What are the main sources for generating singlet oxygen?

The main source for generating singlet oxygen occurs when a photosensitizer molecule interacts with regular oxygen. These sensitized molecules are typically found in photosystems of photosynthetic organisms or in cellular cofactors of non-photosynthetic microorganisms. When they encounter ground state oxygen, they produce singlet oxygen. Singlet oxygen can be also generated through various chemical reactions involving oxygen-containing compounds. For example, non-enzymatic reactions involving ozone (O3) can produce singlet oxygen under certain conditions. Singlet oxygen can also be produced as a byproduct of enzymatic reactions, particularly during processes like phagocytosis in immune cells during inflammation. Lastly, singlet oxygen can be generated as a byproduct of electron transfer reactions, leading to the production of the molecule as a reactive intermediate.