A Pollutant is the major part or component of Pollution, the process by which natural environments are damaged by artificial or harmful objects, actions, or neglect. The pollutant itself might only be damaging in large numbers, or it could cause major toxic damage from only a small dose. Almost all modern pollutants are artificial, or man-made; in earlier times, pollution could be caused by decomposing animals or harmful bacteria, which could pollute and destroy ecosystems. Major issues in pollution include whether the damage is long-term or short-term, and whether the pollutant is biodegradable. Some pollutants, such as plastics, are called stock pollutants, and their damage is generally long-term as they remain undisturbed in the environment. A plastic six-pack ring is a good example of a stock pollutant, because it does not degrade and can kill multiple animals while it is moved around the environment. Other pollutants, such as DDT, are called fund pollutants, since they can be directly absorbed by the environment; DDT is absorbed by fats and oils, so it is easily passed on from parent animals to their children, and through the ecosystem as a whole. Because DDT breaks down to other compounds, it usually causes short-term damage, but its compounds may persist for years. Other pollutants include lead, mercury, ozone, human waste, and carbon monoxide.