Depending on how many electrons oxygen takes up during a reaction, molecules with very different properties are formed. Hydrogen peroxide (H2O2), for example, is an environmentally friendly bleaching agent. The superoxide anion (O2-) is particularly exciting. It is very reactive and can attack most organic compounds. Our immune system uses these substances as "chemical weapons" against invading bacteria and viruses. In low concentrations, the cells also use this compound as a messenger substance for a kind of whisper: Because superoxide can only diffuse a few micrometres before it reacts with any molecule, only cells in the immediate vicinity of the transmitter can register the molecule and thus the information.

The ability of our body to handle such dangerous and reactive compounds is based on a complex and not yet fully understood molecular machinery. Too much of the reactive oxygen compounds (oxidative stress) leads to an attack on our body's own compounds and to premature ageing, too little makes us helpless against bacteria and degenerated cells. As always in life and chemistry: opportunities and risks are very close together."