Why sunburn is dangers?

Answer 1

When the suns rays strike the skin, they penetrate through the uppermost layer of the skin, which sonsists largely of dead, old skin cells, to the dermis, the living, growing part of the skin that creates new skin cells.

These young, newly-formed cells are particularly sensitive to sunlight, which, when over-exposed, can dehydrate them, damage the cell walls, leading to disease and even cause severe damage to the DNA of the cell itself, which is one of the recognised starting points of skin cancer. As the heat persists in the skin, it causes further damage, literally "cooking" these new skin cells as they emerge.