, and nutritional and other dietary 

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p.1 

factors. Absorption was maximal in young animals when growth was rapid; 

absorption decreased as animals aged and gained weight. Fasting for at least 

3 days had no effect on absorption of lead, but fasting for 5 days 

significantly decreased absorption. However, other studies indicate that 

fasting markedly enhanced gut uptake of lead in rats (U.S. EPA, 1986c). An 

iron-deficient diet significantly increased absorption, whereas iron 

supplements, as well as zinc and calcium, decreased the rate of absorption, 

presumably by competing for lead absorption sites in the intestinal mucosa. 

Dietary substances that enhanced the solubility of lead (cystine, cysteine, 

methionine, tyrosine, arginine, and ascorbic acid) also enhanced the absorption 

of lead, probably by maintaining more lead in a physical state capable of 

absorption (Conrad and Barton, 1978). 

Bioavailability of different chemical and physical forms of lead in varied 

diet matrices has been reported. Barltrop and Meek (1975) evaluated eight 

inorganic and four organic lead compounds to determine the relative absorption 

in the rat following dietary exposure to 0.075 percent of lead over a 48-hour 

period. Absorption was compared to that of lead acetate. A relative 

absorption index was determined for the different compounds by comparing the 

lead levels in the blood, bone, and kidney. An absolute total absorption was 

not determined. Basic lead carbonate (164 percent of lead acetate) and tallate 

(121 percent), were the most efficiently absorbed compounds, followed by lead 

sulfide (67 percent), lead naphthenate (64 percent), lead octoate (62 percent), 

lead chromate (44 percent), and metallic lead (14 percent), which was the least 

efficiently absorbed. However, particle size and dietary factors were found.to 

influence the uptake from the gut: for example, an increase in dietary fat 

resulted in an increase in lead absorption. Other reports showed that lead 

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sulfide (Karhausen, 1973) and lead naphthenate (van Peteghem and de Vos, 1974) 

are readily absorbed, because under acidic conditions in the gastrointestinal 

tract, they are converted to lead chloride, which is readily absorbed 

(Karhausen, 1973). Lead oxide and tetraethyl lead are also readily absorbed 

(Karhausen, 1973). Alkyl lead compounds, especially tetraethyl lead, are 

expected to be more efficiently absorbed from the gastrointestinal tract than 

inorganic lead compounds, because of their conversion to trialkyl lead under 

gastric acidic conditions (Jensen, 1983). 

The cancer potency of different lead compounds may vary considerably 

simply due to absorption, which in turn is influenced by a number of factors 

that may help to explain differences in results among animal studies. Also, in 

interpreting these results for human risk assessment, it is important to 

recognize that not all lead compounds are equally prevalent in the environment. 

1.5.2. Distribution and Retention 

1.5.2.1. Human studies—The fraction of absorbed lead retained in the body 

ranges from 1 to 5 percent in adults and from 32 to 34 percent in young 

children. Absorbed lead is transported by the systemic circulation and 

distributed in various organs and tissues. The lead is then gradually 

redistributed to organs and systems according to relative affinity. A number 

of body compartments exist for lead, and the separate kinetic pools have 

different rates of turnover. The greatest fraction of retained lead 

accumulates in the bone: lead in a large and relatively inert bone compartment 

has a half-life greater than 20 years. There is also a fraction of lead in 

bone that is in equilibrium with lead in other tissues. Lead in calcified 

tissues generally builds up throughout life until old age. Blood lead levels 

indicate recent exposure and vary somewhat with age and sex. Lead in blood 

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resides in several distinct pools with most of the blood lead in humans being 

associated with the erythrocyte. The small diffusible fraction of lead in 

plasma and serum is in equilibrium with soft tissue or end organs. Other major 

organs that take up lead are the liver and kidneys, with amounts also found in 

muscle and brain. In the brain the highest concentrations of lead are found in 

the hippocampus, followed by the cerebellum, cerebral cortex, and medulla. 

Lead content in the kidney cortex increases with age and may be related to the 

observation of nuclear inclusion bodies. Lead does not accumulate in the lung. 

U.S. EPA (1986c) concluded that "Mobile lead in organs and systems is 

potentially more active toxicologically in terms of being available to 

biological sites of action. Hence, this fraction of total body lead burden is 

a more significant predictor of imminent toxicity" (p. 10-65). However, this 

mobile lead fraction in organs and systems cannot be measured directly. 

Although the blood lead level is a reasonable indicator of recent exposure, use 

of blood lead as an effective dose or as a measure of total body burden or 

impairment "can have limitations in reflecting both the amounts of lead in 

target tissue and the temporal changes in tissue lead with changes in exposure" 

(p. 10-54). Chelatable lead, using calcium disodium ethylenediaminetetraacetic 

acid (CaNa,"EDTA), is viewed as the most useful reflection of lead body 

burden. The chelatable urinary lead, e.g., the fraction of lead that is 

chelatable from various body compartments, is thought to include "a labile lead 

compartment within bone as well as within soft tissues" (U.S. EPA, 1986c, 

p. 10-65). 

Placental transfer of lead, followed by distribution in the fetus, occurs 

in humans, with lead levels in newborn infants being similar to maternal 

levels. 

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1.5.2.2. Animal studies—Animal studies have helped to sort out some of the 

relationships of lead exposure to in vivo distribution of the element, 

particularly the impact of skeletal lead on whole-body retention. In rats, 

lead dosing results in an initial increase of lead levels in soft tissues 

followed by loss of lead from soft tissue via excretion and transfer to bone. 

Lead distribution appears to be relatively independent of dose. Other studies 

have shown that lead loss from organs follows first-order kinetics except for 

loss from bone, and that the skeletal system in rats and mice is the 

kinetically rate-limiting step in whole-body clearance (U.S. EPA, 1986c). 

The neonatal animal seems to retain proportionally higher levels of tissue 

lead compared to the adult and manifests slow decay of brain lead levels while 

showing a significant decline over time in other tissues. This decay appears 

to result from enhanced lead entry to the brain because of a poorly developed 

brain barrier system as well as from enhanced body retention of lead by young 

animals (U.S. EPA, 1986c). 

