What does the sun actually do to your skin? Part 2

In part 2 of this series about the effect of UV on skin, we look at skin cancer and the most effective ways to protect yourself from the damaging effects of the sun.

 

Skin cancer

Skin cancer is the most common cancer in the western world, in Australia it affects four times as many people as every other type of cancer combined. In fact Australia has the highest rate of skin cancers of anywhere in the world and it has been estimated that between 60-70% of the population will have skin cancer at some point in their life. The major cause of skin cancer is UV radiation, and despite the success of campaigns such as “slip slop slap”, this rate is increasing.

There are 3 main types of skin cancer, basal cell carcinoma (BCC), squamous cell carcinoma (SCC) and malignant melanoma.

 

From left: Basal cell carcinoma, Squamous cell carcinoma, Malignant melanoma. Images courtesy of the Australasian College of Dermatologists


The first two types, BCC and SCC are tumours which are originally keratinocytes which have become sun damaged and now rapidly grow and divide, with BCC forming from keratinocytes deep in the epidermis, while SCC form from keratinocytes closer to the surface. Malignant melanoma however, is originally a sun damaged melanocyte. The BCC and SCC types make up most skin cancers, together around 96%, with melanoma making up most of the remaining 4%. Despite only being a small fraction of skin cancers, melanoma is responsible for around three-quarters of skin cancer deaths in Australia, a result of its fast growth and ability to spread rapidly through the body.

Keratinocytes and melanocytes both become cancerous from damage caused by UV radiation. In fact, the amount of UV radiation needed to cause damage leading to skin cancer is far less than that needed to cause a sunburn. UV penetrates the cell and damages DNA, which is effectively the instructions for how the cell works. This damage can sometimes be repaired by the cell, however if these repair mechanisms don’t succeed, the damage may be permanent. There are several genes (sections of DNA) in particular which when damaged have a high probability of causing cancerous growth. One gene, called Braf is found to be damaged in between 50% and 80% of melanoma cells, and causes increased growth rates of cancerous cells. Other genes which normally hinder cell growth are also often found to be damaged in skin cancer cells, as are genes which normally cause cancer cells to die. Genes which are involved in DNA repair are also sometimes found to be damaged in cancer cells, as damage in these areas means the cell may not be able to repair other DNA damage properly. These changes to the DNA caused by UV radiation result in cells which have fast, unlimited growth which then form tumours.

 

DNA damage by UV radiation

 

UV also causes changes to the cell processes which regulate its growth. When exposed to UV radiation, skin cells can increase the production of chemicals which increase the speed of cellular growth, not only of the cell producing the chemical, but also of cells around it. UV also causes the increased production of chemicals which cause inflammation, and these can also cause damage to the cell and also act to increase its growth.

While melanin does provide some protection to the skin cells from the effects of UV, even if you’re someone who tans darkly it is still vitally important to properly protect your skin. A dark tan on white skin only provides a protection of around SPF 4, compared to most sunscreens which provide protection of around SPF 30. Also, remembering the structure of skin, for UV radiation to reach and activate the melanocytes means it has to travel through all the layers of epidermis, potentially damaging the keratinocyte cells on the way. The tan is effectively a way of the body trying to protect itself against more damage. So while having a tan will partially protect you, your skin cells have potentially been damaged to get the tan in the first place. This is why many sun experts use the saying “There’s no such thing as a safe tan”, because you may have received cancer-causing damage to get that tan.

Ultraviolet light also causes a reduction in the effectiveness of your immune system. Normally, the immune system has a role in preventing cancer by identifying and then removing cancerous cells from the body. However, when suppressed by UV radiation, the immune system isn’t as effective in removing cancer cells, allowing them to remain and form tumours.

 

Sunscreens

It is important when using sunscreens not to think of them as blocking UV. They are in fact only a filter which reduces the amount of UV which reaches your skin by forming a barrier to UV radiation. The strength of this filtering out of UV is shown by their SPF rating. The SPF number is a ratio of the time or dose of UV required for sunburn of protected skin to the time or dose of UV required for sunburn of unprotected skin. For example a sunscreen with an SPF of 20 means that skin protected by the sunscreen will take 20 times longer to burn compared to skin without sunscreen. Another way of thinking about it is that it is an indication of how much UV is allowed to penetrate your skin. For example, in Australia, the highest SPF rating allowed to be shown on sunscreens is 30+, which means it limits the UV reaching your skin to one-thirtieth of the levels than if you hadn’t been wearing the sunscreen.

 

An SPF30+ broadspectrum sunscreen. Image courtesy of the Cancer Council Australia

 

Internationally, there are products sold which claim to be tanning sunscreens. The way these products work is by only blocking some UV radiation, allowing the remaining UV radiation to pass through. UV radiation which reaches the earth’s surface is classified as two types, UVA and UVB. These tanning products work by filtering out UVB but allowing UVA to pass through the skin to activate the melanocytes. The problems with these products are many. Firstly, they sometimes only have a very low SPF factor (I’ve seen one with an SPF of 2), meaning they give poor sun protection to begin with. Secondly, UVA radiation is the type which causes nearly all of the damage to the skin structure, meaning you have no protection against the premature aging effects of UV. UVA is also responsible for the production of chemicals in the skin called “free oxygen radicals”. These chemicals can cause significant damage to cells, damage which can lead to the onset of skin cancer. UVA also causes the reduced immune system activity that helps skin cancer cells stay alive. Finally, UVA is a strong promoter of cancer growth, meaning that if there are any damaged cells in your skin it will make them grow very quickly, and this effect is particularly seen with melanoma. So while you may think you’re using a sunscreen and will be protected against the bad effects of UV, in practice there is effectively no protection given by tanning sunscreens.

When using a sunscreen it is absolutely vital to use broad-spectrum sunscreens, which filter both UVA and UVB at an equal level, giving the maximum amount of protection against all of the damaging effects of UV.

 

Next time you’re in the sun, just remember what the effect it’s having on your skin. The most effective way of avoiding skin cancer is still by being sunsmart – that is, slip on a shirt, slop on broad spectrum sunscreen and slap on a hat, and avoiding the sun by seeking shade as much as possible between 11am and 3pm. By staying smart in the sun you will be able to reduce the harmful effects of sun exposure, not eliminate, but reduce as much as practically possible, while still getting the beneficial effects of the sun.

 

A great resource for skin cancer information is the Cancer Council Australia website

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