The loss of permethrin from impregnated uniforms
due to repeated washings was studied by chromatographic
estimation of the residues. The mean
(± SE
mean) percentage losses of permethrin after one to
five washings were 16.7 ± 2.3, 22.5 ± 3, 29.6 ± 2.9,
40.2 ± 2 and 52.2 ± 2.4, respectively. The reduction in
contact toxicity against mosquitoes after each washing was
studied by World Health Organization tube and cone bioassays
against
Aedes albopictus
mosquitoes. The median
knockdown time for 5-min exposure to the treated uniforms
increased from 5.9 to 71.8 min after five washings.
Mosquito mortality 24 h post-exposure in cone bioassays
was <80 % after the fifth washing, indicating the loss of
efficacy. The uniforms need to be retreated after five
washings so as to ensure adequate protection against disease
vectors. The washing water should be properly disposed
off to prevent environmental contamination and
toxicity to aquatic organisms. Methods for treatment of
military uniforms, which ensure high resistance to washing,
need to be adopted so as to avoid frequent re-impregnations
with permethrin.
