From G. R. Coleman
Jeff Howell’s statement that “the pesticides [permethrin] are water based and
hence less volatile” (Forum, 4 July, p 49) is inaccurate. The vapour pressure of
permethrin is constant whether it is in an organic solvent or in a water-based
emulsion.
The levels of permethrin in the air are extremely low even within two to
three hours after a spray-applied treatment. This is not surprising given the
material’s low vapour pressure.
The levels of permethrin in the air after spraying were obtained from a
number of tests in a variety of buildings, not laboratory simulations. It is
these data on which the independent Advisory Committee on Pesticides bases its
judgment for the use, precautions, restrictions and re-entry times for timber
preservatives.
The 250 cases of suspected permethrin poisoning in Germany are certainly not
reflected in Britain.
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Bourton, Dorset
