Head lice (Pediculus humanus capitis) are common parasites. Many people seek non-chemical treatments, often believing that covering the hair with a thick substance will quickly suffocate the lice. This is an oversimplification, as head lice are remarkably adapted to survive temporary occlusion. They do not die quickly when submerged or covered, making understanding the actual time required for mortality crucial for effective treatment.
Understanding Lice Respiration
Head lice breathe through small openings along their bodies called spiracles, which lead into the insect’s internal respiratory system. Lice can close their spiracles, effectively holding their breath for extended periods when submerged or coated in a substance.
The goal of occlusive treatment is to physically block these spiracles with a thick, non-volatile material, not just deprive the louse of oxygen. Professional treatments, such as dimethicone, can also disrupt the louse’s ability to excrete water, causing internal stress. Household treatments rely primarily on mechanical blockage, which the louse resists by keeping its spiracles tightly closed.
The Actual Time Required for Mortality
The time required for adult lice to die from occlusion is significantly longer than expected, often requiring many hours rather than minutes. Lice can survive submersion in water for up to eight hours by closing their spiracles, which is why briefly applying a thick product or wetting the hair does not work.
For household remedies like petroleum jelly or mayonnaise, experts recommend an application time of eight to twelve hours, often suggesting overnight treatment. This extended duration is necessary to overcome the louse’s ability to hold its breath and ensure the occlusive agent remains fully in place. It is important to distinguish between a “knockdown” effect, where the louse stops moving, and true mortality.
Why Occlusion Does Not Kill Nits
Occlusive agents are effective only against live, active lice and nymphs, but not against nits (head lice eggs). The nit is protected by a hard, shell-like casing that is firmly glued to the hair shaft and is highly resistant to penetration by most topical treatments.
The nit does not breathe through spiracles in the same way as a hatched louse, making it immune to the suffocation mechanism. This protection means that even a successful occlusive treatment must be followed by a second treatment seven to ten days later. The second application kills newly hatched nymphs before they mature and lay new eggs, breaking the reproductive cycle.
Comparing Common Occlusive Treatments
Occlusive treatments create a thick, suffocating layer over the lice, and the substance’s viscosity is a primary factor in effectiveness. Common household agents like full-fat mayonnaise and petroleum jelly are used because their thick consistency helps seal the spiracles. These treatments require a generous coating from the scalp to the hair tips, followed by covering the head with a shower cap to trap the substance and prevent drying.
Professional occlusive products, often based on high-viscosity dimethicone, are generally more effective because they are formulated to spread easily and provide complete coverage. These synthetic silicone oils can sometimes work in shorter timeframes, though application time varies by formulation. Household agents are messier, require many hours, and must still be followed by meticulous combing to manually remove the nits.