The effects of such changes as metabolic stress and nutritional status on 

body redistribution of lead have been noted. Lactating mice, for example, are 

known to demonstrate tissue redistribution of lead, specifically bone lead 

resorption With subsequent transfer of both lead and calcium from mother to pup 

(U.S. EPA, 1986c). 

Essential nutrients have an influence over the body's treatment of lead. 

Certain nutritional factors such as low calcium, phosphate or vitamin D, and 

iron, copper, or zinc deficiency, can enhance lead absorption or decrease renal 

excretion. For example, calcium has been shown to affect the absorption and 

tissue distribution of lead. Whole-body retention of lead (lead chloride given 

by gavage) and tissue distribution were reduced in rats given a calcium 

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supplement to raise the dietary level to 350 mmol of calcium per kg of diet 

(Meredith et al., 1977), and the lead content in the kidney and femur was 

increased in rats placed on low calcium diets (Mahaffey et al., 1975). Low 

dietary calcium enhances toxic effects of lead and increases chromosome 

aberrations in bone marrow cells in mice (Deknudt and Gerber, 1979) and 

peripheral lymphocytes in monkeys (Deknudt et al., 1977a). Dietary calcium 

supplements between 0.3 and 6 percent were shown to cause a decrease in lead 

content in the kidney, but not in the liver (Kasprzak et al., 1985), with a 

surprising increase in renal toxicity and renal tumor formation. 

Lead naphthenate, a compound that is well absorbed through the skin, is 

distributed to the same tissues as lead compounds absorbed from the 

gastrointestinal tract, with the highest levels found in kidneys. Selenium at 

5 and 10 ppm in drinking water administered simultaneously with topical 

applications of lead naphthenate resulted in a dose-related increase in the 

content of lead in kidneys and blood, but not in brain and liver (Rastogi et 

al., 1976). 

Most of the absorbed lead from alkyl lead is found in liver, kidney, and. 

brain; very little is found in bone (Jensen, 1983). Tetraethyl and tetramethyl 

lead are metabolized to their respective trialkyl metabolites by oxidative 

dealkylation by a microsomal cytochrome P450-dependent monooxygenase. The 

trialkyl metabolites may be further converted to dialkyl metabolites and 

finally to inorganic lead. The formation of inorganic lead has been observed 

in various species treated with alkyl lead, although different proportions of 

lead chemical forms are seen across the animal species following alkyl lead 

exposure. 

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1.5.3. Excretion 

1.5.3.1. Human studies--U.S. EPA (1986c) concluded that 

Dietary lead in humans and animals that is not absorbed passes through the 

GI [gastrointestinal] tract and is eliminated with feces, as is the 

fraction of air lead that is swallowed and not absorbed. Lead entering 

the bloodstream and not retained is excreted through the renal and GI 

[gastrointestinal] tracts, the latter via biliary clearance. The amounts 

excreted through these routes are a function of such factors as species, 

age, and exposure characteristics, (p. 10-66) 

Jensen (1983), in a review article, indicated that for inorganic lead about 

three-fourths is excreted in the urine and less than one-fourth in the feces, 

whereas for organic lead (e.g., alkyl lead compounds) the pattern is 'reversed, 

with less than a third in the urine and the majority in the feces. Lead can 

also be excreted in breast milk and thus become a source of exposure for 

nursing infants. 

1.5.3.2. Animal Studies--In experimental animals, both urinary and fecal 

excretion are important routes of elimination, although the relative amounts 

between the two routes is dependent on organic versus inorganic lead, animal 

species, and dose level. 

With regard to species differences, biliary clearance of lead in the dog 

is but 2 percent of that for the rat, while such excretion in the rabbit 

is 50 percent that of the rat. 

Lead movement from laboratory animals to their offspring via milk 

constituents is a route of excretion for the mother as well as a route of 

exposure for the young. (U.S. EPA, 1986c, p. 10-67) 

As seen in the human studies, alkyl lead compounds are excreted primarily 

in feces; inorganic lead, which is converted from dialkyl lead in the 

intestine, dominates. The secondary route of excretion is via urine, with the 

dialkyl lead species dominating (Jensen, 1983). The proportion of chemical 

forms excreted appears to be animal species dependent. 

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1.5.4. Discussion 

The available evidence indicates that lead and lead compounds can be 

absorbed into the systemic circulation from the respiratory tract following 

inhalation exposure and from the gastrointestinal tract following ingestion. 

Consequently, various compounds of lead contribute to. the blood lead level and 

thus have the potential to ultimately cause adverse effects upon distribution 

to the various target organs and tissues. Likewise, limited information shows 

that certain lead compounds can be absorbed into the systemic circulation and 

distributed following dermal absorption. Absorption of lead is influenced by a 

number of factors such as particle size, age of subject, and dietary 

considerations. Even under identical conditions, there may also be differences 

in the bioavailability of different forms of lead due to the unequal rate and 

extent of absorption of individual compounds. However, each of the various 

forms of lead is absorbed to some extent, contributing to the blood lead level 

and thus to the body burden. The evidence that many different lead compounds 

tested in cancer assays induce renal tumors and nonneoplastic renal lesions or 

induce tumors in other organs clearly indicates that they are absorbed into the 

body and distributed to distal target organs. It is reasonable to assume that 

all lead and lead compounds can contribute to adverse effects, including 

cancer-causing activities, because they are absorbed into the systemic 

circulation, contribute to lead body burden, and thus are distributed to target 

organs and tissues throughout the body. The potency of different chemical and 

physical forms of lead may vary, however, due to absorption factors as well as 

other bioavailability pharmacokinetic considerations. It is interesting to 

note that certain rat chows are so suppressive of lead absorption that uptake 

93 

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is less than 1 percent (U.S. EPA, 1986c, p. 10-12). This may help to explain 

some of the different results observed in the animal carcinogenicity studies. 

At the present time, known pharmacokinetic differences, for example, 

uptake and distribution differences between rodents and humans, disallows 

extrapolation of effective target dose for development of a cancer risk 

estimate for humans based on the animal data. Recently available information 

indicates that distribution of lead in the body, in the bone, for example, is 

considerably even more complex than once believed. However, ongoing 

development of pharmacokinetic models, particularly in animals but also 

improved models in humans, makes the possibility of relevant species-to-species 

extrapolation promising for the future. 

