One of the important criteria of UV lens is UV coating. The lens should have the ability to block the UV coating. UV coating is a must if you are going under the sun to protect your eye. UV radiation causes eye cataracts as you grow old. What is the use of sunglasses if it cannot protect you from the harsh UV rays of the sun?
Most of the polycarbonate lenses and high index plastic lenses have inbuilt UV protection. CR-39 plastic lenses and glass lenses do not have the ability to block the UV radiation. To make it UV proof CR-39 and glass lenses should have extra UV coating.