Why does hair lighten in the sun but skin darken?
A curious adult from Brazil asks:
"Why does hair lighten in the sun but skin darken? Does it have to do with melanin?"
That is a great question and worth investigating! You are right in thinking that melanin has something to do with the answer. Melanin is a pigment found in your skin and hair cells that gives each its color.
It does seem weird that the sun bleaches our hair and darkens our skin. This mostly has to do with hair being dead and skin being alive.
The sun bleaches and destroys the melanin in your hair giving you lighter hair. Since hair is dead, the hair will stay that color until new hair comes in.
When sun shines on your skin, it destroys the melanin as well. But since your skin is alive, it can respond to the sun's damage. Your skin cells make more melanin and your skin becomes darker.
That's the simple answer, now let's get into the details.
Special cells in the skin, called melanocytes, make two different types of melanin, eumelanin and pheomelanin. Eumelanin has a brown or black color while pheomelanin is yellowish-red. How much of each kind of melanin we make determines the exact shade of our skin and hair color.
Why do we even need this coloring? Melanin helps protect our skin and hair by filtering out potentially harmful ultraviolet (UV) radiation from the sun.
The UV light from the sun can damage DNA and cause cancer. This is one reason why our body darkens the skin when exposed to sun-- to protect us.
Skin darkening is a two step process. First our cells experience the "immediate pigment darkening" response when exposed to the sun. This happens when our cells are exposed to UVB or ultraviolet type B radiation.
This is a quick response that happens over a period of minutes to days. You can't easily see this happen with lighter skinned people but it is more obvious with people with darker skin.
The second and slower response of our skin cells to sunlight is called "delayed tanning". As the name suggests, it's much slower. This second response is also called melanogenesis and happens when our skin is exposed to UVA radiation.
Melanogenesis just means the cell is ramping up to make more melanin pigment. Part of this is that more melanocytes get made. Also, melanin genes start making more melanin so that the melanocytes are darker. So you end up with darker melanocytes and more of them.
Strangely the sun is not directly causing more melanin to get made. It's the damage that the sun is doing to the cell that starts this process.
When the sun damages a skin cell, the cell releases chemicals alerting the body that it has taken a hit. These chemicals cause more melanocytes and more melanin to get made.
Of course, this process needs to stop at some point. If it kept on, we'd call melanogenesis skin cancer. So the body stops the process with some different chemicals and we eventually turn lighter for the winter.
None of this can happen in hair because hair is dead. So our hair has to take the full brunt of the sun without any defenses.
And sunlight is pretty energetic. For example, look how it ruins plastics -- like vinyl car roofs. In time, sunlight will destroy some plastics causing them to fall apart and crack. In chemical terminology, the ultraviolet radiation in sunlight ruins things by oxidizing them.
Similarly, the UV in sunlight oxidizes melanin into a colorless compound. This is why your hair gets lighter. Blond hair is really just colorless hair or hair with very little melanin in it.
But melanin isn't the only protein taking a beating in your hair. The sun is also clobbering other proteins making hair less manageable as well.
There is a chemical group on hair called a thiol. When thiols are unoxidized, the hairs slide easily across each other. When the sun oxidizes those thiols, it is a whole different story.
When a thiol group is oxidized to a sulfonic acid, the hairs tend to stick together more. So you get tangling that can't be repaired.
Once the thiols on hair have been oxidized to sulfonic acids, there's really no going back. All you can do is to treat the hair with conditioners and wait for new hair to grow in.
So, tanned skin and bleached hair may be a sign that someone is spending too much time in the sun. Try to keep your skin light and your hair dark by wearing sunscreen and a hat!
Author: Dr. Aaron Shafer
When this answer was published in 2006, Aaron was a post-doctoral fellow in the Department of Molecular and Cellular Physiology, studying activation mechanism of g protein-coupled receptors in Brian Kobilka’s laboratory. Aaron wrote this answer while participating in the Stanford at The Tech program.