December 2, 2025 11:32 am
Air pollution refers specifically to the degradation of air quality within the troposphere, the lowest layer of Earth’s atmosphere where weather phenomena occur and where all terrestrial life interacts with the gaseous envelope. Pollution in this layer is mainly anthropogenic and arises from diverse sources such as vehicular exhausts, industrial emissions, and agricultural practices.
The troposphere, extending from the Earth’s surface up to roughly 8–15 km depending on geographical location, acts both as a reservoir and a reactive medium for pollutants. However, due to dynamic meteorological processes like wind, turbulence, and rainfall, most pollutants in this layer tend to have a short residence time. These pollutants are eventually washed out by precipitation, degraded by chemical reactions, or deposited on land or water surfaces.
What Happens when Pollutants reach directly in Stratosphere?
Although the troposphere is the principal site of pollution events, certain extreme phenomena—such as major volcanic eruptions—can inject pollutants directly into the stratosphere, the layer above the troposphere. Unlike the troposphere, the stratosphere is relatively stable and less turbulent, allowing pollutants (e.g., volcanic ash or sulfur aerosols) to linger for years.
This persistence can result in global-scale climatic effects, such as:
- Reduction in solar radiation due to atmospheric scattering and absorption
- Lowering of global surface temperatures, which is the basis of the hypothetical “nuclear winter” scenario—where widespread combustion from nuclear warfare could throw enough soot into the stratosphere to significantly block sunlight, disrupting agriculture and ecosystems.
Such events underline the difference in environmental impact and temporal scale between pollution confined to the troposphere and pollution that reaches the stratosphere.
Mechanism of Atmospheric Air Pollution
Air pollution in the troposphere is typically a result of continuous emissions from anthropogenic sources such as:
- Vehicular exhausts (carbon monoxide, nitrogen oxides, hydrocarbons)
- Industrial chimneys (sulfur dioxide, particulate matter, volatile organic compounds)
These pollutants do not remain inert. Instead, they undergo secondary reactions in the atmosphere, often in the presence of sunlight, forming more toxic compounds. A classic example includes the formation of tropospheric ozone (O₃) through photochemical reactions between nitrogen oxides (NOₓ) and volatile organic compounds (VOCs)—a phenomenon central to photochemical smog.
Thus, air pollution is a dynamic and reactive process, not merely an accumulation of emitted substances.
Dispersion and Variability of Pollutants
Pollutants do not remain static at their point of emission. Their concentration varies depending on:
- Wind speed and direction – Influences horizontal dispersion
- Thermal turbulence – Vertical mixing of pollutants
- Topography – Valleys may trap pollutants, while open plains promote dispersion
- Temperature inversions – Can trap pollutants near the surface, exacerbating pollution levels
Therefore, atmospheric pollution is heterogeneous—its severity changes with location, meteorological conditions, and time.
Defining Air Pollution:
Merely detecting a pollutant in the air does not qualify as air pollution. The threshold for pollution is crossed only when the concentration of contaminants exceeds prescribed standards set by regulatory authorities like the Central Pollution Control Board (CPCB) in India or the United States Environmental Protection Agency (EPA). This distinction is critical in environmental science and policy because not all presence is pollution; excess is.
According to WHO (World health Organization):
“Air pollution is the contamination of the indoor or outdoor environment by any chemical, physical or biological agent that modifies the natural characteristics of the atmosphere.”
According to Air (Prevention and Control of Pollution) Act, 1981 – India
Legal Definition under Section 2(b):
“Air pollution” means the presence in the atmosphere of any air pollutant.
“Air pollutant” [Section 2(a)]:
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.
Key Indicators of Air Pollution
The standard set of pollutants considered indicators of air pollution includes:
- Carbon Monoxide (CO): A colorless, odorless gas released from incomplete combustion of fossil fuels, especially from vehicular exhausts. It interferes with the oxygen-carrying capacity of blood and poses serious health risks at high concentrations.
- Nitrogen Oxides (NOₓ): These include nitrogen dioxide (NO₂) and nitric oxide (NO), predominantly released from fuel combustion in vehicles and power plants. They contribute to the formation of photochemical smog, acid rain, and respiratory issues.
- Sulfur Oxides (SOₓ): Emitted mainly from the burning of sulfur-containing fossil fuels such as coal and oil, particularly in industrial processes. Sulfur dioxide (SO₂), the primary compound, can irritate the respiratory system and also lead to acid rain.
- Particulate Matter (PM): These are tiny solid or liquid particles suspended in the air. Particulates can include dust, smoke, soot, and other microscopic materials. Of particular concern is PM₁₀—particulates with an aerodynamic diameter of 10 micrometers or less—as they can penetrate deep into the lungs and cause severe respiratory and cardiovascular effects.
