What is Air Pollution? Sources and Effects of major Air Pollutants


What is Air Pollution? Sources and Effects of major Air Pollutants

Air Pollution - Introduction 

Air pollution is said to exist if the levels of gases, solids, or liquids present in the atmosphere are high enough to harm humans, other organisms, or materials. The pollution could have natural causes like a forest fire or the eruption of a volcano. Our primary concern, however, is human activities, which are today responsible for most of the air pollution. 

Air pollution can occur at various scales, from local to regional. 

  • Local air pollution - This occurs at a small spatial scale. For example, indoor air pollution caused due to use of unsafe cooking fuel like wood by poorer households in India is a type of local air pollution.  
  • Regional air pollution - It is generally caused by the running of industries, thermal power plants and vehicles. It is mostly a problem in urban areas of the world. However, in developing countries like India, the burning of crop residue after harvest is also a major contributor to regional air pollution. there have been two recent examples where large-scale forest fires (Brazil's forest fires of 2019 and Australian bush fires of 2019- 2020) caused regional air pollution. 

Types of Air Pollutants -

Based on origin, air pollutants can be classified as Primary and Secondary Pollutants.

  • Primary Pollutants - These are emitted directly into the air from sources at the Earth’s surface. Examples- Carbon monoxide, nitrogen oxides, and sulphur dioxide. 
  • Secondary Pollutants -  The regional gases can also react chemically in the atmosphere to form other compounds which are known as secondary pollutants. Examples- are ozone, smog, and photochemical pollutants. 

Particulate Matter-  Apart from gasses, the second type of pollutant is particulate matter which consists of a wide range of liquid and solid particles known scientifically as aerosols. Some of these are visible as smoke, soot or dust; The smallest of these particles are hazardous to human health. Of particular concern are  PM 2.5 and PM 10 as these can be easily inhaled into the lungs and can be absorbed into the bloodstream. Such particulate matter is called Respirable Particulate Matter. As with gases, particles can be directly emitted into the air or can form gases. For example, such particles from wood burning can cause a brown haze over the region and larger particles may interfere with plant growth because they deposit on the leaves. 

What are the sources of Air pollution? 

Air pollution has both natural and human sources

1. Natural Air Pollution 

  •  Dust from Natural sources, usually large areas of land with little or no vegetation
  •  Methane is emitted by various sources.
  • Radon gas from radioactive decay within the Earth’s crust.
  • Smoke and carbon monoxide form Wildfire
  •  Vegetation, in some regions, emits environmentally significant amounts of Volatile organic compounds (VOCs) on warmer days. These volatile organic compounds react with primary anthropogenic pollutants-  specifically, NOx, SO2, and anthropogenic organic carbon compounds- to produce a seasonal haze of secondary pollutants. Black gum, poplar, oak and willow are some examples of vegetation that can produce abundant VOCs. The VOC production from these species results in ozone levels up to eight times higher than the low-impact tree species.
  •  Volcanic activity, which produces sulphur, chlorine and ash particulates

  2. Anthropogenic Sources 

  • Stationary sources include smoke stacks of power plants,  factories and waste incinerators, as well as furnaces and other types of fuel-burning heating devices.
  • In developing and poor countries, traditional biomass burning is the major source of air pollutants; traditional biomass includes wood, crop waste and dung.
  •  Mobile sources include motor vehicles, marine vessels, and aircraft.
  •  Fumes from paint, hair spray, varnish, aerosol sprays and other solvents 
  • Waste deposition in landfills, which generate methane. Methane is also an asphyxiant and displaces oxygen in an enclosed space. Asphyxia or suffocation may result if the oxygen concentration is reduced to below 19.5% by displacement.
  •  Military resources, such as nuclear weapons, toxic gases, germ warfare and rocketry
  •  Particulate matter from mining activities.  

Major Air Pollutants- Their Sources and Impacts  

1. Carbon monoxide as an air pollutant - 

Carbon monoxide is a highly poisonous, colourless, odourless and neutral gas. It is only slightly soluble in water. It is about as dense as air. It has a melting point of -205.1°C and a boiling point of -191.5°C. 

