Monday, June 2, 2014

Sensible sun exposure


Sun exposure leads to the production in the human body of a number of compounds that are believed to be health-promoting. One of these is known as “vitamin D” – an important hormone precursor ().

About 10,000 IU is considered to be a healthy level of vitamin D production per day. This is usually the maximum recommended daily supplementation dose, for those who have low vitamin D levels.

How much sun exposure, when the sun is at its peak (around noon), does it take to reach this level? Approximately 10 minutes.

We produce about 1,000 IU per minute of sun exposure, but seem to be limited to 10,000 IU per day. This assumes a level of skin exposure comparable to that of someone wearing a bathing suit.

Contrary to popular belief, this does not significantly decrease with aging. Among those aged 65 and older, pre-sunburn full-body exposure to sunlight leads to 87 percent of the peak vitamin D production seen in young subjects ().

Evolution seems to have led to a design that favors chronic (every day or so) but relatively brief sun exposure. Most of the sun rays are of the UVA type. However it is the UVB rays, which peak when the sun is high, that stimulate vitamin D production the most. The UVA rays in fact deplete vitamin D. Therefore, after 10 minutes of sun exposure per day when the sun is high, we would be mostly depleting vitamin D by sunbathing when the sun is low.

There is a lot of research that suggests that extended sun exposure also causes skin damage, even exposure below skin cancer levels. Also, anecdotally there are many reports of odd things happening with people who sunbathe for extended periods of time at the pool. Examples are moles appearing in odd places like the bottom of the feet, cases of actinic keratosis, and even temporary partial blindness.


Source: Lifecasting.org

There is something inherently unnatural about sunbathing at the pool, and exponentially more so in tan booths. Hunter-gatherers enjoy much sun exposure by generally avoiding the sun; particularly from the front, as this impairs the vision.

Pools often have reflective surfaces around them, so that people will not burn their feet. They cause glare, and over time likely contribute to the development of cataracts.

When you go to the pool, put your hands perpendicular to your face below you nose so that much of the light coming from those reflective surfaces does not hit your eyes directly. If you do this, you’ll probably notice that the main source of glare is what is coming from below, not from above.

In the African savannas, where our species emerged, this type of reflective surface has no commonly found analog. You don't have to go to the pool to find all kinds of sources of unnatural glare in urban environments.

Snow is comparable. Hunter-gatherers who live in areas permanently or semi-permanently covered with snow, such as the traditional Inuit, have a much higher incidence of cataracts than those who don’t.

So, what would be some of the characteristics of sensible sun exposure during the summer, particular at pools? Considering all that is said above, I’d argue that these should be in the list:

- Standing and moving while sunbathing, as opposed to sitting or lying down.

- Sunbathing for about 10 minutes, when the sun is high, staying mostly in the shade after 10 minutes or so of exposure.

- Wearing eye protection, such as polarized sunglasses.

- Avoiding the sun hitting you directly in the face, even with eye protection, as the facial skin is unlikely to have the same level of resistance to sun damage as other parts that have been more regularly exposed in our evolutionary past (e.g., shoulders).

- Covering those areas that get sunlight perpendicularly while sunbathing when the sun is high, such as the top part of the shoulders if standing in the sun.

Doing these things could potentially maximize the benefits of sun exposure, while at the same time minimizing its possible negative consequences.

27 comments:

dearieme said...

I tried sunbathing one summer when I was a boy. All the skin peeled off my back.

Ever since I've enjoyed sitting in the shade looking out at the sun.

It's different if you happen to enjoy something that exposes you to the sun anyway - sailing, cricket,... - but deliberately sitting in the sun is not for me in summer. It's delicious in spring or autumn though. March this year: mmmm!

Unknown said...

I live in Northern Canada (for 25 years at 62 deg, currently at 55 deg)and I was always under the impression that at these latitudes UVB exposure only produces VitD during a short period during the Summer months since a minimum UVB intensity is required to produce VitD at all. Is this correct?

dearieme said...

My burnt back occurred at 55 degrees North, in the school summer holidays - July I imagine.

It'd be a pity to think that I got little Vitamin D out of my painful experience.

Ned Kock said...
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Ned Kock said...

A great deal of UVB is filtered, even at the equator. The skin seems to be able to produce vitamin D with the slightest amounts of UVB. Higher latitude has a smaller filtering effect for UVB than ordinary glass, which allows most UVA to pass through.

Mandy Wenchel said...

Thank you for sharing this information. We have to take care of our skin so learning this precaution will up help us to know what to do when we have too much exposure under the sun.

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Eva said...

In a natural environment, humans likely spent most of their time outdoors and a lot more time under the sun than we do now. I only avoid sun when it's hot out, otherwise there is no urge to avoid at all. The thing with sunbathers is they often use sunscreen which lets some rays through but not others and many sunscreens have been proven to be toxic themselves. A more natural situation would be people in the sun every day but with a seasonally correct tan which is like natural sunscreen. What is unnatural is pasty white people suddenly going out in the sun too long and becoming a red lobster or people who are genetically really really white like from the cloudy climes like Ireland going out in strong sun, a problem because it's alien to their genetics. Most correlational statistics do not control for those kinds of nuances.

David Isaak said...

Hi, Ned. I assume you've seen all the references to Myerson and Neustadt's 1939 research on sun exposure and testosterone? Brief UVB exposure caused testosterone levels to rise, but UVB exposure on male genetalia caused levels to triple.

I've been unable to find the original article. Or, rather, I can find it, but even though the journal Endocrinology published it in 1939, it is still 'paid content'(!)

Ned Kock said...

This is interesting and makes sense David, since D is a precursor to T.

Ned Kock said...

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minor tanner said...


10 minutes overall, or 10 per side?

dedhy seven said...
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