What Stays True About Tanning Through a Window

Yes, you can get a tan through a window, because ordinary soda-lime glass stops the UVB that reddens skin and still transmits the UVA that darkens pigment. That split is what stays true in a home office, on a balcony, and in a parked car, even while the actual dose changes with the glazing, the UV Index outside, minutes in the sun patch, and whether a film or a laminated interlayer is in the way.

I learned to distrust a single photograph when I was still printing mouth images at a pharmacy counter. A cheek bite on Tuesday looks like a different animal on Friday if you have a neutral color card and a date on the back. A window tan is the same kind of record: the left forearm of a U.S. driver compared with the right. I will not name the lesion. I will date the print.

What rays actually pass through ordinary window glass?

Ultraviolet that reaches the ground arrives as UVB (about 280 to 315 nm) and UVA (about 315 to 400 nm). The National Glass Association's technical bulletin FB34-12 (2023) states the split in one sentence: standard clear glass typically screens out nearly all UVB and does little to screen UVA. The Skin Cancer Foundation's UV window-film page says the same in public language: window glass filters most UVB; UVA wavelengths pass through windows and clouds year-round.

Vitro Architectural Glass's Clear Technical Product Data (April 2020) reports solar performance by ISO 9050:2003. For 3 mm (1/8 in) clear float, ultraviolet transmittance in the 300 to 380 nm band is 73 percent. At 4 mm it is 71 percent. At 6 mm (1/4 in) it is 66 percent. Spectral transmittance on that 6 mm sheet is 0.0 percent at 300 nm and 0.7 percent at 315 nm, the UVB edge. At 340 nm it is 51.5 percent. At 360 nm it is 81.7 percent. At 380 nm it is 82.4 percent. UVB is gone. Long-wave UVA is almost as free as daylight.

ISO 9050:2003 computes Tuv from 300 to 380 nm, the figure glass plants print. ISO 24443:2021 defines UVA-PF for a cream, weighted to persistent pigment darkening from 320 to 400 nm. For a pane, the crude UVA protection factor is 1 divided by UVA transmittance. Cardinal CG's Technical Service Bulletin CG10 (September 2025) lists 73 percent UV transmission for 3 mm monolithic clear, a factor of about 1.4. Dual-pane clear over clear is 58 percent, a factor of about 1.7. A LoĒ³-366 over clear insulating unit is 5 percent, a factor of 20. Cardinal's 7.8L laminate (3 mm / 0.060 in polyvinyl butyral / 3 mm) is less than 1 percent, a factor above 100. Coatings and the interlayer move the number. Thickness of uncoated clear, 3 mm to 6 mm on the Vitro sheet, only walks Tuv from 73 percent down to 66 percent. Tuchinda, Srivannaboon, and Lim (Journal of the American Academy of Dermatology, 2006) published Guardian figures of 72 percent Tuv for 3 mm clear and 0.6 percent for 6 mm laminate.

How does a home window differ from a car windshield and a car side window?

A house window is usually annealed or tempered float in an insulating unit. A U.S. windshield is laminated safety glass because Federal Motor Vehicle Safety Standard No. 205 (49 CFR 571.205) incorporates ANSI/SAE Z26.1-1996: AS-1 laminated glazing in the windshield, AS-2 (commonly tempered) in the side and rear. The polyvinyl butyral interlayer holds shards in a crash and absorbs UVA. Tempering a side lite does not add that interlayer.

Boxer Wachler measured 29 automobiles from 15 manufacturers, model years 1990 to 2014, at Los Angeles dealerships, and published the result in JAMA Ophthalmology in 2016. Average front-windshield UVA blockage was 96 percent (range 95 to 98 percent). Average driver-side blockage was 71 percent (range 44 to 96 percent). Only 4 of 29 cars (13.8 percent) blocked more than 90 percent of UVA at the side.

Bernstein, using an ELSEC UV monitor (300 to 400 nm), is the other field pair Almutawa and colleagues cite in their 2013 Photodermatology, Photoimmunology & Photomedicine review. The windshield blocked 98 percent of the measured UV. The side window blocked 21 percent. The side window plus film blocked 99.6 percent. Hampton, Farr, Diffey, and Lloyd, in the British Journal of Dermatology in 2004, found UK nonlaminated clear transmitting 62.8 percent of UVA and grey laminated transmitting 0.9 percent.

