Lc5

LC50 LC50 LC50 LC50

LC50

4, 0.5 mg/L 0.005 mg/L 1.2-61.0 mg/L 0.008, 0.015 mg/L

aNA, not available; m., mortality. Source: Connell and Miller (1984).

chlorination. PCBs cause liver tumors in rats but are not mutagenic or teratogenic in mammals. Birds, on the other hand, have exhibited teratogenesis from PCBs as well as eggshell thinning and embryotoxicity. The carcinogenic potency of PCBs also varies with number of chlorines. Molecules with five chlorines have the highest potency; those with six are about half as potent; those with four are one-tenth as carcinogenic; molecules with fewer than four or more than six are less than one-tenth as potent.

A number of reproductive effects have been noted in mammals. A single injection of 20 mg PCB per kilogram of body weight lengthens the estrus cycle of mice; 100 mg/kg in the diet of Aroclor 1242 or 1254 decreased survival of rat offspring, and 20 mg/kg of Aro-clor 1254 reduced the number of pups per litter. However, 100 mg of Aroclor 1260 per kilogram showed no effect in rats. Monkeys showed lower fertility and reduced birth-weights at a dosage of 2.5 ppm of Aroclor 1248 in the diet.

PCBs are of concern in ecosystems because of their persistance and tendency to bio-concentrate and biomagnify. In addition, they show chronic toxicity at exceedingly low concentrations to certain species, which can cause significant population effects. As low as 4.4 ppb decreases hatching, survival, and growth of coho salmon; 19 ppb produces a 50% reduction in cell population growth of Daphnia magna. In addition, in an effect that strikes at the heart of a natural aquatic ecosystem's productivity, Aroclor 1242 inhibited net phytoplankton production by 50% at exposures of 10 to 25 ppb. Changes also occur in phytoplankton species distribution, with smaller species favored. This causes changes in zooplankton species distribution, which may in turn affect community structure at higher trophic levels.

Dioxins refer to chlorinated dibenzo-p-dioxins. They are produced as a by-product of various manufacturing processes, such as the production of the herbicide 2,4,5-T or the wood preservative pentachlorophenol, and are impurities in those products. They are also produced in all combustion processes in which the feed material contains chlorine in any form, such as chlorides. Thus, they may be found in fumes from incineration of refuse, and even of plant material. However, they are not produced significantly in exhausts from fossil-fuel combustion. The different congeners have different toxic and physicochemical properties. Of greatest concern is 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). This substance has the reputation of being the most toxic humanmade substance known.

dioxin 2,3,7,8-tetrachlorodibenzodioxin (TCDD)

dioxin 2,3,7,8-tetrachlorodibenzodioxin (TCDD)

Like PCBs, chlorinated dioxins tend to be very stable thermally and environmentally. They photodegrade in the presence of organic material. However, they adsorb strongly to particulate matter such as soil and sediment, protecting them from exposure to the necessary ultraviolet radiation. They have very low solubility in both polar and nonpolar solvents. The solubility in o-dichlorobenzene is 1.4 g/L, in n-octanol it is 0.048 g/L, and in water it is 2 x 10~7 g/L. Thus, the KOW is 2.4 x 105.

Table 21.10 shows the ranges of TCDD bioconcentration factors that have been observed in model ecosystems. Data from the accident in Seveso, Italy (see below) and elsewhere have shown that terrestrial organisms can bioconcentrate dioxin from the soil.

TABLE 21.10 TCDD Bioconcentration Factors

Organism

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