Defining Pollution

• Pollution is the undesirable change in biological, chemical and physical characteristics due to human and natural activities. 
• Pollutants are the agents which cause pollution. 

#1 On the Basis of Degradation

1. Non-Persistent Pollutants

• Pollutants that can easily break down by natural processes. 
• For example, domestic sewage and vegetable waste.

2. Persistent Pollutants

• Pollutants that have the ability to remain in the atmosphere as they takes decades to breakdown.
• For Example, DDT

3. Non-Degradable Pollutants

• These pollutants can’t be degraded by natural processes. Hence, they remain in the atmosphere forever and bioaccumulate if they are released into the atmosphere.
• For Example, Toxic elements like lead, mercury, cadmium, chromium and nickel.

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#2 On the basis of Zones of Influence

Pollutants can also be defined wrt their zones of influence  

2.1 Horizontal Zone

It is defined wrt an area damaged by the pollutant

1. Local Pollutant: They damage area in the vicinity of the source
2. Regional Pollutant: They damage area further from the source

2.2 Vertical Zone

 It is defined wrt the height up to which pollutants show their impact.

1. Surface Pollutant: These pollutants accumulate near the Earth’s surface.
2. Global Pollutants: These pollutants have a higher concentration in the higher atmosphere. 

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#3 Existence in Nature

3.1 Quantitative Pollutants

• Substances which are already present in the environment, but are termed as pollutants when their concentration (quantity) increases in the environment.
• E.g. : CO2 is present in the environment but when it’s concentration increases, it becomes pollutant.

3.2 Qualitative Pollutants

• Substances that are present ordinarily present in the atmosphere and don’t cause any harm. But they become pollutants only when their concentration increases beyond a certain limit. 
• E.g., CO2 is present in the environment, but when its concentration increases, it becomes a pollutant.