1-4.
TOXICITY
All pesticides must be considered potentially toxic (poisonous) to man and
animals. However, the degree of toxicity is only one of several factors in the use of
pesticides that determine the hazard to man. The primary hazard lies in failure to follow
the precautions and directions for use indicated on the pesticide label. Safe handling of
pesticides is discussed in Lesson 3; however, measurement of toxicity will be explained
at this point since these terms will be used throughout the discussion of pesticides.
a.
LD50 Value.
(1) The LD50 (lethal dosage) value of a pesticide -- or any other toxic
substance -- is a statistical estimate of the dosage necessary to kill 50 percent of a
population of test animals (usually white rats) with a single exposure under standardized
conditions in the laboratory. It is expressed in milligrams of poison per kilogram of body
weight (mg/kg) for rodents or micrograms per gram (mg/gm) for insects. The LD50
values do not take into account the physical condition of laboratory animals, nor do they
provide data on the cumulative effects of repeated dosages; however, they do enable
us to compare the relative acute toxicities of various pesticides. The higher the LD50
value, the lower the toxicity since more poison is required to affect a death. Thus, a
pesticide with an LD50 value of 500 mg/kg is more toxic than one with an LD50 value of
1,000 mg/kg.
(2) Toxicity is expressed as an oral or dermal LD50 value, which varies
depending on whether the poison is taken orally (by mouth) or dermally (absorbed
through the skin). Most pesticides have a considerably higher dermal than oral LD50.
Unless otherwise specified, LD50 values quoted in this subcourse are oral values. In
considering LD50 values, it must be remembered that these values have been
determined from experimental data on laboratory animals; therefore, they cannot be
considered exact values nor can they be directly transferred as effects on man.
(3) Various toxicologists have devised tables of relative toxicities based
upon the LD50 derived from test animals and extrapolated (estimated corresponding
effect) to humans. These tables usually begin with an LD50 of more than 5000 mg/kg
(over a quart for a 150-pound man) as relatively harmless or practically nontoxic. Most
tables rate a chemical having an LD50 of less than 50 mg/kg as highly toxic, 50 to 500
mg/kg as moderately toxic, 500 to 5000 mg/kg as slightly toxic, and greater than 5000
mg/kg as practically nontoxic.
b. LC50 Value. The LC50 (lethal concentration) value of a poison is the
concentration in parts per million (ppm) of the poison applied to the environment
required to kill 50 percent of a test population (fish, mosquito larvae, etc.) from a single
exposure under standardized conditions. Like the LD50 value, a high LC50 value
indicates low toxicity and vice versa. It should be emphasized here that LD50 and LC50
values cannot be compared to one another since they are derived from two different
MD0173
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