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Dioxins and PCBs

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Title: Dioxins and PCBs


1
  • Dioxins and PCBs

2
Acute (??) and chronic (??) toxicity
  • Acute toxicity refers to a rapid and serious
    response to a high but short-lived dose.
  • Acute poisons interfere with essential
    physiological processes, leading to a variety
    symptoms of distress, even death.
  • Chronic toxicity refers to a time-lagged response
    to a relatively low but prolonged exposure.
  • Effects are more subtle.
  • Set in motion a chain of biochemical events that
    lead to disease states, including cancer.

3
Index of acute toxicityLD50
LD50 is the lethal dose for 50 of a population.
Dose is expressed as the weight of a chemical
consumed by an experimental animal per kg of body
weight of the animal.
4
LD50 of selected chemicals for rats or mice
Almost every chemical is toxic at some level, and
the difference between toxic and nontoxic
chemicals is a matter of degree.
5
Dioxins (???) General structures
  • The term dioxins denotes a family of chemical
    compounds, known as polycholorinated
    dibenzo-para-dioxins (PCDDs) and polycholrinated
    dibenzofurans (PCDFs)

General structure
6
Dioxins Congeners
Congener a member of the same class, or
group. Each individual PCDD or PCDF is termed a
congener. There are 75 PCDD congeners and 135
PCDF congeners.
7
Toxicity of Dioxins
  • Extraordinary toxic to lab animals (LD50 in male
    guinea pigs is only 0.6 mg/kg)
  • Birth defects
  • Cancer
  • Skin disorders
  • Liver damage
  • Suppression of the immune systems
  • Variation in toxicity among species is large.
  • Male guinea pig, oral, LD50 is 0.6 mg/kg.
  • Hamster, intraperitoneum, LD50 is 3000 mg/kg.
  • Risk to humans is less clear

8
Human risk of dioxin exposure
  • USEPA concludes that dioxins likely increase
    cancer incidence.
  • WHO classifies 2378-TCDD as a probable human
    carcinogen.
  • At high levels, PCDDs cause chloracne (???), a
    painful skin inflammation.
  • Risk from breathing dioxin-laden air is minimal.
  • The main exposure route for human is dietary
    (meat, dairy products, and fish). (Due to
    bioaccumulation of dioxins)
  • Absorption of dioxins by infant through mothers
    milk is efficient.

9
Multiple exposure pathways
10
Biochemistry of dioxin toxicity
  • PCDD/Fs are planar aromatic molecules.
  • The planar structure allows them to bind to Ah
    (Aryl hydrocarbon) receptor protein that is
    present in all animal species.
  • The Ah receptor interacts with the cells DNA.
  • Dioxin toxicity is roughly proportional to the
    strength of binding to the Ah receptor.
  • This explains that 2378-TCDD is the most toxic
    one.

11
Dioxin toxicity varies among the congeners
International Toxicity Equivalency Factors (TEF)
for PCDDs and PCDFs
The TEFs allow the conversion of various congener
concentrations into a single toxicity equivalent
quantity (TEQ)
12
Dioxin toxicity general rules
  • TCDD is the most toxic of the dioxins
  • Toxicity decreases when Cl atoms are removed from
    2,3,7,8 positions or added to the remaining
    positions on the rings.
  • There are similar toxicity pattern for PCDD and
    PCDF series.

13
How much dioxin do we congest?Estimated Dietary
Intakes of PCDD/Fs per day
Source An Assessment of Dioxin Emissions in Hong
Kong, HKEPD, 2000.
14
Average content of 2378-TCDD in various foodstuff
(U.S)
15
Formation of dioxins
  • Combustion
  • In any situation where Cl, C, H, and O come into
    contact with heat, PCDD/Fs could be formed as
    trace by-products.
  • Dioxin emission correlates with the Cl content of
    the combustion feed.
  • Paper pulp bleaching with chlorine
  • Chlorine is used to bleach paper pulp.
  • PCDDs are formed probably through chlorination of
    the phenolic groups in lignin.
  • Manufacture of certain chlorophenol chemicals
  • Dioxin was produced as a contaminant of the
    herbicide 2,4,5-T, a component of Agent Orange

