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 nice gradual tan. With a nice 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 the heck 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 edition of How Stuff 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 the 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.

The dermis is where the action is functionally. As shown in the diagram, the dermis contains sweat glands, hair follicles (each with its own tiny little muscle so that your "hair can stand on end"!), nerve endings and so on. There are several different types of nerve endings:

     Heat sensitive
     Cold sensitive
     Pressure sensitive
     Itch sensitive
     Pain sensitive

All these different nerve endings let you sense the world. They also help you protect yourself from burns, punctures, etc. by warning you when something is damaging your skin.

The epidermis is your interface to the world, and it is actually quite interesting. It has two main layers, the inner of which is living and the outer of which is dead. The dead skin cells of the outer layer are what we can actually see, and they are constantly flaking off and being replaced by new cells being pushed outward.

The living, inner layer is called the malpighian layer. The malpighian layer creates the dead cells that we can see. It is in direct contact with the dermis, which feeds and supports it. The malpighian layer is our focus of attention actually, because it is here that the sun affects the skin during tanning. The malpighian layer is itself layered like this:

In direct contact with the dermis is the basal layer. If you have ever heard of a basal cell carcinoma (cancer), this is where it starts. Above the basal layer is the spinous layer. Above the spinous layer is the granular layer.

Above the granular layer is the stratum corneum. The stratum corneum is the outer layer of dead cells - the cells that we see as our skin. The cells in this layer are filled with a protein called keratin. Keratin is a very interesting protein because it is tough - horns, hair, hoofs, fingernails and feathers all gain their strength from keratin. The same stuff that your fingernails are made of actually forms your visible skin (but in a much thinner and more flexible layer). That is what makes your skin so tough. In parts of the body that get a lot of wear, like the palms and the feet, the stratum corneum is thicker to handle the abrasion.

Living among the basal cells in the malpighian layer is another type of cell called a melanocyte. Melanocytes produce melanin, which is a pigment that is the source of tanning. The melanocytes are actually where a tan comes from. Here is what the Encyclopedia Britannica has to say about melanocytes:

"The appearance of the skin is partly due to the reddish pigment in the blood of the superficial vessels. In the main, however, it is determined by melanin, a pigment manufactured by dendritic cells called melanocytes, found among the basal cells of the epidermis. Their numbers in any one region of the body, which range from about 1,000 to more than 2,000 per square millimetre, are roughly the same within and between races; the blondest whites have as many as the darkest blacks. Colour differences are due solely to the amount of melanin produced and the nature of the pigment granules. When the skin becomes tanned on exposure to sunlight, the melanocytes do not increase in number, only in activity." ("Integumentary Systems, Pigmentation", Britannica CD. Version 97. Encyclopaedia Britannica, Inc., 1997.)

Not only do melanocytes produce a tan, they are also responsible for the form of cancer called melanoma. Melanoma is caused by UV radiation damage to melanocytes. Repeated exposure to UV can cause cancerous mutations.

How Tanning Works
So, now that we know all about the skin we can start to actually understand tans and sunburns. When you get a tan, what is actually happening is that the melanocytes are producing melanin pigment in reaction to ultraviolet light in sunlight. Ultraviolet light stimulates melanin production. The pigment has the effect of absorbing the UV radiation in sunlight, so it protects the cells from UV damage. Melanin production takes a fair amount of time - that is why mostpeople cannot get a tan in one day. You have to expose yourself to UV for a short period of time to activate the melanocytes. They produce melanin over the course of hours. By repeating this process over 5 to 7 days pigment builds up in your cells to a level that is protective.

The previous paragraph applies to Caucasians. In a variety of other races melanin production is continuous, so the skin is always pigmented to some degree. In these races the incidence of skin cancer is much lower because cells are constantly protected from UV radiation by melanin.

Melanocytes actually produce two different pigments: eumelanin (brown) and phaeomelanin (yellow and red). Red heads happen to produce more phaeomelanin and less eumelanin, which is why they don't tan very well (see this article). In albinos, the chemical pathway that produces melanin cannot proceed because an enzyme called Tyrosinase is missing. Therefore albinos have no melanin in their skin, hair or irises. The actual chemistry of melanin creation is described very nicely in this article.

