Can you get a suntan and absorb vitamin D through a window?

The sun emits three types of ultraviolet rays - but ordinary glass only blocks one

As the days get gradually longer and the feeling of sunlight on your skin becomes familiar once again, you might be tempted to roll up your shirt sleeves and bask in the rays - even if they're only coming through a window. 

It is common knowledge that being exposed to the sunlight for too long can expose you to dangerous UV rays that can cause skin cancer. But not getting enough can cause vitamin D deficiency, which can lead to bones becoming soft and weak.

So, if the sun peeks through your window on a wintry day as you sit at your desk or wash up in the kitchen, does this top up your Vitamin D and tan your skin? 

The answer is no - and yes.

The sun emits ultraviolet (UV) A, UVB, and UVC rays. The latter is the most damaging, but is filtered by the atmosphere and doesn’t reach the earth’s surface.

Vitamin D is produced by the body when the skin is hit by UVB rays, with 7-dehydrocholesterol being the resulting chemical. But most glass windows block this wavelength meaning you can't reap the health benefits. 

However, sunlight streaming through a window can still expose the skin to harmful rays which tan and freckle the skin, as ordinary glass does not filter out UVA rays.

Dr Anthony Mancini, head of pediatric dermatology at Children's Memorial Hospital in Chicago, told CNN that people should protect themselves if they are going to be exposed to sunlight through windows – for example in a car. 

A 2010 US study into skin cancer found that people who spend a lot of time driving a car each week are more likely to develop skin cancers on the left side of the body – which is the window side in North American cars.

Data on 1,000 patients showed that 74 per cent of patients with malignant melanoma, an aggressive form of skin cancer, had tumours on their left sides, according to the study by researchers at the St Louis University, the New York Times reported.

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