• Radiation: UV light, X-rays and radioactive material. When exposed to high-intensity visible light, cells in culture are damaged, particularly the mitochondria, which are rich in the protein haem and in flavin co-containing proteins (see Chapter 13 ‘Vitamins and minerals’, p. 93). This is what happens to skin when someone is exposed to excessive UV light.

• Phase I of the liver detoxification system (see Chapter 17 ‘Metabolism’, p. 129) is necessary if other reactions are to take place under the appropriate control. The phase I pathway in the liver involves cytochrome p450 and produces free radicals as a necessary intermediary product. The phase II pathway – the conjugation pathway – adds chemical groups to the free radicals, terminating (quenching) them and returning them to normal (non-radical) molecules. If phase I is upregulated (made to work more efficiently) but phase II does not have the capacity to match this increased production of free radicals, the patient will become ill.

• Oxidative phosphorylation: a series of free radical electron carriers in the mitochondria is responsible for energy production (see Figure 2.5, p. 10).

• Gaining an additional electron: oxygen can gain an extra electron (i.e. it is reduced) to form something called a superoxide, which in turn produces hydrogen peroxide. Hydrogen peroxide is not a free radical because its electrons are paired (Figures 7.2 and 7.3), but it can go on to produce problematic metabolic products. Its potential for damage is compounded by the fact that hydrogen peroxide easily passes through cell membranes and can therefore easily move out of the cell in which is was formed. This mobility means that it can spread around the body easily.

• Atherosclerosis: this is attributed to free-radical-induced oxidation of the chemicals in the body (see Chapter 27 ‘Problems with lipid metabolism, p. 204).