Thread

The UV index is not the warm sensation of sunlight on bare skin

99 points · 79 comments · evakhoury

pazimzadeh · 1790112939 · focus · HN ↗
> the sun may still feel pretty intense in the morning or late afternoon, but the risk of sunburn is considerably lower
This is very misleading. By some coincidence I was just looking into this earlier today.
The lower the UV index, the smaller the window is between the amount of time needed to get enough Vitamin D and the time to get burnt (erythema). So the best thing to do is get sunlight mid-day for a shorter amount of time.
UV Radiation: Balancing Risks and Benefits <a href="https://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.2008.00400.x" rel="nofollow">https://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.2008...
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  As the UVI becomes smaller, it becomes more difficult to produce sufficient vitamin D without inducing erythema. For low sun conditions typical of midday in the mid-latitude winter (SZA ∼65°), where R∼1, the skin area exposed must be more than 12% (see Eq. 5) This means that for these low sun conditions it is improbable that sufficient vitamin D can be produced from exposing the hands and face alone, since that area is less than 12% of the full body.

  Figure 6 shows that for full body exposures, there is a wide window between the time for sufficient UV and the time for too much UV. As the fraction of body that is exposed decreases, the window of optimum UV exposure times also decreases. If only hands and face are exposed, there is generally only a small window between receiving insufficient UV for vitamin D production, and too much UV for skin damage (erythema).
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You would do well to calculate the numbers for your own city, as I did earlier today for mine:
<a href="https://chatgpt.com/share/6ab2f272-a32c-83ea-9fec-49031e4da362" rel="nofollow">https://chatgpt.com/share/6ab2f272-a32c-83ea-9fec-49031e4da3...
1. the__alchemist · 1790115761 · focus · HN ↗
  The statement you quoted is very useful [One of the most useful heuristics], and your statement is misleading. You accumulate damage (Photoaging, cancer risk) dramatically faster at high sun angles (i.e. mid day, close to the summer solstice, closer-to-equatorial latitudes) etc than low ones.
  I am not trying to be negative, but this is very important if you have pale skin. Being outdoors in morning and late afternoon is dramatically less damaging than mid day.
  1. happytoexplain · 1790117021 · focus · HN ↗
    It kind of depends on context.
    If your goal is "hang out in the sun but don't get burned", then morning/evening is better. If your goal is "get exactly as much vitamin D as I need while minimizing UV damage, perhaps using a timer", then it sounds like technically mid-day is better (assuming I'm reading the parent right).
    the__alchemist · 1790117410 · focus · HN ↗
    After reading your reply here and the one I was responding to, I think I understand (You spelled out) the disconnect; the post I replied to was about Vitamin-D-Maxxing, which isn't something I had in my cross check. My primary concern is "Which times should we do outdoors things to prevent sun damage [photoaging + cancer]" (And/or "Do I need to apply sunscreen before going out now / how long can I stay out)

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