- Dust: Often treated as a component of particulate matter, dust includes coarse particles from roads, construction sites, and soil erosion. Though seemingly natural, in high concentrations it contributes to visibility reduction and respiratory problems.
- Ozone (O₃): Unlike the other pollutants, ozone is not directly emitted into the atmosphere. It is formed through complex photochemical reactions involving nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) under the influence of sunlight. Ground-level ozone is a major component of photochemical smog and is harmful to both humans and vegetation.
Baseline Composition of Clean Dry Air
To effectively detect and measure pollution, we must know the baseline composition of unpolluted, clean air. In clean dry air, the gaseous composition (by volume) is approximately:
| Gas | Percentage Volume |
| Nitrogen (N₂) | 78.08% |
| Oxygen (O₂) | 20.95% |
| Argon (Ar) | 0.93% |
| Carbon Dioxide (CO₂) | 0.03% (now ~0.04%) |
In addition to these, trace gases are also present, which play vital roles in climate dynamics and atmospheric chemistry:
- Neon, Helium, Krypton, Xenon – Noble gases with inert behavior
- Methane (CH₄), Nitrous oxide (N₂O) – Greenhouse gases
- Ozone (O₃) – Beneficial in the stratosphere, harmful in the troposphere
- Carbon monoxide (CO) – Highly toxic, especially indoors
- Ammonia (NH₃) – Agricultural emissions, precursor to PM formation
- Sulfur dioxide (SO₂), Nitrogen dioxide (NO₂) – Respiratory irritants and acid rain precursors
- Hydrogen sulfide (H₂S) – Malodorous and toxic
These gases, although minute in concentration, can act as potent pollutants if emitted in large volumes or under specific environmental conditions.
Types of Air Pollutants and Their Sources
Air pollution results from the introduction of harmful substances into the Earth’s troposphere due to both natural processes and human activities. These pollutants vary in form, chemical composition, and environmental behavior. Broadly, air pollutants are classified based on their source, chemical nature, and method of formation.
Air Pollutants from Human Activities (Anthropogenic Sources)
Human activities such as industrial production, vehicular transport, energy generation, and urban development have drastically increased the emission of harmful air pollutants. These are broadly grouped into two major types: particulate pollutants and gaseous pollutants, based on their physical state.
Suspended Particulate Matter (SPM)
Definition and Nature
Suspended Particulate Matter refers to tiny solid or liquid particles that are suspended in the atmosphere. These particles range in size, origin, composition, and reactivity. Depending on their size, they may remain airborne for seconds to days, travel long distances, and affect both outdoor and indoor air quality. Some particles are visible to the naked eye, such as road dust or smoke, while others are microscopic aerosols that require specialized equipment for detection.
SPM is generally classified into:
- PM₁₀ (particles ≤10 micrometers): Can penetrate the upper respiratory tract.
- PM₂.₅ (particles ≤2.5 micrometers): Can reach deep into the lungs and even the bloodstream.
SPM has both natural sources (like dust storms and sea spray) and anthropogenic sources (like vehicular emissions, industrial operations, and combustion).
Components of SPM: Detailed Description
Bio-Particles
These are biological or semi-biological entities suspended in the air, originating from both natural ecosystems and human-modified environments.
- Examples: Bacteria, viruses, fungal spores, pollen grains, insect debris.
- Sources: Agricultural fields, forests, stagnant water bodies, composting sites, and indoor ventilation systems.
- Impacts:
- Cause allergic reactions, including hay fever and asthma.
- Serve as pathogen carriers, contributing to airborne diseases like tuberculosis and influenza.
- In occupational settings (e.g., grain silos or compost plants), they can trigger hypersensitivity pneumonitis and chronic lung disease.
Dust Particles
Dust is a heterogeneous mix of mineral and organic particles, generally coarse in size, though some components fall under the PM₁₀ or PM₂.₅ category.
- Sources: Road transport, construction activities, demolition work, mining, desertification, soil erosion.
- Chemical Composition: Silica, aluminosilicates, calcium carbonate, organic residues.
- Impacts:
- Irritates eyes, nose, and throat.
- Prolonged exposure causes chronic bronchitis and reduced lung function.
- Reduces visibility, contributing to haze in urban areas.
- Acts as a carrier for toxic compounds, including metals and pathogens.
Smoke and Soot
These are by-products of incomplete combustion of carbonaceous materials and are mostly carbon-based ultrafine particles.
- Sources: Vehicle exhaust, biomass burning, industrial chimneys, household cookstoves.
- Chemical Nature: Rich in polycyclic aromatic hydrocarbons (PAHs) and elemental carbon.
- Impacts:
- Major component of black carbon, which absorbs sunlight and contributes to global warming and glacial melting (especially in the Himalayas).
- Penetrates deep into lung alveoli, leading to chronic obstructive pulmonary disease (COPD), lung cancer, and cardiovascular disorders.
