Ultraviolet light comes from the sun at two main wavelenghts: UVA (ultraviolet-A) and UVB (ultraviolet-B). UVB is a "good" form, for helping a skin produce vitamin D. The UVA is considered a "bad" form, penetrating deeper into the skin and causing greater production of free radicals and cellular damage. The problem is that most sunscreens protect only against UVB rays, that is, those that produce the beneficial and protective vitamin D.
Important to clarify that the ultraviolet rays A and B are essential for health, but it is their excess that can cause damage. It is for this reason that I am not an unconditional fan of the use of sunscreens everyday.
First of all, it blocks the natural production of vitamin D through the skin. The second reason is that the person who passes a suncreen belevies to be protected and ends up staying in the sun longer than it should. And that is why, since the onset of suncreens in the 1980s, there has been NO decrease in the incidence (actually increased) of skin cancer. People continue to die of the most aggressive cancers as much as they died in 50 years ago. The truh is that suncreens prevent the most common types of cancer: basal cell carcinoma and squamous cell carcinoma. Although not harmless, they are rarely fatal, but for melanoma, by far the most serious type of skin cancer, suncreens do not offer any protection.