Questions about head lice.

Written by Joe Ballenger
Head Louse note

Image credit: Chez Mummy, via Flikr. License info: CC-BY-SA-2.0

We get a lot of questions about lice, and these are really important. On Biofortified, I wrote an article about head louse treatments that was relatively depressing. There are a lot of companies which market louse control products under loopholes that exempt them from safety and efficacy testing. I also suspect some companies misrepresent which ingredients are the active ingredients. I’ve always wanted to revisit this topic, because that post needs an update, but I don’t think I’d be able to safely do that in today’s blogging climate. Interference, the scary type, is a scary reality in today’s blogging world. All I’m going to say on that topic is this: always use FDA-approved treatments.


So here are a number of questions we’ve gotten through email. Due to the somewhat sensitive nature of lice infestations, they’ve been anonymized as well as decontextualized. These questions are from multiple emails, and any potentially identifying information has been removed.

What is the incubation period for nits?

Lice eggs take about a week to hatch. The entire lifecycle, from egg to egg-laying adult, takes about a month.

How long can a live louse live off of the human body?

How long can un-hatched(not destroyed by chemical treatments) nits live off of the human body?

Louse egg

Egg of the head louse (Pediculus humanus capitis) on a hair. The picture was taken after an anti-lice treatment. This egg is dead, and slightly dehydrated. Scale : egg length= 0.57 mm. Image credit: Gilles San Martin. License info: CC-BY-SA-2.0

Head lice need to be eating constantly to live, and will die from dehydration about 10 hours after falling off a host. It’s not known how long eggs can survive off-host, but they take about two weeks to hatch when kept at room temperature during the day and at body temperature at night. This is really bad for them, because it reduces hatch rates by nearly one-third.

On the host, lice eggs take about a week to hatch. Being off the host is really bad for the eggs, and it’s likely that hatch rates are probably close to 0% when the eggs fall off the host.

Why do my kids have nits without lice being present?

When nits hatch, eggshells are left behind. Under the best conditions, only about 75% of louse eggs hatch. Telling dead eggs and hatched eggs apart from live ones is impossible without inspecting each one under a microscope.

This is why entomologists really don’t like no-nits policies. Nits do not indicate an active louse infestation, so schools which enforce them aren’t really helping their students. They’re needlessly keeping kids out of school, and parents away from work.

Why is it necessary to treat a louse infestation as though it is a bedbug infestation?

It’s really not necessary to treat a louse infestation as if it were a bedbug infestation. Some companies do sell spray for lice, but this isn’t really needed because lice die soon after being separated from a host. When schoolrooms with lice infestations are vacuumed, lice are rarely if ever found.

Adult lice die shortly after leaving the host’s body, and most of the sprays sold for this purpose don’t even kill eggs…which are the only stage which could conceivably be alive after falling off the host. They need to constantly be on a host to survive, and they’re very well adapted to staying on the host.

Would it be effective to soak hair in a swimming pool daily to drown the lice as they hatch?  How long can a louse “hold its breath?”

Is salt water an effective way of controlling lice?

head louse.jpg

Female human head louse, Pediculus humanus capitis, demonstrating how it clings to human hair. License info: Gilles San Martin License info: CC-BY-SA-2.0

Lice are actually very resilient. They can be held underwater for quite some time, half an hour or more, without dying. They can’t spread in public pools, but also aren’t killed by the high chlorine levels. Salt water, even saturated salt water, isn’t great at killing them either.

Lice are very well adapted to clinging to hair, and they appear to have some sort of programmed response to being submerged. When they hit water, they go dormant, and my guess is that they clamp onto the hair. Some get washed off, but enough stay on board to sustain the infestation.

Lice are a unique infestation, because you need to eliminate virtually every individual from the head to cure the infestation. In agriculture, we keep the insects below a certain point…but in head louse-control there is no tolerance for lice.

Why doesn’t a flat iron on a high setting work to kill nits?

A flat iron, on the highest setting, can reach over 400*F. This would definitely kill lice.

However, lice also tend to lay eggs near the scalp. A flat iron needs to be held a little ways away from the scalp, roughly a centimeter or so. Lice typically live just next to the scalp, and lay their eggs where they live. Temperatures hot enough to cook lice will also cause burns to the scalp, or damage to the hair itself.

