SCIENTIFIC QUESTIONS TO THE
CSTEE ON CADMIUM
Note: Following the
terms of reference, this
opinion focuses on specific
questions related to human
health issues and does not
cover ecosystem protection.
Questions to CSTEE
1. What is the most
appropriate study and effect
level to be used as the
starting point for deriving a
limit value for cadmium in air
based on non-cancerous renal
effects?
2. Accepting that
cadmium is a known human
carcinogen, and given the
uncertainty in the unit risks
due to confounding exposure to
arsenic, does the committee
believe it appropriate to
establish a limit value based
on cancer effects, and, if so,
how?
3. Does the committee
consider that an ambient air
quality standard based on
non-cancer effects would also
provide a high level of
protection against cancer
effects?
Opinion:
1. The CSTEE considers
that the studies of Buchet et
al. (1990) and Järup et al.
(2000) provide a valid
scientific basis for the
derivation of a limit value for
the non-cancer renal effects of
cadmium. The appropriateness of
these studies, which were based
on cohorts of the general
population, is further
supported by the compatibility
of the limit value derived from
them with that suggested by
other studies carried out on
workers. Furthermore, the CSTEE
can support the use, as done in
the Position Paper, of an
overall uncertainty factor of
100 in the derivation of a
limit value based from the
above two studies.
2. Given the degree of
confounding by arsenic exposure
in the epidemiological studies,
the CSTEE does not believe that
it is possible to establish a
reliable limit value for
cadmium based on the induction
of cancer effects.
3. The available data do
not permit a reliable
assessment of the degree of
protection against cancer which
would be provided by a limit
value based on the non-cancer
effects of cadmium. The CSTEE
estimates that a limit value of
5-6.5 ng/m3, based on
non-cancer effects, could be
associated with an excess
lifetime risk of up to about 20
cases of lung cancer per
million.
Justification of the
Opinion:
Question 1
Chronic exposure to cadmium causes kidney damage, starting as impairment of tubular reabsorption which, in more severe cases, can develop to alterations in glomerular function. Early tubular damage is detected as increased urinary excretion of low-molecular weight proteins (β2-microglobulin, α1-microglobulin, retinol-binding protein), enzymes such as N-acetyl-β-D-galactosidase), other small molecules and calcium. Proteinuria resulting from relatively high exposure to cadmium (above 5-10 µg/24 hours urinary cadmium) appears to be irreversible and is associated with the development of more severe kidney malfunction (Roels et al., 1989). On the other hand, there is considerable uncertainty over the pathological significance of small increases in proteinuria found after low-level environmental exposure to cadmium (less than 2 µg/24 hours urinary cadmium). Furthermore, recent evidence suggests that that such increases may recede slowly (over a period of years) if exposure to cadmium is reduced (Hotz et al., 1999). However, the CSTEE notes that the discussion on the risks from cadmium in the ambient air concerns chronic exposure, a condition under which reversibility of the effects does not seem relevant. It is further noted that in most areas of Europe it would take a very long time to decrease the exposure to cadmium via food (mainly cereals, roots and vegetables), even if the emissions to the environment could be stopped completely, as the half-time of cadmium in soil is very long.
Data useful for setting
limit values for cadmium have
been provided from studies on
members of the general
population as well as from
studies on occupationally
exposed cohorts.
Studies in the general
population
Many general population
studies have utilised urinary
excretion of cadmium as a
measure of dose. As compared
with the use of the air or food
concentrations as a dose
metric, urinary cadmium has the
advantage of being a biological
indicator of the total body
burden which, because of the
long persistence of cadmium in
the body (half-life of around
20 years), reflects long-term
exposure arising from all
sources and routes.
Furthermore, urinary cadmium
correlates closely with cadmium
concentrations in the kidney
(target tissue), at least at
low exposure levels (Roels et
al., 1981; Orlowski et al.,
1998).
Two studies are particularly useful for risk estimation in the general population. Buchet et al. (1990) carried out a cross-sectional study involving 1699 subjects aged 20-80 years living in two regions of Belgium with different levels of environmental cadmium contamination. Multiple logistic regression analysis provided estimates of urinary cadmium cut-off levels above which at least 10% of the population is expected to have abnormal values of urinary markers, with the limit of the normal range taken as the 95th percentile of values observed in subjects without diabetes, urinary tract disease or treatment with analgesics. Thus the calculated urinary cadmium cut-off levels were associated with a 5% prevalence over and above a 5% background prevalence. Because the whole of the studied population was exposed to cadmium, it is not known how much of this background prevalence may have been due to cadmium. Among urinary protein markers examined, N-acetyl-β-galactosidase showed the lowest cut-off (2.7 µg Cd/24 hours). An even lower threshold (1.9 µg Cd/24 hours) was noted for increased urinary calcium. While the biological relevance of this to kidney toxicity is uncertain, it is of possible relevance to the recently reported association of increased incidence of osteoporosis and bone fragility with low level exposure to environmental cadmium (Staessen et al., 1999; Alfven et al, 2000).
