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.
Over the years, air pollution has greatest environmental challenge being faced by people, especially in urban areas. Most often, it is caused by human activities like the burning of fossil fuels in thermal power plants, and other industrial processes like petroleum refining, smelting, mining, construction, transportation, agriculture etc. In addition to human sources of air pollution, it can also be caused by natural processes.
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 by 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, burning of crop residue after harvest is also a major contributor to regional air pollution. There have been two recent examples where large-scale forests (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.
Primary air pollutants are harmful chemicals that are released directly from a source into the atmosphere. Secondary air pollutants are also harmful chemicals, but they are produced from chemical reactions involving the primary pollutants.
Primary air pollutants include the following:
Particulate matter: This includes both solid particles and liquid suspensions. Soil particles, soot, lead, asbestos, and sulphuric acid droplets are examples.
Oxides of carbon and nitrogen and sulphur dioxide
Hydrocarbons like methane and benzene
Secondary air pollutants include the following:
Ozone: It is a form of oxygen and is a pollutant in the troposphere or the layer of the atmosphere closest to the earth’s surface. It is a beneficial component in the stratosphere from 10 to 45 Km above the earth.
Sulphur trioxide: This is formed when sulphur dioxide reacts with oxygen. In turn, sulphur trioxide combines with water to form sulphuric acid.
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 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. However, 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 pollutants -
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.
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 in producing 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. Lead as an Air Pollutant
Lead is found in different ores such as galena, Anglesite, cerusite etc
Effects of lead as an air pollutants -
- Early symptoms of lead poisoning are headache, depression, nausea, muscle pain, loss of appetite etc.
- Lead poisoning causes colics, abdominal pain, memory loss, kidney failure, skin pigmentation and even paralysis.
- Lead can damage internal organs mainly the brain, and nervous and reproductive systems.
- Lead poisoning in pregnant women causes stillbirths.
- High blood lead levels in children can cause learning problems and mental retardation.
- Lead poisoning affects the populations of several bird species and thus disturbs biodiversity.
6. Particulate matter as an air pollutant
- 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 of asbestos industries. It occurs due to 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 changes 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. 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.
Air Pollution and Human 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 the 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 100,000 inhabitants do not meet WHO air quality guidelines. However, in high-income countries, that percentage decreases to 49%.
3. 4.2 million deaths occur every year as a result of exposure to ambient (outdoor) air pollution.
4. 3.8 million deaths 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 kill 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.
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