Epidermis Structure and Function
Your skin is made up of three layers: the epidermis, the dermis and the subcutaneous tissue. The epidermis is the thinnest and most superficial layer. It contains cells called keratinocytes which continuously divide to replace skin lost through daily wear and tear. The deepest portion of the epidermis also contains cells called melanocytes which produce a dark pigment called melanin. The dermis and subcutaneous tissue provide the blood supply, most of the nerves, strength and support and storage space for fat.
Tanning
Melanin is produced in order to protect your body from the harmful effects of UV radiation. When the DNA in your cells is hit with too much UV radiation it can mutate and either cause the cell to stop functioning or to become cancerous. Melanocytes produce the pigment in response to UV light, store it in capsules called melanosomes and send the melanosomes off to be stored inside keratinocytes. Your skin appears "tan" when more melanosomes are stored inside more keratinocytes. Rather than retrieving and disposing of melanosomes when encountering less light, your skin lightens when the darker keratinocytes simply die and fall off, replaced by lighter cells. The reason your body did not evolve to stay tanned all the time, providing your DNA with as much protection as possible, is because we require some amount of sunlight to produce vitamin D. In a darker environment, tanned skin would be unable to "catch" enough sunlight to produce all the vitamin D we need.
Tanning Beds
Tanning beds attempt to simulate the sun's rays using special light bulbs which emit UV radiation. Since these bulbs can become very warm, most beds also have a fan to keep the tanner cool. There are two main types of UV light: UVA and UVB. Tanning beds predominantly produce UVA light which is safer for your DNA than UVB light. However, according to the World Health Organization, "sunbeds have been manufactured that produce higher levels of UVB to mimic the solar spectrum and speed the tanning process." UV radiation only penetrates a few millimeters into the body and hence does not cause damage to organs other than the skin, but the use of UVB light may lead to a higher risk of sunburn or skin cancer. It has proven difficult to determine a "safe" level of UV radiation since different people are affected by varying amounts. It is possible that using a tanning booth to establish a "base tan" before a large amount of sunlight exposure is safer than the sunlight exposure alone, but the US FDA recommends avoiding all forms of UV light as much as possible to limit your risk.
References
- "Harrison's Principles of Internal Medicine, 17e;" Anthony S. Fauci, Eugene Braunwald, Dennis L. Kasper, Stephen L. Hauser, Dan L. Longo, J. Larry Jameson, and Joseph Loscalzo, Eds.; 2008
- Health Physics Society
- WHO: Artificial tanning sunbeds: risk and guidance
