system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Does darkness of tan determine sun damage?

For example, take two people with the exact same skin complexion. Person 1 wears no sunscreen but stays out in the sun for a shorter period of time than person 2, who wears sunscreen but stays out in the sun for a longer period. Let's assume person 2 applies sunscreen effectively and reapplies the recommended amount of times. If person 1 and 2 obtain the same level of "tanness," have they both equally increased their risk of skin cancer?
gpt
No. There are two main types of UV radiation you get from the sun, UV-A and UV-B, the latter being responsible for sun burns. When you apply sunscreen you are blocking a high percentage of UV-B rays and preventing the damage on the middle and upper layers of skins. UV-A on the other hand penetrates the skin much deeper and isn’t blocked by traditional sunscreen. You still do damage, which presents as tanning, but because this happens at lower levels in your skin it often takes longer for someone to see a tan. Think of it as the color has to work it’s way up as you shed upper layers. 

So the person who wears no sunscreen has absorbed UV-A AND UV-B rays while the sun screen wearer of similar tanness has blocked half of the damaging waves from the sun.