Melanocyte-stimulating hormone (MSH) is produced by the pituitary gland. MSH flows through the bloodstream and reaches the melanocytes, encouraging them to produce more melanin (for example, a person injected with a large dose of MSH will get darker). The pituitary gland is actually quite interesting - it is tied into the optic nerve, which means that it can sense light. If you have ever raised chickens for eggs, you know that in the winter egg production falls way off. You solve this problem by providing light in the chicken coop. The extra light stimulates the pituitary gland in chickens, which causes the gland to produce a hormone essential to egg laying. In humans, light affects the pituitary gland as well and one result is the production of MSH. A funny side-effect of all of this is that wearing sunglasses may make you more susceptible to sunburn! See this page for some thoughts on the subject.
 

How Sunburn works
If you are Caucasian and you don't have a tan, then the cells in your skin are not protected from the sun's ultraviolet radiation. You are therefore an easy target for sunburn if you spend too much time in the sun. As anyone who has a sunburn knows, sunburn leaves your skin red and extremely painful. In severe cases the skin forms blisters.

When you get sunburn, your skin is actually damaged by UV radiation and your body is responding to the damage. Here is a specific medical description of what is happening from this article:

Sunburn is a popular term applied to the marked erythema and pain that commonly follows injudicious sun exposure. A sunburn is really a delayed ultraviolet B-induced erythema caused by an increase in blood flow to the affected skin that begins about 4 hours and peaks between 8-24 hours following exposure [23,30,31]. The underlying cause of this vascular reaction is direct and indirect damage to specific cellular targets from photochemical reactions and the generation of reactive oxygen species [32]. Damage to DNA, and the activation of several inflammatory pathways, particularly involving prostaglandins [27,33-38], are thought to trigger this reaction, ultimately leading to vasodilation and edema. Biologic response modifiers released by both keratinocytes and lymphocytes also play a role [39-55]. The development of erythema therefore implies that enough ultraviolet damage has occured that inflammatory pathways have been activated. Erythema is probably best thought of as a total failure of sun protection, and is a marker for severe UV damage.

Several lines of evidence suggest a relationship between erythema and DNA damage. There is rough correlation between pyrimidine dimer yeild and susceptibility to erythema with sun exposure [56]. Wavelengths that are the most efficient at producing erythema are also the most efficient at producing pyrimidine dimers [56]. From a scientific point of view, a sunburn can be viewed as a marker for a substantial ultraviolet over-exposure that has clinical implications for skin cancer risk. It is now appreciated that there is a linkage between a history of repeated, severe sunburn and increased risk for melanoma [1, 57-62] and non-melanoma skin cancer [63-65].

Translating, what this quote says is that when you get a sunburn, what you are really getting is cellular damage from ultraviolet radiation. The body responds to the damage with increased bloodflow to the capillary bed of the dermis in order to bring in cells to repair the damage. The extra blood in the capillaries causes the redness - if you press on sunburned skin it will turn white and then return to red as the capillaries refill.
 

How Sunscreens Work
Sunscreens (see also this article) block or absorb ultraviolet light. You can block UV with opaque creams like the white zinc oxide cream that you see lifeguards putting on their noses. You can also absorb UV radiation in much the same way that melanin does. The first and most common of the absorpsion chemicals is PABA (para-aminobenzoic acid). It absorbs UVB. Other chemicals include:
 

All sunscreens are labeled with an SPF, or Sun Protection Factor. The SPF acts like a multiplying factor. If you would normally be OK in the sun for 10 minutes and you apply and SPF 10 sunscreen, you will be OK in the sun for 100 minutes. In order for the sunscreen to work, however, you have to apply plenty and it has to stay on. You should apply it about half an hour before going out in the sun (or the water) so it can bind to your skin - if you don't, then it is very easy for the sunscreen to wash off.

The SPF rating, by the way, applies only to UVB radiation.
 

How Vitamin D works
All human beings need Vitamin D in order for their bodies to grow and function properly. Vitamin D helps the body metabolize calcium. When children lack vitamin D, their bodies cannot use calcium properly and they develop rickets. Rickets is marked by "soft bones" and things like bow leggedness and knock knees.

Cod liver oil supplies Vitamin D, and it was commonly prescribed before the advent of Vitamin D fortified milk. The other way to get Vitamin D is to expose your skin to ultraviolet light. Ultraviolet light converts 7-dehydrocholesterol flowing in your bloodstream into Vitamin D. [See this article for details.] [See also this article on skin pigmentation.]