It is pretty common knowledge nowadays that prolonged exposure to solar ultraviolet radiation is dangerous to human skin, potentially resulting in cancers such as melanoma and photoaging.
A bit of background from the Skin Cancer Foundation's webpage Understanding UVA and UVB, the UV radiation reaching the Earth's surface is generally considered to go from a wavelength of 290nm (UVB), and the majority is UVA, as per the diagram (from the link above):
Both UVA and UVB affect the skin, with UVA penetrating further, as per the diagram below (from the link above):
There has also been research in the dangers of tanning booths that often use UVA, for example the CDC web-article Indoor Tanning Is Not Safe and the article Indoor tanning increases melanoma risk, even in the absence of a sunburn (Barton, 2014) among many other articles.
When and how was it confirmed that ultraviolet radiation can be dangerous to human skin?