There’s a lot of confusion surrounding sunlight and the production of Vitamin D. People commonly think that if the sun is up and they are outside in the sun, they are going to produce Vitamin D. This is not the case!
There are many reasons why you won’t make Vitamin D in the sun—even during the middle of the day. But most significantly, the sun cannot stimulate your production of Vitamin D if the sun isn’t high enough in the sky.
UVB Rays And Vitamin D
The reason is that UVB rays—and only UVB rays—catalyze the production of Vitamin D. Yet UVB is reflected and blocked by the atmosphere. The more atmosphere UVB rays have to travel through, the more UVB rays are prevented from reaching the surface of the earth; and the lower the sun is in the sky, the more atmosphere these UVB rays travel through. If the sun is low enough, UVB rays are completely blocked by the atmosphere before they even reach you, and Vitamin D production is not possible.
In addition to the atmosphere, clouds also block UVB rays, so if it is cloudy or overcast you probably aren’t going to produce any Vitamin D. Similarly, UVB rays are also blocked by ozone and pollution. In some cities excessive pollution can be enough to severely limit the amount of UVB rays reaching the ground and, therefore, the potential to make Vitamin D.
As a side note, UVB rays also don’t penetrate glass (or clothing). If you are in your car or behind a glass window Vitamin D production isn’t possible. UVB rays have to strike your exposed skin to make Vitamin D creation possible. Sunscreen lotion also blocks the production of Vitamin D.
UVA Rays And Sun Damage
Conversely, UVA rays pass relatively unfiltered through the atmosphere, so regardless of the angle of the sun or cloud cover, a high percentage of UVA rays are still reaching the surface of the earth. And UVA rays damage skin just like UVB rays. So even in the absence of UVB rays–like during the winter for much of North America, and when it is overcast–UVA rays are still present at high levels and still aging your skin.
Similarly, UVA rays penetrate glass and will still damage your skin while you are behind a window—such as during your daily commute or while you sit next to a window in your office. But UVA rays cannot stimulate Vitamin D production.
You can find some useful information about ultraviolet radiation here: How Sunlight And Ultraviolet Radiation Affect Us
The Angle Of The Sun And Vitamin D Production
In order for the sun to stimulate your Vitamin D production, the sun needs to be at a minimum of about 50 degrees, or greater, above the horizon (90% would be directly overhead). Ideally, the sun should be as close to overhead as possible; the closer to overhead it is, the greater its potential for stimulating Vitamin D production (and the less time you have to be in the sun to produce a given amount of Vitamin D).
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A good way to test if the sun is about 50 degrees above the horizon, or higher, is that your shadow should be slightly shorter than you are tall. If the shadow cast by your body is the same length, or longer, than your height you will not produce Vitamin D–even while your skin is exposed to direct sunlight. And this is under ideal conditions: clear skies without cloud cover or pollution.
Note: this angle depends on many factors and Vitamin D production is not all or nothing. In actuality, you may start to produce a tiny amount of Vitamin D when the sun is slightly lower in the sky, but this amount seems to be inconsequential. It is only when the sun is 50 degrees or higher that the amount of Vitamin D produced starts to become significant. And the higher the sun is above 50 degrees, the higher the rate of your Vitamin D production.
Vitamin D Production And Skin Damage
Of course, UVB rays don’t just stimulate the production of Vitamin D; UVB rays also damage your skin. The more overhead the sun is, the more UVB rays you will absorb, and the more skin damage you will do. So you need to limit the sun exposure you do get.
Since Vitamin D production and sun damage go hand and hand, it’s a trade-off. With every second you are in the sun you are doing damage to your skin—even when you are producing Vitamin D. If you want to produce Vitamin D in the sun, do so when the sun is as close to overhead as possible, spend as little time as possible in the sun to produce Vitamin D, and then get out of the sun.
Also, you should have as much of your skin exposed to the sun as possible, and be at a perpendicular angle to the sun to maximize production. If the sun is directly overhead, for instance, you would lie flat on the ground, and when the sun is at an angle you would lie at the angle that allows you to directly face the sun.
It also doesn’t matter what part of your skin is producing your Vitamin D. It therefore is recommended that you expose the large areas of your body that don’t normally get very much sun, and cover the parts that are often in the sun (such as hands and wrists, face, neck, and shoulders) to protect them. In this way, you can maximize your Vitamin D production without further damaging the sensitive areas of your skin that are already most likely over-exposed. You can also alternate exposing your front and back (flip over halfway through your sun exposure) to decrease the damage to any one area of your body.
The one thing you are able to do to help promote your defenses against the effects of sun exposure, without blocking your production of Vitamin D, is to eat a lot of foods with natural antioxidants in them–like colorful vegetables and fruits. There is also a product called Sunsafe Rx–a pill with specific antioxidants that are shown to defend your skin and eyes against environmental stresses.
In general, aside from the periods when you are producing Vitamin D, make sure you protect yourself from sun exposure!
Vitamin D Production And Sun Exposure
Of course, even if you are producing Vitamin D, you don’t make an endless amount of it. After too much sun exposure your body stops producing Vitamin D, and excess sun exposure beyond this amount can even block Vitamin D production.
