There is something mysterious about the sun and skin. Why is it that if you go out on a bright summer day and spend an hour in the sun, you get a sunburn? You get a sunburn, that is, unless you happen to have taken the time to get a gradual tan. With a tan you can go out in the sun and nothing happens. Of course, that doesn't apply if you have "fair skin." The fair skinned among us never get a tan, so they always get sunburned. Unless of course they are wearing a sunscreen...

Does this make sense? What exactly is a sunscreen? And what is a tan? What is the difference between a tan and a burn? Why can you spread a little blob of lotion on yourself and be protected, but if you forget you are miserable?

If you take the time to look at skin and sunlight in some detail, all of this actually does begin to make sense. You can learn a huge amount about your body in the process. So, that's what we'll do in this article. First, let's take a look at how skin works.

How Skin Works
Skin is one of the most amazing organs in the human body. It is hard for us to think about it as an organ, however. We tend to think of organs as boxy things. Your heart, liver, kidneys - those are obviously organs. But skin is an organ too, especially if you look at the dictionary definition of "organ", like this definition from the Merriam Webster Collegiate Dictionary:

Organ - a) differentiated structure (as a heart, kidney, leaf, or stem) consisting of cells and tissues and performing some specific function in an organism b) bodily parts performing a function or cooperating in an activity

By that definition, skin is definitely an organ. Skin is made up of very specific cells and tissues, and their collective purpose is to act as the boundary between "you" and "the world". One of the neat things about skin that makes it different from a lot of other organs is the fact that it does have to deal with the real world. Therefore it is loaded with sensors, and it also has a very tough layered design so that it can handle realities of the environment like abrasion and sunlight.

If you take a look at a cross section of typical skin (like the skin on your arm or leg) you find that it is made up of two main layers: the epidermis on the outside and the dermis on the inside. The epidermis is the barrier, while the dermis is the layer containing all the "equipment" -- things like nerve endings, sweat glands, hair follicles and so on. Here's a picture to help you see what is going on:

In the subcutaneous layer (you may have heard of subcutaneous fat -- this is where it lives) you can see the blood vessels (shown as two thin red and blue lines). These vessels branch infinitely (not shown) into the dermis to supply the sweat glands, hair follicles, sebaceous glands and erector muscles with blood. They also fan out into the dermis's capillary bed. It turns out that the dermis is loaded with capillaries. Capillaries satisfy the nutritional needs of the cells in the dermis, and they also help the skin perform an important cooling function in humans. The epidermis has no direct blood supply, but instead is supported and fed by the dermis.