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C1 C2

Exposure concentration for fixed time

Figure 19.1 Hypothetical dose-response relationship for (a) a substance required at low dosages and (b) a nonessential substance.

C1 C2

Exposure concentration for fixed time

Figure 19.1 Hypothetical dose-response relationship for (a) a substance required at low dosages and (b) a nonessential substance.

adjusted to the weight of the individual rat. The number of rats with tumors is counted at 80 days.

2. Ten test tubes are prepared, each containing 20 Daphnia, a small crustacean. The concentration of a toxicant in the water of each tube is adjusted to a different concentration. Percent mortality is measured at 96 hours.

In both cases, the individuals in the 10 groups each receive different dosages among the following: 0 (control), 1, 2, 5, 10, 25, 50, 100, 250, and 500. The units in the former case may be milligrams toxicant per kilogram of body weight; for the latter it could be mg/L.

TABLE 19.1 Several Substances That Are Required for Growth But Are Toxic at Higher Levels

Substance

Recommended Dietary Allowances (mg/day)

Toxic Level

Iodine

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