Can you get vitamin D through a window?

Vitamin D is an important vitamin that our body mainly produces through sunlight exposure. It plays a crucial role in our health by supporting strong bones, boosting the immune system, and influencing our mood. Despite its importance, up to 30.2% of adults in Germany do not have sufficient vitamin D, with levels varying depending on the season and sunlight exposure (RKI, 2016). While increased sunlight in the summer can stimulate vitamin D production, the heat often causes people to stay indoors or cover their skin. This raises the question: can the body produce vitamin D through a sunny window? The answer is no, because window glass blocks the UVB rays from the sun that are essential for vitamin D production in the skin. In this article, we explain how vitamin D is produced in the body, the risks of sunlight through a window, and provide helpful tips on how to maintain adequate vitamin D levels while protecting your skin.

Table of Contents

• What is vitamin D?
• How much vitamin D should you consume daily?
• How is vitamin D produced in the body through sunlight?
• Can the body produce vitamin D through a sunny window?
• What are the risks of UV radiation?
• Recommendations for sun exposure
• Summary

What is vitamin D?

Vitamin D is a fat-soluble vitamin, which means it can be stored in the body. This is especially important because the body primarily produces vitamin D through sunlight and relies on these reserves during sun-poor seasons. In addition to sunlight, vitamin D can also be absorbed in small amounts through food. There are two main forms of vitamin D: vitamin D2 (ergocalciferol) is mainly found in plant-based foods, while vitamin D3 (cholecalciferol) occurs in animal-based foods and is also the form produced in the skin via sunlight. Both forms can be utilized by the body, but vitamin D3 is more easily absorbed and contributes to a more effective vitamin D supply (Harvard Health, 2023).

What are the functions of vitamin D?

Vitamin D fulfills several important functions in the body:

• Regulation of calcium and phosphate, which are essential for strong bones and teeth
• Supporting the immune system
• Promoting muscle function
• Influencing mood

Vitamin D significantly contributes to the prevention of osteoporosis, as it is essential not only for bone health but also for muscles that protect the skeletal system. Osteoporosis is a condition characterized by a decrease in bone density and deterioration of bone structure, leading to increased fragility and a higher risk of fractures. Women in menopause should pay particular attention to maintaining sufficient vitamin D levels, as decreasing estrogen levels increase the risk of bone loss and osteoporosis. A vitamin D deficiency can contribute to or worsen osteoporosis (Mayo Clinic, 2023; Mei et al., 2023).

How much vitamin D should you consume daily?

The values in the table below are general recommendations and may vary depending on individual health needs. IU stands for “International Units,” a standardized measure for vitamin quantities. In the case of vitamin D, 1 IU equals 0.025 micrograms (µg).

Excessive intake of vitamin D through supplements can be harmful. Keep in mind that the body can store vitamin D. The upper limit for daily intake is 4,000 IU (100 µg) for adults and children over 9 years of age. Consult your doctor before taking supplements (NHS, 2024).

How is vitamin D produced in the body through sunlight?

Vitamin D is produced in the skin when it is exposed to sunlight. More precisely, it is the ultraviolet B rays (UVB) from the sun that stimulate this process. When the skin receives UVB rays, a substance called 7-dehydrocholesterol in the skin is converted into previtamin D3. Through a temperature-dependent process, this previtamin D3 is then converted into vitamin D3. However, the process is not yet complete, as the vitamin D3 formed in the skin is absorbed into the bloodstream and transported to the liver and kidneys, where it is finally converted into the active form, 1,25-dihydroxyvitamin D (calcitriol), which the body can then utilize (Bikle, 2021).

What factors influence vitamin D production?

Various factors affect how well our body can produce vitamin D from sunlight:

• Age: As we age, the skin produces less 7-dehydrocholesterol, which is needed for vitamin D production, making synthesis more difficult.
• Skin color: Darker skin contains more melanin, a pigment that protects the skin from sunlight. However, melanin also reduces the skin's ability to produce vitamin D. People with darker skin need to spend more time in the sun to produce the same amount of vitamin D as people with lighter skin.
• Season and time of day: Especially in winter, but also during early morning and late afternoon hours, the sun is weaker, and fewer UVB rays reach the skin, reducing vitamin D production.
• Air pollution and cloud cover: Polluted air and thick clouds can block UVB rays, reducing the production of vitamin D in the skin.

