Why lips are a special case
The red part of the lip, the vermilion, is a transitional zone between the skin of the face and the mucosa inside the mouth, and it has the disadvantages of both.
Its stratum corneum is very thin compared with facial skin, so the resistance to water leaving is low. It has effectively no sebaceous glands, so there is no surface lipid film arriving from below to replace what is removed. It has no sweat glands, so it cannot contribute the components of sweat that help hold water at the surface elsewhere. And it is thin enough that the blood supply beneath shows through, which is why it is red and why it looks worse the moment anything happens to it.
Put those together and you have a surface across which water leaves faster than anywhere else on the exposed body, with no capacity to protect itself. This is why lips respond to a cold wind in minutes when a cheek takes hours.
Then the mechanical load
Lips are also under constant mechanical load in a way that no other skin is.
They stretch every time you speak, eat, smile or grimace. The corners, where two directions of movement meet, are the most heavily loaded and are the classic site for splitting. Anyone who has had a split at the corner of the mouth knows how long it takes to close, and the reason is that the site is loaded thousands of times a day.
Add cold, which stiffens the layer, and a stiff thin sheet under repeated stretching fails by cracking. That is exactly what a chapped lip is.
The licking loop
This is the mechanism that matters most, because it is the one people are actively doing to themselves.
A dry lip feels better wet, so people lick. Saliva is mostly water, and thin films of water on a warm surface in moving air evaporate quickly. Evaporation cools the surface and, crucially, takes with it more than it delivered, because the film also carries away water drawn from the layer beneath while it was there.
Saliva also contains digestive enzymes. Repeatedly coating a thin barrier with enzymes intended to break down food is not a neutral act, and the skin around the mouth, not just the vermilion, can be affected by it.
So the loop is: dry lip, lick, temporary relief, faster drying, drier lip, lick more. It is self reinforcing and it is the single commonest reason chapped lips do not resolve in people who are otherwise doing everything right.
Breaking it requires putting something on the lip that lasts, so the impulse is not triggered. That is a genuinely functional argument for an occlusive layer here rather than a cosmetic one.
| Vermilion of the lip | Adjacent facial skin | |
|---|---|---|
| Stratum corneum | Very thin | Thin but substantially thicker |
| Sebaceous glands | Effectively none | Present, and active |
| Sweat glands | None | Present |
| Mechanical load | Stretched thousands of times a day | Moderate |
| Wetted and dried by | Breathing, eating, speaking, licking | Weather and washing |
| How it fails | Splitting, especially at the corners | Tightness, then flaking |
Ordering framework written by this publication from general skin physiology. It is not a measurement, it is not taken from any study, and no number in it is a reading.
The outdoor version
For our readership there are two additional loads.
Wind strips the boundary layer from a surface that had almost no defence to begin with, which is why an hour on the water in a breeze does more to lips than a day indoors.
And salt, which sits on the lips after every session, is a water attracting film applied directly to the thinnest barrier on the exposed body. Licking salt off is the worst possible combination of the two mechanisms above.
Rinsing the face, including the mouth, before drying and before driving home is a small thing that people do not do.
What to look for by function
We do not name products. What we can describe is what the job requires.
The function needed on a lip is occlusion that stays put: a layer that reduces evaporation and that survives speaking, eating and wind for a useful period. Heavier and greasier performs this function better than light and pleasant, which is why the applications people enjoy most are often the ones that do least.
Humectant heavy formulations applied to lips in cold dry air without an occlusive over them can draw water from the layer and let it evaporate, which is the mechanism described in the article on the three jobs. On lips, in a wind, that is a real consideration rather than a theoretical one.
Fragrance, flavour and anything that produces a tingle is being applied to a very thin barrier that already lets things through, on a surface you will then swallow from. That is not an argument that anything is dangerous; it is an argument that this is a poor site for optional ingredients.
Where lips stop being our subject
Persistent cracking at the corners of the mouth, a lip problem that does not settle over a couple of weeks, scaling or crusting that keeps returning, a persistent rough or white patch, any lasting change in a single spot, or anything that bleeds repeatedly, is not something to keep treating with a balm.
Those are reasons to see a GP or a dentist. Lips are also a site where persistent changes need a proper look by someone qualified, and we are not going to describe what any of them might be. Get it looked at.