TOXICOLOGY AND MODE OF ACTION - Agrochemical and Pesticide Safety - page 120

A frequently used measure of acute toxicity is the LD 50 or LC 50 values,
which mean lethal dose or lethal concentration of a poison that kills half the
organisms of a test population. The LD 50 is usually expressed in terms of
milligrams of poison per kilogram of body weight of the experimental ani-
mals (mg/kg). An example might be a pesticide with an LD 50 of 550, which
would indicate that 550 milligrams of this pesticide given to animals that
weigh 1 kilogram each would kill 50% of the test population.
To determine the LD 50 of a pesticide, batches of animals are exposed to a
range of doses and the percentages of organisms dying in each batch are
recorded. The results can usually be represented by a curve (sigmoidal) shown
in Figure 5.1 .
Figure 5.1 Dose-response curve illustrating the log of the dose that causes a
of 50 percent mortality in the test population.
The curve is steepest in the region of 50 percent so that a small change in
concentration in this region of the curve causes a large change in percentage
kill. The dose that kills half of the organisms is thus a more sensitive index
of toxicity than any other dose, and this is why the LD 50 is usually adopted as
a standard for comparing the relative toxicity of substances.
The toxicity data based on the LD 50 values determined from tests on ani-
mal involve several problems and should not be interpreted as exact values for
human toxicity. These values should be used as guides in estimating the rela-
tive toxicities of pesticides and can be also used to determine the level of pro-
tection (PPE) required to perform various tasks such as the application or han-
dling of pesticides.