Agriculture Reference
In-Depth Information
CHAPTER 5
TOXICOLOGY AN D
MODE OF ACTION
Any chemical substance may evoke a toxic effect. A chemical substance
that exerts an injurious effect in the majority of cases when it comes into con-
tact with living organisms is termed a poison. Pesticides, by necessity, are
poisons. Familiar chronic responses to various irritants include silicosis, lung
cancer, and necrosis of the liver or kidneys. However, the toxic effects of
different pesticides and their formulations differ greatly. Two types of effects
can be distinguished:
1.
An
acute effect
normally occurs shortly after contact with a single dose or
exposure to a poison, in this case a pesticide. The magnitude of the effect
depends on the innate toxicity of the substance, duration of exposure, and
the method of application or exposure to a particular organism. Thus, a
smaller dose of arsenious oxide than sodium chloride (table salt) will pro-
duce toxic symptoms in most animal species, and a drop of sulfuric acid
is less dangerous on the skin than it is in the eye. Acute toxicity very of-
ten results from the disruption of an identifiable biochemical or physio-
logical system and, as a consequence, acute toxic responses are more read-
ily quantifiable.
2.
A
chronic effect
, on the other hand, sometimes occurs when an organism
is exposed to repeated small and non-lethal doses of a potentially harmful
substance. Familiar chronic responses to various irritants include silico-
sis, lung cancer, and necrosis of the liver or kidneys.
I.
METHODS TO ESTIMATE THE RELATIVE TOXICITY
OF PESTICIDES TO HUMANS
The toxicity value of a pesticide is a relative measure to estimate its toxic
effect on an organism.
