Why do lice appear on a clean head? - briefly
Lice infest clean hair because they are transferred by direct contact with an already infested person or through contaminated items such as combs, hats, or bedding. Their eggs cling to hair shafts and hatch regardless of scalp cleanliness.
Why do lice appear on a clean head? - in detail
Lice infestations can begin even when hair looks clean because the insects do not require visible dirt to survive. Female head‑lice lay 5–10 eggs (nits) each day, attaching them to hair shafts near the scalp. These eggs hatch in about a week, releasing nymphs that mature within another week. The cycle proceeds irrespective of shampoo use or hair‑care products.
Key factors that enable an infestation on a well‑maintained head:
- Direct contact with an infested person or contaminated items (combs, hats, pillowcases). Transmission occurs within seconds of head‑to‑head proximity.
- Low‑level populations that are not yet noticeable. Adult lice are 2–3 mm long and can blend with hair strands, especially when hair is short or densely packed.
- Resistance to common over‑the‑counter treatments. Some lice strains have evolved mechanisms to survive insecticide‑based shampoos, allowing them to persist despite regular washing.
- Environmental conditions that favor survival: temperature around 30 °C and humidity above 50 % accelerate development and increase egg viability.
The presence of shampoo residues does not deter lice because they feed exclusively on blood, not on scalp oil or debris. Their claws are adapted to grasp hair shafts, enabling them to cling tightly even when hair is freshly washed. Regular cleaning reduces bacterial load but does not affect the lice’s ability to attach and reproduce.
Effective control requires interruption of the life cycle: removing existing lice and nits, treating the scalp with a proven pediculicide, and eliminating sources of re‑infection through laundering of bedding, clothing, and personal items. Re‑inspection after 7–10 days ensures that any newly hatched nymphs are detected and eliminated.