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I realize the answer to this question varies with the time of day (most recommendations are to avoid exposure between 10am and 2pm), latitude, altitude, medication and other factors, and goes as far as claiming that "there's no such thing as a healthy tan".

But let's assume a white person in their 30s without an existing tan, who doesn't take any medication, living at around 35 degrees north latitude (California) at around sea level. For how long can they safely expose one unclothed body area to the sun, without sunscreen, outside peak hours, during the summer without clouds?

By "safely" I mean without non-obvious detrimental changes lasting longer than a week:

  • sunburn is obvious and not safe
  • serotonin increases are non-obvious, but not detrimental
  • DNA damage that does not materially affect the individual but might be passed on to offspring is outside the scope (assume the individual has already reproduced, and/or no longer plans to reproduce)
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    I grew up in south Florida. When I lived there, I could be outside during high summer without a shirt from dawn to dusk without burning. I have now lived in New England for the last 30 years. If I went to south Florida today and spent more than about 20 minutes in full sun, I would be badly burned. The point is, there may be no such thing as a healthy tan, but there is such a thing as a protective tan. You might need to factor that into your question.
    – Carey Gregory
    Jun 24 '17 at 22:00
  • define body - naked? bathing suit? shorts and a tshirt? hat? Define "safe" - won't get a sunburn that puts you in the hospital? won't burn at all? won't contribute to long term damage that may cause melanoma? won't contribute to wrinkies or "cosmetic" damage? And finally, even in California, what time of year and is it sunny or cloudy? Jun 25 '17 at 13:29
  • @KateGregory: I've tried to further clarify my question, though I suspect your point is to show it's unanswerable even with the clarifications. Jun 26 '17 at 8:03
  • Do you define sunburns as permanent damage?
    – Carey Gregory
    Jun 26 '17 at 22:13
  • @CareyGregory: no, but sunburns are detrimental, so I've further clarified the question. Jun 26 '17 at 22:29

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