Whitening or bleaching of materials such as cotton can come from chemicals such as peroxides and chlorines. The mechanism is the same in which oxygen provides the cleaning functionality. Optical brighteners are additives whose function is to make fabrics and other substrates appear visually whiter and hence cleaner.
The mechanism is simple, where residual chemicals are physically adhered to fibres. These chemicals have the property of reflecting UV light, giving fabric a clean, white appearance.
The health issues associated with optical brighteners are allergic reactions and skin irritation. Once entering the environment they are slow to break down and can accumulate in aquatic life.