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Review Draft
EPA-600/8-83-025A
June 1983
Draft
Do Not Quote or Cite
UPDATED MUTAGENICITY AND CARCINOGENICITY ASSESSMENT OF
CADMIUM
Addendum to the Health Assessment Document for Cadmium
(May 1981) EPA-600/8-81-023
NOTICE
This document is a preliminary draft. It has not been formally released by the
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.
Office of Health and Environmental Assessment
Office of Research and Development
U.S. Environmental Protection Agency
Washington, D.C. 20460
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DISCLAIMER
This report is intended 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.
The Health Assessment Document for Cadmium (May 1981;
EPA 600/8-81-023) is available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
Order No.: PB-82-115163
Cost: $28.00 (subject to change)
11
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CONTENTS
Authors and Reviewers iv
Summary and Conclusions 1
Summaryo ...........•••••1
Conclusions 3
Introduction 5
Mutagenicity . 6
Gene Mutations in Prokaryotes 6
Gene Mutations in Yeast 13
Gene Mutations in Mammalian Cell Cultures 15
Studies in Drosophila and Other Insects .16
Chromosomal Aberrations in Humans and Other Mammalian Systems . . .21
Chromosomal Aberrations in Plants 42
Other Indirect Evidence 42
Summary. 44
Carcinogenicity 46
Animal Studies 46
Epidemiologic Studies 62
Ouantitative Estimation 100
Introduction 100
Procedures for Determining the Carcinogenic Potency .... 104
Cadmium Risk Estimates. 113
Relative Potency 123
Appendix A - Comparison of Results by Various Extrapolation Models . 129
Appendix B - International Agency for Research on Cancer (IARC)
Classification for Weight-of-Evidence for Carcinogenicity
of a Suspected Carcinogen 133
References. 136
in
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AUTHORS AND REVIEWERS
The Carcinogen Assessment Group, Office of Health and Environmental
Assessment, was responsible for preparing this document. Participating members
are as follows (principal authors are designated by asterisks):
Roy E. Albert, M.n. (Chairman)
Elizabeth L. Anderson, Ph.D.
*Larry D. Anderson, Ph.D.
*Steven Bayard, Ph.D.
*David L. Bayliss, M.S.
Chao W. Chen, Ph.D.
Maragaret M. L. Chu, Ph.D.
Herman J. Gibb, B.S., M.P.H.
Bernard H. Haberman, D.V.M., M.S.
Charalingayya B. Hiremath, Ph.D.
Robert McGaughy, Ph.D.
Dharm V. Singh, D.V.M., Ph.D.
*Nancy A. Tanchel, B.A.
*Todd W. Thorslund, Sc.D.
The Reproductive Effects Assessment Group, Office of Health and
Environmental Assessment, was responsible for preparing the section on
mutagenicity. Participating members are as follows (principal authors are
designated by asterisks):
John R. Fowle III, Ph.D.
Ernest Jackson, Ph.D.
*K.S. Lavappa, Ph.D.
Sheila Rosenthal, Ph.D.
Carol Sakai, Ph.D.
Vicki Vaughan-Dellarco, Ph.D.
Peter E. Voytek, Ph.D.
i v
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SUMMARY AND CONCLUSIONS
SUMMARY
Qualitative Assessment
As was noted in the Office of Health and Environmental Assessment Health
Assessment Document for Cadmium (May 1981), cadmium and various cadmium salts
have produced injection site sarcomas in rats and interstitial cell testicular
tumors in mice and rats after subcutaneous injection. Specifically,
injections site sarcomas were produced by cadmium powder, cadmium sulfide,
cadmium oxide, cadmium sulfate, and cadmium chloride, and interstitial cell
tumors were produced by cadmium chloride, cadmium sulfate, and ferritin
containing cadmium. However, three drinking water, two gavage, and two
dietary studies using cadmium acetate, cadmium sulfate, or cadmium chloride
have shown no excessive risk of cadmium carcinogenicity in rats and mice. It
was also noted that human exposure to high levels of cadmium dust and/or fumes
in smelter and battery plants produced a slight, but statistically
significant, increase in prostate cancer in several different small-scale
epidemiologic studies.
After the above document was prepared, a lifetime rat inhalation study was
published showing a dose-related induction of lung carcinomas by cadmium
chloride aerosol. The carcinogenic potency of cadmium by chronic inhalation
in the rat appears to be at least two orders of magnitude greater than by
chronic oral administration. In another recent study, lifetime observation of
rats given cadmium oxide by intratracheal instillation did not reveal a
carcinogenic effect; however, only three or fewer treatments were given during
the study, which involved a relatively low dose.
Seven epidemiologic studies reviewed after the May 1981 OHEA document have
not appreciably changed the earlier limited evidence of prostate cancer in
1
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humans. In addition to cancer of the prostate, the risk of lung cancer was
found to be significantly correlated with cadmium exposure in two studies
(Lemen et al. 1976, Holden 1980). The author of one attributes the excess
risk to the presence of arsenic (Holden 1980), although arsenic was present in
both studies. Because of the presence of arsenic along with-cadmium in these
studies, the evidence that cadmium is a causitive agent of lung cancer is
inadequate.
A clear statement on the mutagenicity of cadmium is rather difficult
because of the conflicting results and lack of adequate test protocols used.
However, gene mutation studies in mammalian cell cultures, rec-assays in
bacteria, chromosomal nondisjunction studies in intact mammals, and other
indirect evidence reported in the text suggest that cadmium is weakly
mutagenic.
Quantitative Assessment
Since humans are exposed to cadmium dust or fumes and rats were exposed to
cadmium chloride aerosol, a limitation inherent in the use of the rat study in
estimating human risk is the possible difference between humans and rats in
the lung retention of particulates or between the biological effectiveness of
cadmium chloride aerosol administered to rats and the dust and fumes inhaled
by workers. Since data are not clear on this point, assumptions of equal lung
uptake and equal effectiveness were made in order to get an idea of the human
risk.
Given these assumptions, combined with other assumptions and conventions
used in quantitative risk assessment procedures, the Takenaka et al. (1982)
data on lung carcinomas in rats during lifetime inhalation exposures to
cadmium chloride aerosol were analyzed. The result of the analysis is that
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the upper-bound cancer risk to humans who continuously breathe 1 ug/m3 of
elemental cadmium for a lifetime is 0.15.
Based on respiratory and prostate cancer rates from the Lemen et al.
(1976) study of cadmium smelter workers, the upper-bound cancer risk from
lifetime exposure to 1 ug/m3 of cadmium in the air has a range of
1.9 x 10" 3 to 2.5 x 10~3. These estimates are about 100 times less than
the estimate of lung cancer risks from the rat experiment. Because only
fragmentary information is available concerning cadmium exposures of the
workers, the extreme range of risk, based on the highest and lowest possible
assumptions about several exposure parameters, is a factor of 100 higher and
lower than these estimates. Further detailed analysis and laboratory studies
are needed before the large difference between the estimates based on animal
and human data are resolved.
CONCLUSIONS
Applying the International Agency for Research on Cancer (IARC) approach
(Appendix B) for classifying the weight-of-evidence for carcinogenicity in
experimental animals, the injection site and testicular tumors in mice and
rats given cadmium metal or cadmium salts and the lung carcinomas in rats
exposed to cadmium chloride aerosol by inhalation provide sufficient evidence
for the carcinogenicity of cadmium and certain cadmium compounds in
experimental animals. Although there is a possibility that orally-ingested
cadmium is carcinogenic in rats based on the finding of testicular tumors
induced by injection at a remote site, no response has been observed with
ingested cadmium, and the potency via the oral route is at least 200 times
less than via inhalation.
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The available human epidemioiogic data provide limited evidence, according
to the IARC criteria, that cadmium and certain cadmium compounds are
carcinogenic in humans.
The overall evidence for carcinogenicity, applying the IARC criteriai
places cadmium and certain cadmium compounds in the 2A category, meaning that
they are probably carcinogenic in humans.
The upper-bound unit risk estimate for continuous inhalation exposure at a
cadmium concentration of 1 ug/m3 ranges from 1.9 x 10~3 to 2.5 x 10~3
based on lung clnd prostate cancer, respectively, from one smelter worker
study, although there is considerable uncertainty in these estimates due to
uncertain exposure of the workers. Nevertheless, these estimates are regarded
as more realistic than the estimate based oh the rat inhalation study, which
is about 100 times higher.
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INTRODUCTION
This document is a review and assessment of the current information
relating to the mutagenicity and carcinogenicity of cadmium. It contains a
detailed discussion of information on those subjects which became available
since the earlier Health Assessment Document for Cadmium was prepared by the
Office of Health and Environmental Assessment (OHEA) in May 1981. It includes
all pertinent material from the 1981 document but does not attempt to repeat
details of the animal carcinogenicity studies discussed there.
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MUTAGENICITY
Cadmium has been investigated for its mutagenic potential in both
prokaryotic and eukaryotic systems. The prokaryotic systems include assays
for gene mutation and reparable genetic damage in bacteria. The eukaryotic
systems include gene mutation studies in yeast, Drosophila, and mammalian
cells and chromosomal aberration studies in human and other mammalian cells
exposed to cadmium both in vitro and in vivo. The following is an analysis of
the literature pertaining to the mutagenic effects of cadmium.
GENE MUTATIONS IN PROKARYOTES
Gene mutation studies in prokaryotic systems have been summarized in
Table 1. A discussion of each study follows.
Salmonella Assay
Meddle and Bruce (1977) and Bruce and Meddle (1979) tested the mutagenic
effects of cadmium chloride in the histidine reverse mutation assay using
Salmonella typhimurium tester strains TA100, TA98, and TA1537. The test
compound (purchased from ICN Pharmaceuticals, Plainview, New York) was
dissolved in water and used at concentrations of 0.05, 0.5, 5, 50, and 500
ug/plate with and without the application of a metabolic activation system (S9
mix) derived from phenobarbital-induced rat liver homogenate. According to
these authors cadmium chloride did not induce a significant mutagenic response
over the control value. The criterion set for a positive response was 50% or
1.5-fold increase in the revertant frequency over the negative control or
spontaneous frequency. Revertant counts were given only for strain TA100; the
spontaneous frequency of revertants in this strain was 140 colonies per plate.
The purity of cadmium chloride was not given in this report.
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TARLE 1. MUTAGENICITY EVALUATION OF CADMIUM: RENE MUTATIONS IN PROKARYOTES
Test System
Salmonella
typhimurium
Salmonella
typhimuriuni
Salmonella
typhimurium
Strain
TA98
TA100
TA1535
TA1537
TA1538
TA98
TA1535
TA1537
TA1535
TA1537
Cadmium
Compound
Cadmium
chloride
aqueous
solution
Cadmium
red in OMSO
Cadmium
chloride
(solvent
not specified)
nose
0.05
0.5
5.0
50.0
500 ug/plate
1 ug/ml
10, 20, 30,
45, 90 mM
S9 Activation
System
Phenobarbital-
induced rat
liver
Aroclor 1254-
induced mouse
1 iver
Uninduced mouse
liver
Results
Reported as
negative
Reported as
negative
Reported as
negative
Comments
1. Data are not presented
clearly as revertants/
plate for each strain.
2. Purity of compound not
discussed.
1. Data provided only for
the preincubation or
suspension assay. No data
on the spot test given.
2. Only a single dose was
employed; no dose-response data.
1. Spontaneous reversion data
and the experimental
reversion data have not
been given in terms of numbers.
2. Used uninduced mouse liver
S9 activation system.
3. No positive controls.
Reference
Bruce and Meddle
(1979)
Mi Ivy and Kay
(1978)
Polukhina et al .
(1977)
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TABLE 1. (continued)
oo
Test System
Salmonella
typhimurium
Bacillus
subtilis
Rec-assay
Strain
TA98
TA100
TA1535
TA1537
TA1538
H17
Rec+
M45
Rec-
Cadmium S9 Activation
Compound nose System
Cadmium 1 Aroclor-
diethyl- 5 induced rat
thiocar- 10 liver
hamate in 50
OMSO 100 ug/plate
Cadmium 0.05 M/plate None
chloride
aqueous
solution
Cadmium
nitrate
aqueous
solution
Results Comments Reference
Reported 1. Lowest effective dose was Hedenstedt et al.
positive for 10 ug/plate. (1979)
TA1538 and 2. Reported positive only for
TA98 in the one dose.
absence of 3. No dose-response rela.tonship.
S9 activation.
Reported weakly
positive -both
in the presence
and absence of
S9 activation.
Reported as Nishioka
weakly (1975)
(+) positive
Reported as
negative
(continued on the following page)
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TABLE 1. (continued)
Test System
Bacillus
subtil is
Rec-assay
Strain
H17
Rec+
M45
Rec"
Cadmi urn
Compound
Aqueous
solutions
of cadmium
chloride,
nitrite, and
sulfite .
nose
n.nns
M/plate
S9 Activation
System
None
Results Comments
Reported as 1. Compounds were pure.
weakly (+)
positive
Reference
Kanematsu et al .
(1980)
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In an abstract published by Kalinina and Polukhina (1977), cadmium
chloride was reported to be nonmutagenic in the Salmonella assay. However,
important variables such as the number of strains used, the dosage employed,
and the number of revertants per plate were not reported. Polukhina et al.
(1977) also reported negative results with cadmium chloride on Salmonella
typhimurium strains TA1535 and TA1537 both in the presence and absence of an
S9 activation system derived from uninduced mouse liver homogenate. In this
report a suspension assay with cadmium chloride concentrations of 10, 20, 30,
45, and 90 mM was employed. Positive and negative control data were not
presented in this paper, so it is not possible to know whether or not the
assay system was functioning properly. Toxicity of the test compound was not
reported by these investigators.
Milvy and Kay (1978) studied the mutagenic effects of cadmium red (cadmium
sulfide and selenium), a dye used in the printing industry, using the
Salmonella spot test (Ames et al. 1973) and the preincubation assay (Ames et
al. 1975)i Salmonella typhimurium strains TA1538, TA98, and TA1535 were
employed in these studies. The test compound (10 ug) was dissolved in 0.01 ml
dimethyl sulfoxide (DMSO) and added to 0.9 ml of incubation mixture for 30
minutes at 37°C with shaking before plating 0.1 ml onto minimal plates.
Experiments were carried out both in the presence and absence of an S9
activation system derived from Aroclor 1254-induced mouse liver homogenate.
Cadmium red was reported to be nonmutagenic in both tests. However, data are
presented only for the suspension assay. These investigators used only one
concentration, and hence, no dose-response relationship was demonstrated. The
toxicity of the compound for each strain was not reported. Consequently, this
study may be regarded as inconclusive.
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Hedenstedt et al. (1979) studied the mutagenic effects of cadmium
diethyldithiocarbamate (used in rubber and plastic industries) in Salmonella
typhiumurium strains TA1535, TA1537, TA1538, TA98, and TA100. The
concentrations used were 1, 5, 10, 50, and 100 ug/plate. The compound was
dissolved in DMSO. Concentrations of 50 and 100 ug/plate were toxic in many
of these strains. The concentration of 10 ug/plate exhibited mutagenic
activity in strains TA1538 and TA98 in the absence of a metabolic activation
(S9) system obtained from Aroclor 1254-induced rat liver homogenate (Ames et
al. 1975). In TA 1538 the revertant frequency increased more than twofold at
10 ug/plate, i.e., 26.3 _+ 3.7 revertants/plate compared to a control value of
11.8 +_ 2.6 revertants/plate in the absence of metabolic activation. In the
presence of metabolic activation the revertant frequencies in treated and
controls were the same. In TA98, the revertant frequency was 58.8 _+ 2.3 at 10
ug/plate (almost a twofold increase) compared to the control frequency of 31.5
_+ 4.2 revertants/plate in the absence of metabolic activation. No data were
given for studies in the presence of metabolic activation. Positive control
data were not presented even though the authors indicate that positive
controls were employed in the experiment. These results may be regarded as an
inconclusive because the twofold increase was observed for only one isolated
data point, and there was no evidence of dose-response.
Escherichia coli WP2 Assay
Venitt and Levy (1974), in a report on the mutagenicity of chromates in
the Escherichia coli WP2 forward mutation system, mentioned that they also
tested cadmium compounds for mutagenicity and found them to be negative.
These authors do not mention what types of cadmium compounds they employed or
present data to support their negative conclusions.
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Bacillus subtil is Rec-Assay
Nishioka (1975) investigated the mutagenicity of cadmium chloride and
cadmium nitrate using the rec-assay of Kada et al. (1972). In the rec-assay,
which measures reparable ONA damage, differences in growth sensitivities of
Bacillus subtil is strains H17 (recombination-competent wild type rec+) and
M45 (recombination-deficient rec") to mutagenic chemicals are measured.
When a chemical is more inhibitory to rec~ than to rec+ cells, then it is
suspected of being mutagenic. Concentrations of 2.5 x 10~7 cells/0.1 ml
were streaked on agar plates from the center of the plate in different
directions. Aqueous solutions of cadmium chloride and cadmium nitrate
solutions (0.05 M) were applied in 0.05 ml aliquots to filter paper disks
(diameter of 10 mm) and placed in the center of the agar plate, i.e., the
starting point of the streaks of rec+ and rec~ cells. All plates were
incubated at 37°C for 24 hours. The inhibition of growth was indicated by the
distance (mm) between the paper disk and growth of the bacterial streaks.
This magnitude of inhibition is called "rec-effect" and is expressed as:
no difference between rec+ and rec~ plates (-), less than 5 mm difference
(+), 5-10 mm difference (++), and more than 10 mm difference (+++). Cadmium
nitrate showed no difference in growth inhibition (-), whereas cadmium
chloride exhibited a weak positive response (+). No positive or negative
controls were employed in this experiment. Each experiment was repeated three
times. These experiments did not use a metabolic activation system. The
cadmium compounds used were of reagent grade but their precise purity was not
reported.
Similar results were obtained by Kanematsu et al. (1980) in the rec-assay
of Kada et al. (1972). Cadmium chloride, cadmium nitrite, and cadmium sulfate
were employed at a concentration of 0.005 M in 0.05 ml aqueous solution. All
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these compounds exhibited a weak mutagenic response (+) (of 4-5 mm growth
inhibition zone). According to these authors, test compounds used were of the
highest purity commercially available in Japan.
GENE MUTATIONS IN YEAST
Cadmium chloride has been investigated for the induction of gene mutations
in the yeast Saccharomyces cerevisiae (Table 2) (Takahashi 1972; Putrament et
al. 1977). Takahashi (1972) studied the induction of petite mutations
(p-mutation) and auxotrophs in the Saccharomyces cerevisiae heterozygous
diploid strain C3116. He treated 104 cells with 10 (5.5 x 1Q-5M), 12 (6.6
x 10'5M), and 20 ppm (1.1 x 10~4M) for 2 days (48 hours) at 25°C. After 2
days of growth, the cell number was determined and the cell suspension was
diluted to give a concentration of 2.8 x 103 cells per ml. One-tenth of the
diluted suspension was spread on the YEPD-agar plate and incubated at 28°C.
When small colonies appeared on the plate, they were replica plated onto
YEP-glycerol-agar medium and minimal medium. After overnight incubation at
28°C, induced p-mutants and auxotrophs were scored. At the dose of 12 ppm
(1.1 x 10'4M), no p-mutants or auxotrophs were found in the 786 colonies
counted; at the dose of 10 ppm, 10 p-mutants and three auxotrophs were
detected in the 871 colonies counted; and at the dose of 20 ppm, there were 12
p-mutants and 9 auxotrophs in 1,182 colonies indicating that cadmium chloride
may be mutagenic. In the controls there were five p-mutants and two
auxotrophs in 2,875 colonies counted. According to this paper, however,
mutants were induced at the dosage of 10 ppm and 20 ppm but not at the dosage
of 12 ppm. Such erratic fluctuations in mutation frequency and the low number
of mutants suggest that the positive values may similarly be questionable.
Since p-mutants occur by damage involving the mitochondrial DNA rather than
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TABLE 2. MUTAGENICITY EVALUATION OF CADMIUM: GENE MUTATIONS IN YEAST AND MAMMALIAN CELL CULTURES
Test System
Saccharomyces
cerevisiae
(Yeast)
P-mutants and
auxotrophs
Saccharomyces
cerevisiae
P-mutants
Mouse lymphoma
Chinese hamster
cells
Chinese hamster
cells
Cadmium
Strain Compound
C3116 Cadmium
chloride
197/2d Cadmium
chloride
L5178Y TK+/~ Cadmium
chloride
Lung (Don) cells; Cadmium
resistance to 8- acetate
azoguanine Cadmium
chloride
Ovary cells Cadmium
(CHO) chloride
Activation
Dose System
10 None
12
20 ppm
8 ppm None
0.05 None
0.06
0.08
0.11
0.15 ug/ml
2.5 None
5
10 ug/ml
2.5 None
5
5
10 ug/ml
Results
Reported as
positive
Reported as
negative
Reported as
weakly
positive
Reported as
positive
Reported as
weakly
positive
Comments Reference
1. P-mutants may not represent Takahashi (1972)
true gene mutations because
they arise by damage in
mitochondrial DNA.
2. Vague protocol .
1. Only one concentration of Putrament et al .
test compound was used. (1977)
2. This concentrations was too
toxic for the cells.
3. No mutants observed in those
few suvivors.
1. Application of t-test Amacher and
to determine the Paillet (1980)
significance has been
challenged by Clive et
al. (1981).
1. Very low survival due Casto (1976)
to high toxicity.
2. Observations not repeated
and confirmed.
1. Data not presented. Hsie et al . (1978)
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nuclear DNA, caution should be exercised in assessing the mutagenic potential
of chemicals with this system.
Putrament et al. (1977) also reported a negative result in a test for
induction of p-mutation by cadmium chloride in Saccharomyces cerevisiae. The
concentration of cadmium chloride tested (8 mM) was very toxic, however, and
less than 1% of the cells survived a 6-hour incubation in YEP-glucose medium.
No increase of p-mutants was observed and no data were presented. This study
is regarded as inconclusive.
GENE MUTATIONS IN MAMMALIAN CELL CULTURES
Gene mutation studies in cultured mammalian cells have also been
summarized in Table 2. A discussion of each study follows.
Mouse Lymphoma Assay
Amacher and Pail let (1980) reported that cadmium choloride (ICN
Phamaceuticals) was mutagenic in the mouse lymphoma L5178Y TK+/~ assay.
When cadmium chloride, dissolved in normal saline, was tested at
concentrations of 2.35 x 10~7M (cell survival 100 +_ 11%), 3.57 x 10~7M
(cell survival 78^24%), 4.5 x 10-7*1 (cell survival 0.62 ±4%), 6.00 x
10-7M (cell survival 38 _+!!%), and 8.00 x 10~7M (cell survival 12^1%),
there was a dose-related increase in the mutation frequency. The mutation
frequencies per 104 survivors for the above doses were 0.48 +_ 0.01, 0.58 _+
0.06, 0.56 +_ 0.05, 0.63 _+ 0.16, and 0.68 _+ 0.04, respectively. The mutation
frequency at the highest nontoxic dosage of 6.00 x 10"7M was approximately
1.5-fold higher than the control frequency of 0.40 _+ 0.03 (survival 100% _+ 5).
The dose-response curve obtained by Amacher and Pail let has been criticized by
Clive et al. (1981), who claim that the application of a t-test for low
15
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numbers of samples to determine significance is misleading.
Chinese Hamster Cell Assay
Casto (1976), in a report submitted to Dr. Richard Troast o'f the Office of
Pesticide Program's, U.S. Environmental Proctetid'ii Agency, stated that cadiriiLim
acetate and cadmium chloride are mutagenic in Chinese Hamster-liihg cells (Don)
as determined by induction of mutations that confer resistance to
8-azdgUanihe. Cells were treated with 2.5 (1.36 x 10~8M), 5 (2.72 x
10~^M)i and 10 ug/ml (5.45 x 10"8M) of cadmium acetate and cadniium
chloride, respectively, for 18 hours followed by 48 hours expression time.
