A pollutant is a physical, chemical or biological substance intentionally or unintentionally released into environment which directly or indirectly harmful to living organisms.
Stock pollutants: Pollutants towards which environment has little or no absorptive capacity. (e.g. persistent synthetic chemicals)
Fund pollutants: Pollutants which have absorptive capacity, doesn’t cause much damage unless its limit exceeds to the absorptive capacity of environment. (e.g. CO2)