环境化学英文课件18.ppt

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. 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 of selected chemicals for rats or mice Dioxins (二噁英): General structures The term dioxins denotes a family of chemical compounds, known as polycholorinated dibenzo-para-dioxins (PCDDs) and polycholrinated dibenzofurans (PCDFs) Dioxins: Congeners 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 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. 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 congener