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Assessment of the toxicity of mixtures of halogenated dibenzo-p-dioxins and dibenzofurans by use of toxicity equivalency factors (TEF)
C h e m o s p h e r e , V o l . 1 9 , No.12, P r i n t e d in G r e a t B r i t a i n
pp
1881-1895,
1989
0045-6535/89 $3.00 Pergamon Press plc
+
.OO
ASSESSMENT OF THE TOXICITY OF MIXTURES OF HALOGENATED DIBENZO-P-DIOXINS AND DIBENZOFURANS BY USE OF TOXICITY EOUIVALENCY FACTORS (TEF) i .. 2 Job A. van Zorge , J?op H. van W13nen , Rob M,C. Theelen 3 , Kees Olie I and Martin van den Berg ~. i Directorate-General for Environmental Protection, Directorate for Chemicals and Risk Management, P.O. Box 450, 2260 MB Leidschendam 2 Section Public Health and Environment, Municipal Health Service, P.O. Box 20244, i000 HE Amsterdam 3 Lab. of Toxicology, National Institute of Public Health and Environmental Protection, P.O. Box I, 3720 BA Bilthoven 4 Lab. of Environmental and Toxicological Chemistry, University of Amsterdam~ Nieuwe Achtergracht 166, 1018 WV Amsterdam s Dept. of Veterinary Pharmacology, Pharmacie and Toxicology, University of Utrecht, P.O. Box 80176, 3508 TD Utrecht Abstract In March 1988 the authors completed a report on the risk assessment of mixtures of halogenated dlbenzo-p-dioxins and dibenzofurans at the request of the National Institute of Public Health and Environmental Protection in the Netherlands. Based on a study of the current knowledge of these compounds, a set of toxicity equivalency factors was proposed for use in such risk assessments. This paper presents the section of this report that deals with the literature on the effects of these compounds and the choice of toxicity equivalency factors. The uncertanties in the selection and application of toxicity equivalency factors are discussed. Introduction
Halogenated dibenzo-p-dioxins and of
dibenzofurans are formed as byproducts in the synthesis
a large variety of chemicals (PCB's, chlorophenols, etc.) and in combustion processes.
The formation of chlorinated dibenzo-p-dioxins
and dibenzofurans can, in principle, occur
in any process, that entails the heating of organic compounds in the presence of inorganic chlorides and oxygen. Metals can act as catalysts in this process 2-6. Many years of analytical investigation have shown the worldwide presence of polychlorinated dlbenzo-p-dioxins (PCDD)
and dibenzofurans (PCDF) in air, water, sediment and biota,
including
humans v-11.
Until now the environmental occurrence of other halogenated dibenzo-p-dioxins and dibenzofurans has hardly been studied yet. zo-p-dioxins (PBDD) and dibenzofurans when
Recently the attention was drawn to brominated diben(PBDF).
The formation of these compounds can occur
organobromlne containing chemicals, which are e.g.
extensively used as
fire-retar-
dants in building and isolation materials, are heated 12-14 . Mixtures of chlorobromo diben-
1881
1882
zo-p-dioxins and dibenzofurans were detected in the emissions of a chemical waste
incine-
rator and are expected to occur also in other incinerators Is-16. This paper is primarily focussed on the Furthermore
a
chlorinated
dlbenzo-p-dioxins and dlbenzofurans.
proposal is made for the assessment of the toxicity of PBDD and
PBDF
and
chlorobromo dibenzo-p-dioxins and dibenzofurans. The fluorine or iodine substituted series are expected to be less important and are not considered in this paper. The number of congeners of the chloro-, bromo- and
dibenzo-p-dioxins
amounts to 1700 and for
the similar dibenzofuran series to 3320, in total 5020 compounds.
chlorobromo
Data on the toxicity of
single compounds are very scarce and -
considering the large number of compounds involved
are not expected soon for a considerable number of these compounds.
-
In
general humans are exposed to complex mixtures of these compounds, not only via
envi-
ronmental uptake, mostly from dietary sources v'9 but also through occupational I~-19'22 and accidental 2°'21 routes of exposure. logy there is therefore a great need mixtures.
In
In health risk assessments and in regulatory for
methods
to
toxico-
assess the total toxicity of these
addition, a uniform, internationally accepted method would facilitate
exchange of knowledge.
Especially
because
during
the
the last years there is a tendency to
present the results of analytical studies using different TEF-systems to calculate the toxicity of mixtures of PCDD and PCDF, thus hampering interstudy comparison. The aim of this paper is to propose such a
method by applying - not reviewing - the exis-
ting information on the toxicity of these compounds. For background information and critical evaluation of the toxicity data the
interested
reader is referred to the many excel-
lent reviews that have been published 2'3,Is'23-2s
The toxic congeners The
prototype and most extensively studied congener of the PCDD and PCDF is
trachloro dlbenzo-p-dloxln (2,3,7,8-TCDD), have
an extremely
toxic compound.
2,3,7,8-te-
Various studies
shown that the biologic and toxic properties of PCDD and PCDF are remarkably
dent on their chlorine substitution pattern;
the
depen-
most toxic congeners are substituted on
all four lateral (2,3,7 and 8) positions 29-31. Furthermore,
from the numerous analytical studies that have been carried out with tissues
of fish, birds, mammals and humans it was FCDD
concluded
that
retention occurs only of those
and PCDF in which at least four chlorine atoms are present and in which all
(2,3,7 and 8) positions are
substituted 31-3~.
lateral
In food the fully lateral substituted,
the
"toxic congeners", predominate 7-9'3s. For man this implies a biological filtering of these PCDD and PCDF congeners from the environment. This selective retention might be explained by the influence of multiple factors3~: -
Metabolism
occurs predominantly on the lateral position and is delayed by substitution
with chlorine atoms;
1883
- Selective binding to transport proteins, e.g.
prealbumln can possibly influence
extra-
cellular selective uptake and transport; - On the intracellular level there is a strong affinity of toxic congeners for the cytosolic receptors and certain cytochrome P-450 isoenzymes. The 2,3,7,8- substituted
congeners
show
long
biological
mans 43-45, leading to accumulation in various tissues.
half
lives especially in hu-
Within one species the
biological
half lives vary widely for the different toxic congeners 46 . In general the biological half live of these congeners will increase with increasing chlorination. Remarkable is the long biological half live of 2,3,4,7,8-PeCDF days
in
the liver
of the rat (108-193 days versus 27
for 2,3,7,8-TCDD) 39. This congener is an environmental contaminant which is found in
relatively high concentrations in patients
biological
samples like mothers milk 34'3s'4°
that were exposed to rice oil contaminated with PCB's containing PCDF
Also in the
slow
elimination of this congener has been shown 41-4s.
Toxicity Equivalency Factors (TEF) The large differences in toxicity between otherwise
substituted
the fully lateral chlorinated congeners and the
congeners together with the common toxic and biological
have led to the practice, of assessing
the
toxicity
presence of only the 2,3,7,8-substltuted congeners.
responses
of mixtures of PCDD and PCDF by the Substantial quantitative
differences
in toxicity have been found between the 2,3,7,8-substltuted and otherwise substituted FCDD and PCDF congeners.
