I heard, the goodness in sunlight that helps people make vitamin D does not pass through windows.

Is this true, and if so, does it impact pets as well? If my pet only gets sunlight through windows, how does this impact the health considerations (good and bad) of sunlight?

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    What species of pet? We have a very broad question at the moment
    – Just Do It
    Commented May 13, 2016 at 17:59
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    @JustDoIt Does it matter? According to this link it either gives you Vitamin D or Cancer. I don't think species matters a bunch, but if I am wrong you should be able to make a pretty good answer about why it does matter. Commented May 13, 2016 at 18:04
  • It matters, cause then the answers will be too broad themselves. The link you provided is for humans, not really relevant to your question. Youre asking about pets, but my take is that fishes, reptiles, and mammals have different benefits and/or concerns regarding sunlight. That's why I think that as it stands, your question is too broad.
    – Just Do It
    Commented May 13, 2016 at 18:05
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    i mean is @JamesJenkins going to make three separate questions for each class of animal? Or more for each specific species? I dont think narrowing it down would do much good. I think maybe this question could be moved to the biology exchange, though? They can probably give you a more in depth answer about how vitamin D is made in the first place. I know a ball python and fish dont really need UVB to be healthy but im not sure how this affects an indoor dog/cat. Or a tortoise (where I assume this question spawned from)
    – Ian
    Commented May 13, 2016 at 18:26

2 Answers 2


Glass does block some ultraviolet; exactly how much varies.

However: Most pets have fur, which means they aren't going to be synthesizing a lot of vitamin D that way anyway.

And if they are eating a decent-quality commercial pet food they will be getting the vitamins they need from their diet.

The answer may be different for reptiles, but even so this isn't something I would worry about unless my vet told me there was an issue.

When my cats lie in the sunlight, it's just because they like the warmth. And perhaps because they are converting it to gravity.

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    im pretty sure it blocks all types of UVB which is what cholesterol uses to make vitamin D. the UVA is what varies
    – Ian
    Commented May 13, 2016 at 21:05
  • I believe the degree of blocking varies with the glass. I've gotten quite tanned/sunburned through car windows on long trips.... And certainly UV lamps can be made out of glass to produce a, b, or various mixtures.
    – keshlam
    Commented May 13, 2016 at 21:21
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    UVA rays can tan/ damage you but they arent as damaging as B. Its why driving in your car on a road trip doesnt tan/burn you nearly as much as tanning outside.
    – Ian
    Commented May 16, 2016 at 14:27
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    most glass we know is what is called "soda-lime" glass. bulbs usually use quartz glass or woods glass if they want to allow uv to pass through
    – Ian
    Commented May 16, 2016 at 14:32
  • Downv*ted because only the first sentence directly answers the question, and this isn't enough - I would assume that the person asking such question had already known that glass blocks some UV light. A more elaborate and specific explanation would be nice. What is more, having fur does not necessarily stop animals from photosynthesizing vitamin D via sunlight; oily skin secretions containing 7-DHT are secreted onto the fur and fur is exposed to sunlight; animals then lick their fur and ingest vitamin D (source).
    – lila
    Commented May 3, 2021 at 22:07

Ultraviolet radiation

Ultraviolet radiation is the only known thing that is both beneficial and carcinogenic at the same time. More specifically, part of the ultraviolet spectrum which highly overlaps with UV-B (280 - 315 nm) causes conversion of 7-dehydrocholesterol (a zoosterol, chemical compound naturally occurring in the skin of animals and humans) to previtamin D3, which then spontaneously isomerizes into vitamin D3. As visible in the picture 1., the previtamin D3 synthesis occurs with a peak at around 294 nm, and virtually none at wavelengths above 310 nm.

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Picture 1.: action spectrum of 7-dehydrocholesterol to previtamin D3 conversion; source: www.testudo.cc.

Most of UV-B in the sunlight is already blocked off by atmosphere; the lower the Sun is above horizon, the more thick layer of atmosphere has to be traversed by sunlight and the less UV-B reaches Earth's surface. A rule of thumb says that the Sun has to be at least 45 degrees or higher above horizon (shadows have to appear shorter than objects casting them on the flat ground), or else too little UV-B reaches us for any substantial vitamin D3 production to occur.

Soda lime glass

Windows are made from soda lime glass, which is generally transparent to UV-A but opaque to UV-B, and thus they mostly block off the part of UV spectrum responsible for enabling vitamin D3 synthesis. Pictures 2., 3. and 4. show UV transmittance of soda lime glass from various sources. In this context it could be a problem, but does not necessarily have to be; pet foods are often fortified with minerals and vitamins, including vitamin D. However, vitamin D content present naturally in foods is poor, which could be a concern if you are cooking or otherwise preparing pet food on your own. What is more, vitamin D deficiency sometimes occurs because of gastrointestinal disorders causing its poor gut absorption - however, unlike dietary vitamin D, endogenous synthesis enabled by sunlight is not affected by this, as it bypasses gastrointestinal tract and directly reaches the bloodstream. Additionally, synthesis via sunlight has the bonus property that it is impossible to overdose vitamin D with it - at some point the synthesis reaction reaches equilibrium and vitamin D is degraded as fast as it is synthesized; meanwhile, it is possible to cause vitamin D toxicity via oral ingestion of excessive amounts (it shouldn't be a problem in case of food, though; it is reasonably possible only in case of excessive ingestion of artificial supplements).

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Picture 2.: UV transmittance of soda lime glass; source: publiclab.org.

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Picture 3.: UV transmittance of soda lime glass; source: dichroic.asia.

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Picture 4.: UV transmittance of soda lime glass; source: www.continentaltrade.com.pl.

Additional properties of sunlight

Please also note that enabling vitamin D3 synthesis is not the only benefit of sunlight exposure. UV radiation in sunlight, for example, has been shown to cause nitric oxide production in the skin (source). Nitric oxide is a signalling molecule that causes vasodilation (dilation of blood vessels), which lowers blood pressure and decreases the risk of stroke and cardiovascular disease. Medications like nitroglycerin and amyl nitrite which are taken to alleviate angina (pain caused by insufficient blood flow to the heart muscle) are converted into nitric oxide in the body and this is responsible for their active properties. However, the spectrum enabling nitric oxide production is different than spectrum enabling vitamin D3 synthesis; part of it belongs to UV-A, which passes through soda lime glass.


Sunlight exposure has many potential health benefits beyond enabling vitamin D3 synthesis. For example, it is known to decrease the chance of multiple sclerosis - however, it has been shown that substituting sunlight exposure with vitamin D3 supplementation does not produce such effect.

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