Air pollution refers to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings or cause damage to the climate or materials.
Classification of Air Pollutants
Pollutants are categorized based on their formation process and physical state.
Primary vs. Secondary Pollutants
- Primary Pollutants: Emitted directly from identifiable sources. Examples include Sulphur dioxide (SO2), Nitrogen oxides (NOx), Carbon monoxide (CO), and Particulate Matter (PM).
- Secondary Pollutants: Produced in the atmosphere through chemical reactions between primary pollutants and environmental factors. Examples include Ground-level Ozone (O3), Acid Rain (H2SO4), and Peroxyacetyl Nitrate (PAN), which is a key component of photochemical smog.
Criteria Pollutants
These are common air pollutants regulated by the government because they can harm health and the environment.
- Particulate Matter (PM2.5 and PM10): Fine particles that can penetrate deep into the lungs and enter the bloodstream.
- Sulphur Dioxide (SO2): Primarily from coal combustion; causes acid rain and respiratory issues.
- Nitrogen Oxides (NOx): Contributes to smog and ozone formation; sourced from vehicular exhaust.
- Carbon Monoxide (CO): A colorless, odorless gas from incomplete combustion that reduces the oxygen-carrying capacity of blood.
Major Monitoring Frameworks in India
India utilizes two primary systems to measure and communicate air quality levels to the public and policymakers.
National Ambient Air Quality Standards (NAAQS)
Notified by the Central Pollution Control Board (CPCB) under the Air Act, 1981, these standards cover 12 pollutants:
- PM10
- PM2.5
- NO2
- SO2
- CO
- O3
- NH3 (Ammonia)
- Pb (Lead)
- Benzene
- Benzopyrene
- Arsenic
- Nickel
National Air Quality Index (AQI)
Launched in 2014, the AQI transforms complex air quality data of 8 pollutants (the first eight listed in NAAQS) into a single number and color-coded warning system.
| AQI Category | Numerical Value | Health Impact |
| Good | 0–50 | Minimal impact |
| Satisfactory | 51–100 | Minor breathing discomfort to sensitive people |
| Moderately Polluted | 101–200 | Breathing discomfort to people with lungs/heart disease |
| Poor | 201–300 | Breathing discomfort to most people on prolonged exposure |
| Very Poor | 301–400 | Respiratory illness on prolonged exposure |
| Severe | 401–500 | Affects healthy people and seriously impacts those with existing diseases |
Atmospheric Phenomena and Smog
Pollution behavior is heavily influenced by meteorological conditions, leading to specific phenomena.
Photochemical Smog vs. Classical Smog
- Photochemical Smog (Los Angeles Smog): Occurs in warm, dry, and sunny climates. It is oxidizing in nature and formed by the action of sunlight on NOx and volatile organic compounds (VOCs).
- Classical Smog (London Smog): Occurs in cool, humid climates. It is reducing in nature and consists of a mixture of smoke, fog, and sulphur dioxide.
Temperature Inversion
Usually, air temperature decreases with height. During a temperature inversion, a layer of warm air settles over a layer of cooler air near the ground. This “lid” traps pollutants close to the surface, leading to severe smog episodes, common in Northern India during winter.
Key Government Initiatives
National Clean Air Programme (NCAP)
Launched in 2019, NCAP is a long-term, time-bound national strategy to tackle the air pollution problem across the country.
- Target: A 20% to 30% reduction in PM2.5 and PM10 concentrations by 2024 (base year 2017). This target was later revised to a 40% reduction by 2026.
- Coverage: 131 “non-attainment” cities that did not meet the NAAQS for five consecutive years.
Graded Response Action Plan (GRAP)
Specifically designed for Delhi and the NCR, GRAP is a set of emergency measures that kick in automatically when air quality deteriorates beyond certain thresholds. Measures include banning construction, restricting truck entry, and implementing the odd-even car scheme.
BS-VI Emission Standards
India skipped BS-V and moved directly from BS-IV to BS-VI (Bharat Stage VI) norms in 2020. BS-VI fuel has significantly lower sulphur content (10 ppm) compared to BS-IV (50 ppm), reducing harmful emissions.
Indoor Air Pollution
Often overlooked, indoor air pollution is a significant health risk in rural India.
- Sources: Use of biomass (wood, dung, crop residue) in traditional “Chulhas,” tobacco smoke, and building materials (Asbestos, Radon gas).
- Government Intervention: PM Ujjwala Yojana, which provides LPG connections to BPL households to replace solid fuels with clean cooking fuel.
Quick Facts and Trivia
- Lichens as Bio-indicators: Lichens are highly sensitive to SO2. Their absence in an area is a biological indicator of high air pollution.
- Fly Ash: A byproduct of coal combustion in thermal power plants. It contains heavy metals and silica. It is now mandatorily used in brick making and cement production to prevent air dispersal.
- The Polluter Pays Principle: This is a fundamental principle of Indian environmental law, upheld by the Supreme Court, ensuring that those who produce pollution bear the costs of managing it.