Figure 2. Production of reactive oxygen species (ROS) and cell response. ROS are generated within the cell in mitochondria and peroxisomes, and by cytochrome P450 enzymes (CYP). The intracellular amounts of ROS also increase after exposure to UV light, ionizing radiation, or chemical agents, including dental resin monomers. The defense system against ROS includes endogenous and exogenous anti-oxidants (glutathione GSH, vitamins, N-acetyl-cysteine NAC) plus enzymatic activities. Cellular macromolecules such as lipids, proteins, and DNA may be damaged when the production of ROS is higher than the anti-oxidant capacity of the cells.