1.6. WEIGHT-OF-EVIDENCE CLASSIFICATION 

Although the primary concern about lead has focused on its adverse health 

effects other than cancer, many investigators have studied the potential 

association between lead exposure and cancer. The evidence pertaining to 

cancer includes human studies, long-term animal studies, short-term tests, and 

numerous relevant biochemical data. 

Looking first at the human evidence, there are over a dozen epidemiologic 

studies on lead exposure and cancer. They were conducted in various 

occupational settings involving potential lead exposure, including lead battery 

manufacture and recovery, lead smelter operations, tetraethyl lead production, 

and among plumbers and pipe fitters. One cohort study and one proportional 

mortality study show excesses in kidney cancer, albeit not statistically 

significant, while deficits of kidney cancer were observed in two other 

cohorts, leading to equivocal epidemiologic evidence for kidney cancer. 

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Nevertheless, this circumstance is worth noting since kidney cancer is, by far, 

the most common form of cancer seen in the-animal studies. Several cohort 

studies show an increased incidence of lung cancer; three are statistically 

significant, two are not. All have some type of shortcoming that precludes the 

drawing of a causal inference between lead exposure and cancer. 

Collectively, the human studies suggest that lead may cause cancer in 

humans, but because each study has methodological limitations, they fall short 

of providing a causal link between lead exposure and cancer. The lack of a 

measure of lead exposure is a major problem with all the studies. The presence 

of other carcinogens in some of the occupational settings also makes it 

impossible to determine if the observed increases in cancer are due to lead 

alone, to the combination of lead with other substances, or to the other 

substances solely. For these reasons, the evidence of carcinogenicity from 

human studies must, at this time, be considered "inadequate"' according to EPA's 

Guidelines for Carcinogen Risk Assessment (U.S. EPA, 1986a). Note, however, 

that the role of lead in the occurrence of human cancer cannot be ruled out, 

given the inference that the positive associations seen in the human studies,-

while not adequate for a "limited" or a "sufficient" classification, are 

characterized as suggestive (a descriptor not found in the guidelines). The 

suggestive circumstance raises a concern about the potential carcinogenicity of 

lead in humans. This concern is supported by the suggestion of genotoxic 

effects in lead-exposed workers. 

Turning, then, to the long-term animal evidence, there is strong evidence 

of carcinogenicity from over two dozen studies. Many of these studies involve 

the ingestion of soluble lead salts that, presumably, were selected on the 

basis of ease of administration. In 12 separate studies, involving four rat 

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P.10 

strains and one mouse strain and involving both sexes whenever both sexes were 

tested, the ingestion of lead acetate or lead subacetate has been shown to 

induce kidney tumors. Tumor incidences in animals ingesting maximally 

tolerated doses of lead were often unusually high, almost 100 percent, and the . 

incidences were dose-related in studies using more than one dose. 

Other long-term animal studies and supplementary information provide 

evidence that these observations should not be viewed as a phenomenon specific 

to the ingestion route of exposure, to lead acetate or lead subacetate only, or 

to the kidney in particular. ' A variety of lead compounds has been shown to 

induce tumors by different routes of exposure: lead subacetate caused lung 

tumors by intraperitoneal injection; lead phosphate caused kidney tumors by 

subcutaneous or intraperitoneal injection; lead naphthenate caused kidney 

tumors by topical application; lead dimethyldithiocarbamate caused reticulum 

cell sarcomas by ingestion; and tetraethyl lead caused lymphomas by 

subcutaneous injection. Lead oxide has been shown to have cocarcinogenic 

properties with benzo[a]pyrene in causing lung carcinomas by intratracheal 

instillation, and lead subacetate has been shown to promote kidney tumors by-. 

ingestion after initiation with N-ethyl-N-hydroxyethylnitrosamine. In 

addition, lead acetate has caused testicular tumors and lymphocytic leukemia by 

ingestion, and both lead acetate and lead subacetate have caused cerebral 

gliomas by ingestion. 

Although there are several nonpositive animal studies, they do not weaken 

the pattern of positive evidence. Several nonpositive studies were conducted 

for durations well short of the minimum accepted duration for a cancer study. 

No tumor response is expected in this case, because the study duration may not 

have exceeded the latent period that must pass before tumors can be detected. 

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The other nonpositive studies were conducted at doses well below the maximum 

tolerated dose. Again, no tumor response is expected, based on a linear 

extrapolation of the positive response seen at higher doses. In fact, studies 

that are clearly positive at higher doses show no tumor response at these lower 

doses. (Section 2.3 discusses this point, showing why no tumors are expected 

at the lower doses and explaining why this is consistent with the positive 

response seen at higher doses.) None of the positive studies has been preceded 

or followed by a true replicate negative study. The doses that induce tumors 

do so in study after study. The nonpositive studies cannot, therefore, be used 

to discount the positive evidence. 

Looking at these results across the different forms of lead, several lead 

compounds have been shown to be carcinogenic. Supplementary information has 

shown several other forms of lead to be bioavailable and, therefore, highly 

likely to be carcinogenic at some dose. Considering that no lead compound can 

be called negative for either carcinogenicity or bioavailability, there appears 

to be no evidence at this time to rule out any form of lead as a potential 

carcinogen. 

Alternatively, looking at these results across the different routes of 

exposure, several routes have been shown to induce tumors at distal sites. For 

example, kidney tumors have resulted from exposure to lead in food, in drinking 

water, by topical application, by subcutaneous injection, and by 

intraperitoneal injection. Supplementary information has shown that 

inhalation is an efficient route of systemic absorption of lead, contributing 

to the body burden of lead and, therefore, also highly likely to provide the 

circumstances for carcinogenic activity. Considering that all routes of 

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exposure can result in systemic absorption, there appears to be no evidence at 

this time to rule out any route of exposure as potentially carcinogenic. 