Cadmium acetate induced mutation frequencies of 2.8, 50, and 10 per 10^
survivors, respectively, for above dosages. The survival rate was 0.70, 0.92,
arid 0.43 percent, respectively. Cadmium chloride induced mutation frequencies
of 6, 7, 14, arid 37 per 106 survivors. The negative control rate was 2 per
10^ survivors. According to this investigation both cadmium acetate arid
cadmium chloride are weakly mutagenic, but the results are questionable'
because of low survival rates at these high concentrations. Hsie et al.
(1978) also reported cadmium chloride to be weakly mutagenic at the HGP'RT
locus in the Chinese hamster ovary cells, but no data were presented.
Therefore, although three independent studies have reported cadmium salts to
be weakly mutagenic in cultured mammalian cells, however, in none of the
studies was the demonstration of mutagenicity unequivocal.
STUDIES IN DROSOPHILA AND OTHER INSECTS
Genetic effects of cadmium in Drosophila are summarized in Table 3. A
discussion of each study follows.
16
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TABLE 3. MUTAGENICITY EVALUATION OF CADMIUM: GENE MUTATIONS AND CHROMOSOMAL ABERRATIONS IN OROSOPHILA AND OTHER INSECTS
Test System
Drospphila melanoqaster
sex-linked recessive
lethal test
Drosophila melanogaster
larval development ..
sex chromosome loss
sex-linked recessive
lethal test
Drosophila melanogaster
dominant lethal mutations
Drosophila melanogaster
sex-linked recessive
lethal test
Cadmium
Compound
Cadmium
chloride
Cadmium
chloride
Cadmium
chloride
Cadmi urn
stearate
Dosage
50.0 mq/1
(2.72xlO-4M)
65 mg/1
62 mg/1
+ 3,000 R
x-rays
5
10
20 ppm
10-20 mg/1
50-100 mg/1
100 mg/1 .
3 mg/m3
Treatment
Period
Larvae
feeding
Larvae
feeding
Larvae
feeding
5-10 days
(feeding
larvae)
10-12 days
(feeding
adults)
(feeding
larvae)
(inhalation
adult)
Results
Reported as
negative
Reported as
negative
Reported as
positive
Reported as
negative
Comments
1. Data not presented.
2. Only one dose was used.
1. Treatment was done in larvae
only.
1. Dose-response reported.
2. Confirmation of these results
in an independent laboratory
would be of interest for
comparative purposes.
1. Rationale for selecting
the dosage not given.
Reference
Sorsa and Pfeifer
(1973)
Ramel and Friberg
(1974)
Vasudev and
Krishnamurthy (1979)
Sabalina (1968)
(continued on the following page)
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TABLE 3. (continued-)
CD
Test 'System
Drosophila nelanogaster
chromosomal nondisjuncti on
sex chromosome loss
Cadmium
Compound
Cadmium
chloride
Treatment-
Dosage Period Results
62 ppm- Reported as
negative
Comments
1. No data have
been presented.
Reference'
Ramel and
(1979)
Magnusson
Drosophila melanogaster
sex-linked, recessive
lethal test
Cadmium
. aqueous
chloride 50 ppm
solution
Larvae.
feeding
Reported as
negative
1.,
2.
Toxicity was determined:
Development and survival
was. affected by- cadmium.
Inoue/ and
(1978)
Watanabe
Poekilpcerus pictus
(grasshopper)
testis (meiotic
chromosomal)
Cadmium chloride 0.001%
aqueous solution 0.01%'
0.05%
per animal
Reported as 1.
positive':
2.
The-effect nay be cytotoxic
rather than genetic.
No controls.
Kumaraswamy and
Rajasekarasetty •
(1977)
-------
Sorsa and Pfeifer (1973) reported that cadmium chloride at concentrations
of 1.25 (6.81 x 10-6M), 2.5 (1.36 x 1Q-5M), 5.0 (2.72 x 10-5M), 10.0
(5.45 x 10-5M), 20.0 (1.09 x 10-4M), and 50 mg/1 (2.27 x 10"4M) of media
caused significant delay in the development of larvae compared to controls.
In the sex-linked recessive lethal mutation test (Muller-5 test), only one
concentration of 50 mg/1 (2.72 x 10~4M) was used with no indication of
mutagenic response. The number of chromosomes tested and the criteria set for
scoring the lethals were not reported, however, and no data were presented.
Ramel and Friberg (1974), using a dose of 62 mg'(3.32 x 10~4M) of
cadmium chloride/1 of media, which was the maximum nonlethal dose in the
toxicity test, found a delay in larval development. They also studied the
induction of sex chromosome loss and sex-linked recessive lethal mutations.
In the sex chromosome loss test, a total of 23,360 chromosomes from the
treated group and 28,143 chromosomes from the control group were tested. The
frequencies of sex chromosome losses were 0.3% and 0.2% for the treated and
the control groups, respectively. In the sex-linked recessive lethal test, a
total of 1,045 chromosomes from the treated group and 742 chromosomes from the
control group were tested. The frequencies of sex-linked recessive lethals
were 8.1% and 6.1% for the treated and control groups, respectively. Although
these experiments did not indicate a significant effect of cadmium chloride in
the sex chromosome loss or sex-linked recessive lethal tests, the number of
chromosomes tested in the sex-linked recessive lethal test was too low to
preclude all but a strong mutagenic response.
The mutagenic activity of cadmium sterate was studied by Yu. A. Revazova
quoted by Sabalina (1968) in Drosophila melanogaster using the sex-linked
recessive lethal test. Flies were fed a medium containing
10-20 mg (5.45 x 10~5M to 1.09 x 1Q-4M) and 50-100 mg (2.72 x 10'4 to
19
-------
5.45 x 10~4M) of cadmium stearate/1 substrate for 5-10 and 10-12 days,
respectively. The number of sex-linked recessive lethal mutations in 805
chromosomes analyzed was 1 (0.12%) for the 5-10 day treatment, and the number
of sex-linked recessive lethal mutations in 2,192 chromosomes examined was 8
(0.36%) for the 10-12 day treatment. When larvae were treated with cadmium
sterate concentration of 100 mg/1 substrate for 12 days and scored for
sex-linked recessive lethal mutants in 380 chromosomes, no mutants were
discovered. Cadmium sterate was also administered by inhalation to adult
flies for 32 hours (4 holirs daily for 8 days). The mean cadmium concentration
was 3 mg/m^. The percent sex-linked recessive lethal mutations among the
498 chromosomes was reported to be 0.2%. The control frequency of sex-linked
recessive lethal mutations was not provided in the paper. The number of
chromosomes tested was not adequate in this study. This study provides no
evidence of mutagenicity of cadmium in Drosophila, but the scale of the study
was too small to be considered an adequate test even if appropriate controls
were presented.
Induction of dominant lethal mutations in Drosophila melanogaster with
cadmium chloride has been reported by Vasudev and Krishnamurthy (1979). The
doses used were 5 (2.72 x 10~5M), 10 (5.5 x 10"5M), and 20 ppm
(1.1 x 10~^M). The frequencies of dominant lethals were 11.8, 14.3, and
14.3%, respectively, in 1,244, 1,375, and 1,390 eggs counted. The control
frequency was 4.83% in 1,076 eggs counted. These investigators performed the
experiment according to the procedure described by Shankaranarayanan (1967)
and determined the statistical significance to be at the 5% level, although
they did not mention the type of statistical test employed. Based on these
observations, this study is evaluated as an indicator of a positive response.
A comparable study in an independent laboratory would be of interest for
comparative purposes.
20
-------
Inoue and Watanabe (1978) studied the effects of cadmium chloride in the
sex-linked recessive lethal test (attached-X method) in Drosophila
melanogaster, Oregon-R flies. In this test, the induction of mutations is
measured by the reduction in the proportion of males. The sex ratio (0.528)
in the experimental group treated with 50 ppm (2.72 x 10'^M) was not
statistically different from the sex ratio of controls (0.54), indicating
cadmium chloride is nonmutagenic. The dosage selected was a maximally
tolerated dose. Roth positive (AF-2) and negative controls were used in the
experiment.
Ramel and Magnusson (1979) failed to detect nondisjunction and sex
chromosome loss in Drosophila following treatment of larvae with 62 ppm (3.32
x 10~4M) of cadmium chloride. No data were presented; therefore, this study
cannot be evaluated.
Chromosomal aberrations were observed in the testes of the grasshopper,
Poekilocerus pictus, injected abdominally with 0.001 (5.45 x lO'^M), 0.01
(5.45 x 10-9M), and 0.05% (2.27 x 1Q-7M) cadmium chloride in 0.05 ml
volumes (Kumaraswamy and Rajasekarasetty 1978). Stickiness of chromosomes,
bridge formation at anaphase-I, and tetraploidy at metaphase were noted. The
test cannot be considered adequate because no controls were used and no
tabulated data were presented. The possibility of technical artifacts must
also be considered, particularly because chromosomal preparations were made by
a squash technique, and there were no controls.
CHROMOSOMAL ABERRATIONS IN HUMAN AND OTHER MAMMALIAN SYSTEMS
Chromosomal damage studies of cadmium, both in vitro and in vivo, are
summarized in Tables 4 and 5. A discussion of each study follows.
21
-------
TABLE 4. MUTAGENICITY EVALUATION OF CADMIUM: IN VITRO CHROMOSOMAL ABERRATIONS
Duration of
Test System Cultures
Human 72 hrs
blood
lymphocytes
Human 48 hrs
blood 72 hrs
lymphocytes
Human 48 hrs
blood
lymphocytes
Cell line 24 hrs
WI38 and MCR5
Human 48 hrs
blood
lymphocytes
G0 stage
Cadmium Duration of
Compound Dosage Treatment
Cadmium 6.2 x in~2 4 hrs
sulfide ug/ml 8 hrs
(solvent not
specified)
Cadmium 5 x 10"5M 24 hrs
chloride 5 x 10"6M 48 hrs
aqueous 72 hrs
solution
Cadmium
chloride 48 hrs
aqueous
solution
24 hrs
Cadmium 10"8 3 hrs
acetate 10"7
aqueous 10""
solution 1G"5
Activation
System Results
None Reported as
positive
None Reported as
negative
None Reported as
negative
Reported as
negative
None Reported as
weakly
positive
Comments
1. Blood lymphocytes were
derived from only one
indi vidual .
2. Only 50 metaphases for
each end point were scored.
3. Only one concentration of
the test compound was used.
1. Toxicity was determined.
2. Appropriate dosages used.
3. 100 metaphases scored for
each point.
1. Data are not provided.
2. Concentrations of the
test compound not
speci fied.
1. No dose-response.
2. Experiments were not
repeated to confirm
the positive finding.
Refei ence
Shi raishi
et al. (1972)
Dekundt and
Deminatti
(1978)
Paton and
Al 1 ison
(1972)
Gasiorek and
Bauchinger
(1981)
(continued on the following page)
-------
TABLE 4. (continued)
Duration of
Test System Cultures
Chinese hamster
"Hy" cell line
Chinese hamster
CHO cell line
Mouse Mammary
Carcinoma
FM3A
Cadmium
Compound
Cadmium
sulfate
aqueous
solution
Cadmium
chloride
'in 0.1 MHC1
Cadmium
chloride
aqueous
solution
Duration of Activation
Dosage Treatment System
in-4M 1 hr and None
harvested
at 2,4,6,8,
10,12,15,18,
21,24,27,30,
days
2xlO-6M 12, 24, 36, None
and 48 hrs
6.4xlO-5M 24 and 48 hrs None
3.2xlO-5M 24 and 48 hrs
1.0xlO-5M 24 and 48 hrs
Results
Reported as 1.
positive 2.
3.
Reported as 1.
positive only
in the presence
of newborn calf
(bovine) or
human serum.
Negative in 2.
the presence
of fetal calf
serum. 3.
Reported as 1.
negative 2.
Comments
Colchicine-1 ike effect
The type of sera has
not been specified.
Protocol for chromosome
preparation has not been
specified.
Threshold dosage was
established as 1 xlO^M
for chromosomal
aberration with
newborn calf and
human sera.
Classif icaton of
aberration types not
given.
Active only in the presence
of fetal calf serum.
6.4 x 10"5 too toxic.
Experiments were
repeated to confirm
the results.
Reference
Rohr and
Bauchinger
(1976)
Deaven and
Campbell
(1980)
Umeda and
Nishimura
(1979)
-------
TABLE 5. MUTAGENICITY EVALUATION OF CADMIUM: .IN VIVO CHROMOSOMAL ABERRATIONS IN HUMANS
Number of
Exposed Number .of
Species Workers Controls
Human 14 5
blood
lymphocytes
Human 40 13
blood
lymphocytes
Human 7, 12 6, 9
blood
lymphocytes
Human 5 3
blood
lymphocytes from
cadmium-exposed
workers
Itai-Itai patients 4 4
blood
lymphocytes
Human 24 15
blood
lymphocytes
Duration of Number of
Duration of Culture Metaphases
Exposure (hrs) Analyzed
3 months- 48 2800
26 years (exp)
900
(control )
6 weeks- 48 3740
34 years (exp)
1243
(control )
Not given 72 IBS/
person
5-24 years 48-72 100/
person
72 lOO/
person
3-6.5 years '48 4800
(exp)
1650
(control )
Results .
Reported as
negative
Reported as
negative
Reported as
positive
Reported as
negative
Reported as
negative
•Reported as
posvtive
Comment
.1. Sample size
too small .
1. Study has been
conducted following
good cytogenetic
procedure.
1. The history of the
patients including
exposure to other
drugs was not given
in this paper.
1. Sample size
too sma'l 1 .
.1. The po.ssi bitty of
synergistix
action of various
metals -cannot be
ignored.
Reference
Dekundt et -al .
(1973)
O'Riordan et al .
(1978)
Shiraishi and
Yoshida 1972
Shiraishi 1975
Bui et al.
(1975)
Bauchtnger et al .
(1976)
-------
TABLE 5. (continued)
Species
Mouse bone marrow
Mouse Micronucleus
Rat Embryos
Mouse Dominant
lethal s
Source of
Cells
Bone marrow
cells
Bone marrow
cells
Embryonic
cells
Score dead
proportion of
implants in
the uterus
Cadmium duration of
Compound Dosage Treatment
Cadmium 0.06*, 30 days
chloride in diet
Cadmium 4 mg/kg/ 5 days
chloride day
Kilhman virus
Cadmium
chloride
Cadmium
chloride only
Cadmium 1.35, 2.7, 1 day
chloride 5.4,
mg/kg
Results
Reported as
negative
Reported as
negative
Reported as
positive
Reported as
negative
Reported as
negative
Comments
1. Good technical
procedure.
2. Data were analyzed
statistically.
1. Number of mice per group
was 3.
2. Number of polychromatic
erythrocytes scored was
300 from each mouse.
1. Cadmium enhanced the
effects of vi rus.
2. Cadmium alone was
ineffective.
1. Standard dominant
lethal assay was
performed.
2. The entire spermatogenic
cycle was covered.
Reference
Dekundt and
Gerber
(1979)
Heddle and
Bruce
(1977)
Zasukhina et al .
(1977)
Epstein et al .
(1972)
-------
TABLE 5. (continued)
Species
Mice Dominant
lethal s
Mice (female)
Dominant lethal s
Mice Heritable
Translocation
Mice (female)
Svrian hamsters
(female)
Mice (male)
Source of
Cells
Score dead
proportional
implants in
the uterus
Score dead
and live
implants in
the uterus
Testicular
cells from
Fj males
Oocytes
Oocytes
Spermatocytes
Cadmi urn
Compound
Cadmium
chloride
Cadmi urn
chloride
Cadmium
chloride
Cadmium
chloride
Cadmium
chloride
Cadmium
chloride
Duration of
Dosage Treatment
1.75 mg/kg 1 day
2 mg/kg^ 0.5 to 4.5
days
1.75 mg/kg
1 day
3 mg/kg 12 hrs
6 mg/kg
0.5, 1.75 90 days
3.0 mg/kg
Results
Reported as
negative
Reported as
negative
Reported as
negative
Reported as
positive
Reported as
positive
Reported as
negative
Comments
1. All cell stages
not sampled.
1. Experiments repeated
three times.
1. Technical difficulties
in processing oocytes
not discussed.
1. Technical problems in
processing oocytes not
not discussed.
Reference
Gilliavod and
Leonard (1975)
Suter (1975)
Gilliavod and
Leonard (1975)
Shimada et al.
(1976)
Watanabe et al .
(1979)
Gill iavod and
Leonard (1975)
-------
Studies on Human Chromosomes In Vitro
Shiraishi et al. (1972) tested cadmium sulfide for the induction of
chromosomal aberrations in cultured human blood lymphocytes. Lymphocytes from
a normal human female were cultured for 72 hours at 37°C. Eight and four
hours prior to harvesting, the cultures were treated with cadmium sulfide at a
concentration of 6.2 x 10"2 ug/ml of culture fluid. Control cultures were
incubated similarly without the addition of cadmium sulfide. Three hours
prior to harvesting, cells were treated with 0.02 ug/ml of colcemid to obtain
cells in the metaphase stage of mitosis. Chromosome preparations were made
with the standard procedure (air-drying technique) and stained with Giemsa
stain. Fifty metaphase cells were scored from each treatment group for
chromosome aberrations. The types of aberrations described include chromatid
and isochromatid breaks, and symmetrical and asymmetrical translocations.
Increased incidences of chromosomal aberrations, 52% in the 4-hour treatment
group and 60% in the 8-hour treatment group, were noted over the control value
of 0%. This study utilized a blood sample from only one donor; the history of
the donor was not discussed. Only one concentration of the compound was used,
and hence, no dose-response relationship is available. No information was
given on the solvent used to dissolve the test compound, and the number of
cells scored was small. No indication as to the reproducibility of results
was given, and therefore, this study cannot be regarded as strong evidence for
a cytogenetic effect of cadmium.
Dekundt and Deminatti (1978) investigated the mutagenic effects of cadmium
chloride in cultured human lymphocytes. They treated two batches of cell
cultures and analyzed chromosomes as follows: One batch of cultures was
treated at 0 hour and at 24 hours after the initiation of cell cultures with 5
x 10~5 and 5 x 10~6M cadmium chloride. Chromosome preparations were made
27
-------
48 hours after the initiation of the culture using the standard air-drying
technique. In cultures treated 0 hour after the initiation, one hundred
metaphases were scored for each dosage. There were 3% aberrations (1%
aneuploidy, 2% gaps) at 5 x 10~5M and 7% aberrations (5% aneuploidy, 2%
gaps) at 5 x 10-^M. In cultures treated 24 hours after the initiation of
cultures, there were 5% aberrations (1% aneuploidy, 4% gaps) at 5 x ICT^M
and 2% aberrations (1% gaps and 1% fragments) at 5 x 10~6M. The control
aberration frequency was 5% (3% aneuploidy, 2% gaps). The other batch of
cultures was treated at 0 hour and 24 hours, and chromosome preparations were •
made 72 hours after the initiation of cell cultures. One hundred metaphases
were analyzed for aberration frequencies from each group. In cultures treated
at 0 hour, there were 4% aberrations (3% aneuploidy, 1% gaps) at 5 x 10~5M
and 3% aberrations (3% aneuploidy) at 5 x 10~6M. Cultures treated after 24
hours of initiation exhibited 6% aberrations (2% aneuploidy, 1% fragment, 3%
gaps) at 5 x 10~5M and 4% aberrations (1% aneuploidy, 2% gaps) at 5 x
IQ-fyl. The control frequencies were 1% aneuploidy and 1% gaps. The first
batch of cultures exhibited aberration frequencies similar to the control
level. The second batch of cultures, treated only 24 hours after the
initiation, exhibited aberration frequencies two to three times above the
control level. These aberrations were mostly in the form of aneuploidy and
gaps. The significance of chromosomal gaps is not yet understood and they may
not represent true chromosomal aberrations because they have a tendency of
restitution. Furthermore, the slight increase in the incidence of aneuploidy
may be due to technical difficulties, such as scattering of chromosomes while
preparing slides, leaving uneven numbers. These results may be treated as a
negative response of cadmium in inducing chromosomal aberrations.
28
-------
Paton and Allison (1972) exposed human lymphocyte cultures and cultures of
the established human cell lines WI38 and MRC5 to at least two concentrations
(not specified) of cadmium chloride. The duration of treatment was 48 hours
for lymphocytes and 24 hours for WI38 and MRC5. Chromosomal preparations from
100-200 cells were analyzed for aberrations. No aberrations were recorded 1n
treated cells, but data are not given. In the absence of details of
concentrations and data, this study cannot be critically evaluated.
Gasiorek and Bauchinger (1981) exposed unstimulated human blood
lymphocytes (GQ) in 1 ml quantities to 10~4, 10~5, 10'6, 10'7, and
10~^M of cadmium acetate for 3 hours. The cells were washed free of cadmium
acetate and grown in medium containing fetal calf serum and PHA for 48 hours
at 37°C; chromosome preparations were made with the standard air-drying
technique. Chromosome analysis from 200 cells per treatment indicated a
dose-related increase in the incidence of chromosome gaps. The frequencies of
gaps were 0.160, 0.115, 0.135, 0.085, and 0.055 per cell, respectively, for
the above doses compared to the control frequency of 0.058 per cell. Data
were analyzed by the Mann-Whitney rank U-test to compare the incidence of
chromosome changes in different samples (significance taken as P < 0.05). The
frequencies of structural aberrations (chromatid deletions and acentric
fragments) were 0.025, 0.010, 0.005, 0.020, and 0.010 per cell, respectively,
for the same doses, whereas in controls the frequency of structural
aberrations was 0.005 _+ 0.005 per cell. Analysis by Mann-Whitney rank U-test
indicated that structural chromosome aberrations were significantly higher
than the controls, although no dose-response relationship was evident. No
metabolic activation system was used. Sufficient number of metaphases (200
per dose) was scored and a standard protocol was employed. Although these
suggest a mutagenic response, the lack of a dose-dependent response makes it
29
-------
important that the experiments be confirmed in another study.
Studies on Rodent Chromosomes In Vitro
Rohr and Bauchinger (1976) studied the effects of cadmium sulfate in the
Chinese hamster cell line "Hy" using three types of experiments. In a
long-term experiment without recovery, cells were exposed to cadmium sulfate
at concentrations from of 10'8 to 10~5M. Chromosome preparations were
made following treatment of cells for 16 hours with 0.2 ug/ml of colcemid and
hypotohic solution. The 16-hour time period was chosen in order to analyze
the cells after exposure during a whole cell cycle. Concentrations of 10~^M
were toxic to cells after 16 hours of exposure, and hence, chromosome analysis
could not be made. In a short-term experiment without recovery, cells were
treated only for 3 hours at a concentration range of 10~8 to 10"^ mol and
chromosome preparations were made without the addition of colcemid and
hypotonic solution. This experiment indicated a typical stathmokinetic effect
(spindle inhibition) similar to that caused by colcemid. The mitotic index
increased with higher concentrations of cadmium sulfate. In a short-term
experiment with recovery, a concentration of 10~4M was chosen and cells
grown on coverslips were exposed for 1 hour. Cells with coverslips were
washed free of cadmium sulfate and transfered to fresh medium and grown for 2
to 33 hours. Chromosome preparations were made at 2, 4, 6, 8, 10, 12, 15, 18,
21, 24, 27, 30, and 33 hours after the cells were transferred to the test
medium. A total of 500 cells were scored for each recovery period. The
incidences of aberrations (0.2 to 0.6% structural and 2.4 to 3.7% numerical)
after 2 to 12 hours of recovery were similar to those of control levels (0.1%
structural and 2.4% numerical). Between 15 and 21 hours, the structural
aberrations ranged from 10.2% to 22.8% and the numerical aberrations ranged
30
-------
from 3.0% to 4.9%. From 24 to 33 hours, the aberration frequencies were lower
compared to the interval of 15 and 21 hours. During this period, the
structural aberrations ranged from 1.2 to 4.4% and the numerical aberrations
ranged from 7.8% to 10.8%. Only one dose was used in this study. A common
control was maintained for all these intervals, which may not be satisfactory.
