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According to ThoughtCo (and many other sites):

Glass that is transparent to visible light absorbs nearly all UVB. This is the wavelength range that can cause a sunburn, so it's true you can't get a sunburn through glass.

However, UVA is much closer to the visible spectrum than UVB. About 75% of UVA passes through ordinary glass. UVA leads to skin damage and genetic mutations that can lead to cancer. Glass does not protect you from skin damage from the sun. It affects indoor plants too. Have you ever taken an indoor plant outside and burned its leaves? This happens because the plant was unaccustomed to the higher levels of UVA found outside, compared with inside a sunny window.

For reference, the definition of UV-A and UV-B is:

The UV region covers the wavelength range 100-400 nm and is divided into three bands:

UVA (315-400 nm) UVB (280-315 nm) UVC (100-280 nm)

My skepticism comes from the fact that no one seems to reference a primary source to glass being transparent to UV-A rays (or non-transparent to UV-B rays, for that matter). Is there a lab that actually tested different types of glass/plastic to determine how much UV light passes through each or is this all based on "common sense" without serious measurements to back it up?

NB: Is it possible to get a sunburn if you're behind a (glass) window? addresses a similar question but I don't think its a duplicate per se.

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Summary:

As a general rule, window glass blocks UVB while passing UVA.

If the amount of UV really matters, then you need to look up the appropriate chart for the type of glass you really have.

The UV transmission of glass has been tested in many ways over many years. People have the results of those tests in mind when they speak about UV and glass. It has become common knowledge, to the point that no one feels it necessary to quote a source.

On the other hand, it is good to double check such things.

This article from 1934 used window glass as one filter in a series of experiments on changes in the UV transmission of glass after exposure to sunlight through various filters. Ordinary window glass was used as one of the filters.

Here's the chart of the filter functions for the various filters used:

enter image description here

Note that window glass has next to zero trasmission at 315 nm - that's where UVA ends and UVB begins.

From that, it is clear that ordinary window glass passes UVA while blocking UVB, just as everyone always says.

Glass isn't just glass, though.

I found this test of UV transmission for various materials that are used as or in place of glass.

It also shows glass (smoked glass, in this case) blocking UVB but not UVA.

It contains this chart showing transmission rates:

enter image description here

The upper right corner has the UV data

Polycarbonate (more commonly known as Lexan) blocks nearly all UVA and UVB.

Methacrylate (more commonly known as Plexiglas) blocks all UVB and some UVA.

The 1934 paper also contains these charts:

enter image description here

enter image description here

Those show UV transmission for two different types of glass. Note that both pass more UVB than plain "window glass."

As a general rule, window glass blocks UVB while passing UVA.

If the amount of UV really matters, then you need to look up the appropriate chart for the type of glass you really have.

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  • What is window glass? It isn't hard to imagine that the chemical composition and/or microstructure of glass commonly used for windows changed in the ninety years since that 1932 paper you quote. As an anecdotal data point, I've been using photochromic (transition) glasses for the past seven plus years and they darkened inside a building once, in a in a radiology lab.
    – jaskij
    Mar 19, 2021 at 11:49
  • @JanDorniak: That's why I put information from two sources in there. They show that it varies by the exact type of glass. That's why I suggested looking up the exact sort of glass if it is critical. But, common glass types then and now more or less fit the "pass UVA, block UVB" description.
    – JRE
    Mar 19, 2021 at 11:52
  • True. Also, if it really is critical, presumably whatever product is (to be) bought will come with a datasheet specifying this.
    – jaskij
    Mar 19, 2021 at 11:58
  • JRE, it's important to make clear whether the Ordinate on many of these plots reflect a property of only the material (actually often the front surface), or of the actual object. This would make clear whether the data includes the effect of the object's thickness, an important unknown. When the units involve "per unit length" of material, the term "coefficient" is often used. Common window glass is made from "soda lime glass," obtained by floating molten glass on liquid Tin, which is inexpensive and has much different transmissibility from other glass, such as borosilicate.
    – ttonon
    Mar 20, 2021 at 12:38

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