It is noteworthy that the injection studies show tumors at lower total 

doses than the ingestion studies. This supports the position that lead may be 

carcinogenic to the degree that it is bioavailable for absorption into the body 

and circulation to target tissues. Inhalation studies have shown that 

deposited lead does not accumulate in the lung, indicating that is taken up 

into the bloodstream and circulated to distal target sites. Ingestion studies 

have shown that there is absorption--at different relative rates--of lead and a 

variety of lead compounds, including lead acetate, lead carbonate, lead 

tallate, lead sulfide, lead naphthenate, lead octoate, lead chromate, lead 

chloride, lead oxide, and tetraethyl lead. Acidic conditions facilitate the 

absorption of inorganic lead compounds, some of which otherwise would be 

relatively insoluble .in standard reference water solutions, through conversion 

to more soluble, and hence more bioavailable, compounds such as lead chloride. 

Tetraalkyl lead compounds are metabolized to trialkyl lead, which in turn is 

metabolized to dialkyl lead, which is finally converted to inorganic lead in , 

the intestine. The formation of inorganic lead has been observed in several 

animal species exposed to alkyl lead. Thus, there appears to be no evidence to 

rule out the potential for bioavailability of any lead compound to some degree. 

It is also interesting to note that the ingestion studifs in which lead 

induced cerebral gliomas, testicular tumors, reticulum cell sarcomas, or 

various other tumors were all conducted at exposure levels well below the 

maximum tolerated dose. This suggests that lead may act to induce cancer at a 

variety of sites, at lower exposure levels than those that clearly cause kidney 

tumors. The induction of cancer at other sites is consistent with the 

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P.13 

pharmacokinetics of lead, which indicate that lead is distributed to many sites 

in the body, including the sites of some of these tumors. The induction of 

cancer at lower exposure levels is consistent with the ability of lead to cause 

other adverse health effects at lower exposure levels and with the recent study 

showing that lead stimulates protein kinase C at picomolar concentrations. The 

evidence for these other tumors, however, is less robust than that for kidney 

tumors. 

Taken together, the long-term animal studies form a compelling pattern of 

evidence. Lead causes cancer by several routes of exposure and at a number of 

sites. These findings have been demonstrated in a large number of rodent 

studies, in several species and strains, and in both sexes. A dose-response 

trend has been demonstrated, as tumor incidences increase with exposure to 

reach very high proportions at the highest exposure levels tested. Although 

the studies have limitations in design or conduct when considered separately, 

the limitations do not outweigh the positive evidence from these studies 

considered as a whole. The evidence of carcinogenicity from long-term animal 

studies is, therefore, considered to be "sufficient" according to EPA's 

Guidelines for Carcinogen Risk Assessment (U.S. EPA, 1986a). 

Evidence from short-term tests suggests that lead compounds are genotoxic. 

Briefly, lead compounds have tested negative for point mutations in bacteria, 

but have tested positive for induced gene mutation? in cultured mammalian 

cells. Both positive and negative results have been observed for chromosome 

aberrations in higher organisms. In general, the positive responses for 

genotoxicity appear to depend on factors such as the test system, the duration 

and route of exposure, and the harvest time following exposure. Although the 

mechanism through which lead induces chromosomal aberrations and mutations is 

99 
p.1* 

unknown, the pattern of responses suggests that lead compounds may not directly 

damage the genetic material, but rather may act through an indirect mechanism. 

Nevertheless, the results of the genotoxicity studies collectively lend support 

to the human and animal evidence for carcinogenicity. 

Lead compounds have induced cell transformations in rodent tissue culture 

cells. Lead has recently been shown to activate the enzyme, protein kinase C, 

the putative cellular receptor for tumor-promoting phorbol esters. Lead can 

activate the enzyme to levels that normally require a mi 11ionfold greater 

concentration of the normal endogenous activator, calcium. Animal studies have 

shown that some lead compounds have tumor promoting potential or cocarcinogenic 

activity. Lead compounds have also been shown to stimulate proliferation in 

renal tubular epithelial cells and are considered to be potent mitogens. 

The question of lead's mechanism of action and the presence or absence of 

a practical dose-response threshold has not been demonstrated by dedicated 

scientific studies and thus is a matter for speculation at this time. Such 

uncertainties do not provide a basis to alter the weight of evidence for human 

carcinogenicity. The evidence on toxic effects other than cancer shows that 

the noncancer pathology is similar for four compounds (lead acetate, lead 

subacetate, lead phosphate, and lead naphthenate) that induce similar types of 

kidney tumors. The same kidney pathology, along with the presence of nuclear 

inclusion bodies following these exposures, has been observed in rats and mice 

with subchronic exposure to higher doses of lead dimethyldithiocarbamate, even 

though chronic exposure to lower doses did not induce cancer. The presence of 

nuclear inclusion bodies in the kidney may be related to the carcinogenic 

effects of lead. Although the kidney pathology that has been reported has not 

been definitively linked to kidney cancer, the fact that a variety of lead 

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compounds induce similar oncogenic and nononcogenic pathologic changes suggests 

that lead itself is likely to be the active agent. 

Integrating all sources of information into an overall weight of evidence 

for carcinogenicity, the combination of "sufficient" evidence from animal 

studies and "inadequate" evidence from human studies, along with the relevant 

supportive information from short-term tests, other toxic effects, and 

pharmacokinetic properties, yields an assignment to weight-of-evidence Group B2 

under EPA's Guidelines for Carcinogen Risk Assessment (U.S. EPA, 1986a). The 

overall classification reflects the evidence that lead causes cancer by several 

routes of exposure and at a number of sites; that these findings have been 

demonstrated in a large number of studies, in several species and strains of 

rodents, and in both sexes; that a dose-response trend has been demonstrated, 

as tumor incidences increase with exposure to reach very high proportions at 

the highest exposure levels tested; and that the pharmacokinetics of lead 

support the consideration of lead and its compounds together in an evaluation 

of potential carcinogenicity. Based on this overall evaluation, lead and lead 

compounds are considered to be probable human carcinogens. 