- Interferes with photosynthesis by depositing on plant leaves.
Mist and Spray
Mist consists of liquid droplets suspended in air, while sprays are larger droplets often formed during mechanical or chemical processes.
- Sources: Industrial cooling systems, pesticide spraying, combustion of oil, fogging systems, paint application.
- Chemical Composition: May contain sulfuric acid, oil derivatives, solvents, or toxic metals depending on source.
- Impacts:
- Respiratory irritation due to acidic or toxic content.
- Can form acid aerosols, contributing to indoor and workplace air pollution.
- Inhaled mist from pesticides may lead to neurotoxicity, hormonal disruption, and skin allergies.
Fumes
Fumes are extremely fine solid particles generated by the condensation of vaporized metals during high-temperature operations.
- Sources: Welding, metal smelting, foundries, soldering, electroplating units.
- Composition: Often contain zinc oxide, aluminum, cadmium, lead, or copper particles.
- Impacts:
- Cause metal fume fever, a flu-like illness among industrial workers.
- Long-term exposure leads to lung fibrosis, neurological damage, and increased risk of cancer.
- May accumulate in the liver, kidneys, and bones, leading to systemic toxicity.
Asbestos Fibers
Asbestos refers to a group of naturally occurring fibrous silicate minerals used extensively in insulation, fireproofing, and construction materials.
- Sources: Deteriorating insulation panels, pipe lagging, old roofing materials, demolition of asbestos-containing buildings.
- Properties: Heat-resistant, chemically inert, and very fine—ideal for becoming airborne.
- Impacts:
- Causes asbestosis, a chronic lung condition marked by fibrosis.
- Associated with mesothelioma, a rare but fatal cancer of the lung lining.
- Exposure is especially hazardous in confined indoor environments without proper ventilation.
Pesticide Residues
Pesticides used in agriculture, gardening, and urban pest control often vaporize or adhere to dust particles, forming toxic SPM.
- Sources: Spraying in fields, gardens, mosquito control programs, household insecticides.
- Common Compounds: Organochlorines (e.g., DDT), organophosphates, carbamates.
- Impacts:
- Long-term inhalation may lead to endocrine disruption, neurodevelopmental disorders, and cancer.
- Many pesticides are persistent in the environment and bioaccumulate in the food chain.
- Also pose risk to pollinators, affecting biodiversity and crop yields.
Metallic Dust
Metallic dust refers to fine particles of toxic and non-toxic metals released from industrial processes.
- Sources: Smelting plants, battery manufacturing, electronic waste recycling, welding, and steel industries.
- Toxic Metals:
- Arsenic: Carcinogenic and linked to skin lesions.
- Cadmium: Affects kidneys and bones; causes itai-itai disease.
- Chromium (VI): Potent carcinogen affecting respiratory system.
- Nickel: Allergenic and a probable carcinogen.
- Lead: Damages nervous system, particularly in children.
- Zinc and manganese: Less toxic but may cause metal fume fever.
- Impacts:
- Neurotoxicity, renal failure, cancer, and immune dysfunction.
- Contribute to environmental contamination when deposited on soil or water bodies.
- Pose occupational hazards in industrial zones, especially with poor protective equipment.
Suspended Particulate Matter is a complex and diverse class of air pollutants that pose serious risks to both human health and the environment. Whether they are biological, chemical, or metallic in origin, their size and persistence determine their penetration into the respiratory system and their systemic effects.
Fine particulates (PM₂.₅ and smaller) are especially dangerous due to their ability to bypass the upper respiratory defenses, enter the bloodstream, and affect multiple organ systems. Additionally, many of these particles act as carriers for toxic gases or undergo atmospheric transformation, making their behavior difficult to predict and control.
Understanding the source-specific nature and impact pathway of each component of SPM is essential for:
- Designing air quality standards
- Implementing targeted control technologies
- Formulating regulatory frameworks under environmental policies like the Air (Prevention and Control of Pollution) Act, 1981
Major Gaseous Pollutants
Air pollution from gaseous pollutants is one of the most widespread and damaging forms of environmental degradation. These pollutants are primarily emitted from anthropogenic sources such as vehicles, industries, agriculture, and power plants. Some of these gases are primary pollutants released directly into the atmosphere, while others are secondary pollutants formed through atmospheric reactions. Their impact varies based on their chemical nature, reactivity, and concentration.
Nitric Oxide (NO) and Nitrogen Dioxide (NO₂)
Collectively known as oxides of nitrogen (NOₓ), these gases are released mainly during high-temperature combustion in internal combustion engines, thermal power stations, and industrial furnaces.
- Nitric oxide (NO) is a colorless gas that is relatively inert on its own but quickly oxidizes in the atmosphere to form nitrogen dioxide (NO₂).
