Do I Need Sunscreen in the Car or Indoors by a Window?

The short version: it depends on the glass and the time

Sunlight carries two kinds of ultraviolet light that reach your skin. UVB is the shorter wavelength. It causes sunburn, and it is what most people picture when they think of sun damage. UVA is the longer wavelength. It does not burn. It is the part of sunlight linked to the wrinkling and spotting of aging skin and, in laboratory studies, to the kind of DNA damage that leads to skin cancer.

Glass treats the two differently. A 2013 review put it simply: all types of commercial and automobile glass block the majority of UVB, while how much UVA gets through depends on the type of glass. In a bench experiment on glass of several types, every sample stopped UVB completely. In measurements inside cars, the windows passed only the longer UVA wavelengths. So you will almost never sunburn through a closed window. That is not the same as getting no ultraviolet light.

The honest answer to the question in the title is therefore: it depends. It depends on which glass is between you and the sun, and on how long you sit in the beam. The rest of this page gives the numbers behind that answer. None of the studies measured your car, your office, or your living room, and you cannot tell by looking what your glass blocks.

In the car: the windshield and the side window are not the same

A British study published in 2004 measured UVA passing through a selection of car glass. Its authors noted that windshields are made of laminated glass, while side and rear windows are usually not. Clear non-laminated glass let 62.8 percent of UVA through. Gray laminated glass let 0.9 percent through. Lamination cut UVA, and so did tint.

The best real-world numbers come from a 2016 study that measured UVA behind the windshield and behind the driver's side window of 29 cars from 15 makers, at dealerships in Los Angeles, model years 1990 to 2014. Windshields blocked 96 percent of UVA on average, and every one blocked at least 95 percent. The driver's side window blocked 71 percent on average, but the spread ran from 44 percent to 96 percent, and only 4 of the 29 cars had a side window that blocked more than 90 percent. The chart shows the two results side by side. The study does not say how the cars were chosen or whether any windows were tinted, so the numbers describe those cars, not yours.

Two more points about cars. Opening the window changes everything: in measurements on dummies in seven German cars, the sunburn-weighted dose on the arms was 3 to 4 percent of the outdoor dose with the windows shut, 25 to 31 percent with them open, and 62 percent in an open convertible. Those figures count the burning rays, not UVA, so they do not mean a closed window blocks 96 percent of UVA. And glass changes over time: the 2013 review noted that newer safety rules may put laminated glass in more side windows, so a newer car may block more UVA than the cars in the 2016 study. It may not. You cannot tell by looking.

Do drivers really get more skin cancer on the window side?

Earlier research had already reported more sun damage and precancers on the left side of the face, as a 2010 US study noted. The research supports the observation, with limits. In a study of 148 truck drivers in Turkey, where the driver sits on the left as in the United States, seborrheic keratoses were more common on the driver's side of the face and sun spots, called solar lentigines, on the driver's side ear. In drivers over 40, or with more than ten years of driving, actinic keratoses, the rough precancerous spots, were more common on the driver's side too. One published case describes a woman with severe one-sided sun damage who had worked for 15 years in the same room, facing the same window.

Skin cancers show a smaller tilt. At one US academic Mohs surgery center, 52.6 percent of skin cancers were on the left side of the body and 47.4 percent on the right, a difference that did not quite reach statistical significance overall, though it did in men; among 42 melanomas in situ, 31 were on the left. In the US cancer registry, among 82,587 melanomas of the face, arm, or leg, 53 percent of arm melanomas, 51 percent of face melanomas, and 52 percent of leg melanomas were on the left, as were 55 percent of arm Merkel cell carcinomas. The registry had no information about driving. In one UK hospital, where the driver sits on the right, basal cell carcinomas of the face and scalp were 52 percent right-sided and squamous cell carcinomas 54 percent right-sided; the authors wrote that the finding could contribute to the literature on driving, not that it proved the link. If you have had a basal cell carcinoma or a squamous cell carcinoma on the window side, this is the pattern your dermatologist has in mind.

The honest reading is consistent with driving, not proven to be caused by it. A 2007 study of six cancer registries found more melanomas on the left than the right in every one, including England, Scotland, and Australia, where the driver sits on the right; Scotland had the largest left-sided excess of all. The authors suggested differences in sun exposure, or an asymmetry in how pigment cells are laid down before birth. In the US registry, melanomas on the legs, which are mostly shaded while driving, also tilted left. Our reading of these studies is that the window-side pattern is clearer for the slow-growing cancers and precancers of long, steady sun exposure, actinic keratoses, basal cell carcinoma, and squamous cell carcinoma, than for melanoma, and that driving is a plausible contributor rather than a proven cause. Cancers occur on both sides, and the passenger side gets sun too.