The Skin Cancer Foundation's driving page rounds this into a public figure: laminated windshields can block over 90 percent of UVA; untreated tempered side glass can let up to 50 percent through. That 50 percent is a ceiling. Boxer Wachler's mean side transmission is 29 percent. Bernstein's side sample transmitted 79 percent. Different meters, different glass. Same split.

| Glazing | Construction | UVB | UVA remaining | Source | | --- | --- | --- | --- | --- | | Home, 3 mm clear | Annealed float | ~0% (0.7% at 315 nm on 6 mm Vitro) | 73% Tuv | Vitro TDS, ISO 9050:2003 | | Home, dual-pane clear | Two 3 mm lites | ~0% | 58% Tuv | Cardinal CG10, 2025 | | Home, LoĒ³-366 IGU | Coated dual pane | ~0% | 5% Tuv | Cardinal CG10 | | Home, 7.8L laminate | 3 mm / 0.060 in PVB / 3 mm | ~0% | <1% Tuv | Cardinal CG10 | | Car windshield | Laminated AS-1 (FMVSS 205) | ~0% | 2–5% transmitted | Boxer Wachler; Bernstein | | Car side or rear | Tempered AS-2 | ~0% | 4–56% transmitted (mean 29%) | Boxer Wachler 2016 | | Side glass plus UV film | Tempered + polyester | ~0% | ≤1% (99%+ rejected) | SCF Seal; Bernstein 99.6% |

A driver facing the windshield sits behind a laminate. A driver with a left elbow on the door sits behind tempered glass. An office worker at a 1990s clear dual pane sits closer to Cardinal's 58 percent than to either car row. A later low-e unit can sit at 5 percent Tuv without anyone noticing.

How long does it take to tan through a window?

There is no single clock. Tanning through glass is a UVA dose: transmittance times the UVA in the beam times minutes of skin in that beam. UVB, the waveband that sets outdoor "minutes to burn" charts, is already gone.

Hampton and colleagues timed one medical threshold, not a cosmetic one. A dose of 5 J/cm² of UVA, enough to trigger polymorphic light eruption in some of their patients, came through clear nonlaminated vehicle glass in 30 minutes. The same 5 J/cm² through grey laminated glass would have taken 50 hours.

What does change the minutes is the UV Index outside the pane. The U.S. Environmental Protection Agency's UV Index scale, which follows World Health Organization reporting, runs from 1–2 (low) through 3–5 (moderate), 6–7 (high), 8–10 (very high), and 11+ (extreme). EPA tells people to use broad-spectrum SPF 15 or higher outdoors from moderate upward. NOAA's Climate Prediction Center chart for Phoenix, Arizona, in 2025 shows summer clear-sky peaks near 11 to 12 and 39 extreme forecast days. The same CPC product for New York City in 2025 shows summer peaks near 9 to 10.5, zero extreme days, and 65 very-high forecast days. A Phoenix office in July and a Manhattan office in March are not the same indoor UVA, even on the same 3 mm clear glass.

UVI itself is erythema-weighted. Because glass has already removed the UVB that dominates that weighting, indoor UVI looks low even while indoor UVA remains a large fraction of outdoor UVA. McKenzie, writing on window transmission, put the erythemal protection of ordinary glass near SPF 10: sunburning UV reduced by about a factor of ten. The tan, a UVA story, is not reduced by that same factor. A desk by a window can brown a forearm over a season without the burn a person would have noticed outdoors in 15 minutes.

Gordon and Brieva's 2012 New England Journal of Medicine image is the dated sequence I would pin on a board. A 69-year-old man who had driven a delivery truck for 28 years presented with a 25-year history of thickening and wrinkling on the left side of his face, toward the window. The right side had been shaded by the cab. The clinicians called it unilateral dermatoheliosis.

Does a window screen or balcony glass change the tan?

A standard U.S. insect screen is an 18×16 mesh with about 70 percent open area, the figure U.S. Patent 7,201,208 gives for ordinary residential screening. The mesh is a geometric shade, not a UV absorber. If the glass behind it still transmits 73 percent of the ISO 9050 ultraviolet band, the screen leaves on the order of 0.70 × 73 percent, about 50 percent, of that outdoor UV. A tan through a window screen is still a tan through the glass. The screen only knocks the open fraction.

Balcony glass is the other mix-up. A laminated guard panel is a cousin of the windshield: polyvinyl butyral, Tuv under 1 percent on Cardinal's 7.8L line. A tempered sliding door onto that balcony is a cousin of the car side window or the home insulating unit, depending on coatings. If you cannot see an interlayer at the edge of the lite, do not treat the rail and the door as one system.

What does distance from the glass change, and what does it not?

Sunlight that has already passed the pane is a beam. The sun is about 150 million kilometers away. Inverse-square falloff does not apply between your forearm and a window 40 centimeters farther back if both positions still sit inside that rectangle of sun. Keep the skin in the bright patch at 20 centimeters or at 150 centimeters from the glass, and the UVA in the beam is the same to first order.

