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I never seem to get sunburned in the car although I can certainly feel the heat of the sun through the windows. Does car window glass block out the sun's cancer-causing rays? – Janine in Saskatoon

Understanding your level of exposure to the sun's rays when riding inside a vehicle means understanding a bit about UV radiation – and the types of glass used in automobile windows.

Ultraviolet radiation is commonly divided into UVA, UVB and UVC.

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"They're all just different wavelength bands of UV. The shortest waves being UVC, and UVA being the longest. UVC is the most damaging to organic tissue, but it's also the one that is blocked mostly by the ozone layer and doesn't get to us," says Rob Vandal, vice-president of product engineering and development at Guardian Automotive. Guardian supplies OEM (original equipment) windshields to major auto makers, and its own brand for aftermarket distribution.

UVB causes tanning and burning, and leads to photo-aging or skin damage. UVA also leads to certain types of tissue damage over the long term, but does not cause a suntan or sunburn.

The good news is, your windshield blocks virtually all damaging UV radiation.

"Your windshield is normally quite different from the rest of your glass, in that it's two pieces of glass laminated with a layer of plastic – vinyl – in between," says Vandal. "So that triple-layer system by its nature – because plastics don't like UV – contains UV inhibitors that protect the plastic and as a result also protect any transmission of UV through it. So a windshield, laminated glass, blocks 98 to 99 per cent of all UV – A, B or C."

Due to safety regulations, all windshields in North America are made of laminated glass. The side and back windows, however, are typically a single layer of thicker, tempered glass.

"The tempering process is just to give it strength and make it break in a certain pattern that makes all those tiny little pieces. It blocks a reasonable amount of UVB – even 60 or 70 per cent, but transmits quite large amounts of UVA," says Vandal. "So you get a little UVB through it, but it would still take you say many more times exposure to get a tan or a burn and hence that's why that doesn't happen through that glass. But you are still getting hit with the UVA."

There are certain vehicles, however, which have laminated glass in the side windows and other areas. It's becoming more popular, because laminated glass is stronger and provides increased noise protection.

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So, how do you know if you have laminated glass in your door windows?

A partial list of cars with laminated glass in locations other than the windshield can be found at EPGAA.com. "One can also tell by rolling down the door window and looking at the edge of the glass. With laminated glass a close inspection will reveal the two outer layers of glass and the 0.75-mm plastic layer in the middle," says Vandal.

There are studies which show that exposure to UVA through non-laminated car side windows causes damage over the long term. Some experts say it's not a great concern for those who drive a moderate amount.

"In terms of cancer, the risk is probably very low," says Dr. Henry Lim, chair of the department of dermatology at Henry Ford Hospital in Detroit. "There are studies that have been done to show that individuals have more cancer on the left forearm compared to the right, but it is often associated with the fact that they drive with the windows open. So the risk is very low through window glass filtered sunlight."

If you don't have laminated side windows and are concerned about UV exposure, as Lim recommends for all outdoor activities, use sensible photo-protection: clothing, sunglasses, a hat and sunscreen to cover exposed skin.

Please send your automotive questions to globedrive@globeandmail.com

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