Air Pollution and its Control: Air Pollutants, Effects of air Pollution, Smog, Acid Rain, Bhopal Gas Tragedy, Control of Air Pollution, Prevention of Air Pollution, Practice Problems and FAQs

We all like to walk through the mountain roads when we are on a trip. We love the breeze and want to take the fresh air in. So we will feel fresh and rejuvenated. Can you imagine walking in the same way on a city road in the middle of the traffic. Your answer is ‘no’, because the air is polluted.

                               Fig: Air pollution

Breathing polluted air can increase the risk of heart disease and heart attack. Air pollution also damages the development of the lungs in children. In older people, it makes existing respiratory and cardiovascular conditions bad. Long-term exposure to air pollution in childhood can lead to permanent reduction of lung function.

Dirty air has been shown to affect the development of the foetus as well. Fine particle air pollution directly affects the climate. Many of the causes of air pollution such as traffic emissions are also contributors of climate change. So the best time to run or jog is early in the morning, before the day’s traffic affects air quality. Let us understand more about air pollution in this article.

Table of contents

• Air pollutants
• Effects of air pollution
• Smog
• Acid rain
• Bhopal gas tragedy
• Control for air pollution
• Prevention of air pollution
• Practice Problems
• FAQs

What is air pollution?

Any undesirable change in the chemical, physical or biological characteristics of air is considered as air pollution.

Air pollutants

The pollutants that cause air pollution are known as air pollutants. Examples of air pollutants include gases like carbon dioxide, carbon monoxide, sulphur oxides, nitrogen oxides, hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), methane, aerosols, organic chemicals like hydrocarbons and Volatile Organic carbons (VOCs), particulate matter such as dust, smoke or soot, smog or haze, radioactive dusts, water vapour or steam.

                                                     Fig: Air pollutants

Oxides of carbon

They are formed normally by the combustion of carbon from fuels. It includes Carbon dioxide and carbon monoxide. The amount of carbon dioxide increased drastically by 15% in the last 100 years. This is caused mainly by fuel burning. Carbon monoxide emission originates from automobiles. It is also present in cigarette smoke. It is highly toxic and can impair respiration.

Oxides of sulphur

Oxidation of sulphur that is present in the fossil fuels produces sulphur dioxide and sulphur trioxide. These gases can react with water and form the sulphurous acid and sulphuric acid. This can lead to acid rain.

Oxides of nitrogen

Oxides of nitrogen present in coal and petroleum normally produce nitrogen oxides and nitrogen dioxides on combustion. These gases can also react with water to form nitrous acids and nitric acids.

Hydrocarbons (HCs) or Volatile Organic carbons (VOCs)

These are composed of hydrogen and carbon. They are produced naturally during decomposition of organic matter and by certain types of plants. They are produced normally by the combustion of fossil fuels. Examples include pine trees.

Methane

It is considered the most abundant hydrocarbon in the atmosphere.

Formaldehyde

It is a derivative of benzene. It is emitted from indoor sources like newly manufactured carpeting.

Aerosols

They are chemicals, which enter into the air in the form of vapour or fine mist. For example, jets send aerosols in the form of chlorofluorocarbons.

Particulate lead

Fossil fuels like petroleum used in automobiles contain lead products such as tetra-methyl lead and tetra-ethyl lead. Burning of such fuel emits particulate lead compounds. This lead enters inside the body and hampers the formation of haemoglobin.

Benzopyrenes

This chemical is carcinogenic. It is normally produced in automobile exhausts and tobacco smoke.

Sources of air pollution

The sources of air pollution are either artificial or anthropogenic.

Anthropogenic air pollutants

It includes the air pollutants produced directly by human activities. Examples include aerosols, cigarette smoke, emissions from factories and automobiles.

Natural air pollutants

These air pollutants are created without human intervention. They are of natural origin. Examples include emissions from volcanoes, forest fires and allergens from plants like pollen grains and fungal spores.

