Why have my headlights gone yellow and cloudy?

The lenses are plastic, not glass, and the outer layer has been degraded by sunlight. Almost every car built since the 1990s uses polycarbonate headlight lenses, which are light, cheap and far less likely to shatter than glass — but polycarbonate yellows badly under ultraviolet light.

Manufacturers apply a hard UV-resistant coating at the factory. That coating is a consumable. After roughly five to ten years of sun exposure it thins, cracks and flakes away, and once the bare polycarbonate underneath is exposed it oxidises, turning yellow and hazy. Cars parked outdoors in strong sun degrade fastest, and the lens facing the afternoon sun often goes first, which is why one headlight can look noticeably worse than the other.

This is a safety issue rather than a cosmetic one. A heavily oxidised lens scatters light instead of focusing it, cutting usable beam distance substantially and throwing glare toward oncoming traffic. In several countries it is a test failure.

Restoration kits work by sanding away the oxidised layer with progressively finer grades of abrasive, then polishing the plastic clear again. The step people skip is the last one: applying a new UV sealant. Without it the freshly exposed polycarbonate is completely unprotected and will yellow again within months, which is why home restorations are often said not to last.

Toothpaste and similar household abrasives do produce a visible improvement, and no protection at all, so the haze returns quickly.

If the lens is crazed with fine cracks rather than simply yellow, the damage is inside the plastic and polishing will not reach it. Replacement is the only real fix at that point.