These conclusions can be put into perspective by reviewing the historical 

conclusions reached by other well-respected cancer organizations. In 1980, the 

International Agency for Research on Cancer (IARC, 1980) classified lead 

subacetate, lead acetate, and lead phosphate in Group 2B. IARC's Group 2B is 

similar to EPA's Group B2 in that both classifications require "sufficient" 

evidence from animal studies and "inadequate" evidence from human studies. In 

a subsequent supplement, IARC (1987) extended this classification to include 

lead and all inorganic lead compounds. Organic lead compounds were determined 

not to have an adequate data base for classification under the IARC scheme. 

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Meanwhile, the National Toxicology Program (NTP, 1985), in its Fourth Annual 

Report on Carcinogens, discussed lead and concluded that 

The toxicologic (and carcinogenic) risk from a metal is often a 

property less of the form of the metal administered than of the metal 

per se. The anion or ligand of the metal can determine the physicochemical properties which then affect characteristics such as 

solubility, absorption, distribution to sites believed to be involved 

in carcinogenesis, etc. Thus, if any lead compounds are 

carcinogenic, then all compounds containing lead are potentially 

carcinogenic. This also applies to the elemental metal, given the 

property of most tissues of being able to solubilize most metals to 

some degree. 

The conclusions reached in this report are consistent with and are, in fact, a 

natural extension of the conclusions reached by IARC and NTP. EPA's 

conclusions can be viewed as taking IARC's classification of lead and all 

inorganic lead compounds in Group 2B, and extending it as suggested by NTP to 

include lead and all lead compounds. These conclusions follow from the 

simultaneous consideration of the cancer studies, which show that several lead 

compounds induce cancer, and the pharmacokinetic evidence on lead, which 

indicate that these findings apply to lead and other lead compounds as well. 

As a final note, before beginning work on this report, EPA consulted 

several metal carcinogenicity experts regarding the weight of evidence and 

cancer potency of lead and inorganic lead compounds. In April 1986, a panel of 

experts (Roy E. Albert, M.D., University of Cincinnati Medical Center; 

Paul Hammond, D.V.M., Ph.D., University of Cincinnati Medical Center; 

Max Costa, Ph.D., New York University Medical Center; and Robert P. Beliles, 

Ph.D., D.A.B.T., U.S. Environmental Protection Agency) reviewed evidence on the 

carcinogenicity of lead and inorganic lead compounds. The panel concluded that 

the evidence for inorganic lead compounds satisfies the criteria for a weightof-evidence Group B_ classification, and raised a concern for any form of lead 

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that could add to the overall body burden. The panel further recommended that 

comparative cross-species pharmacokinetic models be developed to provide an 

appropriate basis for extrapolating the cancer potency to humans, if such an 

extrapolation is to be attempted. 

103 
2. POTENCY 

2.1. GENERAL METHODOLOGY 

Once a substance has been determined to be a potential carcinogen, it is 

natural to try to describe its potency, which measures the strength of the 

substance to cause cancer. Potency, for toxic effects in general, is measured 

by the dose required to cause a particular effect or level of effect. 

On different occasions with different objectives, EPA has used two 

different approaches to characterize the potency of carcinogens. One approach 

is to obtain a plausible upper bound on the increased cancer risk resulting 

from low-level exposures. This approach assumes that there may be a linear 

relationship between the level of exposure to a carcinogen and the resulting 

increase in the risk of some form of cancer. The strength of a carcinogen is 

characterized by the slope of this linear relationship. Under this approach, a 

mathematical model is fitted to the dose-response data and used to extrapolate 

down to low doses. Because the slope at low doses can be extremely sensitive 

to small changes in the observed data, an upper bound on the slope is 

estimated. This upper bound can be viewed as the steepest slope that is still 

consistent with the observed dose-response data. This approach is most 

completely described in U.S. EPA, 1980. Some strengths and limitations of this 

approach are discussed in EPA's Guidelines for Carcinogen Risk Assessment 

(U.S. EPA, 1986a). This approach is mentioned here only for perspective, and 

is neither used nor discussed further in this report. 

EPA has chosen an alternative approach for making reportable quantity 

adjustments pursuant to CERCLA Section 102 (U.S. EPA, 1986b). Under this 

alternative approach, the measure of potency is the dose associated with an 

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increased cancer incidence of 10 percent. A mathematical model is fitted to 

the dose-response data and used to estimate the equivalent human dose 

associated with an increased cancer incidence of 10 percent. The 10-percent 

level has been chosen because most animal studies that show an increase in 

cancer include a dose where the increased incidence is near 10 percent. 

This approach has several advantages: 

a. It does not require extrapolation beyond the observed dose range. 

b. It is relatively insensitive to the choice of a mathematical doseresponse model, because all models that fit agree quite well across 

the observed dose range. 

c. It does not introduce the use of statistical upper bounds, which can 

distort comparisons and relative rankings. 

d. It matches the available data in its relative level of 

sophistication — the mathematics do not outstrip the information 

contained in the dose-response data. 

The disadvantages of this approach are: 

a. It estimates the potency "at an increased cancer incidence of 

10 percent and, therefore, does not necessarily reflect 

pharmacokinetic properties or cellular mechanisms that may operate at 

lower doses. 

b. It is not a low-dose extrapolation and, therefore, should not be used 

to make inferences about cancer risks that may result from doses that 

are several orders of magnitude below the observed dose range. 

The dose associated with an increased cancer incidence of 10 percent can 

be used to qualitatively characterize the potency of a carcinogen as high, 

105 
medium, or low, relative to other carcinogens. The distinctions are made as 

follows: 

Group 1. High potency. The dose is below 0.01 mg/kg*d. 

Group 2. Medium potency. The dose is between 0.01 and 1 mg/kg*d. 

Group 3. Low potency. The dose is above 1 mg/kg'd. 

About a quarter of the potential carcinogens evaluated to date are in potency 

Groups 1 and 3, and about half are in potency Group 2. 

2.2. DATA SELECTION 

Lead is certainly one of the more extensively studied toxic substances, 

qualitatively, if not quantitatively. Much is known about what happens when 

humans are exposed to lead, and many of the adverse health effects of lead have 

been well documented. Based on human studies, dose-response relationships have 

been described for several adverse health effects. For cancer, however, the 

human studies cannot be used to describe a dose-response relationship, because 

of the lack of a measure of lead exposure and because of confounding exposures 

to other carcinogens. Therefore, a description of the dose-response 

relationship for cancer or an estimate of the cancer potency of lead must, at 

this time, consider the animal studies. 

