Head lice (singular is “louse”) are a common health condition, especially in children. Symptoms of head lice include the visible detection of white matter in the hair as well as itching and a tingling or tickling sensation of the scalp. These stubborn insects may be difficult to remove from the hair and can be quite contagious.
Learn about the life cycle of head lice and for how long they live.
A louse is a parasitic insect. Parasites need a host to provide nourishment so they can live. Head lice form a parasitic relationship with humans as their host, with blood from the scalp as their source of nourishment. Head lice live close to the scalp, and sometimes the eyelashes and eyebrows. A person infested with lice may encounter an itchy scalp as an allergic response to louse saliva that’s secreted as they feed.
Lice have six claws that are designed to grasp and hold on to a single hair shaft. They also use their claws to crawl down toward your scalp when it’s time to feed. Head lice can’t fly, hop, or swim. However, lice that are in water such as a bathtub or swimming pool are still tightly attached to the hair, and can survive for a few hours. If the insects fall off their host into the water, they’ll die because they become separated from their food source.
The life cycle of a louse begins as an egg, also called a nit. The nit is a 1mm whitish-yellow speck that attaches itself firmly to an individual hair strand close to the scalp. After 7 to 10 days, the nit hatches and becomes what is known as a nymph, or a young louse. Nymphs are usually between 1.1 and 1.3 mm in size and tan or white in color. Nymphs mature into adult head lice within a couple of weeks. Mature adults don’t tend to grow larger than 2mm, and female lice are larger than males.
Immediately after lice eggs hatch, the nymphs need food. Using their claws to crawl from the hair shafts to the scalp, young adult and mature lice feed on the host’s blood multiple times throughout the day. As long as there’s a food source readily available, an adult louse can live for as long as 30 days on a human. However, this short lifespan is perpetuated by the constant laying of eggs. Female lice lay up to six eggs each day.
Nits, or lice eggs, can’t live without a human host. They need the warmth of the scalp for incubation purposes before they hatch. Once they’ve hatched, they need the nourishment they get from human blood. Nits that are dislodged from a hair shaft will most likely die before they hatch.
Nymphs and fully mature adults can survive for only about a day without a food source. Adult lice can’t live past 24 hours or so on non-human surfaces such as carpets, hardwood flooring, clothing, furniture, sports helmets, headphones, or hair accessories.
Head lice don’t live very long without a human host, but can transfer from one person to another easily through close—especially head-to-head—personal contact. It’s important to treat lice immediately and thoroughly to avoid spreading an infestation.
Medicated shampoos are designed to kill adult lice and nits. Vacuuming carpets and upholstered furniture prevents lice from crawling onto a new host before they die. Wash bedding, clothing, and washable toys such as stuffed animals in hot water. Tumble dry on high heat for at least 20 minutes to kill lice.
Getting rid of a case of head lice can be time-consuming and tedious, but it’s not a health hazard. Lice don’t carry disease and don’t harm the head or scalp.
Prevent head lice by not sharing combs, hairbrushes, hair accessories, towels, bedding, hats, or scarves with a person infected with the parasite. In this case, being selfish helps you protect yourself!