Your windshield and your driver's side window are not made of the same glass, and they do not protect you equally. A 2025 study published in Archives of Dermatological Research measured 34 vehicles from model years 2015 through 2025 and found that front windshields attenuated an average of 99.25 percent of UV-A radiation, while driver's side windows attenuated only 88.78 percent. Both effectively blocked UV-B. That gap on the side glass is the single most useful fact a driver can know about sun exposure in a car, and most people have never heard it.
Why the two windows behave so differently
Windshields are laminated: two layers of glass bonded around a plastic interlayer. That interlayer exists for crash safety, so the windshield holds together instead of shattering, and it happens to absorb UV-A very effectively as a side effect. Side windows are usually tempered single-pane glass with no interlayer, chosen because it breaks into blunt pieces and can be shattered in an emergency. Tempered glass blocks UV-B well, as nearly all glass does, but it lets considerably more UV-A through. The difference is a consequence of how each window is engineered for safety, not a manufacturing defect, and it is why the protection you get depends on which window the sun is coming through.
What the measurements actually showed
A second 2025 study, published in Frontiers in Public Health, measured 20 vehicles and reported absolute irradiance rather than percentages, which gives a complementary picture of the same effect.
- Mean UV irradiance outside the vehicle: 3.92 milliwatts per square centimeter.
- Mean UV irradiance behind the front windshield: 0.32 milliwatts per square centimeter.
- Mean UV irradiance behind the side window: 0.82 milliwatts per square centimeter, more than double the windshield reading.
- Mean infrared irradiance outside the vehicle: 124.58 milliwatts per square centimeter.
- Mean infrared irradiance behind the front windshield: 60.42 milliwatts per square centimeter.
- Mean infrared irradiance behind the side window: 46.68 milliwatts per square centimeter.
UV-A is the part that gets through
Ultraviolet light is commonly divided into UV-B and UV-A. UV-B has shorter wavelengths and is strongly absorbed by ordinary glass, which is why both studies found it essentially eliminated by any window. UV-A has longer wavelengths, penetrates deeper into the skin, and is the component that standard side glass handles poorly. So the practical exposure question inside a vehicle is almost entirely a UV-A question, and almost entirely a side-window question. Rear passengers sit beside the same category of glass, which is worth remembering for anyone who regularly carries children.
Heat is a separate problem, and the numbers prove it
Look again at the infrared figures above. Infrared is the band responsible for radiant heat, and both the windshield and the side window still passed roughly a third to a half of the outside infrared load. This is the clearest evidence that UV protection and heat rejection are genuinely different properties of a window. Factory glass already handles UV-B well and the windshield handles UV-A well, yet the cabin still heats up, because the heat is arriving as infrared that the glass does not stop. Any claim that treats darkness, UV protection and heat rejection as the same thing is conflating three separate physical properties.
What this means for drivers in South Florida
Miami drivers accumulate sun exposure in a way that drivers in cooler, cloudier regions simply do not: long commutes, year-round high sun angles, and a left-hand-drive country where the driver sits next to the window that protects least. The research above does not establish that any particular film prevents any particular health outcome, and nobody should claim that it does. What it does establish is that the side glass is the measurable weak point, that the gap is specifically in UV-A, and that infrared heat is a distinct issue the glass barely addresses. Those are the two things a window film can actually be specified to improve, and they are the two things worth asking about before you buy.
Questions worth asking about any film
- What percentage of UV does the film reject, and is that figure for UV-A specifically or for ultraviolet generally?
- What is the infrared rejection figure, and over which wavelength range is it measured, since ranges vary and make numbers hard to compare?
- What is the total solar energy rejected, which combines visible, ultraviolet and infrared into one figure and is harder to present misleadingly?
- Does the film meet Florida's legal limits for the specific window it is going on, since the front side windows have the strictest requirement?
- Will the installer measure the finished windows and show you the reading?
One caution about interpreting all of this: both studies measured a sample of vehicles under specific conditions, and individual results varied considerably between models. The 2016 research that first drew attention to this topic made the same point, noting that side-window protection was not only lower than windshield protection but highly variable from car to car. Your own vehicle may sit above or below these averages. That variability is itself an argument for measuring rather than assuming, whether you are evaluating the glass you already have or the film you are considering adding to it.