An examination of the animal studies summarized in Table 1-1 indicates 

that a response of obvious concern, as well as the most sensitive response, is 

the induction of kidney tumors by lead. This is also the most extensively 

studied cancer response in animals. The human studies are somewhat supportive 

of the choice of this response, with suggestive evidence of an association 

between lead and kidney cancer. Human exposure to high levels of lead has also 

106 
P.21 

been reflected by kidney toxicity and by high levels of lead in the kidney. 

Lung cancer, which has also been suggested by the human studies, has been 

induced in animals by both intraperitoneal injection and intratracheal 

instillation. At this time, however, there is no accepted methodology for 

using these routes of exposure to estimate the cancer potency for routes of 

environmental exposure, and thus the lung tumor response cannot be 

quantitatively assessed. 

For characterizing the cancer potency by environmental exposure, studies 

by a route corresponding to environmental exposure are preferred. Because 

there are no cancer studies on the inhalation of lead, this leaves only 

ingestion by food or drinking water. Although lead has caused kidney cancer by 

topical application, at this time, there is no way to determine the extent to 

which the absorption of lead through the skin was enhanced by the use of a 

benzene solution. Although lead has caused kidney cancer by subcutaneous or 

intraperitoneal injection, at this time, there is no way of inferring the 

environmental exposures that would be equivalent to the directly injected 

doses. 

2.3. POTENCY ESTIMATION AND CHARACTERIZATION 

The dose-response information compiled in Tables 1-1 and 1-2 reveals that, 

with one exception, all groups of rats ingesting a total of at least 10 g of 

lead had an increased cancer incidence of at least 10 percent. The exception, 

the female rats ingesting a total of 14 g of lead in the Azar et al. (1973) 

study, is balanced by the male rats, which had a 50 percent incidence of kidney 

tumors at the same dose and a 10 percent incidence at a total dose of 7 g. 

Animals ingesting no more than 1 g of lead showed no kidney cancer response 

107 
P.22 

(although various other tumors sometimes developed at total lead doses below 

1 g, and kidney tumors were repeatedly induced by injections of lead totalling 

less than 1 g) . Therefore, the total ingested lead dose associated with an 

increased cancer incidence of 10 percent appears to be between 1 and 10 g for 

the rats. 

For comparison with other potential carcinogens, this dose is expressed in 

units of milligrams of lead per kilogram of body weight per day (mg/kg'd). 

Assuming that rats weigh 0.350 kg, a total lead dose of 1-10 g is approximately 

equal to 3000-30,000 mg/kg over the duration of the studies. (Some rats 

weighed more, which would result in proportionally lower doses.) As the 

studies generally lasted up to 24 months, on a daily basis this is 

approximately equal to 4-40 mg/kg'd for the rats. This appears to be a 

relatively high daily dose. 

Analyzing the mouse studies in a similar manner, the information is 

sketchy, but the pattern is similar. Mice ingesting a total of at least 2 g of 

lead had an increased cancer incidence of approximately 10 percent or more. 

Mice ingesting no more than 0.2 g of lead showed no kidney cancer response 

(although other tumors sometimes developed at lower total lead doses). 

Therefore, the total ingested lead dose associated with an increased cancer 

incidence of 10 percent appears to be between 0.2 and 2 g. Assuming that mice 

weigh 0.030 kg, a total lead dose of 0.2-2 g is approximately equal to 7000-

70,000 mg/kg over the duration of the studies. As the studies generally lasted 

up to 24 months, on a daily basis this is approximately equal to 9-90 mg/kg'd 

for the mice. 

108 
P.23 

In the absence of comparative toxicologic, physiologic, metabolic, and 

pharmacokinetic information, EPA's Guidelines for Carcinogen Risk Assessment 

(U.S. EPA, 1986a) call for making cross-species extrapolations based on a 

surface-area adjustment. For reasons that will be more fully discussed in 

Section 2.4, the pharmacokinetic information currently available indicates that 

a surface-area adjustment would probably be misleading, although the 

information is not adequate for validating any alternative adjustment. A 

substantial body of accumulated information indicates that a variety of 

factors, some of which may be unique to lead, are involved in lead-induced 

cancer. Of foremost concern are the potential differences in pharmacokinetics 

between animals and humans, so that a cross-species extrapolation from animal 

studies to estimate the cancer potency for humans cannot credibly be based on 

animal data alone, but should include cross-species pharmacokinetic modeling. 

Therefore, no cross-species extrapolation or quantitative cancer potency 

estimate is being made for lead at this time. 

Nevertheless, because of the consistent pattern of results from a large 

number of studies, in which it has been relatively high concentrations of lead 

If doses are equivalent when expressed on a surface-area basis, then 

2/3 2/3 

m ghuman/kghuman = mganimal/kganimal 

Algebraic manipulation yields 

1/3 

m ghuman/kghuman = mganimal/kganimal x (kganimal/kghuman) 

Assuming humans weigh 70 kg and rats weigh 0.350 kg, this is approximately 

m ghuman/k gh uman = mganimal/kganimal x (1/6) 

Assuming mice weigh 0.030 kg, this is approximately 

m ghuman/k gh uman = mganimal/kganimal x (1/13 > 

109 
P.24 

that have increased the incidence of kidney cancer in animals, it seems 

appropriate at this time to qualitatively characterize the cancer potency of 

lead as low, relative to other carcinogens. This characterization would place 

lead in potency Group 3, as defined earlier in the section on general 

methodology for adjusting CERCLA reportable quantities. 

This characterization reaffirms the position that EPA has previously 

taken. The Air Quality Criteria for Lead (U.S. EPA, 1986c) concluded 

. . . lead has been observed to increase tumorigenesis rates in 

animals only at relatively high concentrations, and therefore it does 

not appear to be a potent carcinogen. . . . J_n vitro studies further 

support the genotoxic and carcinogenic role of lead, but also 

indicate that lead is not potent in these systems either. 