- Nitrogen dioxide (NO₂) is a reddish-brown, highly reactive gas with a pungent smell. It is a strong oxidant that penetrates deep into the lungs and irritates the mucosal lining.
Environmental and Health Impacts:
- Contributes to photochemical smog formation.
- Reacts with water vapor to form nitric acid (HNO₃), leading to acid rain.
- Causes lung inflammation, reduced lung function, and worsens asthma and bronchitis.
- Precursor to ground-level ozone and secondary aerosol particles.
Sulphur Dioxide (SO₂)
A major air pollutant emitted from the combustion of sulfur-rich fossil fuels like coal and petroleum, as well as from the smelting of sulfide ores in metal industries.
- SO₂ is a colorless gas with a sharp, choking odor and is highly soluble in water, forming sulfurous acid (H₂SO₃) and later converting to sulfuric acid (H₂SO₄) in the atmosphere.
Environmental and Health Impacts:
- One of the primary contributors to acid rain, which lowers the pH of lakes, soils, and rivers, damaging aquatic ecosystems and soil fertility.
- Affects plant leaves and stunts growth.
- Causes bronchial irritation, wheezing, coughing, and exacerbates respiratory diseases.
- Damages historical monuments (e.g., the Taj Mahal) through acid deposition.
Carbon Monoxide (CO)
An extremely toxic, colorless, and odorless gas primarily produced from incomplete combustion of carbonaceous fuels in vehicles, household stoves, and industrial boilers.
- CO binds with hemoglobin in red blood cells to form carboxyhemoglobin, which reduces the oxygen-carrying capacity of blood.
Environmental and Health Impacts:
- Even at low concentrations (as low as 100 ppm), it causes headaches, dizziness, fatigue, and at higher levels can lead to coma or death.
- Major contributor to urban pollution, especially in traffic-congested areas.
- Plays a role in tropospheric ozone formation through atmospheric reactions with hydroxyl radicals.
Carbon Dioxide (CO₂)
Though naturally present and non-toxic at ambient levels, CO₂ has become the most significant greenhouse gas due to excessive emissions from anthropogenic sources.
- Major sources include fossil fuel combustion, deforestation, industrial processes, and biomass burning.
Environmental Impacts:
- Primary driver of global warming and climate change.
- Causes glacial melting, sea-level rise, altered rainfall patterns, and increased frequency of extreme weather events.
- Although not directly harmful to human health at atmospheric levels, its long-term environmental effects are catastrophic.
Ozone (O₃) – Tropospheric (Ground-level)
Ozone in the lower atmosphere is a secondary pollutant, not directly emitted but formed by photochemical reactions involving NOₓ and VOCs (Volatile Organic Compounds) in the presence of sunlight.
Formation Mechanism:
- NO₂ → NO + O (under UV light)
- O + O₂ → O₃
Environmental and Health Impacts:
- Powerful oxidizing agent that affects plant metabolism, causes leaf damage, and reduces agricultural productivity.
- Leads to oxidative stress in human lungs, triggering or aggravating asthma, bronchitis, and other respiratory issues.
- Damages materials like rubber, paint, and textiles.
- Note: In the stratosphere, ozone plays a protective role by absorbing harmful UV radiation, but at the ground level, it is a pollutant.
Peroxyacetyl Nitrate (PAN)
PAN is a highly reactive secondary pollutant formed by the reaction of hydrocarbons (especially unburnt VOCs) and nitrogen oxides under sunlight.
Chemical Profile:
- A component of photochemical smog, it is a strong eye irritant and plant toxin.
Environmental and Health Impacts:
- Causes eye irritation, tearing, and discomfort, even at low concentrations (~0.01 ppm).
- Damages foliage and crops, especially sensitive species.
- Reacts with other pollutants to form complex oxidants harmful to both urban ecosystems and human health.
Hydrogen Fluoride (HF)
A corrosive and highly toxic gas emitted during:
- Aluminum smelting
- Glass etching
- Phosphate fertilizer production
Health Impacts:
- Causes fluorosis, damaging bones and teeth.
- Leads to respiratory irritation and, at higher exposures, causes systemic toxicity.
- Can corrode metal and building materials.
Ammonia (NH₃)
Ammonia is released into the atmosphere primarily through:
- Livestock manure
- Fertilizer application
- Composting and sewage treatment
Though used as a fertilizer, in the atmosphere NH₃ is highly reactive.
Environmental and Health Impacts:
- Reacts with nitric and sulfuric acid to form ammonium salts (a major component of PM₂.₅).
- Causes irritation of eyes, nose, and throat.
- In high concentrations, can result in pulmonary edema.
- Contributes to soil and water eutrophication when deposited through rain.
