Air Pollutants: Definition and Classification

Definition

Section 2(a) of the Air (Prevention and Control of Pollution) Act, 1981 define Air Pollutant as any solid, liquid, or gaseous substance (including noise) present in the atmosphere in such concentration as may be or tend to be injurious to human beings or other living creatures or plants or property or environment.

It is important to note that noise was added to this definition via an amendment in 1987, recognizing it as a significant air pollutant.

Classification of Air Pollutants

Air pollutants are broadly classified based on their origin, state of matter, and chemical composition.

1. Based on Origin (Primary vs. Secondary)

This is the most common classification, distinguishing between pollutants emitted directly and those formed in the atmosphere.

• Primary Pollutants: These are emitted directly into the atmosphere from an identifiable source.

  • Examples: Sulfur dioxide ($SO_2$) from factories, Carbon monoxide ($CO$) from car exhausts, and Particulate Matter (PM) from construction sites.

• Secondary Pollutants: These are not emitted directly. They are formed in the air when primary pollutants react with each other or with basic atmospheric components like water vapor or sunlight.

  • Examples: Ground-level Ozone ($O_3$), which forms when $NO_x$ and VOCs react in sunlight, and Acid Rain, formed when $SO_2$ reacts with water to create sulfuric acid.

2. Based on State of Matter (Particulate vs. Gaseous)

• Particulate Pollutants: These consist of tiny solid particles or liquid droplets suspended in the air.

  • PM10: Coarse particles (e.g., dust, pollen) less than 10 micrometers in diameter.

  • PM2.5: Fine particles (e.g., combustion particles, organic compounds) less than 2.5 micrometers. These are more dangerous as they can penetrate deep into the lungs and bloodstream.

• Gaseous Pollutants: These are substances that exist in a gaseous state at normal temperature and pressure.

  • Examples: Nitrogen oxides ($NO_x$), Sulfur oxides ($SO_x$), Carbon dioxide ($CO_2$), and Methane ($CH_4$).

3. Based on Chemical Composition (Organic vs. Inorganic)

• Organic Pollutants: Compounds that contain carbon and hydrogen.

  • Examples: Volatile Organic Compounds (VOCs) like benzene and formaldehyde, which evaporate easily at room temperature and contribute to smog.

• Inorganic Pollutants: Compounds that do not typically contain carbon-hydrogen bonds.

  • Examples: Carbon monoxide, Carbon dioxide, Carbonates, and various metallic dusts like lead or mercury.

4. Natural vs. Anthropogenic (Man-made)

• Natural Pollutants: Resulting from natural phenomena.

  • Examples: Volcanic ash, salt spray from oceans, forest fires caused by lightning, and pollen.

• Anthropogenic Pollutants: Resulting from human activities.

  • Examples: Emissions from thermal power plants, vehicular exhaust, industrial waste, and agricultural activities (like the use of chemical fertilizers or crop burning).