A recent Swedish study (Järup et al., 2000), which is not considered in the Position Paper; examined urinary excretion of α1-microglobulin in 1021 environmentally or occupationally exposed subjects, aged 16-80 years, living in or near a region close to a cadmium battery plant. Multiple logistic regression analysis indicated that a 10% prevalence of increased proteinuria (on top of a 5% taken as background), was associated with urinary cadmium of 1 µg/g creatinine (corresponding to excretion of 1-2 µg cadmium/24 hours). Exclusion from the calculations of the 222 occupationally exposed subjects did not affect the outcome of the calculations. Although certain factors which have been reported in other studies to significantly influence microproteinuria alone or in association with Cd exposure (primary renal disease, diabetes, hypertension, use of analgesics, smoking and other medications), were not controlled for in the statistical analysis, the conclusions of this study are in agreement with those of the study by Buchet et al. (1990) and are compatible with the possibility of an association of microproteinurea with even lower cadmium exposures than suggested by that study. They are also in agreement with a report (Orlowski et al., 1998) indicating that a urinary cadmium concentration of 1.7 µg/ g creatinine corresponds to a cortical kidney cadmium concentration of 50 µg/g, a concentration currently believed to be the threshold for the induction of kidney malfunction (Järup et al., 1998).
Urinary cadmium excretion has been related to chronic cadmium intake using a toxicokinetic model which takes into account both inhalation and oral intakes (US EPA, 1999). The uptake of environmental cadmium depends on the chemical speciation and the route of exposure. Although the speciation of environmental cadmium has not been fully characterised, cadmium oxide is probably the most common form of human exposure, while cadmium sulphide is additionally found in occupational settings. Airborne cadmium is primarily associated with particulates, those found in occupational settings showing a smaller mean aerodynamic diameter (hence a different fractional lung deposition rate) than those found in ambient settings. Systemic absorption after inhalation is high (up to 90% of the amount deposited in the lung) for cadmium oxide, but only about 10% for cadmium sulphide. Absorption after oral ingestion is much lower (3-8%). In the application of the model to estimate the intake necessary to cause the urinary excretion levels of interest, a series of conservative assumptions was made, including that a) exposure involved cadmium oxide (a relatively well absorbed species), b) the fractional lung deposition was 0.21 (half-way between the values corresponding to ambient and occupational cadmium-containing particulates), and c) that the values for the systemic absorption were 90% for the cadmium deposited in the lungs and 5% for that reaching the GI tract. For a population with an oral intake of 0.14 µg/kg.day (the mean dietary intake of the US general population), the model predicts that a urinary excretion of 2.7 µg Cd/24 hours, identified by Buchet et al. (1990) as the threshold for the induction of cadmium-related proteinuria in 5% of the population, would result from life-time inhalation of 650 ng/m3 cadmium. A correspondingly lower exposure level would be expected to correspond to the lower threshold suggested by the Järup et al. (2000) study.
Uncertainty factors: The
data of Buchet et al. (1990)
and Järup et al. (2000) suggest
that the dose-response curve
for the induction of
proteinuria has a positive
slope down to urinary cadmium
excretion levels lower than 2
μg/24 hours. Although the slope
below 2 μg/24 hours is smaller
than above this concentration,
possibly suggesting different
underlying biological
mechanisms, these mechanisms
are currently unknown and
therefore it cannot be assumed
that they are of different
health significance. Given
that, even at the selected
cut-off level of 2.7 µg/24
hours, a significant fraction
of the population (over 5%)
would be expected to be
affected, a factor of at least
10 should be used to derive a
NOAEL.
