Why you can get sunburnt through a window but it won't help with your vitamin D needs

Sun protection shouldn't stop when you go indoors. (Pexels: Tiana)

A warm spot by a sunny window can be the ideal place to ride out a winter cold snap.

But exposure to sunlight through a window has implications for your health. Here's what you need to know.

First, how does sunlight affect the skin? Damage and vitamin D

Sunlight is the main source of ultraviolet (UV) radiation. Made up of UVA and UVB rays, UV radiation penetrates the skin and can cause sunburn, skin cancer, skin ageing and eye damage.

Professor Anne Cust, chair of Cancer Council's National Skin Cancer Committee, says UV radiation is often confused with infrared radiation, which we feel as heat from the sun.

"The temperature does not affect UV levels, and in fact, the UV can be as high on a cool day as it is on a hot one."

Both longer-wavelength UVA and shorter-wavelength UVB cause sunburn and skin cancer.

UVA is primarily responsible for photo-ageing, penetrating deeper into the dermis, breaking down collagen and elastin and causing cellular damage.

"UVB is a shorter wavelength than UVA, so it penetrates the skin more superficially," dermatologist Leona Yip says.

Our bodies also need UVB rays for vitamin D synthesis, which occurs in the skin's epidermis and is essential for calcium absorption and bone health.

Can you get sunburnt through a window?

Yes — although it depends on the type of window glass.

"While all types of commercial and automobile glass block the majority of UVB (vitamin D-generating) radiation, the degree of UVA radiation transmission depends on the type of glass," Professor Cust says.

"Laminated glass used on car windscreens provides better UV protection than tempered glass, blocking 98 per cent of UVA radiation compared to 79 per cent. Similarly, laminated glass used in buildings blocks UVA radiation completely, while tempered building glass can allow around 70 per cent of UVA transmissions.

"While untinted glass can reduce the transmission of UV radiation, it does not completely block it, which means you can still get sunburnt if you spend a lot of time next to an untinted window when the UV [Index] is three or above" (which it regularly is in Australia).

Cancer Council recommends people who spend long periods in a vehicle or close to a window use sun protection, such as a long-sleeved shirt, sunglasses and an SPF 30 or higher sunscreen.

Dr Yip recommends applying sunscreen every morning, even if you spend most of the day indoors.

"For people who work from home … sitting a few metres away from a glass window or a glass door, you still get UVA penetrating through it," she says.

"Or you might go out and get the mail, or go out and walk your dog, or hang out the washing — it might be 10 or 15 minutes, but for many fair-skinned people, 10 to 15 minutes is all you need to start getting a sunburn."

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

The short answer is no.

Glass blocks 95 per cent of UVB rays, which means sun exposure through a window won't trigger vitamin D production.

This matters for your health. Vitamin D is a pro-hormone — a molecule the body turns into a hormone required for calcium absorption.

Vitamin D deficiency can cause rickets, osteoporosis and osteomalacia and affects more than one in three Australian adults.

So, what is a safe way to get enough sun to meet our vitamin D needs?

While small amounts of vitamin D can be found in foods such as oily fish and eggs, the best source is sunlight.

How much time you need in the sun to generate vitamin D depends on several factors, including your skin colour, what you're wearing, where you live, the time of year and the UV index.

"When the UV is three or above, which is almost every day in summer, most Australians maintain adequate vitamin D levels just by spending a few minutes outdoors on most days of the week," Professor Cust says.

"In the cooler months in some southern parts of Australia, when the UV drops below three, it is safe to go outside without sun protection early in the morning and later afternoon, however sun protection is recommended once the UV level reaches three or above."

Dr Yip says a supplement is an option for people with a deficiency in vitamin D, particularly those with very fair skin or a family history of skin cancer.

But for most healthy adults, according to a 2019 review published in the Medical Journal of Australia, they are of little benefit.