can you get vitamin d through a window

Can You Get Vitamin D Through a Window?

Do you spend a lot of time indoors?

Have you ever wondered if all that sunlight you are getting through the windows ​could ​be hurting your health instead of helping you?


​Nowadays, most of us spend a considerable amount of time inside an office building, enclosed in a cubicle.

Being deprived of natural light for hours on end, day after day, can be depressing, don’t you agree?

But lowering your spirits isn’t the only way working in a cubicle harms your health: It also deprives you of vitamin D.

To counteract this, you may feel the need to bargain for a cubicle next to a window.

This will certainly lift your spirits, but, unless you can actually open that window, you won’t be helping your body produce vitamin D in any significant way.


Because glass shields your skin from Ultraviolet B radiation.

Why does this prevent you from getting your vitamin D?

One Sun, May Different “Sunlights”

Most people see the Sun as a source of light and heat. And, although they are not wrong, the topic is far more complex – and fascinating – than it may seem.

Imagine yourself standing in the middle of the street at noon, eyes closed, enjoying the warmth flushing your face.

That warmth you feel is coming from the Sun. Scientists call this warmth infrared radiation.

Warm infrared radiation from the Sun

Infrared radiation leaves the Sun, travels almost 93 million miles (150 million kilometers) and is captured by the skin in your face, penetrating its sub-layers and exciting the molecules inside your body.

Specialized nervous cells in your body detect this invasion of infrared radiation and signal it to your brain. Your brain, in turn, interprets this signal as heat and a pleasant sensation of warmth fills your conscious awareness.

Pretty cool, hey?

But infrared radiation isn’t the only kind of radiation striking your face. Visible light rays are also coming at you. It’s just that your skin cells aren’t designed to interpret their meaning.

Now, imagine opening your eyes.

Visible light spectrum allows you to see

You can see the landscape around you, the luscious trees, the colorful houses and, at a distance, the silhouette of a mountain.


Because, inside your eyes, there are cells that can sense those rays, measure how fast they are vibrating, and relay that information to your visual cortex. Over there, vivid images are constructed based on the electric impulses telegraphed by the photosensitive retina cells lining the inner part of each one of your eyes.

Now... Imagine a big glass window suddenly appears in front of you, standing between you and the Sun.

Glass window between you and the Sun

Did you notice any significant change?

Probably not much. You can still see just fine and feel the Sun’s warmth.

But there’s a strange little insect that would disagree with you. ​

An insect that can perceive ultraviolet B radiation

Image copyright: Mound, L. (2005) American bean thrips (Caliothrips phaseoli)
Updated on 1/12/2008 1:53:51 PM Available online: PaDIL -

It’s known as the American bean trip (caliothrips phaseoli), an insect that feeds of plants.

You see, we humans, have a limited number of sensors in our body.

We can catch and interpret infrared and visible light but not the other kinds of rays the Sun throws at us, such as ultraviolet A (UVA), ultraviolet B (UVB) or even gamma radiation.

Our insect friend, on the other hand, possesses specialized cells that can detect, measure, and interpret, at least, the ultraviolet B rays being sent by the Sun. (1)

It’s this UVB light that glass blocks. The insect perceives the change, but you can’t because your eyes are unable to detect ​UVB either way.

That’s unfortunate because UVB radiation has a vital role in our health.

Why is Ultraviolet B radiation so important?

Just like we can use microwave radiation to cook food, our body uses ultraviolet B radiation to “cook” 7-dehydrocholesterol into vitamin D.

7-dehydrocholesterol is just the name given to one of the many molecules your body can manufacture. And this conversion, from 7-dehydrocholesterol into vitamin D, could ​be just ​like the other countless chemical transformations happening in your body, but it isn't. Unlike other conversions that rely on specific chemicals, this conversion relies on solar radiation.

UVB converts the 7-dehydrocholesterol molecule into a cholecalciferol molecule

Can you see the bottleneck?

If you don’t get enough UVB you won’t be able to produce enough vitamin D to supply the demands of your cells.

It’s as if you had a restaurant, and there was this chef who had this special dessert named “dried banana.”

And how does he make these delicious dried bananas?

He grabs a banana, he peels it, he puts it on a plate, and he lets it dry under the Sun.


Now, imagine, there are a hundred people in your restaurant waiting for their dried banana desserts but there’s none available.

Is it the chef’s fault? Is the problem a lack of bananas? No.

The culprit is the weather. It’s rainy, so there’s not enough infrared radiation ​reaching the bananas. They are not drying. The weather is the bottleneck to the success of your restaurant.

Can you spot the connections?

The chef represents your body.

The peeled bananas are the 7-dehydrocholesterol.

The hungry people at the restaurant represent your cells, which are ravenous for vitamin D.

