Head lice are parasitic insects that usually live on the scalp, feed on human blood, crawl instead of hopping or flying, spread mainly by direct hair-to-hair contact, and carry no disease. The rest is detail worth having: how fast lice develop, what a nit actually is, which familiar warnings the evidence does not support, and how to explain any of it to a worried child.
Head Lice Facts at a Glance
The dependable facts about head lice are few, and they come from the CDC and the AAP.
- What they are: parasitic insects, Pediculus humanus capitis, that feed on human blood. The AAP describes pale, gray bodies whose color can vary.
- Where they live: mostly on the scalp, and also on the eyebrows and eyelashes.
- How they travel: by crawling. The CDC says they cannot hop or fly.
- How many: the CDC estimates 6 to 12 million infestations a year in the United States among children 3 to 11 years of age.
- Who gets them: anyone can, the AAP says, and the CDC adds that getting head lice is not related to cleanliness of you or your environment.
- Not from pets: animals do not get or spread head lice.
- What they do to a child: itching, which the CDC calls the most common symptom, a tickling feeling in the hair, irritability and sleeplessness, and sores from scratching that can become infected.
- What they do not do: carry disease. The CDC does not consider head lice a health hazard, and the AAP says they do not put your child at risk for any serious health problems.
Symptoms can also be absent, especially with a first or light infestation. Our no-panic guide to head lice symptoms and causes covers the signs in more detail.
The Lice Life Cycle, From Nit to Adult
Head lice run through three stages, and the timing is what decides when a second look is worth it. The figures below are the CDC’s; the AAP’s ranges run slightly wider.
| Stage | What you see | CDC timing |
|---|---|---|
| Nit (egg) | Oval, yellow to white, 0.8 mm long, glued to a hair shaft within 1/4 inch of the scalp | Hatches in about a week (6 to 9 days) |
| Nymph | A smaller version of the adult, about the size of a pinhead (1.5 mm) | About seven days to reach adulthood |
| Adult | About 2 to 3 mm long, the size of a sesame seed | About 30 days on a person’s head |
The AAP puts hatching at 7 to 12 days depending on the surrounding temperature and gives an adult louse about 28 days. Egg output is what keeps a case going: about six eggs a day by the CDC’s count, up to 10 by the AAP’s, and a cycle the AAP says can repeat every 3 weeks if head lice are left untreated. Both build in a second treatment about a week later when live lice are still seen.
Can Head Lice Jump, Fly, or Swim?
No on jumping and flying. Head lice move by crawling, and the CDC states plainly that they cannot hop or fly; the AAP says the same about jumping. That is why close, prolonged head-to-head contact is the main route. Sharing combs, brushes, hats or sports helmets carries what the AAP calls a very small chance, and the CDC calls lice from a wig or hairpiece extremely unlikely. Swimming is a separate question: the CDC says chlorine levels in pool water do not kill head lice and that head lice can survive under water for several hours, but are unlikely to spread in a swimming pool. Our piece on how head lice actually spread has the longer version.
Lice Facts for Kids, in Plain Words
Children do better with three plain facts: lice are tiny bugs that crawl, they cannot jump or fly, and they attach to anyone’s hair whether it is clean or dirty. Two habits are worth teaching, both straight from the CDC: avoid hair-to-hair contact during play, and do not share combs, brushes, towels, hats, scarves, hair ribbons or barrettes. The AAP adds that regular checks are a good way to spot head lice before they have time to multiply. If you need the wording, we have age-appropriate scripts for talking to your child about lice.
Myths the Evidence Does Not Support
Four familiar instructions do not survive a look at the CDC and the AAP.
- Replace the mattress and the couch. The CDC calls the risk from a louse that fell onto a rug, carpet or furniture very small, and says spending much time and money on housecleaning is not necessary to avoid reinfestation. The AAP says you do not need to throw these items away.
- Fog or spray the house. The CDC says fumigant sprays and fogs are not necessary and can be toxic if inhaled or absorbed through the skin. The AAP says not to spray pesticides in your home.
- Smother them with oil or mayonnaise. The CDC’s treatment guidance finds no scientific evidence that mayonnaise, olive oil, margarine, butter or similar substances suffocate head lice, and the AAP says no studies prove home remedies work.
- Try gasoline, kerosene or a medicine made for animals. The AAP says never, and it also warns against putting anything made of plastic on a child’s head.
Frequently Asked Questions
How soon does itching start after head lice arrive?
Not right away, and sometimes not at all. The CDC says it may take four to six weeks for itching to appear the first time a person has head lice, since the itch is an allergic reaction to louse bites. The AAP adds that itching can last for weeks after the lice are gone.
Where should I look first for lice and nits?
Start at the hairline at the back of the neck and behind the ears, which the AAP calls the easiest place to find them. Its steps are to seat your child in a brightly lit room and part the hair down to the scalp in small sections. A fine-toothed comb helps, and the CDC says a magnifying lens can too.
How can I tell an old nit from a new one?
By how far it sits from the scalp. The CDC says nits attached more than 1/4 inch from the base of the hair shaft are almost always hatched or dead, and that if those are the only nits you find, the infestation is probably old and does not need treatment.
What purpose do lice serve?
For the person carrying them, none: head lice are parasites. The CDC calls them parasitic insects that feed on human blood, and MedlinePlus says they need to feed on human blood to survive. The CDC adds that they do not transmit any disease, so they are a nuisance rather than a health hazard.
Do nits left in the hair after treatment mean it failed?
No. The CDC says nits may stay in hair after treatment and that successful treatment will kill crawling lice. Its guidance is to treat again only if live, crawling lice appear several days later, and to talk with a healthcare provider before re-treating. It also suggests checking the hair every 2 to 3 days for the next 2 to 3 weeks.
What if a store-bought treatment did not work?
Check with your child’s doctor before trying anything else. The AAP says over-the-counter treatments sometimes are not the most effective, that lice can be resistant to their active ingredients in some communities, and that the likelihood of resistance varies across the country. The CDC lists misdiagnosis, instructions not followed, resistance and reinfestation among the reasons treatment fails. Our post on why treating your kids’ head lice yourself can fail goes through the snags.
How Lice Lifters of Mercer County Can Help
If these facts have you fairly sure someone at home has lice, the next step is a head check. At our clinic we look at the whole family, since the CDC advises checking all household members and close contacts, and one missed head is how a case comes back. The treatment is non-toxic and safe for children, and it is done in one visit; we keep same-day appointments open because lice do not pick convenient days. Our 30 Day Guarantee applies when the whole family is checked. You can see what a visit involves on our page about professional lice treatment, and when you want this settled, book a head check.
