Why does skin react with erythema when it is exposed to sunlight?

Because it is a useful warning mechanism.

Is it wise then to use a UV-B sunprotection cream?

The answer is probably: NO - such a UV-B sunprotection cream is likely to increase the sun-exposure time by the SPF factor with resulting SPF-factor times more skin damage.

Therefore we learned also to include UV-A factors.

How do we measure the effect of UV-A filters? Answer, in the laboratory - and we hope / trust that there is a good corellation between UV-A filtering and reduction of UV-A skin damaging effects like cancer and wrinkles, which appear after years of exposure to sun.

And then what about UV-C and other ionic radiations ?

I do not know - do you ?

I would use white zinc oxide paste, that nothing can get through, or a white bedsheet draped over your head.

Do not trust the sun, in the time to come, as this sun has gone bozo haywire.

It will eventually kill everyone.

Lack of sun causes more problems than not. If prolonged exposure is anticipated, why not just cover up? Prolonged exposure to some SPF ingredients are about as healthy as a sunburn.

I would add to this thread a suggestion someone recently gave me: avoid sun when your shadow is shorter than you.

From everything I've been studying, human bodies require full spectrum lighting and preferably from the sun. Without it, supposedly a number of deficiencies can result. Anyone concerned about too much or too little sun exposure should take the time to research this thoroughly with their personal health and medical history in mind. The simplicity of "easy" answers can be so tempting, but sometimes a seemingly "easy" answer can be as wrong as it is easy, creating problems that can evolve into something difficult to recover from without professional intervention.

A personal example would be that when I was 19 a doctor told me to avoid the sun at all costs to prevent my pregnancy mask or melasma/chloasma from worsening. However, I was treated for months in a dark room with an ultraviolet lamp which did not help at all. Another 5 years later, I was told to avoid all added salt while I was pregnant. Big mistake. Perhaps avoiding added sodium chloride alone in large amounts isn't a bad idea, but seasalt should have been ok. As it was, an adrenal imbalance developed which exacerbated a pre-existing thyroid dysfunction and neither was diagnosed until almost 30 years later. All along, each issue continued to perpetuate the other. But, it was easier to advise me to avoid all added salt rather than encourage typical use of a seasalt complete with trace minerals and iodine which could have been very helpful.

Ignorance? Iatrogenic? Blind trust? The reason then is unimportant now, but this example is intended to encourage others to research information and issues for themselves. Don't believe everything you hear, read and see. Learn to take care of yourself. When needed, share what you've learned about yourself with medical professionals who are willing to help you, without expecting them to know everything about life and medicine as either relates specifically to you. No one deserves the weight of all that, and in my opinion I believe it's unlikely any single person - no matter how well educated - is able to help me in every regard, much less thousands of other people about whom they know relatively little.

I much prefer the age-old british solution to sun protection as employed by my father & grandfather: Find a white cotton handkerchief, tie knots in all four corners. Finally, place on head and lay on scorching beach, drink beer/letch wildly through above averagely black sunglasses for an absolute minimum of 12 hours.

Which one is you?