It has been roughly estimated that just 15 minutes of sun in the middle of the day in the summer, only a few times per week, can be sufficient for you to produce your complete Vitamin D needs. This is for people with fair skin in ideal conditions; darker-skinned individuals may need longer exposure time. The time of day (midday has the highest potential to stimulate Vitamin D production), altitude (higher altitudes increase production potential), and cloud cover and pollution (both decrease production potential) also impact the time it takes to produce Vitamin D. You can find some websites and phone apps that will help you calculate Vitamin D production more precisely. Just understand that the longer you are in the sun the more skin damage you will sustain.
Vitamin D Supplementation
The more you can supplement your Vitamin D production from sun exposure with Vitamin D in your diet, the less sun exposure you need, and therefore the more sun damage you can avoid. You can get Vitamin D in your diet by eating certain foods. And you can also supplement with Vitamin D (Vitamin D3—cholecalciferol—is the best form), either by itself or in a multi-vitamin. This may be a good idea if you aren’t eating the proper foods or during the winter when you can’t produce Vitamin D outdoors anyway.
Dietarily, good sources of Vitamin D include egg yolks, fatty fish such as salmon, tuna, and mackerel, grass-fed beef and other red meat, mushrooms, cod liver oil, fortified milk (including fortified almond and coconut milk), and fortified cereals. For example, just 4 ounces of wild salmon offers about 150% of your daily RDA (recommended daily allowance) of Vitamin D. One tablespoon of cod liver oil can provide 300% of your daily RDA. And since your body stores Vitamin D, just eating a little fish and taking a few tablespoons of cod liver oil fairly regularly can fully satisfy your Vitamin D needs—and preclude the need for Vitamin D from sun exposure.
You can find a little more info in this article about the myths of sun exposure under Myth #8: 17 Myths About Sun Exposure
Vitamin D Testing
If you want to be sure you aren’t Vitamin D deficient, you can get tested. If you’re low, you can supplement with extra Vitamin D3, then get tested again a few months later to see if your levels have changed. Adjust your Vitamin D intake accordingly. And follow your doctor’s advice.
Latitude And UVB Rays
In North America (and anywhere in the northern hemisphere) during the winter, the farther north you are, the lower in the sky the sun is during the day. In fact, in much of North America it isn’t even possible to produce Vitamin D in the winter—even in the middle of the day when the sun is at its highest point.
Cities And Vitamin D Production Potential
Here are date ranges for a few cities and states showing when the sun is not at a high enough angle in the sky to produce Vitamin D at any time during the day:
Los Angeles, California and Atlanta, Georgia: Both are at about 34 degrees latitude north of the equator. You can’t make Vitamin D in these cities or anywhere north of them from the beginning of October until the beginning of March.
Utah, Colorado, Kansas, Missouri, Kentucky, and Virginia, for example, are all entirely above the 35th parallel (35 degrees latitude), and therefore making Vitamin D in these states during this period is also not possible.
New York City: You can’t produce Vitamin D in NYC from the end of September to the end of March.
Madrid, Spain: At about the same latitude as New York City, Vitamin D production isn’t possible from the end of September to the end of March.
Seattle, Washington: It’s not possible to produce Vitamin D outdoors from the beginning of September to the second week of April.
Miami, Florida: You can’t make Vitamin D from the end of October to the middle of February.
London, England: For more than 8 months of the year Vitamin D production is not possible: from the middle of August until late April.
Stockholm, Sweden: For 10 months of the year it isn’t possible to produce Vitamin D in the sun, with late May to late July the only months where it is potentially possible.
Anchorage, Alaska: For more than 10 months of the year Vitamin D production isn’t possible. During less than two month of the whole year, from late May to the middle of July, is the only time when Vitamin D production is possible.
Here are the times of day on the longest day of the year in the northern hemisphere (usually June 20: the first day of summer) in a few cities showing when Vitamin D production is possible:
Los Angeles, CA and Atlanta, GA: On the summer solstice (the first day of summer), Vitamin D production is possible from about 10 AM to 4 PM.
New York City: Vitamin D production is possible on the first day of summer during about the same time frame as Los Angeles and Atlanta: from about 10 AM to 4 PM.
Seattle, WA: Vitamin D production is possible on June 20 from about 10:30 AM to 4 PM.
Now you’ll notice these hours when Vitamin D production is possible on the longest day of the year also happen to be the hours when most people are inside working. And for the days the rest of the year when Vitamin D production is possible, it will still only be possible at closer to the middle of the day. For instance, in New York City on May 1, Vitamin D production is only possible from about 10:30 AM to 3:15 PM. This underscores the fact that, even in the summer, if people who live some distance from the equator don’t get sun exposure during the middle part of the day and/or don’t take in enough Vitamin D via diet and supplements, they can still be deficient.
A Tool For Determining Your Vitamin D Production Potential Wherever You Are
On the following page, you can determine the angle of the sun (degrees above the horizon) for any place in the world on any day and at any time:
In the input box at the upper left corner of the page, you can search for your address or any city. You can then adjust the time of day, and see the respective altitude of the sun.
Any time for which the altitude listing is 50 degrees or more means that you have the potential to produce Vitamin D in the sun in that location under ideal conditions (sunny, not overcast or raining, etc.).
You can also perform the calculation for any given latitude and longitude.
Note: You can easily determine your latitude and longitude in Google Maps by centering your location in the map and looking at the URL: the first number after “@” is the latitude, the second number is the longitude.
With this tool for calculating sun angle, you can determine when during the day you can produce Vitamin D in the sun wherever you are. Just be aware that even when you are producing Vitamin D in the sun you are still aging your skin.