Furthermore all 2,3,7,8-substituted congeners elicit the same toxico-
logical and biological responses, in which situation ners.
only the intensity varies 42.
In practice this
has resulted in toxicity assessments using only the 2,3,7,8-substituted
However, also on those 17 congeners
toxicological
conge-
data are scarce, especially re-
sults of chronic toxicity studies. In
1977 D.L. Grant was the first to
different congeners 4v.
apply weighing factors to grade the toxicity of
These toxicity equivalency
the
factors express the ratio of the toxi-
city of a given congener and the toxicity of 2,3,7,8-TCDD. By summation of the products of the individual congener concentrations with their
respective Toxicity Equivalency Factors
(TEFs), the toxicity of a mixture is calculated as a comparable amount of 2,3,7,8-TCDD. As more toxicological data became available to compare the toxicity of the individual congeners w i t h t h a t o f 2,3,7,8-TCDD, a d a p t a t i o n s but t h e p r i n c i p l e
were made i n G r a n t ' s c o n c e p t ( s e e t a b l e 1),
became more w i d e l y a c c e p t e d , b e c a u s e t h e b a s i c a s s u m p t i o n s were
confir-
med i n many e x p e r i m e n t s . These a r e : - The 2 , 3 , 7 , 8 - s u b s t i t u t e d
c o n g e n e r s show a s i m i l a r mechanism o f t o x i c a c t i o n ;
- Mixtures of 2,3,7,8-substituted
c o n g e n e r s show a d d i t i v e
F u r t h e r m o r e , no o t h e r method o f t o x i c i t y
toxicity.
assessment is available.
t o r y a g e n c i e s have d e v e l o p e d t h e i r own s y s t e m w i t h a d a p t e d T E F ' s . b e r o f t h e s e s y s t e m s 4 s - 5 6 . T h i s has l e d t o t h e c o n f u s i n g s i t u a t i o n ' tical
S i n c e t h e n many r e g u l a T a b l e 1 p r e s e n t s a numthat results
of analy-
s t u d i e s e x p r e s s e d a s e q u i v a l e n t s o f 2,3,7,8-TCDD c a n n o t be compared when
different
s y s t e m s have been u s e d .
1884
Table 1 : Review of a number of Toxicity Equivalent Factor systems in use. Grant* EPA
congener(s)
(1977)
(1981)
47
48
MDCD en DCDD TrCDD 2,3,7,8-TCDD other TCDDs 2,3,7,8-PeCDD other PeCDDs sum PeCDDs 2,3,7,8-HxCDDs other HxCDDs i sum 8xCDD 2,3,7,8-HpCDD other HpCDD sum HpCDD OCDD MCDF, DCDF and TrCDF 2,3,7,8-TCDF other TCDF sum TCDF 2,3,7,8-PeCDFs other PeCDFs [ sum PeCDFs I 2,3,7,8- HxCDFs other 8xCDFs [ sum 8xCDFs l 2,3,7,8-HpCDFs other HpCDFs I sum HpCDFs I OCDF
S w i t z e r l a n d New York Ontario California (1982) (Eadon,1982) (1982) ( G r a v i t z , 49
0 0 i 0 0,1 0
0 0 i I 0 0
0,1 0
0 0
0,1 0
0 0
0
0
0 0,i 0
0 0 0
0,i 0
0 0
1 O
0 0
0,I 0
0 0
0
0
0 0 1 0,01
1983)53
52
50,51
N e t h e r l EPA (1985) !(1986)
54,55
56
0 0 i 0 1 0
0 1 1 0,01 1 0,01
0 0 1 0 1 0
0 0 1 0 0,i 0
0 0 1 0,01 0,5 0,005
0,03 0
1 0,01
1 0
0,i 0
0,04 0,0004
0 0
I 0,01
1 0
0 0
0,001 0,00001
0
0
1
0
0
0 0,33 0
0 0,02 0,002
0 1 0
0 0,1 0
0 0,1 0,001
0,33 0
0,02 0,0002
I 0
0,I 0
0,1 0,001
0,01 0
0,02 0,0002
i 0
0,i 0
0,01 0,0001
0 0
0,02 0,0002
I 0
0 0
0,001 0,0001
0
0
0
0
0
0,I 0,I 0,01 0 0 0,1 0,i 0,1
v-s6 = l i t ref. * = Grant originally
0,01 0
regarded all 2,3,7-chlorlnated PCDD and PCDF as "toxic congeners"
Assessment of the toxicity of mixtures; a proposal for TEFs Toxicity
d a t a on i n v i v o and i n v i t r o
the literature.
The a c t i v i t y
effects
o f t h e most
was a s s i g n e d a v a l u e o f one and t h e a c t i v i t i e s tive
to 2,3,7,8-TCDD.
225'5)-6v
In addition
Most
the
compiled
from
compound - 2 , 3 , 7 , 8 - T C D D - i n t h e s e t e s t s
o f t h e o t h e r compounds were e x p r e s s e d r e l a -
results
to t h e s e d a t a t h e r e s u l t s
t h e t o x i c c o n g e n e r s were a l s o in
of
o f t h e t o x i c c o n g e n e r s were
toxic of
this
practice
of studies
taken into account, e.g.
studies
c h i c k e n embryo 68, i n d u c t i o n o f c h l o r a c n e i n t h e r a b b i t
are presented
with only a limited
in table number o f
on i n d u c t i o n o f c h i c k edema
e a r 69 and on
aryl
hydrocarbon
hydroxylase inducing potency in rats v°'~*. In the assessment of the toxicity of the individual congeners the following preference has been given:
1885
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1886
- Chronic studies over subchronlc studies and the latter again over acute studies; - Studies with mammals over studies with other species; - In vivo studies over in vitro studies.
F u r t h e r m o r e , t h e f o l l o w i n g c o n s i d e r a t i o n s have been t a k e n i n t o a c c o u n t : - Carcinogenic effects
have been d e m o n s t r a t e d
for
2,3,7,8-TCDD v2'v~ and a 1:2 m i x t u r e o f
It
i s not p o s s i b l e to d i s c r i m i n a t e between
1 , 2 , 3 , 6 , 7 , 8 - H x C D D and 1,2,3,7,8,9-HxCDD 6 4 ' 6 s . the
latter
two
isomers.
Other
data
on
the
toxicity
of
the
third
isomer
1 , 2 , 3 , 4 , 7 , 8 - H x C D D , g i v e no c a u s e t o a s s e s s t h i s compound d i f f e r e n t l y ; - The a v a i l a b l e these
d a t a on t h e h e x a c h l o r o
isomers.
dibenzofurans
preclude
a differentiation
Arguments t o a s i g n t h e s e d i b e n z o f u r a n i s o m e r s t h e t o x i c i t y
f a c t o r of the hexachloro d i b e n z o - p - d i o x i n s are based in p a r t i c u l a r
between
equivalency
on t h e r e s u l t s
of s e -
m i - c h r o n i c s t u d i e s w i t h r a t s 6°-62 and t h e s t u d y w i t h rainbow t r o u t e l e u t h e r o embryos63; - Hardly
any d a t a a r e a v a i l a b l e
dibenzofurans. chlorination.
on t h e t o x i c i t y
The c y t o c h r o m e P-450 I n two r e c e n t s t u d i e s
c+d
of the heptachloro dibenzo-p-dioxins
induction
seems
and
to d e c r e a s e with i n c r e a s i n g
t h e i n d u c t i o n o f c y t o c h r o m e P-450 c+d has been a s s o -
c i a t e d w i t h t h e tumor p r o m o t i n g a c t i v i t y
of
dioxins 7v'va.
