How does sunburn actually damage the skin?

The answer

Sunburn is caused by ultraviolet B (UVB) radiation directly damaging DNA in skin cells, creating thymine dimers (abnormal bonds between adjacent DNA bases). When this damage exceeds the cell's repair capacity, the damaged cells trigger apoptosis (programmed cell death) to prevent mutations that could lead to cancer. The redness, swelling, and pain of sunburn are inflammatory responses driven by prostaglandins and cytokines released during this mass cell death, along with increased blood flow to the area as the immune system works to clear dead cells. The damage can extend beyond the superficial skin: UVA radiation penetrates deeper, generating reactive oxygen species that damage collagen and elastin, causing photoaging. A single severe sunburn in childhood doubles the risk of melanoma later in life, because some DNA damage escapes repair and accumulates in surviving cells over decades.