Prolonged exposure to the sun's ultraviolet A (UV-A) rays has long been associated with increased risk for cataracts and skin cancer.

For many Americans who drive each day, their car's front windshield protects them from the harmful rays. Findings of a new study, however, revealed that car door windows do not offer the same protection from the sun.

In a new research published in JAMA Ophthalmology on May 12, Brian Boxer Wachler, from the Boxer Wachler Vision Institute, analyzed the UV protection provided by glass in 29 cars that were produced between 1990 and 2014.

The researcher measured the levels of ambient UV-A radiation behind the cars' front windshield and the side window and found that the windshield windows tend to provide good protection blocking 96 percent of UV-A rays on average. The protection, however, was lower at 71 percent and inconsistent for the cars' side windows.

The research likewise revealed that only 14 percent of the cars have side windows that provide high level of UV-A protection, which could be to blamed in part for the increased prevalence of skin cancer on the left side of people's faces and left-eye cataracts.

Based on his findings, Wachler said that automakers may want to consider boosting the amount of UV-A protection in the side windows of vehicles.

"Auto glass with UV-A protection would be expected to reduce the risks of disorders related to sun damage," Wachler wrote in his study.

Jayne Weiss, from the Louisiana State University Eye Center of Excellence, explained that windshields provide more protection than car door windows because they are made of laminated glass designed to prevent shattering. The car door windows, on the other hand, are only tempered glass.

"Don't assume because you are in an automobile and the window is closed that you're protected from UV light," Weiss said.

Although UV-B rays can be blocked by glass, UV-A is a longer wavelength of light that can go deeper into the skin and this can cause premature aging and even skin cancer.

Experts recommend using sunglasses that block both UV-B and UV-A lights as well as using long sleeve clothing and broad spectrum sunscreen particularly during long drives on sunny days. Drivers with older cars or those whose cars don't have built-in protection can also buy special window tint products that provide shield against UV rays.