Any substance that donates oxygen or that gains electrons in a chemical reaction. Perhaps the most common example of an oxidizing agent is the element oxygen itself. When a substance burns, rusts, or decays, that substance combines with oxygen in the air around it. Oxidizing agents can have both beneficial and harmful environmental effects. Chlorine gas is used as an oxidizing agent to kill bacteria in the purification of water. Ozone damages plant and animal cells by oxidizing them.