Sources of Carbon monoxide as an air pollutant- 

  • Volcanic Activity 
  • Natural forest fires 
  • Biomass burning 
  • Oxidation of methane 
  • Fossil fuel burning in refineries, power plants and motor vehicles with petrol engines
  •  Furnaces and heaters in the home can emit high concentrations of carbon monoxide if they are not properly maintained, 
  • Faulty combustion appliances are used in poorly ventilated rooms. 
  • Tobacco smoke in offices, vehicles and restaurants 

Effects of Carbon monoxide as air pollutant - Symptoms of carbon monoxide poisoning-

  • Headache 
  • Nausea 
  • Vomiting 
  • Blurred vision 
  • Chest pain 
  • Weakness 
  • Difficulty in breathing 
  • Seizures and Coma 
  • Unconsciousness and death at higher levels, 

2. Carbon dioxide as an air pollutant -

 Carbon dioxide is an odourless and colourless greenhouse gas. It traps infrared radiation to keep the earth warm. Carbon dioxide is the by-product of respiration. It is used by plants in the photosynthesis process to prepare food. 

Sources of Carbon dioxide as air pollutant- Fossil fuel combustion especially the burning of coal in refineries and power plants is the main source of carbon dioxide emission. 

Effects of Carbon dioxide as an air pollutant-  It causes no threat to human health. But the high level of carbon dioxide causes toxicity. It increases the acidity of blood by forming carbonic acid. An increase in carbon dioxide concentration leads to global warming, an international problem. 

3. Oxides of Sulphur as air pollution -  

The two major oxides of sulphur are sulphur dioxide and sulphur trioxide. Sulphur dioxide is the highly reactive oxide of sulphur. It is a pungent, irritating and rotten-smelling gas. It is about 2.5 times heavier than air. It remains in the atmosphere for 2-8 days.

When sulphur dioxide combines with oxygen in the air, some sulphur trioxide is slowly formed. Sulphur trioxide rapidly combines with water to produce sulphuric acid.

Sources of sulphur oxides as air pollutants-  

  • Volcanoes and hot springs- A toxic gas is released by marshes and other places on land and oceans due to biological decay. This gas smells like rotten eggs and it is called hydrogen sulphide. It combines with oxygen in the air and produces sulphur dioxide. 
  • Fossil fuel combustion mainly coal burning in electric power generating plants.
  • Natural gas processing 
  • Ore refining 
  • Chemical manufacturing 
  • Motor vehicles emit the least amount of sulphur oxides compared to other sources. 

Effects of oxides of sulphur as air pollutant:- 

  • Exposure of sulphur dioxide to eyes, nose, throat and skin causes irritation 
  • High concentrations of sulphur dioxide cause the irritation of respiratory system, breathing difficulties and decrease the defences of the lungs. 
  • People with lung diseases like asthma, children and older adults are more sensitive to sulphur dioxide.
  • Sulphur dioxide pollution caused the Great London Smog in 1952. It causes lung disease or bronchitis.
  • Sulphur dioxide is converted to sulphuric acid in the presence of sunlight, oxygen and water. Sulphuric acid is mainly present in acid rain. 
  • Sulphur dioxide leads to corrosion and tarnishing of metals. 

4. Oxides of Nitrogen as air pollution  

Oxides of nitrogen (often written as NOx) are a group of different gases made up of oxygen and nitrogen. The two most common nitrogen oxides are nitrogen monoxide (NO) and nitrogen dioxide (NO2). These gases are harmful to human health and the environment. Nitrogen oxides also play an important role to produce ground-level ozone, smog and acid rain. Nitrogen dioxide is converted to nitric acid which is the major component of acid rain. 
Sources of Nitrogen oxides:- 

  • Anaerobic biological processes in soil water
  • Thunderstorms and lighting 
  • Volcanic activity 
  • Photochemical reactions 
  • Fossil fuel combustions in power plants and industrial boilers 
  • Nitric acid manufacturing processes 
  • Using explosives in mining 
  • Agricultural practices (i.e. use of fertilizers) 
  • The main sources of nitrogen dioxide in homes are cigarette smoke and gas-fired appliances. 

