How does shampoo affect lice? - briefly
Most ordinary shampoos do not kill lice; only formulations that include insecticidal ingredients such as permethrin, pyrethrin, or dimethicone can effectively eliminate the insects. Shampoos without these agents may detach some nits but do not provide a complete solution.
How does shampoo affect lice? - in detail
Shampoo can influence head‑lice infestations through several mechanisms that depend on its formulation and mode of application.
The primary actions are:
- Physical removal – vigorous lathering dislodges adult insects and nymphs from hair shafts, allowing them to be rinsed away.
- Chemical toxicity – ingredients such as pyrethrins, permethrin, benzyl alcohol, or dimethicone penetrate the exoskeleton, disrupting nervous function or suffocating the parasite.
- Egg (nit) disruption – certain surfactants and solvents weaken the cement that attaches ova to hair, making them easier to comb out or causing premature hatching.
Effectiveness varies with concentration, contact time, and the presence of resistance‑conferring mutations in lice populations. Standard over‑the‑counter products typically contain 1 % permethrin; studies show 70–80 % mortality after a 10‑minute exposure, decreasing to 40 % when resistance is documented. Dimethicone‑based shampoos act as a physical occlusive, achieving 90 % mortality without reliance on neurotoxic agents, and remain effective against resistant strains.
Application guidelines that maximize impact include:
- Saturate hair completely; ensure scalp coverage.
- Maintain the recommended dwell time (usually 5–10 minutes) before rinsing.
- Follow with a fine‑toothed nit comb to extract remaining insects and eggs.
- Repeat treatment after 7–10 days to target any newly hatched nymphs that survived the initial exposure.
Adverse effects are generally mild, limited to scalp irritation or transient dryness, and are mitigated by using conditioners or limiting frequency to the prescribed schedule. In summary, shampoo affects lice by physically dislodging them, delivering toxic agents that kill adults and nymphs, and compromising the attachment of eggs, with efficacy contingent on formulation, resistance patterns, and proper usage.