Why UVA matters even if you never burn

The glass question matters because UVA damages skin without a burn. In a laboratory study of human squamous cell carcinomas and precancerous solar keratoses, the pattern of DNA damage typical of UVA was found mostly in the basal layer, the deepest part of the outer skin, where the cells that renew the skin live. The damage typical of UVB sat higher up. The authors concluded that the longer ultraviolet wavelengths are an important cause of cancer in that layer. That is mechanism, not a measurement of risk from a car window, and it does not say how much UVA through glass adds to anyone's risk.

Our companion page, Is There Such a Thing as a Safe Amount of Sun?, covers the bigger picture: ultraviolet light is a proven human carcinogen, sunburns and intense on-and-off sun drive melanoma, years of steady exposure drive squamous cell carcinoma and the visible aging of skin, and daily sunscreen reduced squamous cell carcinomas and skin aging in a randomized trial. Nothing on this page changes that. UVA through glass is a smaller, steadier version of the same exposure.

Indoors by a window

Here the research is thinner. The best data are bench tests of glass, not measurements at anyone's desk. In a 2009 experiment, smooth ordinary glass let 74.3 percent of UVA through and blue glass let 56.8 percent through, while laminated glass, green glass, and one sunlight-control film blocked all of it, and every glass blocked UVB completely. The 2004 British study found 62.8 percent of UVA passing through clear non-laminated glass. Neither study tested household windows, and newer coated windows may block more. You cannot tell by looking, and a dark tint is not a guide: the blue glass passed more than half.

What indoor exposure adds up to depends on hours in direct sun. A Danish study fitted 285 volunteers with personal dosimeters for a year. Indoor workers received about 132 standard sunburn doses over the year and gardeners 224, and only the gardeners got most of their dose on working days, which suggests that for indoor workers most of the year's ultraviolet light came from time off, outdoors. Those meters counted sunburn-weighted light, not UVA, so they say little about a window seat. The searches for this article found no study that measured UVA at an ordinary desk or sofa, and no trial of sunscreen used only indoors.

The practical message is proportionate. Ordinary indoor light, away from a direct sunbeam, is a low priority. Hours every day in direct sun through a clear window, on the same side of the face or the same arm, is a real exposure, and the office case above shows what years of it can do. People with sun-sensitive rashes such as polymorphic light eruption may flare behind clear glass: the British study calculated that a UVA dose enough to trigger the rash in some patients could pass through clear non-laminated glass in 30 minutes, but would take 50 hours through gray laminated glass. That calculation is about a rash, not cancer risk.

What the sunscreen trials did and did not test

The strongest evidence that sunscreen prevents skin cancer comes from a trial in Nambour, Australia, that assigned 1,621 adults to daily sunscreen on the head, neck, arms, and hands, or to use sunscreen as they liked. Daily use led to fewer squamous cell carcinomas, a tumor rate ratio of 0.61 with a 95 percent confidence interval of 0.46 to 0.81, and no significant change in basal cell carcinomas. The trial tested daily outdoor use in a subtropical climate. It did not test a car or a window seat, and no trial has. The companion page describes the trial's later melanoma and skin-aging results.

So the case for sunscreen behind glass is reasoning, not trial evidence: UVA gets through some glass, UVA damages skin, a broad-spectrum sunscreen blocks UVA, and the benefit of blocking an exposure is likely to scale with how much of it you get. That reasoning is why many dermatologists think protection is sensible for long drives and for people at higher risk, and why we do too. It is also why a short errand is not worth worrying about.

A proportionate plan, and who it matters most for

This is general education, not a plan for you, and the surgeon or dermatologist who treats you may give different advice. With that said, here is how the evidence sorts out:

• A short drive or an errand: not worth worrying about. The exposure is small, and no study suggests it changes anyone's risk.
• Long or daily drives: the window-side arm, hand, neck, and face get the most. The British study's authors list long sleeves, keeping the arm below the window line, and tinted or laminated windows as helpful. Keeping the windows up on sunny days cuts the dose sharply. A broad-spectrum sunscreen, SPF 30 or higher, on the exposed side is the usual advice; broad-spectrum means it covers UVA as well as UVB.
• Hours each day in direct sun by a window, at a desk or a favorite chair: move the chair, add a shade or blind, or use a UV-blocking window film; one film tested on the bench blocked UVA completely. Sunscreen on the exposed side is reasonable if the sun cannot be moved.
• A history of skin cancer, a weakened immune system, or a sun-sensitive condition: the companion page explains why sun protection stays essential for the highest-risk group, and behind glass is no exception. Some medicines also make skin more sensitive to light; ask your doctor or pharmacist.
• Car window film or tint: check your state's rules before adding anything to car windows, and judge a film by its stated UV blockage, not its shade. Darkness is not a guide to UVA protection.

If you have a healing wound or a new scar

Ask the surgeon who treated you before putting sunscreen on a healing surgical wound or a new scar. A sleeve, a hat, or a dressing may be what they prefer while it heals, and the timing differs from surgeon to surgeon and wound to wound. Follow the instructions from the surgeon who treated you. And whatever side you drive on, point out any new or changing spot, wherever it is, at your skin check.