How long do head lice live outside a human head? - briefly
Adult head lice remain viable off a host for roughly 24–48 hours, with survival extending only under warm, humid conditions. Nymphs and eggs lose viability sooner, generally within a single day.
How long do head lice live outside a human head? - in detail
Head lice (Pediculus humanus capitis) can persist for a limited period without a host. On a dry surface, adult lice typically survive for 24 hours, occasionally extending to 48 hours under optimal humidity (70‑80 %). Nymphs and eggs (nits) are less tolerant; they usually die within 12 hours when removed from a scalp. Moist environments prolong survival: in a humid bathroom or on damp clothing, adults may remain viable for up to 72 hours, while nits may retain embryonic development for 48 hours if temperature stays between 20 °C and 30 °C.
Key factors influencing off‑host longevity:
- Humidity: 70‑80 % maintains cuticular moisture, delaying desiccation.
- Temperature: 20 °C–30 °C supports metabolic activity; lower temperatures accelerate death.
- Substrate: Smooth, non‑porous surfaces (plastic, metal) reduce water loss compared with fabric.
- Age of the insect: Mature adults possess greater internal reserves than freshly emerged nymphs.
Practical implications for control:
- Items that have contacted an infested head should be isolated for at least 48 hours; laundering at ≥60 °C or sealing in plastic bags for three days eliminates surviving lice.
- Vacuuming upholstery and car seats removes detached insects; immediate disposal of vacuum bags prevents re‑infestation.
- Dry heat (dryers, sun exposure) for a minimum of 30 minutes at high temperature effectively kills both adults and nits.
In summary, head lice survive no longer than two days on dry surfaces, with possible extension to three days in humid, warm conditions. Eggs lose viability within two days when separated from the scalp. Proper environmental management exploits these time limits to prevent re‑colonization.