According to the American Academy of Dermatology website,

  • Everyone needs sunscreen
  • Sunscreen should be used every day if you will be outside. The sun emits harmful ultraviolet (UV) rays year round.
  • Even on cloudy days, up to 80% of the sun’s harmful UV rays can penetrate your skin.

I also saw this video a while ago.

The two of these make it sound like we should be wearing sunscreen even if I step outside to get my mail.

Is the sun's damage to skin sufficient to warrant protection anytime I go outside or just for extended periods? What is the general criteria this kind of organization uses to determine what is "sufficiently" damaging to warrant sunscreen protection?

  • $\begingroup$ There are dermatologists who advocate for never exposing the skin to direct sunlight. The interview I saw had terrific looking skin. The increase of UV content of sunlight over the past few decades makes this seem like a better idea now than previously. Clothing, hats, gloves and parasols may be recommended in these cases because of the chemicals in sunblock. $\endgroup$
    – shigeta
    Aug 17, 2015 at 4:27
  • $\begingroup$ Interesting. As an aside though, specific opinions are part of the problem. I don't know what the criteria are by which they are making these recommendations. Doctors are not infallible. I need to know the metric they are making these recommendations based on so I can decide for myself who is being rational and who is just saying stuff. $\endgroup$ Aug 17, 2015 at 4:32
  • $\begingroup$ It is very much dependent on skin type. $\endgroup$
    Aug 17, 2015 at 5:52
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    $\begingroup$ Probably as important, the incidence of skin cancer is up over the past 30 years because of 1) the idea that getting a tan is healthy 2) the intensity of sunlight hitting the earth's surface is going up. An increase in melanoma has been documented. who.int/uv/faq/skincancer/en/index1.html skincancer.org/skin-cancer-information/skin-cancer-facts $\endgroup$
    – shigeta
    Aug 18, 2015 at 14:23
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    $\begingroup$ @shigeta thanks for that comment. Very interesting. I never thought of it like that $\endgroup$ Aug 18, 2015 at 17:12

2 Answers 2


Because the website you link speaks of cancer, they probably go by the rule that a single exposure and even a single photon of UV may be sufficient to cause that one mutation in that one skin cell that will eventually lead to cancer.

In the end, they are right, because mutagens (UV, cigarettes, radiation) do not follow typical toxicological rules. There is no threshold dose of UV that does not cause cancer, as even a single short exposure may do the trick. Obviously, multiple exposures and longer exposures increase the chance. This, as opposed to toxic substances that generally feature a threshold dose. Below this threshold dose they are considered harmless.

  • 3
    $\begingroup$ Let's suppose a person gets cancer from a small does of radiation. This person could just be an outlier, in which case we simple must accept that that person just has bad luck. But is there a general trend --- a statistical threshold --- of radiation exposure? For example, I walk to class which takes about 5 minutes. Am I supposed to put sunscreen on for that walk? Do I need to put it on if I take a bus downtown? Knowing the statistical threshold of sufficiently "safe" sun exposure to determine if it warrants all those precautions is what I am asking about. $\endgroup$ Aug 17, 2015 at 4:38
  • $\begingroup$ 'sufficiently safe' is opinion-based. Obviously, the linked website adheres zero-tolerance policy. As I say above, there is no safe threshold. It's a gradual thing. $\endgroup$
    – AliceD
    Aug 17, 2015 at 4:40
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    $\begingroup$ There should be objective measures I would think. Rates of skin deterioration. Risk of cancer. These are objective measures. What one considers safe is subjective. Am I thinking about this question the wrong way? Just asking. I haven't really asked a question like this before. $\endgroup$ Aug 17, 2015 at 4:41
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    $\begingroup$ @StanShunpike - And I normally don't answer these kind of questions :) +1 for the question btw. It's a good one. $\endgroup$
    – AliceD
    Aug 17, 2015 at 4:51
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    $\begingroup$ AFAIK, other than acute radiation sickness (where someone gets a massive dose of radiation within a short period of time) there is no way to tell. When we talk about radiation damage, usually we end up using metrics equivalent to "x dose will increase your chances of cancer within y years." $\endgroup$
    – jzx
    Aug 17, 2015 at 6:40

Since your questions seems to be motivated by personal health protection, consider an alternate view, which is that not only is the sun's damage to the skin not "sufficient to warrant protection anytime I go outside or just for extended periods", but that lack of sufficient unblocked exposure to the sun may be, overall, even more harmful--not only for the risk of various cancers, but for a large number of serious diseases.

Dr. Michael Holick is a Professor of Medicine, Physiology and Biophysics at Boston University School of Medicine, and has researched vitamin D--which is produced by our skin when exposed to the sun's UVB radiation--and its critical role in health for over 30 years. In addition to his book, The Vitamin D Solution and an entertaining and informative (and at times, zany) lecture, he recently (Jul 24, 2015) wrote about this in The Washington Post. For example:

the risks associated with sensible sun exposure have been exaggerated by well-meaning health authorities, and the measures to guard against them often have nothing to do with the sun’s occasionally malignant effects. Contrary to the paranoia generated by years of messaging, the sun is not our enemy. It’s safe to step back outside — and, please, go easy on the sunscreen.

The book and lecture provide much more detail, but the main points are that 1) vitamin D, a hormone, is critical to many bodily functions, 2) that lack of it in a majority of people in many countries is a "pandemic" related to many important diseases, 3) that the current United States 400 IU/day guidelines are too low, 4) that it is easily produced by sufficient exposure to unblocked mid-day summer sun, and, importantly, 5) that there may be additional "photoproducts" (other molecules our skin manufactures in response to UVB rays) that we may find are similarly important to cellular processes and so he prefers sun exposure to taking supplements. In regards to diseases that are related to vitamin D deficiency, he writes:

A lack of vitamin D is associated with increased risk for Type 1 and 2 diabetes, multiple sclerosis, rheumatoid arthritis, Crohn’s disease, cardiovascular disease, stroke, depression, Alzheimer’s disease, schizophrenia, colon and breast cancer, influenza and tuberculosis....Of the 30 leading causes of deaths in the United States in 2010, 19 were linked to low vitamin D status.

He does not deny that one needs to limit exposure to the sun reasonably, and recommends sun block for the face and basically exposing arms and legs something like 10-20 minutes at a time, several times a week, from 10am--3pm (when UV-B is sufficiently strong, else one only gets UV-A, which does nothing for vitamin D but is linked to skin damage), though exact "dosing" will be dependent on latitude, skin color, time of year, and perhaps other factors (which he covers in the book).


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