How long can a lice egg live? - briefly
A louse egg stays viable for roughly 7–10 days, and under optimal temperature and humidity it can survive up to about 14 days before hatching.
How long can a lice egg live? - in detail
A louse egg, often called a nit, remains viable for a limited period that depends on temperature, humidity, and exposure to the host. Under typical indoor conditions (approximately 30 °C / 86 °F and 70 % relative humidity), the embryonic development lasts 7–10 days before the nymph emerges. Cooler environments slow metabolism; at 20 °C (68 °F) the incubation can extend to 14 days or slightly longer.
If an egg is detached from the scalp and kept in a dry, cool setting, it may survive without hatching for up to 10 days, after which the embryo usually dies. Prolonged exposure to temperatures above 50 °C (122 °F) or to chemical treatments such as permethrin, dimethicone, or isopropyl alcohol destroys the chorion within hours, rendering the egg non‑viable.
Key factors influencing egg longevity:
- Temperature: 30 °C → 7–10 days; 20 °C → up to 14 days; >50 °C → immediate death.
- Humidity: 70 % RH supports normal development; very low humidity can cause desiccation and premature mortality.
- Host contact: Eggs attached to hair shafts receive warmth and moisture from the scalp, maintaining optimal conditions for development.
- Mechanical removal: Detaching nits eliminates the heat and moisture source, causing most to perish within a week if not reattached.
In summary, a louse egg typically hatches within one to two weeks when positioned on a host. Outside the host, its viability declines sharply after about ten days, with extreme temperatures and chemical agents accelerating mortality.