 Effects of nitrogen oxides as air pollutants:- 

  • Low levels of nitrogen oxides irritate the eyes, nose, throat and lungs. 
  • It also causes breathing problems, tiredness and nausea.
  • Nitrogen dioxide or its products can remain in the lungs for a long time. This causes lung infection, bronchitis, flu and other respiratory diseases. 
  • Higher levels of nitrogen oxides cause swelling of the throat, decreased oxygen intake and even death. People suffering from asthma and small children are more affected by nitrogen oxides.
  • Nitrogen oxides are converted to nitric acid. Acid rain mainly contains nitric acid.
  • Nitrogen oxides lead to an increase in ground-level ozone concentration. Ozone forms photochemical smog. Smog harms human health and also disturbs the ecological balance.
  • Nitrous oxide is a greenhouse gas. It causes global warming. 

5. Particulate matter as air pollution 

Particulate matter (PM) causes serious air pollution problems in the whole world. The term particulate matter is used for a mixture of solid particles and liquid droplets suspended in the air having a diameter size of 0.0002 micrometres to 500 micrometres. It is also named SPM (Suspended Particulate Matter) 

Sources of particulate matter:- 

  • Volcanic eruptions
  • Blowing of dust and soil by wind 
  • Forest and grassland fires 
  • Burning of fossil fuels in vehicles and power plants 
  • Industrial processes 
  • Dust and asbestos particles 
  • Fly ash from smelters and mining operations
  • Smoke from incomplete combustion processes 

Effects of Particulate matter as an air pollutant:- 

Of particular concern are PM 2.5 and PM 10 as these can be easily inhaled into the lungs and can be absorbed into the bloodstream. Such particulate matter is called Respirable Particulate Matter. 

  • Chronic respiratory disease 
  • Asthma increase 
  • Lung cancer
  • Bronchitis
  • Birth defects 
  • Asbestosis 

Asbestosis is an occupational disease in workers in asbestos industries. It occurs due to the deposition of fine asbestos materials in the lungs as particulates. 

  • Particulate matter can change the amount of incoming solar radiation and outgoing terrestrial long-wave radiation. Particulates scatter radiations and change the reflection power of the earth’s atmosphere. 
  • They can also change the amount of rainfall in an area.
  • Particulate matter decreases the growth of plants. They settle on the leaves and coat and clog the stomata. The coating of leaves decreases the amount of sunlight reaching the leaves. Clogging of leaves decreases the amount of carbon dioxide needed for the photosynthesis process. 

Role of Automobiles in Creating Air pollution and other Environmental Problems -

The automobile is one of the most desired items of consumption today and the demand seems insatiable. From about 50 million in 1950, the numbers increased to more than 1 billion in 2010. The US alone has 25% of all cars in the world. China is now the second-largest car market in the world. 

The automobile contributes ta a range of environmental problems by increasing air and noise pollution, adding to solid waste, accelerating global warming, taking a heavy toll on life through accidents, using up natural non-renewable resources like oil and metals etc. 

Lead pollution caused by automobiles has been a serious problem. Lead was added to the petrol for preventing the ‘knocking’ of the engine. Lead, however, is extremely poisonous and tends to accumulate in most biological systems, Large accumulation of lead in the body can result in paralysis, blindness, and even death. It can also affect the mental development of unborn children. Lead addition in petrol is now banned in most countries. 

Automobiles need roads and highways, which have many adverse environmental effects. They use up land and consume resources like steel and cement, which require heavy energy inputs and pollute the environment. Congestion and traffic jams cause enormous loss of man-hours as well as fuel. In cities, automobile emissions are a major cause of smog.    

What are the effects of outdoor Air Pollution? 

At low levels, air pollutants irritate the eyes and cause inflammation of the respiratory tract. If the person already suffers from a respiratory illness, air pollution may lead to the condition becoming chronic at a later stage. It can also accentuate skin allergies. 

Many pollutants also depress the immune system, making the body more prone to infections. Carbon monoxide from automobile emissions can cause headaches at lower levels and mental impairment and even death at higher levels. 

Particulate matter can reduce visibility, soil clothes, corrode metals, and erode buildings. On a larger scale. Air pollution leads to acid rain, ozone layer depletion, and global warming. 

