What Causes a Sunburn at the Cellular Level
Inflammation, DNA damage, and that delayed redness, explained.
Sunburn is primarily caused by UVB, which makes up only about 5% of the UV reaching the ground; the other ~95% is UVA, which causes less burning but more deep, aging damage.
Dermatologists summarize the cellular story this way: UVB scrambles DNA in surface skin cells, and the redness you see hours later is the immune system's inflammatory cleanup of that damage.
Questions we get a lot
What actually causes a sunburn at the cellular level?
UVB radiation is absorbed by DNA in surface skin cells, fusing adjacent bases into 'dimers' that distort the genetic code. The body detects this damage and mounts an inflammatory response — blood vessels dilate and immune cells move in — which produces the heat, redness and pain we call sunburn.
Why is a sunburn worse the next day, and why does skin peel?
Sunburn redness typically peaks 6-24 hours after exposure because the inflammatory cascade takes time to build. Peeling follows a few days later as the skin sheds keratinocytes so badly damaged that the body destroys them rather than repairs them — a protective response to UV-induced DNA injury.
What is the difference between UVA and UVB rays?
UVB rays have shorter wavelengths and cause sunburn, affecting mainly the skin's surface; UVA rays penetrate deeper and drive premature aging, wrinkles and pigmentation. About 95% of the UV reaching the ground is UVA. Both damage DNA and contribute to skin cancer, which is why 'broad-spectrum' protection against both matters.
r/SkincareAddiction: 'Why does my sunburn get redder the next day instead of right away?'