Nit combs are fine-toothed combs designed to remove head lice as well as their eggs, or 'nits'.
Nit combs can be used on their own, or in conjunction with some other treatment, such as
insecticidal gel/shampoo/lotions, tea tree oil or even vegetable oil.
The chemical approach was the firm favourite when I was a small boy,
and I remember the smell of 'nit lotion' fondly. These days insecticides are often avoided,
sometimes because parents fear for their children's safety. The prescription medications
such as Malathion and Lindane, while fairly safe, 
can be dangerous if ingested or applied to open sores. Think 'blood-sucking creatures puncturing your
scalp'. Now think 'applying toxic organic compounds directly to broken skin'. You can see why
people would be wary. Furthermore, overexposure has perhaps already caused resistance to some
conventional insecticides. 
There are many alternatives when removing head lice, but here
I want to focus on the mechanical approach; physically removing lice and eggs from the scalp.
This, when done regularly, is a very effective method of stopping an infestation of lice.
Nit combs come in a three distinct forms.
- Plastic combs
Regular combs are not good enough to remove nits.
Longer, finer teeth are required. The plastic nit comb is the budget
option, and will often come free with a bottle of over-the-counter lotion.
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Fig. 1 - a plastic nit comb
shimmer reminds us of dust combs, which are cheap plastic combs suitible for brushing dust (or nits) out of hair. Because they are cheap (~ 50p) they handy and disposable. Good in an emergency, when no other comb can be found. Their cheap plastic constuction probably makes them worse than regular nit combs for removing nits though.
- Steel combs
Nit combs with stainless steel teeth won't bend or break as easily as plastic combs, making them more effective.
Furthermore, they can be boiled after use to easily ensure that any lice and nits are dead
and will not be transferred to the next brushed head.
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Fig. 2 - a steel-toothed nit comb
- A 'LiceMeister' stainless steel-toothed comb ("LiceMeister: it's not just another lousy nit comb") will cost you around $15.
It has 1.5" stainless steel teeth. See http://www.headlice.org
- A 'Nit Grabber' will set you back only $10. It has similar (though slightly shorter) stainless steel teeth, but a big handle for easy use
with slippery hands. See http://shop.store.yahoo.com/headliceinfo/noname.html
- Electrical combs
LiceGuard make a device called the Robi Comb, which retails for around $30.
Powered by one AA cell, the comb uses two rows of teeth which carry a small electrical charge, killing nits and lice on contact.
As the comb slides through a child's hair, it makes a soft humming sound until it encounters a louse. At that time, the sound stops and a small electrical charge passes from one of the comb's teeth through the louse to the next tooth, killing the louse.
Much as I love this device (my wife works with 4 year olds. I am very familiar with its ways)
I have two small grievances with it. Firstly, the noise it emits is not a "soft humming sound" at all.
My model shrieks with a high-pitched wail. Secondly, the electrical current means that
the Robi Comb cannot be used on wet hair. Nit-combing dry hair can be more painful,
especially if your hair is thick. Other than these minor issues (you might almost say I'm nitpicking)
the Robi Comb is a wonderful piece of technology.
Head lice enjoy the warmth they find in the hair behind our ears, so that's a particularly important place to comb,
though obviously the whole head should be systematically combed.
The best time to do this is just after hair has been washed and conditioned. Hair is easier to comb when it is wet.
Conditioner is sometimes thought to help the nit casings slide more easily off the hair shaft.
This is what I've always been told anyway. I tried to read around this subject, to
bring you the best advice on optimum nit comb usage. I found an amazing paper, published in The Archives of Pediatric Medicine.
The effects of several substances, including WD-40, bleach, vinegar and liquid nitrogen on the nit casing were tested.
While nothing was great, "products that seemed to assist mildly in nit removal included bleach, vodka, dental plaque remover solution and vinegar, both at room temperature and heated." 
The next line is the best in the article though:
"Interestingly, 20% potassium hydroxide in dimethyl sulfoxide dissolved the hair shaft without any discernible effect on the cylindrical nit cast."
My mother still recommends conditioner. At the very least it will help detangle your hair, making
the combing easier.
- 'Treating Head Lice'
The Centers for Disease Control and Prevention - Division of Parasitic Diseases
- 'Differential Permethrin Susceptibility of Head Lice Sampled in the United States and Borneo'.
Pollack RJ, Kiszewski A, Armstrong P, Hahn C, Wolfe N, Rahman HA, Laserson K, Telford SR III, Spielman A.
- Archives of Pediatrics and Adolescent Medicine. 1999;153:969-973
- 'The Adherent Cylindrical Nit Structure and Its Chemical Denaturation In Vitro: An Assessment With Therapeutic Implications for Head Lice'
Archives of Pediatric Medicine, Vol 152, July 1998
- 'Head Lice in British children'
Brian J Ford
- 'Head Lice Information'
Harvard School of Public Health
Journal of Pesticide Reform, Winter 1992, Vol. 12, No. 4