Dioxins And Pcbs

Dioxins And Pcbs - Transcript

Dioxins and PCBs

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.

Index of acute toxicity:LD50
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.

LD50 of selected chemicals for rats or mice
Chemical LD50 (mg/kg) Chemical LD50 (mg/kg)
Sugar 29,700 Caffeine 130
ethanol 14,000 DDT 100
Vinegar 3,310 Arsenic 48
Table salt 3,000 Parathion 3.6
Atrazine 1,870 Nicotine 1
Malathion 1,200 Dioxin (TCDD) 0.001
Aspirin 1,000 Botulin toxin 0.00001
Almost every chemical is toxic at some level, and the difference
between toxic and nontoxic chemicals is a matter of degree.

Dioxins (??? ): General structures
• The term dioxins denotes a family of chemical
compounds, known as polycholorinated dibenzo-
para-dioxins (PCDDs) and polycholrinated
dibenzofurans (PCDFs)
O
O
Dibenzodioxin
PCDD
O
Dibenzofuran
PCDF
Clx Cly
Clx
Cly
General structure

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.
O
O
Dibenzodioxin
PCDD
O
Dibenzofuran
PCDF
1
2
3
4
7
8
9
6
9
8
7
6
1
2
3
4
O
O
2378-TCDD
O
2378-TCDF
Cl
ClCl
Cl Cl
Cl
Cl
Cl

Toxicity of Dioxins
• Extraordinary toxic to lab animals (LD50 in male guinea
pigs is only 0.6 µg/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 µg/kg.
– Hamster, intraperitoneum, LD50 is 3000 µg/kg.
• Risk to humans is less clear

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.

Multiple exposure pathways

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 cell’s 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.

Dioxin toxicity varies among the congeners
Congener PCDD series PCDF series
2378 1 (defined) 0.1
12378 0.5 0.05
23478 0.5
123478,
123678,
123789
0.1 0.1
(also include 234678 congener)
1234678 0.01 0.01 (also include 1234789 congener)
octachloro 0.001 0.001
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)

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.

How much dioxin do we congest?
Estimated Dietary Intakes of PCDD/Fs per day
Country Dietary Intake (pg I-TEQ/day)
Canada 92
Germany 62-100
Italy 260-480
Japan 63
Netherlands 70
Norway 51-85
Russia 139
UK 70
US 18-192
Hong Kong 105
Source: An Assessment of Dioxin Emissions in Hong Kong, HKEPD, 2000.

Average content of 2378-TCDD in various
foodstuff (U.S)
Food TCDD (pg/g)
Ocean fish 500
Meat 35
Cheese 16
Sour cream 10
Cream 7.2
Ice cream 5.5
Cottage cheese 2.1
Milk 1.8
Coffee 0.1
Orange juice 0.2

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

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.
PCDF contamination from PCB oxidation
High-
temperature
step

Dioxin emission inventory (US)

Agent orange
• 50:50 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 mother’s 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.

Estimated Dioxin emission inventory
(1997 in HK)
Sources Emission (g I-TEQ)
MSW combustion 21-27
Coal combustion 0.4-20
Clinical waste
combustion
0.4-1.8
Land fill gas combustion 0.2-0.3
Non-ferrous metals 0.1-1.0
Cement manufacture 0.32
Crematoria 0.024
Cars 0.005-0.51
Goods vehicles 0.061-0.11
Buses 0.016-0.023
Motorcycles 0.0001-0.006
Source: An Assessment of
Dioxin Emissions in Hong
Kong, HKEPD, 2000.

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.

Man-made sources outweigh natural sources
Dioxin deposition on Lake Siskiwit
on an island in Lake Superior
From 1940 to 1970
•deposition increased
eightfold.
•Great expansion in the use
of Cl
•Forest fires diminished by
more than a factor of four
After 1970
•Deposition decreased
•2,4,5-T phaseout

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

Toxicity of PCBs
Test scores of 4-year-old children
vs. PCB concentrations in the
umbilical cord serum at birth.
Effects of in utero PCB exposure
•Less toxic than PCDD/Fs, but
more abundant than PCDD/Fs
in the environment.
•Probable carcinogen
•Cause skin chloracne.
•Learning deficits.

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.
…..

Study questions
1. What is acute toxicity? What is the index to express
acute toxicity?
2. What is chronic toxicity? Give an example of
chronic health effect.
3. Draw chemical structures of dioxins and PCBs.
What are congeners?
4. Does dioxins bioaccumulate? Why?
5. What are the major sources for dioxins?