16
Formation of dioxins in combustion processes
  • Incomplete combustion of organic wastes in the
    combustion chamber leads to the formation of
    organic fragments which serve as organic
    precursors to the PCDD/F molecule.
  • The waste provides a source of chlorine, and of
    metals. The latter are incorporated into fly ash,
    which carries over to the cooler (250-400oC)
    post-combustion zone of the incineration system.
  • The organic precursors adsorb onto the surface of
    the fly ash in the post-combustion zone, and
    following a complex sequence of reactions which
    are catalyzed by metals (primarily copper) in the
    fly ash, lead to the formation of PCDD/Fs along
    with other chlorinated trace organics.

17
High-temperature step
PCDF contamination from PCB oxidation
18
Dioxin emission inventory (US)
19
Agent orange
  • 5050 mix of two herbicides 2,4-D and 2,4,5-T.
  • Used as a defoliant for broad-leaf trees during
    Vietnam war.
  • Contains dioxins as contaminants.
  • In 1966, dioxin residues were found in fish,
    shell-fish, and mothers milk in the Vietnam
    target areas.
  • In 1978 and 1979, Vietnam veterans in the U.S.
    began complaining of a variety of symptoms such
    as numbness, skin rashes, liver problems, and
    birth defects in their children.

20
Estimated Dioxin emission inventory(1997 in HK)
Source An Assessment of Dioxin Emissions in Hong
Kong, HKEPD, 2000.
21
Dioxin Are there natural sources?
  • Analysis of archived UK soil samples dating from
    the pre-1900s confirmed the presence of dioxins.
    (Large scale manufacture and use of chlorinated
    chemicals started after 1900s.)
  • Wood burning emits dioxin
  • NaCl in wood ? HCl in combustion zone
  • Organochlorines are widely produced by a variety
    of microorganisms.

22
Man-made sources outweigh natural sources
  • From 1940 to 1970
  • deposition increased eightfold.
  • Great expansion in the use of Cl
  • Forest fires diminished by more than a factor of
    four

Dioxin deposition on Lake Siskiwit on an island
in Lake Superior
  • After 1970
  • Deposition decreased
  • 2,4,5-T phaseout

23
Polychlorinated biphenyl (PCB)
Excellent insulators, chemically stable, low
flammability and vapor pressure.
Used as coolant in power transformers and
capacitors.
  • a total of 209 congeners
  • Additional uses
  • Heat transfer fluids in machinery
  • Plasticizers for PVC and other polymers
  • De-inking agents for recycled newspapers

24
Toxicity of PCBs
  • Less toxic than PCDD/Fs, but more abundant than
    PCDD/Fs in the environment.
  • Probable carcinogen
  • Cause skin chloracne.
  • Learning deficits.

Effects of in utero PCB exposure
Test scores of 4-year-old children vs. PCB
concentrations in the umbilical cord serum at
birth.
25
EPA orders General Electric to dredge PCBs from
Hudson River (Source Pollution Online
newsletter, 12/4/2001) WASHINGTON, Dec 04, 2001
(AP WorldStream via COMTEX) The Bush
administration ordered tons of PCBs removed from
New York's upper Hudson River, setting in motion
one of the largest dredging operations in the
nation's history. General Electric Co. dumped
1.3 million pounds (585,000 kilograms) of PCBs
into the river before the federal government
banned the pollutants in 1977. The company
bitterly opposes dredging, which expected to cost
GE dlrs 500 million. . PCBs, used as insulation
and a coolant, have been linked to cancer in
laboratory animals. The EPA classifies the oily
substance as a probable carcinogen and says PCBs
pose risks to wildlife and to people who eat fish
from the Hudson. GE released PCBs from its
plants in Fort Edward and Hudson Falls, about 40
miles (64 kilometers) north of Albany. The
cleanup will include locations in that stretch,
reaching almost down to the capital city. ..
26
Study questions
  • What is acute toxicity? What is the index to
    express acute toxicity?
  • What is chronic toxicity? Give an example of
    chronic health effect.
  • Draw chemical structures of dioxins and PCBs.
    What are congeners?
  • Does dioxins bioaccumulate? Why?
  • What are the major sources for dioxins?
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