The c a r c i n o g e n i c p o t e n c y in
l a b o r a t o r y a n i m a l s o f c o r r e s p o n d i n g h e x a c h l o r o d i b e n z o - p - d i o x i n s s u g g e s t s t h a t the t o x i c h e p t a c o n g e n e r s a l s o may e x e r t a expected
to
tumor
promoting
activity.
This a c t i v i t y
i s however
be l e s s p o t e n t t h a n t h a t o f t h e HxCDDs, but more p o t e n t t h a n t h a t
of
the
OCDD; - An
important
activity. 2,3,7,8-
a s p e c t of the t o x i c i t y
The
EDs0 for
immunosuppressive
response
in
mice
after
immunosuppressive one o r a l d o s e o f
TCDD, 1 , 2 , 3 , 6 , 7 , 8 - H x C D D and 1 , 2 , 3 , 4 , 6 , 7 , 8 - H p C D D a r e r e s p e c t i v e l y
85 ~ g / k g b.w. ~4'v5 of
of the t o x i c congeners i s t h e i r
0.65,
7.1 and
Based on t h e s e s t u d i e s and on t h e o t h e r d a t a i n t a b l e 2, TEF v a l u e s
0.1 and 0.01 f o r r e s p e c t i v e l y
1,2,3,6,7,8-HxCDD and 1 , 2 , 3 , 4 , 6 , 7 , 8 - H p C D D seem
reaso-
nable; - The o c t a c h l o r o compounds show h a r d l y l e v e l s 2s.
s o r p t i o n o f t h e s e compounds. nic
any
activity
T h i s s u g g e s t s a low t o x i c i t y ,
in acute studies,
The l a t t e r
possibility
The EROD a c t i v i t y
ximum o n l y a f t e r
was r i s i n g
e i g h t weeks.
d e n t EROD a c t i v i t y .
i n c r e a s e o f c y t o c h r o m e P-450 c+d
s l o w l y d u r i n g t h e e x p e r i m e n t and r e a c h e d i t s ma-
2,3,7,8-TCDD i s a s t r o n g i n d u c e r o f P-450 c+d and
The b i o c h e m i c a l
re-
i s s u p p o r t e d by a r e c e n t s e m i - c h r o -
s t u d y w i t h OCDD i n t h e r a t 76 showing a s u b s t a n t i a l
isozymes.
even a t h i g h dose
which may a l s o be c a u s e d by t h e v e r y slow
activity
of
depen-
OCDD i s e s t i m a t e d to be two to t h r e e
o r d e r s o f m a g n i t u d e l o v e r t h a n t h a t o f 2,3,7,8-TCDD. OCDD has a v e r y l o n g b i o l o g i c a l gener in biotic larly
tissues.
half life.
Due to t h e
Quantitatively
chronic
i n l o n g l i v i n g s p e c i e s l i k e humans.
it
i s t h e most i m p o r t a n t con-
exposure it accumulates readily,
been d e t e c t e d i n human a d i p o s e t i s s u e and human b r e a s t m i l k 3 4 ' 3 8 ' 4 ° ' 4 s . tions
particu-
R e l a t i v e l y h i g h c o n c e n t r a t i o n s o f OCDD have These c o n s i d e r a -
have l e d t o t h e p r o p o s a l o f a TEF o f 0.001 f o r OCDD. T o x i c o l o g i c a l d a t a a r e p r e -
s e n t l y not a v a i l a b l e
f o r OCDF. Because
OCDF and OCDD t h e same t o x i c i t y
of
the
planar structural
relationship
e q u i v a l e n c y f a c t o r i s a p p l i e d t o b o t h compounds.
between
1887
Based
on
the data and considerations mentioned above we propose the use of the
equivalency factors presented in table 3 to PCDF.
assess
the
The factors used in this proposal are 0.5 - 0.1 - 0.05 - 0.01 and 0.001.
the current scientific information a further within
numerical
the different isomer groups seems not possible.
toxic pentachlorodibenzofurans,
toxicity
toxicity of mixtures of PCDD and Based
on
differentiation of the TEF values An exception is made for the
two
which have been studied in more detail.
In comparison with the congeners presented in
table 3 the toxicity of all other chlorina-
ted dibenzo-p-dioxins and dibenzofurans is considered to be negligible. Consequently their TEF-values are set at zero.
Table 3 : Proposed toxicity equivalency factors
compound
TEF
2,3,7,8 1,2,3,7,8
-TCDD -PeCDD
1 0,5
1,2,3,4,7,8 1,2,3,6,7,8 1,2,3,7,8,9
-HxCDD -HxCDD -HxCDD
0,1 0,1 0,1
1,2,3,4,6,7,8
-HpCDD
0,01
1,2,3,4,6,7,8,9-OCDD
compound
2,3 7,8 1,2,3 7,8 2,3 4 , 7 , 8 1,2,3 4 , 7 , 8 1,2,3 6,7,8 1,2,3 7,8,9 2,3,4,6,7,8 1,2,3,4,6,7,8 1,2,3,4,7,8,9
TEF
-TCDF -PeCDF -PeCDF -HxCDF -HxCDF -HxCDF -HxCDF -HpCDF -HpCDF
0,i 0,05 0,5 0,I 0,i 0,I 0,I 0,01 0,01 0,001
1,2,3,4,6,7,8,9-0CDF
0,001
Hardly any data are published on the toxicity of the brominated and mixed chlorinated brominated homologues of the PCDDs and PCDF. the toxicities of these compounds are arralogues.
The few data available give the impression that
comparable
with
those of the corresponding chloro
Therefore, we suggest to use the same toxicity equivalency factors for
these
series of compounds, until new data allow a better evaluation of their toxicity. To
check
the proposed toxicity equivalency values in table 3, these have
with the toxicity data of table 2. given
been
compared
Ideally, the quotients of the relative activities of a
compound and the corresponding toxicity equivalency factors should be one for
each
test system. These quotients are presented in table 4. In fact they often differ from one. However,
it is more important,
that the overall picture of the deviations is not unbalan-
ced, and that for the most important criteria the value of one is not exceeded.
Toxicity equivalency factors~ limitations and uncertainties
The
use of toxicity equivalency factors in hazard assessments must be regarded as a prag-
matic approach to assess the toxicity of mixtures of PCDDs and PCDF
based on their chemi-
cal composition. At present, it is the only available approach for such an assessment. The toxicity equivalency factors presented here, are
identical
to
in
use since March 1988 in the Netherlands,
the later published international toxicity equivalency
factors
by
1888
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1889
NATO/CCMS (Committee on the Challenges of become
Modern Society) 94 .
available they will have to be re-evaluated.