Subgroups with increased
sensitivity to cadmium include
persons with degenerative
kidney damage (e.g. the elderly
and diabetics), as well as
persons with lower iron stores
(e.g. women) who may take up
cadmium more readily (Järup et
al., 1998). Additional
considerations which need to be
taken into account in deriving
a limit value are the
following:
a) Mean dietary intake of cadmium in some countries is higher than the value of 0.14 µg/kg.day assumed above, appearing to be closer to or even exceed 0.2 µg/kg.day in some European countries (Järup et al., 1998; Biego et al., 1998; Muller et al., 1998; Coni et al., 1991). For subjects near the top of the consumption range, dietary exposure is likely to be 2-3fold higher. Higher dietary exposure would also apply to specific population subgroups, e.g. shellfish eaters and vegetarians. Increased dietary intake would leave a smaller margin for inhalation exposure. For example, the toxicokinetic model employed by US EPA predicts that, for subjects consuming 0.36 µg/kg-day (roughly representing the intake of shellfish eaters), the threshold of urinary excretion would be reached at approximately 2fold lower inhalation exposure than indicated above. A further source of exposure that needs to be considered is tobacco smoking, which can result in an amount of absorbed cadmium approximately equal to that absorbed from the diet.
b) Cadmium
concentrations in the kidney
cortex of the middle-aged
general population appear to be
in the range 15-50 µg/g, close
to the critical value of 50
µg/g already mentioned as the
threshold for the induction of
kidney malfunction. This
implies that, if the critical
kidney cortex concentrations
are to be avoided, further
exposure needs to be strictly
limited.
In view of the
considerations discussed above,
the adoption of an additional
uncertainty factor of 10 to
derive a limit value from the
NOAEL, as suggested in the
Position Paper, seems
justified, leading to a limit
value of 6.5 ng/m3.
Occupational exposure
studies
Many occupational
studies utilised cumulative
airborne exposure to cadmium as
the dose metric, and therefore
suffer from the disadvantage of
depending on the estimation of
past workplace exposures.
Nevertheless, the data obtained
from a large number of such
studies demonstrate clearly
increased incidence of
low-molecular weight
proteinuria at cumulative
exposures greater than 500
μg/m3 x years (TWA
concentrations over 8 hours per
day, 5 days per week), and are
compatible with a threshold of
not less than 100 μg/m3 x
years. While rare reports of
effects at a lower cumulative
exposure cannot be completely
ignored, they are difficult to
assess because of the very
small numbers of affected
individuals and the occurrence
of microproteinuria in
non-exposed populations.
Uncertainty factors: A
cumulative occupational
exposure of 100 μg/m3 x years
is equivalent to a lifetime,
continuous exposure of the
general population to an
atmospheric concentration of
270 ng/m3. For the derivation
of a NOAEL from this, it should
be remembered that the figure
of 100 μg/m3 x years is not a
LOAEL but a threshold below
which effects are thought
unlikely to occur. Thus an
uncertainty factor smaller than
10 may be used. On the other
hand, as already indicated,
evidence from studies utilising
urinary cadmium as a measure of
dose suggests that the
dose-response curve has a
positive slope down to
background exposures. For this
reason, and having in mind
occasional reports of effects
occurring at lower occupational
exposures (Ellis et al., 1985),
it would be prudent to use an
uncertainty factor of 5, as
suggested in the Position
Paper.
The cohorts of the above
mentioned studies were
frequently limited to healthy
males of working age, and may
not fully reflect the full
range of individual
susceptibilities likely to be
encountered in the general
population. Although a recent
attempt (TERA 2000) to compare
the quantitative outcomes of
studies in workers and the
general population did not find
evidence of a "healthy worker"
effect, the reliability of its
conclusions is limited
(heterogeneity of data
reporting in different studies,
absence of consideration of age
effects). On the other hand,
examination of data obtained
from comparable cohorts of
workers and members of the
general population (Buchet et
al., 1980) provides evidence
that the latter may have higher
sensitivity. For this reason
the use of an additional
uncertainty factor of 10, as
suggested in the Position
paper, seems justified. This
would lead to a limit value of
5 ng/m3, in agreement with that
derived from the general
population studies which made
use of urinary cadmium
excretion.
Conclusion: The studies
of Buchet et al. (1990) and
Järup et al. (2000) provide a
valid scientific basis for the
derivation of a limit value
based on the non-cancer renal
effects of cadmium. The
appropriateness of these
studies is further supported by
the compatibility of the limit
value derived from them with
that suggested by other
studies. Furthermore, the CSTEE
supports the use, as done in
the Position Paper, of an
overall uncertainty factor of
100 in the derivation of a
limit value based from the
above two studies.
Question 2
Cadmium is classified as
a category 1 carcinogen (known
human carcinogen) by IARC and
US EPA. This classification is
based largely on the findings
of epidemiological studies in
the US showing increased
incidence of lung cancer in
cadmium-exposed smelter workers
(Thun et al., 1985; Stayner et
al., 1992). Although the
presence of confounding by
arsenic and tobacco smoking in
these studies was recognised,
it has been concluded by the
above agencies that such
confounding is unlikely to
account for all the excess of
cancers observed, and a
cadmium-specific unit risk
factor was calculated by the US
EPA using the data of Stayner
et al. (1992) [US EPA, 1999].
