Can UVB Go Through Glass? And What About UVA?

Standard glass blocks nearly all UVB radiation but lets a large share of UVA pass right through. In one experimental study, smooth ordinary glass transmitted about 74% of UVA while completely blocking UVB.¹ That single fact explains a lot of everyday puzzles, from why you cannot make vitamin D sitting next to a sunny window to why truck drivers still develop sun damage on the side of their face nearest the glass. But the story gets more complicated once you consider the type of glass, whether it is laminated or tempered, whether it is tinted, and whether the material is actually glass at all.

Why Glass Stops UVB but Lets UVA Through

Ultraviolet light spans a range of wavelengths. UVB sits between roughly 280 and 315 nanometers, while UVA covers the longer stretch from 315 to 400 nanometers. Ordinary soda-lime glass, the stuff in most residential windows, absorbs wavelengths below about 320 nanometers very efficiently because of its chemical composition. That absorption zone swallows virtually all UVB. But UVA wavelengths are longer and carry less energy per photon, so they slip through the glass much the way visible light does. The result is a window that looks perfectly transparent and feels warm in the sun yet is silently filtering out one part of the UV spectrum while passing another.

Several factors shift exactly how much UV a given pane blocks. Glass type, glass color, whether there is an interlayer sandwiched between panes, and any surface coatings all play a role.² A sheet of clear float glass in a single-pane window will let through far more UVA than a double-glazed unit with a low-emissivity coating. Laminated glass, which bonds two panes around a plastic interlayer, can block UVA almost entirely. That same experimental study found that laminated glass totally blocked UVA, in stark contrast to the 74% transmission of plain smooth glass.¹

Car Windows Tell Two Different Stories

If you drive frequently, this is probably the section that matters most to you. Your windshield and your side windows are made differently, and that difference has real consequences for UV exposure.

Windshields are laminated. They consist of two layers of glass with a clear plastic interlayer, typically polyvinyl butyral (PVB), sandwiched between them. That plastic layer is there primarily to keep the glass from shattering into your lap during a collision, but it also happens to absorb most UVA radiation. Front windshields offer consistent protection against UVA for the driver’s face and eyes.³

Side windows are a different matter. They are typically single-pane tempered glass, meaning they lack that UV-absorbing plastic interlayer. The result is measurably higher UV transmission. In a study that assessed cars ranging from a 2001 Toyota Camry to a 2008 Isuzu, the highest UV readings through side windows reached 1.70 mW/cm², compared with 0.80 mW/cm² through front windshields.⁴ That gap matters because most of your daily driving UV exposure hits you from the side, not head-on through the windshield.

Side windows may or may not contain UV-blocking polymers, depending on the manufacturer and model year. Some luxury vehicles come with UV-absorbing side glass standard. Many do not. The inconsistency is the problem: unlike windshields, where lamination provides a reliable baseline of protection, side windows are a gamble.³

Can You Get a Sunburn Through Glass?

Technically, yes, though the circumstances would be unusual. UVB is the primary wavelength band responsible for sunburn, and ordinary glass blocks it effectively. But researchers studying various common materials, including glass, found that in the UVB range from 300 nanometers, most materials still transmit somewhere between 6 and 14% of the biologically relevant radiation. Calculated exposure times showed that sunburn could occur after prolonged exposure to solar radiation through these materials.⁵ The key phrase is “prolonged.” You are not going to burn sitting at your desk for an hour. But someone spending an entire day pressed against a sunny window in midsummer could, in theory, accumulate enough erythemal UV to cause mild reddening.

UVA also contributes to sunburn at high doses, though far less efficiently than UVB. Since ordinary glass transmits substantial UVA, a person spending hours in direct window light is getting a meaningful dose of the longer-wavelength UV, even as UVB is almost entirely stripped away. The practical risk of an actual burn through glass remains low for most people. But “glass blocks UV” as a blanket statement oversimplifies what is happening.

Skin Aging on One Side of Your Face

Sunburn is dramatic but relatively rare through glass. Chronic UVA exposure is the quieter and arguably more significant concern. UVA penetrates deeper into the skin than UVB and is a major driver of photoaging: the wrinkles, sagging, and uneven pigmentation that distinguish a sun-worn face from a protected one.

