Dermal penetration of avermectin B1a in the rhesus monkey.

Forearms of rhesus monkeys were treated with [3H]avermectin B1a in three different vehicles and concentrations so that the penetration of avermectin B1a through skin could be determined. In order to simulate exposure of farm workers, such as mixer-loaders, applicators, and harvesters, to this pesticide, avermectin B1a was applied to the forearms of the monkeys as an emulsifiable concentrate (300 micrograms/monkey), a diluted emulsifiable concentrate (4.5 micrograms/monkey), and as a suspension in water (216 micrograms/monkey). After 1 or 10 hr of exposure, the treatment area was washed. The levels of radioactivity were determined in the urine, feces, plasma, and wash. On the basis of the amounts of radioactivity excreted in the urine and feces and the levels of radioactivity in the plasma after dermal application compared to those found after intravenous administration of the compound, less than 1% of the doses was absorbed. These data indicate that avermectin B1a would not readily penetrate the skin of farm workers exposed to it. Therefore, the hazard to farm workers exposed to this compound would be substantially mitigated.