Why is it that when you go out on a bright summer day and spend an hour or so in the sun, you most likely get a sunburn? That is, unless you've taken the time before that to get a nice gradual tan. If you have a tan, you can go out in the sun and not get burned. Of course, none of this applies if you have "fair skin." The fair-skinned among us never get a tan, so they always get burned. If you take the time to look at skin and sunlight in some detail, all of this begins to make sense, and you can learn a lot about your body in the process.
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. @Merriam-Webster Collegiate Dictionary defines an organ as a) a differentiated structure (as a heart, kidney, leaf, or stem) consisting of cells and tissues and performing some specific function in an organism; or 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 interesting things about skin that makes it different from the internal organs is that it has to interact extensively with the real world. Therefore, it is loaded with sensors, and it also has a very tough, layered design so it can handle diverse environmental factors, such as 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 between you and the world, while the dermis is the layer containing all the "equipment" -- such as nerve endings, sweat glands, hair follicles, and so on. Here's a description to help you understand what is going on:
In the skin's subcutaneous layer you can see the blood vessels. These vessels branch extensively into the dermis to supply the sweat glands, hair follicles, sebaceous glands, and erector muscles with blood. They also fan out into the dermis' capillary bed. It turns out that the dermis is loaded with capillaries, very small blood vessels. 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 also contains nerve endings, which let you sense heat, cold, pressure, pain, etc. These help you protect yourself from burns, punctures, and so on by warning you when there is danger to your skin. Functionally speaking, therefore the dermis is where the action is.
The epidermis is your interface to the world. 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. It is in direct contact with the dermis, which feeds and supports it. It is here that the sun affects the skin during tanning. The malpighian layer is itself layered like this: The bottom layer, in direct contact with the dermis, is the basal layer. Above the basal layer is the spinous layer. Above the spinous layer is the granular layer. And 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 very strong protein called keratin. Horns, hair, hoofs, fingernails, and feathers all gain their strength from this remarkable structure. It is the keratin that 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 forms much thicker layers, 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 both tanning and naturally dark skin color, such as in @African-Americans. The Encyclopedia @Britannica describes melanocytes in the following way:
"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. Color 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",CD. Version 97. Encyclopaedia Britannica, Inc., 1997.)
Melanocytes are also responsible for the form of cancer called melanoma. Melanoma is caused by ultraviolet û @UV û radiation damage to melanocytes. Repeated exposure to UV can cause cancerous mutations.
In non-Caucasian 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). Redheads happen to produce more phaeomelanin and less eumelanin, which is why they don't tan very well. In albinos, the chemical pathway that produces melanin does not function because an enzyme called tyrosinase is missing. Therefore albinos have no melanin in their skin, hair, or irises.
So now we can begin to understand both tans and sunburns. When you get a tan, what is actually happening is that the melanocytes produce melanin pigment in reaction to the ultraviolet light present in natural sunlight. Ultraviolet light stimulates melanin production. Melanin pigments absorb the UV radiation in sunlight, effectively protecting the cells from radiation damage. Melanin production takes a fair amount of time, though, which is why most people cannot get a tan in one day. You have to expose yourself to UV for a short period of time to activate the melanocytes; then, they produce melanin over the course of a number of hours. By repeating this process over five to seven days, the pigment builds up in your cells to a protective level.
Melanocyte-stimulating hormone (or @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 tied into the optic nerve, which means that it can sense light. The more light your eyes see, the greater the activity of your pituitary gland. 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!
If you are @Caucasian and you don't have a tan, then the cells in your skin are not primed with melanocytes, and they are therefore not protected from the sun's ultraviolet radiation. You become 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. These responses are actually cellular damage caused from UV exposure. The body responds to the damage with increased bloodflow to the dermis' capillary bed 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.
You can block or absorb UV with opaque creams like the white zinc oxide cream that you see lifeguards putting on their noses. These are commonly known as sunscreens. The first and most common of the absorption chemicals is @PABA (para-aminobenzoic acid). It absorbs UV-B, one form of ultraviolet light.
All sunscreens are labeled with an @SPF, or Sun Protection Factor. This applies to cream's UV-B blocking ability. The SPF acts like a multiplying factor. If you would normally be OK in the sun for 10 minutes and you apply an SPF 10 sunscreen, you will be OK in the sun for about 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.
Regardless of the SPF sunscreen you use, however, and regardless of the degree of your tan, moderation is the key. Even with these protective factors, sunlight can still cause a variety of problems from skin drying to skin cancer. Use your judgment with how long you stay out in the sun before taking a break.