When new toxicological data
Furthermore it is possible that
the
use of toxicity equivalency factors, which is based on the principle of additlvity, may be of
limited use.
As it is known that some compounds, e.g.
antagonlstic e6 effect on the activity of 2,3,7,8-TCDD,
PCBs, have a synergistic 83
or
the toxicity of samples containing
such mixtures cannot be assessed properly by this approach. The
following comments may illustrate the limitations of the TEF-concept as such
and
of
the proposed TEF: 1. Only
two chronic toxicity studies have been published which allow an estimation of the
carcinogenic potency of these isomers was used.
compounds 64'v2. In one of these studies a mixture of two
The carcinogenicity of
these compounds is based on an extremely po-
werful promotor effect (see also under 5.).
Quantitative data on this cancer promoting
effect for the other congeners are lacking; 2. Comparable
seml-chronic
2,3,7,8-TCDD,
1,2,3,7,8-PeCDD,
1,2,3,4,8-PeCDF relationships substituted
toxicity
data
are
1,2,3,7,8-PeCDF,
i n one a n i m a l s p e c i e s 6 ° - 6 2 .
synergism between the
laterally
substituted
congener did not contribute
to the toxicity
by
certain effect
2,3,4,7,8-PeCDF not
certain
and PCBs i n mice 82.
if a synergistic
now
in
interaction
studies
receptors.
the otherwise substituted
trout eleuthero
activity
embryos v2. Such a
as h a s been shown i n a
the current
of
scientific
study
knowledge,
with it
is
non-coplanar
i n a number o f p u b l i s h e d s t u d i e s a l - s s ;
is
observed
between effect
the laterally
substituted
was a l s o r e p o r t e d
between
w i t h PCDDs, PCDFs and PCBs h a v e n o t o f f e r e d mechanism o f t o x i c i t y
S.
S a f e and
covorkers
the
expected
o f t h e s e compounds.
i s c a u s e d by i n t e r a c t i o n have studied extensively
It
w i t h o n l y one t h e r o l e of t h e
regulating
d e p e n d i n g i s o z y m e s 89
the
synthesis
of
cytochrome
P1-450 (P-450 c+d,
The i n d u c t i o n o f t h e s e i s o z y m e s h a s been a s s o c i a t e d
on tumor i n i t i a t o r s
vv'vs.
between the binding of the different biochemical
However, t h e r e a p p e a r s to be l i t t l e PCDD and PCDF-congeners to t h e
and t o x i c e f f e c t s .
with cor-
Ah-recep-
A much h i g h e r c o r r e l a t i o n
found between t h e i n d u c t i o n o f t h e c y t o c h r o m e P1-450 d e p e n d i n g i s o e n z y m e s and t o x i c
effects the
on
R e c e n t l y an a n t a g o n i s t i c
t o t and t h e p o t e n c y o f t h e i r is
whereas
was
i n t h e mechanism o f a c t i o n o f t h e s e compounds 42. I t h a s been shown t h a t t h e
the promoting effect relation
rainbow
not understood whether or not lethality
Ah-receptor is at least P-448)
c o n g e n e r s and t h e o t h e r w i s e
1,3,6,8-TCDF88;
toxicity
Ah-receptor
and
structure-activity
may a l s o be e x p e c t e d f o r c e r t a i n
results
b r e a k t h r o u g h i n t h e u n d e r s t a n d i n g of t h e or several
1,2,3,6,7,8-HxCDF
of a mixture6°;
Based
interaction
PCDDs/ PCDFs and PCBs s 6 . S v . 2,3,7,8-TCDD and
congeners:
In the semi-chronic study additivity
i s however n o t always p r e s e n t ,
PCBs b e c a u s e o f t h e c o n t r a d i c t o r y
is still
substituted
six
c o p l a n a r PCBs h a s b e e n o b s e r v e d on t h e t e r a t o g e n i c
4. Sometimes an a n t a g o n i s t i c
5. U n t i l
for
2,3,4,7,8-PeCDF,
congeners,
2,3,7,8-TCDD i n mice 81 and on l e t h a l i t y synergistic
available
Based on t h e a v a i l a b l e
congeners is not expected 3°'v9-81.
observed for the laterally 3. S y n e r g i s m
only
like
teratogenicity,
number o f b i o l o g i c a l
the intensity
t h y m i c a t r o p h y and l o s s o f body w e i g h t s v ' 9 ° ' 9 ~ . effects
of these effects
o f t h e s e compounds and t h e poor
and t h e
Ah-receptor
binding,
Because of
correlation
the existence
between
of at least
1890
two different receptors has been postulated 42'43 . Strong arguments for this postulation are found in the synergistic interaction of certain coplanar PCBs and of chloronaphthalenes with 2,3,7,8-TCDD activity. The lack of knowledge of I) the biological receptors which mediate in the effects of PCDDs and PCDFs and 2) the toxicological relevance of many of these effects, especially for
hu-
mans, precludes an unequivocal scientific basis of the TEF approach. Nevertheless there is a great need for a uniform method to assess the health risks involved. In the TEF proposal presented here it has been tried to judge the toxicological importance of these effects.
Acknowledgement The authors wish to thank Mrs. H.J.H. Brachel (M.H.S.) for typing the manuscript.
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antagonist:
Effects
on enzyme i n d u c t i o n and i m m u n o t o x i c i t y . T o x i c o l o g y 46, 29-42 ( 1 9 8 7 ) . 8~ B a n n i s t e r ,
R. e t a l .
The e f f e c t s
of receptor
antagonists
on AHH i n d u c t i o n a c t i v i t y
of
2,3,7,8-TCDD i n C57BL/6 and DBA/2 mice. Chemosphere 16, 1739-1742 ( 1 9 8 7 ) . 8, D a v i s , D. ners:
et al.
Immunosuppressive Activities
Quantitative
Structure-Activity
of Polychlorinated
Relationships
DibenZofuran
and I n t e r a c t i v e
Effects.
CongeToxicol.
Appl. P h a r m a c o l . 94, 141-149 ( 1 9 8 8 ) . 89 S a f e , 90 P o l a n d
S. 2,3,7,8-TCDD B i o c h e m i c a l E f f e c t s . A.
and
Glover,
E.
Chemosphere 12, 447-451 ( 1 9 8 3 ) .
Genetic Expression of Aryl
2,3,7,8-Tetrachlorodibenzo-p-dloxin:
Evidence
for
Hydrocarbon
Hydroxylase
by
a Receptor Mutation in Genetically
N o n r e s p o n s i v e Mice. Mol. P h a r m a c o l . 11, 289-398 ( 1 9 7 5 ) . 9, P o l a n d , A. and G l o v e r , E. w i t h t h e Ah l o c u s .
92 McKinney, J . D . e t a l . for Polychlorinated
2,3,7,8-Tetrachlorodibenzo-p-dioxin:
S e g r e g a t i o n of t o x i c i t y
Mol. P h a r m a c o l . 17, 8 6 - 9 4 ( 1 9 8 0 ) . Structure
- Induction
versus Structure
- Toxicity Relationships
B i p h e n y l s and R e l a t e d A r o m a t i c H y d r o c a r b o n s .
