Head lice, scientifically known as Pediculus humanus capitis, are small, wingless insects that live exclusively on the human scalp. These parasites sustain themselves by feeding on human blood. Understanding their life cycle and how long they can survive under various conditions is important for managing and preventing infestations.
Lifespan on a Human Host
The life cycle of head lice begins with eggs, commonly called nits, which are typically laid by adult female lice on hair shafts very close to the scalp. These tiny, oval-shaped eggs are firmly cemented to the hair, usually within a quarter-inch of the scalp, where the temperature is optimal for their development. Nits generally hatch within 6 to 10 days of being laid.
Once a nit hatches, an immature louse, called a nymph, emerges. Nymphs resemble adult lice but are much smaller, often the size of a pinhead. These nymphs must feed on human blood to grow and undergo three molts before maturing into adult lice. This developmental stage from nymph to adult takes about 7 to 12 days.
Adult lice, about the size of a sesame seed, can live for 30 to 35 days on a human host. Female lice are capable of laying multiple eggs each day, typically between 4 to 8, contributing to the continuation of the infestation.
Survival Off the Human Body
Head lice are obligate human parasites, meaning they require a human host for survival. Adult lice quickly face challenges when removed from the scalp, as they cannot fly or jump. Without a consistent source of blood meals, adult lice dehydrate and starve, leading to their death within 24 to 48 hours.
Nits, or lice eggs, are dependent on the conditions of the human scalp to hatch successfully. They require the warmth and humidity provided by the scalp for proper incubation. If nits fall off or are removed from the hair, they are unlikely to hatch. These dislodged nits die within a week due to the lack of suitable temperature and moisture.
Factors Influencing Lice Survival
Environmental conditions influence the survival duration of head lice, both on and off a human host. Lice thrive in warm, humid environments, such as the human scalp, which provides a stable temperature and necessary moisture. They are sensitive to temperature extremes.
High heat can kill lice and their eggs. Exposure to temperatures above 130°F (54.4°C) for at least 5 to 10 minutes is sufficient to eliminate them. Similarly, extreme cold can also be lethal; temperatures of 0°F (-18°C) or lower can kill lice within a few hours. Humidity also plays a role in their survival, as dry environments accelerate dehydration, shortening their lifespan off the host.
A regular food source is primary for adult lice survival. Head lice feed on human blood multiple times a day. Without these consistent blood meals, adult lice cannot sustain themselves and will quickly perish.