Mosquito repellent tests
DEET works, most others don't Science Mag
Forum Discussion
As a retired chemist: Permethrin is a type of Pyrethrin. Pyrethrins were first derived from a plant, a species of Chrysanthemum and have been in use since the year 1900 or so. Permethrin is a synthetic derivative that was first made in 1973. So that makes it's use in Viet Nam unlikely.
