Can UV radiation be safe for the skin?

Question

It is well known that UV radiation can damage the DNA and generally harm our skin. We also know that UV radiation helps on the production of melanin and Vitamin D.

From what I could find, the DNA absorption spectrum goes to almost zero for wavelengths higher than 300 nm. This seems to suggest that we would be safe to use UV radiation between 300 and 340 nm in our skin (as long as the power or exposure is not too high/long to make burns), for therapeutic purposes such as the stimulation of Vitamin D production.

Is this assumption correct? Are there any evidences that we could use this UV wavelength range safely?

Answer 1

You're talking about long-wave UV, or UV-A radiation. In the 80s, experts claimed that this was a safe wavelength. Protection against UV-A was not part of sunscreen in the early days. Consequently, UV-A was (and still is) used in tanning beds due to its perceived safety over UV-B. However, a lot of research has been done since.

UV-A is well understood now to also be unsafe in unreasonable amounts. Currently, UV-A protection is a typical feature of sunscreen and tanning beds are still not a healthy alternative to moderate, healthy doses of sun. Here is a recent review covering some of the aspects comparing different UV range effects on skin. I really suggest you put a search engine to good use here; it makes little sense for us to expound on the literature when it is so clear and easily available.

In summary,

• UVA certainly contributes to the development of skin cancer.
• UVA penetrates deeper into the skin than UV-B (which is largely responsible for 'burning' of the topmost layer of skin, without directly affecting the deeper layers). For this reason, UV-B is associated primarily with burning and UV-A is primarily associated with aging and aging diseases like cancer.
• It is important to note that 95% of UV light in every day life is UV-A, because it does not vary seasonally and can penetrate clouds and windows. Therefore, in spite of the fact that short wavelengths carry more energy per photon, the ratios of UV-A and UV-B exposure are far from equal.

These are only a few of the explanations as to why we observe an incidence of aging and skin damage and disease upon UV-A exposure.

Answer 2

There are (at least) two sides to this story. One is direct DNA damage being caused by UV-B light which happens to have photons with just the right amount of energy to interact with thymine. This has been known, and assumed not just during the 80s, but until the late 2000s to be The One major thing that causes cancer, or trouble in general (there's papers from ~2008 claiming that).

Meanwhile, it is known that UV-A can excite an electron in a chromophore (i.e. some more or less random unspecified molecule, or part of it) in a way which turns out being less desirable than one would wish.

Photons exciting electrons isn't something particularly new or stunning (think photosynthesis or our ability to see), it happens all the time, almost every time a photon hits something. But some molecules react more favorably, and some... don't.
For example, melanin happens to be our "sun protector" molecule because it has the property that it almost instantly drops its excitement into heat, which is quite harmless.

Other "random" molecules do not necessarily have such favorable properties and may either form radicals or stay excited for a long time until they finally bump into something and interact with it in some way, giving off that energy in some unspecified, haphazard manner. When that happens, well, things happen, whatever they may be. The damage, finally, doesn't even have to be at the exact location where the sun was shining.