&EPA
United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington DC 20460
EPA/600/8-89/045A
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(Do Not
Cite or Quote)
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 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.
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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DO NOT QUOTE OR CITE March 1989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.
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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.
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CONTENTS
Tables v
Preface vi
Abstract ix
Authors and Reviewers x
1. WEIGHT OF EVIDENCE l
1.1. HUMAN STUDIES l
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 73
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
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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 24
1-2 Summary of nonpositive animal studies 32
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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 ;.02. 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
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appearing in one of these sources up through 1988 have been included. The
following data bases were used:
CHEMLINE National Library of Medicine
TOXLIT National Library of Medicine
TOXLINE National Library of Medicine
Subfiles:
EMIC Oak Ridge National Laboratory
ETIC Oak Ridge National Laboratory
EPIDEM Oak Ridge National Laboratory
PESTAB U.S. Environmental Protection Agency (Pesticide
Abstracts)
HMTC Department of the Army
TOXNET National Library of Medicine
Subfiles:
CCRIS National Cancer Institute
HSDB National Library of Medicine
RTECS National Institute of Occupational Safety and
Health
Chemical
Abstracts (CA) Chemical Abstracts Service
BIOSIS Biological Abstracts, Inc.
International
Pharmaceutical
Abstracts (IPA) American Society of Hospital Pharmacists
CIS Abstracts International Labour Office, International
Occupational Safety and Health Information
Center
CRISP National Institutes of Health
NTIS National Technical Information Service
TSCATS 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.
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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 develops 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, Section 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.
VI 1 1
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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 informa'ion 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 th'is 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.
Tables.
William E. Pepelko, Ph.D.
David H. Reese, Ph.D.
Dharm V. Singh, D.V.M., Ph.D.
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.
Section 1.5.
William E. Pepelko, Ph.D.
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
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1. WEIGHT OF EVIDENCE
1.1. HUMAN STUDIES
1.1.1. Epidemloloqic 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); and 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. DinqwaTl-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
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of 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
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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 case-
control design, but the results are discussed in terms of observed and
expected deaths with no elaboration of how the expected deaths were
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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 Gaffev. 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 neoplasms 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
the 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
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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 wa<; cross-
checked 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
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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.
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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 1950 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
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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 4393 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
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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. SHRs 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
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1.1.1.4. Studies in Tetraethvl 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 TEL-
area 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-sex-
age 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
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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 monomer:;, 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
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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
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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. Mil kins and Sinks, 19841. 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
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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
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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 v
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
leul 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 the1potential 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
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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 thesi 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
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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 was 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 a!.,
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
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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 ,c
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 cohorts of thr 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
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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
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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 tetracthyl 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 le'ad 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. Bovland 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
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exposure period, the animals were killed when moribund or when palpable tumors
were detected. t .
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
-------
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 1'ver 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
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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 remaining rati, 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
-------
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 Ha, 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
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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 a!., 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.
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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.
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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 peer-
reviewed 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
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in proximal tubule epithelial cells, appearing as early as week 4 in lead-
exposed 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. Nogueira, 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. Kam'sawa 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
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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
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ethylnitrosourea precursors. The limitations of the study include small group
size and use of only one sex.
1.2.1.2. Studies in Hice--
1.2.1.2.1. Blaklev, 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
were 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 arid 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.
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1.2.1.3. Studies in Doas--
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 was 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 rmall, 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 Monkeys--
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.
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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., ?962. 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
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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 djse-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
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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 52 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
['
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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
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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. Kasorzak et al.. 1985. Male Sprague-Dawley rats (30 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
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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.
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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 0.1 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
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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 diet 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 two:
males. 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
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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.
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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. Mass 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,
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or a combination of 1 mg of each suspended in 0.2 ml of 0.5 percent
carfaoxymethylcellulose-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
aderocarcinoma 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
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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 none 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. Zollinqer, 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 rug 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 autppsied,
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
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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
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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 Naohthenate (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 for 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
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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 high-
and 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.
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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
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suspended in 0.5 percent gelatin. These animals were also killed vifhen 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 1!?. 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 trtal number of
controls for test compounds injected subcutaneous!y 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 (NCIK 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
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lead dimethyldithiocarbamate at doses ranging up to 500 ppm (the lower dose
range was not reported). Toxic.ity 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 anirals
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
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definitively evaluating the potential carcinogenicity of lead
dimethyldithiocarbamate.