The exact type of serum used in this study was not indicated and the type of
serum can influence the yield of aberrations.
Deaven and Campbell (1980) studied the effects of cadmium chloride on
chromosomes in CHO cells grown in the presence of bovine serum and fetal calf
serum. A concentration of 2 x 10~6M cadmium produced 17, 26, 62, and 74%
damaged cells, respectively, at 12-, 24-, 36-, and 48-hour analyses of
metaphase chromosomes. However, the presence of fetal calf serum and 2 x
10-6M cadmium chloride did not induce growth inhibition or chromosome
aberrations. According to these investigators, fetal calf serum appears to
protect the cells from the damaging effects of cadmium, whereas newborn calf
serum and human serum actively transport cadmium ions into the nucleus, thus
damaging the chromosomes. These authors also examined the frequencies of
sister chromatid exchanges (SCEs) in cells grown in F-10 containing 15%
newborn calf serum at a concentration of 4 x 10"^M cadmium chloride (low,
marginal toxicity). The SCE rate was not elevated above control levels (10
SCEs/cell). The range was 2 to 18 for cadmium-treated cells and the range for
controls was 4 to 19 per cell. This report is assessed as inconclusive
because more information on the exact role of serum in causing chromosome
aberrations is still not known.
Umeda and Nishimura (1979) investigated the clastogenic effects of cadmium
chloride in FM3A cells derived from C3H mouse carcinoma. Cells were grown in
Eagles minimal essential medium supplemented with 10% bovine serum. Cells
31
-------
were exposed to 6.4 x 10-5, 3.2 x 1Q-5j anc) Io0 x 1Q-5M of aqueous
cadmium chloride. After 24 and 45 hours of exposure, chromosome preparations
were made and analyzed. One hundred metaphases were scored for each dose. No
significant increase in the aberration frequency was noted in treated cultures
as compared to control cultures. There were no metaphases in cells treated
with 6.4 x 10'^M either at 24 hours or at 48 hours, indicating toxicity. At
3.2 x 10~5M the aberration frequencies were 2% and 3%, at 24 and 48 hours
respectively. At the lowest concentration of 1.0 x 10~5, the aberration
frequencies were 1%, each for the 24- and 48-hour treatment. The control
cultures exhibited 2% aberrations at 24 hours and 1% aberrations at 48 hours.
Experiments were performed using the accepted procedures. Three
concentrations of the test compound were used and 100 metaphases were scored
for evaluation. This report is assessed as a negative response of cadmium in
inducing chromsomal aberrations.
Zasukhina et al. (1977) reported increased aberration yields in rat
embryos exposed to virus and cadmium chloride. They infected rat embryo
cultures with Kilhman virus and then introduced cadmium chloride (3.5 x
10"6M) into the cell cultures. Chromosome preparations were performed 24
hours after the infection. Examination of metaphase cells revealed 10%
aberrations as compared to the control value of 2% aberrations. In control
cultures infected with virus only, the aberration frequency was 6%, and in
cultures treated with cadmium chloride only, there were 3% aberrations. These
results indicate that cadmium chloride may enhance virus-induced chromosomal
aberrations but does not induce chromosomal aberrations by itself.
Studies on Human Chromosomes In Vivo
Shiraishi and Yoshida (1972) and Shiraishi (1975) obtained markedly
32
-------
positive results from Japanese Itai-Itai patients. The Itai-Itai disease is
believed to have been induced by cadmium contamination. Analysis of blood
lymphocytes from 72-hour cultures derived from these patients exhibited a high
degree of chromosomal aberrations (26.7%) compared to the control aberration
rate (2.6%). Blood cadmium level was not given in this paper. See Table 5
for exposure parameters.
The results of Shiraishi and Yoshida (1972) and Shiraishi (1975), were
contradictory to the results of Bui et al. (1975) who performed chromosomal
analysis in four Itai-Itai patients (blood cadmium level 15.5-28.8 ng/g), five
Swedish workers exposed to cadmium (blood cadmium level 24.7-61 ng/g), four
Japanese controls (blood cadmium level 4.4-6.1 ng/g), and three Swedish
controls (blood cadmium level 1.4-3.2 ng/g). The incidences of aberrations
after 72 hours of culture were 2.3% numerical aberrations and 6.6% structural
aberrations in the Itai-Itai patients as compared to the Japanese controls
having frequencies of 4.5% numerical and 6.0% structural aberrations,
indicating there was no difference between the controls and Itai-Itai
patients. In the five Swedish workers exposed to cadmium, the incidences of
chromosomal aberrations were 1.0% numerical and 2.0% structural aberrations,
while in the three Swedish controls the frequencies were 0.7% numerical and
4.7% structural aberrations indicating nonmutagenic response.
The reason for the discrepancy between the results of Shirashi and Yoshida
(1972) and Bui et al. (1975) in Itai-Itai patients could possibly be due to
some other factor, such as the time of initiation of cultures after the blood
was drawn. In the experiment of Bui et al., the subjects were not exposed to
drugs and x-rays, nor did they suffer from viral infections at the time of
venepuncture. These factors were not controlled for in the study by Shirashi
and Yoshida.
33
-------
Dekundt et al. (1973) Investigated the incidence of chromosome aberrations
1n 14 workers exposed to zinc, lead, and cadmium 1n a zinc smelting plant.
The workers were classified Into three groups based on the degree of exposure.
Group 1 consisted of five workers exposed to high levels of zinc
(concentration not specified), low levels of lead (1% w/w of the mineral), and
cadmium (concentration negligible). Group 2 consisted of five workers exposed
to dust with high levels of all three metals: zinc (concentration not
specified), lead (4% .w/w), and cadmium (1% w/w). Group 3 consisted of four
workers exposed to mud and dust containing high levels of lead (60% w/w) and
cadmium (1% w/w). The control group consisted of three normal (control)
Individuals. Chromosomal analysis from blood lymphocytes cultured for 72
hours indicated 3.87%, 1.6%, and 2.76% aberrant cells, respectively, in groups
1, 2, and 3, while the control frequency was 1.55%. Since the incidence of
aberrations in group 3 was less than the incidence of aberrations 1n group 1,
it does not appear that cadlmum enhanced the aberration frequency in this
study. In addition, analysis of their data using the t-test indicated that
cadmium exposure did not induce a significant increase in the aberration
frequency. The blood cadmium level was not determined in this experiment.
Bauchinger et al. (1976) studied 24 workers (25-53 years of age) exposed
to lead (mean blood lead level 1 _+ 7 ug/100 ml) and cadmium (mean blood
cadmium level 0.40 _+ 0.27 ug/ml). The workers were exposed to these metals
for approximately 3 to 6.5 years at a smelting plant. Of the 4,800 metaphases
scored from lymphocytes cultured for 48 hours, an increase in both chromosome
and chromatid type aberrations (1.354 +_ 0.994%) was noted compared to a
frequency of 0.467 _+ 0.916% aberrations in 1,650 metaphases derived from 15
controls (mean blood cadmium level 0.15 ug/ml). The authors point out that
"the observed chromosome aberrations cannot be causally related to cadmium
31*
-------
because the workers were also exposed to lead and zinc." Dekundt and Leonard
(1975) reported a significant (P < 0.02) increase in the incidence of "complex
chromosomal aberrations" in a group of 23 men exposed to high levels of
cadmium and lead (23.5 to 75.9 ug/100 ml) compared to controls.
O'Riordan et al. (1978) studied chromosomal aberrations in blood
lymphocytes from 40 workers exposed to cadmium salts, (name not specified,
mean blood cadmium level 1.95 ug/100 ml range < 0.2 to 14.0 ug/100 ml) for a
period of 6 weeks to 34 years. In 3,740 cells examined from these workers,
there were four chromatid interchanges. In the control population of 1,243
cells derived from 13 normal subjects (mean blood cadmium level less than 0.2
ug/100 ml in 8 donors and 0.6 to 2.9 ug/ml in 5 donors), there were no
aberrations at all. Data were pooled from all 40 workers. It is not clear
whether the four chromatid interchanges came from one exposed worker or more
than one worker. The occurrence of chromatid exchanges though small in
number, 4/3,740 cells, does not necessarily indicate a negative response but
does indicate that the study may be considered inconclusive.
Most of these studies on workers reflect mixed exposure to other metals
such as zinc and lead. Since smelters also commonly process relatively crude
materials, exposure to other metals such as chromium, nickel, etc., cannot be
eliminated as possible contributors to the observed effect. Synergistic
effects may also confuse the results.
Studies on Rodent Chromosomes In Vivo
Dekundt and Gerber (1979) investigated the in vivo cytogenetic effects of
cadmium chloride (3.27 x 10~7M, 0.06%) in mice. Mice were maintained on a
standard diet (1.1% calcium) or on a low calcium diet (0.03%) for one month.
In both cases the diet was supplemented with cadmium chloride. Cadmium is
35
-------
known to be absorbed through the gut. Cadmium chloride did not induce
chromosome aberrations in bone marrow cells significantly above the control
level either in normal or low calcium diet groups. The frequency of
aberrations in animals treated with cadmium chloride in the standard diet
(1.1% calcium) was 2.20%, and the frequency of aberrations in animals treated
with cadmium chloride in the low calcium diet (0.03%) was 1.60%. The control
frequencies were 1.8% and 2.0%, respectively. These results indicate that
cadmium chloride does not induce chromosomal aberrations in mice. -
Micronucleus Assay
The micronucleus assay is based on the observation that chromosome
fragments are produced by mutagenic chemicals. Those fragments lacking
centromeres are unable to segregate normally but lag behind during the cell
division to form small nuclei or micronuclei in daughter cells. Heddle and
Bruce (1977) studied the ability of cadmium chloride to induce micronuclei in
the mouse. Three groups of mice (Fj of C57B1/6X C3H/He), each group
containing three animals, were given daily intraperitoneal injections of
cadmium chloride for 5 days with total doses of 1, 6, and 16 mg/kg,
respectively. Mice were sacrificed, bone marrow smears were prepared, and 333
polychromatic erythrocytes from each mice were scored for the presence of
micronuclei. No increase in the incidence of micronuclei was observed. This
report deals with three mice in each group and only 333 cells from each mouse;
thus, a total of 1,000 cells were analyzed for each dose group. The
spontaneous frequency of micronuclei was 0.5%. An observation of 1% over the
control value was considered a positive response. According to these
authors, the frequency of micronuclei in the experimental groups was not
different from the control level. Confirmation of the results are required
36
-------
using a larger sample of animals (10) per dose group and an analysis of at
least 2,000 polychromatic erythrocytes per dose group. Consequently, this
report is regarded as inconclusive.
Dominant Lethal Assay
The ability of cadmium chloride to induce dominant lethal mutations, which
result in the death of fetuses during various stages of development, has been
investigated (Epstein et al. 1972; Gilliavod and Leonard 1975; Ramaiya and
Pomerantseva 1977; Suter 1975; Sutou et al. 1980 a, b).
Epstein et al. (1972) evaluated the dominant lethal effects of cadmium
chloride in ICR/Ha mice. Groups of seven or nine male mice, 8 to 10 weeks of
age, were injected intraperitoneally with 1.35, 2.7, 5.4, and 7.0 mg/kg of
cadmium chloride in distilled water. Treated males were bred with virgin
females, 8 to 10 weeks of age. Each male was allowed to mate with three
virgin females per week for 8 weeks. Mated females were sacrificed on the
13th day and analyzed for dead (dominant lethals) and live implants.
According to these authors, cadmium chloride did not induce a statistically
significant increase in dominant lethal mutations over the control value.
Gilliavod and Leonard (1975) investigated the dominant lethal effects of
cadmium chloride in another strain of mice BALB/c. Only one dose of 1.75
mg/kg cadmium chloride was injected into male mice (11-13 weeks of age)
through the intraperitoneal route. The treated males were bred with three
virgin females every week for 3 weeks. The mated females were sacrificed on
the 10th day and the number of corpora lutea, and dead and live implants were
counted and compared with the controls. No dominant lethal effect was
observed in treated and control groups. These investigators treated the
parental male mice with only one acute dose of the test compound.
37
-------
Furthermore, they bred the treated mates with normal females for only 3 weeks,
which is too short to sample stages of spermatogenesis. The standard way of
doing an dominant lethal test is to breed the treated males for 8 weeks.
Consequently, this report is treated as inconclusive.
Suter (1975) studied the mutagenic effects of cadmium chloride using the
dominant lethal assay in female mice (Fl progeny of C3H and C57BLA).
According to this investigator cadmium chloride had no dominant lethal effects
in female mice. Female mice, Fj (10 x C3H) stock, were injected
intraperitoneally with 2 mg/kg cadmium chloride and bred with untreated males
0.5 to 4.5 days postinjection. Mated females, as evidenced by the vaginal
plug, were sacrificed 12-15 days later and the number of corpora lutea, number
of total implants, number of living implants, and the percent of dead implants
per female were counted. No differences were noted between the treated and
control groups. In the treated group, the frequency of corpora lutea, total
implants, living implants, and dead implants per female were 8.2, 7.8, 6.9,
and 6.9, respectively, as compared to control frequencies of 7.6, 6.8, 6.4,
and 6.1% per female.
Ramaiya and Pomerantseva (1977) investigated the mutagenic effect of
cadmium chloride using the dominant lethal test. Fj hybrid mice (CBA x
C57BL) aged 2.5 to 3 months, were selected for these studies. Males were
given a single intraperitoneal injection of aqueous cadmium chloride solution.
Three doses, 1.0, 2.0, and 4 mg/kg, were employed. 105^ was determined to
be 6.9 mg/kg. Treated males were mated with untreated females for a period of
6 weeks covering the entire spermatogenic cycle. Dominant lethals, as noted
by preimplantation and postimplantation losses and the ratio between the dead
and live implants, were recorded. No significant (P > 0.01) increase in the
dominant lethal frequency was recorded. These results are regarded as
38
-------
negative since the authors followed appropriate protocols, the dosage
selection was based on 1059, and data were analyzed statistically.
From these studies it appears that cadmium chloride has no mutagenic
potential in the mammalian dominant lethal test. The dominant lethal test has
been extensively used in screening for the mutgenic activity of chemicals.
Heritable Translocation Assay
Gilliavod and Leonard (1975) evaluated the mutagenic effects of cadmium
chloride in BALB/c mice using the Fj heritable translocation assay. Male
mice (number not specified) were treated with 1.75 mg/kg of cadmium chloride
intraperitoneally and each treated male was bred with three nontreated virgin
females each week for 3 weeks. The spermatocytes of the resulting 120 Fj
male progeny were analyzed for the presence of heritable chromosomal
translocation by standard cytogenetic methods. No evidence of heritable
translocation was noted in the spermatocytes of Fj males. Only a single
concentration was used in this experiment. Mating of treated males was done
only for 3 weeks instead of 8 weeks. No controls were maintained.
Consequently, this report is treated as inconclusive.
Direct Effects on Germ Cells
Effects of cadmium chloride on oocytes of mice (Shimada et al. 1976),
Syrian hamsters (Watanabe et al. 1979), and on spermatocytes of mice
(Gilliavod and Leonard 1975) have been investigated.
Shimada et al. (1976) induced superovulation by injecting female mice,
ddy strain, with 5 international units (I.U.) of pregnant mare's serum (PMS)
followed 48 hours later by 5 I.U. of human chorionic gonadotrophin (HCG).
Mice were given 3 mg/kg or 6 mg/kg of cadmium chloride 3 hours after the
39
-------
administration of HCG and were dissected 12 hours after the cadmium chloride
administration. Chromosome preparations were made from unfertilized obcytes
at the second meiotic metaphase as described by Tarkow'ski (1966). No
structural chromosome abnormalities were found. However, numerical
aberrations (aneuploidy) were found to be statistically significant (P =
0.015) in the dose group of 3 nuj/kg group as compared to controls. The
authors postulated that this hondisjunction may be due to the spindle
inhibiting effects of cadmium.
Watanabe et al. (1979), using Syrian hamster oocytes and cadmium
chloride, reported even more pronounced incidence of aneuploidy. Cadmium
chloride at concentrations of 1.0, 2.0, and 4 iiig/kg were subctitarieolisly
injected to groups of 20 female Syrian hamsters 5 hours before ovulation.
Matched controls were given equal volume of normal saline. Females were
sacrificed 12 hours after the treatment, and the oocytes were recovered from
the ampulla. Analyses revealed that 6 females out of 20 from the 1.0 trig/kg
group, ll females out of 20 from the 2.0 mg/kg group, and 12 females out of 20
from the 4.0 mg/kg group had numerical chromosomal abnormalities, such as
hyperhaploidy and dipldidy, in oocytes as compared to 3 out of 20 in control
females. The results were statistically significant (P < 0.05 and P < 0.01)
in the treated groups compared to the control group. Cadmium-treated animals
were also analyzed for cadmium accumulation in the ovary usirtg atomic
absorption spectrophotometry. The results indicated statistically significant
(P < 0.05) increases in the accumulation of cadmium in the ovaries of treated
females as compared to control females. Both of these results appear to
indicate a positive response of cadmium in inducing numerical chromosomal
abnormalities in mammalian oocytes.
-------
Gilliavod and Leonard (1975) investigated the mutagenic effects of cadmium
chloride in BALB/c mice using the spermatocyte assay. Males in groups of 10
were treated with 0.5, 1.75, and 3.0 mg/kg of cadmium chloride
intraperitoneally. After 3 months, treated males were sacrificed and
spermatocytes (100 cells per animal) in the testes were analyzed for
translocations that may have been passed on from treated spermatogonia. No
translocations were found, i.e., the frequency of translocations were 0 in
both the treated and control animals. This is not a very sensitive test, and
hence, it is not commonly employed in mutagenicity tests.
Sperm Abnormality Assay in Mammals
Heddle and Bruce (1977) evaluated the effects of cadmium using the sperm
abnormality assay. The sperm abnormality assay is based on the observation of
increased incidence of sperm heads with abnormal shapes as a result of
exposure to chemical mutagens (Wyrobek and Bruce 1975). Three groups of mice
of the genotype (C57BL/6 x CSH/HejFj, each consisting of three mice, were
given daily intraperitoneal injections of cadmium chloride for 5 days with
doses of 1, 4, and 16 mg/kg, respectively. Sperm suspensions were made from
sperm collected from the cauda epididymis following the sacrifice by cervical
dislocation of mice. The sperm suspension was stained with 1% eosin-Y in
water, and smears were made, dried, and mounted under a coverslip with
permount. One thousand sperm heads were evaluated for morphological
abnormalities. The background frequency of sperm head abnormality in the
control population was 1%. Under the conditions of the test, there were no
increases in sperm head abnormalities in the treated over the control groups.
-------
CHROMOSOMAL ABERRATIONS IN PLANTS
Levan (1945) reported that treatment of Alllum cepa root-tips with cadmium
chloride induced C-mitosis. This observation was later confirmed by Avanzi
(1950) using cadmium chloride concentrations ranging from 2 x 10"6M to 5 x
lO-fyl. Qehlkers (1953) reported that cadmium nitrate induced chromosomal
aberrations in Vicia faba. Van Rosen (1953, 1954) reported the genotoxi'city
of cadmium as evidenced by chromosomal aberrations in the root-tips of plants
such as Alii urn cepa, Beta vulgaris, Pi sum abysinnicum, and Vicia sativa.
Similar observations were made by Degraeve (1971) in Horedeum sativum and by
Ruposhev and Garina (1977) in Crepis capillaris. Aberrations reported in
these studies were of both chromatid and chromosome type with a dose-related
response. Since many of these publications are in foreign languages, the
material presented here is a summary derived from the review article published
by Degraeve (1981).
OTHER INDIRECT EVIDENCE
Some information is available on the effects of cadmium on animals, and
although it is not strictly mutagenicity test data, it may be useful in
evaluating the ability of cadmium to reach and damage the gonads. Dixon et
al. (1976) reported that cadmium chloride at 2.24 mg/kg administered
intraperitoneally caused damage to rat testes. A single 10 mg/kg
intraperitoneal injection causes selective destruction of rat testes. Cadmium
chloride, when administered intraperitoneally at 1 mg/kg, reduced fertility of
male mice at sperm cell stages, except spermatozoa (Lee and Dixon 1973).
However, single oral doses up to 25 mg/kg had no effect on the fertility of
male rats (Dixon et al. 1976), and cadmium chloride at 0.1 mg/1 in drinking
water for up to 90 days had no effect on the fertility of rats.
-------
Intraperitoneal injection of 1 mg/kg, cadmium chloride decreased the
incorporation of thymidine into spermatogonia in mice (Lee and Dixon 1973).
These authors (Lee and Dixon 1973) also found the binding of cadmium to late
spermatids in vivo or in vitro. Friedman and Staub (1976) studied the effects
of cadmium chloride on DNA synthesis in Swiss mice. Cadmium chloride at 10
mg/kg inhibited DNA synthesis significantly. An aqueous solution of cadmium
chloride was injected intraperitoneally at the above dose into five male mice
and the mice were sacrificed 3.5 hours later. Thirty minutes prior to
sacrifice, mice were injected with 10 uCi [^H] thymidine. Controls received
only 10 uCi [^H] thymidine. Testes were removed following cervical
dislocation, DNA was isolated, and the specific activity was determined.
Cadmium chloride induced a statistically significant (P < 0.01) inhibition of
[^H] thymidine uptake (1.90 +_ 0.58) in the testes as compared to controls
(7.45^1.44).
Mitra and Bernstein (1977, 1978) reported that when E_. coli cultures were
exposed to 3 x 10'6M cadmium (Cd2+), 82 to 95% of the cells lost their
ability to form colonies on agar plate. Analysis of DNA strands from cells
treated with various doses of Cd2+ indicated that there was a dose-related
increase in the breakage of single strand DNA. These investigators believe
that the loss of viability in cadmium-treated cells is due to the
single-strand DNA breakage. Cadmium-treated cells recovered viability when
grown in Cd2+-free liquid medium containing 10 mM hydroxyurea.
Si rover and Loeb (1976) investigated the infidelity of DNA synthesis
brought about by cadmium chloride and cadmium acetate. This assay measures
the perturbations in the fidelity of DNA synthesis in vitro caused by soluble
metal salts. Cadmium chloride and cadmium acetate decreased the fidelity of
DNA synthesis. Cadmium chloride also induced concentration-dependent
-------
inhibition of RNA synthesis (Hoffman and Niyogi 1977).
SUMMARY
Cadmium has been investigated for mutagenic activity in both prokaryotic
and eukaryotic systems. Gene mutation studies in Salmonella typhimurium and
£. coli have produced inconclusive results. In yeast, gene mutation studies
are also inconclusive. However, gene mutation studies in mammalian cell
cultures, mouse lymphoma cells, and Chinese hamster lung and ovary cells, have
resulted in a weak mutagenic response. Rec-assay, which is a test for DNA
repair in Bacillus subtilis, also resulted in a weak mutagenic response. In
the Drosophila sex-linked recessive lethal test, cadmium has been found to be
nonmutagenic. However, the negative response may be due to inadequate test
control. In contrast, the dominant lethal test in Drosophila resulted in a
positive response with a dose-response relationship. Chromosomal aberration
studies in human lymphocytes and human cell lines treated with cadmium are
conflicting and contradictory. In Chinese hamster cells, chromosomal
aberrations were noted following treatment with cadmium, however, in mouse
carcinoma cells no aberrations were recorded in response to cadmium treatment.
In rodents, treatment with cadmium induced no chromosomal aberrations or
micronuclei in bone marrow cells. Similarly, no dominant lethal mutations
were noted in mice treated with cadmium. Induction of chromosomal
nondisjunction in female germ cells of mice and Syrian hamsters by cadmium has
been reported. Chromosomal aberrations and gene mutations in plants exposed
to cadmium have also been recorded.