Researchers have studied this by comparing the left and right sides of people’s faces, using the reasoning that in countries where drivers sit on the left, the left cheek gets more window-filtered sunlight over years of commuting. The results are consistent with what you would expect. Significant differences showed up in wrinkle depth, skin roughness, and skin color heterogeneity between the sun-exposed and the shaded side of the face. Differences in skin hydration and laxity were also observed.⁶ The asymmetry points directly at UVA coming through car side windows, since UVB would have been blocked. It is one of the cleaner demonstrations that UVA alone, without UVB, causes real skin damage over time.

This finding has practical implications. Wearing sunscreen on your face for a long commute is not vanity; it addresses a well-documented source of cumulative UV exposure. Broad-spectrum sunscreen, specifically, matters here, since “broad spectrum” means it covers UVA as well as UVB.

The Tinted Window Effect

Aftermarket window tinting and factory-installed tinted glass reduce UVA transmission substantially. A field study comparing tinted and untinted cars found that the erythemal UV measured on a horizontal surface inside a tinted car was reduced by a factor of 42 compared with an untinted car. For UVA specifically, measured with a broadband meter, exposure in a car with tinted windows was about 3.8 times lower than in a similar untinted vehicle.⁷

That same study also showed that vehicle size matters. The average normalized UVA exposure inside a large family sedan was about 1.3 times higher than in a small hatchback, likely because of the larger window area and different angles of glass.⁷ If you spend hours a day driving and are concerned about skin or eye exposure, tinted side windows are one of the more effective interventions available, short of wearing sunscreen every time you get behind the wheel. Many regions have legal limits on how dark side-window tinting can be, though, so check local regulations before adding film.

Vitamin D and the Window Paradox

Your skin produces vitamin D when UVB hits a cholesterol-related compound in the outer layers of the epidermis and triggers a chemical conversion. Since glass blocks UVB, sitting in a sunny room does not help you make vitamin D. Research has confirmed that previtamin D3 production only occurs outdoors where sufficient UVB is available.⁸

This creates an ironic situation. A person who works all day next to a large, south-facing window might feel like they are getting plenty of sun. They will absorb a meaningful dose of UVA, which contributes to skin aging and potentially to DNA damage. But they will produce essentially zero vitamin D from that exposure. They get the downsides of sun without the biochemical benefit that requires UVB. For people who rely on sunlight rather than supplements for their vitamin D, this distinction matters. A 15-minute walk outside at midday does what eight hours by a window cannot.

Plastics Behave Very Differently from Glass

Not everything transparent is glass, and the substitution matters enormously for UV transmission. Many skylights, greenhouse panels, bus shelters, stadium roofs, and patio covers use plastic sheeting instead of glass, and plastics vary wildly in how much UV they let through.

Methacrylate (the material in Plexiglas-type sheets) and smoked glass both transmitted about 56% of UVB from 300 nanometers in summer measurements. Polycarbonate, another widely used transparent plastic, let through about 30%.⁵ Compare that with ordinary glass, where UVB transmission is near zero in most formulations. If you are sitting under an acrylic skylight on a sunny day, you are getting a UV exposure profile much closer to being outdoors than you would be behind a glass window. The assumption that any transparent overhead barrier “blocks UV like glass” is wrong and potentially harmful for people who are photosensitive or taking medications that increase sun sensitivity.

Polycarbonate, while it transmits less UVB than methacrylate, still lets through a third of it, which is a meaningful amount over hours of exposure. It also had markedly lower visible-light transmission, around 45% compared with methacrylate’s 95%, which means the material that looks more tinted is not necessarily blocking the wavelengths you most need to worry about.⁵ Visual darkness is a poor proxy for UV protection in plastics.

Eyes and Cataracts

UVA exposure is linked to eye problems as well as skin damage. Chronic UVA reaching the lens of the eye contributes to cataract formation over years. Since car side windows often lack the laminated interlayer that absorbs UVA, drivers accumulate more UV exposure in the eye on the side nearest the window. The finding that windshields protect the eyes consistently while side windows do not has led ophthalmologists to recommend UV-blocking side windows or sunglasses for frequent drivers.³ Sunglasses with genuine UV400 ratings block wavelengths up to 400 nanometers, covering the full UVA range. Cheap sunglasses that only darken visible light without UV filtering can actually make things worse by dilating your pupils behind dark lenses, letting in more UVA.

