The Science of Sunlight: Why Do you absorb vitamin D through Windows?

October 5, 2025 •

4 min read

Most people get 80-90% of their vitamin D from sun exposure, leading many to assume that soaking up sun indoors is just as effective. However, the common question, "Do you absorb vitamin D through windows?", has a scientifically straightforward and often surprising answer.

Quick Summary

Standard window glass effectively blocks the ultraviolet B (UVB) rays required for the body to synthesize vitamin D. The article clarifies this process, explores dietary alternatives and supplementation, and offers guidance for a proper nutrition diet to maintain healthy vitamin D levels.

Key Points

Window Glass Blocks UVB Rays: Standard window glass prevents the necessary UVB radiation from reaching your skin, making indoor sun exposure ineffective for vitamin D production.
UVA Rays Still Pass Through: While UVB is blocked, harmful UVA rays can still penetrate glass, causing skin aging and increasing cancer risk, without the benefit of vitamin D synthesis.
Dietary Sources are Key: Since sunlight through windows is not an option, focus on a diet rich in fatty fish and fortified foods like milk, cereals, and certain mushrooms.
Supplements are a Reliable Option: For many, especially those with limited sun exposure, supplements containing vitamin D3 are a reliable way to maintain adequate levels.
Take Supplements with Fat: To maximize absorption, consume your vitamin D supplements with a meal that contains fat, as it is a fat-soluble vitamin.
Safe Outdoor Exposure is Effective: Short, sensible periods of direct sun exposure on the skin (10-30 minutes, midday) remain the primary natural way to produce vitamin D.
Vitamin D Deficiency Causes Health Problems: Long-term deficiency can lead to serious bone disorders like rickets and osteomalacia, as well as affecting immune function.

The Science of Sunlight and Vitamin D Production

To understand why glass interferes with vitamin D synthesis, it's crucial to distinguish between the two main types of ultraviolet (UV) radiation that reach the Earth's surface: UVA and UVB. Both can be harmful, but they affect the body differently. The sun's UVB rays, with their shorter wavelengths, interact with a compound in your skin called 7-dehydrocholesterol to produce pre-vitamin D3, which then thermally isomerizes into vitamin D3. This process is the primary natural source of vitamin D for humans.

In contrast, UVA rays have longer wavelengths and penetrate deeper into the skin. While UVA can cause premature aging and is linked to skin cancer, it does not trigger the chemical reaction required for vitamin D production.

Why Window Glass is a Barrier

Standard window glass is a highly effective filter that separates UV radiation. It allows visible light to pass through, creating the warmth and brightness we associate with sunshine. Unfortunately for indoor sunbathers, this same glass blocks virtually all of the essential UVB rays, while permitting a significant amount of UVA to pass through. As a result, sitting by a sunny window, whether at home or in a car, will not provide the UVB exposure needed for your skin to produce vitamin D. Some people mistakenly believe that since they can feel the warmth or even get a sunburn through a window (caused by residual UVA or if the glass is untinted and exposure is prolonged), they are also producing vitamin D, but this is incorrect.

The Role of a Balanced Nutrition Diet

Since indoor sun exposure is ineffective, a well-planned nutrition diet is critical for maintaining adequate vitamin D levels. Very few foods naturally contain significant amounts of this nutrient, but those that do are important to include. Fortified foods also play a key role in public health initiatives to prevent deficiency.

Key dietary sources of vitamin D

Fatty Fish: Excellent natural sources include salmon, mackerel, and sardines.
Fortified Foods: Many dairy products, plant-based milk alternatives (like soy and almond milk), orange juice, and breakfast cereals are fortified with vitamin D.
Mushrooms: Some commercially grown mushrooms are exposed to UV light to boost their vitamin D2 content.
Egg Yolks: A smaller, but still notable, source of vitamin D.
Beef Liver: Contains a modest amount of vitamin D.

