Car door windows don't stop UV rays

Car windows don't protect against harmful sun exposure, so it might be a good idea to wear sunglasses and sun block while driving, a new study suggests.

While windshields blocked the vast majority of ultraviolet (UV) radiation from the sun, car door windows offered varying levels of protection from the rays that are tied to cataracts and skin aging.

SUNSCREEN

• Car door windows offer varying levels of protection from sun’s rays
• In US, UV rays blamed for higher number of cataracts, skin cancers on the left side as that side of the driver is exposed to sunlight
• Some car windows give as low as 50 per cent protection from UV rays
• There is no guarantee that  even cars that came with factory tint would protect against UV rays

"Some cars were as low as 50 per cent blockage," said researcher Brian Boxer Wachler of the Boxer Wachler Vision Institute in Beverly Hill, California. "Even cars that came with factory tint, there was no guarantee that would protect against UV rays."

UV rays account for a small portion of the sun's rays but are the most damaging to human skin. UV-A rays are the most common and penetrate most deeply, according to the Skin Cancer Foundation. Because drivers in the US have their left side exposed to sunlight, UV rays have been blamed for the increased number of cataracts and skin cancers that occur on the left side, Boxer Wachler writes in JAMA Ophthalmology.

To see whether car windows are protective, Boxer Wachler took a UV-A light meter to a number of Los Angeles car dealers on a cloudless May day in 2014. He tested 29 cars from 15 manufacturers, made between 1990 and 2014. On an average, car windshields blocked about 96 per cent of UV-A rays. The protection afforded by individual cars ranged from 95 to 98 per cent. But side-door windows were far less dependable. The percentage of UV-A rays blocked varied from 44 per cent to 96 per cent. Only four of the 29 cars had windows that blocked more than 90 per cent of UV-A rays. "It had no correlation at all with the cost of the car, high-end car or low-end car," said Boxer Wachler.

Windshields are more protective than car door windows because they must be made of laminated glass to prevent shattering, writes Jayne Weiss in a commentary published with the study. Car door windows, however, are usually just tempered glass.