What does the sun actually do to your skin? Part 1.

This is part 1 of thatscienceguy’s articles about the effects that the sun has on your skin. This week we look at skin itself, the damage the sun does to the structure of your skin and the health benefits of sun exposure.

In Australia we’re encountering another typically sunny summer, and it’s extremely important to properly protect ourselves from the harmful effects of the sun. This makes it the perfect time to examine just what the sun does to your skin and why it’s important to protect ourselves.

Before we can talk about the effects that the sun has on skin, let’s first look at skin itself. Skin is the largest organ of the human body and its main job is to provide protection for the bones, tissues and internal organs and prevent too much water being lost from the body. It is made up of two main areas, the surface epidermis and the deeper dermis layer.

Cross section of human skin (Adapted from Stulberg et al)

The epidermis is the topmost layer, and is made up mainly of cells called keratinocytes. Keratinocytes begin in the deepest layer of the epidermis and are pushed upwards by the production of more cells beneath them. By the time they reach the surface they have died, and it is these layers of dead cells which provide protection and waterproofing to the layers underneath.

The dermis makes up most of the skin and acts as a base for the epidermis. As it acts as a base, the dermis is rich in a substance called collagen, which provides strength and resistance to damage. As well as collagen, the dermis is also rich in elastin fibres, which as the name suggests allows skin to stretch and recoil. Hair follicles and sweat glands are also found in the dermis.

Ultraviolet (UV) light from the sun has many effects on the skin. Australia receives some of the highest levels of UV of anywhere in the world – the UV levels during winter in Australia are equal to the UV levels during summer in The Netherlands. The map below shows the UV levels in Australia during a typical summer day, with every populated location on mainland Australia in the “extreme” level of UV radiation.

Map of summertime UV levels in Australia. Purple areas represent extreme UV levels. (Image courtesy of the Australian Bureau of Meteorology)

In between the epidermis and dermis are cells called melanocytes which, when exposed to ultraviolet radiation from the sun, produce a dark substance called melanin. Melanin is absorbed by the keratinocytes, which then use it to protect themselves from UV as a kind of sunshade. This melanin response to UV has evolved as a natural sunscreen, providing protection to the keratinocyte cells by absorbing incoming ultraviolet radiation and produces the visible tanning effect. People who tan darkly from sun exposure have a lesser risk of skin cancer because of the protection given by melanin.

In addition to the tanning effect, UV light also attacks the elastin and collagen of the skin, damaging collagen and causing tangling of elastin. This damage results in the loss of skin’s structure, which is what causes sagging, wrinkling and increased roughness of sun-damaged skin. UV also increases the production of free oxygen radicals, small molecules which attack and damage the skin cells. These effects are permanent and lead to noticeable premature aging.

Vitamin D

As well as protecting your body, skin also has a vital role in the production of a chemical in your body called vitamin D. Up to 90% of vitamin D is produced by the skin by a mechanism which actually requires UV radiation. Vitamin D is beneficial to health, and in fact the World Health Organisation recommends UV exposure to reduce diseases such as bone defects including osteoporosis and rickets, diabetes, some cancers and heart disease.

Because the production of vitamin D requires UV, theoretically there is the chance that using sunscreens or avoiding sun exposure may lead to vitamin D deficiency. However, this very rarely happens as the amount of  sun exposure needed to make enough Vitamin D is extremely small. Only a couple of minutes of exposure to the sun are enough to produce sufficient vitamin D, so just normal day-to-day exposure when people don’t normally wear sunscreen is satisfactory. Also, when applying sunscreen it is normal that people will accidentally miss a small area, or cover some areas lighter than others. The exposure to UV from these areas is also often sufficient for the production of vitamin D. While there may be rare cases of people not getting enough vitamin D, it is incredibly unlikely to happen due to using sunscreen or by being sunsmart and people should not be concerned, nor should they seek out sun specifically for the reasons of vitamin D production.

The ozone layer

Just a final note about the importance of the ozone layer. UV light is separated into 3 types, UVA, UVB and UVC, although effectively all UVC and most UVB is absorbed by the ozone layer. This means that around 90-99% of UV radiation reaching the surface of the earth is UVA, with the remaining 1-10% being UVB. Although the ozone layer does filter out a lot of UV radiation, it has been calculated that a 1% decrease in ozone actually results in a 2% increase of UV dose, so the integrity of the ozone layer is vital for human health.

The hole in the ozone layer over Antarctica. Purple and blue areas are regions of very low levels of ozone (Image courtesy of NOAA)

Over the last decade the breakdown of the ozone layer has been slowing due to the ban of CFC-containing aerosols. CFC’s contain chlorine which is very reactive in breaking down ozone, and human activities are responsible for around 75-85% of the chlorine in the atmosphere. Estimates suggest however that chlorine levels in the atmosphere may be peaking at the moment, and as levels begin to drop the ozone hole may begin to close. However, this is a slow process and it will be at least a decade before any measurable changes to the size of the ozone layer can be seen.

Part 2 will be published next week looking at skin cancer and the importance of sunscreens.

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