Environ.
Health Pers-
p e c t . 60, 57-68 ( 1 9 8 5 ) . 93 V i c k e r s , A.E.M. Health Perspect. 94 NATO/CCMS. Compounds.
Pilot
et al.
Mechanisms of A c t i o n o f T o x i c H a l o g e n a t e d A r o m a t i c s .
Environ.
59, 121-128 ( 1 9 8 5 ) . s t u d y on I n t e r n a t i o n a l
International
Exchange
on D i o x i n s and R e l a t e d
Toxicity Equivalency Factor ((I-TEF).
Method o f R i s k A s s e s s -
ment f o r Complex M i x t u r e s o f
Dioxins
Information and
Related
Compounds.
O r g a n i s a t i o n / C o m m i t t e e on t h e C h a l l a n g e s o f Modern S o c i e t y .
North
Atlantic
R e p o r t number 176.
1988.
(Received in Germany 31 August 1989; accepted 25 September 1989)
Treaty August
pp
1881-1895,
1989
0045-6535/89 $3.00 Pergamon Press plc
+
.OO
ASSESSMENT OF THE TOXICITY OF MIXTURES OF HALOGENATED DIBENZO-P-DIOXINS AND DIBENZOFURANS BY USE OF TOXICITY EOUIVALENCY FACTORS (TEF) i .. 2 Job A. van Zorge , J?op H. van W13nen , Rob M,C. Theelen 3 , Kees Olie I and Martin van den Berg ~. i Directorate-General for Environmental Protection, Directorate for Chemicals and Risk Management, P.O. Box 450, 2260 MB Leidschendam 2 Section Public Health and Environment, Municipal Health Service, P.O. Box 20244, i000 HE Amsterdam 3 Lab. of Toxicology, National Institute of Public Health and Environmental Protection, P.O. Box I, 3720 BA Bilthoven 4 Lab. of Environmental and Toxicological Chemistry, University of Amsterdam~ Nieuwe Achtergracht 166, 1018 WV Amsterdam s Dept. of Veterinary Pharmacology, Pharmacie and Toxicology, University of Utrecht, P.O. Box 80176, 3508 TD Utrecht Abstract In March 1988 the authors completed a report on the risk assessment of mixtures of halogenated dlbenzo-p-dioxins and dibenzofurans at the request of the National Institute of Public Health and Environmental Protection in the Netherlands. Based on a study of the current knowledge of these compounds, a set of toxicity equivalency factors was proposed for use in such risk assessments. This paper presents the section of this report that deals with the literature on the effects of these compounds and the choice of toxicity equivalency factors. The uncertanties in the selection and application of toxicity equivalency factors are discussed. Introduction
Halogenated dibenzo-p-dioxins and of
dibenzofurans are formed as byproducts in the synthesis
a large variety of chemicals (PCB's, chlorophenols, etc.) and in combustion processes.
The formation of chlorinated dibenzo-p-dioxins
and dibenzofurans can, in principle, occur
in any process, that entails the heating of organic compounds in the presence of inorganic chlorides and oxygen. Metals can act as catalysts in this process 2-6. Many years of analytical investigation have shown the worldwide presence of polychlorinated dlbenzo-p-dioxins (PCDD)
and dibenzofurans (PCDF) in air, water, sediment and biota,
including
humans v-11.
Until now the environmental occurrence of other halogenated dibenzo-p-dioxins and dibenzofurans has hardly been studied yet. zo-p-dioxins (PBDD) and dibenzofurans when
Recently the attention was drawn to brominated diben(PBDF).
The formation of these compounds can occur
organobromlne containing chemicals, which are e.g.
extensively used as
fire-retar-
dants in building and isolation materials, are heated 12-14 . Mixtures of chlorobromo diben-
1881
1882
zo-p-dioxins and dibenzofurans were detected in the emissions of a chemical waste
incine-
rator and are expected to occur also in other incinerators Is-16. This paper is primarily focussed on the Furthermore
a
chlorinated
dlbenzo-p-dioxins and dlbenzofurans.
proposal is made for the assessment of the toxicity of PBDD and
PBDF
and
chlorobromo dibenzo-p-dioxins and dibenzofurans. The fluorine or iodine substituted series are expected to be less important and are not considered in this paper. The number of congeners of the chloro-, bromo- and
dibenzo-p-dioxins
amounts to 1700 and for
the similar dibenzofuran series to 3320, in total 5020 compounds.
chlorobromo
Data on the toxicity of
single compounds are very scarce and -
considering the large number of compounds involved
are not expected soon for a considerable number of these compounds.
-
In
general humans are exposed to complex mixtures of these compounds, not only via
envi-
ronmental uptake, mostly from dietary sources v'9 but also through occupational I~-19'22 and accidental 2°'21 routes of exposure. logy there is therefore a great need mixtures.
In
In health risk assessments and in regulatory for
methods
to
toxico-
assess the total toxicity of these
addition, a uniform, internationally accepted method would facilitate
exchange of knowledge.
Especially
because
during
the
the last years there is a tendency to
present the results of analytical studies using different TEF-systems to calculate the toxicity of mixtures of PCDD and PCDF, thus hampering interstudy comparison. The aim of this paper is to propose such a
method by applying - not reviewing - the exis-
ting information on the toxicity of these compounds. For background information and critical evaluation of the toxicity data the
interested
reader is referred to the many excel-
lent reviews that have been published 2'3,Is'23-2s
The toxic congeners The
prototype and most extensively studied congener of the PCDD and PCDF is
trachloro dlbenzo-p-dloxln (2,3,7,8-TCDD), have
an extremely
toxic compound.
2,3,7,8-te-
Various studies
shown that the biologic and toxic properties of PCDD and PCDF are remarkably
dent on their chlorine substitution pattern;
the
depen-
most toxic congeners are substituted on
all four lateral (2,3,7 and 8) positions 29-31. Furthermore,
from the numerous analytical studies that have been carried out with tissues
of fish, birds, mammals and humans it was FCDD
concluded
that
retention occurs only of those
and PCDF in which at least four chlorine atoms are present and in which all
(2,3,7 and 8) positions are
substituted 31-3~.
lateral
In food the fully lateral substituted,
the
"toxic congeners", predominate 7-9'3s. For man this implies a biological filtering of these PCDD and PCDF congeners from the environment. This selective retention might be explained by the influence of multiple factors3~: -
Metabolism
occurs predominantly on the lateral position and is delayed by substitution
with chlorine atoms;
1883
- Selective binding to transport proteins, e.g.
prealbumln can possibly influence
extra-
cellular selective uptake and transport; - On the intracellular level there is a strong affinity of toxic congeners for the cytosolic receptors and certain cytochrome P-450 isoenzymes. The 2,3,7,8- substituted
congeners
show
long
biological
mans 43-45, leading to accumulation in various tissues.
half
lives especially in hu-
Within one species the
biological
half lives vary widely for the different toxic congeners 46 . In general the biological half live of these congeners will increase with increasing chlorination. Remarkable is the long biological half live of 2,3,4,7,8-PeCDF days
in
the liver
of the rat (108-193 days versus 27
for 2,3,7,8-TCDD) 39. This congener is an environmental contaminant which is found in
relatively high concentrations in patients
biological
samples like mothers milk 34'3s'4°
that were exposed to rice oil contaminated with PCB's containing PCDF
Also in the
slow
elimination of this congener has been shown 41-4s.
