Where do reactive oxygen species (ROS) come from?

The ROS can be produced from either endogenous or exogenous sources. The endogenous sources of ROS include different cellular organs such as mitochondria, peroxisomes, and endoplasmic reticulum, where the oxygen consumption is high. Phagocytic cells such as neutrophils, eosinophils, and mononuclear phagocytes can also produce ROS when stimulated. On the other hand, ROS are also generated in the biological systems by various exogenous source, such as pollution, alcohol, tobacco smoke, heavy metals, transition metals, industrial solvents, pesticides, certain drugs like halothane, paracetamol, and radiation.