Does glass block UVA? Windows, cars and UV film

The short answer: glass blocks UVB almost completely, and UVA only partly. That single asymmetry explains why you can sit in a sunny window or drive for hours without ever burning, and still be accumulating the exposure that drives photoaging, pigmentation and long-term skin damage. It is probably the most counter-intuitive fact about ultraviolet light, and it is why a sunburn-weighted UV Index tells you so little about indoor and in-car exposure.

Why glass treats UVA and UVB so differently

Ordinary window glass is soda-lime glass, and its transmission does not fall off gradually across the ultraviolet — it has a fairly sharp cut-off. Iron oxide impurities and the silica network itself absorb strongly below roughly 320 nm, then become progressively more transparent above it. Since UVB is 280–315 nm and UVA is 315–400 nm, that cut-off lands almost exactly on the boundary between the two bands.

The consequence is that the cut-off clips the whole of UVB and only the very bottom edge of UVA. The longer wavelengths — UVA-I (340–400 nm), which penetrate skin most deeply — pass through with the least resistance of all. Glass is not a neutral filter that dims UV evenly; it is a filter that removes the band that warns you and keeps the band that ages you.

How much gets through: glass by glass

Two things are worth noticing in that table. First, the range for ordinary clear glass is wide because transmission depends on thickness, iron content and age. Second, the range for tempered car glass is enormous — which is the subject of the next section.

Cars: the windshield is not the problem, the side window is

Car glass is the clearest real-world case of this asymmetry, because the two kinds of glass in a car are built differently and perform differently.

• Windshields are laminated. Two panes of glass are bonded around a polyvinyl butyral (PVB) interlayer, a plastic film whose original job is to hold the glass together in a crash. That interlayer happens to be an excellent UVA absorber. A windshield typically blocks 96–99% of UVA — it is, incidentally, one of the best UV filters most people own.
• Side and rear windows are tempered. Tempered glass is heat-treated for strength so it crumbles rather than shards, and it has no interlayer. Without one, it is just clear glass. A widely cited survey of 29 vehicles found side-window UVA blocking ranging from about 44% to 96%, averaging around 71% — against 96% for the windshields in the same cars. More recent measurements on newer model years are better, averaging near 89% for side glass, but the spread between vehicles remains large.

The evidence written on drivers' faces

If side-window UVA were negligible, drivers' skin would not record it. It does.

In countries that drive on the right, skin cancers are consistently more common on the left side of the body — the side by the window. A US series found the majority of unilateral facial skin cancers occurred on the left; the pattern reverses in left-hand-drive countries such as the UK and Australia, which is exactly what you would expect if the cause is the driver's window rather than anything biological about left and right. Measurements support the mechanism: the left side of the head, neck, arm and hand can receive several times the UV dose of the right during a commute.

The most striking single illustration is a widely reproduced clinical photograph of a US delivery driver with 28 years on the road, whose left face shows deep unilateral dermatoheliosis — thickened, furrowed, sagging skin — beside a right side that looks decades younger. He had, in effect, run a controlled experiment on himself, and the variable was a tempered side window.

Offices and homes: the desk-by-the-window problem

The same physics applies indoors, with a different exposure pattern: lower intensity, but many more hours. If you sit near a large window for six hours a day, five days a week, the cumulative UVA dose is not trivial — and it lands repeatedly on the same side of your face, just as it does in a car.

Three points make this less alarming than it sounds, and one makes it worse:

• Indoor UVA falls off quickly with distance from the glass and with the angle of the window to the sun. A desk facing away from a north-facing window (in the northern hemisphere) receives very little.
• Modern commercial glazing is often tinted, coated or laminated, and blocks considerably more UVA than the old clear float glass in the table above.
• Neither indoor lighting nor your screen contributes meaningfully — see UVA indoors for why the "blue light is UV" claim is wrong.
• But: people almost never wear sunscreen indoors, so whatever does come through is landing on unprotected skin, every working day, for years.

Choosing UV film: what the numbers mean

UV-blocking film is a clear polyester layer with UV absorbers, applied to the inside of the glass. It is the single most effective fix, and it does not need to be dark to work — UV rejection and visible tint are independent properties. A completely clear film can block over 99% of UVA, which matters because most jurisdictions legally restrict how dark a driver's-side window may be.

The one specification to scrutinise is the cut-off wavelength. A film quoted as "99% UV" measured to 380 nm says nothing about the 380–400 nm slice, which is real UVA-I and the most deeply penetrating part of the band. Films and lenses specified to 400 nm close that gap — the same logic behind the UV400 standard for sunglasses.

Estimating your exposure behind glass

Because glass simply scales UVA down by a roughly fixed fraction, you can get a usable behind-glass estimate from an outdoor figure. Take the UVA Index for your location and time, then multiply by the transmission of the glass you are sitting behind:

This is a back-of-envelope adjustment, not a measurement. The UVA Index on this site models UVA on an open horizontal surface outdoors — glass, window angle, distance and shading are explicitly not modelled. But it makes the point quantitatively: at a high outdoor UVA Index, an ordinary window still leaves you in the moderate range, indefinitely, with no sunburn to tell you.

What to actually do

• Do not treat "indoors" or "in the car" as protected. It is protected from burning, not from UVA.
• Film the side windows if you drive a lot, and the windows you sit beside if you work near glass. Clear, legal, ~99% films exist.
• Wear sunscreen on the drive, not just at the beach — especially on the left hand, forearm and side of the face (right, in left-hand-drive countries). Look for real UVA protection, not just SPF: see how to read UVA labels.
• If you are photosensitive — from medication or a condition — glass is not a shelter. That article covers it in detail.
• Check the outdoor UVA Index first. If it is low outside, it is lower still behind glass. Scaling a real number beats guessing.