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Sun exposure

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Although the sun exposure is an unequivocal risk factor for melanoma, the relation between sun exposure and melanoma is complex (1):

the connection between the sun exposure and MM is further supported by the higher melanoma incidence in
- populations that live in lower latitudes (2) - the risk of malignant melanoma in Australia is three to four times higher than the risk in Britain (1)
- white populations and lower incidences among dark-skinned populations (2)
short, intermittent sun exposure assessed indirectly by history of burning after unaccustomed sun exposure is a particular risk factor for the development of MM (3) (in contrast, cumulative sun exposure is thought to be important in non-melanomatous skin cancers (2))
- the risk is doubled in people who have had significant skin burning after sun exposure when compared to people who have never had a sunburn (2)
  - people who can remember significantly burning their skin on three occasions have a significantly increased risk of malignant melanoma
  - age at sunburn appears to be an important factor with the highest risk seen in sunburns occurring during childhood (2)
  - growth of melanocytic naevi (which it self is a risk factor for MM) is thought to be associated with childhood sun exposure (4)
the role of UVR as a risk factor is further supported by studies of immigrant populations in Australia where risk of developing melanoma was proportional to the length of stay and inversely to age at arrival (2)
the UV wavelengths in sunlight which is responsible for MM is unknown (4)
- UVB has shown a direct connection with the development of MM e.g. - a 10% increase in average annual UVB irradiation is associated with a 19% increased risk of melanoma
- the role of UVC is thought to be negligible (atmospheric ozone layer prevents UVC from reaching the surface of the earth) (3)
- over the last 10 years there has been a 7% increase in UVB radiation in Europe which may in part be responsible for the increasing incidence of malignant melanoma

The role of non solar sources of UVR (fluorescent light, sunlamps, sunbeds and tanning beds) in MM is controversial. A meta-analysis of 12 case-control studies and 1 cohort study have shown that

there might be a slightly higher risk of developing melanoma in patients using sunbed and sunlamp
longer duration of exposure and earlier age at exposure was associated with a higher risk (2)
current recommendations are that sunbeds should be avoided
- before the age of 20 years
- in people with red hair and freckles or multiple atypical naevi with or without a family history of melanoma (1)

Theories as to how such exposure precipitates melanoma include:

the production of a circulating chemical factor
the production of pre-malignant clones of abnormal melanocytes
an altered immune response in a susceptible person

Reference:

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