Bicycle helmet tests for most standards use one wet sample, immersed for at least four hours before the wet test. So you know that the helmet can be immersed in water and still perform. But do not leave it immersed for long periods of time, since the materials will absorb some water, and that may affect the adhesives used in construction. In addition, foam saturated in water usually gives a harder landing on flat surfaces.
Never use solvents or harsh cleaning materials on your helmet, since some of them may attack the foam or the shell and weaken the helmet. Gasoline, for example, dissolves EPS foam so well that in some developing countries the resulting gunk is used as glue.
Never put your helmet in a washing machine, dryer, microwave or dishwasher to clean it!
If you are concerned that someone who has worn your helmet may have had head lice, put the helmet in a plastic bag for two weeks until any nits left behind have hatched and died. (See our page on lice for more solutions).
So it seems you’re more or less likely to have killed off the lice? 😀