Air Pollution: Health Impact, Causes, AQI Monitoring & Pollution Control

Air pollution has become one of the most pressing environmental challenges of the 21st century. Rapid urbanization, industrial growth, expanding transportation networks, and growing energy demands have significantly deteriorated air quality in many parts of the world. Cities like Delhi, Mumbai, Beijing, and Los Angeles frequently record dangerous levels of pollutants that threaten human health, ecosystems, climate, and the economy.

Understanding what causes air pollution, how it affects our health, and how air quality is monitored and controlled is essential for policymakers, researchers, and citizens alike. This article explores air pollution in depth—its sources, impact, AQI system, and effective control strategies.

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What Is Air Pollution?

Air pollution refers to the presence of harmful substances in the atmosphere at levels that negatively affect human health, the environment, or property. These substances may be natural or man-made, and can exist in the form of gases, liquids, or solid particles.

Common Air Pollutants

• PM2.5 (Fine particulate matter)
• PM10 (Coarse particulate matter)
• Nitrogen oxides (NO & NO₂)
• Sulfur dioxide (SO₂)
• Carbon monoxide (CO)
• Ozone (O₃ at ground level)
• Volatile Organic Compounds (VOCs)
• Lead and heavy metals

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Causes of Air Pollution

Air pollution arises from numerous sources. These can be broadly classified into anthropogenic (human-made) and natural sources.

1. Vehicular Emissions

Transportation contributes significantly to urban air pollution. Vehicles emit:

• PM2.5 and PM10
• NOx (Nitrogen oxides)
• CO (Carbon monoxide)
• VOCs
• Unburnt hydrocarbons

Heavy traffic congestion, old vehicles, and diesel engines make pollution worse.

2. Industrial Activities

Factories, refineries, power plants, and manufacturing units release enormous amounts of:

• SO₂
• NOx
• PM
• VOCs
• Toxic metals

Thermal power plants, especially coal-based, are major contributors in countries like India and China.

3. Construction and Demolition

Dust from construction sites contributes heavily to PM10 and PM2.5 levels. Demolition waste, cement particles, and road dust also play a major role.

4. Agricultural Activities

• Crop residue burning
• Use of chemical fertilizers
• Pesticide spraying

In northern India, stubble burning is a major seasonal cause of smog.

5. Household and Commercial Pollution

• Solid fuel cooking (wood, cow dung, coal)
• Burning waste
• Use of Diesel generators
• Indoor smoking

Indoor pollution is often worse than outdoor levels.

6. Natural Sources

• Wildfires
• Dust storms
• Volcanic eruptions
• Pollen
• Sea spray

While natural, these sources can significantly reduce air quality during extreme events.

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Health Impacts of Air Pollution

Air pollution is directly linked to millions of deaths and chronic illnesses globally. According to WHO, 99% of the global population breathes air that exceeds safe limits.

1. Respiratory Problems

• Asthma
• Chronic Obstructive Pulmonary Disease (COPD)
• Bronchitis
• Reduced lung function
• Lung infections

Fine particles (PM2.5) can penetrate deep into the lungs and enter the bloodstream.

2. Cardiovascular Diseases

Polluted air increases the risk of:

• Heart attacks
• High blood pressure
• Stroke
• Atherosclerosis

PM2.5 triggers inflammation and blood vessel damage.

3. Cancer

Long-term exposure to pollutants like benzene, formaldehyde, and PM2.5 increases risk of lung cancer and potentially other cancers.

4. Effects on Children

Children are particularly vulnerable because their lungs and immune systems are still developing.

• Reduced IQ
• Stunted lung growth
• Behavioral problems
• Increase in pneumonia and infections

5. Effects on Pregnant Women

Pollution can lead to:

• Low birth weight
• Preterm delivery
• Developmental issues for the fetus

6. Mental Health Effects

Recent studies suggest links between air pollution and:

• Anxiety
• Depression
• Cognitive decline
• Alzheimer’s and Parkinson’s diseases

7. Other Impacts

• Eye irritation
• Skin aging
• Reduced immunity
• Worsening allergies

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Air Quality Index (AQI): Monitoring Air Pollution

The Air Quality Index (AQI) is a standardized system used worldwide to measure and communicate air quality levels. It converts complex pollutant concentrations into a simple scale that indicates how polluted the air is.