In order to counter criticisms of inadequate control of arsenic confounding, Thun et al. (1985) and US OSHA (1999) [using the data of Stayner et al. (1992)] made use of estimates of the cumulative exposure of the study cohorts to arsenic, and the known unit risk for arsenic, to calculate the expected contribution of such exposure to the observed lung cancers, and came to the conclusion that it could account only for a small fraction (under 10%) of them. The validity of this conclusion has been criticised on account of inadequacies in the estimation of the arsenic exposures employed in the calculations (Lamm et al., 1992; Doll, 1992; Sorahan and Lancashire, 1994). This criticism has been further developed by Sorahan and Lancashire (1997) who, using improved estimates of exposure to cadmium and arsenic, concluded that elevated lung cancer risks are associated only with mixed exposure to cadmium and high concentrations of arsenic, and that at this time it is not possible to distinguish between the contributions of cadmium and arsenic to the overall cancer risk.
Attempts have also been
made to derive unit risk
factors from both animal
experiments and epidemiological
data, using the data of
Takenaka et al. (1983) for
induction of lung cancers in
male Wistar rats exposed by
inhalation to cadmium chloride.
These data have been
quantitatively analysed and
used to derive an upper bound
95% value of the unit risk
factor which corresponds to a
limit value, for <1 in a
million risk, of approximately
0.03 ng/m3 (US EPA, 1999). This
limit value is nearly 10fold
lower than that derived from
the epidemiological data [0.2
ng cadmium/m3, US EPA (1999)]
which, as already pointed out,
are likely to overestimate the
risk associated specifically
with cadmium. In considering
these risk estimates, it should
be remembered that, in addition
to the difficulties related to
inter-species dosimetric
extrapolation and
susceptibility differences,
which normally complicate the
use of animal data for human
risk assessment, the situation
is further complicated in the
case of cadmium by questions of
chemical speciation and the
limited understanding of the
mechanistic basis of cadmium
carcinogenicity. Therefore the
available animal studies are
not considered suitable for
deriving a reliable limit value
for the protection of humans.
Conclusion: It is
concluded that the available
data do not permit the
estimation of a reliable unit
risk or the establishment of a
limit value for cadmium based
on the induction of cancer
effects.
Question 3:
Based on the arguments
presented in the response to
Question 1, a limit value in
the range 5-6.5 ng/m3 would be
derived, based on protection
against cadmium effects on the
kidney.
The risk calculations
carried out by US EPA (US EPA,
1999) using the data of Stayner
et al. (1992) resulted in a
unit risk of approximately
4.3x10-3 (µg/m3)-1. At an
exposure of 5 ng/m3, this would
give rise to an excess
life-time risk of about 20
cases of lung cancer per
million. To the extent to which
the cancers observed in the
epidemiological studies on
which the calculation of the
unit risk was based might have
been due in part to confounding
factors (arsenic, smoking),
this excess would be expected
to be lower.
An addition factor that
should be considered in
assessing low-dose risks
relates to the genotoxicity of
cadmium. While cadmium is
capable of inducing a range of
genotoxic effects (including
mutations and cytogenetic
damage) directly, there is
evidence that indirect
mechanisms, such as inhibition
of DNA repair and induction of
oxidative stress, may also play
a role in its genotoxicity.
This would suggest that cadmium
genotoxicity has a non-linear
component, and that cancer
risks at low doses may be
smaller than the above
calculation suggests.
References:
- Alfven T, Elinder CG,
Carlsson MD, Grubb A, Hellstrom
L, Persson B, Pettersson C,
Spang G, Schutz A, Järup L
(2000) Low-level cadmium
exposure and osteoporosis. J
Bone Miner Res 15, 1579-86
- Biego GH, Joyeux M,
Hartemann P, Debry G (1998)
Daily intake of essential
minerals and metallic
micropollutants from foods in
France. Sci Total Environ 217,
27-36
- Buchet JP, Roels H,
Bernard A, Lauwerys R (1980)
Assessment of renal function of
workers exposed to inorganic
lead, calcium or mercury vapor.
J Occup Med 22, 741-50
- Buchet JP, Lauwerys R,
Roels H, Bernard A, Bruaux P,
Claeys F, Ducoffre G, de Plaen
P, Staessen J, Amery A, Lijnen
P, Thijs L, Rondia D, Sartor F,
Saint Remy A and Nick L (1990),
Renal effects of cadmium body
burden of the general
population. Lancet 336,
699-702.
