United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington DC 20460
Research and Development
EPA/600/S8-84/014F August 1988
&EPA Project Summary
Health Assessment
Document for Polychlorinated
Dibenzo-p-Dioxins
The full health assessment
document on polychlorlnated
dibenzo-p-dioxlns discusses mul-
timedia environmental issues per-
taining to the most toxic chlorinated
dioxins, namely 2,3,7,8-tetrachloro-;
1,2,3,7,8-pentachloro-; 1,2,3,6,7,8-
hexachloro-; and 1,2,3,7,8,9-
hexachlorodibenzo-p-dioxins.
Scientifically valid data essential for
human health risk assessment
purposes from an extensive literature
search were compiled and discussed
critically. Discussions are based on
physiochemical properties and ana-
lytical methodologies; stability and
degradation; production, use, syn-
thesis; environmental resources and
environmental levels; environmental
fate and transport; degradation; bio-
accumulation and bloconcentration
factors; ecological effects; various
aspects of toxic effects from acute,
subchronic and chronic exposure in
experimental animals and humans;
pharmacokinetics and mechanism of
toxic effects; teratogenicity and
reproductive effects; mutagenicity
and carcinogenicity. Based on this
review, critical studies have been
identified and utilized for estimating
the unit risk.
This Project Summary was
developed by EPA's Environmental
Criteria and Assessment Office,
Cincinnati, OH, to announce key
findings of the research project that
is fully documented In a separate
report of the same t/t/e (see Project
Report ordering information at back).
Introduction
Dioxins are a class of compounds that
contain the dibenzo-p-dioxin nucleus.
In chlorinated dioxins, the dibenzo-p-
dioxin nucleus is substituted with
chlorine at different positions of the fused
benzene rings. Depending on the number
and position of chlorine substitution, 75
congeners are possible for the
chlorinated dioxins. The full document
deals with the most toxic chlorinated
dioxins, namely, 2,3,7,8-tetrachloro-,
1,2,3,7,8-pentachloro-, 1,2,3,6,7,8-
hexachloro- and 1,2,3,7,8,9-hexa-
chlorodibenzo-p-dioxin. Of these four
congeners, the 2,3,7,8-tetrachloro-
dibenzo-p-dioxin has been studied
extensively and is often described in both
popular and technical literature as
"TCDD" or simply "dioxin".
Few documents exist at the present
time that deal with selected aspects of
polychlorinated dibenzo-p-dioxins m
the environmental media. The full
document, however, provides compre-
hensive multimedia assessment of the
analytical methodologies, environmental
levels and ecological and health effects
of the four chlorinated dioxins Table 1
lists the acronyms used when discussing
the polychlorinated dibenzo-p-dioxins
Discussion
Polychlorinated dibenzo-p-dioxins
are a class of chlorinated tricyclic
aromatic hydrocarbons consisting of two
benzene rings connected by a pair of
oxygen atoms. According to the position
and number of chlorine atoms it is
possible to form 75 different congeners
of chlorinated dioxins. The word
"dioxins" is often used to refer to this
class of compounds, especially with
respect to the highly toxic and
environmentally widely distributed
2,3,7,8-tetrachlorodibenzo-p-dioxin
(TCDD). This class of compounds is
rather stable toward heat, acids and
alkalis. The solubility of 2,3,7,8-TCDD in
water is 0.2 ng/l. This isomer and the
three other PCDDs discussed in the full
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Table 1. Acronyms for polychlorinated dibenzo-p-
dioxins
Acronym Full name
PCDDs Polychlorinated dibenzo-p-dioxions
2,3,7,8-TCDD 2,3,7,8-Tetrachlorodibenzo-p-dioxin
1,2,3,7,8-PeCDD 1,2,3,7,8-Pentachlorodibenzo-p-dioxin
1,2,3,6,7,8-HxCDD 1,2,3,6,7,8-Hexachlorodibenzo-p-dioxin
1,2.3.7.a.9-HxCDD 1,2,3.7.8.9-Hexachlorodibenzo-p-dioxin
document are soluble in certain aromatic
and aliphatic solvents. The PCDDs are
chemically relatively stable and start to
decompose at temperatures >500°C;
the percent of decomposition depends
upon the residence time in the high
temperature zone and the proportion of
oxygen in the heated zone.