Pollutants Measured for AQI

Most countries measure six main pollutants:

1. PM2.5
2. PM10
3. NO₂
4. SO₂
5. CO
6. Ozone (O₃)

AQI Categories (India Example)

AQI Range	Category	Health Impact
0 – 50	Good	Minimal impact
51 – 100	Satisfactory	Minor breathing discomfort
101 – 200	Moderate	Discomfort for sensitive groups
201 – 300	Poor	Breathing issues for most people
301 – 400	Very Poor	Respiratory illness likely
401 – 500	Severe	Health emergency; everyone affected

Why AQI Matters

• Helps citizens plan outdoor activities
• Guides government pollution-control actions
• Alerts hospitals for potential health surges
• Encourages industries to reduce emissions

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Air Pollution Control Strategies

Reducing air pollution requires combined efforts from governments, industries, and individuals.

1. Government and Policy-Level Measures

a. Emission Standards

• Bharat Stage VI (BS VI) norms for vehicles
• Industrial emission norms
• Cleaner fuels like CNG, LNG

b. Renewable Energy Transition

• Solar, wind, hydro
• Electrification of public transport

c. Urban Planning

• Green belts
• Pollution-free zones
• Improved public transport

d. Strict Monitoring and Penalties

• Real-time air quality monitoring stations
• Fines for non-compliant industries
• Ban on open waste burning

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2. Technological Measures

• Electrostatic precipitators (ESP) for industries
• Scrubbers to remove SO₂
• Catalytic converters in vehicles
• HEPA-based air purifiers for indoor spaces
• Dust suppression systems at construction sites

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3. Community and Individual-Level Actions

• Avoid burning waste
• Use public transport, carpooling, or EVs
• Adopt LPG or electric stoves
• Use indoor plants that improve air quality
• Regular maintenance of vehicles
• Wearing N95 masks during high pollution days

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Impact of Climate Change on Air Pollution

Air pollution and climate change are interconnected:

• Rising temperatures increase ground-level ozone
• Heatwaves worsen pollution episodes
• Wildfires release huge amounts of PM2.5
• Changing wind patterns influence pollutant dispersion

Mitigating pollution helps slow climate change—and vice versa.

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Chemistry of Air Pollution

Air pollution is not only a physical problem of dust and smoke; it is fundamentally a chemical phenomenon involving complex reactions in the atmosphere. These reactions lead to the formation, transformation, and removal of pollutants. Understanding the chemistry behind air pollution is crucial for predicting its behavior, designing monitoring systems, and implementing effective control strategies.

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Primary vs. Secondary Pollutants

Primary Pollutants

These are emitted directly from sources into the atmosphere:

• SO₂ from coal combustion
• NO (nitric oxide) from vehicles and industries
• CO (carbon monoxide) from incomplete combustion
• VOCs (volatile organic compounds) from fuels and solvents
• Particulate matter (PM2.5, PM10) from dust, combustion, construction

Secondary Pollutants

These form through atmospheric chemical reactions:

• Ozone (O₃)
• Peroxyacetyl nitrate (PAN)
• Secondary organic aerosols (SOA)
• Sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)

Secondary pollutants are often more harmful than primary pollutants.

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Chemistry of Photochemical Smog

Photochemical smog forms when sunlight interacts with NOx (NO + NO₂) and VOCs.

Key Reactions

• NO₂ photolysis (initiated by sunlight):
  NO₂ + hv → NO + O

• Formation of ozone (O₃):
  O + O₂ → O₃

• VOCs + NO → Peroxy radicals → Formation of PAN and other oxidants

This chain reaction leads to high levels of:

• Ozone (O₃)
• PAN
• Aldehydes
• Free radicals

This chemical mixture is responsible for irritation of eyes, lungs, and respiratory distress.

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Chemistry of Acid Rain

Acid rain forms when SO₂ and NOx react with water, oxygen, and oxidants in the atmosphere.

