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Dear Mark: Sun Exposure and Glass

Posted By Mark Sisson On November 16, 2010 @ 10:26 am In Dear Mark,Sun Exposure | 53 Comments

These days many of us go to work in the dark and leave greeted by the same. Those lucky enough to have windows next to their desks or work stations might think, “Hey, at least I get some sun exposure during the day.” But how does sun exposure through glass compare with direct sunlight? One reader brought up the topic this week.

Dear Mark,

I work in an office and have a big picture window in front of my desk. Don’t get me wrong – I love the light and all, but my friend told me you don’t get any real sunlight benefits (vitamin D, etc.) through glass. Is that right? I’m guessing a tan is out as well. What’s the story on what gets through and what doesn’t? Love the site by the way.

Thanks to reader Rob for this week’s question. Sorry to rain on anyone’s parade here, but the friend is right and then some. Window light just isn’t the same as direct exposure – on many fronts. I think a view to the outside – particularly to a natural space – has its own benefits, but don’t count on healthy sun exposure [7] being one of them.

Let’s talk UV rays, particularly UVA and UVB.

UVB rays are the triggers for vitamin D [8] production in our bodies. Obviously, we need this. UVB rays penetrate the epidermis, the upper layers of our skin. UVA rays, on the other hand, penetrate more deeply into the basal section of the dermis, which is where most skin cancer develops. Excessive UVA exposure also associated with wrinkling, immune suppression, oxidative stress, and related aging.

When we’re outside in direct sunlight, our bodies absorb both rays. Although we’re exposed to the more damaging UVA rays, research shows [9] that our concurrent exposure to UVB – and the subsequent vitamin D production – actually serves to counteract skin damage and inflammation.

When we’re sitting in front of a window, however, the exposure is different [10]. Glass blocks the shorter-wave UVB rays while allowing most of the longer UVA rays to pass through. In short, we’re receiving the more damaging rays without the normal protective benefit of vitamin D production. This information might cause the corner window office lose a bit of its luster in your mind, but it’s important to keep the various risk factors in perspective here. Excessive UVA exposure is indeed a skin cancer risk factor, but this risk factor typically presents in the form of sunburn episodes in the first 18 years of life. Insufficient vitamin D levels, poor immune function, poor diet, insufficient sleep [11] and overly stressful lifestyle habits are of far more concern than the UVA rays hitting your corner office or window of your delivery truck. In fact, most cases of melanoma are found in areas of the body that receive little or no sun exposure, in people who receive minimal sun exposure, and in people with insufficient vitamin D levels.

My philosophical disagreement with the medical community and Conventional Wisdom is often over an alarmist approach to health concerns. This mentality is often fueled by the media, where scare tactics and lowest common denominator sound bites deliver bigger ratings. Those standing to make a profit also fuel the hysteria, as we have seen with the dermatological and “skinceutical” communities promoting a blanket policy of avoiding (or lathering up against) the sun – to the great detriment of society.

In my next book Reconnect (working title), I’ll deliver an extensive chapter on the vitamin D/sun exposure/skin cancer risk, complete with detailed guidelines for how to maintain optimal vitamin D levels with absolutely no increased risk of cancer. For now, I’ll assert that living Primally, including obtaining a reasonable amount of exposure to direct sunlight to maintain a slight tan (and never burning), will dramatically reduce your risk of all forms of cancer and debilitating health problems common in modern life.

Regarding the windows of the offices, cars, autos, buses, trains and homes that we live with daily, I think enjoying the light (albeit filtered), stimulating view (hopefully!), and expanded perspective provide significant psychological health benefits (imagine commuting, working, or being at home all day without any window to the outside world!) that outweigh any material health concerns about skin damage from UVA exposure. That said, if you have a bunch of accumulated risk factors, such as being extremely fair skinned, living in the lower half of the continental US, with work space or work conditions that blast you with bright sunlight through glass during the peak sun intensity hours of 10am-3pm every day, you may want to consider using a “full spectrum” sunscreen that blocks both UVA and UVB rays during those summer months when UVA radiation is particularly strong. The term “full spectrum” indicates that the product blocks both UVA and UVB rays. This is the only type of sunscreen that you should ever use, since “regular” sunscreen blocks only UVB rays (preventing sunburn) but can often promote excessive UVA exposure by allowing you to stay outside longer than you ordinarily would without the deterrent of potential sunburn. Check out the Skin Cancer Foundation’s chart on sunblock ingredients and their UV properties [12].

You can also consider using tinted window films specially designed to block UVA rays. Although you’ll still get something like 80% of the visible light coming in, more than 99% of UVA rays won’t make it through. This might be especially prudent to use in a car window where infants or young children might be hit with lots of sunlight, since youngsters are especially vulnerable to skin damage from excessive sunlight.

The most important take-away on the topic is to be sure that you obtain enough vitamin D – from direct sunlight exposure and your diet. And of course load up on antioxidants to promote optimal immune function and cancer protection. As I’ll detail further in Reconnect (coming June 2011), it’s not difficult for most people to get enough sunlight to ensure healthy vitamin D levels, but in the colder months a vitamin D supplement [7] can help ensure your levels are up to snuff.

Thanks for reading today. Be sure to share your thoughts and questions in the comment boards. Have a great week, everyone!


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[7] healthy sun exposure: http://www.marksdailyapple.com/vitamin-d-sun-exposure-supplementation-and-doses/

[8] vitamin D: http://www.marksdailyapple.com/deconstructing-vitamin-d/

[9] research shows: http://www.nature.com/jid/journal/v125/n5/full/5603599a.html

[10] the exposure is different: http://www.marksdailyapple.com/vitamin-d-confounding-factors/

[11] sleep: http://www.marksdailyapple.com/category/sleep/?submit=view

[12] Skin Cancer Foundation’s chart on sunblock ingredients and their UV properties: http://www.skincancer.org/understanding-uva-and-uvb.html

[13] The Primal Blueprint Starter Kit: http://primalblueprint.com/products/The-Primal-Blueprint-Starter-Kit.html

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