each UV photon that hits exactly in the right spot will cause permanent DNA changes that eventually lead to cancer
Pretty sure this is incorrect. It’s not the damage that causes cancer, but the failure of the body to heal/repair it. Such failures can be caused for example by you being very old, and therefore healing slower, or by getting a sunburn (= too much exposure in a short time, overwhelming repair capability).
I think the most important thing here is that things scale very much not linearly.
See also this, which argues/claims that more sun exposure (without getting sunburnt) actually leads to less cancer than getting less UV total, but with sunburns:
Pretty sure this is incorrect. It’s not the damage that causes cancer, but the failure of the body to heal/repair it. Such failures can be caused for example by you being very old, and therefore healing slower, or by getting a sunburn (= too much exposure in a short time, overwhelming repair capability).
I think the most important thing here is that things scale very much not linearly.
See also this, which argues/claims that more sun exposure (without getting sunburnt) actually leads to less cancer than getting less UV total, but with sunburns: