Do you still get vitamin D from the sun through glass? The scientific truth

According to the National Institutes of Health, your skin does not produce vitamin D from sunlight when it's filtered through a window. This is a critical point that many people misunderstand, potentially leading to vitamin D deficiency, especially for those who spend extensive time indoors.

Quick Summary

Glass effectively blocks the specific UV rays required for vitamin D synthesis in human skin. While some UV light passes through, it is the wrong type to trigger production. This fact highlights the importance of direct sun exposure, supplementation, or dietary sources for maintaining adequate vitamin D levels.

Key Points

• UVB is Blocked: Standard window glass filters out the essential UVB rays required for vitamin D synthesis in the skin.

• UVA Penetrates: While you won't get vitamin D, damaging UVA rays still pass through glass, contributing to skin aging and cancer risk.

• Direct Sun is Required: To produce vitamin D, you need to expose bare skin to direct, unfiltered sunlight.

• Supplements are an Alternative: For those with limited sun exposure, dietary sources and supplements are the most reliable ways to get enough vitamin D.

• Deficiency Risks: Relying on sun through glass can lead to vitamin D deficiency, which can cause bone health issues like osteoporosis.

Why Glass Blocks Vitamin D Production

The synthesis of vitamin D in the human body is a fascinating biological process that hinges on one crucial element: UVB radiation. When sunlight hits our skin, a cholesterol-like molecule called 7-dehydrocholesterol converts into previtamin D3, which is then further converted into vitamin D3. However, this entire process relies on the energy and specific wavelength of UVB light, which is filtered out by standard window glass.

The Difference Between UVA and UVB Rays

To understand why glass hinders vitamin D production, you must understand the two primary types of UV radiation that reach Earth's surface: UVA and UVB. Both are part of the sun's spectrum, but they have very different effects on human skin and different abilities to pass through materials like glass.

• UVB Rays: These are the shorter, more energetic rays responsible for stimulating vitamin D synthesis in the skin. They are also the primary cause of sunburn. Crucially, standard window glass is an effective blocker of UVB, preventing it from reaching your skin. This is why you can't get sunburnt by sitting next to a window for a couple of hours, but it's also why you can't get your daily dose of the sunshine vitamin this way.

• UVA Rays: These are longer wavelength rays that penetrate deeper into the skin than UVB. They are associated with premature aging, wrinkles, and skin damage, and can also contribute to the development of skin cancer over time. Unfortunately, most standard glass allows a significant percentage of UVA rays to pass through. This means that while sitting by a sunny window, you may not be producing vitamin D, but you are still exposing your skin to damaging UVA radiation.

Can special glass make a difference?

Some high-performance or specialized glasses, such as laminated glass used in car windshields and buildings, offer better UV protection by blocking a higher percentage of both UVA and UVB rays. For example, laminated glass may block almost 100% of UVA, while tempered glass might only block around 70%. While this is beneficial for skin protection, it does not change the fact that you still cannot produce vitamin D through a window, as the necessary UVB wavelengths are blocked regardless.

The Health Implications of Not Getting Vitamin D Through Glass

Over time, relying on indoor sun exposure for vitamin D can lead to a deficiency. This can have significant health consequences, as vitamin D plays a vital role in numerous bodily functions. A severe deficiency can manifest as issues with bone health, immune function, and overall wellness.

The Risks of Vitamin D Deficiency

Without adequate vitamin D, the body cannot effectively absorb calcium, which is essential for strong bones. In children, a prolonged deficiency can cause rickets, a condition that leads to soft and weak bones. In adults, it can result in osteomalacia, a painful softening of the bones, and osteoporosis, a disease characterized by fragile bones and an increased risk of fractures. Given that more than one in three Australian adults is affected by vitamin D deficiency, this is a serious health consideration.

Other Factors That Affect Vitamin D Production

Beyond glass, several other factors influence how effectively your body produces vitamin D from sunlight:

• Latitude and Season: In many parts of the world, especially northern latitudes, the sun's angle is too low during winter to provide the necessary UVB radiation. This means that even direct sun exposure is not enough during certain times of the year.
• Time of Day: The sun's UVB rays are most intense between 10 a.m. and 3 p.m.. Morning and late afternoon sun contain much less UVB, so exposure during these times yields minimal vitamin D production.
• Skin Tone: Melanin, the pigment that gives skin its color, acts as a natural sunscreen. People with darker skin tones require significantly more sun exposure to produce the same amount of vitamin D as those with lighter skin tones.
• Age: The body's ability to produce vitamin D from sunlight decreases with age, making older adults more susceptible to deficiency.

Safe Alternatives to Get Your Vitamin D

Since sitting by a window won't help, here are some safe and effective ways to boost your vitamin D levels:

• Get Direct Sun Exposure (Safely): Aim for short periods of direct sun exposure, around 15 minutes a few times a week, especially around midday. After this brief period, use sun protection like sunscreen and protective clothing to reduce the risk of skin damage.
• Incorporate Vitamin D-Rich Foods: Some foods are natural sources of vitamin D, while others are fortified. Fatty fish like salmon, mackerel, and sardines are excellent sources. Other options include cod liver oil, egg yolks, and fortified milk, cereals, and plant-based milks.
• Consider Supplements: For many, especially during winter months or if sun exposure is limited, taking a vitamin D supplement is the most reliable way to ensure adequate intake. Consult a healthcare provider to determine the right dosage for your needs.
• Use a UV Lamp: In some cases, and under medical supervision, a UVB-emitting lamp can be used to stimulate vitamin D production.

