A Better Time until Sunburn Calculator

by Josh Jacobson2 min read17th Aug 202111 comments

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Practical
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This is a preliminary review (more on what this means).

I became curious about sun exposure recently. I did an abbreviated review of some of the relevant literature and came away thinking that whether sun exposure is beneficial or not was unclear at that level of investigation (this seems to be an open question for most LWers as well).

One consensus that did seem to exist was that sunburn is bad and a thing to avoid. In order to be able to avoid it, and especially to be able to avoid it while still getting the potential benefits of non-burning sun exposure, I figured it would be helpful to know how long I could be in the sun without getting burnt.

This could be determined by trial and error, and maybe that would be a valid approach if I felt a good sample size could be obtained in an enjoyable, low-risk way. I don’t like sunburn though (and it probably wouldn’t be low-risk), so I was personally interested in finding a calculator that would predict this.

If you google something like “sunburn calculator” you’ll get a lot of results. But they are incredibly bad. Most seem highly untrustworthy, many clearly have bad inaccuracies, some don’t gather sufficient information for meaningful calculation, and ~none are transparent about their methodology or how to interpret their results.

So I’ve built a better, bad, sunburn calculator (view, copy).

Sunscreen Calculator

It’s better than others because:

  1. At least some of the results have high interpretability. It can calculate how many minutes it takes for 1% of people in this situation to develop a sunburn, and what percent of people in these conditions will get a sunburn after the indicated number of minutes.

  2. There’s transparency. Sources are cited (in notes), calculations are available on-sheet (formulas & hidden rows), and I’ve added relevant notes and context where appropriate to help with interpretability.

  3. Relatively feature-rich. It accounts for sunscreen protection typically not working as advertised, and includes some information about how much sun exposure to actually aim for.

  4. Information quality is probably better. I didn’t spend much time vetting studies or calculations used, but I still strongly suspect that the information quality is significantly better than otherwise readily available. I think it probably mostly asks for the right inputs as well, given the state of knowledge today.

But despite being better, this calculator is still quite bad:

  1. Information is only lightly vetted. As said in #4 above, the studies weren’t highly vetted and some certainly seemed to have issues but were still included when I couldn’t find something better quickly and easily. There may be better info out there, or existing info just may not be that great (I mainly lean toward the latter).

  2. Most results are limited. You’ll see that a number of calculations will throw warnings in many circumstances. Most of the best info I found only pertained to a subset of conditions, without a clear way to apply them to other situations.

  3. This UX isn’t the best. Maybe I’ll make a web or mobile app someday, but for now, you need to make your own copy of this sheet for use.

Still, I think it’s good to have a better version (I find myself using it often), even if it’s still what I would call bad.

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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.

This is really interesting; I’d never heard of it before… thanks for sharing. I’m excited to research it more.

Sunscreen lengthens the amount of sun exposure needed to synthesize a given amount of Vitamin D; I wonder if this does as well.

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. 

I think that’s a quite interesting topic / question. I may see if I can find any info on it, but for now am less informed than you.

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.

Yeah that may be an interesting extension of it for version 2. Not sure how straightforward it would be to implement; haven’t looked into that yet.

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.

Indeed, the results for which warnings are thrown should be disregarded; the non-monotonicity of out-of-bounds results is a situation I noticed as well.

The authors were quite clear about the equation only being useful in certain conditions, and it does seem to act reliably in those conditions, so I think this is just an out-of-bounds quirk that can be disregarded.

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.

Yes, latitude and more.