A Better Time until Sunburn Calculator

Interestingly there's a pill you can take which has sun protection effects - polypodium leucotomos extract. It reduces UV damage, equivalent to about SPF 4. So not a replacement for sunscreen, but a useful addition to it, as it protects spots you may have missed. Eg my girlfriend goes on long daily runs, for which it's hard to cover all exposed skin adequately with sunscreen.

It doesn't seem to be widely available, but I get it from Super Smart (order online from Portugal).

The recommended dosage is 1 x 500 mg first thing in the morning (as it takes 30-120 minutes to take effect). If you'll be out in the sun all day, maybe take another one mid-morning. And preferably also take one the day before sun exposure - so not a bad idea to take one (or two) daily all summer.

The lowdown on scientific research into it is here: https://examine.com/supplements/polypodium-leucotomos/ This seems somewhat tentative, but in a more detailed (?subscription only) report elsewhere on the site they recommend it quite strongly.

Thanks for doing this! Just seeing the concept makes me realize how subjective my assessments of sunburn risk are.

One thing I've been wondering lately is the effects of interrupted vs uninterrupted sun exposure. E.g., if I spend an hour outside, an hour inside, and then another hour outside, how does that compare to the effects of two continuous hours outside? I've tried a bit of googling, but the information is surprisingly hard to find.

What I have learned is that UV-induced DNA damage is mostly single-strand breaks that can be repaired via nucleotide excision repair, but I'm not sure how long that takes. I did find this on the simpler related process of base excision repair:

BER reactions in cells are extremely fast, and in many cases, an individual BER event may take only a few minutes (10,11). The repair of acute DNA damage requires several rounds of BER and can take several hours, as the amount of BER enzymes is limited.

(Dramatically-titled source)

To me that suggests a model where sun damage accumulates at a rate depending on exposure, is repaired at a fixed rate, and damage reaching a certain threshold triggers a sunburn. 

This initially seemed to give implausibly low numbers for optimal daily sun exposure, which seemed inconsistent with rates of Vitamin D deficiency (https://pubmed.ncbi.nlm.nih.gov/21310306/), though after thinking about it more I may just have unusually high sun exposure and be overestimating how much the median American gets. 

This isn't a criticism that would be easy to improve your calculator based on. Still, I will report that the main impediment to its practicality for me is that I usually try to stay in the shade. I don't have a good way of adjusting the weather report's UV index to account for being in the shade. For example, today I spent 2.5 hours in the shade at midday without sunscreen and got a slight burn on my chest.

Thanks for sharing this! I did notice a weird non-monotonicity: if I go from 90 minutes exposure to 120 minutes, the "Percent of Population w/ Sunburn Degree 1 at Time Exposed" drops from 96.8% to 72.7%. There is a warning in both cases that it's outside normal range, but it still seems odd that more exposure gives lower risk.

Does the UV index already account for Lattitude effects? It is much quicker to burn in San Diego than New York, even at the same time of year.