How Your Skin Tans and Why It’s Natural
Tanning is the human body’s natural and intended response to ultraviolet light exposure. Throughout human evolution a tan has served as the body’s natural acquired protection against sunburn and overexposure. Today we know that a suntan achieved in a non-burning fashion, combined with proper use of sunscreen outdoors when sunburn is a possibility, is the best way to maximize the potential benefits of regular sun exposure while minimizing the risks that are associated with overexposure.

This section will explain how your skin develops a tan by first introducing ultraviolet light, introducing parts of the skin and then showing how UV light works with the skin to develop a tan.

Part 1: Understanding UV Light
Natural light actually is composed of energy waves that are transmitted 93 million miles from the sun to the Earth. Each energy wave (or light ray) occurs in a different part of a complex light spectrum based on its length in nanometers (nm), which is one-billionth of a meter.

• Light is energy.

• Light travels in waves.

• Different forms of light are differentiated by the length of the waves – the wavelength.

That means that no two types of light are the same. For example, ultraviolet light used in tanning salons cannot possibly be the same as an X-ray because of the difference in the length of their energy waves. Therefore, these two waves will behave and affect the human body in completely different ways.

For our purposes, let’s divide light into three categories: infrared, visible and ultraviolet. Not all light waves reach Earth, however. Many are filtered out by the atmosphere, which protects us from harmful rays. The light waves that tan people are invisible, but let’s briefly look at all three categories for comparison.

Infrared Light. Infrared waves (above 700nm) include electric waves, radio waves, infrared and shortwave infrared, but only this last type reaches the Earth. Shortwave infrared waves, which give us heat, make up about 49 percent of the solar radiation we receive on Earth.

Visible Light. Visible rays (400nm to 700nm) cause illumination we can see as colors, including red, yellow, green, blue and violet. These account for about 46 percent of the Earth’s solar radiation.

Ultraviolet Light. Ultraviolet rays (200 nm to 400 nm) comprise the remaining 4 or 5 percent radiation we receive on Earth. Of all invisible rays—including cosmic rays, gamma rays, X-rays and three forms of ultraviolet light—only two of the ultraviolet light rays actually penetrate the Earth’s atmosphere. These are the same two invisible light rays used in tanning equipment.

Ultraviolet light rays are energy waves that are shorter in length than visible light rays. Because of this, the human eye cannot see ultraviolet light. However, we can see the effects of ultraviolet light, such as tan skin, on the human body. Three categories of ultraviolet light exist:

• Ultraviolet A. UVA rays are the longest (320nm to 400nm).

• Ultraviolet B. UVB rays are shorter than UVA rays (290nm to 320nm).

• Ultraviolet C. UVC rays are the shortest (200nm to 290nm).

Of these three categories of UV rays, only UVA and UVB pass through the Earth’s atmospheric filter. More UVA hits Earth than UVB because the filter prevents the passage of shorter wavelengths of UVB that resemble UVC. If UVC light also passed through, it would have devastating effects on this planet, which is why the depletion of the ozone layer in the atmosphere is of great concern.

UVA and UVB light waves cause skin to tan. As previously mentioned, these same light rays can be replicated in special lamps used in tanning equipment. How they work together to create a suntan is a process we’ll discuss after we introduce the parts of the skin.

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