The WHO Global Burden of Disease Assessment for 2010 estimated that 627000 premature deaths in India could be attributed to outdoor air pollution. Of the pollution-related risks, a substantial increase was observed in heart diseases, cerebrovascular diseases, chronic obstructive pulmonary diseases, lower respiratory infections, and cancers (in the trachea, lungs, and bronchitis). These estimates do not include acute impacts such as asthma attacks, eye irritations, and other respiratory ailments. We do not yet know the long-term health impacts of air pollution on vulnerable populations such as children and the elderly. 

What are the Cause and effects of indoor Air Pollution? 

We give importance and attention to outdoor air pollution, but we do not realize that indoor pollution can be equally damaging. 

Pesticides, mosquito repellents, cleaning agents, etc. used in urban households can cause toxic conditions. Building materials like asbestos, glass fibre, paints, glues, and varnishes are all health hazards. They can cause irritation of the eyes and skin, respiratory ailments, and cancer. 

Air-conditioned rooms and offices cause a broad spectrum of health complaints because the sealed space accumulates various contaminants. Cigarette smoke affects both smokers and non-smokers. The concentration of pollutants indoors may be five times more than outdoors.

 The most common pollutants in urban interiors are cigarette smoke, gases from stoves, formaldehyde (from carpets and furniture), pesticides, cleaning solvents, and ozone (from photocopiers). Organisms like viruses, bacteria, fungi, dust mites, and pollens also thrive in the many ducts found in office buildings.

Urban indoor pollution results in ailments like colds, influenza, and upset stomachs. Since these are common ailments, the connection with indoor pollution is often missed.  Indoor pollution can also cause eye irritations, nausea, depression, etc, collectively called the ‘sick Building Syndrome’.

Some forms of Air Pollution - 

1. Acid Rain/ Acid Precipitation 

It is the precipitation with a pH of less than 5.6, which is more acidic than normal rain. Even normal rain is slightly acidic with average pH of 5.6, as water combines with naturally occurring carbon dioxide in the air thus forming weak carbonic acid. It includes both: 

  1. Wet acidic depositions - This can be in the form of acid rain or acid for or even acid snow can occur in colder areas. 
  2. Dry acidic depositions - These are acidic particulate matter. 

It is formed majorly by sulphur dioxide or nitrogen oxides present in the air which combine with water to form much stronger sulphuric acid or nitric acid respectively. Although these oxides are the main contributors to acid rain, other acidic like hydrochloric acid may also be involved.

Acid rain affects all elements of the environment, surface- and groundwater, soils and vegetation. It negatively affects food chains, reduces biodiversity and damages our world as discussed below: 

  • When soil becomes acidified, essential nutrients such as calcium and magnesium are leached out before the trees and plants can use them to grow. This reduces the soil’s fertility. 
  • It leads to acidification of water bodies. Some 14000 Swedish lakes, located in acidic crystalline rocks, have been affected by acidification with widespread damage to plant and animal life as a consequence.
  • Acid precipitation does not usually kill trees directly. Acid deposition destroys the surface of the leaves of trees and plants. This damage causes uncontrolled water loss and slows photosynthesis. It reduces the rate at which leaf litter decomposes, causes the death of useful microorganisms present in tree roots and reduces the rate at which soil organisms(including bacteria) respire. 
  • Soil acidification releases metals that can harm microorganisms in the soil as well as birds and mammals higher up in the food chain. The most sensitive groups include fish, lichens, mosses, certain fungi and small aquatic organisms. Some organisms may be completely eliminated, reducing biodiversity. 
  • Acid rain also disturbs the natural cycles of sulphur and nitrogen. 

2. Smog 

The word ‘smog’ was used to describe the combination of smoke and fog in 1911 by Dr H.A. des Voeux. Smog is the combination of various gases with water vapour and fine particles. Smoke particles get trapped in the gog giving it a yellow/ black colour. This smog settles over cities for many days. It is dangerous for plants, animals, human beings and materials. 

There are two different types of smog

  1. Photochemical Smog 
  2. London Smog 

Photochemical Smog 

This smog is produced when sunlight acts upon motor vehicles exhaust gases such as nitrogen oxides and volatile organic compounds.

NOx+VOCs+ Sunlight Photochemical Smog

Ozone, nitrogen oxides, peroxyacetyl nitrates (PAN), and volatile organic compounds are the main components of photochemical smog.