Chlorine (Cl₂)
An industrial gas used in:
- Disinfectants
- Plastic and paper manufacturing
- Water treatment
Toxicological Profile:
- Extremely corrosive and toxic when inhaled.
- Reacts with moisture in the mucous membranes to form hydrochloric acid, damaging the lungs and throat.
- At high concentrations, it can be fatal, especially in enclosed spaces.
Hydrogen Sulphide (H₂S)
H₂S is a colorless gas with a characteristic rotten egg smell, produced from:
- Petroleum refining
- Sewage treatment
- Anaerobic decomposition of organic matter
Health Impacts:
- Neurotoxic in high concentrations.
- Even at low levels, it causes eye irritation, coughing, and nausea.
- At higher concentrations (~1000 ppm), it can lead to loss of consciousness and death.
Hydrocarbons (HCs)
These are organic compounds composed of hydrogen and carbon, many of which are volatile and contribute to the formation of photochemical smog.
Sources:
- Vehicular exhaust
- Crude oil refining
- Fuel evaporation
- Chemical solvents
Common Hydrocarbons:
- Methane (CH₄) – a potent greenhouse gas, also emitted from landfills and agriculture.
- Ethane, Butane, Propane, Isopentane, Acetylene, Ethylene – released from fuel storage and combustion.
Environmental and Health Impacts:
- Serve as precursors for ozone and PAN formation.
- Some (like benzene) are carcinogenic.
- Interfere with oxygen transport and contribute to smog formation, reducing air clarity and harming vegetation.
Aldehydes and Alcohols
These are organic oxygenated compounds released through:
- Incomplete combustion
- Paints and solvents
- Industrial chemical reactions
Key Effects:
- Formaldehyde, a common aldehyde, is a Group 1 carcinogen.
- Cause eye and respiratory tract irritation.
- Participate in photochemical reactions leading to smog.
The gaseous pollutants described above constitute the core of urban and industrial air pollution. Each gas varies in terms of:
- Source (natural or anthropogenic),
- Reactivity (whether it participates in atmospheric chemistry),
- Health impact, and
- Environmental consequence.
While some like CO₂ pose long-term climate risks, others like CO and SO₂ cause immediate health hazards. The growing complexity of pollutant interactions, particularly under sunlight, makes it essential to understand both primary and secondary effects. Efficient pollution control demands not just emission reduction but also a deep grasp of the chemical lifecycle of these gases in the atmosphere.
Air Pollutants from Natural Sources
While most attention in environmental discourse focuses on air pollution from anthropogenic sources, it is important to recognize that natural processes also contribute to atmospheric pollutants. These emissions occur as part of Earth’s ecological and geological cycles and are usually sporadic, spatially diffused, and low in concentration. However, in some conditions—especially when combined with human activities—these natural pollutants can become significant environmental and health hazards.
Pollen and Volatile Organic Compounds (VOCs) from Plants
Pollen grains are microscopic biological particles released by flowering plants during pollination. While they are part of natural reproductive cycles, in high concentrations they contribute to bioaerosols, especially during spring and early summer seasons.
Volatile Organic Compounds (VOCs) from plants include:
- Isoprene
- Terpenes
These compounds are emitted naturally as a form of plant communication or defense, especially under stress or high temperature.
Environmental and Health Impacts:
- Pollen grains are major allergens and trigger asthma, hay fever, and seasonal allergic rhinitis, particularly in urban areas where they mix with vehicular emissions.
- VOCs participate in atmospheric photochemistry, reacting with nitrogen oxides under sunlight to form tropospheric ozone, a secondary pollutant.
- High VOC emissions in forested areas like the Amazon or Deccan plateau may significantly alter regional air chemistry, especially during hot seasons.
Sulphur Dioxide (SO₂) and Hydrogen Sulphide (H₂S) from Volcanic Activity and Organic Decay
Volcanoes are potent natural emitters of gases, particularly sulphur dioxide (SO₂) and hydrogen sulphide (H₂S), both of which contribute to atmospheric sulfur load.
- Sulphur dioxide (SO₂) is released in vast amounts during volcanic eruptions and can linger in the stratosphere, forming sulfate aerosols that reflect solar radiation and temporarily cool the planet.
- Hydrogen sulphide (H₂S) is produced during the anaerobic decomposition of organic matter in wetlands, swamps, and bogs, and is also emitted from geothermal vents and hot springs.
Environmental and Health Impacts:
- High levels of SO₂ and H₂S near volcanic zones can cause respiratory distress, irritation of eyes and skin, and contribute to acid rain formation.
- Volcanic eruptions such as Mount Pinatubo (1991) injected large quantities of SO₂ into the stratosphere, temporarily reducing global temperatures.
- Although these emissions are episodic, they can have global climatic effects depending on the magnitude and altitude of emission.