Toxicity Equivalency Factors (TEF) The large differences in toxicity between otherwise
substituted
the fully lateral chlorinated congeners and the
congeners together with the common toxic and biological
have led to the practice, of assessing
the
toxicity
presence of only the 2,3,7,8-substltuted congeners.
responses
of mixtures of PCDD and PCDF by the Substantial quantitative
differences
in toxicity have been found between the 2,3,7,8-substltuted and otherwise substituted FCDD and PCDF congeners.
Furthermore all 2,3,7,8-substituted congeners elicit the same toxico-
logical and biological responses, in which situation ners.
only the intensity varies 42.
In practice this
has resulted in toxicity assessments using only the 2,3,7,8-substituted
However, also on those 17 congeners
toxicological
conge-
data are scarce, especially re-
sults of chronic toxicity studies. In
1977 D.L. Grant was the first to
different congeners 4v.
apply weighing factors to grade the toxicity of
These toxicity equivalency
the
factors express the ratio of the toxi-
city of a given congener and the toxicity of 2,3,7,8-TCDD. By summation of the products of the individual congener concentrations with their
respective Toxicity Equivalency Factors
(TEFs), the toxicity of a mixture is calculated as a comparable amount of 2,3,7,8-TCDD. As more toxicological data became available to compare the toxicity of the individual congeners w i t h t h a t o f 2,3,7,8-TCDD, a d a p t a t i o n s but t h e p r i n c i p l e
were made i n G r a n t ' s c o n c e p t ( s e e t a b l e 1),
became more w i d e l y a c c e p t e d , b e c a u s e t h e b a s i c a s s u m p t i o n s were
confir-
med i n many e x p e r i m e n t s . These a r e : - The 2 , 3 , 7 , 8 - s u b s t i t u t e d
c o n g e n e r s show a s i m i l a r mechanism o f t o x i c a c t i o n ;
- Mixtures of 2,3,7,8-substituted
c o n g e n e r s show a d d i t i v e
F u r t h e r m o r e , no o t h e r method o f t o x i c i t y
toxicity.
assessment is available.
t o r y a g e n c i e s have d e v e l o p e d t h e i r own s y s t e m w i t h a d a p t e d T E F ' s . b e r o f t h e s e s y s t e m s 4 s - 5 6 . T h i s has l e d t o t h e c o n f u s i n g s i t u a t i o n ' tical
S i n c e t h e n many r e g u l a T a b l e 1 p r e s e n t s a numthat results
of analy-
s t u d i e s e x p r e s s e d a s e q u i v a l e n t s o f 2,3,7,8-TCDD c a n n o t be compared when
different
s y s t e m s have been u s e d .
1884
Table 1 : Review of a number of Toxicity Equivalent Factor systems in use. Grant* EPA
congener(s)
(1977)
(1981)
47
48
MDCD en DCDD TrCDD 2,3,7,8-TCDD other TCDDs 2,3,7,8-PeCDD other PeCDDs sum PeCDDs 2,3,7,8-HxCDDs other HxCDDs i sum 8xCDD 2,3,7,8-HpCDD other HpCDD sum HpCDD OCDD MCDF, DCDF and TrCDF 2,3,7,8-TCDF other TCDF sum TCDF 2,3,7,8-PeCDFs other PeCDFs [ sum PeCDFs I 2,3,7,8- HxCDFs other 8xCDFs [ sum 8xCDFs l 2,3,7,8-HpCDFs other HpCDFs I sum HpCDFs I OCDF
S w i t z e r l a n d New York Ontario California (1982) (Eadon,1982) (1982) ( G r a v i t z , 49
0 0 i 0 0,1 0
0 0 i I 0 0
0,1 0
0 0
0,1 0
0 0
0
0
0 0,i 0
0 0 0
0,i 0
0 0
1 O
0 0
0,I 0
0 0
0
0
0 0 1 0,01
1983)53
52
50,51
N e t h e r l EPA (1985) !(1986)
54,55
56
0 0 i 0 1 0
0 1 1 0,01 1 0,01
0 0 1 0 1 0
0 0 1 0 0,i 0
0 0 1 0,01 0,5 0,005
0,03 0
1 0,01
1 0
0,i 0
0,04 0,0004
0 0
I 0,01
1 0
0 0
0,001 0,00001
0
0
1
0
0
0 0,33 0
0 0,02 0,002
0 1 0
0 0,1 0
0 0,1 0,001
0,33 0
0,02 0,0002
I 0
0,I 0
0,1 0,001
0,01 0
0,02 0,0002
i 0
0,i 0
0,01 0,0001
0 0
0,02 0,0002
I 0
0 0
0,001 0,0001
0
0
0
0
0
0,I 0,I 0,01 0 0 0,1 0,i 0,1
v-s6 = l i t ref. * = Grant originally
0,01 0
regarded all 2,3,7-chlorlnated PCDD and PCDF as "toxic congeners"
Assessment of the toxicity of mixtures; a proposal for TEFs Toxicity
d a t a on i n v i v o and i n v i t r o
the literature.
The a c t i v i t y
effects
o f t h e most
was a s s i g n e d a v a l u e o f one and t h e a c t i v i t i e s tive
to 2,3,7,8-TCDD.
225'5)-6v
In addition
Most
the
compiled
from
compound - 2 , 3 , 7 , 8 - T C D D - i n t h e s e t e s t s
o f t h e o t h e r compounds were e x p r e s s e d r e l a -
results
to t h e s e d a t a t h e r e s u l t s
t h e t o x i c c o n g e n e r s were a l s o in
of
o f t h e t o x i c c o n g e n e r s were
toxic of
this
practice
of studies
taken into account, e.g.
studies
c h i c k e n embryo 68, i n d u c t i o n o f c h l o r a c n e i n t h e r a b b i t
are presented
with only a limited
in table number o f
on i n d u c t i o n o f c h i c k edema
e a r 69 and on
aryl
hydrocarbon
hydroxylase inducing potency in rats v°'~*. In the assessment of the toxicity of the individual congeners the following preference has been given:
1885
.~ ,., ;~.~,~o
~n
o
u~4a
N~a
'~
.o.
u
~o
~
+
~,
.~
~
~
. . o . .~
o
~
o
o
o
o
o
o
o
,-4
o
o
-,-I = . ~
~o
o
o
r~
o
.r4
- . ~
. s~
o~ m,4
~, 0
0
.
.
o
.
.
o
o
.
o
o
. o
o
I~
o
o.Q ~
o
o
o
o
o
o
o
o
o
o
o
o
o
0
v
~
1
v
o"
- ~
o o
~
:
o
v
o
o
o
~
~
o
o v
o
o ^
o
..
