Can You Tan Through a Window?

Sunshine is good for the mood and good for health (vitamin D) and bright sunny interiors are desirable spaces. Does anything feel better than being curled up in a chair next to a large window with the sun pouring in whilst you read and drink a coffee?

Sitting in a conservatory allows you to benefit from the winter sun even when it’s cool outside and to get a dose of sunshine all year round. The solar gain from direct sunshine can also heat a room and help with energy efficiency in your home.

But can you get a tan through the glass, or can you get sunburn? Should we wear sunscreen if we have a desk next to a large window?

How UVA and UVB Rays Affect the Skin

The sun naturally emits ultraviolet radiation (UV), consisting of UVA and UVB rays. It’s not perceptible as visible light but can be felt by the skin.

Ultraviolet A (UVA) has a longer wavelength of 320-400 nanometers and is known for skin ageing. UVA can penetrate through glass and cloud cover.

UVA is a more consistent radiation than UVB and is prevalent everywhere. UVA is not as intense as UVB, but it penetrates more deeply into the skin. Used in sunbeds, it was thought to be a safer UV but is now understood to cause skin cancer by damaging the innermost part of the top layer of skin.

Ultraviolet B (UVB) has a shorter wavelength of 280-320 nanometres and is known for skin burning. UVB can be filtered by cloud cover and glass.

UVB damages the outer layer of the skin. Whilst it causes a sun tan, it also causes sunburn. UVB intensity can fluctuate year-round and is more intense at high altitudes.

Both UVA and UVB are proven to contribute to the risk of skin cancer, but UVA has a higher incidence of malignant tumours. They can both cause ageing of the skin and eye damage.

UV damage doesn’t happen quickly, it builds up over time. Skin does repair, but it can’t repair all damage and the more that builds the more the risk of skin cancer builds.

How a Tan Develops

When the body is exposed to UV light, melanin forms in the skin as a defensive mechanism to absorb the radiation and protect the skin from DNA damage.

• UVA oxidizes melanin already present in the skin and produces a tan quickly.
• UVB stimulates the body to produce melanin and takes 2 days to produce a tan.

Melanin is the body’s reaction to protect the skin but is prized for its colour as a tan.

Does Glass Block UV Radiation?

Standard glass (without coating) filters out almost all UVB rays, but between 50% and 63% of UVA longer-wave rays can pass through, depending on the glass thickness and type of window.

Laminated glass reduces UVA transmission and is often used in windows to reduce UV bleaching on furniture and furnishings in south-facing rooms.

Car windscreens also have laminated glass for safety reasons to hold the glass together if it breaks whilst driving. This blocks the majority of UVA and UVB rays. The side windows are not made from laminated glass and they do allow UVA light to pass through. How much depends on the type of glass, but similar to standard glass.

It has been reported that in countries with a high sunshine rate, people who spend excessive time driving can develop excessive sun damage and skin cancer on the side of their body that faces the window.

Can You Absorb Vitamin D Through Glass Windows?

Vitamin D is synthesized by the skin through exposure to UV and only UVB can stimulate this process.

Because 97% of UVB rays are blocked by glass, your body can’t synthesize vitamin D from sunlight that is filtered through the glass.

Vitamin D is essential for good health, but many people are deficient because they don’t spend enough time outdoors in daylight. Most people would benefit from a supplement, especially in winter to keep their levels topped up.

Can You Get a Tan Through a Window?

Yes, you can get a tan through a window with standard glass because it allows UVA rays to pass through. However, it would take a long time, so don’t expect a glowing tan from sitting in a conservatory for a few hours.

The most important thing to watch out for when sitting in the direct sun through a window is the risk of sunburn or skin cancer. This would only develop after an excessive time sitting in direct sun over a prolonged period. However, if you are sitting in a bright conservatory every day, then we recommend you take the same precautions as sitting in the garden and applying sunscreen.