Effects of photochemical smog 

  • Ozone is the main component of photochemical smog. It is a powerful oxidising gas. It attacks our tissues. It causes sore throats, inflammation, discharges in nasal passages etc. 
  • Ozone affects lung function and the lower respiratory tract.
  • Ozone also attacks plants mainly citrus fruits, potatoes, legumes, soybeans etc. 
  • Other photochemical smog components and ozone cause damage to buildings and degrades rubber, clothing etc. 
  • Peroxybenzoyl nitrate produced in photochemical smog is 100 times more toxic than PAN. 

London Smog 

When sulphur is emitted in large amounts and air contains high liquid water contents, then London smog is formed. In 1952, about 4,000 deaths were caused in London due to yellow fog. It lasted for 5 days. This was called the Great London Smog.

In 1952, there was cold weather and windless conditions. Low-grade coals were burnt. This increased the amount of sulphur dioxide in the smoke. Air pollutants formed a thick layer of smog over the city.

Due to the smog, people could not see anything to a distance of a few yards. Respiratory diseases and lung function disorders were caused due to this smog. 

Air Pollution and Health  

With large-scale urbanisation and industrialisation in present times, the challenge of air pollution has assumed unprecedented dimensions. World Health Organisation (WHO) calls it an invisible killer that lurks around, attacking the body’s defences and causing mortality from heart attacks, strokes, lung diseases and cancer. As per Global Air Pollution Database released by WHO in May 2018 : 

  1. 91% of the world’s population lives in places where air quality exceeds WHO guideline limits.
  2. 97% of cities in low and middle-income countries with more than 100000 inhabitants do not meet WHO air quality guidelines. However, in high-income countries, that percentage decreases to 49%. 
  3. 4.2 million death occur every year as a result of exposure to ambient (outdoor) air pollution. 
  4. 3.8 million death occur every year as a result of household exposure to smoke from dirty cookstoves and fuels. 

This means that outdoor and indoor air pollution together kills more than 7 million people every year globally! 

WHO has set International Standards for Air Quality by recommending limits of key air pollutants for both outdoors and inside buildings and houses. 

For ambient (outdoor air pollution), the guidelines were set in 2005. Pollutants covered for outdoor air pollution are annual and daily concentrations of fine particulates, nitrogen dioxide, sulphur dioxide, carbon monoxide and ozone. 

What is Air Pollution? Sources and Effects of major Air Pollutants

Negative Effects of Air Pollution  

Air pollution has an impact on both local and global scales. Harmful substances which are emitted into the atmosphere in one country are transported by the wind and cross over national borders. Global negative effects of air pollution include the enhanced greenhouse effect and the ozone hole. Smog and acid rain are the best-known local effects and smog, in particular, affects people living in urban areas. Air pollution is a threat to our health and can also cause economic losses.

  • Humans:- It is detrimental to human health causing major respiratory disorders. Asthma and bronchitis are caused due to air pollution. Sulphur dioxide is responsible for cough and reddening of the eye due to irritation of membranes in the eye. Haemorrhage and pulmonary disorders have resulted even with very low concentrations of ozone. Dusts, grit and smokes cause tuberculosis and silicosis whereas heavy metals are carcinogenic and develop dermatitis and ulcers of skin. 
  • Animals:- The forage crops are sometimes contained metallic pollutants, such as lead, arsenic and molybdenum in mining and thermal power plants area due to air pollution. Domestic animals feeding on contaminated fodder suffer from different diseases. Animals feeding on fluoride compound-containing fodder may suffer from fluorosis. Cattle and sheep are the most frequently affected animals. 
  • Plants:- Plants are affected by various air pollutants. Excessive sulphur dioxide makes the cells inactive and finally killed. Tomato is affected by ammonia and radish, cucumber and soybean are affected due to hydrogen sulphite. Ethylene causes epinasty and early maturation of plants. 
  • Materials and Atmosphere:-  Increase in carbon dioxide concentration increase the temperature of the earth. Depletion of the ozone layer due to fluorocarbon of aerosol causes exposure to U.V. radiation which is lethal. Different metals, such as iron, aluminium and copper are corroded when exposed to contaminated air. Building and other materials are disfigured by the deposition of soot, 
  • Smog:- The term ‘smog’ was first used in 1905 for the mixture of smoke or soot and fog that produces unhealthy air. Smog can have hazardous health impacts. Minor exposure to smog can cause eye irritation cause inflammation of lung tissue thus aggravating asthma and breathing difficulties. Ground-level ozone in the smog inhibits plant growth and causes immense damage to crops and forests. 