A clear analysis of the mutagenicity of cadmium in the studies discussed
herein is rather difficult because of the conflicting results and lack of
adequate test protocols used. However, the results of gene mutations studies
-------
in mammalian cell cultures, rec-assays in bacteria, chromosomal nondisjunction
studies in intact mammals, as well as other indirect evidences suggest that
cadmium is weakly mutagenic.
-------
CARCINOGENICITY
The topic of the carcinogenicity of cadmium has been reviewed critically
in earlier documents (IARC 1973, 1976; U.S. EPA 1977, 1981; Sunderman 1977,
1978; Hernberg 1977). This section updates findings mentioned previously and
discusses recent findings not mentioned in earlier reviews.
ANIMAL STUPIES
Injection Studies in Mice and Rats
Injection of cadmium metal or certain salts of cadmium has been shown to
produce sarcomas at the site of the injection as well as testicular tumors
(Leydig cell, interstitial cell) in experimental animals as summarized in
Table 6.
Tomatis (1977) reviewed the appropriateness of the subcutaneous (s.c.)
injection route as a carci'nogenesis bioassay by comparing s.c. with other
routes of administration. He surveyed a number of chemicals tested by the
s.c. route in rodents to see if there was a correlation between their
capacities to induce local and/or distant tumors in one species and their
capacities to induce tumors by another route in another species. A total of
102 chemicals, which have been reviewed by the International Agency for
Research on Cancer (IARC) and have been tested by the s.c. route as well as by
other routes of administration, were surveyed. Of those, 69 were positive for
carcinogenic activity when administered by s.c. injection and by another
route, and 18 were negative or inconclusive whether given by s.c. injection or
another route. Nine were positive only when administered by s.c. injection,
and six were negative by s.c. injection and positive by another route. The
author concludes that "administration of a chemical by the s.c. route produced
what one could call false negative results for six (5.6%) of the 102 chemicals
-------
TARLE fi. ANIMAL TUMORIGENESIS INDUCED RY CADMIUM INJECTION
Author
Haddow
et al. (1961)
Heath (196?)
Species
Rats
Mice
Hooded Rats
Compound
ferritin containing
Cd
-
Cd powder
Route Tumor and Incidence
s.c. Sarcomas 8/20
Interstitial cell tumors
Sarcomas 0/20
i.m. Sarcomas 2/10
10/20
o*J [.iwmici
0.28 g in 0.4 ml fowl serum
0.014 g in 0.4 ml fowl serum
(later in the study 10 more
rats developed tumors; which test
group they were in is not stated)
Sarcomas 3/10
Heath and
Daniel (1964)
Kazantzis (1963)
Kazantzis and
Hanbury (1966)
Haddow et al.
(1964)
Hooded Rats
Chester-Beatty
Rats
Wistar Rats
Rats
Mice
Cd powder
0.014 g in 0.4 ml fowl serum i.m.
0.02R g in 0.4 ml fowl serum
25 mg CdS in 0.25 ml s.c.
physiological saline
25 mg CdS in 25 ml s.c.
physiological saline i.m.
25 mg CdO in 0.25 mg s.c.
physiological saline
0.25 ml physiological saline alone
0.5 mg CdS04.H20 s.c.
in 1.0 ml sterile
distilled water once weekly
for 10 weeks
0.05 mg CdSO^^O in 0.2 ml
H20 once weekly for 11 weeks
Sarcomas 9/10
Sarcomas 6/8 (2 were killed early)
Sarcomas 6/10
Sarcomas 6/10, 6/26
Sarcomas 5/14
Sarcomas 8/10
Sarcomas 0/10
Sarcomas 14/20
control 0/15
0/20 injection site tumors
control 0/15
[continued on the following page)
-------
TARLT 6. (continued)
Author
Species
Compound
'Route
Tumor and Incidence
Roe et al. (1964)
Rats
Gunn et al. (1963a) Albino Mice
Wistar Rats
Gunn et al. (1964) Wistar Rats
oo
Gunn et al. (1967)
Knorre (1970)
Knorre (1971)
Lucis et al. (1972)
Wistar Rats
Wistar Rats
Wistar Rats
Wistar Rats
0.5 mg CdSn4.H2n in .1.0. ml
once weekly for 10 weeks
0.05 mg CdS0.4.4H20 in 0.2 ml H20
0.03 mM/kg CdCl ?
0.03 mM/kg CdCl 2
0.03 mM/kg CdCl 2
1.8 mg CdCl2
0.003 mM CdCl2/100 g b.w.
0.003 mM CdCl2/100 g b.w.
0.02-0.03 mM/kg CdCl2 in
isotonic NaCl solution
Reddy et al. (1973) Fischer 344 Rats 0.03 mM/kg CdCl2
Furst and
Cassetta (1972)
Favino et al. (1968)
Fischer 344 Rats 5 mg Cd powder
(suspended in 0.2 ml)
synthetic trioctanoin)
s.c.
s.c.
s.c.
s.c.
Sprague-Dawley
Rats
1 mg/100 g CdCl 2
simultaneous s.c.
and i.m.
single s.c.
single s.c.
single s.c.
single s.c.
2 monthly i.m.
injections
single s.c.
Interstitial cell tumors 11/15
control 0/15
Interstitial cell tumors 0/16
Interstitial cell tumors 20/26
control 0/25
Interstitial cell tumors 17/25
control 0/25
Sarcomas 9/22; control 0/18
Interstitial cell tumors 21/24
control 0/18
Sarcomas 10/23
Sarcomas 3/26
Sarcomas 6/45
Interstitial cell tumors 10/25
Interstitial cell tumors 13/15
Sarcomas 2/15 (two animals died
early)
Interstitial cell tumors 16/20
control 0/10
Sarcomas 26/50
Interstitial cell tumors 6/6
(continued on the following page)
-------
TABLE fi. (continued)
Author
Species
Compound
Route
Tumor and Incidence
Malcolm (1972)
Levy et al. (1973)
Rats
C.B. Hooded Rats
CdCl2
0.2 mg 3CdS04.H20 in
0.2 ml H20
0.1 mg 3CdS04.8H20 in
0.2 ml H20
0.05 mg 3CdS04.8H20 in
0.2 ml H20
control - 0.2 mg H20 only
s.c.
weekly s.c. injection
into alternate flanks
for 2 yrs
Sarcomas (?)
Interstitial cell tumors (?)
Experiment not completed at
time of publication
Sarcomas 4/25
Interstitial cell tumors 17/25
1 lung adenoma
Sarcomas 1/25
Interstitial cell tumors 17/25
1 malignant lymphoma
Sarcomas 1/25
Interstitial cell tumors 16/25
1 adenocarcinoma of pancreas
Sarcomas 0/75
Interstitial cell tumors 48/75
1 squamous carcinoma of tongue
1 benign liver cell tumor
-------
tested arid, if we accept all the criticisms of this route of administration,
false ppsjtive pesults for pine (8.7%) of the 102 chemicals tested. Even, so,
according to £he author, it appeals that the s.c. rpu.te of administration is
pot top muc|i wppse th,an apy other; rpijte of administration.
Inhalation Study in Rats
Takenaka et al. (J982)--
A carcinogenicity study of cadmium administered to male Wistar r^ts by
inhalation has been reported, by Takenaka et al. (1982). The sninials were
placed in a 225 1 inhalation chamber for exposure to cadmium chloride.
(CdCl2) aerosol. Aerosol was generated by atorr)izif]g a spjijtipn pf CdClz,
and airflow through the atomizer was 0.7 1/min. Analytical measurements of
cadm,iu.m levels were made by collecting aerosol samples in menibfane filters in
t(ie intake a.n
-------
TABLE 7. NOMINAL AND MEASURED CADMIUM CONCENTRATIONS OF
CdCl2-AEROSOLS USED FOR INHALATION
(Takenaka et al. 1982)
Nominal concentrations
Measured concentrations
Standard deviation
Number of measurements
ug/m3
ug/m3
ug/m3
• _ •
50.0
50.8
5.9
212
25.0
25.7
3.6
220
12.5
13.4
2.1
210
-------
Differences in body weights (Table 8) and mean survival times (Table 9)
among control and treated animals were not statistically significant (P >
0.05).
A dose-related increase in the incidence of primary lung carcinomas in
treated animals was evident as shown in Table 9. The first epidermoid
carcinoma and the first adenocarcinoma were found 20 and 22 months,
respectively, after treatment commenced. Several treated rats also developed
adenomas and nodular hyperplasia in the lung. Metastases to the regional
lymph nodes and the kidneys and invasion into the regional lymph nodes and the
heart occurred in some rats with lung carcinomas. No lung tumors were found
in control animals.
Nonneoplastic lesions and various tumors in other organs were found in
both control and treated animals. None of these additional tumor types and
nonneoplastic lesions was significantly (P > 0.05) different among the four
groups.
The data in Table 10 show that cadmium was retained in lung, liver, and
kidney of survivors for as long as 13 months after cessation of exposure.
Anaylsis of these tissues indicates that cadmium was absorbed and circulated
throughout the body and that, although the lung was the target organ for
carcinogenicity, the kidney retained the largest amount of cadmium. Increases
in cadmium levels were dose-related in liver in all treatment groups and in
lung and kidney in the mid-dose and high-dose groups. Pathologic changes
apparently were not observed in kidney and liver, thus suggesting that the
cadmium levels found did not have a toxic effect in these tissues.
The authors attributed their success in demonstrating cadmium
carcinogenicity to: 1) performance of a long-term study using CdCl2
aerosols that were retained at a rather high level in the lungs after
52
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TABLE 8. AVERAGE BODY WEIGHTS OF RATS EXPOSED TO CADMIUM CHLORIDE
(Takenaka et al. 1982)
Exposure groups
Control
12.5 ug/m3
25 ug/m3
50 ug/m3
Control
12.5 ug/m3
25 ug/m3
50 ug/m3
Average
0
135.2*
(4.8)
135.1
(6.6)
133.4
(6.7)
133.3
(6.7)
18t
434.9
(32.4)
424.6
(41.0)
437.6
(38.1)
424.3
(40.6)
Body Weights (Months After the Beginning
3
333.3
(27.4)
320.9
(29.4)
326.6
(28.6)
323.5
(29.0)
21
428.2
(31.4)
421.8
(41.2)
441.2
(37.7)
424.9
(43.8)
7
385.2
(30.5)
375.3
(37.1)
382.1
(32.1)
375.1
(32.2)
24
406.2
(41.3) -
409.5
(45.9)
429.2
(45.9)
415.2
(42.6)
10
411.6
(31.2)
405.2
(39.4)
410.0
(32.8)
403.2
(34.8)
27
405.7
~ (31.3)
408.4
(40.9)
423.9
(37.6)
398.4
(35.8)
of the Inhalation)
12
422.9
(31.7)
417.2
(41.4)
425.7
(35.9)
417.0
(36.6)
30
367.3
(39.8)
372.5
(41.8)
375.4
(47.8)
357.8
(41.5)
15
425.1
(31.8)
420.0
(38.8)
428.3
(36.1)
422.0
(38.5)
*Mean value (_+ S.D.).
tEnd of the inhalation.
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TABLE 9. SURVIVAL TIMES AND LUNG CHANGES OF WISTAR RATS AFTER THE EXPOSURE TO CdCI2 AEROSOLS
(Takenaka et al. 1982)
Exposure
Groups
No. of Rats With Lung
Survival No. of
Time in Rats Adeno- Adenomas Carcinomas
Initial Weeks Examined matous
No. of Mean Value Histo- proli-
Rats + S. D. logically feration
adeno epidermoid combined muco- Total
epidermoid epider (%)
and adeno moid
Control
41
12.5 ug/m3 40
25 ug/m3 40
50 ug/m3 40
122+19
119+17
125^15
116+23
38*
39t
38§
3511
1
6
5
3
0
1
0
1
0
4
15**
14**
0
2
4
7
0
0
1
1
0 0
0 6(15.4%)#
0 20(52.6%)**
3 25(71.4%)**
*Two rats died during the first 18 months; another rat was not examined because of autoTysis.
tOne rat was not examined because of autolysis.
§Two rats were not examined because of autolysis.
lIThree rats died during the first 18 months; two other rats were not examined because of autolysis.
#P = _< 0.01.
**P < 1.0 x 10-5.
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TABLE 10. CONCENTRATION OF CADMIUM IN LUNGS, LIVER, AND KIDNEYS OF RATS EXPOSED TO
CdCl2 FOR 18 MONTHS (13 MONTHS AFTER THE END OF THE INHALATION)
(Takenaka et al. 1982)
No. of Cadmium concentration (ug/g wet weight) in
Exposure Groups Rats Lungs Livers Kidneys
Control 9 0.03 0.1 +_ 0.1* 0.3 _+ 0.1
12.
25
50
5 ug/m3 6 5.6 +_ 1.0 2.2 +. 0.6 13.5 _+ 3.2
ug/m3 9 4.7^1.5 5.9^1.5 16.4 +. 3.6
ug/m3 9 10.4 +_ 4.2 13.5 +_ 3.0 33.6 _+ 10.7
*Mean value + S.D.
-------
cessation of exposure, and 2) continuous observation of the animals over an
extended duration (31 months). Most of the lung carcinomas were detected
after the 27th month of the study.
Intratracheal Studies in Rats
Sanders and Mahaffey (1982)--
Sanders and Mahaffey (1982) evaluated the carcinogenicity of cadmium oxide
(CdO) in male Fischer 344 rats. Four groups of 46 to 50 rats each were
treated as follows: Group 1 (untreated controls) received one intratracheal
instillation of 0.9% sodium chloride solution (the dosing vehicle); Group 2
was given an intratracheal instillation of 25 ug CdO when 70 days old; Group
3 received intratracheal instillation of 25 ug CdO when 70 and 100 days old
for a total dose of 50 ug; Group 4 was given intratracheal instillations of 25
ug CdO when 70, 100, and 130 days old for a total dose of 75 ug. The authors
stated that the 25 ug dose was 75% of the 1.050 by the route of
administration used. Instilled CdO had a count median diameter of 0.5 urn.
The animals were allowed to survive until spontaneous death. All animals
were necropsied, organs were weighed, and tumors, lesions, and major tissues
and organs from all rats except 12 lost due to autolysis or cannibalism were
examined histopathologically.
Median survival times were 793, 824, 785, and 788 days for Groups 1, 2, 3,
and 4, respectively. Survival times and organ weights (body weights were not
obtained) were similar (P > 0.05) between control and treated groups.
Statistical analysis of tumor data by life table and contingency table methods
revealed no significant (P > 0.05) differences among the four groups. Lung
tumor findings consisted of adenocarcinomas in two rats in Group 3.
56
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Cadmium, as CdO, was not carcinogenic under the conditions of this study;
however, the protocol used may not be as sensitive an indicator of the
carcinogenic potential of cadmium as a design with lifetime exposures by
inhalation, particularly in reference to the carcinogenicity study by Takenaka
et al. (1982) discussed herein. Lung tissue was not analyzed for cadmium
content in the Sanders and Mahaffey (1982) study. However, clearance of 80%
of an intratracheally instilled dose of 15 ug 109CdO from the lung in male
Fisher 344 rats, with an elimination half-life of 4 hours and an experimental
duration of 2 weeks, has been observed (Hadley et al. 1980) as well as repair
of lung tissue to Type 1 alveolar epithelium by 2 weeks following an
intratracheal instillation of 25 ug CdO into male Fischer 344 rats (Hadley et
al. 1980). Hence, a lifetime inhalation exposure to CdO also might have
presented a stronger challenge for carcinogenicity by providing a greater
cumulative dose of cadmium within target (lung) tissue.
Drinking Mater Studies in Rats and Mice
Schroeder et al. (1964, 1965)
Schroeder et al. (1964) conducted two lifetime exposure studies (survival
up to 21 months) in Swiss mice. The animals were given drinking water
containing 5 ppm of cadmium acetate. The exposure level was purposely low to
simulate the human experience, according to the authors. Only males
experienced decreased longevity in comparison with the controls. The exposed
males had fewer "visible" tumors (1/>50) than the controls (11/>50), possibly
related to shortened lifespan (P < 0.005).
In another lifetime exposure study by Schroeder et al. (1965), male and
female Long-Evans rats ingested 5 ppm cadmium acetate in water as the sole
source of fluid; the treated group developed 28/84 tumors versus 24/70 in
57
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controls. The authors stated that "no significant differences appeared among1
the Various groups as to type of tumor." This study, like th'e 1964 study, was
compile a'tec! by being performed in a low metal environment with a diet low in
many trace metals. When the essential trace element Cr(III) Was added to the
diet of bhe ^roO|j Of rats devoid of cadmiunii they thrived better than the
control grbtip arid had 34/71 tumors (SchrOeder et al. 1965).
Halcblni (1972)--
Halcoim (1972) gave male Chester-Beatty hooded rats up to 0.2 nig of
cadmium1 sulphate subctitarieOusly arid up to 0.8 mg weekly by stomach tube for 2
years. In another experiment, Swiss mice were given doses of cadmiiJm sulfate
in distilled Water Up1 to 0.02 mg/5g of body weight subcu'tanebusly at weekly
intervals for 2 years. Except for a few sarcomas see'ri iri the rats cjive'ri
sUbcUtaneous injectibris and Leydig cell tumors (both also seen in the
controls), these studies were negative at the time 'reported.
Levy arid 'Clack (1975)--
Experitiients with male specified pathogen-free Chester-Beatty h'boded rats,
usiri(j doses of 0.087, 0.18, arid 0.35 mg/kg of cadm'idm sulphate' in distilled
water given by gastric iristillatibri brice weekly for 2 years, were carried but
by Levy and Clack (19*75) with rib difference iri tumor iriciderice in exp'bsed and
coritrbl groups. It is noted, h'Owever, that this particular strain of rats has
a very high lifetime incidence of spbntiarieou's interstitial cell tumbr
formatibri (75% in the untreated coritrol group), such that "if exposure to Cd
had any effect on the incidence of the lesions it was entirely overshadowed by
their spontaneous occurrence," according to the authors. Effects on the
prostate were especially scrutinized, with no neoplastic lesions observed.
'58
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Dietary Studies in Rats
Loser (1980) —
A 2-year oral carcinogenicity study of Wistar rats given cadmium chloride
(CdCl2) was carried out by Loser (1980). Doses of 1, 3, 10, and 50 ppm were
given in food to 50 male and 50 female rats, with 100 controls of each sex.
Food consumption was similar in all the test groups. The mean body weights of
treated males were significantly reduced (P < 0.01) at the highest dose level.
Other than reduced weight in the high-dose males, the male and female
treatment and control groups were comparable for weight, mortality, and tumor
incidence.
U.S. Food and Drug Administration (1977)--
An unpublished chronic toxicity study of CdCl2 was conducted at the U.S.
Food and Drug Administration (U.S. FDA 1977). The compilation of animals
examined pathologically shows that six groups of Charles River COBS (SD) rats,
each consisting of 26 to 32 males and 26 to 29 females, were studied. These
groups were given 0 (untreated controls), 0.6, 6, 30, 60, or 90 ppm CdCl£ in
the diet for 103 weeks. Five males and five females per group were sacrificed
at 24 and 52 weeks. All animals were necropsied, and tissues, organs, and
tissue masses were examined histopathologically. Kidney tissue from five or
fewer males in each sacrificed group was evaluated by electron microscopy;
sections of liver and kidney from these animals were stained to assess
fibrosis, lipid content, liver glycogen, and the basement membrane of tubuli
and Bowman's capsules in kidney.
No significant (P > 0.05) differences in survival between control and
treated groups were reported, and, excluding interim sacrificed animals, no
more than two animals per group died before 77 weeks. Results of necropsy and
59
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histopathologic and histochemical evaluations did not show treatment-related
effects. Electron microscopy revealed dose-related changes as small
cytoplasmic lipid droplets in renal tubular epithelium, increased numbers of
residual bodies in renal nephron cells, and swelling and sloughing of cells in
distal tubular epithelium and the collecting ducts of the kidney.
Cadmium, as CdCl2» was not carcinogenic in this study, but a stronger
evaluation of cadmium carcinogenicity possibly could have been made through
the use of larger group sizes and, as suggested by the lack of overt toxicity
in treated groups, higher doses.
Cadmium appears to be much less potent by ingestion than by inhalation in
terms of the overall carcinogenicity regardless of the site of cancer
induction. For example, the total dose of inhaled cadmium in the Takenaka et
al (1982) study, where the rats developed a 71% incidence of lung cancer, was
about 7 mg (0.25 m3/day x 0.05 mg/m3 x 365 days/year x 1.5 years). By
contrast, in the Schroeder et al. (1965) drinking water study in rats, which
had one of the smallest total doses of all the ingestion studies, a total dose
of about 60 mg (5 ppm x 0.05 x 0.35 kg x 730 days) induced no cancer response.
If we assume a 10% upper-limit of detection of tumors in the Schroeder et al.
(1965) study, the highest reasonable cadmium potency via ingestion is about
0.0017 (0.1/60), campared with a potency of about 0.1 (0.7/7) for inhalation.
While it is possible that cadmium is not carcinogenic by ingestion at all,
the negative animal evidence can only set an upper-limit on the carcinogenic
potency of ingested cadmium, which in the rat appears to be about two orders
of magnitude less than for inhalation.
60
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Summary
Evidence for the carcinogenicity of cadmium and certain cadmium salts in
experimental animals has been obtained from studies of cadmium and cadmium
salts subcutaneously administered into mice and rats, as summarized in Table
9, and the inhalation exposure study of cadmium chloride aerosol in rats by
Takenaka et al. (1982). Oral-intake and inhalation studies were termed
inadequate by the IARC in 1976, apparently on the basis of the relatively
small doses employed and the small percentage of absorption from the gastro
intestinal tract; however, the studies by Takenaka et al. (1982), Sanders and
Mahaffey (1982), and Loser (1980) were not reviewed by the IARC. Schroeder's
work was specifically designed to simulate human exposure, and, for the most
part, the doses given seem realistic for this purpose; however, the doses used
were apparently below the maximum tolerable doses usually used today in
attempting to establish the carcinogenic potential of various substances.
61
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EPIDEMIOLOGIC STUDIES
Epidemioiogic information dealing with the relationship between cadmium
exposure a'nd prostate cancer has been obtained from mortality studies. A
major difficulty with mortality studies involves survivorship. As
survivorship has been improving since the 1940s through the present time, . ,
detecting an increased prostate cancer incidence in any group based on
mortality from this disease has become increasingly difficult; prostate cancer
victims are living longer and, more often than in the past, dying from other
causes (Robbins and Angell 1976).
One of the lowest death rates for this site is in Japan where the
age-adjusted rate in 1974-75 was 2.4/100,000, although Japan has the highest
per capita intake of cadmium in the world. On the other hand, Sweden has the
highest age-adjusted death rate from prostate cancer (21.9/100,000 in 1974-75)
in the world, but a low daily intake and low body burdens of cadmium
(Kjellstrom et al. 197R). Per capita rates, of course, include many people
who are not exposed to the agent in question.
There are 12 epidemiologic studies reviewed here that deal specifically
with cancer risks resulting from cadmium exposure. Although five of these
were reviewed in the OHEA Health Assessment Document for Cadmium (May 1981),
they are covered here also for the convenience of the reader.
Potts (1965)
Potts (1965) reported the results of a clinical study of an unstated
number of current and former employees of a British alkaline battery factory
who were exposed to cadmium oxide dust beginning in 1920 and ending in 1963.
In 1946 the manufacture of these batteries was moved to a new location not far
from the site of the earlier factory. The first measurements of cadmium dust
62
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in the air were made in 1949. At this time, cadmium in the air varied from
0.6 to 2.8 mg/m3 of air in the platemaking and assembly shops and 236
mg/m3 in the negative active material department. After the installation of
local exhaust ventilation in 1950, cadmium in the air was reduced to less than
0.5 mg/m3. Improvements to the exhaust system in 1956 further reduced the
dust to less than 0.1 mg/m3. The policy at the time of the study's
publication was to take steps to reduce the exposure whenever the measurement
of cadmium dust exceeded 0.5 mg/m3.
