Malondialdehyde (MDA) is the dialdehyde of malonic acid and a biomarker of oxidative damage to lipids caused by smoking. Oxidized lipids are able to produce MDA as a decomposition product. The mechanism is thought to involve formation of prostaglandin-like endoperoxides from polyunsaturated fatty acids with two or more double bonds. An alternative mechanism is based on successive hydroperoxide formation and β-cleavage of polyunsaturated fatty acids. MDA is then directly formed by β-scission of a 3-hydroperoxyaldehyde or by reaction between acrolein and hydroxyl radicals. While oxidation of polyunsaturated fatty acids is the major source of MDA in vivo, other minor sources exists such as byproducts of free radical generation by ionizing radiation and of the biosynthesis of prostaglandins. Aldehydes are generally reactive species capable of forming adducts and complexes in biological systems and MDA is no exception although the main species at physiological pH is the enolate ion which is of relative low reactivity. Consistent evidence is available for the reaction between MDA and cellular macromolecules such as proteins, RNA and DNA. MDA reacts with DNA to form adducts to deoxyguanosine and deoxyadenosine which may be mutagenic and these can be quantified in several human tissues. Oxidative stress is an imbalance between oxidants and antioxidants on a cellular or individual level. Oxidative damage is one result of such an imbalance and includes oxidative modification of cellular macromolecules, induction of cell death by apoptosis or necrosis, as well as structural tissue damage. Chemically speaking, oxidants are compounds capable of oxidizing target molecules. This can take place in three ways: abstraction of hydrogen, abstraction of electrons or addition of oxygen. All cells living under aerobic conditions are continuously exposed to a large numbers of oxidants derived from various endogenous and exogenous sources. The endogenous sources of oxidants are several and include the respiratory chain in the mitochondria, immune reactions, enzymes such as xanthine oxidase and nitric oxide synthase and transition metal mediated oxidation. Various exogenous sources of ROS also contribute directly or indirectly to the total oxidant load. These include effects of ionizing and non-ionizing radiation, air pollution and natural toxic gases such as ozone, and chemicals and toxins including oxidizing disinfectants. A poor diet containing inadequate amounts of nutrients may also indirectly result in oxidative stress by impairing cellular defense mechanisms. The cellular macromolecules, in particular lipids, proteins and DNA, are natural targets of oxidation. Oxidants are capable of initiating lipid oxidation by abstraction of an allylic proton from a polyunsaturated fatty acid. This process, by multiple stages leading to the formation of lipid hydroperoxides, is a known contributor to the development of atherosclerosis. (PMID: 17336279). MDA has been identified as a uremic toxin according to the European Uremic Toxin Working Group (PMID: 22626821).