Originally Posted by
Cookiemonsta
That is not even the worst of it. They provide no UV protection. A lot of people think that just because the glasses are dark they provide a similar amount of protection compared to more expensive sunglasses. However, you are actually better off not wearing sunglasses at all, since at least you will squint and avoid looking directly into the sun. Wearing "fake sunglasses" is something that is just generally not advisable. You are better off wearing a pair of cheap safety glasses.
Plastic is inherently UV absorbing (even clear plastic). As a result almost any pair of sunglasses provides decent UV protection. You would actually have to work fairly hard to find glasses that didn't block UV. The only way sunglasses wouldn't block UV is if they were really really thin and obviously flimsy.
» Blog Archive » UV Measurement of Sunglasses using a Back-thinned CCD Spectrometer
Basically, every single pair they tested blocked >99% of UV (OD>2 for wavelength <400 nm).
Cheap sunglasses can suck for all sorts of reasons, but UV protection is likely the least important reason.