Head lice (Pediculus humanus capitis) are tiny, wingless insects that live exclusively on the human scalp and hair. As obligate parasites, they require a human host and regular blood meals to survive and reproduce. Understanding the duration of their life stages is important for effective eradication and prevention. Head lice cannot hop or fly, spreading primarily through direct head-to-head contact. While they do not transmit disease, they cause discomfort and itching due to an allergic reaction to their bites.
The Full Life Cycle Duration
The life cycle begins with the nit, which is the egg firmly cemented to the hair shaft close to the scalp. This oval-shaped egg typically requires seven to ten days to hatch. After hatching, the immature louse, called a nymph, emerges and immediately begins feeding on blood.
The nymph stage involves three molts as the insect sheds its exoskeleton and grows. This developmental period lasts approximately seven to twelve days. Once the third molt is complete, the louse matures into a reproductive adult. The total time from the egg being laid to the emergence of a sexually mature adult is roughly two weeks.
Adult Louse Survival on the Head
Once a louse reaches the adult stage, it can live for up to 30 days, provided it remains on the human host and feeds regularly. Adult females are slightly larger than males and are responsible for reproduction.
A female louse can lay between three and eight eggs daily for about 16 days. This high reproductive capacity allows an infestation to escalate quickly if not treated promptly. The female uses a strong, glue-like substance to attach the nits close to the scalp, where the temperature is optimal for development.
Off-Host Survival and Environmental Risk
Head lice are highly dependent on the warmth and consistent blood supply of the human scalp. When removed from the host, survival time for adult lice and nymphs is limited due to lack of food and dehydration. They can survive for a maximum of 24 to 48 hours off the human head.
Most lice separated from the host die much sooner, often within 24 hours, especially in low humidity environments. Nits present an even lower risk of environmental re-infestation. They require the sustained temperature of the human scalp (approximately 98.6°F) to successfully hatch.
Nits that fall onto furniture or clothing are extremely unlikely to hatch or cause a new infestation. Therefore, extensive cleaning of the entire home is generally not necessary. Focus should be placed on treating the infested person.
Machine washing items that have been in direct contact with the head, like bedding and recently worn clothing, is recommended. Non-washable items can be sealed in a plastic bag for a minimum of 48 hours to ensure any remaining lice perish.