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. foota · 1790113245 · focus · HN ↗
  Isn't think only true if getting enough vitamin D is as important to you as cancer?
  1. pazimzadeh · 1790115315 · focus · HN ↗
    Sunlight is associated with lower risk of almost every cancer except skin cancer, and skin cancer is the easiest to detect early.
2. happytoexplain · 1790113256 · focus · HN ↗
  I really appreciate learning this fact, but I can't tell how it makes the quoted sentence misleading. The author is just talking about the surprising disparity between how warm it feels and how fast you will burn. They don't mention vitamin D.
  1. pazimzadeh · 1790113795 · focus · HN ↗
    I know they don’t mention vitamin D, but I think it’s important for people to know the context and not think that late day sunlight automatically = better
    andrewflnr · 1790115316 · focus · HN ↗
    That's good to know as an additional factor to consider, but it's not enough to justify your accusation that the original statement is "misleading".
    You're allowed to just say "cool, here's another important thing to think about it". You don't have to pick a fight.
    pazimzadeh · 1790115467 · focus · HN ↗
    I think the original statement was misleading wrt sunburn for the reason I stated. I think we have different definitions of fight.
    happytoexplain · 1790116724 · focus · HN ↗
    Ohh, I see. I'm not sure it was "very misleading", but I agree somebody trying to decide when to get sunlight for the purposes of vitamin D could easily misapply the lesson from this article.
3. prometheus1992 · 1790114295 · focus · HN ↗
  it really only takes about 5 minutes of exposure to get the vitamin D your body needs. i would happily do it between 7-9AM when the UV is low than doing it at mid day.
  1. an_account · 1790116044 · focus · HN ↗
    You only need 5 minutes at midday in the summer. At 7am, you get essentially no vitamin D.
  2. pazimzadeh · 1790117822 · focus · HN ↗
    What you are saying is exactly the wrong takeaway. Please read the article:
    <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...
    Beneficial UV is UVB, which is low when the sun is low because it can’t get through the ozone and atmosphere as well when the sun is at an angle. Instead, you are getting mostly UVA, which has an easier time passing through the skin (and atmosphere) and doesn’t stimulate vitamin D production.
4. 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)
    pazimzadeh · 1790117529 · focus · HN ↗
    Yes, mid-day for a shorter amount of time is better for optimal Vitamin D. The article I linked above goes into great detail about it, more than the two quotes I pulled out

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