Dioxin

TCDD is the most toxic of the chlorinated dioxins, while octachlorodioxin may be the least so. However, there are large differences in the susceptibility of species to suffering toxicity from TCDD. The guinea pig, for example, is extremely sensitive to TCDD, thousands of times more so than the hamster.

Short-term or acute toxicity is often indicated by a laboratory assay known as LD₅₀, or the dose of a chemical required to kill one-half of a test population of organisms over a period of several days. Guinea pigs have a LD for TCDD in food of only 0.0006 mg/kg (that is, 0.0006 mg of TCDD per kg body weight). In comparison, rats have a LD₅₀ for TCDD in food of 0.022-0.045 mg/kg, while hamsters have a LD₅₀ of 1 mg/kg. Clearly, TCDD is a highly toxic chemical, although species vary greatly in sensitivity.

Depending on the dose and biological sensitivity, the symptoms of TCDD toxicity in mammals can include severe weight loss, liver damage, lesions in the vascular system, stomach ulcers, a persistent acne known as chloracne, birth defects, and ultimately, death.

Much of what is known about the toxicity of TCDD to humans has come from studies of: (1) industrial exposures of chemical workers, (2) people living near a toxic waste dump at Times Beach, Missouri, and (3) an accidental event at Seveso, Italy, in 1976. The latter case involved an explosion at a chlorophenol plant that released an estimated 2.2-11 lb (1-5 kg) of TCDD to the surroundings, and caused residues as large as 51 ppm to occur in environmental samples. This accident caused the deaths of some livestock within 2-3 days, but remarkably it was not until 2.5 weeks had passed that about 700 people were evacuated from the severely contaminated residential area near the factory. The exposure of humans to TCDD at Seveso caused 187 diagnosed cases of chloracne, but there were apparently no statistically detectable increases in the rates of other human diseases, or of deformities of children born to exposed women.

Overall, studies of humans suggest that they are among the least-sensitive mammals to suffering toxicity from TCDD. While chloracne is a common symptom of an acute human exposure to TCDD, the evidence showing increased rates of TCDD-related disease, mortality, cancer, or birth defects are equivocal, and controversial. Some scientists believe there is no evidence that a human has ever died from an acute exposure to TCDD. However, there is unresolved scientific controversy about the possible effects of longer-term, chronic exposures of humans to TCDD, which might result in increased rates of developmental abnormalities or cancers. Unless large, these effects would be difficult to detect, because of the great environmental and genetic variations that must be overcome in epidemiological studies of humans.