The commonly used method for the
determination of these compounds in
different samples consists of solvent
extraction, followed by sulfuric acid and
base washes to remove lipids and other
impurities from the solvent extract. The
extract is then subjected to two liquid
chromatographic clean-up procedures.
The cleaned-up extract is finally
analyzed for the PCDDs by the gas
chromatographic-mass spectrometric
methods. Despite the specialized
methods used for the determination of
PCDDs, the results of analysis at very
low levels (possibly <9 ppt in biological
matrices) can be questionable unless
special precautions including addition of
internal standards, are made.
None of the PCDDs are either
commercially manufactured or have any
known use. They are produced as
unwanted contaminants primarily during
the manufacture of chlorophenols and
their derivatives. The primary sources of
PCDD contamination in the environment
result from the industrial manufacture of
chlorophenols and their derivatives and
the subsequent disposal of wastes from
these industries. Municipal incineration
may also produce some environmental
emission of PCDDs. From the available
data, it is difficult to ascertain the
comparative importance of these three
sources in contributing to environmental
emissions. The 1,2,3,7,8-PeCDD found
in environmental samples has only been
reported in emissions from incinerators.
The monitoring data to date indicate
that the maximum level of PCDDs is
likely to be found in soil and drainage
sediment samples near chlorophenol
manufacturing industries and chemical
waste disposal sites.
The environmental fates of the four
PCCDs are not known with certainty.
Most of the investigations in this field
have been conducted with 2,3,7,8-
TCDD, and the conclusions regarding the
environmental fate of the other three
PCDDs have been drawn by analogy.
Few data exist in the literature that would
indicate significant chemical and
biological transformation of these com-
pounds in atmospheric, aquatic or soil
media. The role of photochemical
transformation in determining the fates of
these chemicals in various ambient
media is not known with certainty, but
the PCDDs are susceptible to pho-
tochemical reactions in the presence of
hydrogen donors. In the aquatic media, a
substantial proportion of the PCDDs may
be present in the sediment-sorbed state
or in the biota. In the atmosphere, the
PCDDs are expected to be present in the
vapor-phase and particulate-sorbed
states. The atmospheric transport of
these compounds can be predicted from
dispersion modeling equations. In the
case of the accidental release of
2,3,7,8-TCDD at Seveso, Italy, it has
been estimated from laboratory
experiments that 2,3,7,8-TCDD depo-
sition from air to soil follows an ex-
ponential decay pattern along the
downward wind direction. The most
probable transport mechanisms of the
PCDDs from soils are transport to at-
mosphere by contaminated dust parti-
cles, direct volatilization from the surface
or near surface zones (£ 5 cm), and
transport to surface water by eroded soil.
Both the calculated and the
experimental results show that the
PCDDs will concentrate in sediments and
biota present in aquatic media. In
mammals, 2,3,7,8-TCDD is readily
absorbed through the gastrointestinal
tract, and absorption through intact skin
has also been reported. Absorption may
decrease dramatically if 2,3,7,8-TCDD
is absorbed to particulate matter such as
activated carbon or soil. After absorption,
2,3,7,8-TCDD is distributed to tissues
high in lipid content; however, in many
species, the liver is a major storage site.
Metabolism of 2,3,7,8-TCDD occurs
slowly, with the polar metabolites
excreted in the urine and feces
Unmetabolized 2,3,7,8-TCDD can b(
eliminated in the feces and in the milk. I
is metabolized by the P-450 mono
oxygenase system through a reactiv*
epoxide intermediate. The metabolism o
2,3,7,8-TCDD seems to be ;
detoxification process resulting in thi
production of metabolites that are les
toxic than the parent compound. Avail
able scientific data support the contentioi
that the toxic response to 2,3,7,8-TCDI
exposure is mediation through cytosoli
Ah-receptor site binding.
