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Which time of the day does the sun have highest UVB and lowest UVA

  1. May 6, 2017 #1
    Hi all,

    which sun has highest concentration of UVB, but least concentration of UVA (skin cancer culprit).
    Right now, I'm in equator area (Singapore) with only 2 seasons (just in case if geography has any effect local sun's UV ray distribution).
    I've heard a lot of people saying that morning sun (between 6-9 AM is best), while noon sun (10AM - 3PM) only causes skin cancer. but lately, I've been hearing the opposite, it's the noon sun that's best for UVB, but need to beware of sunburn.
    so, which one is right?

  2. jcsd
  3. May 6, 2017 #2

    jim mcnamara

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    Staff: Mentor

    Your answer for overall UV:
    Relative levels of UVA and UVB by latititude and time of day:

    Both of these sites are aimed at the non-scientist. I chose these two over the much more technical sites.
    And you have the damage potential of UVA and UVB reversed. B is at least a thousand times more damaging to your skin - per the second link , see the second graph.
  4. May 6, 2017 #3
    hi jim,

    thanks for the reply. now this is very interesting. if you type "vitamin d sun" in google, all the result will say the opposite, it's UVB that you need to make vit D.
    like this one:
    which is NHS, UK government official health website
    so, not only the time of exposure is contradictory, but also the UV type.
  5. May 7, 2017 #4


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    As far as I can tell, that's right. UVB is what your body uses to generate vitamin D. But UVB is also FAR more damaging than UVA. So you need to be getting enough exposure to get enough vitamin D but avoiding too much exposure that increases the risk of skin cancer.
  6. May 7, 2017 #5
    I see. any guide on how often per week and how long per session staying under noon sun to achieve optimal vit D while avoiding skin cancer, melanoma, sunburn, etc? to maximize are of coverage, I'll be only wearing swimming trunks and cover my face with books cause pretty sure genital area and face dont generate vit D. in fact, it's wise to keep genital area (for men) from heat and sun since both components are harmful to sperm cells.

  7. May 7, 2017 #6


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    To be honest, I don't know. A quick google search turned up a value of once a day for about 10-30 minutes depending on your skin color and age. But I have no idea how reliable that information is. I doubt there's any need to strip down and sunbathe just to get vitamin D.
  8. May 7, 2017 #7
    trust me, that's what I thought at first. I have a fair complexion, with normal working lifestyle, full office clothing, 10 min walk outdoor under morning sun to MRT station at 7AM, arrived at office (indoor, no sunlight) before 8AM, 5 min walk outside under noon sun to lunch at 12PM and then same 5 min under the noon sun walk back to the office, and then stay in office until 7PM before going home. as for Sats and Suns, I wear T-shirts and shorts and sandals, hang out in the mall, hotel, clubs, etc, and perhaps exposure to sun of around 30 mins total under noon sun. I took Vit D-25 OH panel along with my routine annual MCU, just to check if I'm healthy, the result? I only have 15 ng/mL in my blood. the benchmark values are:
    40 - 100 ng/mL --> normal
    21-39 ng/mL --> insufficient
    <21 ng/mL --> deficient, and this is where I am, not even insufficient, but already in the category of deficient.
    I suggest you also take this test in your next MCU, just to make sure if you're really as healthy as you think.

    btw, I;m not vegetarian. I eat meat, vegetable, fruits, basically everything that;s edible and drink low fat milk that's fortified with vit D.

  9. May 7, 2017 #8


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    This is something you need to take up with your doctor. No one here can tell you whether you actually need more vitamin D or not, and whether you have a related medical condition. Since this is getting towards "medical advice" I'm afraid I'm going to have to lock this thread.
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