Source: Harvard Health, 2018

Can the body produce vitamin D through a sunny window?

No, this is not possible. The sun emits different types of radiation, including UVA and UVB rays. While UVB rays are essential for vitamin D production in the skin, they cannot penetrate window glass. Windows filter out UVB rays, meaning vitamin D production through a closed window is not possible (NHS, 2024).

Would you like an overview of your vitamin status?

If you are interested not only in your vitamin D level but also in the levels of other vitamins, the Homed-IQ Vitamin Deficiency Test offers a simple way to check your vitamin status conveniently from home. With this test, you can measure not only your vitamin D level but also the levels of vitamins B12 and B9. If you would like to focus on individual vitamins, there are also specific tests available, such as the Active Vitamin B12 Blood Test (Holo-TC).

Can UVA rays penetrate window glass?

Yes, UVA rays are not blocked by window glass. However, this is not necessarily good news. These rays do not contribute to the production of vitamin D but are associated with skin aging. They can cause wrinkles and a loss of elasticity and contribute to the development of skin cancer types through indirect DNA damage in skin cells (Cleveland Clinic, 2022).

Interesting fact: Truck and long-haul drivers are especially at risk on the left side of their body, which is more exposed to UVA rays through the window while driving. Studies show that the left side of the face and the left arm are more prone to sun damage, resulting in a higher risk of skin cancers like melanoma and other types of skin cancer on those areas (DGUV, 2024).

What are the risks of UV radiation?

Both UVA and UVB rays from the sun can be harmful to the skin by causing sunburn, a skin inflammation, and contributing to premature skin aging. More importantly, UV rays are closely associated with the development of skin cancer, such as melanoma, a particularly aggressive form of skin cancer. Early skin damage significantly contributes to this risk. The more sunburns a person experiences throughout life, especially at a young age, the greater the risk of developing skin cancer. Repeated sunburns lead to an accumulation of damage in the skin, making it more vulnerable to cancerous changes (Cleveland Clinic, 2022; CDC, 2024).

Recommendations for sun exposure

The risks associated with UV rays therefore require a balance between ensuring sufficient vitamin D production and protecting the skin from sun-induced damage. To produce enough vitamin D, unprotected sun exposure of ideally 5–30 minutes daily or at least twice per week is recommended, depending on skin type and season. Shorter sun exposure times apply to people with lighter skin and during sunny seasons, while longer exposure is recommended for people with darker skin or during sun-poor seasons. After a short time in the sun, sunscreen should be applied to avoid sunburn and the associated risk of skin cancer. The sun protection factor (SPF) of a sunscreen indicates how long the skin is protected from sunburn. An SPF of 15, for example, means that you can stay in the sun 15 times longer without burning than without protection (Raymond-Lezman& Riskin, 2023; NHS, 2022).

Note: SPF protects against UVB rays, but not directly against UVA rays. Therefore, it is important to choose a sunscreen labeled “broad-spectrum protection,” which blocks both UVA and UVB rays. Below, you will find more tips to help you protect yourself in the sun.

• Avoid direct sunlight between 11 a.m. and 3 p.m.
• Use sunscreen with SPF 30+ and UVA/UVB protection.
• Reapply sunscreen regularly.
• Wear sunglasses that offer 100% UVA/UVB protection.
• Protect yourself around reflective surfaces such as water and sand.
• Avoid tanning beds and artificial UV radiation.

Summary

Vitamin D is important for health, especially for strong bones, the immune system, and muscle function. Although UVB rays are necessary for vitamin D production, they cannot penetrate window glass, which is why direct sun exposure is essential. However, it is important to find a balance between the necessary sun exposure and protection from harmful UV rays in order to prevent skin damage and skin cancer. Use broad-spectrum sunscreen and avoid being in the sun during midday heat to protect your skin’s health.