The PCDDs discussed in the fu
document are among some of the moj
toxic compounds known, with the (owes
LD5q level for male guinea pigs, the mo;
sensitive species, being 0.6 ug/kg fc
2,3,7,8-TCDD. The other congeners ar
somewhat less toxic; however, the LDg
values are still in the ug/kg range
Although 2,3,7,8-TCDD is highly toxic i
all species tested, there are large specie
differences in sensitivity, with the LD£
for hamsters being 1157-5052 ng/k<
The characteristic signs and symptorr
of lethal poisoning are severe weight los
and thymic atrophy. Death usually occui
many days after the exposure. In rat
rabbits and mice, 2,3,7,8-TCDD pn
duces an acute liver injury that is n
observed in either monkeys, hamsters, <
guinea pigs. In mice, the immur
response is also suppressed. After sul
chronic or chronic exposure to 2,3,7,
TCDD in rats or mice, the liver appears
be the most severely affected orga
although systemic hemorrhage, eden
and suppressed thymic activity are all
observed. The limited data available f
the other PCDDs indicate that the:
chemicals produce the same sympton
as 2,3.7,8-TCDD in a given specie
however, the doses required are higher
Humans have been exposed
herbicides and other chlorinated chei
icals containing 2,3,7,8-TCDD as
contaminant. The symptoms of toxicity
many cases are similar to those observ
in animals, with exposure leading
altered liver function and lip
metabolism, porphyria cutanea tare
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neurotoxicity and pathologic changes in
hematologic parameters. In addition,
exposure of humans to 2,3,7,8-TCDD
produces skin lesions such as chloracne
and hyperpigmentation. Although some
signs such as chloracne are attributed to
the PCDDs, the other signs of toxicity
may arise, at least in part, from the other
chemical of which PCDDs are a minor
contaminant.
Animal studies have demonstrated
that 2,3,7,8-TCDD is teratogenic and
fetotoxic in rats, mice, rabbits and ferrets;
and fetotoxic in monkeys. Exposure to
2,3,7,8-TCDD in mice produces facial
clefts, while exposure in rats results in
edema, hemorrhage and kidney anom-
alies; rabbits have a higher incidence of
extra ribs. In rats, a reduction in the
gestation index, decreased fetal weight,
increased liver-to-body weight ratio
and increased incidence or dilated renal
pelvis in the offspring have been
observed. Certain human epidemiology
studies have shown positive associations
with exposure to chemicals contaminated
with 2,3,7,8-TCDD and birth defects
and abortions, while others have not.
There is a limited data base with
conflicting evidence for 2,3,7,8-TCDD's
mutagenic potential; therefore, the
available evidence is judged to be
inconclusive. There are no studies in the
published literature regarding the
mutagenicity of HxCDD or any other
congeners of PCDD.
There is evidence from chronic animal
cancer bioassay studies that 2,3,7,8-
TCDD and HxCDD are probable human
carcinogens. There are no chronic
cancer bioassay studies available that
evaluate the carcinogenic potential for
the other PCDDs. The available data for
2,3,7,8-TCDD and HxCDD come from
gavage and feeding studies, there being
no studies available for inhalation
exposure. The epidemiologic evidence
for the carcinogenicity of 2,3,7,8-TCDD
alone is inadequate while the evidence
for phenoxyacetic herbicides and/or
chlorophenols with 2,3,7,8-TCDD as an
impurity is limited. There have been no
epidemiologic evaluations, as yet, for
HxCDD as the sole compound of
concern.