Before proceeding with the discussion in the next section, some 

observations should be made about the apparent absence of kidney tumors at 

lower doses. This absence of tumors does not demonstrate a threshold for leadinduced cancer. As has been noted earlier, other tumors sometimes developed at 

total lead doses below 1 gram, and kidney tumors were repeatedly induced by 

injections of lead totalling less than 1 gram. Additional information can be 

developed from the Azar et al. (1973) study, which, with seven different lead 

concentrations, is the most comprehensive study of lead ingestion at lower 

doses. The study includes the highest total lead doses at which no tumors were 

reported. The linearized multistage model (U.S. EPA, 1980) can be used to 

estimate how many rats would be expected to develop kidney tumors -f the doseresponse relationship is truly linear at low doses. As the following table 

indicates, the observed results are virtually identical to the results expected 

with a linear model. None of the small differences are statistically 

significant (p > 0.10). This indicates that the animal studies did not, in 

general, have adequate power to detect an increase in kidney cancer at lower 

110 
doses. This analysis, therefore, demonstrates that the results of the animal 

studies are not inconsistent with a linear dose-response relationship, even at 

the lowest doses tested. It should be noted, however, that this inference does 

not rule out a threshold response, and that the data are also not inconsistent 

with such an inference. 

Comparison of the Azar et al. (1973) study and a linear model 

Lead acetate 

concentration 

(ppm in food) 

Male rats 5 

18 

62 

141 

548 

1130 

2102 

Number exoected = 

(1.14xl0"4 x Cone + 

Female rats 5 

18 

62 

141 

548 

1130 

2102 

Number expected = 

(2.23xl0"5 x Cone 

Number 

2.07x1 

Number 

+ 4.30 

Number 

of rats 

tested 

120 

50 

50 

50 

50 

20 

20 

tested x 

0""7 x Cone 

120 

50 

50 

50 

50 

20 

20 

Number 

observed 

with tumors 

0 

0 

0 

0 

5 

10 

16 

2 + 6.62x10" 

0 

0 

0 

0 

0 

0 

7 

tested x 

xlO"21 x Cone6 

Number 

expected 

from model 

0.1 

0.1 

0.4 

1.0 

6.8 

9.8 

N.C. 

1 1 x Cone3 . 

0.0 

0.0 

0.1 

0.2 

0.6 

0.7 

8.4 

2.4. DISCUSSION 

The calculations in the preceding section are somewhat simplistic. A 

variety of complicating factors, some of which may be unique to lead, are 

111 
involved in lead-induced cancer. Of foremost concern are the differences in 

pharmacokinetics between animals and humans, so that a cross-species 

extrapolation from animal studies to estimate the cancer potency for humans 

cannot credibly be based on animal data alone, but should include cross-species 

pharmacokinetic modeling. While such studies are now under way, they are not 

yet available for use. These and other considerations are discussed below. 

The injection studies show cancer at lower total doses than the 

ingestion studies. This suggests that less-than-complete absorption by 

ingestion, as well as by other routes of exposure, may mitigate the 

carcinogenic response to some extent. In addition, the injection and ingestion 

routes of exposure induce tumors at a common site. This suggests that lead is 

distributed to the same target sites, despite differences in the route of 

exposure. Although it appears that all routes of exposure can result in 

absorption of lead to some extent, the route of exposure affects the degree of 

absorption. The absorption of inhaled lead further depends on particle size 

and ventilation rate. Because the characterization of lead as being of low 

potency, relative to other carcinogens, is based on ingestion studies, the 

potency by other routes of exposure could be either higher or lower. 

Similarly, although it appears that all lead compounds can be absorbed 

to some extent, several studies have shown that the specific form of lead 

affects the degree of absorption somewhat. Because the characterization of 

lead as being of low potency, relative to other carcinogens, is based on 

studies using lead acetate or lead subacetate, the potency of other lead 

compounds could be either higher or lower, depending on the extent that the 

nonlead moiety of a lead compound affects its absorption. 

112 
P.27 

In certain environments, the absorption or metabolism of lead is 

facilitated by chemical conversion, which makes the lead more bioavailable. 

Under acidic conditions,- several lead compounds are converted to compounds such 

as lead chloride, which is more readily absorbed than the parent compound. 

Tetraalkyl lead compounds are metabolized to trialkyl lead, which in turn is 

metabolized to dialkyl lead, which is finally converted to inorganic lead in 

the intestine. The formation of inorganic lead has been observed in several 

animal species exposed to alkyl lead. 

The absorption of lead has also been shown to depend somewhat oh the 

dose. Absorption in rats 4 hours after gavage administration of lead is about 

5 percent over a wide range of doses from 1 ng to 10 ug, but falls off to 

1 percent at a dose of 1 mg. 

The preceding factors are not surprising and, in fact, are common to 

other families of compounds. There are, however, several additional and 

complicating factors that seem to be specific to lead. 

As has been mentioned already, the pharmacokinetics of lead differ both 

within and across animal species. Lead absorption, metabolism, distribution ,v 

patterns, and excretion through the renal and gastrointestinal tracts have all 

been shown to vary across animal species. For example, the structure and 

dimensions of the respiratory tract affect the deposition and absorption of 

inhaled lead. In cases where the range of cross-species differences in 

absorption, metabolism, distribution, or excretion has been quantified, the 

results indicate the importance of cross-species modeling. For example, 

biliary clearance of lead in rabbits is 50 percent of that in rats; in dogs, 

the relative clearance is only 2 percent of that in rats. In another example, 

the absorption of ingested lead typically ranges between 10 and 15 percent in 

113 
adult humans, and about 50 percent in children, but in rats may be no higher 

than 5 percent. This shows that without cross-species pharmacokinetic 

modeling, cancer potency estimates based on rat studies may underestimate human 

risks, perhaps substantially in the case of children. Although absorption, 

metabolism, distribution, and excretion also vary across routes of exposure and 

forms of lead, it is the differences across animal species that have the most 

profound implications for the extrapolation of animal studies to project human 

cancer risks. 

The relationship between lead and nutrition is rather complicated. 

