Photo-oxidation is the combined action of UV-light and oxygen and is the most significant factor in the weathering of plastics. Although many polymers do not absorb UV-light, they often contain impurities like hydroperoxide and carbonyl groups introduced during thermal processing, which do. These act as photoinitiators to give complex free radical chain reactions where the mechanisms of autoxidation and photodegradation combine. Photo-oxidation can be held back by light stabilizers such as hindered amine light stabilizers (HALS).