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

99 points · 79 comments · evakhoury

  1. YesBox · · focus · HN ↗
    I've heard a quick way to determine if the UV index is dangerous is if your shadow is shorter than you. Though location matters too.

    I moved from Vermont to Texas and had to purchase an umbrella with UV protection to walk around during the day. The highest the UV index hits in the Vermont's summer is 6. In Texas, it's 11+ !!! 15 minutes and you're toast

    1. jandrewrogers · · focus · HN ↗
      In some parts of South America the UV index reaches 15-20 due to near-equatorial latitude and extremely high elevation. It is strong enough that you need protection from indirect UV radiation.
    2. pazimzadeh · · focus · HN ↗
      Not quite. Shorter exposure when your shadow is short is better than longer exposure when your shadow is long, in terms of sunburn/vitamin D tradeoff

      <a href="https:&#x2F;&#x2F;onlinelibrary.wiley.com&#x2F;doi&#x2F;10.1111&#x2F;j.1751-1097.2008.00400.x" rel="nofollow">https:&#x2F;&#x2F;onlinelibrary.wiley.com&#x2F;doi&#x2F;10.1111&#x2F;j.1751-1097.2008...

      1. AmazingEveryDay · · focus · HN ↗
        From conclusion of paper, I read the opposite of what you state. &quot;A better strategy would be to expose a larger fraction of the body for a shorter time period, preferably when the sun is lower in the sky to allow a greater margin of error.&quot;

        A sun lower in the sky means your shadow is longer. Maybe I&#x27;m confused?

        1. pazimzadeh · · focus · HN ↗
          Good catch, it is kind of confusing as they seem to contradict themselves, and also depends on the season.

            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).

  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.
          If you are able to expose a lot of your body at a time then yes later in the day is workable (in the summer). In the winter, (depending on your latitude) you still need mid day sun to make any meaningful Vitamin D.
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