Of 70 battery workers for which Potts's clinic had medical records and who
were exposed for at least 10 years, proteinuria was observed in 44%. Although
no comparison group was provided, this number is probably excessive since
proteinuria is the result of renal tubular dysfunction. A 200-248 ug/day
cadmium dietary intake over a 50-year exposure period is required to produce a
critical renal cortex concentration that is associated with renal dysfunction.
Only 1% of Americans ingest more than 50 ug/day (U.S. EPA 1981). However, the
author did note that the earlier studies of the urine protein of
cadmium-exposed workers in this same plant revealed "similar characteristics"
as those of the present study. Four individuals with persistent proteinuria
were examined further. Two of them ultimately died. Kidney function tests
prior to death revealed no abnormalities nor were any gross abnormalities
observed following microscopic examination of the kidneys of the deceased.
In a second phase of this study, Potts claimed that a "careful search"
produced a total of 74 men who had been exposed to cadmium dust for more than
10 years and eight of them had died. The author did not reveal what was
searched, whether it was his clinic's medical records or employment records of
the factory, nor did he say how these 74 men relate to the 70 battery workers
mentioned earlier. Furthermore, the source of his death information on the
63
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eight was not available. Five of the eight deaths were due to cancer and
three of these were cancer of the prostate. The death data from Potts's paper
is summarized in Table 11. Presumably, the death information on these eight
came from Potts' clinical files, although it is not stated. That the author
made any attempt to determine the vital status on the remaining 66 is unclear.
Since all of the deaths occurred in the early 60s and nearly all had lengthy
exposures, this implies that they were all at risk to the highest cadmium dust
levels during their earlier years of employment prior to 1950. No information
is given on workers exposed for fewer than 10 years.
In the absence of selection bias (a distinct possibility if clinical
records were used), the distribution of the eight deaths is striking as was
noted by the author. But because of the possibility of selection bias, lack
of a comparison group, and an unknown age structure of the population of 74,
it is impossible to say if the observation of three prostate cancer deaths is
statistically significant. Therefore, this study provides only the suggestion
of an association of prostate cancer and exposure to cadmium.
Kipling and Waterhouse (1967)
Kipling and Waterhouse (1967), in a letter to the Lancet, reported on 246
workers who had been exposed for a minimum of one year to cadmium oxide dust.
The authors compared the number of cancers observed from several sites with
the number expected from those sites based on incidence rates from the
Birmingham Regional Cancer Registry. The number of observed cancer deaths of
the prostate was significantly greater than expected (4 observed vs. 0.58
expected, P < 0.003). Three of the four prostate cancer cases are the same as
those reported in Pott's paper (personal communication from Kipling to the
IARC in 1976) indicating that there is some acknowledged overlapping, and
-------
TABLE 11. MORTALITY DATA FOR CADMIUM WORKERS EXPOSED FOR MORE THAN 10 YEARS
(Potts 1965)
Length of
Cadmium
Year of Death Age Exposure (yrs) Cause of Death
1960 65 31 Auricular Fibrillation
1960 75 14 Carcinoma of Prostate
1961 65 37 Carcinoma of Prostate
1962 63 34 Bronchitis and Atheroma
1962 78 18 Bronchitis
1963 53 35 Carcinoma of Bronchus
1964 65 38 Carcinoma of Prostate
1964 59 24 Carcinomatosis
65
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therefore they cannot be said to be independent studies. No significant
difference between observed and expected was found for cancer of the bronchus,
bladderj testis, or for cancers of all sites.
Latency period, although obliquely referred to in the letter, is poorly
addressed. Furthermore, the letter states that expected cases were calculated
by "computing the number of cases of cancer which would be expected to occur
in such a group of men of known age" and by excluding the time spent in other
jobs or retirement. It is not clear how the latter was to be done; the
discussion is sketchy at best. The authors mention that "judging from work in
similar fields, fairly short exposure may be sufficient to initiate a tumor."
Whether this generalized conclusion by the authors can be extended to the
specific case of cadmium exposure and cancer remains uncertain. Failure to
allow for a sufficient latency period weakens the significance of the
findings. Because of these problems and the lack of an adequate discussion of
the derivation of expected deaths, the results, although statistically
significant, cannot be considered definitive with respect to the
carcinogenicity of cadmium.
Humperdinck (1968)
Humperdinck (1968) reported on mortality among 536 people who worked or
had worked at an alkaline dry cell battery plant during the period 1949-67 and
who had been exposed to cadmium hydroxide and "to a large extent nickel
hydroxide." Seventeen of the 536 had died, five from cancer. Of the five who
died from cancer, two died from lung cancer, one from liver cancer, one from
prostate cancer, and one from cardiac cancer. The length of exposure to
cadmium for these cases was: lung, 2.3 years and 9.3 years; liver, 3.5 years;
prostate, 6.4 years; and cardiac, 3.0 years.
66
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There was no comparison group for the 1949-67 time period. The author did
compare, however, the average of the cancer death rates for the years 1963-66
of the city where the plant is located with the average 1963-66 rate of the
whole plant and the average 1963-66 rate for the departments of the plant
where there was exposure to cadmium hydroxide. The author did not state
whether these rates were age-adjusted, race-adjusted, or sex-adjusted. No
difference among the three rates was found. No difference in the proportion
of lung cancer deaths between the city population and the plant population was
found either. The proportion of lung cancer deaths for the department where
cadmium exposure occurred was not reported.
Previously, Baader (1951) had reported on "20 to 30 males and females"
suffering from chronic cadmium poisoning at the same dry cell plant. Of this
group, Humperdinck reported that four of eight had died, one of lung cancer;
these four are included in the seventeen deaths described previously. No
mention is made of any of the other "20 or 30" workers.
Because Humperdinck found no excess cancer mortality amoung workers
exposed to cadmium when compared to the city population or to the plant
population as a whole, he concluded that there was insufficient information to
establish an association between cadmium and cancer.
A major weakness of this study is that it did not include an appropriate
comparison group for the years of the study, 1949-67. Comparison of average
death rates for the years 1963-66 among the city, plant, and cadmium
departments is not appropriate since it is not known whether all workers in
the cadmium departments in 1963-66 had experienced a latency period of
sufficient duration to have developed cancer. Second, there is no indication
that the city population or the population of the rest of the battery plant is
similar to the cadmium-exposed group in terms of race, sex, smoking habits,
67
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a,ge, etc. tP make an objective comparison. Third, had a proper comparison
group be.e.q used and an increase in cancer among workers exposed to cadmium
been demonstrated, a, possible confounding variable would have been the nickel
exposure, since nickel has previously been associated with cancer of the lung,
nasal sinus, Urge intestine, mouth, and pharynx (Fraumeni 1975).
In conclu.sion,, the design and methods of Humperdi.nck are inadequate to
assess whether an association between cadmium exposure and cancer exists for
the workers in hi? study.
Hoi den (1969)
Hqlden. (1969), in 3 letter to the Lancet, reported on 42 men exposed to
cad.m.iu.m fumes from 2 to 40 years. He stated that six of the men had been
exposed, to cpn,ce.n,tratiqn,s of cadmium in excess of 4 mg/m3 and the remainder
had been exposed to a,n average concentration of Q.I mg/m3. The author
reported, that there w,a,s one case of carcinoma of the prostate and one case of
carcinoma, of the bronchus.
No evaluation of the cancer risk of cadm/ium can be made from this article
since the Author did not report important variables such as the age, time
s,in,ce first exposure, and smoking history of each worker.
Kolopel (1975)
Ko^one'l (1976) compared the. cad,m,iiU,m. exposure of 6.4 cases of ren.al cancer
to 197 non,mal,ijga,a,n,t cfigje.s.tlve. disease controls and 72 colon, cancer controls.
According, to th.e, author, "a ca.n,ce.r contra! gro.up w,as included to, address, the
problem o,f potential! n,oin(com.p.arab,i,liiity" between ca,ses $n,d contraljS when, a
aonjca.nce.r con.trolj g,ro,UP "is usedj. Ca.se;s an,4 controls w,e,re taken, from, pati,en,ts
adm,i,tted from 19.57 to 1(96,4 to. ljto.swe.llj Pa,rk M,emo,ri;al; lastitute, 8u,ffalO:, New
6.8
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York. Cadmium exposure was assessed using data on occupational exposure,
cigarette smoking, and dietary intake. A person was considered to have
experienced occupational exposure to cadmium only if he had worked for one or
more years at a high risk job within a high risk industry. High risk
industries included electroplating, alloy-making, welding, and the manufacture
of storage batteries. A person was considered to be exposed to cadmium
through smoking if he had at least 10 "pack-years" of cigarette use during a
lifetime. Dietary exposure to cadmium was determined by applying reports of
cadmium content in foods to individual dietary histories based on a frequency
recall for a one-week period. An individual was considered exposed through
diet if his mean daily intake exceeded the third quartile, determined from the
distribution of intakes for the noncancer control group.
The author found that the odds* of developing renal cancer in
occupationally-exposed patients who smoked was 4.4 when compared to controls
who also smoked and had nonmalignant diseases of the digestive system. This
is significant at P < 0.05. The odds of developing renal cancer in patients
who were occupationally-exposed was 2.5 (P < 0.05) when compared to colon
cancer controls. The latter is not significant (0.05 < P < 0.10). Because of
the finding of a greatly increased risk2 when the effects from smoking and
occupational exposure are added together, the author concluded that the
effects of smoking and occupational exposure must be synergistic.
The odds of developing renal cancer when consideration is given to cadmium
exposure through cigarette smoking only, and separately through diet only
•••Although the author referred to relative risk in his article, it is
more correct to use the term odds ratio or estimated relative risk.
2Risk in this context is an estimated relative risk derived by use of
the odds ratio.
69
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(utilizing colon cancer controls), were 1.2 and 1.6, respectively, neither of
which were significant (0.05 < P < 0.10, two-tailed).
A major criticism of this study is the confounding exposures to other
industrial materials in the electroplating, alloy-making, welding, and
manufacture of storage batteries industries. The author had stated that renal
cancer, as a result of cadmium exposure, is biologically plausible because, of
all the organs, the kidney concentrates cadmium most. Furthermore, Kolonel
pointed out, from an earlier study by Ellman (1959), that the kidney contains
the body's highest concentration of sulfhydryl groups which are often found in
zinc-containing enzymes. Cadmium inhibition of a variety of sulfhydryl-
containing enzymes has been reported, the author notes, and this may be the
mechanism of action. The kidney concentrates many trace metals, however, and
a variety of metals are found in the industries mentioned above, including
nickel, lead, and zinc. Also, it is interesting to note that the odds ratio
for occupational exposure to cadmium is significant (P < 0.05) only when
compared to noncancer controls, but not significant (0.5 < P < 0.10) when
compared to colon cancer controls. This indicates that the renal cancer cases
may not be comparable to the noncancer cases, and selection bias may have
occurred.
Smoking has previously been associated with kidney cancer (Wynder et al.
1974, Schmauz and Cole 1974, Kahn 1966, Hirayama 1977) as well as cancers of
other sites. Although cadmium may be the carcinogen in the tobacco smoke
which causes kidney cancer, the issue is confounded by the presence of many
other carcinogens in the smoke. Although the smoke may serve only as a
possible synergist or as a carrier mechanism for cadmium exposure from other
sources, it remains to be demonstrated, as indicated above, that cadmium is
the agent of concern in smoking.
70
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In conclusion, Kolonel's study provides suggestive, but not sufficient
evidence that cadmium is a renal carcinogen. More studies, epidemiologic and
animal, are necessary to adequately address the issue.
Lemen et al. (1976)
Lemen et al. (1976) conducted an historic prospective study on 292 white
male employees of a cadmium smelter who had worked a minimum of 2 years in the
smelter sometime during the period from January 1, 1940 to December 31, 1969.
Vital status was determined for this group through January 1, 1974. Death
certificates listing the cause of death were acquired on 89 of a reported 92
deceased. Some 20 (6.8%) remained lost to follow-up. For comparison,
expected deaths by cause were generated through a modified life table
technique based upon the product of person-years times the corresponding age,
calendar time, and cause-specific mortality rates for the total United States
white male population.
The authors stated that the smelter was engaged in the production of
cadmium metal and cadmium compounds. However, they reported that some lead
was also produced. The plant ceased full-scale lead production in 1918 and
began to produce arsenic instead. In 1925, arsenic production ceased with the
start of cadmium production at that time. The authors cited a previous
industrial hygiene survey in 1947 that reported average air concentrations of
cadmium fumes ranging from 0.04 to 6.59 mg/m3 and cadmium dust at 17.23
mg/m3. The authors state that most operations had concentrations lower than
1.5 mg/m3. The present study included a 1973 industrial hygiene evaluation
of cadmium dust levels which stated that 8-hour time-weighted average (TWA)
gross concentrations of cadmium ranged infrequently up to 24 mg/m3, but
generally remained below 1 mg/m3. The authors reported that in 1973 a
71
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respirator program was in use at the plant which allegedly reduced exposure by
a factor of 10. However, respirators have not usually been found to be useful
in other settings as workers tend to remove them because of their
inconvenience. Two air measurements taken in the preweld department showed
that besides air concentrations of 74.8 and 90.3 mg/m^ of cadmium, arsenic
was measured at 0.3 and 1.1 ug/m^. This is about 1% of the cadmium
measurement. In the retort department, however, where cadmium concentration
measured 1,105 ug/m^, arsenic measured 1.4 ug/m^, which was about
l/l,000t(l that of cadmium. On the other hand, analyses of bulk samples
revealed 42.2% to 70% cadmium, 3.53% to 6% zinc, 0% to 4.3% lead, and 0.02% to
0.3% arsenic. The remaining ingredients were not identified. The authors
concluded that the exposures from the remaining ingredients were
insignificant.
A statistically significant excess of total malignant neoplasms (27
observed vs. 17.6 expected, P < 0.05) was found, as was also the category of
malignant respiratory disease (12 observed vs. 5.1 expected, P < 0.05).
Without regard to latent effects, an excess of prostatic cancer was reported
by the authors to be not significant (4 observed vs. 1.15 expected). However,
utilizing a One-tailed Poisson variable, the CAG found this observation to be
statistically 'significant (P < 0.05). After a lapse of 20 years from initial
exposure, the finding of a statistically significant excess in prostatic
cancer (4 "observed vs. 0.88 expected, P < 0.01) was even stronger.
Information concerning exposure and latency of the four prostatic cancer
cases is given in Table 12.
Of the 12 malignant respiratory cancer cases, the cell types of eight were
known. Three were squamous cell carcinoma, one was undifferentiated small
cell .carcinoma, three were anaplastic, and one was an oat cell carcinoma,
72
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TABLE 12. PROSTATE CANCER DEATHS AMONG CADMIUM SMELTER WORKERS
WITH MORE THAN 2 YEARS EXPOSURE
(Lemen et al. 1976)
Case Age
1 71
2 77
3 79
4 64
Exposure
4
13
18 -
17
Latency
32
25
31
26
Date of Death
2/26/72
3/19/68
12/10/60
4/3/51
73
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according to the authors. Unfortunately, smoking histories were not available
for any members of the cohort. Therefore, the confounding of the results due
to smoking cannot be assessed. Only in one other study by Hoi den (1969) of
cadmium-exposed workers does the author take note of a single case of
bronchogenic -ca.ncer appearing in 42 workers exposed to cadmium on which the
author did not comment. Furthermore, Lemen et al. reported the presence of
other substances in the smelter either known or suspected of causing cancer.
Arsenic, for example, has been shown to be a human lung carcinogen. Thus, any
conclusions that are made regarding the carcinogenic potential of cadmium
should be tempered with the knowledge that arsenic, lead, and .zinc were also
known to be present in the atmosphere of the smelter. Other constituents of
the processed ore are left unidentified (the percentage content of the ore
does not add up to 100%).
However, when consideration is given to the fact that the vital status of
6.8% of the study cohort remains unknown, additional causes of death found in
this group of 20 people potentially may add a prostate cancer or two to the
observed deaths. In contrast, expected deaths are overestimated by counting
person-years to the cut-off date for these same individuals. This could bias
downward the finding of an excess risk of prostate cancer and bronchogenic
cancer.
This study provides evidence that exposure to cadmium leads to a
significant excess risk of prostate cancer. The other metals known to be
present have not been shown to be associated with an elevated risk of prostate
cancer. On the other hand, the presence of arsenic in the atmosphere of the
smelter and the lack of information on smoking habits of the workers casts
doubt on the significant association of bronchogenic cancer in these workers
with cadmium exposure.
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McMichael et al. (1976)
McMichael et al. (1976), as part of an historic prospective study of
cancer mortality among rubber workers, followed 18,903 active and retired male
workers, aged 40 to 84, for a period of 10 years. They were divided into four
separate cohorts each consisting of workers from the four tire manufacturing
plants of the companies under study.
The mortality experience during the 10-year observation period was
determined from death claims filed with the companies under the group life
insurance policy in effect. In three of the four plants, workers were
included if they were employed on January 1, 1964, whereas in the fourth plant
it was January 1, 1963. About 1% were lost to follow-up according to the
authors, and death certificates listing cause of death were obtained on 98% of
the deceased. Expected deaths were calculated based on the 1968 U.S. male
race-age-specific death rates. The calculation of standard mortality ratios
(SMRs) utilizing such rates produces an underestimate of the risk. This bias,
known as the "healthy worker effect," is a consequence of the selection of the
healthiest individuals into a given workforce from the general population from
which the expected deaths were derived. Apparently, little turnover occurred
in these four plants because the former employees who switched to another
place of employment formed the group of 1% lost to follow-up during the
10-year follow-up period.
The total number of deaths equaled 5,106 for an overall SMR of 94. The
total number of cancer deaths equaled 1,014 for an SMR of 100, while that for
prostate cancer was 119 (103 observed, nonsignificant at 0.05 < P < 0.1). The
author noticed an association of prostate cancer with the compounding and
mixing areas of the four plants, work areas that entail contact with metallic
oxides (including cadmium oxides). McMichael et al. also noted an association
75
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of prostate cancer with three additional work areas (cement mixing,
janitoring, and trucking) of one particular plant after "exploratory
work-history" analyses were completed for stomach, bladder, and prostate
cancer; lymphosarcoma; and Hodgkin's disease at this plant.
The object of this study was not to single out the association of prostate
cancer with cadmium exposure as the main topic of study, but rather to examine
site-specific cancer mortality, in general, in rubber workers. Hence, the
authors found excesses in cancer mortality at a number of different sites but
did not test the significance of any. These data (McMichael et al. 1976) are
summarized in Table 13. The tests of significance were calculated by the
Carcinogen Assessment Group (CAG) using the method of Chiang (1961).
One major problem with this study is that rubber workers are potentially
exposed to numerous organic and inorganic chemicals, some of them known or
suspected carcinogens, including benzene, which is a known human carcinogen.
While cadmium is present in these factories, SMRs may be confounded by
exposures other than cadmium. Exposure levels of the many different compounds
found in these plants are not given.
A second problem with this study is the relatively short observation time
(10 years) from the beginning of the study to its cut-off. This is an
insufficient period in which to assess latent effects, and in fact, no data is
presented in which latency is considered. This cohort should be followed for
several additional years before concluding that there is no effect from
cadmium exposure.
While this paper is of interest as a basis for further studies, it is
inadequate evidence for associating or not associating cadmium with prostate
cancer.
76
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TABLE 13. STANDARD MORTALITY RATIOS (SMRs) BY SITE
(McMichael et al. 1976)
Site
Lymphatic leukemia
Stomach
All leukemias
Hodgkin's disease
Prostate
Colon
Pancreas
Bladder
Respiratory
Rectum
Brain,
central nervous system
All cancer
All causes
Observed
Deaths
20
80
46
32
103
103
57
32
252
27
14
1014
5106
SMRs
158
148
130
129
119
116
103
92
85
82
78
100
94
Probability of
Occurrence*
0.039
< 0.001
0.073
0.150
0.077
0.131
0.826
0.638
0.002
0.303
0.352
1.000
< 0.001
Taken from Chiang (1961).
77
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Kjellstrom et al. (1979)
Kjell strom et al. (1979) reported on an historic prospective cohort study
of 269 male Swedish cadmium nickel battery factory workers and 94 Swedish male
cadmium-copper alloy factory workers having more than 5 years exposure since
the factories began production. As an internal reference group, 328 alloy
factory workers not exposed to cadmium were also studied. They were employed
in the alloy factory for at least 5 years but not exposed to cadmium. It has
been estimated that the average cadmium levels for one of the two factorys
were as follows: exceeded 1 mg Cd/m3 prior to 1947; 200 ug Cd/m3 between
1962 and 1974; 50 ug Cd/m3 in 1974; decreased to below 5 ug Cd/m3 at the
time of the study. At the other factory, concentrations were in the range of
100 to 400 ug/m3 in the mid-sixties and 50 ug Cd/m3 in 1971 and after.
The battery study population was also exposed to nickel hydroxide dust.
National average age-cause-specific death rates and cancer incidence rates
were used to generate expected deaths and expected new cancer cases in the two
separate study groups. New cases of cancer were found in the battery factory
by matching the names of the 269 workers with those of the Swedish National
Cancer Register. This was not done with the alloy factory workers. With
respect to mortality in the battery factory, 43 deaths occurred between 1949
and 1975 of which eight were due to cancer. This contrasts with 67 expected
total deaths during the same period. No further breakdown is given of the
cancer deaths and no expected cancer mortality is given. However, the authors
state that there was no increase in "general" cancer mortality. Furthermore,
the total number of new cases of cancer equaled 15 during the period 1959 to
1975, while the expected number of new cases equaled 16.4 based in incidence
data provided by the Swedish National Cancer Register. A breakdown by site is
given in Table 14. Only cancer of the nasopharynx was found to be
78
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TABLE 14. EXPECTED AND OBSERVED NEW CASES OF CANCER BETWEEN 1959-1975
IN THE WHOLE GROUP OF BATTERY FACTORY WORKERS (N = 228)
(KjellStrom et al. 1979)
Site
Prostate
Lung
Kidney
Bladder
Colon-rectum
Pancreas
Nasopharynx
Other
All sites
Cancer
Expected*
1.2
1.35
0.87
1.07
2.25
0.60
0.20
9.81
16.4
Cases
Observed
2
2
0
1
5
0
2
3
15
Risk Ratios
1.67
1.48
0
0.93
2.22
0
10. Ot
0.31
0.91
*Expected deaths based on Swedish National Cancer Register.
tStatistically significantly greater than 1 (P < 0.05).
79
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significantly in excess (2 observed vs. 0.2 expected, P < 0.05) possibly due
to exposure to nickel dust.
In the alloy factory, only "preliminary" calculations of prostatic cancer
mortality were done according to the authors; cause-specific mortality or
incidence was not examined in these workers. Among 94 exposed workers, four
prostatic cancer deaths were noted versus 2.69 expected (P = 0.29). In the
reference group of 328 nonexposed workers, four prostatic cancer deaths were
noted versus 6.42 expected (P = 0.23) (Table 15). A corrected "healthy worker
effect" risk ratio was derived by dividing the risk of developing prostatic
cancer in the exposed by that of the reference group. This equals 2.4 (P =
0.087), which is still nonsignificant.
Although the results of these two studies are nonsignificant with respect
to prostatic cancer and basically inconclusive due to the small sizes of the
study groups, they do suggest a positive association of prostate cancer with
exposure to cadmium.
Two problems with this work are apparent. The first is that terminated
employees are apparently not included in any of the study cohorts unless they
are terminated by death. The resulting cohorts are healthier than the general
population because former employees, who would be expected to carry the
greatest burden of potential disease, are not represented. These employees
are represented in the general population death rates however. The net result
is to overestimate the expected deaths, thus masking a potential risk in
battery workers.
Second, in the incidence studies, cancer cases occurring in the 1950s
would be missed since the Swedish Cancer Registry didn't begin .until 1959,
thus leading to an underestimate of new cancer cases.
