Tanning and burning: the science of the Sun

The end of another year is upon us and with it comes thoughts of summer and all it entails. Some of us may be lucky enough to spend some or most of the holidays abroad but even those staying in the country will (hopefully!) be able to enjoy a week or two of sunshine. Regardless of location, when the Sun is out most enjoy basking in it and building up a tan. Spend too much time outside though and, instead of a tan, you end up with a sunburn. But why do we tan and how does it work?

The light we see coming from the Sun is just a small fraction of its total radiation output. Approximately 10 percent of the radiation emitted is in the form of ultraviolet (UV) radiation, which is more energetic than visible light. In fact, UV rays have enough energy to cause chemical changes to many molecules that are crucial to living beings. Ultraviolet light is usually subdivided into three categories, with UVA being the least energetic and UVC the most. Thankfully our atmosphere, particularly the ozone layer, absorbs much of the UV spectrum (UVC light is fully absorbed) and so only about three percent of the total sunlight reaching the ground is in the form of UV rays.

Our skin is made up of two different layers. The outer layer is called the epidermis and the inner layer is called the dermis. Located at the bottom of the epidermis are cells called melanocytes, which are responsible for creating melanin, the pigment responsible for skin colour and tanning. The melanin is then transferred to other skin cells, where it sits atop the cell nucleus to protect the DNA from mutations caused by UV light.

Two different types of tanning can occur, depending on whether UVA or UVB light hits the skin. UVA radiation simply causes a chemical change in existing melanin, which is what causes the darkening of skin, but it does not lead to the production of extra melanin and so it does not offer more protection against UV radiation and sunburn. Only UVB radiation has enough energy to cause the cell damage required to stimulate the creation of new melanin. This process takes a few days to complete but the tan produced lasts longer and actually protects the skin against damage.

This adds another concern to the already long list regarding the use of tanning beds. Customers might leave the salon believing that they are better protected against the Sun because of their tan when in fact they are not.

Our skin is not fully protected against the Sun even with a tan, so the use of sunscreen is essential. Sunscreens usually contain molecules that reflect UV radiation, such as titanium oxide and zinc oxide, and other molecules that absorb ultraviolet and release lower energy radiation. The sun protection factor (SPF) is a well-known measure of the effectiveness of sunscreens and is defined as the ratio between how much UV radiation is required to cause sunburn on skin with the sunscreen on and without it.

Most people know what they should do to protect themselves against the dangers of the Sun, such as staying in the shade when the sun is at its highest and always wearing sunscreen, but choose to ignore it. The scientific consensus is that even as few as five cases of sunburn can double the risk of developing melanoma, the most serious type of skin cancer. So this year if you want to enjoy the Sun, make sure you do it safely.