1.2.8. Tetraethvl 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 very 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
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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 historically 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 high-
dose 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
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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 a!.,
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 a!., 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 delta-
ami no! evulinic 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
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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 (Mass et a'i., 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
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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
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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 very 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
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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 Mass 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 Mass 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
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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., 1980; 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
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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 Nielsen's technique. Ultrastructurally, the bodies have a dense
central core and outer fibrillary region. The bodies are composed of non-
histone 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, i971, 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
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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 membrane 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
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tightly regulated by the availability of both calcium and diacylglycerol.
Tumor promoters 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 promoters 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 promoters (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 promoter or facilitates promotion. It is possible, for example,
that the high affinity of PKC for lead (compared to that of calcium) causes the
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requirement for endogenous diacylglycerol and/or environmental promoters 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
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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 tumorigenie 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
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.
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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.
Ir. 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.
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1.3. SHORT-TERM TESTS
1.3.1. Mutagenicitv
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-
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.
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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
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this document include the following: subcellular effects, effects on heme
biosynthesis and erythropoiesis/erythrocyte physiology, neurotoxicity,
nephrotoxicity, reproductive and developmental toxicity, immunbtoxicity, 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
i
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
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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)
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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 (Boy!and et a!., 1962; API, 1971; Tanner and Lipsky, 1984;
Nogueira, 1987), lead subacetate (Mass 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),
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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 3H-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 after *
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 3H-thymidine into DNA) occurred 36 hours after administration of lead and
the peak in the number of cells entering mitosis occurred 48 hours after
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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 a!., 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 Board 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
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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 forms 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 It-ad particles 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 cor-pounds; 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).
Host 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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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. Pistrjbution 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 (CaNa2*ELTA), 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).
Placenta! 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 alky! 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 alky! lead, although different proportions of
lead chemical forms are seen across the animal species following alky! 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
61 [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
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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.
i
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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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 oistal 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
tall ate, 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 alky! 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 studies 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 thoso that clearly cause kidney
tumors. The induction of cancer at other sites is consistent with the
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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 nutations 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
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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 millionfold 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. lARC'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 physico-
chemical 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 lARC'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, I'h.D., New York University Medical Center; and Robert P. Bellies,
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 weight-
of-eyidence Group B2 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.
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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 dose-
.response 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,
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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 exposu.-e to high levels of lead has also
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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
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(although various other tumors sometimes developed at total lead doses below
1 g, and kidney tumors were repeatedly induced by in.lections 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 dos;es).
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.
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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.2 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
Algebraic manipulation yields
m9human/k9human = ^animal/^animal x
Assuming humans weigh 70 kg and rats weigh 0.350 kg, this is approximately
m9human/k9human = m9animal/k9animal x W6)
Assuming mice weigh 0.030 kg, this is approximately
m9human/k9human = m9animal/kganimal x (1/13)
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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. ... In 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 lead-
induced cancer. As has been aojted 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 if the dose-
response 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
I
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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
Male
Lead acetate
concentration
(ppm in food)
rats 5
18
62
141
548
1130
2102
Number exoected = Number t
(1.14xlO~4 x Cone + 2.07x10"
Female
rats 5
18
62
141
548
1130
2102
Number
of rats
tested
120
50
50
50
50
20
20
qsted x
x Cone
120
50
50
50
50
20
20
Number
observed
with tumors
0
0
0
0
5
10
16
2 + 6.62xlO~
0
0
0
0
0
0
7
Number
expected
from model
0.1
0.1
0.4
1.0
6.8
9.8
N.C..
11 x Cone3 ,
0.0
0.0
0.1
0.2
0.6
0.7
8.4
Number expected = Number tested x
(2.23X10"13 x Cone + 4.30xlO"ZI x Cone6
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
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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.
t
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.
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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 on 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, distributions
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 whero 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
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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 a,so 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
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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 lead-
induced 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
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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. Lead-
induced 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
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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 cross-
species pharmacokinetic models for lead.
117
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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 strength of the evidence that the
substance causes cancer. Substances are classified into five weight-of-
evidence 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 strength 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
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Potency Group
1 2
Weight-of-
Evidence
Group
A
B
C
D
E
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
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