Sulfur Cycle in Acid Rain

SO₂ + OH → HOSO₂
HOSO₂ + O₂ → SO₃
SO₃ + H₂O → H₂SO₄ (sulfuric acid)

Nitrogen Cycle in Acid Rain

NO₂ + OH → HNO₃ (nitric acid)

These acids dissolve in rainwater, reducing the pH of precipitation to harmful levels (below 5.6).

Effects include:

• Soil and water acidification
• Damage to crops and forests
• Corrosion of metals and monuments

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Chemistry of Ozone Formation and Depletion

Tropospheric Ozone (Bad Ozone)

Formed by reactions involving sunlight, NOx, and VOCs (as explained in photochemical smog).
Highly reactive and harmful to:

• Respiratory system
• Plants
• Materials (rubber, plastics)

Stratospheric Ozone (Good Ozone)

Destroyed by chlorine radicals from CFCs:
Cl + O₃ → ClO + O₂
ClO + O → Cl + O₂

This catalytic cycle reduces the ozone layer, increasing UV radiation reaching Earth.

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Chemistry of Particulate Matter (PM2.5 & PM10)

PM is not a single chemical substance. It is a complex mixture of:

• Sulfates (SO₄²⁻)
• Nitrates (NO₃⁻)
• Ammonium (NH₄⁺)
• Organic carbon
• Elemental carbon (soot)
• Metals (Pb, Ni, Cd)

Formation of Secondary PM

SO₂ → H₂SO₄ → Sulfate particles
NOx → HNO₃ → Nitrate particles
VOCs → SOA (Secondary Organic Aerosols)

These particles penetrate deep into lungs and enter blood.

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Chemistry of Carbon Monoxide (CO)

CO forms from incomplete combustion of carbon-containing fuels.

Key Reaction

2C + O₂ → 2CO (when oxygen is limited)

CO binds with hemoglobin more strongly than oxygen:
Hb + CO → HbCO (carboxyhemoglobin)

This reduces the oxygen-carrying capacity of blood, leading to hypoxia.

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Heavy Metals and Toxic Chemical Reactions

Pollutants like lead (Pb), mercury (Hg), arsenic (As), and cadmium (Cd) interact with atmospheric components forming:

• Oxides
• Chlorides
• Organic-metal complexes

These compounds deposit on soil and water and enter the food chain.

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Atmospheric Oxidants and Radical Chemistry

The atmosphere is governed by radicals like:

• OH (hydroxyl radical)
• HO₂ (hydroperoxy radical)
• RO₂ (organic peroxy radicals)

These radicals determine:

• Lifetime of pollutants
• Rate of smog formation
• Pathways of pollutant conversion

OH is often called the “detergent of the atmosphere” because it controls the breakdown of pollutants.

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Scientific Conclusion

Air pollution is often described as a silent killer, but in reality, it is a complex atmospheric adversary—shaping everything from human health to ecological balance, climate dynamics, and economic stability. Its true danger lies not merely in the smoky haze we see, but in the invisible network of chemical reactions unfolding above us. Understanding the origins, transformations, and consequences of these pollutants equips society with the power to intervene intelligently and effectively.

Clean air is not an aspirational luxury; it is a fundamental prerequisite for long, healthy, and sustainable living. The chemistry of the atmosphere constantly evolves—primary pollutants are transformed into more toxic secondary species, SO₂ and NOx convert into acidic compounds that fall as acid rain, and photochemical reactions drive the formation of ozone and fine particulates that infiltrate the deepest regions of the human lung. These molecular interactions magnify the severity of pollution and ripple across scientific, medical, economic, and policy domains.

Addressing air pollution therefore demands more than awareness; it requires coordinated scientific insight and decisive societal action. Cleaner energy technologies, rigorous emission standards, comprehensive air quality monitoring, and sustainable urban planning form the backbone of an effective response. As we advance our understanding of atmospheric chemistry, we refine our ability to design interventions that are not only corrective but preventive.

Ultimately, the path to cleaner air is a collaborative journey—uniting science, governance, industry, and citizens. By embracing evidence-based strategies and fostering a culture of environmental responsibility, we can build communities that are healthier, more resilient, and capable of thriving in harmony with the atmosphere that sustains us.