Most skin cancer is caused by ultraviolet (UV) light damaging the DNA in skin cells. The main source of UV light is sunlight.
Sunlight contains three types of UV light:
- ultraviolet A (UVA)
- ultraviolet B (UVB)
- ultraviolet C (UVC)
UVC is most dangerous to the skin but is filtered out by the Earth's atmosphere. UVA and UVB damage pale skin over time, making it more likely for skin cancers to develop. UVB is thought to be the main cause of skin cancer overall, but it isn't yet known whether UVA also plays a role in causing melanoma.
Artificial sources of light, such as sunlamps and tanning beds, also increase your risk of developing skin cancer.
Repeated sunburn, either by the sun or artificial sources of light, increases the risk of melanoma in people of all ages.
You're at an increased risk of melanoma if you have lots of moles on your body, particularly if they're large (over 5mm) or unusually shaped.
For this reason, it's important to monitor your moles for changes and avoid exposing them to intense sun.
Read more about checking your skin.
Other risk factors
You're also more likely to develop melanoma skin cancer if you have:
- a close relative who's had melanoma skin cancer
- pale skin that doesn't tan easily
- red or blonde hair
- blue eyes
- a large number of freckles
- previously damaged your skin through sunburn or radiotherapy treatment
- a condition that suppresses your immune system, such as HIV, or you take medicines that suppress your immune system (immunosuppressants)
- a previous diagnosis of skin cancer
The risk of developing skin cancer also increases with age.
Cancer Research UK has more information about melanoma risks and causes.