It mandates that children with eggshell nits or living lice be taken from school until no evidence of them remains. The egg shell is not the louse, and the nit can stay attached to a hair shaft long after the lice are dead. By the time head lice are discovered, it's almost always weeks after they've been there. Nits are white, usually stay stuck to the hair shaft near the head, and are not easy to wash out. Even trained nurses can have a very difficult time separating what are nits and what is difficult dandruff.
Viable eggs are brown and are more challenging to detect.
The real work falls on the parents. In an effort to banish head lice as quickly as possible, many run for the nearest insecticide. Pyrethrins and permethrin kill crawling lice, but not unhatched nits. In 1998, three OTC manufacturers were enjoined by the Federal Trade Commission from advertising one-application success against the eggs. In one of Pollack's recent experiments, lice were picked off the heads of 209 children in Massachusetts and Idaho.