•
4.1
o~ •
,'4 •
°
.
~.~ u
.,4 II
o
o
o
o
o
o
o
o
",4 ~a
U
ii
r~
.
.
.
.
N I
.
I
o
1886
- Chronic studies over subchronlc studies and the latter again over acute studies; - Studies with mammals over studies with other species; - In vivo studies over in vitro studies.
F u r t h e r m o r e , t h e f o l l o w i n g c o n s i d e r a t i o n s have been t a k e n i n t o a c c o u n t : - Carcinogenic effects
have been d e m o n s t r a t e d
for
2,3,7,8-TCDD v2'v~ and a 1:2 m i x t u r e o f
It
i s not p o s s i b l e to d i s c r i m i n a t e between
1 , 2 , 3 , 6 , 7 , 8 - H x C D D and 1,2,3,7,8,9-HxCDD 6 4 ' 6 s . the
latter
two
isomers.
Other
data
on
the
toxicity
of
the
third
isomer
1 , 2 , 3 , 4 , 7 , 8 - H x C D D , g i v e no c a u s e t o a s s e s s t h i s compound d i f f e r e n t l y ; - The a v a i l a b l e these
d a t a on t h e h e x a c h l o r o
isomers.
dibenzofurans
preclude
a differentiation
Arguments t o a s i g n t h e s e d i b e n z o f u r a n i s o m e r s t h e t o x i c i t y
f a c t o r of the hexachloro d i b e n z o - p - d i o x i n s are based in p a r t i c u l a r
between
equivalency
on t h e r e s u l t s
of s e -
m i - c h r o n i c s t u d i e s w i t h r a t s 6°-62 and t h e s t u d y w i t h rainbow t r o u t e l e u t h e r o embryos63; - Hardly
any d a t a a r e a v a i l a b l e
dibenzofurans. chlorination.
on t h e t o x i c i t y
The c y t o c h r o m e P-450 I n two r e c e n t s t u d i e s
c+d
of the heptachloro dibenzo-p-dioxins
induction
seems
and
to d e c r e a s e with i n c r e a s i n g
t h e i n d u c t i o n o f c y t o c h r o m e P-450 c+d has been a s s o -
c i a t e d w i t h t h e tumor p r o m o t i n g a c t i v i t y
of
dioxins 7v'va.
The c a r c i n o g e n i c p o t e n c y in
l a b o r a t o r y a n i m a l s o f c o r r e s p o n d i n g h e x a c h l o r o d i b e n z o - p - d i o x i n s s u g g e s t s t h a t the t o x i c h e p t a c o n g e n e r s a l s o may e x e r t a expected
to
tumor
promoting
activity.
This a c t i v i t y
i s however
be l e s s p o t e n t t h a n t h a t o f t h e HxCDDs, but more p o t e n t t h a n t h a t
of
the
OCDD; - An
important
activity. 2,3,7,8-
a s p e c t of the t o x i c i t y
The
EDs0 for
immunosuppressive
response
in
mice
after
immunosuppressive one o r a l d o s e o f
TCDD, 1 , 2 , 3 , 6 , 7 , 8 - H x C D D and 1 , 2 , 3 , 4 , 6 , 7 , 8 - H p C D D a r e r e s p e c t i v e l y
85 ~ g / k g b.w. ~4'v5 of
of the t o x i c congeners i s t h e i r
0.65,
7.1 and
Based on t h e s e s t u d i e s and on t h e o t h e r d a t a i n t a b l e 2, TEF v a l u e s
0.1 and 0.01 f o r r e s p e c t i v e l y
1,2,3,6,7,8-HxCDD and 1 , 2 , 3 , 4 , 6 , 7 , 8 - H p C D D seem
reaso-
nable; - The o c t a c h l o r o compounds show h a r d l y l e v e l s 2s.
s o r p t i o n o f t h e s e compounds. nic
any
activity
T h i s s u g g e s t s a low t o x i c i t y ,
in acute studies,
The l a t t e r
possibility
The EROD a c t i v i t y
ximum o n l y a f t e r
was r i s i n g
e i g h t weeks.
d e n t EROD a c t i v i t y .
i n c r e a s e o f c y t o c h r o m e P-450 c+d
s l o w l y d u r i n g t h e e x p e r i m e n t and r e a c h e d i t s ma-
2,3,7,8-TCDD i s a s t r o n g i n d u c e r o f P-450 c+d and
The b i o c h e m i c a l
re-
i s s u p p o r t e d by a r e c e n t s e m i - c h r o -
s t u d y w i t h OCDD i n t h e r a t 76 showing a s u b s t a n t i a l
isozymes.
even a t h i g h dose
which may a l s o be c a u s e d by t h e v e r y slow
activity
of
depen-
OCDD i s e s t i m a t e d to be two to t h r e e
o r d e r s o f m a g n i t u d e l o v e r t h a n t h a t o f 2,3,7,8-TCDD. OCDD has a v e r y l o n g b i o l o g i c a l gener in biotic larly
tissues.
half life.
Due to t h e
Quantitatively
chronic
i n l o n g l i v i n g s p e c i e s l i k e humans.
it
i s t h e most i m p o r t a n t con-
exposure it accumulates readily,
been d e t e c t e d i n human a d i p o s e t i s s u e and human b r e a s t m i l k 3 4 ' 3 8 ' 4 ° ' 4 s . tions
particu-
R e l a t i v e l y h i g h c o n c e n t r a t i o n s o f OCDD have These c o n s i d e r a -
have l e d t o t h e p r o p o s a l o f a TEF o f 0.001 f o r OCDD. T o x i c o l o g i c a l d a t a a r e p r e -
s e n t l y not a v a i l a b l e
f o r OCDF. Because
OCDF and OCDD t h e same t o x i c i t y
of
the
planar structural
relationship
e q u i v a l e n c y f a c t o r i s a p p l i e d t o b o t h compounds.
between
1887
Based
on
the data and considerations mentioned above we propose the use of the
equivalency factors presented in table 3 to PCDF.
assess
the
The factors used in this proposal are 0.5 - 0.1 - 0.05 - 0.01 and 0.001.
the current scientific information a further within
numerical
the different isomer groups seems not possible.
toxic pentachlorodibenzofurans,
toxicity
toxicity of mixtures of PCDD and Based
on
differentiation of the TEF values An exception is made for the
two
which have been studied in more detail.
In comparison with the congeners presented in
table 3 the toxicity of all other chlorina-
ted dibenzo-p-dioxins and dibenzofurans is considered to be negligible. Consequently their TEF-values are set at zero.
