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Difference between Primary Pollutants and Secondary Pollutants

Difference between Primary Pollutants and Secondary Pollutants


Pollutants are classified into two categories:

  1. Primary pollutants
  2. Secondary pollutants.

Primary Pollutants

These are produced directly from the source of pollution. Significant primary pollutants are as follows:

Carbon monoxide

  • Carbon monoxide (CO) arises from incomplete burning (due to lack of oxygen) of carbon-based fuels such as petrol, diesel, wood, etc. When there is sufficient oxygen, the carbon-based fuels on burning produce CO2.
  • Carbon monoxide is a colourless and odourless gas. Hence, it can be inhaled without being noticed.
  • Carbon monoxide has 250 times more affinity than oxygen to get attached with haemoglobin.
  • When CO is attached with haemoglobin, oxygen supply to various parts of the body is cut off.
  • The haemoglobin is replaced by carboxy haemoglobin. Inhalation of CO can slow down our reflexes and make us feel dizzy to the extent of death.
  • Burning of carbon fuels such as wood or charcoal in a closed room generates high amount of CO. This is because of two reasons:
  1. In a closed room, oxygen from the atmosphere is less available and
  2. Carbon monoxide produced from incomplete combustion remains in the closed room.

Sulphur dioxide

  • Sulphur dioxide (SO2) is formed when sulphur-containing fuel is burnt. The released sulphur reacts with oxygen to form SO2.
  • SO2 is released during natural processes such as eruption of volcanoes and industrial processes such as production of paper, smelting of metals, etc.

Nitrogen dioxide

  • Nitrogen dioxide (NO2) is released when high temperature combustion occurs.
  • Thus, it can arise from both natural sources (such as volcanoes) and man-made sources (such as burning of fossil fuels).
  • NO2 is reddish brown in appearance and has sharp odour.

Particulate matter

  • It refers to fine particles of dust, other solids and liquids suspended in the air. Particulate matter includes fly ash.

Volatile organic compounds

  • Volatile organic compounds (VOCs) have low boiling point and so they are converted into vapour form even at room temperature.
  • The vapour so formed mixes with air. There are numerous volatile organic compounds, both natural as well as man-made.
  • Certain naturally occurring VOCs play an important role in plant communication.
  • Man-made VOCs include perfumes, hair sprays, furniture polishes, dyes, air fresheners, moth repellents and wood preservatives.
  • VOCs do not have short-term effects but some VOCs exhibit harmful effects over long term.

Secondary Pollutants

  • Primary pollutants undergo chemical and physical reactions to get transformed into secondary pollutants.
  • Important secondary pollutants are ground level ozone, smog and photochemical smog.

Ground level ozone

Ozone is found in two regions: Upper atmosphere and ground level.Both the regions have similar chemical composition (03).

  • While upper atmospheric ozone protects us from the harmful effects of UV rays of the sun, the ground level ozone is the main component of photochemical smog.
  • Ground level ozone is also called troposphere ozone.
  • Ground level ozone is a secondary pollutant, because it is not directly emitted into air, but it emerges due to chemical reaction between oxides of nitrogen and VOCs.
  • In the presence of sunlight, formation of ozone increases. Thus, ground level ozone increases during sunny days.
  • It emerges in urban areas, but it can travel long distances under the influence of wind.
  • Ground level ozone has various harmful effects on health. People with lung diseases, children and older adults are particularly vulnerable.
  • Children are at the highest risk due to exposure to ground level ozone, as their lungs are still under development.
  • Inhalation of ground level ozone reduces oxygen availability and thus can cause breathing disorders.


  • Smog is an urban phenomena that arises due to heavy emissions of harmful gases from vehicles and industries.
  • Harmful gases such as SO2, NO2 and CO combine with haze or fog to form smog.
  • Smog can create multiple disorders including respiratory problems and even death.

Photochemical smog

  • It emerges when harmful gases which constitute smog react in the presence of sunlight.
  • Ground level ozone is one of the primary components of photochemical smog. Photochemical smog is even more dangerous than smog.

Environment & Biodiversity

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