Particulates from Wildfires, Sea Spray, and Wind Erosion
Wildfires, whether naturally occurring (due to lightning) or triggered by humans, release vast amounts of particulate matter and gaseous pollutants into the air.
- Smoke, soot, and ash generated by forest fires contain carbon-rich particles that contribute to both local air quality deterioration and long-range transport of pollutants.
- Sea spray contributes aerosolized salts and minerals, including sodium chloride, magnesium, and calcium particles into coastal and marine air.
- Wind erosion in arid and semi-arid regions like Rajasthan, Sahara, and the Middle East lifts mineral dust into the atmosphere, often reaching global scales (e.g., Sahara dust traveling to the Amazon basin).
Environmental and Health Impacts:
- Wildfire smoke can raise PM₂.₅ concentrations far beyond safe limits, causing acute respiratory distress, particularly in vulnerable populations.
- Sea salt aerosols are less toxic, but they can affect cloud formation and precipitation patterns.
- Wind-blown dust contributes to reduced visibility, soil loss, and may carry pollutants and pathogens over long distances.
Natural Radioactivity: Radon Gas
Radon (Rn) is a radioactive noble gas that is produced by the decay of uranium present in the Earth’s crust. It seep through cracks in rocks and soils and can accumulate in enclosed spaces such as basements and underground dwellings.
- It is odorless, colorless, and chemically inert, making it difficult to detect without specialized instruments.
- Radon exposure is geologically dependent, being higher in granite-rich regions or areas with uranium-rich soil profiles.
Environmental and Health Impacts:
- Radon is a Group 1 carcinogen, and long-term exposure is the second leading cause of lung cancer after smoking.
- Particularly problematic in indoor air pollution, especially in poorly ventilated houses in high-radon-emission zones.
- Radon decay products attach to airborne particulates and are inhaled, depositing radioactive particles directly into the lungs.
Natural sources of air pollution are integral parts of the Earth’s biogeochemical and atmospheric cycles. While these emissions are often transient, geographically localized, and lower in intensity, they may:
- Combine with anthropogenic pollutants to intensify air quality degradation
- Act as precursors to complex chemical reactions in the atmosphere
- Cause episodic pollution events like volcanic smog (vog), dust storms, and wildfire smoke plumes
Understanding natural pollutants is essential not just for environmental monitoring, but also for disentangling their effects from human-induced emissions in climate modeling, public health policy, and sustainable urban planning.
Primary and Secondary Pollutants
Air pollutants are also distinguished based on their behavior and transformation in the atmosphere.
Primary Pollutants
Primary pollutants are those directly emitted into the atmosphere from identifiable sources. They retain their chemical form as originally released and are the building blocks for more complex pollutants. These include:
- Particulate matter (dust, soot, ash)
- Sulphur dioxide (SO₂)
- Nitrogen oxides (NO, NO₂)
- Carbon monoxide (CO)
- Carbon dioxide (CO₂)
- Volatile hydrocarbons
Primary pollutants are directly responsible for airborne toxicity, respiratory irritation, and environmental acidification.
Major Secondary Pollutants: Formation and Impact
Secondary air pollutants are not emitted directly into the atmosphere. Instead, they are formed through chemical reactions involving primary pollutants and atmospheric components such as sunlight, water vapor, or oxygen. These pollutants often have greater toxicity, reactivity, and persistence than their precursors, making them a significant focus in air pollution control and policy.
Ozone (O₃)
Formation Mechanism
Ozone in the troposphere is a secondary pollutant formed through photochemical reactions. The process begins when nitrogen dioxide (NO₂), emitted from vehicles and industrial activities, is broken down by ultraviolet radiation (sunlight):
- NO₂ + sunlight (UV) → NO + O
- O + O₂ → O₃
This newly formed ground-level ozone becomes a major component of photochemical smog.
Environmental and Health Impacts
- Acts as a strong oxidizing agent, causing damage to crops, inhibiting photosynthesis, and reducing agricultural productivity.
- Reacts with rubber and textiles, degrading materials over time.
- Inhalation of ozone leads to respiratory tract inflammation, throat irritation, chest tightness, and worsening of asthma and bronchitis.
- Even low concentrations (0.001 ppm) can trigger symptoms in sensitive individuals.
Note: While ozone is harmful at ground level, it is beneficial in the stratosphere, where it forms the ozone layer that shields Earth from harmful UV-B radiation.
Photochemical Smog
Definition and Formation
Photochemical smog is a brownish haze observed in many urban and industrial regions, particularly under conditions of high solar radiation, dry weather, and stagnant air. It results from complex interactions between primary pollutants such as nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) in the presence of sunlight.
The reaction produces a cocktail of harmful substances, including:
- Ozone (O₃)
- Formaldehyde
- Peroxyacetyl Nitrate (PAN)
- Acrolein and organic peroxides
Common Settings
- Urban centers with high vehicular traffic (e.g., Delhi, Los Angeles, Beijing).
