Safe inside?

How much UV gets through your windows?

Now for something lighter than last time …

Some commonly-used window glasses transmit somewhat into the UVB region. But by how much? Is it important?

The view below is through our lounge windows. It looks a lot brighter outside than in. That’s partly because of the double-glazing and tinted glass. The indoor/outdoor contrast is much larger at UV wavelengths. There’s no risk of skin damage in the situation shown.

While indoors - behind windows - you’re exposed to only a small fraction of the sky, so the amount of UV you receive is already reduced by that fraction. And at each glass-air interface, there are reflection losses of a few percent (and smaller losses at other interfaces). So, with double-glazing and other window-coatings or tints that further block the amount of UV, you’re usually pretty safe from skin damage. But not always.

A few years ago, I was asked to evaluate the transmission of a coating applied to windows to prevent UV damage to carpets and furniture.

Results of those tests are shown below. The blue curve is the unobstructed spectrum of sunlight up to wavelength 450 nm (blue light). The red and black curves are spectra taken immediately after through a sample of uncoated glass (red), and a sample of coated glass (black).

The transmissions shown in the lower panel, are just the ratios of the red and black curves to the blue curve, multiplied by 100 to convert to a percentage.

As you can see, as you go to shorter and shorter wavelengths, the uncoated glass blocks more and more of the UV. Its transmission is about 50 percent at 330 nm, reducing to about 5 percent at 315 nm (the boundary between UVB and UVA). And at all wavelengths less than 300 nm it’s zero.

If skin-damaging radiation were confined to just UVB wavelengths, the transmission of less than 5 percent in that region means that the Skin Protection Factor (SPF) of the glass would be greater than 20.

But, because skin-damaging UV includes some UVA radiation, it turns out to be closer to SPF10, meaning that sunburning UV has been reduced by a factor of 10 (because 90 percent of it has been absorbed).

The reduction in our our ability to produce vitamin D - where shorter wavelengths are involved - is even larger. Only 1percent of that radiation gets through the window, meaning 99 percent has been absorbed.

And only a fraction of the available radiation from the sun even reaches the window. Their vertical orientation, with roof and wall obstructions makes sure of that.

I’ve tested the UV transmission of several other glasses. None of those transmit more UV than this one, and for several its much less. My client had no-doubt supplied me with a worst-case example to highlight the value of his product. Notice that I didn’t name the product being tested (I might if he pays me 😊). But it was very good. As you can see in the plot, it blocked virtually all the UV radiation at wavelengths less than about 390 nm.

Whatever glass you have in your windows, while the UVA may brown your skin slightly after long indoor exposures, little if any vitamin D will be made. You need to be outdoors for that. Usually for only a few minutes per day.

If any of your loved ones are confined to indoor living, make sure they’re receiving vitamin D supplements.