Head lice eggs, known as nits, are tiny, oval casings securely glued by the female louse to an individual hair shaft. Because they are firmly attached and do not move, distinguishing a live, viable egg from one that is dead or an empty shell requires careful visual inspection. This guidance provides specific cues to help differentiate between an active infestation and a harmless remnant of a past one.
What Defines a Viable Lice Egg
A nit is considered viable when it exhibits specific characteristics related to color, texture, and location. Live eggs are generally oval-shaped and possess a yellowish-white or translucent, pearly appearance. They have a plump, uniform shape and are incredibly difficult to slide off the hair shaft because of the durable, cement-like substance secreted by the louse.
The most reliable indicator of a viable nit is its proximity to the scalp. Eggs require warmth to successfully incubate and hatch, which typically takes about seven to ten days. Live nits are almost always found cemented within a quarter-inch (about 6 millimeters) of the scalp’s surface. Any egg found firmly attached within this warm zone should be treated as viable, even if its color is ambiguous.
Visual Cues for Non-Viable Nits
The easiest way to determine a nit’s non-viability is by observing its distance from the scalp. Since human hair grows at a predictable rate, an egg located more than a half-inch (about 12 millimeters) from the scalp is almost certainly dead or has already hatched. This distance indicates the nit was laid at least a week or more ago, and it has moved further down the hair shaft as the hair grew.
Non-viable eggs show distinct changes in color and appearance. A nit that has successfully hatched is left as an empty shell, called a casing, which appears dull, translucent, or white against the hair. If a nit died before hatching, it often develops a dark, opaque, or brownish-black coloration because the embryo inside has stopped developing.
Non-viable nits often look flattened, shriveled, or dented. While live nits are firmly secured, dead nits may feel more brittle or appear chalky, occasionally crumbling when pressed. Finding a large number of these dull, white, or dark casings further down the hair after treatment confirms that the eggs were successfully killed.
Why Physical Removal is Still Necessary
Even after an egg is confirmed dead or hatched, physical removal is necessary. The female louse uses a powerful, water-resistant adhesive to secure the egg to the hair shaft. This cement does not dissolve with water, shampoo, or most chemical treatments, meaning the dead casing remains glued to the hair until it is manually removed or the hair is cut.
The continued presence of non-viable nits can lead to misdiagnosis. Removing all nits with a specialized fine-toothed metal nit comb is the only way to confirm the hair is completely clear and prevent confusion about whether an infestation is active. Combing is most effective when the hair is wet and conditioned, which helps the comb glide smoothly and pulls out the remaining debris.