Another potential source of selection bias involves the exclusion of all
80
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TABLE 15. CUMULATIVE EXPECTED AND OBSERVED NUMBER OF PROSTATIC CANCER DEATHS
IN 1940-1975 AMONG ALLOY FACTORY WORKERS
(Kjellstrom et al. 1979)
Prostatic Cancer Deaths
Expected Observed Risk Ratios P value
Exposed group 2.69 4 1.49 0.29
Reference group
(N = 328) 6.42 4 0.62 0.23
81
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members with incomplete information in the factory files. However, since
there is no reason to assume differential selection of subjects for study
through this procedure, it may not be a problem.
Hoi den (1980)
Holden (1980) reported the results of a preliminary cohort mortality study
of workers employed sometime from August 1940 to August 1962 and followed to
December 31, 1979, in a British cadmium factory, while iron and brass foundry
workers in a second factory served as a control. The cadmium factory was
further subdivided into two sections for analysis purposes. One section of
the building contained the copper cadmium alloy department. In that
department, 347 men worked a minimum of 12 months while another 624 men worked
a minimum of 12 months in the remaining part of the factory. These were
dubbed "vicinity" workers by the author because they worked within the
building but not in the copper cadmium alloy department. Another 537 brass
and iron workers were employed in the second British factory for a minimum of
12 months and were reported by the author to be similar to workers of the
first factory with respect to their social and physical environment.
Industrial hygiene surveys carried out at this factory in 1953 and 1957
indicated a mean level of 70 ug/m3 (S.D. = 62 ug/m3) based on 12-hour
sampling data in the cadmium copper alloy department, while a mean level of 6
ug/m3 (S.D. = 8 ug/m3) was measured in the vicinity. Vicinity workers
were manufacturing arsenical copper, and during refining they were exposed to
silver and nickel. The author reports that those workers were exposed to
considerably less cadmium than were the cadmium copper alloy workers.
Follow-up was over 95% complete on all three subcohorts. Expected deaths were
generated based upon death rates for England and Wales in 5-year age
intervals.
82
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A statistically significant elevated risk of dying from all causes
(observed = 158, SMR = 112) was evident in cadmium copper alloy workers. This
was repeated even when malignant neoplasms were excluded (observed = 122, SMR
= 113). Mortality from neoplasms was not significant in cadmium copper alloy
workers except for leukemia (observed = 3, SMR = 441). The author contends
that the excess overall was due to pulmonary disease deaths. On the other
hand, a statistically significant elevated risk of cancer in general (observed
= 72, SMR = 120) was apparent in "vicinity" workers due chiefly to an excess
of cancer of two sites, lung cancer (observed = 36, SMR = 138) and cancer of
the prostate (observed = 8, SMR = 267). The author attributes the elevated
risk of lung cancer in these workers to the presence of other metals such as
arsenic. With respect to prostate cancer, he correctly noted the absence of a
dose-effect relationship since five of the eight prostate cancers occurred to
individuals with less than 15 years exposure; three were exposed only one year
if one assumes that "years of exposure" means years of exposure throughout the
entire plant. The author attributes only three of the prostate cancer deaths
to cadmium exposure. This last observation is somewhat strong in view of the
fact that every prostate cancer case occurred more than 15 years after initial
exposure. Latency as a factor was not considered in calculating expected
deaths so that the actual risk may be greater.
It should be noted that the workforce of any factory may be rotated many
times during its operating life. The fact that cadmium alloy workers, under
the author's definition, apparently experienced a lower risk of prostate
cancer than did "vicinity" workers may not be so unusual. It is possible that
several of the eight cases worked in the copper cadmium alloy department as
well as in the remaining part of the plant at some time during their working
career.
83
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The observed risk of cancer may actually be greater than calculated
because of the presence of the healthy worker effect. The fact that this
factor is '.operating in these .cohorts is
-------
fjg Cd/m3 air
10,000
1000
100
10
1946 1956 1966 1976 Tear
Figure 1. Concentration of cadmium in the air (ug Cd/m3) from 1949 to 1976,
Arithmetic mean of stationary and personal samples.
(Kjell Strom 1982).
85
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From T94'6 t'O 1967 there appears to ''have -been -a .-1,000-fdTd drop i'n average
exposure levels. A detailed analysts frf past arvd present ca'dmi-um expbWres in
this factory has been published '('Adams's-on 1979% The author report's t!h;at
nickel exposure levels have been at least the same as that "of 'cd'cUntum and
oftentimes as much as 10 times hi'gher.
Problems with this study are as follows; The records >of te'rfhinatetl
employees prior to 1945, a group in which the greatest ris'k is likely to be
found, are nonexistent. Almost 31% of this group hatl less than '2 years
duration of exposure to cadmium. Almost 50% of the tohbrt (3'01 wb'rkers)
received their first exposure to cadmium after 1959, which means a large
proportion of the cohort had not been followed even 20 years, an'd thus, there
was probably not enough time to evaluate a cancer risk. Furthermore^ smbkifig
information was not available for the older workers, a subgroup of the cohort
in which the greatest cancer risk is likely to be found* This conceivably
could have been the reason why no results evaluating the effects of smbkifig
are presented in the study, although a detailed data-base is reported By the
author to be in the development stages as an extension of the study for future'
follow-ups. Additionally, the author reports that for cancer 6f the p'rostatei
the rate ratio increases with increasing latency and increasing dose* He
reports rate ratios of 1.27, 1.33, and 1.55 corresponding to exposure
categories > 0 years, > 1 year, and > 5 years. In the > 1 y£eif exposure
duration category, prostate mortality rate ratios of l«33j l«44j arid l:8lj
corresponding to latency periods of 1, 10, and 20 years* respectively,- are
given. However, since no tabular data is presented* it is7 n~bt pbssiBle to
determine how the four observed prostate cartcer deaths <¥re distributed into
the subcategories alluded to by the author. The autlfbr did fibCe1 that the
numbers were too small for the detection of statistically s'igriificant
differences.
-------
Kjellstrom repeated the above exercise for urogenital diseases. For those
with more than 20 years exposure and 20 years latency, 4 observed urogenital
deaths occurred versus 0.93 expected (P < 0.05). This type of disease was
exclusively nephritis of the kidney. Again it is difficult to conclude
without evaluation that cadmium exposure was related, although the author
himself states that it is "clear that cadmium exposure increases mortality
from kidney diseases" after high exposure intensity and long duration of
exposure. The author notes a tendency in his data for a slightly increased
nonsignificant risk of prostate cancer from exposure to cadmium.
In addition to the main study discussed above, Kjellstrom included
discussions of four Japanese studies [Japanese Public Health Association
(JPHA) 1979; Shigematsu et al. 1981; Nogawa et al. 1978, 1981] and a
description of another planned ecological study by himself and the Department
of Epidemiology at the University of Tokyo for which only preliminary findings
are available. In this latter ecological study age-standardized death rates
in cadmium polluted areas for persons 35 to 84 years of age were compared with
the respective rates in non-cadmium polluted areas. Preliminary data,
according to Kjellstrom, suggested a nonsignificant tendency toward higher
mortality rates in cadmium-exposed areas compared to control areas (an
age-adjusted mortality rate of 176 per 1,000 in cadmium-exposed areas versus
139 in the control areas). Prostate cancer and kidney disease mortality rates
were also higher in the cadmium-exposed areas, but most of the prostate cancer
mortality excess occurred in individuals 85 and over. No significant tests
were done. This analysis was reported by Kjellstrom as tending to support the
hypothesis of a cadmium effect, but "definite conclusions have to be left
until all the analysis are completed."
87
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The first of these (JPHA 1979, Shigematsu et al. 1981) was an analysis of
cadmium exposure and mortality in the general environment. According to the
author, people in many areas of Japan endure high cadmium exposure up to
several micrograms/day from consumption of contaminated rice. In each of four
"prefectures" of Japan, age standardized mortality rates were calculated in a
cadmium-exposed area and compared to those calculated in a nonexposed
reference area of the same prefecture. It was found that cancer mortality
rates were generally about the same in the non-polluted areas compared to the
polluted areas, but no significance tests were run. Only kidney disease death
rates and diabetes death rates were found to be lower in non-cadmium polluted
areas compared to cadmium polluted areas. With respect to prostate cancer
mortality, two of the polluted areas had higher death rates than did their
controls, while in two others the reverse was true according to the author.
The author noted that the two prefectures with higher death rates of prostatic
cancer compared to their controls are the areas with the "highest likely
cadmium exposure to the population." The remaining two prefectures had lower
cadmium exposures. The former two prefectures tended to have higher rates of
mortality from kidney disease and hyperplasia of the prostate as well. Again,
this is an ecological study and thus can only be considered as suggestive of
areas for future research.
The second Japanese study (Nogawa et al. 1978) found that in 2,689 men and
women over age 50, the village-specific prevalence of low molecular weight
proteinuria (LMWP) increased with an increase in the village-specific average
cadmium concentration in rice. LMWP was measured by urinary retinol binding
protein. Since this ecological study more than likely includes more persons
who have never been exposed to cadmium in rice as well as prevalence rates
that include persons with prior-existing conditions, possibly introduced long
88
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before they were exposed to relatively high concentrations of cadmium in rice,
the positive association noted by the author should not be construed to
signify a causal association.
Nogawa et al. (1981) conducted a mortality study of the 81 men and 124
women identified in the earlier study as having LMWP. They were followed from
1974 to 1979 as well as the remaining men and women not found to have LMWP.
He found a nonsignificant (P < 0.05) twofold excess risk of death to men with
LMWP and a nonsignificant 1.2-fold excess risk of death to women with LMWP.
Mortality rates were based on 27 deaths of males with LMWP and 30 deaths of
females with LMWP. A positive association of LMWP with heart disease,
cerebrovascular disease, nephritis, and nephrosis was noted. This association
raises the spector of a possible confounding effect of hypertension with LMWP.
If hypertension is a cause of LMWP, the higher mortality in individuals with
LMWP described above may have been a consequence of the hypertension and not
necessarily that of LMWP brought on by cadmium exposure as suggested by the
author. Thus, the correlation with LMWP may be spurious. Hence, conclusions
drawn from this study regarding an association of higher mortality with
cadmium exposure must be characterized as certainly no more than suggestive.
Kazantzis and Armstrong (1982 unpublished)
In a recently completed but unpublished cohort mortality study of 6,995
male cadmium workers with at least one year service in one of five British
industries (primary producers, copper-cadmium alloy, silver-cadmium alloy,
pigments, and plastic stabilizers), the only finding of any statistical
significance was that of bronchitis deaths occurring to employees classified
as having "high" exposure to cadmium (12 observed versus 2.5 exposed,
P < 0.01). The excess of bronchitis (chronic respiratory disease) occurred
89
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among workers exposed mostly to Cadmium fumes* although the author "reports a
nonstatistically significant excess among those exposed to Cadmium dust only-
no data is provided to support this finding. Workers wha Were borfi after 1940
or who were exposed after 1970 were excluded from the study* The remaining
cohort was followed to 1979 when it was reported that 1*902 deaths had
occurred versus 2,056 expected (SMR = 93), a shortfall of deaths attributed
most likely to the "healthy worker effect*" No information is provided
concerning the degree of success of the follow-up* Standard mortality rates
were presumably based upon British death rates* the entire cohort was divided
into three categories* "high*" "medium/ and "low*" according to severity of
exposure* some 3% fell into the high category 17% were in the medium
category* and 80% were in the low category* according to the author* These
categories of exposure were devised by industrial hygienist§ based upon
knowledge of past and present processes and working procedures*, and upon
biological and environmental monitoring results where available* The number
of persons who fell into the highly exposed category (certainly no more than
210 persons total) was a rather small number upon which to estimate a Msk of
prostate cancer*
No other excessive risks of death by cause (including cancer) were present
according to the authors* Unfortunately, only one table of tabular results of
cause«specific mortality are available in this Study by exposure Category* and
this table has no breakdown by time Since onset Of exposure* Hence* no
consideration of latent effects are given in the Study* Therefore* it is
impossible to assess the risk Of cancer in just that group followed for more
than 15 or 20 years after Initial exposure to cadmium*
It Should be noted that this study 1§ not independent of an iariier §tudy
by Sorahan (1981, unpublished). The author acknowledges the inclusion of
-------
3,026 male (43% of the cohort) nickel-cadmium workers who are the subject of a
cohort mortality study by Sorahan. (The Sorahan study is critically reviewed
following this discussion.) Sorahan also found essentially negative results.
Kazantzis himself admits that with respect to both kidney disease and
prostatic cancer, the "numbers are too small to exclude an increased risk
emerging with time," although he states that if such a risk existed, it "could
only be small in terms of extra deaths expected." It is difficult to see how
he could make the latter statement since he made no effort to sort out latent
effects. This study is, in the opinion of the CAG, inconclusive in terms of
finding no risk of prostate cancer or cancer of other sites.
Sorahan (1981)
Sorahan (1981), in a letter to Dr. Roy Albert (dated 2/18/81), related the
findings of an historic prospective mortality study of 3,026 nickel-cadmium
battery workers employed during the period from 1946 to June 30, 1980, who had
worked at least one month. A subset of these same workers was studied earlier
by Kipling and Waterhouse (1967), according to the authors. The cohort was
derived from workers employed in two separate factories that amalgamated in
1974. The earliest mention of cadmium in the air was reported in 1949. In
the platemaking assembly shops, the range was 0.6 to 2.8 mg/m3, but in the
"negative active material" department where cadmium oxide powder was prepared,
the levels were reported to be "considerably higher." No numbers are
provided. Extensive local exhaust ventilation was installed in 1950, and as a
consequence, cadmium in the air was reduced to below 0.5 mg/m3 in most parts
of the factories. By 1967, when a new platemaking department was built, the
cadmium oxide dust was reduced to less than the threshold limit value (TLV) of
0.2 mg/m3. From 1975 on, the plant levels were within the current TLV of
0.05 mg/m3.
91
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For the purposes of the analysis the author excluded 566 female employees
and divided the remaining all male cohort into two subcohorts: 1,066 who were
first employed before the amalgamation (1947) and 1,494 who were first
employed after the amalgamation.
Standard mortality ratios (SMRs) were computed. Expected deaths were
generated assuming that the general population rates were operating on the
study cohorts. Overall, the observed numbers of deaths, all causes combined,
were slightly less than expected (observed = 591, SMR = 97). With respect to
all forms of cancer, there was virtually no difference between observed and
expected deaths (observed = 152, SMR = 100). On the other hand, a deficit of
cancer deaths occurred to the subcohort of employees employed prior to the
amalgamation (observed = 80, SMR = 84). But, in those employed for the first
time after the amalgamation, a significantly increased risk of total cancer
deaths was apparent (observed = 72, SMR = 129, P < 0.05). This increased risk
was attributed by the author to a survivor population effect in the latter
subcohort. However, in both subcohorts, before 1947 and after 1947, an
excessive but nonsignificant risk of cancer of the bronchus was evident
(observed = 45, SMR = 114; observed = 32, SMR = 134, respectively). No
significant excess risk of prostate cancer occurred in either group (observed
= 4, expected = 4.1; observed = 3, expected = 1.9, respectively). Even after
consideration was given to the time since first employed, no significant
excessive risk was seen in workers 15 years after first employment in any of
the following cause of death categories: all causes combined, all cancers
combined, cancer of the bronchus, and cancer of the prostate. Nor was there a
significant risk of cancer to employees having left employment before
completion of one full year of employment and one to 14 years of employment.
In an apparant inconsistency, Sorahan reported no prostate cancer deaths in
92
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the length of employment category 1 to 14 years; yet in the earlier study by
Potts (presumably based upon the same data), three employees whose deaths were
attributed to prostate cancer were exposed to cadmium for 14, 37, and 38
years, respectively. Given the cohort definition of Sorahan, at least one of
these three cases should be included.
Furthermore, in Sorahan's analysis of latent effects, implicit is the
assumption that only terminated employees were considered in the analysis.
Presumably, person-years of individuals still employed with the company were
not enumerated. Only if the individual left the employment of the company
(through death or other cause) would his person-years be counted. This has
the effect of reducing the expected deaths by the non-inclusion of
person-years of individuals who are at risk of death but are still alive and
working. This would tend to bias the SMRs upward.
On the other hand, without regard to latent effects, the study suffers
from a type of selection bias known as the "healthy worker effect" brought
about by comparison of the observed deaths with expected deaths based upon the
mortality rates of England and Wales, thus biasing the result toward the null.
Additionally, some 82 persons remain untraced with respect to their vital
status. The non-inclusion of the causes of death of the decreased members of
this subgroup would tend to bias the SMRs downward.
More seriously, tabular data presented classifies the cohort into two
categories of exposure: "exposed" and "non-exposed," although in the
"population section" the author described all the jobs in the factories in
terms of "high," "slight," and "minimal" exposure to cadmium. It cannot be
determined from the text how the three latter categories were reconstituted as
"exposed" and "non-exposed" for the purposes of presenting the findings in
tabular form. The author's treatment of the subject suggests that some
93
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portion of the study population received little or no exposure to cadmium, and
if this is so, they should have been excluded from the study group, or else a
better definition of intensity of exposure should have been utilized to
present the tabular findings. It would have been more appropriate and
informative to present the tabular findings in terms of "high, slight, and
minimally" exposed groups. Otherwise, the cancer incidence cannot be
adequately related to cadmium exposure.
Overall, this paper presented no evidence of an increased risk of prostate
cancer in cadmium-exposed workers. However, since many questions remain
unanswered concerning the structure of the study, it cannot be said to provide
conclusive evidence that cadimum is not carcinogenic.
Summa ry
Of the 12 epidemiologic studies of cancer in cadmium-exposed people
reviewed by the CAG, three (Kipling and Waterhouse 1967, Lemen et al. 1976,
Holden 1980) provide evidence of a significant association with prostate
cancer. Although the numbers are very small, four cases in each study, a
statistically significant positive association was observed (P < 0.05) in all
three studies.
Three other studies (Potts 1965, McMichael et al. 1976, Kjellstrom 1978)
provide the suggestion of a risk of prostate cancer (although statistically
nonsignificant) with exposure to cadmium. Potts's clinical study (which
provided three of the four prostate cancer cases in the Kipling and Waterhouse
study) suffers from a lack of a comparison group. The McMichael et al. study
of mortality in rubber workers suggested a positive correlation of exposure to
cadmium with prostate cancer. However, the lack of significance even with
large numbers (observed = 103, SMR = 119) and the concomitant exposure to
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other known and suspected carcinogens make this study equivocal. Kjellstrom's
"corrected healthy worker effect" risk ratio of 2.4 is still nonsignificant
because of small numbers, although it approaches borderline significance at
P < 0.09, offering the suggestion of a possible association of prostate cancer
with cadmium exposure.
Two other studies (Humperdinck 1968, Holden 1969) did not report evidence
of an association of prostate cancer with cadmium exposure chiefly because the
comparison population was either inadequate to assess a risk (Humperdinck) or
absent entirely (Holden). The ninth study (Kolonel 1976) did not evaluate the
risk of prostate cancer in cadmium-exposed people but did evaluate the risk of
renal carcinoma and cadmium exposure.
An update by Kjellstrom (1982) of his earlier 1979 study again failed to
demonstrate a significant risk of cancer of any site. One of the failings of
this study was that members of his cohort were not observed long enough to
evaluate latent effects. More than half of his cohort received no exposure to
cadmium prior to 1959 and thus could not have been followed for even 20 years.
The study by Kazantzis and Armstrong (1982) of 6,994 workers, which
included the entire Sorahan cohort, also failed to demonstrate an increased
risk of cancer of any site. This study combines cohorts from several
different plants each with their own unique exposure history, none of which
are necessarily comparable. The main failing of this study, however, is the
lack of consideration of latent effects.
Kolonel (1976) found a statistically significant elevated risk of renal
cancer in persons occupationally exposed to cadmium, and an even greater risk
in occupationally exposed people who smoke, thus raising the possibility of a
synergism. The chance of selection bias and concurrent occupational exposures
to nickel, lead, zinc, and a variety of metals minimizes the importance of the
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findings. None of the other human epidemiologic studies reviewed have
demonstrated an elevated risk for the development of renal cancer and exposure
to cadmium.
Lemen et al. (1976), in addition to a significant risk of prostate cancer,
found a statistically significant elevated risk of bronchogenic carcinoma in
cadmium smelter workers (12 observed vs. 5.1 expected, P < 0.05). But these
same workers were simultaneously exposed to low levels of arsenic, lead, and
zinc. Arsenic is a known pulmonary carcinogen and thus, confounding cannot be
eliminated. Furthermore, smoking habits among workers in the rather small
study cohort are not known.
Holden (1980) also noted a statistically significant elevated risk of
bronchogenic carcinoma in British cadmium workers who worked in the "vicinity"
of the cadmium copper alloy department. However, the author attributed the
excess risk to the presence of other metals, such as arsenic.
The Sorahan (1981) study, which reportedly includes the same group of
workers evaluated by Kipling and Waterhouse, is negative for an increased risk
of prostate cancer. Separation of the study group according to "exposed"
versus "unexposed" is unclear. Since the control group constitutes the
population of England and Wales, the "healthy work effect" may bias the result
downward. Furthermore, although it is not clear whether the first four tables
include only terminated employees, the rest of the tables explicitly state
that is the case. If only terminated employees were analyzed, the SMRs would
be biased upward. For these and other reasons, the study is inconclusive to
determine an association between cadmium exposure and cancer.
It might be of interest to estimate the likelihood that positive results
in the five independent study populations discussed above could occur by
chance alone under the hypothesis that cadmium had no effect on deaths due to
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prostate cancer. To test this hypothesis, a formal statistical test of the
joint information contained in all the studies can be made in the following
manner. Often the same hypothesis is tested in a series of independent
studies where an estimate of the probability of rejecting a true hypothesis,
the "P" value, is calculated for each study.
The results of any one individual study may be only marginally
significant, but the series of consistent results among the studies gives the
appearance of much stronger evidence than any one study. It would be
desirable to combine the results of the studies in some manner to obtain some
overall statement about the "P" value of the series of experiments in order to
obtain a joint result. However, often due to differences in how the data were
analyzed and obtained in the studies, this approach was not possible. Sir
R.A. Fisher developed a simple, straightforward method to analyze data in this
situation where the only information needed is the "P" value for each of the
studies. He showed that the statistic
m
S = -2 E loge Pj
is distributed as an X2 value with 2m degrees of freedom, where Pj is the
"P" value for the jth study, and m is the total number of studies. However,
caution must be taken in utilizing this approach to be sure that all
methodologically acceptable studies that have been used to test the required
hypothesis are included in the series and that all of these studies are
independent of each other. If studies that did not produce a "significant"
result are not reported in the literature, or one study uses part or all of
the actual data from another study, the proposed use of the Fisher method
could lead to a biased result.
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In the present situation a total of five independent studies have been
conducted testing the hypothesis of no association between exposure to
airborne cadmium and prostate cancer. These studies, along with the resulting
"P" valueSj are as follows:
Study
Kazantzis and Armstrong (1982)
Lemeh et al. (1976)
McMichael et al. (1976)
Kjellstrom et al. (1978)
Hoi den (1980)
Source of Exposure "P" Value
Combined-Primary Production
Cadmium Copper Alloy
Cadmium Silver Alloy
Plastics Stabilizer
Pigments 0.52
Cadmium Smelter 0.013
Rubber Plants 0.077
Cadmium Nickel Battery Plant
Cadmium Copper Alloy Plant 0.375
Cadmium-Copper Alloy 0.010
Applying Fisher's method, we find that the statistic
S = 2{LN {'0*52) + LN {0.013) + LN (0*077) + LN {0.375) + IN {0.01-0)3 = 26.28
has a X^ distribution with 2 x 5 = 10 degress of freedom and a joint "P"
value of '< 0.01 associated with it.
Although, this calculation indicates that the association of prostate
cancer and cadmium exposure in these five studies is not likely to be 'due to
chance alone, a definite statement about caus'ation cannot be .made considering
the limitations of these studies.
