Dioxins and PCBs
Acute (急性) and chronic (慢性) 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.
Dioxin toxicity varies among the congeners
International Toxicity Equivalency Factors (TEF) for PCDDs and PCDFs
Congener
2378
PCDD series PCDF series
1 (defined) 0.1
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.
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.
Food Ocean fish Meat TCDD (pg/g) 500 35
Cheese
Sour cream Cream Ice cream Cottage cheese Milk Coffee
16
10 7.2 5.5 2.1 1.8 0.1
Orange juice
0.2
Formation of dioxins
Atrazine
Malathion Aspirin
1,870
1,200 1,000
Nicotine
Dioxin (TCDD) Botulin toxin
1
0.001 0.00001
Almost every chemical is toxic at some level, and the difference between toxic and nontoxic chemicals is a matter of degree.
• 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.
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.
• 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
How much dioxin do we congest?
Country
Estimated Dietary Intakes of PCDD/Fs per day
Dietary Intake (pg I-TEQ/day)
Canada
Germany Italy Japan Netherlands Norway Russia
9 8 1 9 2 8 1 2
O
7 6
O
4
3
7 6
O
4
3
Dibenzodioxin PCDD
Dibenzofuran PCDF
Cl
O
Cl
Cl
Cl
Cl
O
2378-TCDD
Cl
Cl
O
2378-TCDF
Cl
Toxicity of Dioxins
• Extraordinary toxic to lab animals (LD50 in male guinea pigs is only 0.6 mg/kg)
• 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.
• 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 (氯痤倉), 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.
LD50 of selected chemicals for rats or mice
Chemical Sugar ethanol Vinegar Table salt LD50 (mg/kg) Chemical 29,700 14,000 3,310 3,000 Caffeine DDT Arsenic Parathion LD50 (mg/kg) 130 100 48 3.6
Dioxins
(二噁英):
General structures
• The term dioxins denotes a family of chemical compounds, known as polycholorinated dibenzopara-dioxins (PCDDs) and polycholrinated dibenzofurans (P-480 63 70 51-85 139
UK
US Hong Kong
70
18-192 105
Source: An Assessment of Dioxin Emissions in Hong Kong, HKEPD, 2000.
Average content of 2378-TCDD in various foodstuff (U.S)
• 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.