                                         Anthropogenic air pollutants Natural air pollutants

Effects of Air Pollution

The quality of air people breathe can affect their health. The rapid increase in industries and vehicular traffic is polluting the air more. Air pollution is a serious issue in congested and industrialised cities. It can affect all living beings in the ecosystem including humans, animals, birds and plants. It also affects the buildings, material and climate. Effects of air pollution are listed under the four headings as follows:

• Effects on human health
• Effects on vegetation
• Effects on animals
• Effects on materials
• Aesthetic loss
• Effects on climate

                                       Fig: Effects of air pollution on ecosystem

Effects on human health

The effects of air pollutants on the human body depends on the fate of particles. Particles less than 2 µm are normally trapped in nasal hair and bronchial mucus. It is either coughed out or swallowed. The smaller particles enter the alveoli and are engulfed by the special cells in the blood.

Short term effects

In humans air pollution can cause short term effects like pneumonia or bronchitis, headaches, dizziness, nausea, and irritation in nose, throat, eyes and skin.

                           Fig : Short term effects of air pollution

Long term effects

It can be respiratory diseases such as emphysema, lung cancer, nerve or brain damage, effect on kidneys, liver, and other organs. It can also affect the path of other organs, such as lungs to blood, blood to heart and heart to all organs.

                          Fig : Long term effects of air pollution

Effects on vegetation

Dust, smoke and other particulate matter will reduce sunlight and settle on leaves. This can affect photosynthesis. Chemicals like fluorides can destroy tissues and cause necrosis. PAN (Peroxyacyl nitrates) damages chloroplasts and reduces the photosynthetic rates. Chemicals like hydrocarbons cause premature leaf fall, fruit drop etc. Chemicals like sulphur dioxide cause chlorosis, plasmolysis, membrane damage and metabolic inhibition. Lichens act as pollution indicators as they are sensitive to pollution. Acid rain also affects vegetation.

Effects on animals

Ingestion of chemicals like fluorides can can cause fluorosis. It results in the excessive calcification of bones and teeth. It can lead to diarrhoea and weight loss. They are also susceptible to air bone diseases.

Deterioration of materials

Acid rains produced by oxides of sulphur and nitrogen have a corrosive effect on the buildings. For example, Taj Mahal at Agra is suffering the same. Ozone has a deteriorating effect on rubber goods. Photochemical fog has a deteriorating effect on metals, textiles and marbles.

                                               Fig: Effect of air pollution

Aesthetic loss

Smoke causes blurred vision. Foul odours of air pollutants emitted from the different types of industries, automobiles, dirty drains and garbage heaps make life unpleasant. Dusts and other materials settle on houses, monuments and plants give a bad or dirty look.

Effects on climate

Climate on Earth depends on various factors. It includes the composition of atmosphere and balance of gases. Heat produced by the industries can raise the local temperature. Global warming is a result of air pollution. Melting of glaciers and polar ice caps can lead to floods. Changes in rainfall can affect agriculture output. Ozone depletion will have serious effects on all organisms.

Smog

It is considered as the mixture of fog and smoke. It pollutes the atmosphere. Smog is formed by a chemical reaction of carbon monoxide, nitrogen oxides, and heat from sunlight. Smog leads to lower visibility. Pollutants that contribute to the formation of smog come from a variety of sources. It includes vehicle emissions, paints, smokestack emissions, and solvents.

                                                                       Fig: Smog

In 1952 in London, lethal smog had spread for 4 days. The polluted fog was so thick that people could not see a few metres in front of them. It had led to the death of 12,000 people and sickened 100,000 people. The reason was that this occurred in winter, and people burnt coal to warm themselves. Smoke from coal and other pollutants trapped in the thick fog settled all over the city.

Acid Rain

When sulphur dioxide (SO₂) and nitrogen oxides (NO_X) are released into the atmosphere and transported by wind and air currents, this results in acid rain. The SO₂ and NO_X react with water, oxygen and other chemicals to form sulphuric acid and nitric acid respectively. These then mix with water and other materials before falling to the ground in wet or dry forms. This can include rain, fog, hail, snow, or even dust particles that are acidic. The pH of acid rain is less than 5.6 only. It can go upto pH 4.

                                                                 Fig: Acid rain

Bhopal gas tragedy

In December, 1984, a chemical, methyl isocyanate (MIC) which is highly reactive to heat, spilled out from Union Carbide Corporation India Ltd’s pesticide factory, turning the city of Bhopal into a colossal gas chamber. Approximately 45 tonnes of methyl isocyanate gas leaked. India’s industrial disaster killed more than 15,000 people and affected over 600,000 workers. The Bhopal gas tragedy is known as one of the world's worst industrial disaster.