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Conclusion
Although problems exist in the design and methodology of all 12
epidemiologic studies evaluating the association of cadmium exposure with
prostate cancer, six of them are nevertheless consistent in their findings of
an elevated risk of prostate cancer. But only in three are the data
statistically significant. The remaining six studies are inadequate to
evaluate this risk. Because of small sample sizes and a negative
dose-response curve in one of the significant studies, the evidence is limited
that cadmium is a human carcinogen.
In two studies (Lemen et al. 1976, Holden 1980) where a significant risk
of bronchogenic cancer was apparent, there is a suggestion of an association
of exposure to cadmium with bronchogenic carcinoma. However, the confounding
effects of exposure to arsenic in both these studies makes it questionable
that the association is real with respect to renal cancer, and there is
sufficient reason to doubt the finding of a significant positive association
in the only study that demonstrated such a risk (Kolonel 1976).
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QUANTITATIVE ESTIMATION
INTRODUCTION
This quantitative section deals with the unit risk for cadmium in air and
the potency of cadmium relative to other carcinogens that the CAG has
evaluated. The unit risk estimate for an air pollutant is defined as the
lifetime cancer risk occurring in a hypothetical population in which all
individuals are exposed continuously from birth throughout their lifetimes to
a concentration of 1 ug/m3 of the agent in the air that they breathe. This
calculation is done to estimate in quantitative terms the impact of the agent
as a carcinogen. Unit risk estimates are used for two purposes: 1) to
compare the carcinogenic potency of several agents with each other, and 2) to
give a crude indication of the population risk which might be associated with
air or water exposure to these agents, if the actual exposures are known.
The data used for the quantitative estimate are taken from one or both of
the following: 1) lifetime animal studies, and 2) human studies where excess
cancer risk has been associated with exposure to the agent. In animal studies
it is assumed, unless evidence exists to the contrary, that if a carcinogenic
response occurs at the dose levels used in the study, then responses will also
occur at all lower doses with an incidence determined by the extrapolation
model.
There is no solid scientific basis for any mathematical extrapolation
model that relates carcinogen exposure to cancer risks at the extremely low
concentrations including the unit concentration defined above. For practical
reasons such low levels of risk cannot be measured directly either by animal
experiments or by epidemiologic studies. We must, therefore, depend on our
current understanding of the mechanisms of carcinogenesis for guidance as to
which risk model to use. At the present time the dominant view of the
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carcinogenic process involves the concept that most agents that cause cancer
also cause irreversible damage to DNA. This position is reflected by the fact
that a very large proportion of agents that cause cancer are also mutagenic.
There is reason to expect that the quantal type of biological response, which
is characteristic of mutagenesis, is associated with a linear nonthreshold
dose-response relationship. Indeed, there is substantial evidence from
mutagenicity studies with both ionizing radiation and a wide variety of
chemicals that this type of dose-response model is the appropriate one to use.
This is particularly true at the lower end of the dose-response curve; at
higher doses, there can be an upward curvature probably reflecting the effects
of multistage processes on the mutagenic response. The linear nonthreshold
dose-response relationship is also consistent with the relatively few
epidemiologic studies of cancer responses to specific agents that contain
enough information to make the evaluation possible (e.g., radiation-induced
breast and thyroid cancer, skin cancer induced by arsenic in drinking water,
liver cancer induced by aflatoxin in the diet). Some supporting evidence also
exists from animal experiments (e.g., the initiation stage of the two-stage
carcinogenesis model in rat liver and mouse skin). Linearity is also
supported when the mode of action of the carcinogen in question is similar to
that of the background cancer occurrence in the exposed population.
Because it has the best, albeit limited, scientific basis of any of the
current mathematical extrapolation models, a linear nonthreshold model has
been adopted as the primary basis for estimating risk at low levels of
exposure. The risk estimates made with this model should be regarded as
conservative, representing the most plausible upper-limit for the risk, i.e.,
the true risk is not likely to be higher than the estimate, but it could be
lower.
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For several reasons, the unit risk estimate based on animal bioassays is
only an approximate indication of the absolute risk in populations exposed to
known carcinogen concentrations. First, there are important species
differences in uptake, metabolism, and organ distribution of carcinogens, as
well as species differences in target site susceptibility, immunological
responses, hormone function, dietary factors, and disease. Second, the
concept of equivalent doses for humans compared to animals on a mg/surface
area basis is virtually without experimental verification regarding
carcinogenic response. Finally, human populations are variable with respect
to genetic constitution and diet, living environment, activity patterns, and
other cultural factors.
The unit risk estimate can give a rough indication of the relative potency
of a given agent compared with other carcinogens. The comparative potency of
different agents is more reliable when the comparison is based on studies in
the same test species, strain, and sex, and by the same route of exposure,
preferably by inhalation.
The quantitative aspect of the carcinogen risk assessment is included here
because it may be of use in the regulatory decision-making process, e.g., in
setting regulatory priorities, evaluating the adequacy of technology-based
controls, etc. However, it should be recognized that the estimation of cancer
risks to humans at low levels of exposure is uncertain. At best, the linear
extrapolation model used here provides a rough, but plausible estimate of the
upper-limit of risk; i.e., it is not likely that the true risk would be much
more than the estimated risk, but it could very well be considerably lower.
The risk estimates presented in subsequent sections should not be regarded as
an accurate representation of the true cancer risks even when the exposures
are accurately defined. The estimates presented may be factored into
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regulatory decisions to the extent that the concept of upper risk limits is
found to be useful.
The mathematical formulation chosen to describe the linear nonthreshold
dose-response relationship at low doses is the linearized multistage model.
This model employs enough arbitrary constants to be able to fit almost any
monotonically increasing dose-response data, and it incorporates a procedure
for estimating the largest possible linear slope (in the 95% confidence limit
sense) at low extrapolated doses that is consistent with the data at all dose
levels of the experiment.
In addition to the multistage model currently used by the CAG for low-dose
extrapolation (a detailed description of the procedure is given in Appendix
B), three more models, the probit, Weibull, and one-hit, are employed for the
purpose of comparison. These models cover almost the entire spectrum of risk
estimates that could be generated from the existing mathematical extrapolation
models. These models are generally statistical in character and are not
derived from the biological arguments, except for the multistage model which
has been used to support the somatic mutation hypothesis of carcinogenesis
(Armitage and Doll 1954, Whittemore 1978, Whittemore and Keller 1978).
The main differences among these models is the rate at which the response
function P(d) approaches zero or P(0) as dose d decreases. For instance, the
probit model would usually predict a smaller risk at low-doses than the
multistage model because of the difference of the decreasing rate in the
low-dose region. However, it should be noted that one could always
artificially make the multistage model have the same (or even greater) rate of
decrease as the probit model, by making some dose transformation and/or by
assuming that some of the parameters in the multistage model are zero. This,
of course, is not reasonable without knowing, a priori, what the carcinogenic
process for the agent is.
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Although the multistage model appears to be the most reasonable or at
least the most general model to use, the point estimate generated from this
model is of limited value because the issue remains as to what the shape of
the dose-response curve is beyond the experimental exposure levels.
Furthermore, the point estimates at low doses extrapolated beyond the
experimental dose could be extremely unstable and could differ drastically,
depending on where the lowest experimental dose is. Since the upper-bound
estimates at low doses from the multistage model are relatively more stable
than the point estimates, we suggest that the upper-bound estimate of the risk
(or the lower-bound estimates of the dose) be used in evaluating the
carcinogenic potency of a suspect carcinogen. The upper-bound estimate can be
taken as a plausible estimate if the true dose-response curve is actually
linear at low doses. The upper-bound estimate means that the risks are not
likely to be higher but could be lower if the compound has a concave upward
dose-response curve or a threshold at low doses. Another reason one can, at
best, obtain an upper-bound estimate of the risk when animal data are used is
that the estimated risk is only a conditional probability under the assumption
that an animal carcinogen is also a human carcinogen. Therefore, in reality,
the actual risk could range from a value near zero to an upper-bound estimate.
PROCEDURES FOR DETERMINING THE CARCINOGENIC POTENCY
Description of the Low-Dose Animal Extrapolation Model
Let P(d) represent the lifetime risk (probability) of cancer at dose d.
The multistage model has the form
P(d) = 1 - exp [-(q0 + qxd + q^2 + ...
where
q >.0, i = 0, 1, 2 k
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Equivalently,
Pt(d) = 1 - exp
where
Pt(d) P(d) - P(o)
tV - 1 - P(o)
is the extra risk over background rate at dose d or the effect of treatment.
The point estimate of the coefficents q., i = 0, 1, 2, ..., k, and
consequently the extra risk function Pt(d) at any given dose d, is
calculated by maximizing the likelihood function of the data.
The point estimate and the 95% upper confidence limit of the extra risk
Pt(d) are calculated by using the computer program GLOBAL79 developed by
Crump and Watson (1979). At low doses, upper 95% confidence limits on the
extra risk and lower 95% confidence limits on the dose producing a given risk
are determined from a 95% upper confidence limit, q*, on parameter
q... Whenever q1 > 0, at low doses the extra risk Pt(d) has
approximately the form Pt(d) = q.^ x d. Therefore, q^ x d is a 95%
upper confidence limit on the extra risk, and R/q* is a 95% lower
confidence limit on the dose producing an extra risk of R. Let LQ be the
maximum value of the log-likelihood function. The upper-limit q. is
calculated by increasing q- to a value q* such that when the
log-likelihood is remaximized subject to this fixed value q* for the
linear coefficient, the resulting maximum value of the log-likelihood Lj
satifies the equation
2 (L0 - L!) = 2.70554
where 2.70554 is the cumulative 90% point of the chi-square distribution with
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one degree of freedom, which corresponds to a 95% upper-limit (one-sided).
This approach of computing the upper confidence limit for the extra risk
Pt(d) is an improvement on the Crump et al . (1977) model. The upper
confidence limit for the extra risk calculated at low doses is always linear.
This is conceptually consistent with the linear nonthreshold concept discussed
earlier. The slope, q* is taken as an upper bound of the potency of the
chemical in inducing cancer at low doses. (In the section calculating the
risk estimates, P^(d) will be abbreviated as P). In fitting the
dose-response model, the number of terms in the polynomial is chosen equal to
(h-1), where h is the number of dose groups in the experiment including the
control group.
Whenever the multistage model does not fit the data sufficiently well,
data at the highest dose is deleted and the model is refit to the rest of the
data. This is continued until an acceptable fit to the data is obtained. To
determine whether or not a fit is acceptable, the chi-square statistic
is calculated where N-j is the number of animals in the in dose group,
Xj is the number of animals in the ith dose group with a tumor response,
P.J is the probability of a response in the itn dose group estimated by
fitting the multistage model to the data, and h is the number of remaining
groups. The fit is determined to be unacceptable whenever X^ is larger than
the cumulative 99% point of the chi-square distribution with f degrees of
freedom, where f equals the number of dose groups minus the number of non-zero
multistage coefficients.
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Selection of Data--
For some chemicals, several studies in different animal species, strains,
and sexes, each run at several doses and different routes of exposure, are
available. A choice must be made as to which of the data sets from several
studies to use in the model. It may also be appropriate to correct for
metabolism differences between species and absorption factors via different
routes of administration. The procedures used in evaluating these data are
consistent with the approach of making a maximum-likely risk estimate. They
are listed as follows.
. 1. The tumor incidence data are separated according to organ sites or
tumor types. The set of data (i.e., dose and tumor incidence) used in the
model is the set where the incidence is statistically significantly higher
than the control for at least one test dose level and/or where the tumor
incidence rate shows a statistically significant trend with respect to dose
level. The data set that gives the highest estimate of the lifetime
carcinogenic risk, q*, is selected in most cases. However, efforts are
made to exclude data sets that produce spuriously high risk estimates because
of a small number of animals. That is, if two sets of data show a similar
dose-response relationship, and one has a very small sample size, the set of
data that has the larger sample size is selected for calculating the
carcinogenic potency.
2. If there are two or more data sets of comparable size that are
identical with respect to species, strain, sex, and tumor sites, the geometric
mean of q* estimated from each of these data sets, is used for risk
assessment. The geometric mean of numbers Aj, A2, ..., A,,, is defined as
x A2 x ... x
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3. If two or more significant tumor sites are observed in the same study,
and if the data are available, the number of animals with at least one of the
specific tumor sites under consideration is used as incidence data in the
model .
Calculation of Human Equivalent Dosages from Animal Data —
Following the suggestion of Mantel and Schneiderman (1975), we assume that
mg/surface area/day is an equivalent dose between species. Since, to a close
approximation, the surface area is proportional to the 2/3rds power of the
weight as would be the case for a perfect sphere, the exposure in mg/day per
2/3rds power of the weight is also considered to be equivalent exposure. In
an animal experiment this equivalent dose is computed in the following manner.
Let
Le = duration of experiment
le = duration of exposure
m = average dose per day in mg during administration of the agent
(i .e. , during le), and
W = average weight of the experimental animal
Then, the lifetime average exposure is
,
Le x W2/3
Inhalation—When exposure is via inhalation, the calculation of dose can
be considered for two cases where 1) the carcinogenic agent is either a
completely water-soluble gas or an aerosol, and is absorbed proportionally to
the amount of air breathed in, and 2) where the carcinogen is a poorly
water-soluble gas which reaches an equilibrium between the air breathed and
the body compartments. After equilibrium is reached, the rate of absorption
of these agents is expected to be proportional to the metabolic rate, which in
turn is proportional to the rate of oxygen consumption, which in turn is a
function of surface area.
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Case I—Agents that are in the form of participate matter or virtually
completely absorbed gases, such as sulfur dioxide, can reasonably be expected
to be absorbed proportional to the breathing rate. In this case the exposure
in mg/day maybe expressed as
m = I x v x r
where I = inhalation rate per day in m3, v = mg/m3 of the agent in air,
and r = the absorption fraction.
The inhalation rates, I, for various species can be calculated from the
observations (FASEB 1974) that 25 g mice breathe 34.5 liters/day and 113 g
rats breathe 105 liters/day. For mice and rats of other weights, W (in
kilograms), the surface area proportionality can be used to find breathing
rates in m3/day as follows:
For mice, I = 0.0345 (W/0.025)2/3 m3/day
For rats, I = 0.105 (W/0.113)2/3 m3/day
For humans, the values of 20 m3/day is adopted as a standard breathing
rate. The equivalent exposure in mg/W2/3 for these agents can be derived
from the air intake data in a way analogous to the food intake data. The
empirical factors for the air intake per kg per day, i = I/W, based upon the
previous stated relationships, are tabulated as follows:
Species W i = I/W
Man 70 0.29
Rats 0.35 0.64
Mice 0.03 1.3
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Therefore, for participates or completely absorbed gases, the equivalent
exposure in mg/W^/3 is
. m Ivr iWvr
d =
In the absence of experimental information or a sound theoretical argument to
the contrary, the fraction absorbed, r, is assumed to be the same for all
species.
Case 2--The dose in mg/day of partially soluble vapors is proportional to
the 02 consumption, which in turn is proportional to W2/3 anc| js aiso
proportional to the solubility of the gas in body fluids, which can be
expressed as an absorption coefficient, r, for the gas. Therefore, expressing
the 62 consumption as 02 = kW2/3, where k is a constant independent of
species, it follows that
m
= k W2/3vr
or
d = -^577 = kvr
w2/3
As with Case 1, in the absence of experimental information or a sound
theoretical argument to the contrary, the absorption fraction, r, is assumed
to be the same for all species. Therefore, for these substances a certain
concentration in ppm or ug/m3 in experimental animals is equivalent to the
same concentration in humans. This is supported by the observation that the
minimum alveolar concentration necessary to produce a given "stage" of
anesthesia is similar in man and animals (Dripps et al. 1977). When the
animals are exposed via the oral route and human exposure is via inhalation or
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vice-versa, the assumption is made, unless there is pharmacokinetic evidence
to the contrary, that absorption is equal by either exposure route.
Calculation of the Unit Risk from Animal Studies—
The 95% upper-limit risk associated with d mg/kg2/3/day is obtained from
GLOBAL79 and, for most cases of interest to risk assessment, can be adequately
approximated by P(d) = 1 - exp-(q*d). A "unit risk" in units X is simply
the risk corresponding to an exposure of X = 1. To estimate this value we
simply find the number of mg/kg2/3/day corresponding to one unit of X and
substitute this value into the above relationship. Thus, for example, if X is
in units of ug/m3 in the air, we have that for case 1, d = 0.29 x 701/3 x
10~3 mg/kg2/3/day, and for case 2, d = 1, when ug/m3 is the unit used to
compute parameters in animal experiments.
If exposures are given in terms of ppm in air, we may simply use the fact
that
1 ppm = 1.2 x molecular weight (gas) mg/m3
molecular weight (air)
Note, an equivalent method of calculating unit risk would be to use mg/kg for
the animal exposures and then increase the jth polynomial coefficient by an
amount
(Wn/wa)J/3 j = 1, 2, .... k
and use mg/kg equivalents for the unit risk values.
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Model for Estimation of Unit Risk Based on Human Data
If human epidemiologic studies and sufficiently valid exposure information
are available for the compound, they are always used in some way. If they
show a carcinogenic effect, the data are analyzed to give an estimate of the
linear dependence of cancer rates on lifetime average dose. If they show no
carcinogenic effect when positive animal evidence is available, then it is
assumed that a risk does exist, but it is smaller than could have been
observed in the epidemiologic study, and an upper-limit to the cancer
incidence is calculated assuming hypothetically that the true incidence is
just below the level of detection in the cohort studied, which is determined
largely by the cohort size. Whenever possible, human data are used in
preference to animal bioassay data.
Very little information exists that can be utilized to extrapolate from
high exposure occupational studies to low environmental levels. However, if a
number of simplifying assumptions are made, it is possible to construct a
crude dose-response model whose parameters can be estimated using vital
statistics, epidemiologic studies, and estimates of worker exposures.
In human studies, the response is measured in terms of the relative risk
of the exposed cohort of individuals compared to the control group. The
mathematical model employed assumes that for low exposures the lifetime
probability of death from lung cancer (or any cancer), Pg, may be
represented by the linear equation
P0 = A + BHx
where A is the lifetime probability in the absence of the agent, and x is the
average lifetime exposure to environmental levels in some units, say ppm. The
factor BH is the increased probability of cancer associated with each unit
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increase of r, the agent in air.
If we make the assumption that R, the relative risk of cancer for exposed
workers, compared to the general population, is independent of the length or
age of exposure but depends only upon the average lifetime exposure, it
follows that
R - JL - A + BH (XT + x?)
- P0 - A + BH Xl
or
RP0 = A + BH (Xl + x2)
where xj = lifetime average daily exposure to the agent for the general
population, X2 = lifetime average daily exposure to the agent in the
occupational setting, and PQ = lifetime probability of dying of cancer with
no or negligible exposure.
Substituting P0 = A + BH Xj and rearranging gives
BH = P0 (R - l)/x2
To use this model, estimates of R and X2 must be obtained from the
epidemiologic studies. The value PQ is derived from the age-cause-specific
death rates for combined males found in 1973 U.S. Vital Statistics tables
using the life table methodology.
CADMIUM RISK ESTIMATES
Unit Risk Estimate Based on an Animal Study
The bioassay by Takenaka et al. (1982) using male Wistar rats and cadmium
chloride aerosol was chosen for estimating the quantitative unit risk of
cadmium. This was the only positive animal inhalation study with cadmium
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and/or cadmium compounds, with a dose-response trend of primary lung
carcinomas to animals continuously exposed to CdCl2 aerosols for 18 months.
The primary lung carcinomas were histologically differentiated as
adenocarcinomas, epidermoid carcinomas, combined epidermoid and
adenocarcinomas, and mucoepidermoid carcinomas, but were combined for this
unit risk analysis. The incidences of total primary lung carcinomas was 15%
(6/39), 53% (20/38), and 71% (25/35) for the low (12.5 ug/m3), medium (25
ug/m3), and high (50 ug/m3) exposure groups, respectively. No tumors were
found among 38 controls.
In calculating an upper-limit unit risk estimate for humans, dose is
calculated on a lifetime continuous basis with 2 years considered a full
lifetime exposure for rats. Thus, by multiplying by 0.75, the measured
concentrations of 13.4 ug/m3, 25.7 ug/m3, and 50.8 ug/m3 for the three
dose groups, the lifetime continuous exposure can be estimated as 10.05
ug/m3, 19.3 ug/m3, and 38.1 ug/m3, respectively. The corrections for
animal to human weight differences are given below.
In transforming from animal exposure to human equivalence, the method for
treating inhalation of an aerosol [presented earlier in the section for
calculation of human equivalent dosages from animal data (Case 1)], assumes
aerosols to be absorbed proportional to the breathing rate. This breathing
rate is also given there for 113 g rats as 0.105 m3/day. For the Wistar
rats used in the Takenaka et al. bioassay the average weights at 18 months
were 424.6 g (for the 13.4 ug/m3 group), 437.6 g (for the 25.7 ug/m3
group), and 424.3 g (for the 50.8 ug/m3 group). Adjusting for these weights
we use the formula
I = 0.105 (W/0.113)2/3 m3/day
-------
where I = the daily inhalation rate of a rat weighing W kilograms. For the
three groups the I values are 0.254 m3/day, 0.259 m3/day, and 0.254
m3/day, respectively. Combining these with the lifetime continuous exposure
estimates above, we estimate daily exposure as 2.55 ug/day, 5.00 ug/day, and
9.68 ug/day, respectively. Equivalently, we can estimate dose on a ug/kg/day
basis as 6.0 ug/kg/day, 11.4 ug/kg/day, and 22.8 ug/kg/day.
Based on the above data, the 95% upper-limit unit risk for induced cancer
based on cadmium chloride exposure is q* = 6.3 x IQ-^ug/kg/day)"1
based on animals using the linearized multistage model. When transformed to
equivalent human dose, the CAG method requires multiplying q* by the
weight ratio factor (Wn/Wa)l/3, where W^ = weight of a human, which we
assume to be 70 kg. Thus,
=
-------
that a 70 kg human breathes 20 m3 air/day. Thus,
q* = 3.4 x lO-^ug/kg/day)-1 x 1 x 20 m3 =
" ToTg" liy
9.7xlO-2(ug/m3)-1
for cadmium chloride exposure, and
q* = 9.7 x 10-2(ug/m3)-l/0.613 = 1.6 x lO-
based on inhalation exposure to the cadmium ion. Therefore, the unit risk
from the inhalation of one microgram of elemental cadmium per cubic meter of
air is approximately
R = 1 - exp -(0.16 x 1) = 0.15
This is an upper-bound estimate of risk based upon the best direct
experimental evidence presently available. Using other dose-response models
to estimate risk (as shown in Appendix A) can give considerably lower
estimates than obtained using the upper-bound multistage model. However,
there is no direct evidence suggesting that these alternative models provide a
more rational basis for estimating risk than the upper-bound multistage model.
It must be kept in mind that the alternative models have the potential for
seriously underestimating the true risk at low levels of environmental
exposure to cadmium.
Unit Risk Estimate Based on a Human Study
Data Base--
At the present time the strongest evidence in humans suggesting a cadmium-
induced carcinogenic response is found in the Lemen et al. (1976) study. It
was observed in a cohort of cadmium smelter workers with more than 2 years of
116
-------
exposure that 12 deaths were due to a malignant neoplasm of the respiratory
system, while only 5.11 would have been expected based upon the age,
calendar-time, and cause-specific mortality rates for the total United States
white male population (relative risk = 2.34). Assuming that the U.S. white
male population is a valid control population for the cohort of cadmium
smelter workers, we find that the probability of obtaining 12 or more
respiratory cancer deaths is P = 0.0063 using the exact Poisson Test.
A number of problems exist in using these data to obtain a quantitative
estimate of human respiratory cancer risk due to cadmium exposure. Among them
are the following:
1. The smoking histories of the cadmium workers are unknown, and if their
smoking rates were substantially higher then white U.S. male averages,
this fact alone could explain the twofold increase in relative risk.
2. The exposure to cadmium is confounded with exposure to arsenic, a
known respiratory carcinogen.
