How do you know if enough UVB is actually able to penetrate the atmosphere to allow for vitamin D production in your skin?
The key point to understand is that sunlight is composed of about 1500 wavelengths, but the only wavelength that will have your body make vitamin D are UVB-rays when they shine on unexposed skin. The key is that the UVB-rays from the sun actually have to pass through the atmosphere and reach where you are on the earth. This obviously does not occur in the winter for most of us, but the sun’s rays are also impeded during a fair amount of the year for people living in temperate climates. There are factors, such as ozone concentration, altitude, air pollution, ground covered by snow or ice, and cloud cover that also contribute to the amount of UVB that is ultimately reaching your skin.
It should be noted that this represents a very small portion of the total radiation from the sun that reaches the earth’s surface. Much is filtered out by our atmosphere. So due to the physics and wavelength of UVB rays it will only penetrate the atmosphere when the sun is above an angle of about 50° from the horizon. When the sun is lower than 50°, the ozone layer reflects the UVB-rays but let through the longer UVA-rays.
The first step is to determine the latitude and longitude of your location. You can easily do this on Google Earth, or if you are in the U.S. you can use the TravelMath Latitude Longitude Calculator to find your latitude and longitude. Once you have obtained that you can go to the U.S. Navy site to calculate a table to determine the times and days of the year that the sun is above 50 degrees from the horizon.
View this video on use of the US Naval Observatory Azimuth Table :
Translated to the date and time of some places on the globe, it means for example: In my hometown of Chicago, the UVB rays are not potentially present until March 25, and by September 16th it is not possible to produce any vitamin D from the sun in Chicago. Please understand it is only theoretically possible to get UVB rays during those times. If it happens to be cloudy or raining, the clouds will also block the UVB rays.
This is one of the reasons I now spend most of my winters in the sunshine state of Florida because in the center of the state there are more than two extra months of UVB as the sun doesn’t disappear for winter until October 23 and comes back again much earlier, around February.
Testing Your Blood is the Only Way to Know Your Vitamin D Levels
The above guidelines are the most comprehensive details that I know of that will allow you to optimize your vitamin D levels by natural sun exposure. However, there are other variables that you can’t control, such as the darkness of your skin and your age, which can add further confusion to the ability to accurately predict what your vitamin D level will be after appropriate sun exposure.
I can tell you that these guidelines work and have personally used them to maximize my vitamin D levels. I have not taken any oral vitamin D for years now. Last spring, with one to two hours of sun exposure regularly, I was able to get my vitamin D level to 105, which is typical of lifeguards that are not using sun screens.
I would strongly encourage you to have your blood level checked to confirm that your sun exposure is putting you in the right level. If it isn’t, or if sun exposure is not a practical option for you, then you should consider supplementing with oral vitamin D3. (You want to avoid vitamin D2 as it is clearly inferior to D3.) The following chart shows the therapeutic levels of vitamin D you’ll want to reach and maintain.
Read full article by By Dr. Mercola
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