                                                     Fig: Bhopal gas tragedy

Air quality index

Air quality index (AQI) shows the quality of atmospheric air. Higher the AQI, the greater the air pollution and health concern. AQI value below 100 is considered satisfactory while above 100 it is unhealthy.

                                                                Fig: Air Quality Index

Control for Air Pollution

There are several ways to control air pollutants from getting released into the atmosphere. Among them three are major methods which include the following:

• Catalytic converter
• Electrostatic precipitator
• Scrubber.

Electrostatic precipitator

Smokestacks of thermal power plants, smelters and other industries release particulate and gaseous air pollutants. These pollutants must be separated or filtered out before releasing the gas into the atmosphere. There are several ways of removing particulate matter from the emissions of smokestacks, the most widely used is the electrostatic precipitator.

                                                  Fig : Electrostatic precipitator

Method of working of electrostatic precipitator

Electrostatic precipitator removes 99% particulate matter present in the exhaust from a thermal power plant. It has electrode wires that are maintained at several thousand volts and are negatively charged. It also has many positively charged collection plates.

                                                       Fig: Electrostatic precipitator

Emissions from smokestacks are made to pass through electrostatic precipitators. The emission contains dust particles which are later filtered out by electrostatic precipitators. Negatively charged electrode wires produce a corona that releases electrons. These electrons attach to the dust particles and give them a net negative charge.

                                                   Fig: Electrostatic precipitator

Since the collecting plates are positively charged, these attract the negatively charged dust particles. The velocity of air between the plates must be maintained low enough. It will allow the dust to fall. These dust particles are collected and removed.

                                                    Fig: Electrostatic precipitator

Clean air from the electrostatic precipitator is let out to the smokestack which is later released into the atmosphere.

                                               Fig: Electrostatic precipitator

Air purifier

Electrostatic precipitators cannot remove PM 2.5 (particulate matter of size 2.5 microns).

As per the Central Pollution Control Board (CPCB), particulate size 2.5 micrometers or less in diameter (PM 2.5) are dangerous. They are the ones causing the greatest harm to human health. These fine particulates can be inhaled deep into the lungs and can cause breathing and respiratory problems, coughing, irritation to eyes and lungs, sneezing, asthma etc.

PM 2.5 can be removed by air purifiers with high efficiency filters. In order to remove PM 2.5, one can install air purifiers with high efficiency filters at homes. These high efficiency filters have the ability to filter out very small particulate matter leaving behind clean air.

                                        Fig: Air purifier

Scrubber

Scrubber removes gases like sulphur dioxide. It is the most effective sulphur removing technique.

Method of working of scrubber

Scrubber consists of a tower-like structure with nozzles to spray lime or water. Dirty air containing particulate matter such as sulphur dioxide is passed into the tower.

                                                                 Fig: Scrubber

As the flue gases (sulphur rich stack gas) rises up, it is made to pass through a spray of water or lime. Sulphur dioxide present in flue gases reacts with water or lime solution, leaving the clean air to escape.

                                                                  Fig: Scrubber

Sulphur dioxide along with water or lime is collected back at the bottom of the tower. Sulphur dioxide reacts with lime to form calcium sulphite. Thus, sulphur dioxide is collected as calcium sulphite and removed from the scrubbing tower.

                                                                 Fig: Scrubber

Then the clean air, free of sulphur, leaves the tower from the top and is let out into the atmosphere through smokestacks. Spray-tower scrubbers are more efficient. They can remove upto 90 percent of particulates larger than about 8 μm.

                                           Fig: Scrubber

Catalytic Converter

Lack of enough oxygen to completely burn fuel in the vehicle engines causes vehicles to emit toxic by-products such as unburnt hydrocarbons, carbon monoxide and nitric oxide. Catalytic converters provide a site for conversion of toxic by-products into less hazardous substances.

Method of working of catalytic converter

Emissions from the engine are passed into the catalytic converter.

                                                              Fig: Catalytic Converter

Catalytic converters have expensive metals as the catalysts for reducing emission of poisonous gases. Two types of catalysts found in catalytic converters are platinum-palladium and platinum-rhodium.These catalysts help in oxidising and reducing the toxic by-products into non hazardous substances.

