Not all nits will hatch – only those laid by a female which has successfully mated will be viable. Research in America suggests as many as 50% of nits could be infertile. And the British Medical Journal reports that “less than 20% of school children with nits will go on to develop infestation within 14 days”.

It’s possible to tell whether a nit is empty simply by touch, but the most accurate method is to use a magnifying glass. Viable eggs are plump and shiny. Hatched and empty nits will be shrunken and transparent.

The remains of empty nits are unsightly, but they don’t cause any harm. Any unhatched nits should be quickly removed. This can be done using finger nails or a nit comb. Or simply by snipping off the hair with scissors.

Nits are difficult to pull off because the glue that binds them has a very similar chemical structure to the surface of human hair. This is also why no head lice lotions are available which would remove them – any effective chemicals would also damage the hair.

A typical case of lice infection will number anything between two and 10 lice, although there can be far more. Particularly if no active measures have been taken to remove them.