The rainy weather does to the peeled bananas the same thing that the glass does to your skin: it acts as a barrier.

Without enough infrared light passing through the clouds there won’t be any dried bananas, and without enough ultraviolet B radiation striking the 7-dehydrocholesterol there won’t be any vitamin D.

These hungry people would be better off ordering some pre-made dried bananas from the supermarket instead of relying on the chef.

In the same way, if all the sunlight you can get is coming at you through a glass window, you’d be better of ordering a bottle of “pre-cooked” vitamin D, because your skin isn’t going to be able to convert any 7-dehydrocholesterol for you.

Moreover, trying to get more exposure to sunlight that is being filtered by a glass window, in the hopes of getting UVB, would be like leaving the bananas in the rainy weather all day. It just won’t work.

Worse yet, the rain will have the opposite effect. Instead of drying the bananas it will make them wetter.

In the same way, besides blocking the much-needed UVB rays, the glass will not filter all the UVA radiation. So you can get a tan, or even get a sunburn, but your skin won’t produce any vitamin D.

How can you solve this problem?

Get outside. That’s a start.

But it may not be enough.

You see, just like UVB rays have a real hard time with glass, they also have major difficulties crossing the Earth’s atmosphere. Unless the Sun is directly above you. Why is that?

When the Sun rises in the morning, its rays have a greater amount of dense atmosphere to cross. As a result, most of the UVB radiation ends up being absorbed by the many particles composing the several layers of our atmosphere.

Because of this, if your goal is to increase your vitamin D levels, you won’t benefit much from being in the Sun in the early hours of the day.

As time moves forward so does the Sun moves west.

At noon is the best time of the day to get UVB from the Sun

At around noon it will be directly above you. This is the best time to catch the precious UVB rays.

Miss noon by a few hours and you’ll be too late.

As ​the Sun continues to move west, ​​its rays start to hit the earth at a greater angle. This means that in the afternoon you’ll have the same challenge as you ​had in the early hours of the day.

There'll be a greater lenght of atmosphere for the Sun's rays to cross and, as a result, the UVB ​radiation will dissipate long before reaching your skin.

Therefore, if you want to give your body the greatest chance of “cooking” 7-dehydrocholesterol into vitamin D, be sure to get out on the Sun as close to noon as you can.

Of course, this comes with a caveat: At noon, UVA, UVB, and infrared radiation will be at their peak. This means that, even though you’ll want to be outdoors, you shouldn’t be doing so for too long.

Your goal is drying the bananas, not roasting them.

How much time should you stay in the Sun to optimize Vitamin D production?

According to Dr. John Cannel, from the vitamin D council, you’ll want to sunbathe for half the time it takes for your skin to burn. Or, in other words, you should sunbath until your skin begins to get rosier. (2)

That’s great advice.

When you expose your skin to the sun, you feel a pleasant warmth.

However, after a few minutes, the temperature starts rising, switching on your internal thermostat.

At this time, your body commands the blood vessels on the surface of your skin to expand. Thus, a larger amount of blood will make contact with the surface of your skin thereby moving the heat away from the skin, preventing it from burning right away.

When the skin starts getting pink, it is as if your body is telling you: "It’s time to go find a shade."

At this point, choosing to remain in the sun longer may result in a mild sunburn. This risk, and the severity of the burn increase with the extra time you remain under the sunlight.

This is especially true when your skin isn't used to sunbathing.

But, if you are used to being in the sun, you’ll have developed a protective coating called melanin — a pigment that darkens your skin while providing you with an extra layer of protection against sunburns.

Logically, the darker the skin, the longer you’ll need to stay in the sun to get your skin pinkish and reap the benefits of sunbathing.

Windows aren’t the only barriers between your skin and UVB rays

Unless you are at the beach, chances are you are wearing clothes covering most of your body.

Clothes protect your skin from the damages inflicted by ultraviolet radiation. But, as you may imagine, this also means that they won’t let your skin absorb any UVB whatsoever.

This means that you should, keeping modesty in mind, expose as much skin as you can to the Sun’s rays.

You can think of your whole skin as a giant solar panel. If a part of the solar panel is covered, it won’t be able to produce electrical energy. In the same way, if your skin is covered, you won’t be able to "cook" 7-dehydrocholesterol into vitamin D.

The same holds true for sunscreen.

Sunscreen protects our skin from the negative effects of solar radiation, while also blocking the positive effects. (3)

So what should you do?

Remember the goal is to spend just a few minutes under the hot noon sun — an amount of time considered reasonable and safe for most people.

Still on the topic of sunscreen, Dr. Manuel Pinto Coelho, in his book “Growing Younger As You Grow Old”, has a great tip on how to use sunscreen in a smart way.