Step out of the patch and the dose collapses to the diffuse remainder: sky seen through the aperture, plus bounce off the floor. Zhou and colleagues, in Indoor Air in 2021, measured indoor ultraviolet from 330 to 400 nm and found reflected UV decaying exponentially with distance from sunlit surfaces (about −2 m⁻¹ off the floor). They modeled photochemistry at 10 centimeters and at 1 meter from the window center because those two stations are not the same sky view. Is the skin inside the bright rectangle, or only in the room's general daylight? Distance matters for the second. It barely matters for the first.

A driver is almost always in the patch. The left forearm sits centimeters from tempered glass. An office worker can leave the patch by rolling the chair one meter.

What SPF and window film numbers actually move the UVA dose?

Sunscreen SPF, as the U.S. Food and Drug Administration labels it, is an erythema test. Broad-spectrum SPF 15 or higher is the FDA floor for a product that may claim to help protect against skin cancer and early skin aging, not only against sunburn. The agency tells people to apply about one ounce to the body and to reapply at least every two hours. The Skin Cancer Foundation's driving note is stricter on the road: at least SPF 30 for short drives, SPF 50 or higher for longer exposures, on the skin the side window still sees. SPF is not a UVA-PF. ISO 24443 exists because a high-SPF cream can still have a modest UVA-PF; the standard's own example is SPF 50 with UVA-PF 3 to 4. Behind glass, where UVB is already gone, a broad-spectrum cream is doing UVA work. A UVB-only product is doing almost nothing.

Window film is the construction change that moves the pane's own factor. The Skin Cancer Foundation puts its Seal of Recommendation on a film only when the maker's data show it blocks 99 percent or more of solar UVA and UVB. Clear film is enough; darkness is not a UV rating. Almutawa's 2013 review cites two monitor studies in which film on tempered side glass cut UV transmission by more than 99 percent. Bernstein's side-plus-film figure was 99.6 percent. A 99 percent rejection is a UVA protection factor near 100 on the crude 1/T scale, in the same band as Cardinal's laminate, without replacing the lite.

I prefer a dated sequence of the same forearm to one dramatic close-up. The NEJM photograph is real. It is also one man, 28 years, and a window that was often open. The numbers that stay true without that photograph are shorter: ordinary clear glass, 66 to 73 percent Tuv; a U.S. windshield, about 2 to 5 percent UVA transmitted; a typical side window, tens of percent; a Seal-rated film, 1 percent or less. If the skin is in the patch, those four figures decide the tan.

Frequently asked questions

Can you tan through a car window?

Yes, through the side and rear glass, which are usually tempered and still transmit a large share of UVA. Boxer Wachler found mean side-window UVA blockage of 71 percent (29 percent still entering) across 29 cars. The laminated windshield blocked 96 percent on average. A tan through a car window is a side-window problem in left-hand drive.

Can you tan through a window screen?

Yes. A common 18×16 insect screen has about 70 percent open area, so it shades by geometry rather than by absorbing UVA. The glass behind it still sets the spectrum: UVB out, UVA in. The screen trims the open fraction. It does not turn a 73 percent Tuv pane into a laminate.

Can you tan through glass on a balcony?

It depends which lite. Laminated balcony guards behave like windshields, with Cardinal 7.8L Tuv under 1 percent. A tempered sliding door onto the balcony behaves like home insulating glass or car side glass. Check the edge for an interlayer before treating the rail and the door as one pane.

How long does it take to tan through a window?

There is no fixed number of minutes. Hampton timed 5 J/cm² of UVA through clear nonlaminated vehicle glass at 30 minutes, a photosensitivity dose. UV Index, glazing Tuv, and minutes in the sun patch set the clock. Phoenix summer UVI near 11 is a different indoor UVA than a New York winter day at UVI 1 to 2.

Do UV rays go through window glass?

UVA does; UVB almost does not. Vitro's 6 mm clear sheet transmits 0.7 percent at 315 nm and 81.7 percent at 360 nm. ISO 9050 Tuv for 3 mm clear is 73 percent. Laminated interlayers and Seal-rated films are the constructions that stop UVA as well.

Can you get sunburned through a window?

A burn is unlikely through closed ordinary glass because UVB is gone and erythemal protection sits near SPF 10 in McKenzie's reading of the spectrum. UVA can still darken pigment and age skin. Open the sash, or sit in a sunroof, and UVB returns.

Should I wear sunscreen at my office desk?

Wear a broad-spectrum SPF 30 on the skin that sits in the sun patch, the same rule the Skin Cancer Foundation writes for short drives, if that patch is a daily station. FDA's cancer-and-aging claim starts at broad-spectrum SPF 15. Rolling the chair out of the rectangle of sun does more than a cream on a shaded wrist.