A number of chronic animal cancer
bioassays show that 2,3,7,8-TCDD is an
animal carcinogen. In rats, oral exposure
to 2,3,7,8-TCDD resulted in an
increased incidence of hepatocellular
carcinomas, squamous cell carcinomas
of the tongue and hard palate/nasal
turbinates, and squamous cell car-
cinomas of the lung. In both male and
female mice, increased incidences of
liver tumors were observed. A mixture of
the two isomers of HxCDD, discussed in
the full document has been tested for
carcinogenicity and shows increased
incidences of liver tumors in rats and
mice. Also, 2,3,7,8-TCDD has produced
fibrosarcomas at the site of application
after dermal administration, although
there was no significant increase in
dermal tumors when the mixture of
HxCDDs was tested. Since both
compounds produce statistically
significant increased incidences of
tumors in two species of animals, there is
sufficient evidence, according to the EPA
weight-of-evidence classification cri-
teria, to conclude that both 2,3,7,8-
TCDD and HxCDD are animal carcino-
gens. The 2,3,7,8-TCDD has been
shown to be a promoter as well as an
initiator in rodent test systems. Evidence
is available from epidemiologic studies
that implicate exposure to herbicides
contaminated with 2,3,7,8-TCDD with a
significantly elevated risk of soft tissue
sarcomas and to a lesser extent non-
Hodgkins lymphomas; however, the
exposures to 2,3,7,8-TCDD were always
compounded with exposures to the
herbicides chemicals.
Assuming that 2,3,7,8-TCDD and
HxCDD are carcinogenic in humans,
upper bound incremental unit cancer
risks have been estimated for both
ingestion and inhalation exposure. The
unit risks have been estimated using a
multistage extrapolation model that is
linear at low doses. Available metabolism
and pharmacokinetic data are insufficient
to alter typically used assumptions for
estimating the human equivalent dose.
Since incidence data exist only for oral
studies in animal test systems, the
inhalation risk estimates are based upon
the cancer potency derived from the oral
studies along with appropriate conversion
assumptions.
Using data from a feeding study with
female rats, the upper limit incremental
cancer risk for 2,3,7,8-TCDD is es-
timated to be 1.56x10'1 per ng/kg/day.
The upper limit estimate of incremental
cancer risk is 4.5x10-3 for a continuous
lifetime exposure to 1 ng/l of 2,3,7,8-
TCDD in drinking water and 3.3x10'5 for
a continuous lifetime exposure to 1
pg/m3 of 2,3,7,8-TCDD in ambient air.
Using data from an ingestion study
with female rats and male mice, the
cancer potency for HxCDD is estimated
to be 6.2x10~3 per ng/kg/day. The upper
limit estimate of incremental cancer risk
is 1.8x1(H for a continuous lifetime
exposure to 1 ng/l of HxCDD in drinking
water and 1.3x10-6 for a continuous
lifetime exposure to 1 pg/m3 of HxCDD in
ambient air.
Conclusions
The PCDDs, 2,3,7,8-TCDD, 1,2,3,
7,8-PeCDD, 1,2,3,6,7,8- and 1,2,3,7,
8,9-HxCDD, are highly toxic following
acute exposure. All animal species
administered high levels of these
compounds developed weight loss and
thymic atrophy. In some species liver
damage, edema, hair loss and immuno-
suppression were also observed. Chronic
toxicity studies were conducted only on
2,3,7,8-TCDD and a mixture of the two
isomers of HxCDD. In these studies, the
primary nonneoplastic lesion was fatty
and necrotic change in the liver.
In the species studied, the fetus has
been shown to be highly sensitive to the
toxic effects of 2,3,7,8-TCDD. In rats the
fetotoxicity observed included hemor-
rhage, edema and kidney anomalies,
while in mice the predominant lesions
were cleft palate and kidney anomalies.
The lowest reported exposure in rats, 1
ng/kg, produced a significant (by some
analyses but not others) effect on the
fetus, and was similar to the lowest-
observed-adverse-effect level (LOAEL)
observed in chronic studies.
