[graeme]

This is not a direct answer, but:

1. Uva is the cause of most skin cancer

2. Uvb generates vitamin d and sunburns

3. When uvb is present, you can make sufficient vitamin d quite quickly

This would suggest you would want sunscreen for longer exposures, or with much uva exposure. And could get vitamin d from a briefer exposure pre sunscreen.

I can’t say how completely sunscreen blocks vitamin d however.

[dralley]

Given that sunburn is a response to cellular damage of your skin cells, why would it be the case that the two are de-coupled w/r/t UVA and UVB?

[graeme]

Actually, it seems I was wrong. I thought it was just UVA, but uvb also causes melanoma.

I’d delete the above comment if I could. The main point that you can get vitamin d from short exposure without sunburn is correct, but I was incorrect on the risk. But he fact I was wrong about uvb and melanoma also makes me think I may be incorrect on the risk of uva during periods without uvb.

https://en.wikipedia.org/wiki/Melanoma#UV_radiation