- Peak during late mornings and afternoons when sunlight intensity is maximum.
Environmental and Health Impacts
- Reduces air visibility and causes the characteristic brown-orange sky.
- Damages trees and plant foliage, especially in agricultural zones near cities.
- Causes eye irritation, coughing, headaches, and respiratory distress.
- Exerts synergistic toxicity, meaning the combined pollutants are more harmful than individual ones.
Peroxyacetyl Nitrate (PAN)
Formation
PAN is formed during photochemical smog episodes, through the oxidation of hydrocarbons (VOCs) in the presence of NO₂ and sunlight:
- Hydrocarbons + NO₂ + sunlight → PAN
It is a type of peroxyacyl nitrate, a group of highly reactive secondary pollutants.
Chemical Properties
- PAN is a stable oxidant at lower temperatures and can be transported long distances.
- At warmer temperatures, it breaks down, releasing NO₂ and other reactive radicals, contributing further to ozone formation.
Environmental and Health Impacts
- Causes eye irritation, even at concentrations as low as 0.01 ppm.
- Known to injure leaf tissue, particularly in vegetables and cash crops like spinach, beans, and cotton.
- Prolonged exposure may suppress plant growth and reduce crop yield.
- Affects mucous membranes, leading to headaches, fatigue, and lung irritation in humans.
Acid Rain
Formation Process
Acid rain is formed when primary pollutants like sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) react with atmospheric oxygen and water vapor, forming strong acids:
- SO₂ + O₂ → SO₃
- SO₃ + H₂O → H₂SO₄ (sulfuric acid)
- NOₓ + H₂O → HNO₃ (nitric acid)
These acids are incorporated into rain, snow, dew, fog, or mist, resulting in precipitation with a pH lower than 5.6.
Types of Acid Deposition
- Wet deposition: Acid rain, fog, and snow.
- Dry deposition: Acidic gases and particles settling on surfaces.
Environmental and Structural Impacts
- Aquatic ecosystems: Lowers the pH of lakes and rivers, killing fish and altering species composition.
- Soil degradation: Leaches essential nutrients (like calcium and magnesium), reducing soil fertility.
- Forests: Damages foliage, weakens trees, and increases vulnerability to pests and diseases.
- Built structures: Corrodes metals and erodes limestone, sandstone, marble, and other building materials (e.g., blackening of Taj Mahal).
- Human health: Indirectly contributes to respiratory diseases through inhalation of acid aerosols.
Acid Fog and Aerosols
Formation Mechanism
Acid fog forms when fog droplets—tiny suspended water particles near the ground—absorb acidic gases like SO₂ and NOₓ from the surrounding air. This leads to the formation of acidic water droplets. When the fog clears, acid residues and aerosols remain in the air.
Acid aerosols are ultrafine solid or liquid particles suspended in the air containing sulfates, nitrates, and hydrogen ions.
Environmental and Health Impacts
- When inhaled, these particles can penetrate deep into the lungs, bypassing natural respiratory defenses.
- Increase the risk of chronic respiratory diseases such as asthma, bronchitis, and even lung cancer.
- Acid fog in mountainous or urban valleys persists longer and is more concentrated due to temperature inversions and low wind movement.
- Contribute to visibility reduction and acid deposition on nearby soil, water, and vegetation.
Major Air Pollutants: Sources and Effects on Human Beings and Environment
| Pollutants | Primary Sources | Major Effects on Human Health | Major Environmental Effects |
| Oxides of Carbon (COₓ) – Carbon Dioxide (CO₂) – Carbon Monoxide (CO) | – Combustion of fossil fuels (coal, oil, natural gas) – Biomass burning – Industrial and vehicular emissions | – CO₂: Non-toxic directly, but contributes to climate-related health risks (heatstroke, disease spread) – CO: Binds with haemoglobin 250x more than O₂ → reduced oxygen transport → fatigue, unconsciousness, death in high doses | – CO₂: Primary greenhouse gas → intensifies global warming, alters precipitation patterns, and leads to sea-level rise – Forms weak carbonic acid, contributing mildly to acid rain – CO indirectly forms tropospheric ozone through secondary reactions |
| Oxides of Sulphur (SOₓ) – Sulphur Dioxide (SO₂) – Sulphur Trioxide (SO₃) – Sulfates (SO₄²⁻) | – Combustion of sulphur-containing coal & oil – Ore smelting – Petroleum refining – Paper & pulp industries – Municipal incinerators | – SO₂ causes bronchoconstriction, asthma, emphysema, and other chronic respiratory diseases – Irritates eyes, skin, and mucous membranes – Exacerbates cardiovascular diseases in vulnerable populations | – Forms acid rain after reacting with water vapor → damages forests, acidifies lakes – Causes corrosion of buildings, metals, and monuments – Damages plant leaves and inhibits photosynthesis |