Deficiencies in certain essential nutrients, such as low levels of calcium, 

phosphate, vitamin D, iron, copper, or zinc, can enhance lead absorption, 

influence distribution, or decrease excretion. Somewhat surprisingly, however, 

the Kasprzak et al. (1985) study shows that the addition of calcium to a diet 

containing lead can simultaneously increase the incidence of kidney tumors and 

decrease kidney lead concentrations in rats. Another example of the 

complicated relationship between lead and nutrition concerns fasting. The 

absorption of ingested lead typically ranges between 10 and 15 percent in adult 

humans, but may increase to between 21 and 63 percent when fasting. Some 

studies in rats also indicate that fasting enhances lead absorption. On the 

other hand, one study shows that in young animals absorption is maximal when 

growth is rapid, but absorption is significantly decreased after fasting for 

5 days. 

Age has other pronounced effects. Children absorb approximately 

50 percent of ingested lead, compared to between 10 and 15 percent for adults. 

The fraction of absorbed lead retained in the body ranges from between 1 and 

5 percent in adults to between 32 and 34 percent in young children. It is not 

114 
clear how lifetime-exposure studies can be used to make inferences about cancer 

risks arising from childhood exposure. 

It appears from the Azar et al. (1973) study and the Zawirska and 

Medras (1968) study that male Wistar rats are more sensitive to lead-induced 

kidney tumors than are female rats. In the Van Esch et al. (1962) study, 

however, male and female Wistar rats appear to be about equally sensitive. It 

is not known whether pharmacokinetic differences play a role in the apparent 

difference between the sexes. 

A few studies conducted at doses well below the maximum tolerated dose 

suggest that lead may also induce cerebral gliomas, testicular tumors, 

reticulum cell sarcomas, and various other tumors. The evidence for these 

other tumors, however, is much weaker than that for kidney tumors. Although 

these effects lack the robustness of the kidney response, these studies 

nevertheless suggest that lead may act to induce cancer at a variety of sites, 

at lower exposure levels than those that clearly cause kidney tumors. This has 

implications for low-dose extrapolation, as it raises the possibility of more 

than one cancer response. Measurement of tissue concentrations and some 

understanding of cellular mechanisms would aid in the proper interpretation of 

these possible responses. 

From the above discussion it is clear that many factors influence leadinduced cancer. The relatively profound knowledge of lead chemistry indicates 

the difficulties in selecting an appropriate measure of dose and scaling 

equivalent doses across animal species. Although pharmacokinetic models have 

been developed to help explain some of the adverse health effects observed in 

humans, comparative pharmacokinetic models across species are needed for the 

extrapolation from high-dose animal studies to project human cancer risks at 

115 
environmental exposures. While such studies are now under way, they are not 

yet available for use. In the meantime, any model or cancer potency estimate 

that does not take factors such as nutrition and age into consideration would 

be highly unrepresentative for large segments of the population. For these 

reasons, a specific cancer potency estimate for lead has not been calculated at 

this time. 

It is, admittedly, a fine distinction to say that the information 

currently available is adequate to qualitatively characterize the cancer 

potency but not to quantitatively estimate it. To understand this distinction, 

it is useful to recall that the qualitative characterization is based on the 

dose associated with an increased cancer incidence of 10 percent, and does not 

necessarily reflect pharmacokinetic properties or cellular mechanisms that may 

operate at lower doses. A credible quantitative cancer potency estimate, 

however, should be based on cross-species pharmacokinetic modeling. 

Nevertheless, the distinction is somewhat ambiguous, and pharmacokinetic 

properties can influence the qualitative characterization as well. Leadinduced cancer can reasonably be expected to depend on the form of lead, route 

of exposure, animal species, and target site. Because there may be a form of 

lead more potent than lead acetate, a route of exposure more potent than 

ingestion, an animal species more sensitive than the rat, or a target site more 

sensitive than the kidney, the currently available information cannot rule out 

that the potency of lead may be higher in some exposure circumstances or for 

some sensitive groups in the population. 

Although the Agency believes that adequate information is not currently 

available for a credible quantitative assessment of the cancer potency of lead, 

there is hope that such estimates may be made in the future. The Agency is 

116 
P.31 

aware of ongoing research to develop pharmacokinetic models for rats. Such 

efforts are important and are to be encouraged, because they will make a 

significant advance in a process that will eventually make possible crossspecies pharmacokinetic models for lead. 

117 
3. HAZARD RANKING 

3.1. GENERAL METHODOLOGY 

The hazard ranking is a relative measure of overall concern about a 

potential carcinogen. It combines two factors—weight of evidence and 

potency—that the Agency considers important in characterizing a potential 

carcinogen. Weight of evidence refers to the strenqth of the evidence that the 

substance causes cancer. Substances are classified into five weight-ofevidence groups: 

Group A. Human carcinogen 

Group B. Probable human carcinogen 

Group C. Possible human carcinogen 

Group D. Not classifiable 

Group E. Evidence of noncarcinogenicity 

Potency refers to the strenqth of the substance to cause cancer. Potential 

carcinogens are classified into three potency groups: 

Group 1. High potency (relative to other potential carcinogens) 

Group 2. Medium potency 

Group 3. Low potency 

Combining the weight of evidence and the potency, potential carcinogens 

receive a hazard ranking of either high, medium, or low, relative to other 

carcinogens. These adjectives should be interpreted not as an absolute measure 

of concern, but as a ranking relative to other carcinogens. That is, a 

carcinogen with a low hazard ranking may, in fact, be a serious problem; it is 

considered low only when compared with other carcinogens. The hazard ranking 

increases as either the weight of evidence or the potency increases, according 

to the following table: 

118 
P.33 

Weight-ofEvidence 

Group 

A 

B 

C 

D 

E 

Potency Group 

1 2 3 

High 

High 

Medium 

High 

Medium 

Low 

Medium 

Low 

Low 

No hazard ranking 

No hazard ranking 

3.2. OVERALL CHARACTERIZATION 

Based on their classification into weight-of-evidence Group B2 and potency 

Group 3, lead and lead compounds receive a low hazard ranking among potential 

carcinogens. 

119 
P.34 

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