Understanding Vitamin D Supplements

For many people, particularly those with limited sun exposure due to living in northern latitudes or spending most of their time indoors, dietary sources alone may not be enough. In these cases, supplements can be a simple and effective way to meet daily requirements. Vitamin D supplements are available in two forms: vitamin D2 and vitamin D3. Studies suggest that D3 may be more effective at raising and sustaining blood levels of the vitamin.

An important tip for improving absorption is to take vitamin D supplements with a meal that contains some fat, as vitamin D is a fat-soluble vitamin. Research has shown that taking supplements with the day's largest meal can increase blood levels by a significant margin.

Health Consequences of Deficiency

Chronic vitamin D deficiency can have serious health implications, primarily affecting bone health. A lack of vitamin D hinders the body's ability to absorb calcium and phosphate, leading to conditions like rickets in children and osteomalacia (softening of the bones) in adults. It's also been linked to other potential issues, including increased risk of osteoporosis, muscle weakness, and a compromised immune system. The best way to prevent these issues is a proactive approach to ensuring adequate vitamin D intake through diet, sensible sun exposure, and potentially supplementation.

Safe and Effective Sun Exposure

While windows are not a viable option, safe sun exposure outdoors remains a potent source of vitamin D. The amount of sun needed varies depending on several factors, including time of day, season, latitude, skin tone, and the amount of skin exposed.

It's recommended to get about 10–30 minutes of midday sun exposure to the arms and legs a few times per week during sunnier months for most people with fair to medium skin. Those with darker skin require more exposure due to higher melanin content, which acts as a natural sunscreen. However, this should always be balanced with proper sun protection to reduce the risk of skin cancer from harmful UVA and UVB rays.

Comparison of UV Rays and Their Effects


Feature	UVA Rays	UVB Rays
Wavelength	Longer (315-400 nm)	Shorter (280-315 nm)
Penetration	Penetrates deep into the skin's dermis.	Affects the skin's top layer, the epidermis.
Primary Effect	Skin aging, wrinkles, DNA damage.	Sunburn, skin cancer, vitamin D synthesis.
Passes Through Window Glass?	Mostly passes through, depending on glass type.	Almost completely blocked by standard glass.

Conclusion

The simple answer to the question "Do you absorb vitamin D through windows?" is a definitive no, as standard glass blocks the essential UVB radiation. Relying on indoor sun exposure is a futile effort for boosting your vitamin D status. A comprehensive nutrition diet incorporating fatty fish, fortified products, and supplements is the most reliable strategy, especially for those with limited outdoor time. Combining this with short, safe periods of direct sun exposure outdoors during appropriate seasons offers the best approach to maintaining healthy vitamin D levels and supporting overall health. For persistent concerns about vitamin D, consulting a healthcare provider for personalized advice is always recommended.

Visit the Office of Dietary Supplements website for detailed information on vitamin D.

Frequently Asked Questions

No, you cannot. Standard car windows, like home windows, are made of glass that blocks the UVB rays necessary for vitamin D production. While some UVA rays may get through, they do not trigger the process.

For most individuals with fair to medium skin, about 10 to 30 minutes of midday sun exposure to the face, arms, and legs a few times per week is often sufficient during spring and summer months. This can vary based on skin tone, latitude, and time of year.

Clouds can reduce the amount of UVB radiation that reaches the Earth's surface, but they do not block it completely. Vitamin D synthesis is still possible on cloudy days, though it may take longer.

Yes. Sunscreen is designed to block UV rays, including the UVB rays needed for vitamin D synthesis. An SPF as low as 8 can reduce production by 95 percent, though experts still recommend using sunscreen to prevent skin damage from prolonged exposure.

Few foods are naturally rich in vitamin D, but key sources include fatty fish like salmon and mackerel, cod liver oil, and some mushrooms. Many dairy products, juices, and cereals are also fortified.

There is no conclusive evidence that a specific time of day is better for taking vitamin D supplements. The most important factor is consistency and taking it with a meal containing fat to maximize absorption.

Yes. While sunlight is a primary source, it is possible to achieve and maintain healthy vitamin D levels through a diet rich in fortified foods and with the use of supplements, especially in winter or for those with limited outdoor time.