[giwook]

This is a clever idea.

Unfortunately, UV exposure is cumulative. So your skin is still getting damaged by the UV rays even if you're not getting burnt.

[TurdF3rguson]

Cumulative but with a repair mechanism. There's not necessarily "rollover exposure".

[littlexsparkee]

There are temporary as well as permanent effects and the repair mechanism can be overwhelmed: photoaging, age spots, mutations.

[TurdF3rguson]

Right but there are common precautionary rule-of-thumb heuristics which lead normal people to believe that every sun exposure brings them a little closer to death. That's exaggerated. Or at least, the claims are widely contested.

[chiply314]

Lets be honest here, every additional sun exposre means you lived longer which directly leads you closer to death sooooo ;P

[TurdF3rguson]

That part really is cumulative

[smallpipe]

So how many Joules can you safely sustain in say, a month, or a lifetime ?

[giwook]

I don't think it's really helpful (nor possible) to get into this level of specificity.

Generally, just apply sunscreen and limit prolonged sun exposure when possible (particularly when UV index is high).

Avoiding sunburns (i.e. prolonged sun exposure, particularly when skin is unprotected) is probably the main thing to focus on as I believe studies have shown there is a high correlation between getting burnt and developing skin cancer.