*2.4.1 Singlet oxygen*

Singlet oxygen is the common name for the two metastable states of molecular oxygen, but the singlet O2 1 Δg is the most active in biological systems. It has no unpaired electrons and therefore is not a radical, but upon excitation, one of the O2 electrons shifts to a higher and unstable orbit, which makes it chemically more active than regular triplet O2. *In vivo* O2 to O2 1 Δg excitation occurs when the sunlight fells on human skin containing several biological pigments, such as porphyrins or flavins. This is the reason of early skin aging and skin damages in people with abnormal porphyrin metabolism. Formation of singlet O2 by sunlight occurs also *in vivo* in both lens and retina of the mammalian eye, which causes cataract and loss of vision [52]. Thus, wearing dark glasses and a hat, even in not very bright weather, will protect eyes from cataracts early development and skin from early aging. Studies of plants suggested that in photosynthetic systems one of the functions of polyenes and carotenoids is to protect plants from damages caused by singlet oxygen. Therefore, a carrot salad dressed with vegetable oil may have protective effect against complications caused by bright sunlight.