This is especially useful if you tend to avoid the Sun because of the potential it has to prematurely age your skin. (4)

If that’s you, here’s a smart trade-off:

Before sunbathing for UVB, apply sunscreen to your hands and face.

In this way, you would still allow your legs and your arms to absorb the much-needed UVB rays while, at the same time, preventing them from making your skin look older in the long run.

Sunscreen isn't evil, is a tool

Sunscreen isn’t evil, it’s just a tool. And just like a tool, in certain cases, when used in a smart way, it can be useful.

How to make the most out of UVB and Vitamin D

​Adapt these instructions to the particularities of your skin, geographical location, and climate.

Just as the amount of solar infrared radiation varies throughout the day, so does the amount of UVB radiation.

Infrared radiation is easy to perceive because we feel it in our skin as heat, not so much for UVB. But, as it turns out, the times of higher UVB ray intensity coincide with the times we have always been warned to avoid sun exposure: between 11.00 am and 3.00 pm when there are more heat and light.

This means the best time to sunbathe in UVB rich rays is around noon.

Of course, remember that we are talking about spending just a few minutes bathing our skin in sunlight and not an excessive amount of time, maybe even to the point of burning our skin.

If your skin is lighter, making use of UVB radiation will be easier for you. This means you will not need much time to start getting pink.

However, darker skins — with more melanin — will be better protected from solar radiation and, consequently, will require longer periods of sunbathing, perhaps even more than an hour, until an optimal exposure is reached.

According to the Vitamin D council, you should seek to adjust the time you spend sunbathing to half the time it would take to burn your skin. (5)

If your skin burns after 10 minutes, you should expect it to turn pink after 5 minutes in the Sun. Then, you should seek a shade.

However, if your skin takes two hours to burn you may need an hour of exposure to get the same effect.

This approach is especially useful for those with darker skin tones because the darker the skin the harder it will be to notice the slight pink color shift caused by the dilation of the blood vessels in the epidermis.

As for location, consider that the closer to the equator line, the more effective the UVB radiation reaching your skin. Similarly, an elevated location, like a mountain, will provide you with a greater exposure to UVB rays.

Finally, the weather and the seasons also exert a great influence.

If it’s a cloudy day or a typical winter day, the amount of UVB reaching your skin will be little to nonexistent.

By contrast, a summer day, at noon, with clear skies will provide you with the ideal levels of UVB radiation.

Finally, take some precautions if you intend to shower afterward. 

There's no consensus, but there are some concerns among vitamin D enthusiasts that if you shower immediately after having exposed your skin to the sun you may remove some of the pre-vitamin D newly formed in the epidermis — the outer layer of your skin — especially if you use more than just water to bath.

Thus, knowing that about 50% of the pre-vitamin absorption happens within the first 2 hours, it would make sense to wait a few hours before passing water on your skin, to maximize the absorption of pre-vitamin D.

Do you need to supplement if you sunbathe?

As you can see, there are many variables influencing the quality of your sun exposure and the amount of vitamin D you’ll produce.

Of course, all this provided your body can properly perform each of the steps in the sequence of events that culminates with the activation of vitamin D in your kidneys.

Thus, it's easy to understand why, not too long ago, it was estimated that one billion people ​were deficient in vitamin D.

Because of all these issues, many turn to supplementation. That’s understandable, and in many cases may even be a necessity – like, for example, if you suffer from a health problem that responds well to vitamin D supplementation.

​Still, any amount of sunlight you can get will be beneficial for your health, so it's always beneficial to strive to follow these steps whenever you can.

​Can you absorb vitamin D 
through a window?

According to Google, this is a common question, very similar to the one titling this article. As it seems, more than 1300 queries like this get typed into the search engine every year.

It’s actually a funny question, I believe.

When I think about “absorbing vitamin D through a window,” I imagine the molecule literally being produced inside the Sun, crossing almost 93 million miles and hitting your skin at nearly the speed of light.

Well, unless there’s a window in front of you.

In that case, vitamin D might just hit the window and fall to the floor. Unless, of course, it’s able to shatter ​the glass, like a bullet.

If you got here by googling this question, don’t worry, though. Just keep in mind that glass blocks vitamin D production, not its absorption.

Glass prevents you from getting vitamin D by shielding your skin against the much-needed ultraviolet B radiation.

Tiago Henriques

With more than 10 years of experience as a Public Speaker, Tiago Henriques has done hundreds of public talks. He was born in 1987, in Portugal, and is the Author of 3 books — and counting — and the creator and editor of the Portuguese Science Project, "Ciência Desenhada," where complicated science is explained in a simple way, using whiteboard animation techniques. Tiago developed his own practical and down-to-earth teaching method inspired by his experiences with the Portuguese Deaf Community and their use of highly visual, descriptive and easy-to-understand language.