- Coni E, Baldini M,
Stacchini P, Zanasi F (1992)
Cadmium intake with diet in
Italy: a pilot study. J Trace
Elem Electrolytes Health Dis 6,
175-81
- Doll R (1992) Is
cadmium a human carcinogen? Ann
Epidemiol 2, 335-7
- Ellis KJ, Cohn SH,
Smith TJ (1985) Cadmium
inhalation exposure estimates:
their significance with respect
to kidney and liver cadmium
burden. J Toxicol Environ
Health15, 173-87
- Hotz P, Buchet JP,
Bernard A, Lison D, Lauwerys R
(1999) Renal effects of
low-level environmental cadmium
exposure: 5-year follow-up of a
subcohort from the Cadmibel
study. Lancet 354,1508-13
- Järup L, Berglund M,
Elinder CG, Nordberg G, Vahter
M (1998) Health effects of
cadmium exposure--a review of
the literature and a risk
estimate. Scand J Work Environ
Health, 24 (Suppl 1), 1-51
- Jarup L, Hellstrom L,
Alfven T, Carlsson MD, Grubb A,
Persson B, Pettersson C, Spang
G, Schutz A, Elinder CG (2000)
Low level exposure to cadmium
and early kidney damage: the
OSCAR study. Occup Environ Med
57, 668-72
- Lamm SH, Parkinson M,
Anderson M, Taylor W (1992)
Determinants of lung cancer
risk among cadmium-exposed
workers. Ann Epidemiol 2,
195-211
- Muller M, Anke M,
Illing-Gunther H, Thiel C
(1998) Oral cadmium exposure of
adults in Germany. 2: Market
basket calculations. Food Addit
Contam 15, 135-41
- Orlowski C, Piotrowski
JK, Subdys JK, Gross A (1998)
Urinary cadmium as indicator of
renal cadmium in humans: an
autopsy study. Hum Exp Toxicol
17, 302-6
- Roels HA, Lauwerys RR,
Buchet JP, Bernard A, Chettle
DR, Harvey TC, Al-Haddad IK
(1981) In vivo measurement of
liver and kidney cadmium in
workers exposed to this metal:
its significance with respect
to cadmium in blood and urine.
Environ Res 26, 217-40
- Roels HA, Lauwerys RR,
Buchet JP, Bernard AM, Vos A,
Oversteyns M (1989) Health
significance of cadmium induced
renal dysfunction: a five year
follow up. Br J Ind Med 46,
755-64
- Sorahan T, Lancashire
R (1994) Lung cancer findings
from the NIOSH study of United
States cadmium recovery
workers: a cautionary note.
Occup Environ Med 51, 139-40
- Sorahan T, Lancashire
R (1997) Lung cancer mortality
in a cohort of workers employed
at a cadmium recovery plant in
the United States: an analysis
with detailed job histories.
Occup Environ Med 54, 194-201
- Staessen JA, Roels HA,
Emelianov D, Kuznetsova T,
Thijs L, Vangronsveld J, Fagard
R (1999) Environmental exposure
to cadmium, forearm bone
density, and risk of fractures:
prospective population study.
Public Health and Environmental
Exposure to Cadmium (PheeCad)
Study Group. Lancet 353, 1140-4
- Stayner L, Smith R,
Thun M, Schnorr T, Lemen R
(1992) A dose-response analysis
and quantitative assessment of
lung cancer risk and
occupational cadmium exposure.
Ann Epidemiol 2,177-94
- Takenaka S, Oldiges H,
Konig H, Hochrainer D,
Oberdorster G (1983)
Carcinogenicity of cadmium
chloride aerosols in W rats. J
Natl Cancer Inst 70, 367-73
- TERA (Toxicology
Excellence for Risk Assessment)
(2000) Cadmium Nephrotoxicity
in occupational and general
populations. Report prepared
for US EPA.
- Thun MJ, Schnorr TM,
Smith AB, Halperin WE, Lemen RA
(1985) Mortality among a cohort
of U.S. cadmium production
workers--an update. J Natl
Cancer Inst, 74, 325-33
- US EPA (1999)
Toxicological Review: Cadmium
and compounds. External Review
Draft.
- US OSHA (1999), OSHA
Preambles: Cadmium. Available
online at
http://www.osha.gov/pls/oshaweb/owasrch.search_form?p_doc_type=PREAMBLES&p_toc_level=1&p_keyvalue=Cadmium
(revision date: July 30,
1999)