Concentration of pollutants has been expressed by fractions. In case of gas mixture, the unit is ppm by volume (parts per million, mg/ml), while in case of liquids and solids; the unit is ppm by weight (mg/kg).
Environment can be divided into four segments
(1) Atmosphere is the protective blanket of gases, which are surrounding the earth. The atmosphere has been the source of oxygen (for respiration), carbon dioxide (for photosynthesis), nitrogen (for nitrogen fixation) etc.
(2) Hydrosphere includes all type of water resources such as oceans, seas, rivers, lakes, streams, glaciers, polar ice caps, reservoirs and ground water.
(3) Lithosphere covers the outer part of the solid earth (minerals, organic matter, and soil).
(4) Biosphere is the environmental segment comprising living organisms and their interactions with the rest of the environmental segments (atmosphere, hydrosphere and lithosphere).