How long can lice survive outside the head? - briefly
Head lice can survive away from a human scalp for about 24 hours, extending to up to 48 hours under cool, humid conditions. After this period they die from dehydration and lack of blood meals.
How long can lice survive outside the head? - in detail
Lice are obligate ectoparasites that require a human host for feeding and reproduction. When removed from a scalp, their survival depends on environmental conditions, primarily temperature, humidity, and access to food.
Survival limits under typical indoor conditions
- At 20 °C (68 °F) with relative humidity around 70 %, adult head lice can remain alive for 24–48 hours. They become increasingly lethargic after the first 12 hours and will die within two days if they cannot find a host.
- In cooler, drier air (below 15 °C or 59 °F, humidity under 40 %), mortality accelerates; most adults perish within 12–24 hours.
- Warmer, moist environments (25–30 °C, humidity above 80 %) extend viability to roughly 48 hours, occasionally reaching 72 hours, but such conditions are uncommon in most homes.
Nymphs and eggs
- Nymphs (immature lice) are more vulnerable; they typically survive only 12–24 hours without a host, regardless of moderate humidity.
- Lice eggs (nits) are not viable outside the hair shaft. If detached, they dry out and become non‑infectious within a few hours. The embryo inside requires the warmth and moisture of a scalp to develop.
Factors that shorten survival
- Direct sunlight or exposure to UV radiation kills lice within minutes.
- Low humidity (<30 %) causes rapid desiccation.
- Temperatures above 35 °C (95 °F) or below 5 °C (41 °F) are lethal within a few hours.
Implications for control
- Items that may have come into contact with an infested head (combs, hats, pillowcases) should be isolated for at least 48 hours in a dry environment, or washed in hot water (≥60 °C) and dried on high heat.
- Vacuuming carpets and upholstery removes stray lice but does not guarantee elimination unless the area is treated and left unused for the maximum survival period.
In summary, adult head lice can persist for up to two days under favorable indoor conditions, while nymphs and detached eggs survive for significantly less time. Environmental extremes and proper sanitation dramatically reduce their lifespan outside a host.