What causes lenses to turn cloudy, hazy, or yellow?

Although poly carbonate is a very hard plastic, it is very porous. While there are many advantages to using poly carbonate, one major drawback is that over time oxidation can change the physical properties of the headlight lens surface. The lenses have a factory applied UV resistant .03 mil thick silicone coating to help protect and preserve the porous plastic from the environment. Once this protective barrier weakens and wears, the lenses are exposed to many elements. Daily exposure to the sun’s harsh UV rays is the biggest contributor to the deterioration (oxidation) and discoloring of the poly carbonate lens surface. However, UV rays and heat from the sun isn’t the only culprit; intense heat from the halogen bulb also has a negative effect over time…as does acid rain, humidity, insects, road debris, chemicals, vehicle exhaust, and car wash brushes