3. Very limited evidence exists concerning the exposure rate and the
duration of exposure for the members of the cohort.
4. No exposure estimates exist for individuals.
5. To obtain an estimate of risk, a mathematical model must be assumed
that cannot be evaluated for goodness of fit in any reasonable manner
using the available data.
In spite of these considerable shortcomings, it was still felt that a risk
estimate based on the limited and potentially biased data base could be of use
for the following reasons:
1. The observed human respiratory cancer response corresponded to the
animal response in regard to site, which increases the likelihood that
the response is real.
2. All the factors that are potentially biasing would work to increase
the apparent cadmium-induced cancer risk. Thus, such a risk estimate
should be considered an upper-bound estimate. If this upper-bound
estimate is lower than the one obtained in the animal experiment, it
should be used in preference to the animal estimate.
117
-------
Another approach for obtaining a risk estimate would be to use the
information on prostate cancer. In the cohort studied by Lemen et al. (1976),
after a lapse of 20 years from initial exposures, 4 cases were observed versus
0.88 expected (SMR = 4.55), which is a statistically significant finding
(P < 0.01). Prostate cancer was also marginally significant in the Holden
(1980) study, but exposure information for individuals in his cohort was not
well-enough defined to permit a risk estimate. Use of the prostate cancer
data avoids the difficulties in numbers 1 and 2 relating to respiratory system
cancers, but the other difficulties remain.
Risk Model--
We make the following assumption to obtain a simple risk model. It is
assumed that an average daily lifetime exposure increases the relative risk
over the entire lifespan by an amount that is linearly related to the
exposure. This model would tend to overestimate a risk unless early exposures
in one's lifetime are much more critical than exposure during the working
lifetime. Under this model, the unit risk estimate has the following simple
form:
p = PQ (R-D
X
where P0 is the U.S. lifetime respiratory or prostate cancer risk in the
absence of cadmium exposure, R is the relative risk for cadmium smelter
workers, and X is the lifetime average exposure received by the cadmium
workers.
For respiratory cancer the value for P0 is 0.036 based upon 1973 U.S.
Vital Statistics data. The term R, the relative risk, is 2.34 with a 95%
confidence interval of 1.35 to 4.10 based upon the exact Poisson distribution.
118
-------
For prostate cancer the value of P0 is 0.0187 using the same data source and
the relative risk is 4/0.88 = 4.55.
To estimate the average lifetime exposure on a continuous basis, a number
of factors need to be estimated. They are:
The average age, t, of the cohort at the end of the observation period.
The average duration, d, of exposure for the cohort in years.
The average exposure rate, e, on the job.
The fraction, f, of time per year exposed on the job.
Given these factors, an estimate of the average exposure rate over the
cohort's lifetime is
X = def/t
Unfortunately, information is very limited concerning any of the above
factors. The approach to be taken is to use the limited information to make
an educated guess as to what is the best estimate of each quantity. In
addition, an upper- and lower-bound is given for each quantity, which most
likely brackets the true value. We shall discuss each of the following terms
in order.
Average age, t, at the end of the observation period—The only hints on
the age of the cohort is that the people were employed for at least 2 years
during the period 1940 to 1970 and were followed up to the start of 1974.
As an upper-bound it is assumed that the average age at the end of the
observation period is 70. The lower-bound is set at 40 and the best guess is
taken be the midpoint, or 55.
119
-------
Average duration, d, of exposure—We know that each individual was exposed
for at least 2 years and that the four individuals that died of prostatic
cancer were exposed for 4, 13, 17, and 18 years. If we make the potentially
biased assumption that these four observations are a random sample from the
total smelter worker population and obtain a 95% confidence interval around
the mean of the sample, we have that best estimate is 13 with bounds from 3 to
23.
Average exposure rate, e, on the job--Information concerning exposure
rates during the appropriate tie frame that we have at our disposal is taken
from the Lemen et al. (1976) study.
In 1947, a previously cited study by Princi reported an average air
concentration of cadmium fumes that ranged from 0.04 to 6.59 mg/m3 and
of cadmium dust at 17.23 mg/m3, with one man exposed 2-3 hr/day to an
average cadmium dust concentration of 31.3 mg/m3. Most operations,
however, had concentrations lower than 1.5 mg/m3.
As a low estimate we take the geometric mean of the range of the cadmium
fumes, (0.04 x 6.59)1/2 = 0.5 mg/m3. As a best estimate we use the 1.5
mg/m3 and as a upper-bound, 6 mg/m3.
Fraction, f, of time per year exposed on the job--We will assume that as a
low, "best," and high estimate the number of hours per day exposed on the job
is 2, 4, and 8 hours, respectively, and the number of days exposed per year as
120, 180, and 240 days, respectively.
Estimation of Unit Risk—
The values assumed for each parameter used in the estimation of the unit
risk based on respiratory cancer data is shown in Table 16. For prostate
cancer the "best" estimate of risk can be obtained, using the equation
120
-------
TABLE 16. "BEST" ESTIMATE AND BOUNDS ON TERMS USED TO ESTIMATE UNIT RISK
FROM RESPIRATORY CANCER DATA IN THE LEMEN et al. STUDY
Factor
t = age at end of
observation period
d = years duration
exposure
e = exposure rate
on job (mg/m3)
h = hours in exposure
per day
n = days worked
per year
f = hn*(24 x 365)
fraction of time per
years exposed
x = 1,000 def/t
average lifetime
exposure x ug/m3
R = relative risk
Value
Maximizing
Risk
70
3
0.5
2
120
0.0274
0.587
4.10
Value Giving
"Best" Estimate
of Risk
55
13
1.5
4
180
0.0822
29.1
2.34
Value
Minimizing
Risk
40
23
6
8
240
0.2192
756
1.35
Unit Risk Based on Above Values
P = 0.036(R-1)/I =
1.9 x 10-1
1.6 x 10
-3
1.7 x 10-5
121
-------
given previously for estimating the unit risk, by multiplying the risk for
respiratory cancer by the factor
(Rp - l)P0,p/(Rr - l)P0,r = (4-55 - l)0.0187/(2.34 - 1)0.036
where the subscripts p and r refer to prostate and respiratory cancer data,
respectively. Therefore, the "best estimate of the unit risk based on
prostate cancer data is 1.6 x 10'3 x 1.38 = 2.2 x 10~3. The effect of
compounding, especially multiplying together, a series of assumptions that
consistently overestimates or underestimates the true values of parameters
used to estimate risk leads to very different estimates. This is true even
though the mathematical model itself, a major source of uncertainty, remains
the same. However, it is highly unlikely that either extreme is close to the
true value. We take as our estimate the value obtained by compounding the
series of best guesses. Even though a single term may not be conservative,
the overall result is probably more reasonable than either of the extremes.
One final correction is needed. We assume that human exposure was to
cadmium oxide, CdO; thus, the risk to elemental cadmium is increased by the
ratio
(CdO/Cd) = (128.4/112.4) = 1.14
with a corresponding unit risk estimate of
P = 1.65 x 10~3 x 1.14 = 1.88 x 10~3 from respiratory data
and
P = 2.2 x 20-3 x 1.14 = 2.51 x 10~3 from prostate data.
This estimate is two orders of magnitude lower than the estimate based on
the rat inhalation sutdy, which was 0.156. Some of this difference might be
explained as a generalized difference in the biological activity between
122
-------
in rats and Cd dust and fumes in humans. In any event, our final
estimate is based on the human study, which is used for the relative potency
calculation.
RELATIVE POTENCY
One of the uses of unit risk is to compare the potency of carcinogens. In
this section the potency of cadmium is compared with that of other chemicals
that the CAG has evaluated as suspect carcinogens. To estimate the relative
potency on a per mole basis, the unit risk slope factor is multiplied by the
molecular weight and the resulting number expressed in terms of
(mMol/kg/day)-l. This is called the relative potency index.
Figure 2 is a histogram representing the frequency distribution of potency
indicies of 53 chemicals evaluated by the CAG as suspect carcinogens. The
actual data summarized by the histogram are presented in Table 17. When human
data were available for a compound, they have been used to calculate the
index. When no human data are available, animal oral studies and animal
inhalation studies have been used in that order. Animal oral studies are
selected over animal inhalation studies because most of the chemicals have
animal oral studies; this furnishes a more consistent basis for potency
comparisons.
The potency index for cadmium based on the Lemen et al . (1976) study of
cadmium smelter workers is 7.4 x 10+2 (mMol /kg/day)-1. This is derived as
follows: the slope estimate from the human study, 1.88 x 10~3
(ug/m3)~l, is first converted to units of (mg/kg/day)~^, assuming a
breathing rate of 20 m3 of air per day and a 70 kg person.
1.88 x 10-3(ug/m3)-1 x l daV x 1 "9 x 70 kg
20 m3 10"3 mg
23
-------
4th
quartile
3rd
quartile
2nd
quartile
1st
quartile
1x10
+i
4x10
2x10
CM
CM
°n
-2
i
0
2 4
Log of Potency Index
i
6
8
Figure 2. Histogram representing the frequency distribution of the potency
indices of 53 suspect carcinogens evaluated by the Carcinogen
Assessment Group.
-------
TABLE 17. RELATIVE CARCINOGENIC POTENCIES AMONG 53 CHEMICALS EVALUATE!!
BY THE CARCINOGEN ASSESSMENT GROUP AS SUSPECT HUMAN CARCINOGENS1'2'3
Compounds
Acrylonitrile
Aflatoxin B^
Aldrin
Ally! Chloride
Arsenic
B[a]P
Benzene
Benzidine
Beryl 1 ium
Cadmium
Carbon Tetrachloride
Chlordane
Chlorinated Ethanes
1,2-dichl oroethane
hexachl oroethane
1 ,1 ,2,2-tetrachl oroethane
1,1,1-trichl oroethane
1,1,2-trichl oroethane
Chloroform
Chromium
DDT
Dichlorobenzidine
1 ,1-dichloroethylene
Dieldrin
Slope
(mg/kg/day)'1
0.24(W)
2924
11.4
1.19x10-2
15(H)
11.5
5.2xlO-2(W)
234(W)
4.86
6.65(W)
1. SOxlO-1
1.61
6.90x10-2
1.42x10-2
0.20
1.6xlO-3
5.73x10-2
7x10-2
41
8.42
1.69
1.47x10-1(1)
30.4
Molecular
Weight
53.1
312.3
369.4
76.5
149.8
252.3
78
184.2
9
112.4
153.8
409.8
98.9
236.7
167.9
133.4
133.4
119.4
104
354.5
253.1
97
380.9
Potency
Index
1X10*1
9xlO+5
4xlO+3
9x10-1
2xlO+3
3xlO+3
4x10°
4xlO+4
4x10+1
7xlO+2
2xlO+1
7x10+2
7x10°
3x10°
3x10+1
2x10"!
8x10°
8x10°
4xlO+3
3xlO+3
4x10+2
1x10+1
1x10+4
Order of
Magnitude
(1 og^Q
Index)
+1
+6
+4
0
+ 3
+3
+ 1
+ 5
+ 2
+ 3
+ 1
+ 3
+1
0
+1
-1
+1
+1
+4
+3
+3
+ 1
+4
125
(continued on the following page)
-------
TABLE 17. (continued)
Compounds
Dinitrotoluene
Diphenylhydrazine
Epichlorohydrin
Bis(2-chloroethyl )ether
Bis(chloromethyl )ether
Ethylene Dibromide (EDB)
Ethylene Oxide
Formaldehyde
Heptachlor
Hexachlorobenzene
Hexachlorobutadiene
Hexachl orocycl ohexane
technical grade
alpha isomer
beta isomer
gamma isomer
Nickel
Nitros amines
Dimethyl nitrosamine
Diethylnitrosamine
Dibutylnitrosamine
N-nitrosopyrrol idine
N-nitroso-N-ethylurea
N-nitroso-N-methylurea
N-nitroso-diphenylamine
PCBs
Slope
(mg/kg/day)-1
0.31
0.77
9.9x10-3
1.14
9300(1)
8.51
0.63(1)
2. 14xlO-2(I)
3.37
1.67
7.75X10-2
4.75
11.12
1.84
1.33
1.15(W)
25.9(not by q*)
43.5(not by q})
5.43
2.13
32.9
302.6
4.92xlO-3
4.34
Molecular
Weight
182
180
92.5
143
115
187.9
44.0
30
373.3
284.4
261
290.9
290.9
290.9
290.9
58.7
74.1
102.1
158.2
100.2
117.1
103.1
198
324
Potency
Index
6x10+1
1x10+2
9X10-1
2xlO+2
1x10+6
2xlO+3
3xlO+1
6X10-1
lxlO+3
5x10+2
2xlO+1
1x10+3
3x10+3
. f\
5x10+2
4x10+2
7xlO+1
2xlO+3
4x10+3
9x10+2
2x10+2
4xlO+3
3x10+4
JLAiU
lxlO+3
r-l f 1 q —
Order of
Magnitude
(Io9l0
Index)
+2
+2
0
+2
+6
+3
+1
0
+3
+3
+1
+ 3
+3
+ 3
+3
+2
+ 3
+4
+ 3
+2
+4
+4
0
+ 3
— T— i
126
-------
TABLE 17. (continued)
Compounds
Phenols
2,4,6-trichlorophenol
Tetrachlorodioxin
Tetrachl oroethylene
Toxaphene
Trichloroethylene
Vinyl Chloride
Remarks:
Slope
(mg/kg/day)'1
1.99xlO'2
4.25xl05
5.31x10-2
1.13
1.26xlO-2
1.75xlO-2(I)
1. Animal slopes are 95% upper-limit
Molecular
Weight
197.4
322
165.8
414
131.4
62.5
slopes based
Potency
Index
4x10°
lxlO+8
9x10°
5xlO+2
2xlOn
1x10°
on the lineari
Order of
Magnitude
Oogm
Index)
+ 1
+8
+1
+3
0
0
zed multistage
2.
3.
They are calculated based on animal oral studies, except for those indicated by
I (animal inhalation), W (human occupational exposure), and H (human drinking water
exposure). Human slopes are point estimates based on the linear non-threshold
model.
The potency index is a rounded-off slope in (mMol/kg/day)"l and is calculated by
multiplying the slopes in (mg/kg/day)"^ by the molecular weight of the compound.
Not all of the carcinogenic potencies presented in this table represent the same
degree of certainty. All are subject to change as new evidence becomes available.
127
-------
Multiplying by the molecular weight of 112.4 gives a potency index of
7.4 x 10+2. Rounding off to the nearest order of magnitude gives a value of
10+3, which is the scale presented on the horizontal axis of Figure 2. The
index of 7.4 x 10+^ lies in the second quartile of the 53 suspect
carcinogens.
Ranking of the relative potency indicies is subject to the uncertainty of
comparing estimates of potency of different chemicals based on different
routes of exposure to different species using studies of different quality.
Furthermore, all the indicies are based on estimates of low-dose risk using
the linearized multistage extrapolation model fitted to the data at relatively
high doses. Thus, relative potencies could be different at high exposures,
where non-linearities in the dose-response curves could exist.
128
-------
APPENDIX A
COMPARISON OF RESULTS BY VARIOUS EXTRAPOLATION MODELS
The estimate of unit risk from animals presented in the body of this
document is calculated by the use of the linearized multistage model.
The reasons for its use have been detailed therein. Essentially, it is part
of a methodology that estimates a conservative linear slope at low extrapolation
doses and is consistent with the data at all dose levels of the experiment.
It is a nonthreshold model holding that the upper-limit of risk predicted by
a linear extrapolation to low levels of the dose-response relationship is the
most plausible upper-limit for the risk.
Other models have also been used for risk extrapolation. Three
nonthreshold models are presented here: the one-hit, the log-Probit, and the
Weibull. The one-hit model is characterized by a continuous downward curvature
but is linear at low doses. It can be considered the linear form or first
stage of the multistage model because of its functional form. Because of this
and its downward curvature, it will always yield estimates of low level risk
which are at least as large as those of the multistage model. Further,
whenever the data can be fit adequately by the one-hit model, estimates from
the two procedures will be comparable.
The other two models presented below are the log-Probit and the Weibull.
They are often used to fit toxicological data in the observable range, because
of the general "S" curvature. The low-dose upward curvatures of these two
models usually yield lower low-dose risk estimates than those of the one-hit
or multistage models.
The log-Probit model was originally used in problems of biological assay
such as the assessment of potency of toxicants and drugs and has usually
129
-------
been used to estimate such values as percentile lethal dose or percentile
effective dose. Its development was strictly empirical, i.e., it was
observed that several log dose-response relationships followed the
cumulative normal probability distribution function, . In fitting the cancer
bioassay data, assuming on independent background, this becomes
P(D;a,b,c) = c + (1-c) $ (a+blogjo D) a,b > 0 _< c < 1
where P is the proportion responding at dose D, c is an estimate of the
background rate, a is an estimate of the standarized mean of individual
tolerances, and b is an estimate of the log dose-Probit response slope.
The one-hit model arises from the theory that a single molecule of a
carcinogen has a probability of transforming a single normal cell
into a cancer cell. It has the probability distribution function
P(D;a,b) = l-exp-(a+bd) a,b > 0
where a and b are the parameter estimates. The estimate a represents the
background or zero dose rate, and the parameter estimated by b represents
the linear component or slope of the dose-response model. In discussing the
added risk over background, incorporation of Abbott's correction leads to
P(D;b) = l-exp-(bd) b > 0
Finally, a model from the theory of carcinogenesis arises from the multihit
model applied to multiple target cells. This model has been termed here the
Wei bull model. It is of the form
P(D;b,k) = l-exp-(bdk) b,k > 0
130
-------
For the power of dose only, the restriction k > 0 has been placed on this model.
When k > 1, this model yields low-dose estimates of risks usually significantly
lower than either the multistage or one-hit models, which are linear at low
doses. All three of these models usually project risk estimates significantly
higher at low exposure levels than those from the log-Probit.
The estimates of added risk for low doses for these models are given in
Table 18 for the cadmium chloride rat inhalation studies by Takenaka et al.
Both maximum likelihood estimates and 95% upper confidence limits are presented.
The results show that the maximum likelihood estimates of risk for the
log-Probit model are all less than those for the other models and this
difference increases sharply at low doses. The one-hit model yields maximum
likelihood estimates slightly higher than from the multistage model, with the
Wei bull somewhat lower.
131
-------
TABLE 18. ESTIMATES OF LOW-DOSE RISK TO HUMANS EXPOSED TO CADMIUM CHLORIDE BASED ON MALE WISTAR
RATS FROM THE TAKENAKA ET AL. INHALATION STUDY DERIVED FROM FOUR DIFFERENT MODELS
Maximum Likelihood Estimates of 95% Upper Confidence Limit of
Additional Risks Additional Risks
Dose Multistage One-Hit Weibull Log-Probit Multistage One-Hit Weibull Log-Probit
ug/m3 Model Model Model Model Model* Model Model Model
10-4 5.5xlO-6 S.lxlO-6 1.9xlO-7 0 9.7xlQ-6 l.OxlO-5 1.3xlO'6 1.2xlQ-38
10-3 5.5xlO-5 8.1xlO-5 4.1xlO-6 0 9.7xlQ-5 l.OxlO-4 2.6xlO~5 8.9x10-25
lO-2 5.5xlO'4 S.lxlO'4 8.8xlO-5 2.0xlO-15 9.7xlQ-4 l.OxlO-3 S.SxlO'4 4.4X10'14
ID'1 5.5x10-3 S.lxlO-3 1.9x10-3 l.SxlO'7 9.7xlQ-3 LOxlO"2 5.9xlQ-3 1.5X10'6
1 5.5x10-2 7.8x10-2 3.9x10-2 7.0x10-3 9.2xlQ-2 9.5xlQ-2 S.lxlO"2 2.3X10'2
* q* = 9.7 x 10-2(ug/m3)-1 for the multistage model; P(d) = 1 - e -
-------
APPENDIX B
INTERNATIONAL AGENCY FOR RESEARCH ON CANCER (IARC) CLASSIFICATION FOR
UEIGHT-OF-EVIDENCE FOR CARCINOGENICITY OF A SUSPECTED CARCINOGEN
ASSESSMENT OF EVIDENCE FOR CARCINOGENICITY FROM STUDIES IN HUMANS
The degrees of evidence for carcinogenicity from studies in humans are
categorized as:
1. Sufficient evidence of carcinogenicity, which indicates that there is
a causal relationship between the agent and human cancer.
2. Limited evidence of carcinogenicity, which indicates that a causal
interpretation is credible, but that alternative explanations, such as chance,
bias, or confounding, could not adequately be excluded.
3. Inadequate evidence, which indicates that one of three conditions
prevailed: (a) there were few pertinent data; (b) the available studies,
while showing evidence of association, did not exclude chance, bias, or
confounding; (c) studies were available which do not show evidence of
carcinogenicity.
ASSESSMENT OF EVIDENCE OF CARCINOGENICITY FROM STUDIES IN EXPERIMENTAL ANIMALS
These assessments are classified into four groups:
1. Sufficient evidence of carcinogenicity, which indicates that there is
an increased incidence of malignant tumors: (a) in multiple species or
strains; or (b) in multiple experiments (preferably with different routes of
administration or using different dose levels; or (c) to an unusual degree
with regard to incidence, site, or type of tumor, or age at onset. Additional
evidence may be provided by data on dose-response effects, as well as
information from short-term tests or on chemical structure.
133
-------
2. Limited evidence of carcinogenicity, which means that the data suggest
a carcinogenic effect but are limited because: (a) the studies involve a
single species, strain, or experiment; or (b) the experiments are restricted
by inadequate dosage levels, inadequate duration to exposure to the agent,
inadequate period of follow-up, poor survival, too few animals, or inadequate
reporting, or (c) the neoplasms produced often occur spontaneously and, in the
past, have been difficult to classify as malignant by histological criteria
alone (e.g., lung and liver tumors in mice).
3. Inadequate evidence, which indicates that because of major qualitative
i
or quantitative limitations, the studies cannot be interpreted as showing
either the presence or absence of a carcinogenic effect; or that within the
limits of the tests used, the chemical is not carcinogenic. The number of
negative studies is small, since, in general, studies that show no effect are
less likely to be published than those suggesting carcinogenicity.
4. No data indicates that data were not available to the working group.
EVALUATION OF CARCINOGENIC RISK TO HUMANS
At present, no objective criteria exist to interpret data from studies in
experimental animals or from short-term tests directly in terms of human risk.
Thus, in the absence of sufficient evidence from human studies, evaluation of
the carcinogenic risk to humans was based on consideration of both the
epidemiological and experimental evidence. The breadth of the categories of
evidence defined above allows substantial variation within each. The
decisions reached by the group regarding overall risk incorporated these
differences, even though they could not always be reflected adequately in the
placement of an exposure into a particular category.
13*
-------
The chemicals, groups of chemicals, industrial processes, or occupational
exposures were thus put into one of three groups:
Group 1
The chemical, group of chemicals, industrial process, or occupational
exposure is carcinogenic to humans. This category was used only when there
was sufficient evidence from epidemiological studies to support a causal
association between the exposure and cancer.
Group 2
The chemical, group of chemicals, industrial process, or occupational
exposure is probably carcinogenic to humans. This category includes exposures
for which, at one extreme, the evidence of human carcinogenicity is almost
"sufficent," as well as exposures for which, at the other extreme, it is
inadequate. To reflect this range, the category was divided into higher
(Group A) and lower (Group B) degrees of evidence. Usually, category 2A was
reserved for exposures for which there was at least limited evidence of
carcinogenicity to humans. The data from studies in experimental animals
played an important role in assigning studies to category 2, and particularly
those in Group B; thus, the combination of sufficient evidence in animals and
inadequate data in humans usually resulted in a classification of 2B.
In some cases, the working group considered that the known chemical
properties of a compound and the results from short-term tests allowed its
transfer from Group 3 to 2B or from 2B to 2A.
Group 3
The chemical, group of chemicals, industrial process, or occupational
exposure cannot be classified as to its carcinogenicity to humans.
135
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