I believe humans have evolved to accept certain damage as natural and it just repairs itself.
When I expose myself to moderate amounts of light, my body reacts making my skin darker, which protects my skin against light.
Think for example gravity, that is exerting acceleration that damages our bodies, so our body has to react making bones and muscles. If we go to space and do not apply damage, the human becomes weaker, as it atrophies.
If we make a person to breathe in a total clean environment(filtered air), the immune system atrophies, then if you make the person breath normal air, you can kill her.
Of course we also know that too much is very bad, because it exceeds the limits of the human body to repair and something breaks down.
Common sense is key. If you go to the mountain or the sea in summer in Spain, Italy, Greece, Morroco, New Zealand... you should be extremely careful. This exposure in thousands of times more than indoors(or countries like UK) and UV light skyrockets.