How long can lice live outside a human head? - briefly
Lice can survive without a host for approximately 24 – 48 hours; in cool, humid conditions they may persist up to two days, after which they usually die.
How long can lice live outside a human head? - in detail
Head lice (Pediculus humanus capitis) require human blood for nourishment; without a host they quickly exhaust their energy reserves. Under average indoor conditions (20‑25 °C, 40‑60 % relative humidity) adult insects survive roughly 24–48 hours. Survival extends to 48–72 hours when temperatures drop to 15 °C and humidity rises above 70 %. Laboratory experiments have recorded occasional survival up to 7 days in cool (10 °C), moist environments, but such cases are exceptional.
Key factors influencing off‑host longevity:
- Temperature: Below 10 °C metabolic activity slows, allowing prolonged survival; above 30 °C dehydration accelerates death.
- Humidity: Relative humidity above 70 % prevents desiccation; below 30 % leads to rapid water loss.
- Life stage: Nymphs die faster than adults; eggs (nits) remain viable for 7–10 days if kept moist, but will not hatch without a blood meal.
Practical implications:
- Items that may harbor live insects (combs, hats, pillowcases) should be washed in hot water (≥60 °C) or sealed in a plastic bag for at least 48 hours.
- Vacuuming carpets and upholstery reduces the chance of accidental transfer but does not eradicate lice, as most survive only a short period away from a scalp.
- Environmental control is unnecessary beyond routine cleaning; the short off‑host lifespan makes prolonged infestation of surroundings unlikely.