Air Pollution Control and Abatement 

There exist some basic strategies for controlling air pollution. 

  1. Modification of internal combustion engines of vehicles can decrease the release of pollutants. 
  2. Catalytic converters are used in almost all vehicles to decrease air pollution. These convert pollutants into less harmful products. CO, nitrogen oxides and volatile organic compounds are converted to nitrogen, carbon dioxide and water. Finally, divided platinum is the most active catalyst and is widely used. But, it is of high coast. The reduction catalysts are platinum and rhodium metals. They convert nitrogen oxide to nitrogen and oxygen. Some of new converters have also started to use gold mixed with other catalysts.
  3.  Other substitute fuels instead of gasoline can be used to decrease the pollutants.
  4. Complete fuel combustion in vehicles is very important. It decreases the formation of pollutants. This is done by developing exhaust system reactors in vehicles.
  5. Lead-free petrol is used in modern cars to decrease pollutants.
  6.  Supercritical Technology-based Coal Power Plants - These power plants operate at higher temperatures and pressure than traditional power plants. Thermodynamically, such power plants operate at pressure and temperature above the critical point of water. This results in better thermal efficiency of power plants and reduced fuel consumption. Improved efficiency also means fewer emissions of greenhouse gases and pollutants like NOx. SOx and particulate matter. 
  7. Emphasis on clean energy sources like solar and wind to reduce dependence on fossil fuel burning for generating electricity.
  8. Particulate pollution control can be done through a variety of settling chambers or collectors. The collectors have a mechanism that causes particles in gases to settle out in a location where they can be collected for disposal. This is an effective way to control particulate pollution from power plants and industry. 
  9. Electrostatic precipitator is the most efficient method among all these methods and it can separate very small sized particulates. 

Frequently Asked Question


Question:  What is Air Pollution?

Answer- It is the accumulation of gaseous, particulate wastes and toxic compounds in the atmosphere. It is two types - Indoor air pollution and Outdoor air pollution 

Question: What are primary and secondary air pollutants? 

Answer- Primary air pollutants are harmful chemicals that are released directly from a source into the atmosphere and Secondary air pollutants are also harmful chemicals, but they are produced from chemical reactions involving the primary pollutants. 

Question: What are the Sources of Outdoor Air Pollution?

Answer- The sources of outdoor air pollution are the Burning of fossil fuels in automobiles and domestic cooking, Mining activities leading to dust as well as fires. 

Question: What is Acid Rain? 

Answer- It is the precipitation with a pH less than 5.6 that is more acidic than normal rain.

Question: What is smog? 

Answer - Smog is a form of outdoor pollution and the term was originally used to describe a combination of smoke, fog, and chemical pollutants that poisoned the air in industrialized cities like London.

It is used to describe air pollution that is localized in urban areas, where it reduces visibility.

Question: What is Vog?

Answer- It refers to volcanic smog produced by sulphur dioxide and other pollutants released during volcanic eruptions. 

Question: What is Meant by the Urban Heat Island Effect?

Answer -It is a phenomenon when a city (such as a business district) region experience much warmer temperatures (usually 1-5 degree Celsius higher) than nearby rural areas.

Urban heat islands result from the following causes: 

  • The dark and tall buildings and the asphalt roads absorb a significant amount of light and radiation and emit it as heat, warming the city area.
  • The lack of trees and other vegetation prevents evaporative cooling that occurs in other green areas.
  • Waste heat from vehicles, air conditioners, and generators releases heat into the air. 

Question: What is Particulate Matter?

Answer - It is a varying mixture of particles (solid or liquid) less than 10mm suspended in the air. Mining, construction, agriculture, vehicles, volcanic eruptions etc. add considerable particulates into the atmosphere. 

Some of these are visible as smoke, soot or dust; while others may not be easily visible. 

Question: What is Biomagnification?

Answer - It is the increase in the concentration of a harmful or toxic substance in the living tissue of an organism as it moves through the food chains and webs.

Also Read - Soil Pollution | Consequences of Soil Pollution


Post a Comment

Previous Post Next Post