system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do sunburns generally take a few hours to 'set in'?

Pretty simple question. But to give an example, today I spent about an hour and a half or so at the pool. I intentionally sat out to get a tan. Unfortunately, though, you never know how long is long enough, until a couple hours later when you start getting red and sore.

What causes this delay? What is happening in those few hours that changes everything? 


btw, I searched the sub for this question, didn't see anything pertaining to the sunburn delay. Most other sunburn questions were about skin cancer and the warmth your skin retains when sunburned.
gpt
UV radiation is absorbed by the outermost layers of your skin and does all the damage there. This does not create immediate damage around nerve endings so you don't feel pain. However the cells that die in the outer layers leave a lot of debris and signaling molecules that summon a cleaning crew (immune system) to pick up the debris and start the healing process. This is causes inflammation in the area which stimulates nerves to generate a pain response. So you experience a delay until the inflammation starts.