1. Why Skin Turnover Can Be Said as 28 Days or About Two Weeks
The outermost part of your skin is called the epidermis. New cells are born in the deep layers of the epidermis, pushed upward, and finally fall off as dead skin flakes. The time this full cycle takes is generally explained as "about 28 days" [1].
However, if you look only at the stratum corneum, which is the very outermost layer of the epidermis, cells are said to finish their job and shed in about two weeks (14 days) [2]. There is also an explanation that counts from the point where new cells are made in the deepest basal layer as about 45 days, while counting from when cells begin to grow within the epidermis is about 28 days [2].
To organize this, the difference between 28 and 45 days is mainly "where you start counting." On the other hand, the "about two weeks" figure is not a different starting point but an estimate of how long cells stay inside the stratum corneum. There is a range depending on the measurement method and body part. None of these are wrong; they just measure different things.
You can check this relationship with addition. A summary of studies using autoradiography (a method that uses radioactive markers) shows that time passing through the living cell layers of the epidermis is about 13 days, and the stratum corneum takes about 13 to 14 days, totaling about 26 to 27 days [3]. This is close to "about 28 days." Accept 28 days as the value generally used in explanations.
2. The Stratum Corneum: A Thin Layer Made of Stacked Dead Cells
The cells in the stratum corneum, called corneocytes, are flat cells without nuclei. They are no longer alive. Like tiles on a roof, they stack up in many layers to protect against outside stimuli and prevent body moisture from escaping.
The number of layers varies by source, ranging from about 10 to 20 layers. It is natural to think this changes depending on the location on the body or the individual.
In a study examining the inner forearm and upper arm, the average number of layers was about 17 in the forearm and about 14 on the inner upper arm [4]. The number of layers is also related to the time it takes for cells to pass through.
3. Measuring Stratum Corneum Turnover Days with Fluorescent Dye
An old method for measuring turnover days in the stratum corneum uses a fluorescent dye called dansyl chloride. The dye is stained onto the stratum corneum and illuminated with an ultraviolet lamp (Wood's lamp) to count the days until the light disappears [4]. When skin cells shed, the dye is lost with them, so the day the light is no longer visible marks the turnover time.
Using this method, the stratum corneum transit time in young adults was reported as 19.8 days on the forearm and 17.7 days on the inner upper arm. This means about 18 to 20 days [4]. This is slightly longer than the about 13 to 14 days found in studies using radioactive markers [3], showing that "about two weeks" is a rough estimate that varies by method and location.
The time per layer was also estimated in the same study. For young people, it is about 30 hours (about 1 day and 6 hours) [4]. Since the forearm has about 17 layers, 30 hours times 17 equals 510 hours, which is about 21 days. If it were 28 hours, 17 times 28 equals 476 hours, or about 19.8 days. This multiplication is an approximation, but it gets close to the measured value of 19.8 days.
Age also affects these results using this method, as we will see in the next section.
4. How Skin Turnover Time Changes with Age
Older people were also studied using the same method. On the forearm, it took about 28 days, and on the inner upper arm, about 25 to 26 days. This is about 8 to 9 days longer than for younger people [4].
What is notable is that the number of layers did not change with age. This means the difference caused by aging was not because there were more layers, but because each layer took longer to turn over [4].
One possible reason for this is a joint study published in 2021 by Menard Cosmetics and Fujita Health University. They reported that when the protein JAG1 decreases due to aging, the flow of cells moving to upper layers weakens, causing epidermal turnover to stall. This included experiments using artificial skin models [1]. Because this is a joint study with a company and still at an "expected" stage, it cannot be stated as absolute fact.
Both stratum corneum measurements and epidermis research point in the same direction: turnover slows down as you age.
5. How Enzymes Cut the Fasteners Between Skin Cells
Corneocytes are connected to each other by fasteners called corneodesmosomes. In the middle of the stratum corneum, these fasteners remain. However, as cells move toward the outside, protein-decomposing enzymes like kallikrein cut these fasteners. On the outer side, it is reported that the places where the fasteners were are replaced by gaps called lacunae [5].
It is important that they do not break in the middle but only on the outside. If all fasteners broke at once, the stratum corneum would collapse. If they did not break, cells would remain and the stratum corneum would become thicker.
Because this shedding mechanism is regulated, the stratum corneum stays thin even as cells shed little by little every day. The balance between maintaining adhesion and breaking it down in specific places helps keep the stratum corneum thin [5].
6. How to Read the 14-Day and 28-Day Numbers and Check Them
You can mark your calendar. Mark 14 days from today and 28 days from today. Compare these visually: "The estimate for stratum corneum stay time is 14 days, and the general explanation for the whole epidermis is 28 days." Then, for practice, assume 10 layers and calculate with 30 hours per layer (10 times 30 equals 300 hours, or 12.5 days). Note that 10 layers is not a standard value but just a number for calculation practice.
When reading numbers in ads or news, if you see "28 days," "45 days," or "renews in X days," ask yourself: "Where do they start counting?" There is a 45-day theory that counts from basal cells and a 28-day theory that counts from within the epidermis. The difference may just be in the counting method.
If you want to know more, the primary source for turnover measurement is the paper on dansyl chloride disappearance methods [4]. In this English-language paper, tables show layer numbers and time per layer. The mechanism of shedding can be seen with photos in the study on corneodesmosome degradation [5].