                                             Fig: Catalytic Converter

As these by-products reach catalysts, they undergo redox reactions. Unburnt hydrocarbons are converted into carbon dioxide and water. Carbon monoxide is converted into carbon dioxide. Nitric oxide is converted into nitrogen gas. Clean exhaust which has non-hazardous carbon dioxide, water, and nitrogen gas is released from the vehicle. Motor vehicles equipped with catalytic converters should use unleaded petrol because lead in the petrol inactivates the catalyst.

Air (Prevention and Control) Act

On realising the importance of preventing air pollution, the Government of India enacted the Air (Prevention and Control) Act in 1981 to provide for the prevention, control and abatement of air pollution in India. Under the Air Act, the government encourages various measures to prevent air pollution such as solar and wind energy.

Prevention of Air Pollution

Various methods to prevent air pollution are enlisted below with details:

Alternate energy sources

Coal power plants which are utilised to generate electricity is one of the major sources for air pollution. Hence, finding alternatives of fossil fuels for energy sources is important. Technologies for alternative energy, including solar and wind, are still being developed and can be a major step towards preventing air pollution.

                              GIF: Solar energy

Use of electric vehicles

Developing vehicles that do not use fossil fuels such as electric vehicles will reduce pollution.

                                GIF: Electric vehicles

Public transport

Using public transport, as the number of vehicles increases, it causes pollution. Hence, using public transport can reduce the number of vehicles on the road and thereby reduce air pollution.

                                        GIF: Public transport

Norms for fuels

Application of stringent norms for fuels, that is the use of fuels with low sulphur and aromatic content. The Government of India, by the new auto fuel policy, has laid out a roadmap which will cut down vehicular pollution in Indian cities. More stringent norms for fuels were laid down like reducing the sulphur and aromatic content in petrol and diesel fuels. It is necessary to reduce sulphur and aromatic content. When fuels containing sulphur are burned, sulphur is emitted as sulphur dioxide (SO₂) which is one of the major air pollutants.

Mass Emission Standards

Several norms were laid down like Euro III or Bharat stage III norms, stipulating that sulphur be controlled at 350 parts-per-million (ppm) in diesel and 150 ppm in petrol. Aromatic hydrocarbons should be limited to 42 per cent of the concerned fuel as per this norm. Euro IV or Bharat stage IV norms were adopted in 2017 which were even more stringent. Sulphur content was targeted to be reduced to 50 ppm in both petrol and diesel and aromatic hydrocarbons were targeted to be brought down to the level of 35 per cent. Application of stringent pollution level norms for vehicles are now implemented. According to the fuel, vehicle engines will also need to be upgraded. Older Mass Emission Standards are no longer applicable in any of the cities of India. By 2017, all the vehicles were to follow Bharat Stage IV Emission Standards.

                                       Fig: Mass Emission Standards in India

Practice problems

1. Read the given statements and identify the correct option.

Statement 1: In India, the Air (Prevention and Control of Pollution) Act came into force in 1981 and was amended in 1987.

Statement 2: According to the CPCB (Central Pollution Control Board), particulates of size 100 µm are responsible for causing great harm to human health.

1. Both the statements are correct
2. Statement 2 is correct while statement 1 is incorrect
3. Statement 1 is correct while statement 2 is incorrect
4. Both the statements are incorrect

Solution: In India, the Air Act came into force in 1981 to control and prevent air pollution. It was amended in 1987 after including noise as an air pollutant. This was the first attempt by the Government of India to combat air pollution. According to the Central Pollution Control Board (CPCB), particulates of size 2.5 micrometers or lesser in diameter are responsible for causing the greatest harm to human health. These fine particulates can normally be inhaled deep into the lungs. It can cause breathing and respiratory symptoms. It causes irritation, inflammations, damage to the lungs and premature deaths. Thus statement 1 is correct whereas statement 2 is incorrect. Hence option ‘c’ is correct.

2. Which of the following can be a method used to get rid of particulate matter present in thermal power plant exhaust?

1. Magnetic separation
2. Chromatography
3. Electrostatic precipitation
4. Mass spectrometry

Solution: Electrostatic precipitators are devices that are capable of removing 99% particulate matter present in the exhaust from industries. These devices possess negatively charged electrode wires which are maintained at several thousand volts. The electrodes produce a discharge corona that releases electrons which attach to the dust particles and give them a net negative charge. The collecting plates in this device are grounded to attract the negatively charged dust particles.

