1. Sunburn Redness and Skin Darkening Are Two Separate Reactions
The Japanese word for sunburn actually covers two different reactions. One is redness, known as sunburn (sunburn), which appears a few hours after exposure to sunlight. The other is darkening, known as tanning (tan), which appears a few days after the redness has disappeared [1]. Tanning can last for several weeks or months.
In this article, "darkening" refers to this later stage, tanning. The light that causes the initial redness reaction is thought to be mainly ultraviolet rays with wavelengths shorter than 320 nanometers, known as UVB [2].
2. How Ultraviolet Rays Damage the DNA in Skin Cells
Sunlight contains invisible ultraviolet rays. When these rays hit skin cells, they can damage the DNA stored inside the cell nucleus. DNA is like a blueprint that contains the information needed to build the body. However, it is not made of paper; it is actually very small substances connected in long chains.
This "DNA damage" is thought to be the starting point for the darkening reaction [3]. The next section explains how cells report that they are damaged.
3. How Damaged DNA Leads p53 to Send a Signal for Darkening
When DNA is damaged by ultraviolet rays, a protein called p53 starts working inside the epidermal cells. It is said that p53 is involved in making a hormone called alpha-MSH through a gene called POMC [3].
When this alpha-MSH reaches the pigment-producing cells, melanin is made. Think of it like a fire alarm. However, no sound actually rings; instead, specific substances are passed along one after another.
In 2007, David Fisher and others at Dana-Farber Cancer Institute, Harvard Medical School, reported that p53 triggers this flow [4]. When they stopped p53 from working, alpha-MSH did not increase. Mice without p53 did not get dark ears or tails even when exposed to ultraviolet rays [4].
4. How Melanocytes Make Melanin and Deliver It
The cells that make pigment are called melanocytes. When melanocytes receive the signal from alpha-MSH, they produce a dark pigment called melanin.
Melanin is put into small bags called melanosomes. These bags are passed to neighboring cells called keratinocytes through projections that extend from the melanocyte like tree branches [3].
It is explained that melanin works mainly inside the keratinocytes where it is delivered [3]. The cell that makes the pigment and the cell that needs protection are different.
5. How Melanin Lines Up Around the Cell Nucleus to Protect It
The received melanin lines up to surround the cell nucleus. A research group including Kyoto University reported this observation in 2016 [6].
It is thought that melanosomes enter the cell and surround the nucleus, protecting it from DNA damage [6].
Comparing this to an umbrella makes it easy to understand. However, because this observation was made in experiments using skin tissue grown inside chicken eggs, we cannot say for certain that it is exactly the same as human sunburn.
6. Why Darker Skin Still Does Not Mean Safe Skin
A document from the Ministry of the Environment explains sunburn (tanning) as a defense reaction where the body tries to prevent damage from ultraviolet rays. However, it says the protective effect of tanning is small, about SPF 4 [1]. The MSD Manual also states that while tanning provides slight natural protection, there are no health benefits [2].
In other words, darkening is the result of cells trying to protect themselves. But it does not seem to be a sign that it is okay to get sunlight.
However, the protective power is small, and being dark is not evidence that you are safe. If you are worried about your skin condition, please talk to an adult or a doctor.
If You Want to Look It Up Yourself
Please do not try to get sunburned on purpose. If you have severe redness, strong pain, or blisters, tell an adult and consult a doctor.
The first chapter of the Ministry of the Environment's "Ultraviolet Radiation Environmental Health Manual" contains information about the difference between sunburn and tanning, as well as the protective effects of tanning.
If you are worried about your skin condition, talk to an adult or a doctor.