How Tanning & Sunlight Work

How Tanning Works

When you get a tan, what is actually happening is that your 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 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. 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.

In a variety of 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 cannot proceed because an enzyme called Tyrosinase is missing.
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. The pituitary gland is tied into the optic nerve, which means that it can sense light resulting in the production of MSH. In fact a side effect of all of this is that wearing sunglasses may make you more susceptible to sunburn!

How Sunlight Works

Sunlight arrives on earth in three forms: infrared (heat), visible light and ultraviolet. Ultraviolet light is classified into three categories:UVA (315 to 400 nm), also known as black light, which causes tanningUVB (280 to 315 nm), which causes damage in the form of sunburn UVC (100 to 280 nm), which is filtered out by the atmosphere and never reaches us.

99% of the sun's UV radiation at sea level is UVA. It is the UVB that causes most of the problems related to sun exposure: things like aging, wrinkles, cancer and so on, although research is increasingly implicating UVA as well. One of the interesting things about UV radiation is that it is reflected by different surfaces.

These reflections can amplify the effects of UV exposure. For example, snow reflects 90% of UV light. That is why you can get snow blindness and severe sunburns from skiing on a sunny day. Sand can reflect up to 20% of UVB that hits it, meaning that you can get extra UV exposure at the beach.

On the other hand, certain things absorb almost all UV radiation partially or completely. Glass is one of these substances, which is why you cannot get sunburn in a greenhouse. Most sunscreens use chemicals that have the same UV-absorbing properties.