If you spend a lot of time in a car, be aware that side windows do not protect well against ultraviolet-A (UVA) radiation, the kind that penetrates deeply and is primarily responsible for skin cancer and cataracts. Clear glass screens out nearly all ultraviolet-B (UVB, most responsible for sunburn, as well as contributing to skin cancer) but, depending on how the glass is made and treated, varying amounts of UVA pass through. And as an investigative study in JAMA Ophthalmology found, protection against UVA is reliably high in front windshields but lower and highly variable in side windows.
Using a handheld UVA light meter, the researcher tested 29 cars from 15 manufacturers on a cloudless day in Los Angeles, finding that front windshields blocked, on average, 96 percent of these harmful rays (with a range of 95 to 98 percent), while side windows blocked only 71 percent (44 to 96 percent). This helps explain why other studies have found an increased prevalence of left-sided cataracts and of skin cancer (including malignant melanoma) on the left side of the face and on left arms in countries where the steering wheel is on the left side of the car, and more on the right side of the body in countries where the steering wheel is on the right side.
“The high level of variability may be explained by the type of glass used by the automaker,” the paper explained, with front windshields required to have a clear plastic layer in between two panes of glass to lessen damage in case of shattering. This has an extra benefit of providing excellent UVA protection (similar to sunscreen with an SPF of about 50, as the accompanying commentary noted). In contrast, side windows are typically not laminated but rather are composed of single-pane glass, which “may or may not have a high level of UVA-blocking polymers and other protective components.”
Don’t assume tinted windows block more UVA or that newer or higher-end car models provide more protection. In fact, Porsche and BMW models were among the cars with the worst side-window protection, and a 1990 Buick was more protective than some newer cars.
For the most protection, when the sun is at your side, keep the window up and flip the front visor to the side. In addition, use a broad-spectrum sunscreen and wear long sleeves and sunglasses. You can also have the side windows treated to block nearly all UV rays.
Also see Pollution Inside Your Car.