PRINCIPLES OF TOXICOLOGY

354 PROPERTIES AND EFFECTS OF PESTICIDES
the insecticidal component of the extract. The pyrethrins jasmolins I and II, cinerins I and II, and
pyrethrins I and II are extracted from the powder for formulation into commercial aerosols and spray
products. These compounds are often formulated with a synergist such as piperonylbutoxide or n-octyl
bicycloheptene dicarboximide. These synergists are incorporated in order to slow down the degradation
of the pyrethrin compounds. This class of compounds are commonly used in household insecticides
and in pet products (e.g., flea and tick dips and sprays).
Pyrethrins and pyrethrum are very rapidly metabolized and excreted from humans and have very
low mammalian toxicity. Crude pyrethrums have been associated with allergic responses in individuals,
although this action is most likely due to the noninsecticidal components of this compound. A study
of 59 workers who had been employed in a pyrethrum factory ranging from 1 to 25 years showed
essentially no adverse health effects, with the exception of inflammatory pleural lesions in some
individuals exposed to high air levels of pyrethrums. Treatment of pyrethrin and pyrethrum exposure
is primarily symptomatic.
Synthetic Pyrethroids
Pyrethrins and pyrethrum insecticides are unstable in light and heat. Because of this instability,
synthetic pyrethroids, which have better stability to light and heat, have been developed and are used
in agricultural settings as well as for home pest control. Over 1000 synthetic pyrethroids have been
developed over the years and include compounds such as cyfluthrin, cypermethrin (Cymbush),
deltamethrin, fenpropathrin (Danitol), fluvalinate (Mavrik), permethrin (Ambush), resmethrin
(Chryson), and tralomethrin (Scout) (Figure 15.3).
The action sites of the pyrethroids are the voltage-dependent sodium channels in nerves. The general
basis for nerve impulse generation and conduction is the ionic permeability of the membrane combined
with the sodium (high levels outside the cell) and potassium (high levels inside in the cell) concentration
gradients. The resting cell membrane is maintained by the sodium–potassium pump, and the inside of
the cell is negatively charged with respect to the outside of the cell. A normal nerve impulse is caused
by a quick transient increase in the permeability of the membrane to sodium ions, causing an inward
Cl
Cl
Cl
Cl
C
C
CH
CH
H 3C
CH 3
CH 3
O
C
Cypermethrin
CH 3
O O
C
Permethrin
CN
O CH
CH 2
Figure 15.3 Synthetic pyrethroids.
O
O