Comparison Table: Direct Sun vs. Through Glass

Feature	Direct Sun Exposure (Outdoors)	Sun Exposure Through Glass (Indoors)
UVB Radiation	Allows UVB rays to reach the skin, triggering vitamin D synthesis.	Blocks virtually all UVB radiation, preventing vitamin D production.
UVA Radiation	Exposes skin to UVA rays, which contribute to aging and skin cancer.	Allows a significant amount of UVA rays to pass through, causing skin damage.
Vitamin D Production	Yes, the body synthesizes vitamin D.	No, no vitamin D is synthesized.
Skin Damage Risk	Moderate to High with prolonged exposure; proper protection is needed.	Low for Sunburn, but High for Aging and long-term damage due to UVA.
Recommended Exposure	Short, controlled bursts (approx. 15 mins) around midday.	Not Recommended for vitamin D benefit; risk of UVA damage remains.
Overall Health Impact	Beneficial for vitamin D, but requires caution due to UV risk.	Provides no vitamin D benefit and exposes skin to damaging UVA rays.

Conclusion

The idea that you can still get vitamin D from the sun through glass is a persistent myth that can lead to health issues. Standard window glass effectively blocks the vital UVB rays needed for your skin to produce this essential nutrient. While it may feel warm and sunny indoors, you are only receiving the damaging UVA rays, which contribute to skin aging. To maintain optimal health and ensure you are getting enough vitamin D, it's crucial to seek safe, direct sun exposure, incorporate fortified foods, or consider taking supplements. For those who spend a lot of time indoors or live in northern latitudes, supplementation is often the most reliable strategy. Taking proactive steps to address your vitamin D intake can help safeguard your bone health and overall well-being. For more information on vitamin D, consult the Office of Dietary Supplements at the National Institutes of Health.

How to safely increase your vitamin D intake

Get Safe, Direct Sun Exposure

• Timing is Key: Optimize your body's vitamin D production by spending a short amount of time outdoors between 10 a.m. and 3 p.m. when UVB radiation is at its peak.
• Go Bare: Expose bare skin to the sun. This means rolling up sleeves or wearing shorts to increase the surface area available for UVB absorption.
• Follow the 15-Minute Rule: Limit unprotected exposure to about 15 minutes to reduce skin cancer risk, especially for fair-skinned individuals.
• Prioritize Safety: After a brief period, apply sunscreen, wear protective clothing, and seek shade to prevent sun damage.

Incorporate Vitamin D-Rich Foods

• Eat Fatty Fish: Include fatty fish like salmon, mackerel, and sardines in your diet, as they are among the best natural sources of vitamin D.
• Choose Fortified Products: Look for foods fortified with vitamin D, such as milk, certain cereals, and plant-based milk alternatives.
• Look for D3: Opt for vitamin D3 supplements, as they are generally more effective at raising blood levels than vitamin D2.
• Explore Other Sources: Incorporate cod liver oil, egg yolks, and beef liver into your meals for additional dietary intake.

Supplement When Necessary

• Consult a Professional: Speak with a healthcare provider to determine if you need a supplement and what dosage is appropriate for your individual needs.
• Don't Guess: Get your vitamin D levels checked, as a blood test is the most accurate way to assess your status and avoid over-supplementation.
• Embrace Supplements During Winter: For those living in northern latitudes where UVB is scarce during winter, supplementation is the most reliable strategy.

Understand Your Unique Needs

• Consider Your Skin Tone: Recognize that if you have a darker skin tone, you will need more sun exposure to produce the same amount of vitamin D due to higher melanin levels.
• Factor in Age: Acknowledge that the body's ability to synthesize vitamin D decreases with age, which might necessitate increased dietary intake or supplementation for older adults.

Frequently Asked Questions

Can you get vitamin D from the sun through a car window?

No, you cannot get vitamin D from the sun through a car window. Car windows, like most glass, block the UVB rays necessary for vitamin D production. While UVA rays may still penetrate, they do not help with vitamin D synthesis and can still cause skin damage.

Does being in a conservatory or sunroom produce vitamin D?

No, a conservatory or sunroom will not enable your body to produce vitamin D. The glass ceiling and walls block the UVB rays, meaning you need to step outside to get the direct sunlight required for synthesis.

Is it possible to get a sunburn through a window?

While it's unlikely you will get a traditional sunburn through standard window glass because UVB is blocked, you can still experience skin damage from the UVA rays that pass through. This can lead to skin aging and increases your risk of skin cancer over time.

How long do I need to be in the sun to get enough vitamin D?

For most people, a short, controlled exposure of about 15 minutes of direct midday sun several times a week is enough to produce sufficient vitamin D. However, this can vary based on skin tone, latitude, and time of year.

What happens if I don't get enough vitamin D?

A vitamin D deficiency can cause various health problems, including weakened bones, reduced immune function, and a higher risk of conditions like osteoporosis and osteomalacia. A balanced diet and supplements are often recommended to prevent this.

Are there any glasses that let UVB through for vitamin D?

While some specialized glasses or materials exist that allow a portion of UVB to pass, standard residential and vehicle glass effectively blocks these rays. For most practical purposes, you should not rely on any glass to get your vitamin D.

Can you get vitamin D from the sun on a cloudy day?

Yes, but it is less efficient. Thick clouds and air pollution can significantly reduce the amount of UVB light reaching the surface. You will need more time outside on a cloudy day to produce the same amount of vitamin D as on a clear day.