In general, substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. A pollutant may cause long- or short-term damage by changing the growth rate of plant or animal species, or by interfering with human amenities, comfort, health, or property values. Pollutants may be classified by various criteria: (1) By the origin: whether they are natural or man-made (synthetic). (2) By the effect: on an organ, specie, or an entire ecosystem. (3) By the properties: mobility, persistence, toxicity. (4) By the controllability: ease or difficulty of removal.