Skin Cancers

Causes and Risk Factors

The major cause of skin cancer is exposure to UV radiation, such as that from natural sunlight: it is estimated that more than 66% of melanomas, and up to 90% of non-melanoma cancers, are caused by heavy sun exposure.

UV radiation penetrates the skin and damages the DNA of the cells, either directly or by generating reactive chemicals within the skin that then go on to cause DNA damage. Some of this damage is repaired by the natural processes in the skin, but intense or repeated exposure can overwhelm the skin’s ability to repair itself and the damage builds up over time, increasing the likelihood of cancerous tumours forming later in life.

The incidence of skin cancers has increased significantly over the past few decades, and is continuing to do so. This is likely to be due to a number of factors:

People are generally living longer than they used to, allowing much greater cumulative exposure to the sun over a lifetime, and more time for cancer to develop. Many serious diseases have been defeated by modern medicine: previously the poor in particular, who were most likely to spend a lot of their working life outside, died of other causes before they could develop skin cancer.
At least among the wealthier northern countries, people generally have more time and money to spend on outdoor leisure activities and holidays in sunnier locations, leading to greater, and often more intense, sun exposure.
A tan is now widely considered to be healthy, attractive and fashionable, leading to deliberate sunbathing and relatively infrequent but intense sun exposure.

Worldwide, the lifetime risk for an average person for skin cancer of any type is 1:5, while the lifetime risk for melanoma is 1:74. That is, on average, 1 in 5 people will develop some sort of skin cancer at some point in their life, and 1 in 74 people will develop melanoma. However, individuals do not all have the same risk: some have a much higher risk than others.

Inherited Factors

One of the most important natural risk factors for skin cancer development is skin type. Those with a naturally dark skin colour have a 20 times lower risk of melanoma than the average Caucasian. At higher risk still are those with:

Very pale skin – the melanoma risk for very pale skin is about 3 times that for the darkest white skin,
Red or blonde hair – the melanoma risk for red hair is about 4 times that for dark brown or black hair, for blonde hair it is about 2 times,
Blue, green or hazel eyes,
Freckles,
A large number of moles, especially abnormal ones – those with 100 or more normal moles (over their whole body) have about a 7 times higher melanoma risk than those with 15 or fewer moles,
Poor tanning ability, and who burn easily – those who tan poorly have about 3 times higher melanoma risk than those who tan easily,
Albinism – the lack of natural skin pigment means little natural protection to UV,
A family history of skin cancer – the risk of melanoma for those with a family history of it is about twice that of other families.

Where people live also has a major influence on the level of risk associated with their skin type: those with light skin, and especially any of the additional factors listed above, who live in a tropical or near-tropical climate (i.e. one with much stronger sun intensity than that for which their skin colour evolved) are at much higher risk than those who live in their natural, temperate climate. The risk is even higher if they migrate as a child.

This is probably why the skin cancer rates in Australia are so high: the Australian population has a high proportion of immigrant pale-skinned people, and an intensely sunny climate, especially towards the north of the country.

Behavioural Factors

Voluntary or careless exposure of a person’s skin to the sun (or other artificial sources of UV radiation) in conjunction with their skin type is a major factor in increasing their risk of developing skin cancers.

Outdoor workers, such as farm workers, builders, gardeners, etc., are at higher risk due to their prolonged exposure to the sun. The increase in risk, however, is mainly for non-melanoma cancers.

People who intermittently expose themselves to intense sun for relatively short periods, such as during holiday sunbathing, significantly increase their risk of developing melanomas.

People who suffer sunburn are at even higher melanoma risk. The risk is strongly related to the number of sunburn episodes: from 1.4 times for 1–2 sunburns, to 2.4 times for 10 or more sunburns. When the sunburns occur is also important: risk doubles if sunburn occurs in childhood.

Regular use of sunbeds and sunlamps also increases a person’s cancer risk, particularly for melanomas. Sunbed use by under 35s increases their melanoma risk by 75%.

Medication

Some types of medication, prescription or otherwise, can increase a person’s sensitivity to UV radiation, and therefore increase their risk of developing skin cancer if they expose themselves to UV while taking it. These include:

Some antibiotics,
Some antidepressants, including St. John’s Wort,
Some diuretics,
Some retinoids (acne treatments).

If you are taking any form of medication, including herbal remedies, check the instructions that come with it (or your doctor, pharmacist or supplier) for photosensitivity side effects, and if necessary take additional precautions when going out into the sun.

Diseases

A number of diseases and other conditions can also increase a person’s sensitivity to UV radiation, and therefore increase their risk of skin cancer. These include:

Lupus – inflamed tissues are extremely sensitive to UV,
Porphyrias – production of abnormally large amounts of porphyrins that cause skin damage and scarring when stimulated by UV,
Vitiligo – causes patches of white, de-pigmented skin that lack melanin and are therefore extremely sensitive to UV,
Xeroderma pigmentosum – lack of natural ability to repair UV DNA damage causes hundreds of times greater risk of developing skin cancers.

Your doctor should inform you of the increased UV sensitivity associated with these conditions, and advise you what additional precautions you need to take: follow that advice.

Other Causes

There are also other causes of skin cancers besides UV exposure, particularly for melanomas. These include:

Badly burned skin
Long-term skin inflammation
Radiotherapy treatment
Use of immunosuppressant drugs
Over-exposure to certain chemicals
Genetic (i.e. inherited) conditions

However, this site is concerned with the effects of the sun, so these other causes will not be discussed further.

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