Do you still get vitamin D through a window? The surprising truth about sunlight and glass

October 5, 2025 •

4 min read

According to the National Institutes of Health, your skin does not make vitamin D from sunlight when you are indoors by a window because glass blocks the necessary ultraviolet B (UVB) rays. So, while the warmth of the sun streaming in might be comforting, do you still get vitamin D through a window? The scientific answer is unequivocally no.

Quick Summary

Glass blocks the crucial UVB rays required for the body to synthesize vitamin D from sunlight. This can lead to deficiency for people with limited outdoor access, highlighting the importance of alternative nutritional sources and supplements.

Key Points

Glass Blocks UVB Rays: Standard glass absorbs the UVB radiation necessary for your skin to produce vitamin D, meaning indoor sun exposure offers no vitamin D benefit.
UVA Rays Still Penetrate: While UVB is blocked, UVA rays pass through glass, contributing to skin aging and increasing the long-term risk of skin cancer without providing any vitamin D.
Outdoor Sun is Required: To synthesize vitamin D from sunlight, you must be outdoors with bare skin exposed directly to the sun.
Food Sources are Essential: Excellent dietary sources include fatty fish like salmon and mackerel, fortified milk and cereals, and cod liver oil.
Supplements Fill the Gap: For those with limited sun exposure or dietary intake, vitamin D$_3$ supplements are a reliable way to maintain healthy levels.
Risk of Deficiency Increases Indoors: People who spend most of their time indoors are at a higher risk of vitamin D deficiency, especially in winter or at higher latitudes.

The Science Behind Sunlight and Glass

To understand why you don't get vitamin D through a window, you must first know how the body produces it. Our skin contains a precursor molecule, 7-dehydrocholesterol. When exposed to sunlight's UVB radiation, this molecule is converted into pre-vitamin D$_3$, which then rapidly transforms into vitamin D$_3$ through a heat-sensitive process. The liver and kidneys then convert this into the active form the body can use.

The crucial difference between UVA and UVB

The sun emits two main types of ultraviolet (UV) radiation that reach the Earth's surface: UVA and UVB. This is where the glass barrier comes into play.

UVB Rays (280-320 nm): These are the high-energy rays that are primarily responsible for stimulating vitamin D synthesis in the skin. They are also the rays that cause sunburn. Standard window glass is highly effective at absorbing and blocking UVB rays, preventing them from penetrating indoors.
UVA Rays (320-400 nm): These are longer-wavelength rays that penetrate deeper into the skin than UVB. Unlike UVB, a significant portion of UVA rays (about 75%) can pass through standard glass. While they do not trigger vitamin D production, UVA rays contribute to skin aging (photoaging) and increase the risk of skin cancer over time.

This means that when you sit by a sunny window, you're exposing your skin to UVA rays without getting any of the vitamin D benefits. This is particularly concerning for people who spend a lot of time indoors near windows, as they may be at risk for skin damage without realizing it. The only way to get vitamin D from the sun is by exposing your bare skin directly to sunlight outdoors.

Dietary and Supplemental Sources for Vitamin D

Since relying on indoor sunlight is not an option, a balanced approach to getting enough vitamin D is essential, especially for those living in northern latitudes or with limited outdoor access.

Foods rich in vitamin D

Several foods can help you meet your daily vitamin D needs. While few foods naturally contain significant amounts, many are fortified.

Fatty Fish: Excellent natural sources include salmon, mackerel, trout, and sardines.
Cod Liver Oil: A single tablespoon is a potent source of vitamin D.
Fortified Foods: Many dairy and plant-based milks, orange juices, and cereals are fortified with vitamin D.
Egg Yolks: Contain a small amount of vitamin D, depending on the hen's diet and sun exposure.
Mushrooms: Some mushrooms, particularly those treated with UV light, can be a good plant-based source.

The role of supplements

For many, especially during winter months, dietary intake alone may not be enough. Vitamin D supplements are a reliable and effective way to ensure adequate intake.

Vitamin D$_3$ (cholecalciferol): This is the same form your body produces in response to sunlight and is generally more effective at raising blood levels than vitamin D$_2$.
Daily Needs: The recommended dietary allowance for adults is 600-800 IU, though some individuals may require higher doses, as advised by a healthcare provider.

Maximizing Vitamin D Intake

To ensure you are getting enough vitamin D, consider a multi-pronged approach:

Get outdoor exposure: Aim for 10-15 minutes of direct sun exposure on bare skin (hands, arms, legs) during peak daylight hours (10 am to 3 pm), especially in warmer months.
Incorporate dietary sources: Regularly eat vitamin D-rich foods, focusing on fatty fish and fortified options.
Consider supplements: If your sun exposure is limited or your dietary intake is insufficient, a daily supplement can help maintain healthy levels.
Protect your skin: Balance sun exposure with sun-protective measures to minimize the risk of skin cancer. After your brief, unprotected exposure, cover up and apply sunscreen.

Comparing Vitamin D Sources: Sunlight vs. Indoor Light


Feature	Direct Sunlight	Sunlight Through Window	Fortified Foods / Supplements
Source	Solar UVB rays	Solar UVA rays (mainly)	D$_3$ or D$_2$ via ingestion
Vitamin D Synthesis	Yes (Efficient)	No	Yes (Controlled, predictable)
Skin Damage (Aging)	Risk (with prolonged exposure)	Risk (UVA penetrates glass)	No
Skin Cancer Risk	Risk (with prolonged exposure)	Risk (UVA damages skin)	No
Dependence on Weather/Season	Yes	Yes	No
Convenience	Variable	Convenient (but ineffective)	High

Conclusion

While the idea of passively absorbing the "sunshine vitamin" from a sunny window is appealing, the reality is that glass acts as a barrier, blocking the specific UVB rays needed for vitamin D synthesis. This means that for adequate vitamin D, you must either get direct, sensible sun exposure outdoors or rely on nutritional sources, including fortified foods and supplements. For most people, a balanced nutrition diet that includes these sources, combined with supplements as needed, is the most reliable way to maintain healthy vitamin D levels year-round. Given the documented health risks of prolonged indoor UVA exposure, a proactive approach to supplementing or getting outdoor sun is a crucial aspect of overall health.

For more information on recommended vitamin D intake and sources, consult the NIH Office of Dietary Supplements.

Frequently Asked Questions

Yes, just like home and office windows, the glass in car windows blocks the UVB rays that trigger vitamin D synthesis. Therefore, you do not produce vitamin D while driving or sitting in a car.

Fatty fish, such as salmon, trout, and mackerel, are some of the best natural food sources of vitamin D. Fortified foods like milk, orange juice, and cereals are also excellent sources.

The necessary time varies based on skin tone, latitude, and season. For fair-skinned individuals, a few minutes of direct sun exposure on hands, arms, or legs around midday during summer is often sufficient. People with darker skin need more time.

Yes, vitamin D$_3$ is the same form your body produces and is generally more effective at raising blood levels than vitamin D$_2$, which is plant-derived. Both can be absorbed, but D$_3$ is preferred.

Risk factors include older adults, people with darker skin, individuals with limited sun exposure, and those with certain medical conditions that affect fat absorption or obesity.

Symptoms can include fatigue, bone pain, muscle weakness, and, in severe cases, more serious conditions like osteomalacia or osteoporosis.

Yes, heavy cloud cover can significantly reduce the amount of UVB radiation reaching your skin, making vitamin D synthesis less efficient. However, lighter cloud cover may still allow some UVB penetration.