Evidence from oral animal cancer
bioassays is "sufficient" (according to
EPA and IARC criteria) to conclude that
2,3,7,8-TCDD and a mixture of the two
isomers of HxCDD are animal car-
cinogens. The 2,3,7,8-TCDD has
increased the incidence of a variety of
tumors, including liver tumors in rats and
mice, while the mixture of HxCDD tested
increased the incidence of liver tumors in
both sexes of rats and mice. In addition,
squamous cell carcinomas of the tongue
and hard palate/nasal turbinate and
squamous cell carcinomas of the lung
were observed in rats. The available
epidemiologic evidence for the car-
cinogenicity of 2,3,7,8-TCDD alone is
inadequate and there have been no
epidemiologic evaluations, as yet, for
HxCDD as the sole compound of
concern. Considering the animal evi-
dence together with the epidemiologic
data, the overall weight-of-evidence
classification for 2,3,7,8-TCDD using
EPA's classification scheme is category
B2 meaning that 2,3,7,8-TCDD should
be regarded as a "probable" human
carcinogen. The overall weight-of-
evidence classification for HxCDD is also
category B2 meaning that it should be
regarded as a "probable" human
carcinogen. In terms of low dose
potency, 2,3,7,8-TCDD and the HxCDD
mixture are the two most potent
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carcinogens evaluated by the EPA's
Carcinogen Assessment Group. Epi-
demiologic studies of workers exposed
to chemicals contaminated with 2,3,7,8-
TCDD such as 2,4,5-trichlorophen-
oxyacetic acid and 2,4,5-trichlorophenol
have produced positive findings that are
suggestive of an elevated risk of cancer
in humans. These epidemiologic findings
are not inconsistent with the premise that
2,3,7,8-TCDD is probably carcinogenic
for humans. There are no chronic studies
available regarding the carcinogenicity of
1,2,3,7,8-PeCDD.
Needs for Future Research
• The basic physical properties such as
water solubilities and vapor pressures
of the PeCDOs and HxCDDs need to
be determined. These parameters are
important in predicting the environ-
mental fate of these compounds.
• New analytical methodologies must
be established to determine the low
levels of these compounds in envi-
ronmental matrices without ambigu-
ity.
• More monitoring data, particularly in
air aquatic media as well as in
vegetables grown near urban incin-
erators, should be developed by a
diversity of research groups.
Isotopically labeled internal standard
compounds (37CI or 13CI) should be
prepared for PeCDDs and HxCDDs.
More research efforts should be
directed to determining the envi-
ronmental fate of the PeCDDs and
HxCDDs. The determination of the
fate of these chemicals with respect to
the possibility of photochemical
transformations in different environ-
mental matrices needs special
attention.
Pharmacokinetic studies should be
conducted to demonstrate, more
clearly the degree of absorption of the
PCDDs by all routes. In particular,
studies are needed on respiratory
absorption and on PCDDs absorbed to
environmental media.
Although a number of studies
demonstrate that 2,3,7,8-TCDD is a
teratogen, the other congeners should
be tested for teratogenic potential.
There is no information on the effects
of chronic exposure to 1,2,3,7,8-
PeCDD, and studies should be
conducted to determine both the toxic
effects of this compound and its
carcinogenic potential.
Further epidemiology data on the,
effects in human populations to
PCDDs might assist in determining
which effects observed in animals are
also present in humans. In these
studies, careful quantitation of PCDD
levels in humans and industrial
hygiene samples might provide
dose-response data necessary for
health assessment.
Bioavailability studies from
contaminated soil, fly ash, etc., are
needed.
Mechanism-of-action studies should
be conducted to determine the
fundamental mode of action of the
PCDDs.
New destruction methods should be
investigated in order to provide
feasible methods for decontaminating
environmental sites where PCDDs
have been detected.
Determine the BCF for all the most
toxic PCDDs in state-of-the-art
test systems.
The EPA Project Officer and author of this summary, Debdas Mukerjee, is with
Environmental Criteria and Assessment Office, Cincinnati, OH 45268.
The complete report, entitled "Health Assessment Document for Polychlorinated
Dibenzo-p-Dioxins," (Order No. PB 86-122 546/AS; Cost: $50.95, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, V'A 221'61
Telephone: 703-487-4650
The EPA Protect Officer can be contacted at:
Environmental Criteria and Assessment Office
U. S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S8-84/014F
0000329 PS
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