A recent analysis of the glass in car windows has shown that front windshields consistently provide a high level of ultraviolet (UV) protection whereas the UV protection afforded by side windows was much lower and more variable.
Drivers who frequently make long journeys may thus be unwittingly exposing themselves to damaging UV rays.
We are now aware of the damage exposure to the sun can cause to the skin and eyes. However, since sunburn does not commonly occur through glass, many people consider glass to protect them from the dangerous effects of the sun.
Although UVB, which causes skin redness and sunburn, typically cannot pass through glass, the UVA rays that cause skin ageing and probably also skin cancer can pass through glass. Some windows do have added UV protection but we do not know by looking which ones they are, so glass should not be considered as a form of sun protection.
This is of particular importance to car drivers since the same side of their face is always exposed to the car side window. With facial cancers and eye diseases occurring more commonly on the left side than the right side in the US, the level of UV-A protection in the front windshields and side windows of cars was assessed.
Levels of UV-A radiation were measured outside, behind the front windshield, and behind the driver's side window in 29 vehicles from 15 manufacturers. The date of manufacture of the cars assessed ranged from 1990 to 2014, with the average being 2010.
On average, the front-windshield blocked 96% of UVA. In contrast, the side-window provided, on average, 71% UVA protection. Of the 29 car tested, only 4 (14%) provided a high level of UVA protection from the driver side window.
These findings may contribute to explaining the reported increased rates of left-eye cataract and left-sided facial skin cancer.
Dr Boxer Wachler, the author of the paper, commented "Automakers may wish to consider increasing the degree of UV-A protection in the side windows of automobiles".