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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):

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Both UVA and UVB affect the skin, with UVA penetrating further, as per the diagram below (from the link above):

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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?

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I was able to trace some information back to The Ultraviolet Disinfection Handbook, by James R. Bolton and Christine A. Cotton. The relevant passages can be found here on Google Books.

A timeline of ultraviolet disinfection begins on page two of the introduction. It notes Ritter's 1801 discovery of UV light, and then several other early experiments. However, the first study of the effects of UV light on organisms appears to be a test conducted in 1877 by Downes and Blunt:

1877: Downes and Blunt (1877) observed that test tubes filled with a broth containing bacteria, when exposed to sunlight, eventually becomes sterile.

The next year, the pair determined that light towards the blue side of the spectrum had the greatest effect on bacteria.

Unfortunately, the book does not appear to properly address the dangers of UV light to human skin. The closest it gets is citing an experiment done around 1900 by Niels Finsen:

~1900: the Danish physician Neils Finsen, considered the founder of modern phototherapy, discovered a UV treatment for lupus vulgaris, a form of skin tuberculosis.

However, other sources seem to indicate that Finsen did research on he effects of UV light on skin. He eventually won a Nobel Prize. From a page in the Nobel prize website,

In beautiful but simple experiments Finsen demonstrated that the most refractive rays from the sun («the chemical rays») or from an electric arc may have a stimulating effect on the tissues. If the irradiation is too strong, however, it may give rise to tissue damage, but this may to some extent be prevented by pigmentation of the skin as in the negro or in those much exposed to the sun.

I haven't been able to confirm that this was the first experiment addressing UV light and human skin, but it appears that it was, at the least, one of the first.

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