Table 3 : Proposed toxicity equivalency factors
compound
TEF
2,3,7,8 1,2,3,7,8
-TCDD -PeCDD
1 0,5
1,2,3,4,7,8 1,2,3,6,7,8 1,2,3,7,8,9
-HxCDD -HxCDD -HxCDD
0,1 0,1 0,1
1,2,3,4,6,7,8
-HpCDD
0,01
1,2,3,4,6,7,8,9-OCDD
compound
2,3 7,8 1,2,3 7,8 2,3 4 , 7 , 8 1,2,3 4 , 7 , 8 1,2,3 6,7,8 1,2,3 7,8,9 2,3,4,6,7,8 1,2,3,4,6,7,8 1,2,3,4,7,8,9
TEF
-TCDF -PeCDF -PeCDF -HxCDF -HxCDF -HxCDF -HxCDF -HpCDF -HpCDF
0,i 0,05 0,5 0,I 0,i 0,I 0,I 0,01 0,01 0,001
1,2,3,4,6,7,8,9-0CDF
0,001
Hardly any data are published on the toxicity of the brominated and mixed chlorinated brominated homologues of the PCDDs and PCDF. the toxicities of these compounds are arralogues.
The few data available give the impression that
comparable
with
those of the corresponding chloro
Therefore, we suggest to use the same toxicity equivalency factors for
these
series of compounds, until new data allow a better evaluation of their toxicity. To
check
the proposed toxicity equivalency values in table 3, these have
with the toxicity data of table 2. given
been
compared
Ideally, the quotients of the relative activities of a
compound and the corresponding toxicity equivalency factors should be one for
each
test system. These quotients are presented in table 4. In fact they often differ from one. However,
it is more important,
that the overall picture of the deviations is not unbalan-
ced, and that for the most important criteria the value of one is not exceeded.
Toxicity equivalency factors~ limitations and uncertainties
The
use of toxicity equivalency factors in hazard assessments must be regarded as a prag-
matic approach to assess the toxicity of mixtures of PCDDs and PCDF
based on their chemi-
cal composition. At present, it is the only available approach for such an assessment. The toxicity equivalency factors presented here, are
identical
to
in
use since March 1988 in the Netherlands,
the later published international toxicity equivalency
factors
by
1888
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NATO/CCMS (Committee on the Challenges of become
Modern Society) 94 .
available they will have to be re-evaluated.
When new toxicological data
Furthermore it is possible that
the
use of toxicity equivalency factors, which is based on the principle of additlvity, may be of
limited use.
As it is known that some compounds, e.g.
antagonlstic e6 effect on the activity of 2,3,7,8-TCDD,
PCBs, have a synergistic 83
or
the toxicity of samples containing
such mixtures cannot be assessed properly by this approach. The
following comments may illustrate the limitations of the TEF-concept as such
and
of
the proposed TEF: 1. Only
two chronic toxicity studies have been published which allow an estimation of the
carcinogenic potency of these isomers was used.
compounds 64'v2. In one of these studies a mixture of two
The carcinogenicity of
these compounds is based on an extremely po-
werful promotor effect (see also under 5.).
Quantitative data on this cancer promoting
effect for the other congeners are lacking; 2. Comparable
seml-chronic
2,3,7,8-TCDD,
1,2,3,7,8-PeCDD,
1,2,3,4,8-PeCDF relationships substituted
toxicity
data
are
1,2,3,7,8-PeCDF,
i n one a n i m a l s p e c i e s 6 ° - 6 2 .
synergism between the
laterally
substituted
congener did not contribute
to the toxicity
by
certain effect
2,3,4,7,8-PeCDF not
certain
and PCBs i n mice 82.
if a synergistic
now
in
interaction
studies
receptors.
the otherwise substituted
trout eleuthero
activity
embryos v2. Such a
as h a s been shown i n a
the current
of
scientific
study
knowledge,
with it
is
non-coplanar
i n a number o f p u b l i s h e d s t u d i e s a l - s s ;
is
observed
between effect
the laterally
substituted
was a l s o r e p o r t e d
between
w i t h PCDDs, PCDFs and PCBs h a v e n o t o f f e r e d mechanism o f t o x i c i t y
S.
S a f e and
covorkers
the
expected
o f t h e s e compounds.
i s c a u s e d by i n t e r a c t i o n have studied extensively
It
w i t h o n l y one t h e r o l e of t h e
regulating
d e p e n d i n g i s o z y m e s 89
the
synthesis
of
cytochrome
P1-450 (P-450 c+d,
The i n d u c t i o n o f t h e s e i s o z y m e s h a s been a s s o c i a t e d
on tumor i n i t i a t o r s
vv'vs.
between the binding of the different biochemical
However, t h e r e a p p e a r s to be l i t t l e PCDD and PCDF-congeners to t h e
and t o x i c e f f e c t s .
with cor-
Ah-recep-
A much h i g h e r c o r r e l a t i o n
found between t h e i n d u c t i o n o f t h e c y t o c h r o m e P1-450 d e p e n d i n g i s o e n z y m e s and t o x i c
effects the
on
R e c e n t l y an a n t a g o n i s t i c
t o t and t h e p o t e n c y o f t h e i r is
whereas
was
i n t h e mechanism o f a c t i o n o f t h e s e compounds 42. I t h a s been shown t h a t t h e
the promoting effect relation
rainbow
not understood whether or not lethality
Ah-receptor is at least P-448)
c o n g e n e r s and t h e o t h e r w i s e
1,3,6,8-TCDF88;
toxicity
Ah-receptor
and
structure-activity
may a l s o be e x p e c t e d f o r c e r t a i n
results
b r e a k t h r o u g h i n t h e u n d e r s t a n d i n g of t h e or several
1,2,3,6,7,8-HxCDF
of a mixture6°;
Based
interaction
PCDDs/ PCDFs and PCBs s 6 . S v . 2,3,7,8-TCDD and
congeners:
In the semi-chronic study additivity
i s however n o t always p r e s e n t ,
PCBs b e c a u s e o f t h e c o n t r a d i c t o r y
is still
substituted
six
c o p l a n a r PCBs h a s b e e n o b s e r v e d on t h e t e r a t o g e n i c
4. Sometimes an a n t a g o n i s t i c
5. U n t i l
for
2,3,4,7,8-PeCDF,
congeners,
2,3,7,8-TCDD i n mice 81 and on l e t h a l i t y synergistic
available
Based on t h e a v a i l a b l e
congeners is not expected 3°'v9-81.
observed for the laterally 3. S y n e r g i s m
only
like
teratogenicity,
number o f b i o l o g i c a l
the intensity
t h y m i c a t r o p h y and l o s s o f body w e i g h t s v ' 9 ° ' 9 ~ . effects
of these effects
o f t h e s e compounds and t h e poor
and t h e
Ah-receptor
binding,
Because of
correlation
the existence
between
of at least
1890
two different receptors has been postulated 42'43 . Strong arguments for this postulation are found in the synergistic interaction of certain coplanar PCBs and of chloronaphthalenes with 2,3,7,8-TCDD activity. The lack of knowledge of I) the biological receptors which mediate in the effects of PCDDs and PCDFs and 2) the toxicological relevance of many of these effects, especially for
hu-
mans, precludes an unequivocal scientific basis of the TEF approach. Nevertheless there is a great need for a uniform method to assess the health risks involved. In the TEF proposal presented here it has been tried to judge the toxicological importance of these effects.
Acknowledgement The authors wish to thank Mrs. H.J.H. Brachel (M.H.S.) for typing the manuscript.
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(Received in Germany 31 August 1989; accepted 25 September 1989)
Treaty August