Indoor Plants and Greenhouse Glass

The UV-filtering properties of glass have consequences beyond human health. Plants use UV radiation as a signaling cue that affects growth form, flowering, pigment production, and even pest resistance. Behind standard glass or laminated greenhouse panels that strip out UV, plants tend to grow taller and leggier, produce fewer protective pigments, and may flower differently than they would outdoors.

Research into UV-transmitting laminated films for greenhouse use has found that allowing more UV through the glass changes plant behavior in several useful ways. Plants grow more compactly, flowering can be stimulated, concentrations of nutritionally valuable secondary metabolites increase, and flower color may improve. Pollination by bees is also better when UV is present, since bees use UV patterns on petals to locate flowers. Plant resilience to pests and diseases improves as well.⁹ For indoor gardeners frustrated by leggy houseplants near a window, the issue may be less about total light quantity and more about the missing UV component that glass strips away.

The 1920s Attempt to Fix the Problem

The idea that ordinary glass blocks beneficial UV is not new. In the mid-1920s, a physiologist, a glass technologist, and the London Zoo collaborated on a product called Vitaglass, specifically designed to transmit ultraviolet radiation into buildings. The pitch was ambitious: Vitaglass would “let health into the building” in an era when sunlight therapy was a mainstream medical treatment for rickets and tuberculosis. Its modified chemistry allowed UV wavelengths that soda-lime glass blocked to pass through.¹⁰

Vitaglass never became standard. Vitamin D supplementation in food proved simpler and cheaper than rebuilding every window, and antibiotics replaced sunlight therapy for tuberculosis. But the episode illustrates that the UV-blocking nature of glass has been recognized for over a century, and that people have tried, repeatedly, to engineer around it. Today’s low-e coatings and laminated interlayers represent the opposite engineering goal: blocking even more UV, not less. The conversation has flipped from “how do we let UV in” to “how do we keep UVA out,” reflecting a modern understanding that the risks of chronic UVA exposure generally outweigh the lost benefit of indoor UVB.

What This Means for Everyday Decisions

If you work near a window or drive frequently, you are getting essentially no UVB through glass, which means no vitamin D production and minimal sunburn risk. You are, however, getting a potentially significant dose of UVA, especially through untreated single-pane windows and tempered car side glass. Over months and years, that UVA adds up in ways that show up as skin aging and may contribute to eye damage.

A few practical responses make a difference. Broad-spectrum sunscreen on your face and hands addresses cumulative UVA from driving and desk work near windows. Sunglasses with UV400 protection handle the eye exposure. If you are shopping for a car and spend a lot of time driving, checking whether the side windows have UV-absorbing properties is worth the effort. For home windows, modern double-glazed and low-e coated glass blocks far more UVA than the single-pane windows common in older buildings. And if you are sitting under a skylight or patio cover made of acrylic or polycarbonate rather than glass, treat it more like being outdoors from a UV standpoint. The material overhead may be transparent and look protective, but it is letting through UV wavelengths that glass would have stopped.

1
PubMed. The role of glass as a barrier against the transmission of ultraviolet radiation: an experimental study
2
PubMed. Photoprotection by window glass, automobile glass, and sunglasses
3
JAMA Ophthalmology. Assessment of Levels of Ultraviolet A Light Protection in Automobile Windshields and Side Windows
4
PubMed Central. Assessment of ultraviolet and infrared radiation transmission through automobile windshields and side windows
5
PubMed. Spectral transmission of solar radiation by plastic and glass materials
6
PubMed Central. Assessment of cumulative exposure to UVA through the study of asymmetrical facial skin aging
7
Physics in Medicine & Biology. Ultraviolet radiation penetrating vehicle glass: a field based comparative study
8
PubMed. Increased UVA exposures and decreased cutaneous Vitamin D(3) levels may be responsible for the increasing incidence of melanoma
9
Challenging Glass. UV Transmission in Laminated glass: Effects on Plant Growth and Development
10
Architectural Research Quarterly. The healthful ambience of Vitaglass: light, glass and the curative environment