There are some devices which are basically blow-driers which kill lice by using hot air to dry them out, but they’re not widely available. I’m not entirely sure why, but I’d imagine there are economic reasons for this.

Why doesn’t tea-tree oil or diatomaceous earth work on lice?

Tea tree oil doesn’t have a whole lot to do with the plant used to make the stuff I drink in the morning, it’s made from a completely different species. The oil preparation is made by blasting the leaves of the plant with steam, and collecting the stuff which evaporates.

The resulting mixture, called Tea Tree Oil, is highly fragrant and smells really nice. It contains a whole host of things which can kill bugs, including lice. I suspect some of the compounds in the mixture interfere with insect-specific hormones, although it’s known that some components hit the same targets as agricultural pesticides. It’s a very complex mixture, with a lot of components which aren’t well researched.

Tea tree oil does have insecticidal properties, and a small (but well performed) clinical trial showed promise against lice. However, tests run in petri dishes can show conflicting results when different products are compared. This is most likely because different products can be prepared in ways that are slightly different. Different formulations may contain different chemicals, so it’s the product itself which needs to be tested.

With Tea Tree oil, there are also safety concerns which go beyond potential allergies. There’s some evidence of feminizing effects in males, but different scientists have gotten conflicting results when testing this hypothesis in cell cultures. Cell culture studies are very sensitive, and there’s no shortage of reasons different research teams could get different answers. Animal tests are needed to sort this out, and (so far as I know) these haven’t been done yet.

So don’t take that as a final word on safety of Tea Tree Oil, because it’s hardly a concrete statement. It is something which needs to be mentioned, and a line of research I’ve been curious about for awhile.

Diatomaceous earth is the leftover skeletons of diatoms which died long ago. It’s used as an insecticide in gardens, and it works by rubbing off the protective wax coats of insects. Eventually, they die of dehydration. Diatomaceous earth is a lung irritant, so clumping it over someone’s hair doesn’t seem especially safe.

Will smoothing down the cuticles of the hair shaft with oil make the hair less susceptible to nit attachment?

Lice don’t really have any preference for clean or oily hair. Smoothing the hair down with oil may make it easier to comb out nits, but it’s unlikely to eliminate an infestation by itself.

The Bottom Line

Lice are pretty cool critters, but that doesn’t make them something you want to have around. Infestations are on the rise during the winter months, and we’re leaving head louse season right now. I think that’s a good thing.

We do get a lot of louse questions, and they’re a bit iffy for us to answer because they’re right at that intersection of entomology and medicine. As far as treatment advice goes, I’ll just re-iterate what I said at the beginning of the post. There are a lot of loopholes which companies can take advantage of so they can skip safety and efficacy testing that is a part of the FDA approval process. If you have head lice, speak to your physician before you buy anything, and make sure to buy something that is FDA approved.

Works Cited:

  1. Burgess, I. F. (2004). Human lice and their control. Annual Reviews in Entomology, 49(1), 457-481.
  2. Canyon, D. V., & Speare, R. (2010). Indirect transmission of head lice via inanimate objects. the open dermatology Journal, 4(1).
  3. Canyon, D., & Speare, R. (2007). Do head lice spread in swimming pools?. International journal of dermatology, 46(11), 1211-1213.
  4. Cueto, G. M., & Picollo, M. I. (2010). Response of Pediculus humanus humanus (Pediculidae: Phthiraptera) to water or 70% ethanol immersion and determination of optimal times for measuring toxic effects. Parasitology research, 106(6), 1503-1506.
  5. Frankowski, B. L., & Bocchini, J. A. (2010). Head lice. Pediatrics, 126(2), 392-403.
  6. Heukelbach, J., Canyon, D. V., Oliveira, F. A., Muller, R., & Speare, R. (2008). In vitro efficacy of over‐the‐counter botanical pediculicides against the head louse Pediculus humanus var capitis based on a stringent standard for mortality assessment. Medical and veterinary entomology, 22(3), 264-272.
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1 Response to Questions about head lice.

  1. The hot air devices do work! There’s the original Airalle (formerly the lousebuster) and then there’s a knockoff. We have found that it’s highly effective in killing the eggs, which are susceptible to temperature changes. We haven’t heard of anything else that can kill eggs.


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