| Oxides of Nitrogen (NOₓ) – Nitric Oxide (NO) – Nitrogen Dioxide (NO₂) – Nitrous Oxide (N₂O) – Nitrate (NO₃⁻) | – Burning of fossil fuels – Biomass burning – Fertilizer production – Lightning (natural source) | – Causes eye and throat irritation, cough, reduced lung function – Increases risk of influenza, bronchitis – Chronic exposure linked to cardiopulmonary disorders | – Forms secondary pollutants: ozone, nitric acid, PAN – Major contributor to acid rain – Reduces visibility (via nitrate aerosols) – N₂O is a potent greenhouse gas, 298x more effective than CO₂ |
| Hydrocarbons (VOCs) – Methane, Benzene, Ethylene, Benzopyrene etc. | – Evaporation from fuel tanks – Incomplete combustion – Biomass burning – Landfills – Industrial solvents | – Many are carcinogenic (e.g., benzene, benzopyrene) – Cause central nervous system depression, dizziness, nausea – Some cause liver and kidney damage | – Participate in photochemical smog formation – Reduce air quality, visibility – Damage crops and plant growth through oxidative stress |
| Other Organic Compounds – CFCs, Formaldehyde, Vinyl chloride, Trichloroethylene | – Aerosol sprays – Foam manufacturing – Refrigeration units – Paints and solvents – Plastic industry | – Formaldehyde is a probable carcinogen and strong irritant – Vinyl chloride causes liver cancer – Trichloroethylene affects central nervous system | – CFCs destroy the ozone layer, increasing UV-B radiation → skin cancer, cataracts in humans – UV radiation also harms phytoplankton and disrupts food chains |
| Heavy Metals and Inorganic Compounds – Lead (Pb), Mercury (Hg), Arsenic, Cadmium, Hydrogen fluoride (HF), Hydrogen sulfide (H₂S) | – Oil refining – Transport fuels (especially leaded gasoline) – Pesticides and fungicides – Metal smelting and coal combustion – Fluoride industry | – Lead causes neurological damage, especially in children (IQ loss, developmental delays) – Mercury causes kidney and brain damage – H₂S causes olfactory fatigue, dizziness, unconsciousness – Many are carcinogenic or teratogenic | – Accumulate in soil and water, entering the food chain – Toxic to fish, birds, livestock – Damage crops and reduce soil fertility – Fluoride accumulation leads to fluorosis in grazing animals |
| Acid-forming Liquid Droplets – Sulphuric acid (H₂SO₄) – Nitric acid (HNO₃) – Oil mist | – Secondary reactions in the atmosphere – Oil refineries – Agricultural pesticide spraying – Fumigation activities | – Corrosive to skin, eyes, and respiratory tissues – Prolonged exposure can lead to chronic bronchitis, dental erosion, skin lesions | – Causes acid rain → acidifies water bodies and soils – Deteriorates stone buildings and monuments – Acid fog may deposit acids directly into lungs when inhaled |
| Suspended Particulate Matter (SPM) – Dust, soot, fly ash, heavy metals, silica, asbestos, mist, spray | – Fuel combustion – Construction and mining – Stone crushing – Thermal power plants – Industrial emissions – Forest fires – Incinerators | – PM₂.₅ and PM₁₀ cause deep lung penetration, leading to COPD, asthma, lung cancer – Asbestos causes mesothelioma – Long-term exposure leads to cardiovascular diseases | – Deposits on plant leaves → inhibits photosynthesis – Reduces visibility, causes haze – Settles on soil and water, affecting soil respiration and water oxygenation |
| Photochemical Oxidants – Ozone (O₃), PAN, Formaldehyde, Acetaldehyde, Hydrogen peroxide, Hydroxyl radicals | – Formed by reactions between NOₓ, VOCs, and sunlight – Especially in urban traffic zones and sunny weather | – Ozone causes chest tightness, shortness of breath, and worsens asthma – PAN causes eye irritation and damages lung tissue – Formaldehyde & acetaldehyde are carcinogens and respiratory irritants | – Photochemical smog formation – Inhibits plant growth, damages foliage – Contributes to material degradation (rubber, fabrics) – Reduces solar radiation penetration (blocking sunlight for plants) |
Conclusion
This table reveals that air pollutants are multi-dimensional in nature—with interlinked sources, health effects, and environmental consequences. While primary pollutants are directly emitted, many undergo complex chemical transformations to become even more hazardous secondary pollutants. Addressing air pollution thus requires:
- Source-specific interventions (e.g., cleaner fuels, vehicular control),
- Pollutant-specific standards (like PM₂.₅ and ozone limits), and
- Integrated policy mechanisms across agriculture, industry, and urban planning.