Low velocity of air is maintained between the collecting plates to allow the dust to fall in this device. Hence option ‘c’ is correct.

3. Select the incorrect statement/s among the following with respect to the acid rain.

I. Acid rain means the rainfall and other forms of precipitation which have a pH of less than 5.

II. Oxides of nitrogen and sulphur are released from the automobile exhaust and power plants.

III. The oxides of nitrogen and sulphur may react with water in the air and form nitric acid (HNO₃) and sulphuric acid (H₂SO₄) respectively.

IV. Acid rain has harmful effects on animals and human beings but no characteristic impact on plants.

1. II and III only
2. II only
3. III only and IV only
4. IV only

Solution: Acid rain refers to any form of precipitation which contains acidic components, such as sulphuric or nitric acid and falls to the ground in wet (rain, fog and snow) or dry (acidic gases and chemicals blown by wind) forms.

Pollutants such as oxides of nitrogen and sulphur dioxide (SO₂) are added in large concentrations to the air. They are released from automobile exhausts, power plants etc. The oxides of nitrogen and sulphur dioxide (SO₂) react with oxygen, atmospheric water vapour and other chemicals to form acids such as nitric acid and sulphuric acid. Sulphur dioxide (SO₂) reacts with atmospheric water (H₂O) to form Sulphurous acid (H₂SO₃) and when dioxide reacts with oxygen it forms sulphur trioxide (SO₃). SO₃ or sulphur trioxide reacts with water to form sulphuric acid (H₂SO₄). Nitric oxide (NO), contributes to the natural acidity of rainwater. It is formed during lightning storms when a reaction of nitrogen and oxygen, two common atmospheric gases, occurs. In air, NO is oxidised to nitrogen dioxide (NO₂), which in turn reacts with water to give nitric acid (HNO₃). Before they fall on to the ground in wet form or dry form, these acids mix with rain water or other substances. The pH of soil and water bodies is changed by acid rain. This affects the growth of plants and results in killing aquatic organisms too. Hence option ‘d’ is correct.

FAQs

1. How do automobiles pollute the air?

Answer: Internal combustion engines, which run on gasoline or diesel, are common in automobiles. When gasoline or diesel is burned, various harmful gases are released, including carbon dioxide, carbon monoxide, sulphur dioxide, nitrogen dioxide, and others. Furthermore, automotive engines emit microscopic particles. As a result automobiles pollute the atmosphere.

2. What are the natural sources of air pollution?

Answer: Natural dirt, usually large areas of ground with little or no plant growth. When animals, like cattle, break down their food by digestion, methane is emitted. Smoke and carbon monoxide (CO) results from jungle fires. Sulphur, ash particles, and chlorine are emitted by volcanic activities.

Humans, their habitats and the ecosystem are badly affected by air pollution. Dense particles, fluids, and gases can all be components of air pollution. Contamination might be either natural or artificial. The two types of contaminants include primary and secondary contaminants.

3. Is acid rain an air pollution?

Answer: Acid rain is caused by certain air pollutants, a subject that received a lot of attention in Europe and North America in the 1980s and 1990s. Sulphur dioxide (SO2) and nitrogen oxides (NOx) combine with water in the atmosphere to form sulphuric acid and nitric acid, which fall as ‘acid rain’ on the ground. Acid rain harms plant leaves and reduces their photosynthetic output. It depletes the soil nutrients which are required for the existence and growth of plants. Acidification of ground and river water can harm fishes and insects. It can also harm other animals that eat them. Buildings and monuments are also harmed by acid rain. For example, the Taj Mahal.

4. Why does air pollution need to be prevented?

Answer: When we breathe, pollutants enter our lungs. It can reach our bloodstream from there. This can contribute to minor disturbances like coughing or itchy eyes. It can also cause or worsen many diseases involving the organs of lungs and affect breathing. It may cause pneumonia, lung fibrosis, pneumoconiosis, silicosis etc. This may lead to hospitalizations, cancer, or even premature death. It affects nature and natural resources in a bad way.

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